Guadalupe Peak, the highest summit of the Guadalupe Mountains, lies at the crest of their wedge-shaped southern end (fig. 2). From its top, one may on clear days look out over a large section of the trans-Pecos region of Texas and New Mexico, to a horizon 100 miles or more away. One's most lasting memory of the view from the peak is the contrast that it reveals between the country to the east and to the west.
Eastward the mountains descend in a long slope to the Pecos River, 50 miles away, whose valley may be seen as a dark band in the distance. On the sky line beyond the river is the white rim of the Llano Estacado; there are no more mountains in this direction (fig. 1). As the eye scans the land on the nearer side of the river, diverse elements in the sloping surface of the mountains become evident, which may be distinguished by their form, color, and height. To the south are the flat-topped, brown, desolate ridges of the Delaware Mountains, standing several thousand feet lower than the peak. East of them, down the slope, are the gray, rounded hills of the Gypsum Plain (fig. 2). Northeastward are the much higher, sharper ridges of the Guadalupe Mountains, with white limestone ledges here and there, interspersed with darker patches of forest.
Erosion has left its mark over the whole sloping surface. The Delaware Mountains and Gypsum Plain are penetrated by an intricate network of water courses, and the Guadalupe Mountains are trenched by steep-sided canyons. The dip of the rocks on the sloping surface appears to be gentle and unbroken. In the Delaware Mountains, one can distinguish thin, straight, bedding planes, inclined at a low angle to the east.
Toward the west, the observer finds a land of entirely different aspect. He is standing on the edge of a precipice, of which the peak is the highest point, and looks out, not over plains and plateaus, but over an expanse of varied, irregularly placed mountain ranges and intervening desert basins. The effects of erosion are not as impressive as toward the east. The mountain sides are gashed by many short, steep water courses, but the eye fails to distinguish any canyons penetrating deeply into the mountains. In the desert basins, instead of long drainage lines and a network of tributaries, one sees a host of alkaline flats and ephemeral lakes, whose white crusts gleam in the sun. One notes also the steep-sided, rectilinear edges of the mountain ranges, and the occasional outcrops of tilted strata. One infers that the land to the west may derive its form more from the raising and lowering of blocks of the earth's crust than from the wearing down of the basins between the mountains.
Toward the west, the gently sloping surface of the Guadalupe and Delaware Mountains breaks off abruptly in a west-facing escarpment. The precipice on the west side of Guadalupe Peak forms the local rim of the escarpment. Below it, the declivity continues across steep rock slopes, and then over more gently sloping alluvial aprons, which extend out into the Salt Basin, the desert basin that flanks the escarpment on the west. The precipice ends a short distance south of the peak in the monolithic rock of El Capitan, but the steep slopes below continue southward along the same trend in a steplike escarpment that forms the western edge of the Delaware Mountains. The outer ends of the rock spurs of the escarpment meet the alluvial apron along an even base line, as though they had been outlined by faults.
Projecting from the alluvial apron, between the base of the escarpment and the floor of the Salt Basin, are occasional low ridges, whose conspicuous ledges indicate that they are islands of bedrock, not quite engulfed by alluvium. The color and texture of the outcrops leads one to suspect that the ridges are composed of essentially the same rocks as those in the Guadalupe and Delaware Mountains. Many of the ridges are cuestas whose steep faces are toward the east, indicating that the strata dip more steeply here than in the mountains, and that the dip is in a reverse direction, or westward toward the basin.
Viewed from the peak, the Guadalupe and Delaware Mountains are thus seen to be a great asymmetrical block of the earth's crust, elongated north and south, with a gentle slope on the east and a steep escarpment on the west. Apparently the block has been uplifted, the uplift having been sufficiently recent for the surface form to reflect rather well the underlying structure. It is therefore probably of Cenozoic age. The uplift appears to have tilted the east side of the block but to have faulted off its western side, leaving a narrow, downthrown flank, remnants of which project here and there from the alluvial apron to the west. East of the Guadalupe and Delaware Mountains, the rocks flatten out beneath the Llano Estacado, but to the west, to judge from the ranges in view from the peak, are other, similar, faulted uplifts.
The strata of the Guadalupe and Delaware Mountains are tilted eastward, away from the crest of the mountain uplift with a dip sufficiently low to produce plateaulike and cuestalike land forms. The faults that fringe the base of the west-facing escarpment of the mountains lie parallel to the north-south elongation of the uplift. These relations suggest that the tilting and faulting of the rocks are features related to the uplift of the mountain area and that the uplifted rocks had hitherto been little disturbed.
Detailed study of the Guadalupe Mountains and adjacent ranges, however, indicates that the region was disturbed several times, in various degrees of intensity, before the mountains were uplifted, in their present form. The effects of three of these earlier disturbances (during the late Carboniferous, during the Permian, and during the early Mesozoic) are suggested by figures 15 and 16.
FEATURES OF PRE-PERMIAN AGE
The oldest known tectonic features near the Guadalupe Mountains are displayed in the pre-Cambrian rocks exposed in the south part of the Sierra Diablo (shown by special patterns on figure 15, A, and plate 21). These features have been summarized in another paper;57 their strikes are suggested on plate 21. The pre-Cambrian rocks are exposed over too small an area for much to be learned about their regional pattern or their relation to later tectonic features.
The next important tectonic features are of late Pennsylvanian, pre-Wolfcamp age. They are widely exposed in the Sierra Diablo, and some evidence regarding them can be obtained from wells drilled near the Guadalupe Mountains. The nature of the features is suggested by figure 16, B, which is a paleogeologic map of the surface on which the Wolfcamp series was deposited. As indicated by this map, the Sierra Diablo area was uplifted, faulted, and deeply eroded before Wolfcamp time, but the Guadalupe Mountains area was little disturbed. Of special interest are the west-northwestward trending faults and belts of outcrop in the Sierra Diablo, which lie parallel to younger tectonic features described below.
FEATURES OF PERMIAN AGE
The Permian rocks of the Guadalupe Mountains, the Sierra Diablo, and nearby ranges contain a number of features, partly of depositional and partly of tectonic origin, that are apparently of Permian age. The features in the southern Guadalupe Mountains have been described in the chapter on Permian stratigraphy (pp. 18-86) and are illustrated in the sections on plate 17. Their regional relations are summarized in figure 16, A, which is a map showing the positions of monoclinal flexures and reef zones in the Permian rocks and their relation to the northwest part of the Delaware Basin.
One of the features, the Bone Spring flexure, is exposed in the southern Guadalupe Mountains, and is overlain by the Goat Seep and Capitan reefs, of Guadalupe age, which form zones with the same northeastward trend. Two other features, the Babb and Victorio flexures, are exposed in the Sierra Diablo and trend west-northwest. Through parts of their courses, these flexures are followed by reefs of Leonard age. To the east, in the Apache Mountains, a reef zone is formed by the Capitan limestone which likewise trends west-northwest. On the Victorio flexure exposures extend into the basement rocks, and these basement beds are flexed downward in the same manner as the younger beds.
All three flexures appear to have been in existence during the deposition of the Permian rocks that now cover them. Not only are they followed by Permian reef zones of the same trend, but the deposits on the lower sides, seem to have been laid down in deeper water than those on the upper sides. Moreover, on the Bone Spring flexure, the Delaware Mountain group overlaps the Bone Spring limestone; near the Babb flexure the Bone Spring overlaps the Hueco limestone; and on the Victorio flexure the Bone Spring contains conglomerates apparently derived from the Hueco. The flexures apparently outlined the margins of the northwest part of the Delaware Basin, which was an area of subsidence in Permian time. Perhaps they were caused by deformation along local lines of weakness during the sinking of the crust in the basin area.
The flexures and reef zones of Permian age are crossed by the dominant later tectonic trends; those in the Guadalupe Mountains are cut cleanly by the younger north-northwestward trending faults and are unrelated to them or other tectonic features. Those in the Sierra Diablo, however, are parallel to prominent sets of west-northwestward trending faults and joints, as shown on plate 21. As indicated by figure 16, B, this trend was already in existence during the late Pennsylvanian, pre-Wolfcamp disturbance.
FEATURES OF EARLY MESOZOIC AGE
Further disturbances probably took place in post Permian and pre-Cretaceous time. At any rate, the Cretaceous rocks that are extensively exposed south of the Guadalupe Mountains lie on a variety of Permian formations, and in the southern Sierra Diablo they overlap onto the pre-Cambrian formations. These relations are shown on figure 15, B, which is a paleogeologic map of the surface on which the Cretaceous rocks were deposited. The map is partly hypothetical, in that the Cretaceous is now missing entirely in some areas, such as the Guadalupe Mountains. It was assumed that the summit peneplain that bevels the rocks of the Guadalupe Mountains was approximately the surface on which the Cretaceous sediments were once deposited.
The features shown on figure 15, B, are largely a reflection of those of Permian time, as may be seen by comparison with figure 16, A. The semicircular outline of the Delaware Basin is still evident, and also the positive area of the southern Sierra Diablo. This persistence raises a question as to what extent the features shown on figure 15, B, were merely formed by peneplanation of Permian and older Paleozoic features. Positive areas of Paleozoic time were covered by thinner sequences of deposits than the negative areas, hence early Mesozoic erosion penetrated the older rocks more readily in those places than in others. Most of the features shown on figure 16, B, could therefore have been formed by erosion of Paleozoic structural features during the early Mesozoic, without the aid of any early Mesozoic movement. Some movement during early Mesozoic time, however, is suggested by the fault indicated on the north side of the pre-Cambrian inlier in the southern Sierra Diablo.
Whatever their cause, the early Mesozoic features have had an important influence on the aspect of the modern mountain ranges. As indicated by the westward overlap of the Cretaceous, which lies on the Ochoa series to the east and in places on the pre-Cambrian to the west, the dip away from the east flank of the Guadalupe and Delaware Mountains uplift is in part of pre-Cretaceous age. Further, the Guadalupe Mountains now stand several thousand feet higher than the Sierra Diablo, yet they expose only later Permian rocks, where as the latter exposes older Permian, older Paleozoic, and pre-Cambrian rocks. Structure contours drawn on the top of the pre-Cambrian indicate that the pre-Cambrian58 in the south part of the Sierra Diablo stands higher than in any other area in trans-Pecos Texas. Most of this uplift resulted from the greater structural height of the Sierra Diablo in early Mesozoic time, for the range was not uplifted as much as the Guadalupe Mountains in Cenozoic time. Similar conclusions were reached by Adams.59
FEATURES OF EARLY CENOZOIC AGE
The Permian rocks now exposed in the Guadalupe Mountains had a broadly warped structure by Cretaceous time. Afterwards, as shown on figure 15, A, and plate 21, they were broken into tilted fault blocks. The close relation of the fault blocks to the present topography suggests that most of the post-Cretaceous disturbance took place in later Cenozoic time, which implies that the region was little deformed during early Cenozoic time.
Some movements probably took place in early Cenozoic time, however, for elsewhere in the Cordilleran province, even nearby in trans-Pecos Texas, there were important disturbances during this period. Cretaceous rocks that were closely folded and overthrust during the early Cenozoic crop out, for example, in Devil Ridge, about 70 miles southwest of the Guadalupe Mountains, (shown in the southwest corner of pl. 21).60 These rocks are a part of a larger area of deformed rocks that includes the Malone, Quitman, and Eagle Mountains (fig. 1).61
In southern trans-Pecos Texas, there appear to have been at least two early Cenozoic movements, one older and the other younger than such volcanic rocks as are found in the Davis Mountains area. The volcanics have been shown by plant and vertebrate fossils to be of Eocene and Oligocene age.62 The older movement therefore took place in late Cretaceous or early Tertiary time, and corresponds to the Laramide movements of other parts of the Cordilleran province. The younger movement took place in post-Oligocene time, and perhaps in the mid-Tertiary, because the folded rocks are cut by normal faults that are presumably of late Tertiary age.63
If disturbances took place in the Guadalupe Mountains during these epochs, they were of small magnitude. Broadly considered, the total result of all the Cenozoic movements in the area studied is not great. If most of the movements are of later Cenozoic age, those of early Cenozoic age were only a small fraction of the whole.
FORM OF THE UPLIFT
The area studied is a typical part of the uplifted block of the Guadalupe and Delaware Mountains, whose broader features have already been noted in the view from Guadalupe Peak. It includes a segment of the crest of the uplift about 18 miles long, and extends 12 miles east and 7 miles west from the crest.
The broader tectonic features of the area are suggested by the topography, for the higher parts of the area are those which have been uplifted, and the lower parts are those which have been depressed. The original form of the uplift, however, has been considerably modified by surface agencies. The higher parts have been worn away by erosion, and the lower parts have been more or less filled by alluvial deposits.
The tectonic features are shown by the four sections on plate 3, and by the contour lines on the tectonic map (pl. 20). The contours have been drawn on the base of the middle part of the Guadalupe seriesthat is, on the contact between the Cherry Canyon and Brushy Canyon formations of the Delaware Mountain group to the southeast and on the contact between the sandstone tongue of the Cherry Canyon formation and the Bone Spring limestone to the northwest. This key horizon lies near the middle of the exposed section, or above the prominently flexed lower beds and below the irregular reef deposits of the younger beds. Contours drawn on it thus show mainly the warping and faulting associated with the uplift of the mountains. Most of the features of Permian age are eliminated, except possibly the mild flexing of the latter part of the period.
As shown by the contours, the strata rise from a low position at the east and west edges of the area studied to a high position near the center. The altitude of the base of the middle part of the Guadalupe series at the east edge of the area is 3,000 feet above sea level, and near the west edge is 2,000 feet. Near the center of the area, not far north of Guadalupe Peak, it rises to more than 6,750 feet above sea level. The crest of the uplift extends north and south from this place along the escarpment at the west edge of the Guadalupe and Delaware Mountains.
The simple, archlike form suggested by these figures is greatly complicated by faulting. The rocks along the crest and western flank of the uplift in a belt about 10 miles wide, are broken by numerous faults whose general trend is parallel to that of the crest. East of the belt, as shown by wide exposures, the rocks are not faulted. The west edge of the fault belt is not known, as the bedrock on this side is overlapped by the alluvial deposits of the Salt Basin. From the latitude of Guadalupe Peak southward, the crest of the uplift is flanked by one of several faults, here called the Border fault zone because the faults serve to outline the western border of the mountains. In a narrow belt on the west, or downthrown side of the zone, the key horizon sinks to 2,500 feet above sea level, or to about its altitude at the east and west edges of the area mapped.
CREST AND EASTERN FLANK
Within the area studied, the crest of the Guadalupe and Delaware Mountain uplift lies at the western edge of the mountains, and along the east side of the Border fault zone. Its highest point is a short distance north of Guadalupe Peak, where the altitude of the key horizon is more than 6,750 feet above sea level (pl. 20). Here the rocks are bent into a half dome, convex to the east. Northward and southward along the crest the altitude of the key horizon sinks to a little more than 5,000 feet.
The half dome may have its origin in movements older than the faulting, for its crest lies near the upper end of the Bone Spring flexure, of Permian age. Other, less-definite, much-faulted, high-standing areas to the northeast and southwest may lie on the extension of the same older tectonic trend. There is also a vague suggestion of northeast-trending cross-folds to the south. Thus, the low point on one fault block is likely to be adjacent to the low point on the next block, although it has a different structural height. Further, on the unfaulted eastern flank of the uplift, local variations may be observed in the angle of dip and direction of strike. Most of them are too small to influence the trend and spacing of the contour lines, but in the vicinity of Frijole Post Office there is more pronounced warping, which apparently has a northeast trend.
East of the crest of the uplift the strata dip at an angle of 2° or 3° east-northeast, or at the rate of about 250 feet per mile. (Some of these tilted strata appear in the foreground of plate 4, A.) The continuity of the slope is much disturbed by faults for about 4 miles east of the Border fault zone, but farther east it extends unbroken past the edge of the area studied, and far beyond to the eastern base of the uplift along the Pecos River (pl. 21).
The faults that disturb the strata in the 4-mile belt east of the Border fault zone have straight or gently curved traces which trend generally north-northwest, parallel to the crest of the uplift (pl. 20). Most of them are of small displacement, and many are down thrown westward. Through most of the area studied, the easternmost faults of the group are a part of the feature here called the Lost Peak fault zone, which pursues a remarkably straight, north-northwestward course across the area, and makes a sharp separation between the faulted tract to the west and the undisturbed tract to the east (as shown in section DD, pl. 3).
Within the faulted belt one large tract in which there are no faults stands out prominently. It is here named the Guadalupe Peak horst, from the peak that lies near its center. The horst is about 9 miles long and 2 to 3 miles wide, and is elongate north-northwestward. Within it is the half dome that is the highest part of the uplift. It includes the highest mountains of the area, carved from the Capitan limestone and associated formations. The horst is bounded on the west by the Border fault zone and on the north and east by smaller faults, all of which are downthrown away from it. To the southeast, it is not bounded by a single fault, but is penetrated by numerous, small, north-northwestward trending minor faults that die out northwards in the horst.
The areal relations of the Guadalupe Peak horst may be seen on the two geologic maps, plates 3 and 20. On the latter, note the greater structural height of the horst than its surroundings, as indicated by the contours. An idea of the structure of the horst may be gained from section BB', plate 3, although this section crosses its southern end where the continuity of the strata is interrupted in the middle by a pair of minor faults. The topographic features of the horst, including the lofty cliffs and peaks of limestone, can be seen on plates 1 and 5, A (as viewed from the south) and plate 5, B (as viewed from the west).
Between the Guadalupe Peak horst and the Lost Peak fault zone at the eastern edge of the faulted belt is a graben, or strip of downfaulted rocks, as much as 1-1/2 miles wide and cut by several minor faults. North of the horst the graben is followed by the north-draining depression of West Dog Canyon. South of the horst it forms the Getaway graben. Near Getaway Gap, from which the graben is named, the downfaulted rocks have been carved into a prominent, longitudinal topographic depression.
The areal relations of the graben are shown on plates 3 and 20. On the former, note the outliers of the Bell Canyon formation along it in the south part of the area, far to the west of their normal position on the east flank of the mountains. On the latter, note how its structurally low position in relation to its surroundings is indicated by the contours. For the structure of the graben, see sections AA' and DD', plate 3, in each of which it appears to the left of the Lost Peak fault zone.
Near the north edge of the area studied, a large fault appears east of the Lost Peak fault zone, and continues northward into New Mexico along the east side of Dog Canyon. The fault trends north-northeast in this area, but to the north, in New Mexico, it curves to a northerly, or even a north-northwesterly course (pl. 21). In New Mexico it and the associated faults, here called the Dog Canyon fault zone, form the eastern boundary of the faulted belt; the crest of the uplift lies along their eastern side. To the west, in the depression drained by Dog Canyon and in the somewhat higher Brokeoff Mountains beyond, the rocks are structurally lower, and are faulted into many narrow slices. Some of the fault slices of the Brokeoff Mountains extend southward, west of the Lost Peak fault zone, into the area studied. In this direction, the strata rise toward the Guadalupe Peak horst, which bounds the fault slices on the south.
The Dog Canyon fault zone extends for only a few miles into the area covered by the two geologic maps, plates 3 and 20. Its structure in this segment is shown on section AA', plate 3. For its extension northward into New Mexico, see the regional tectonic map, plate 21. Compare this with the topographic relations shown on figure 2, where the position of the zone is suggested by a west-facing escarpment that extends northward from El Paso Gap. For a view of the region crossed by the Dog Canyon fault zone, see the panorama, plate 14, A, where the escarpment above noted stands out prominently in the middle distance. On figure 2, note also the topography of the Brokeoff Mountains, which reflects the structure to a large degree.
BORDER FAULT ZONE
Beginning somewhat north of the latitude of Guadalupe Peak, and extending southward, the crest of the Guadalupe and Delaware Mountains uplift is broken off on the west by the Border fault zone. The faults of the zone drop the strata westward from 2,000 to 4,000 feet. In places, as west of Guadalupe Peak, the fault separates the uplifted bedrock on the east from alluvial deposits that cover the depressed rocks on the west; in places the alluvium covers the trace of the fault itself; elsewhere, as in the Delaware Mountains, the fault separates uplifted rocks from downfaulted, much disturbed rocks, which crop out in low hills to the west.
The displacement on the zone is especially well revealed for several miles northwest of the point where it is crossed by United States Highway No. 62. Here, one may stand on ledges of downfaulted rocks near the fault and, looking northward, see the same beds projecting from the slopes of Guadalupe Peak and El Capitan, 2,000 feet higher.
The areal relations of the Border fault zone are shown on the two geologic maps, plates 3 and 20. On the former, the displacement on the zone is suggested by the relatively young Permian rocks which project through the alluvium to the west of it, as compared with the relatively old rocks on its east side. The displacement is more strikingly shown on the accompanying structure sections BB', CC', and DD', and by means of structure contours on plate 20. For a view of an exposure of one of the faults in the zone, see plate 14, B, where the Bone Spring limestone is upfaulted against older Quaternary gravel deposits. The displacement of the gravel in this vicinity is relatively slight, as compared with that in the underlying bedrock, as is shown on figure 17, B.
The exposures north of United States Highway No. 62 may be seen on plate 5, A. Those of the downfaulted rocks, including formations of the Delaware Mountain group, fringe the outer bench of the escarpment below Pine Top Mountain, and the same beds in the upfaulted block form the slopes below El Capitan, a little to the left.
At most places the large displacement of the rocks along the zone takes place along a single fault. None of these single faults is continuous along the entire course of the zone, and the greatest break lies now to the east and now to the west of its general north-northwestward course. Unlike the smaller faults to the east, the faults of the Border zone trend in highly varying directions. Some are straight, others curved, some trend north-northwest, and others east of north. Some of the offset parts of the zone are connected by west-northwest trending faults. The zone bends to the west around the Guadalupe Peak horst, whose western side projects as a blunt salient into the downfaulted area beyond.
CUTOFF MOUNTAIN AREA
A mile or so north of the latitude of Guadalupe Peak, the Border fault zone passes into the interior of the mountains, and near the north end of the Guadalupe Peak horst splits into several branches, no one of which has a large displacement. The high escarpment that rises east of the Border fault persists northward to the northwest corner of the horst, but fades out beyond. The bedrock west of the zone in the latitude of Guadalupe Peak is mostly covered by alluvium, but it rises northward into low mountains that fringe the western base of the high escarpment. Where the high escarpment fades out, the low mountains themselves form the edge of the Guadalupe range. They are a part of the Brokeoff Mountains, which are better developed to the north, in New Mexico. Their highest summit within the area studied is Cutoff Mountain.
These relations can be seen on the panorama, plate 5, B, a view of the Guadalupe Mountains from the west, but the features shown on it should be compared with their appearance on the two maps, plates 3 and 20. On plate 5, B, note the high escarpment that rises east of the Border fault, which extends north from El Capitan to the Blue Ridge, beyond which it disappears. Below and in front of it, note the foothills of downfaulted rock, which to the south (as near point 4909) are low and discontinuous, but to the north (as near points 5284 and 6305) stand in high ridges. The Cutoff Mountain section is to the north (left) of the Blue Ridge. To see the difference in structure between this segment of the escarpment and that farther south, compare sections AA' and BB', plate 3.
Near Cutoff Mountain, and elsewhere in this area, the rocks bend over from a nearly horizontal position on the rim of the mountains to an inclined position on the face of the escarpment, and at the base dip beneath the alluvial deposits of the Salt Basin. The beds on the face of the escarpment dip as steeply as 45°, and many of the resistant layers are carved into dip slopes (such as those. below point 5443, pl. 5, B). The inclined beds are crossed diagonally by interlacing, northwest-trending faults of small displacement, many of which are downthrown to the northeast in the opposite direction to the dip of the beds. Each fault originates to the southeast as a branch of the Border zone and disappears to the northwest by passing under the alluvial deposits beyond the escarpment. The escarpment in this area is thus bordered by no single fault, and has been outlined more by flexing than by faulting.
WESTERN FOOTHILL AREA
South of the latitude of Guadalupe Peak and west of the Border fault zone the bed rock projects in many low foothill ridges. The ridges are surrounded and separated by alluvial deposits, and the structure is less easy to decipher than in the mountains to the east where the exposures are continuous. Some of the alluvial cover is thin, but in places it has apparently been deposited to a considerable thickness in deeply depressed fault blocks. The highest foothills are southwest of Guadalupe Peak, where the Capitan and associated limestones form the steep-sided ridges of the Patterson Hills. Southeast of the Patterson Hills the foothills are lower, but more because they are composed of poorly resistant sandstones (Delaware Mountain group) than because of any diminution in their structural height.
The rocks of the foothill area are the same as those that form the mountains to the east, but they have been depressed to a much lower position. They dip generally west-southwest at an angle of about 15°, but in some places they dip at angles as low as 5° or as high as 45°, In general, the older rocks of the succession lie to the east and the younger rocks to the west, in harmony with the prevailing dip. Along United States Highway No. 62 the first rocks seen west of the Border fault zone are thus the prominently exposed, massive sandstones of the Brushy Canyon formation, tilted westward, away from the mountains. Farther west the higher rocks of the succession are encountered, such as the Capitan limestone, and are seen to dip in the same direction until they pass beneath the alluvial deposits of the Salt Basin beyond. One thus receives the impression at first that the rocks of the Delaware and Guadalupe Mountains bend over to the west as a great fold, with little or no faulting,64 but this impression is modified by further study. Older rocks are found in the foothills considerably west of their anticipated positions, and younger rocks are found close to the Border fault zone. Closely adjacent exposures are discovered that consist of rocks many hundreds of feet apart stratigraphically. It is thus clear that the structure of the foothill area is greatly complicated by faulting.
For the areal relations of the foothill zone, see the two geologic maps, plates 3 and 20. Note on the former the disconnected nature of the exposures of Permian rocks, and the extensive areas of alluvium. Note also that the same beds are exposed in the area as in the mountains to the east, but in more confused, less regular order. On plate 20, the structure contours show that the beds stand at a much lower height than in the mountains.
The panorama, plate 5, A, shows the rocks of the foothill area that are exposed near United States Highway No. 62. The Border fault zone follows the bench at the foot of the mountains in the middle distance, in the center and right-hand parts of the view. Note that the rocks beyond it are either horizontal or dip gently to the east (right), whereas those on the nearer side, projecting in occasional hills, dip more steeply to the west (left). The apparent anticlinal structure is suggested by the outcrops designated by letter symbols on the view, such as those of the Brushy Canyon formation near the Border fault and of the Capitan limestone farther west, in the Patterson Hills. Note, however, that one outcrop of Bell Canyon formation is indicated close to the Border fault, which suggests that the relations are more complicated. The true structure of the nearby hills is shown in section CC', plate 3, and of the more distant hills in section BB'.
The faults of the foothill area are not easy to map, as their traces are widely covered by alluvium. So far as they have been worked out, their general trend is north-northwest, but there are some of west-northwest and some of north-northeast trend. Most of them are downthrown to the east, opposite to the direction of dip and in the opposite direction from the faults of the Border zone. The fault blocks immediately west of the Border zone thus stand much lower than those on either side of them, somewhat after the fashion of a sunken keystone (as in section DD', pl. 3).
The sunken tract west of the Border zone is expressed prominently in the topography west and southwest of Guadalupe Peak. Here, a straight-sided trench 4 miles long and 1 mile wide lies between the even base of the Guadalupe Mountain escarpment on the east and the straight front of the eastern ridge of the Patterson Hills on the west. It is shown just west of Shumard Peak on plate 5, A. The trench is floored by coarse fanglomerate washed down from the Guadalupe Mountains, which probably fills it to a great thickness, and the bed rock beneath may be deeply depressed (as suggested in sec. BB', pl. 3). At the south end of the trench, bed rock crops out in patches in the space between the Guadalupe Mountains and the Patterson Hills, and the graben beneath the trench apparently ends against higher-standing fault blocks.
On one of the higher-standing blocks south of the end of the trench and close to the Border zone, the N. B. Updike, Williams No. 1 well reached the Bone Spring limestone within less than 100 feet of the surface (sec. 47, pl. 8), or less than 500 feet below its position on the Guadalupe Peak horst to the east (sec. CC', pl. 3). Between this block and the horst, however, are a number of deeply depressed, narrow wedges, which lie in the angle formed by the Border zone where it turns westward around the blunt salient of the Guadalupe Peak horst. The wedges stand at unlike structural heights, some nearly level with the rocks to the east and west, and others as much as 2,000 feet lower (as indicated by the structure contours on the structure map, pl. 20).
West of the Delaware Mountains, the sunken tract on the west side of the Border fault zone is again well defined. Between the Border fault and another a mile and a half to the west, the surface rocks are much younger than those on either side and include the Castile formation, or highest member of the bedrock succession preserved in the area (sec. DD', pl. 3). The rocks of the tract are broken by numerous branching and intersecting faults of various trends. Its east side, next to the Border fault zone, is more deeply depressed than the rest, and forms a graben less than half a mile wide.
As already suggested, and as indicated on the tectonic map, plate 20, the faults of the area lie in a belt about 10 miles wide which follows the crest and west flank of the Guadalupe and Delaware Mountains uplift. To the east, the rocks are not faulted, and to the west the structure of the bedrock is concealed by the alluvial deposits of the Salt Basin.
Most of the faults trend north-northwest, parallel to the axis of uplift of the mountains and to the trend of the fault belt as a whole. Faults of this trend east of the Border zone are remarkably straight and parallel for long distances, and depart from the general course only in gentle curves. Those of the Border zone and the foothill area west of it are somewhat less regular, with many curves and some sharply bent offsets. Parts of the more strongly curved faults trend north or east of north. The larger curved faults, such as those in the Dog Canyon and Border zones, are concave toward the downthrow. Some short faults in the Border zone and foothill area trend west-northwest, north-northeast, and east-northeast. In the Cutoff Mountain section the faults that extend diagonally across the escarpment have more of a northwestward than a north-northwestward course.
The faults of the belt are spaced, on the average, about three-quarters of a mile apart, but the belt extends around the large, unfaulted tract of the Guadalupe Peak horst, and includes some intensely shattered tracts where there are 6 or more faults to the mile (pl. 20). None of the faults continues across the whole length of the area. Some are only a few miles long, others extend 10 miles or more without a break. The discontinuity of the faults is caused partly by a dying out of the displacement at their ends, and partly by branching. Branching faults are more, common in and west of the Border zone than east of it. In places, as in the Cutoff Mountain section, the branching and rejoining of the faults gives them an interlacing pattern. The zones of displacement that form the Dog Canyon, Lost Peak, and Border fault zones are longer than the faults that constitute them. When one fault dies out, another with similar displacement makes its appearance, lying en echelon with it. There are, however, no systematically arranged belts of echelon faults in the area.
DISPLACEMENT ALONG FAULTS
A large number of the faults in the area are downthrown in a direction opposite to the dip of the strata, and toward the axis of the uplift. They have moved, therefore, in opposition to the general uplift of the mountain area.65 Most of the faults west of the Border zone are thus downthrown to the east, and many of those east of it are downthrown to the west. The faults of the Border zone itself, by contrast, have moved in harmony with the general uplift. East of the Border zone the faults downthrown to the west alternate with those downthrown to the east, thereby producing a horst and graben structure (as shown in sec. DD', pl. 3). The displacements along the faults range from a few hundred to several thousand feet. The largest displacements are along faults of the Border zone, which are downthrown to the west as much as 4,000 feet.
Movements on the faults appear to have been largely down the dip as suggested by nearly vertical slickensides observed on the occasional exposures of the fault surfaces. Baker66 reports that an exposure of the surface of the Border fault not far southwest of El Capitan displays well-developed slickensides inclined slightly to the vertical. Any large amount of horizontal movement on the Border faults or others in the area is unlikely. Not only are there no consistent, well-developed belts of echelon faults, but the angular offsets in the trace of the Border zone would prevent the blocks on either side from moving past one another horizontally for any appreciable distance. Moreover, the facies boundaries in the Permian rocks, and especially the southeast edge of the Capitan limestone (lines C, D, and E, figure 10), are not offset by the fault belt, but extend in straight lines across the area.
DIPS OF FAULTS
The planes of most of the faults in the area either dip steeply in the direction of downthrow or stand vertical. This attitude is indicated both by occasional outcrops of the fault surfaces and by the straightness of the fault traces, even where the faults extend through mountainous country. The observations that have been made suggest that the fault planes tend to lie nearly perpendicular to the bedding planes, and that where the beds are most steeply tilted, the faults dip at the lowest angles.
Steep dips may be inferred if not proved for most of the faults east of the Border zone; in fact, the plane of the fault on the east side of the Getaway graben which has been observed at many places near Getaway Gap and to the north, stands vertical in each exposure. Also, the two faults that bound a narrow graben near the head of Guadalupe Canyon (shown just east of Guadalupe Peak in sec. BB', pl. 3), extend through country with 2,000 feet of relief, and yet their traces are no closer in the lower places than in the higher suggesting nearly vertical dips of the fault planes. An exception to the generalization of steep dips is the Dog Canyon fault, the trace of whose outcrop indicates that it dips 60° toward the downthrown side.
The planes of the faults of the Border zone are exposed in many of the canyons and ravines that cross it, as shown in plate 14, B, and dip at angles of 70° or more toward the downthrown side.
In the area west of the Border fault zone, the dips of the fault planes may be less than farther east. Here, the beds are more steeply tilted than east of the zone, and the joints of the region are mostly perpendicular to the bedding. Possibly the fault planes are parallel to the joint planes. In the Cutoff Mountain section, where the fault traces are well exposed on the mountain sides, many of the faults appear to dip at angles of 60° or less toward the downthrown side. In the foothill area farther south, the fault planes are mostly covered by alluvial deposits and no observations have been made regarding their dips.
MINOR DEFORMATIONAL FEATURES
No crumpling has been observed near the faults of the area and very little dragging of the beds. On the downthrown sides of some of the faults, the beds dip outward at a low angle, and in some narrow fault blocks the beds are much more steeply tilted. On the upthrown sides, the beds generally extend horizontally even to the fault lines; for example, the black limestones of the Bone Spring, which form the upthrown sides of the faults of the Border zone, are undisturbed even at the planes of the faults themselves. Near most of the faults the joints parallel to them are very abundant and closely spaced.
Vein deposits and breccias are common along the faults. At many exposures of the faults of the Border zone the fault surface of the upthrown block, cut on black limestones of the Bone Spring, is covered by a straight-sided mass of calcite 5 or 10 feet thick, in which angular fragments of black limestone are embedded. Similar calcite veins and masses have been seen on other faults in the area, especially on the one that bounds the east side of the Getaway graben. It is not known whether the veins are lenticular or continuous bodies.
RELATION TO QUATERNARY DEPOSITS AND TOPOGRAPHIC FEATURES
Some faults within a few miles west of the Border fault zone have displaced Quaternary gravels and fanglomerates. For instance, 2 miles southwest of El Capitan, older fanglomerate deposits project in low hills whose eastern edges are straight, north-northwestward trending scarps 25 to 50 feet high which stand out prominently on aerial photographs. These scarps are probably fault scarps. Farther south, west of the Delaware Mountains, Quaternary gravels laid down on an old pediment surface stand at different heights in adjacent fault blocks. These differences are results of faulting, and the actual planes along which the gravels have moved are exposed in places (fig. 17, A). The displacement of the gravels, however, is only about a tenth as great as the displacement of the bedrock beneath (as shown on figure 17, B). These faults. therefore, underwent at least two movements, the first of which was the larger. Faults that appear to have offset the Quaternary deposits are shown by a different color than the rest on the map of Cenozoic deposits and land forms (pl. 22).
There may have been movements at the same time along the faults of the Border zone. The older fanglomerates and gravel deposits west of it, whose displacement along faults has been described above, consist of fragments of rocks derived only from the upper part of the escarpment to the east, and contain none from its lower, outer bench. Later fanglomerates of the same type contain rocks from the lower bench abundantly. In places the older deposits lie in fault contact with the rocks of the outer bench along the Border zone (as shown in plate 14, B). The outer bench ends along an even, little-dented base line, which follows the trace of the Border zone. It seems to be less eroded than the upper part of the scarp, as though it had been only recently raised. On the upper part of the scarp near Guadalupe Peak are remnants of an old, well-rounded topography, now deeply dissected, which probably was carved at the time when the older fanglomerates to the west were being laid down and before the last faulting. The possible history of the Border zone and the faults to the west of it is shown diagrammatically on figure 22, B.
The faults farther east and west of the Border zone are generally followed on their upthrown sides by escarpments. Some of these escarpments, such as those along the Getaway graben, are resequent fault-line scarps; that is, they were formed by the more rapid erosion of the rocks on the downthrown side than those on the upthrown (pl. 22). The scarps are younger than any movements along the faults because the Quaternary gravel deposits in places cross the traces of the faults without displacement. Other scarps along faults east and west of the Border zone are of less certain origin. The scarps in the Guadalupe Mountains are approximately of the same height as the throw of the faults along which they lie. They may be old, greatly eroded fault scarps, or they may be resequent fault-line scarps from whose faces weak beds have been carried away by erosion. Whatever their origin, their character suggests that no recent displacements have taken place along the faults that fringe their bases.
At nearly every exposure in the area, the rocks are cut by joints, which are in part closely and in part widely spaced, and which generally trend in two or more directions. Observations were made on them during the field work because of the possible information they might furnish as to the origin of the larger tectonic features.
Observations made on the joints were incidental to other field work and are therefore incomplete. In some areas many observations were made, in other areas none were made, although joints were present in the rocks. Stations at which observations were made are shown by black circles on plate 20. At these stations, only qualitative information was obtained on the relative abundance and perfection of the different sets of joints, and on their dip. It was assumed that their most important feature was their trend, and measurements of the trends of the different sets therefore constitute the bulk of the information obtained on them. The notes on the joints contain 1,141 such measurements, made at 407 stations.
Most of the joints are straight and smooth in all sorts of rock, though some in the sandstones of the Delaware Mountain group are curved and some poorly developed joints are jagged and irregular. The surfaces of the straight joints are smooth, even where they cut through irregularly bedded rocks, or alternations of hard and soft layers. No slickensides have been observed on them. At the surface, many of the joints are open fissures, some are narrow cracks, and a few are filled by vein calcite. The open fissures were probably formed by weathering, and give little indication of the nature of the joints at depth.
Single joints commonly extend across the entire length of any exposure, although some close and come to an end. The joints have a great vertical as well as a great lateral extent. Individual joints can be traced from the tops to the bases of the limestone cliffs near Guadalupe Peak and El Capitan, or through a distance of 1,000 feet or more.
Where several sets of joints are present, they commonly cross one another without deflection, although in places subordinate sets may either end against or branch from the dominant sets. The observations made on the intersections of the joint sets are not sufficient to show whether some are of different ages than others. Such differences might be revealed by closer scrutiny.
The spacing of the joints (which is only imperfectly suggested by those plotted on the tectonic map, pl. 20) is quite variable. It depends to a certain extent on the nature of the rocks, for thin-bedded, brittle rocks are likely to be more jointed than massive rocks. To a larger extent it depends on the tectonic relations, for the joints of one area tend to be more abundant in all types of rock than those of another area.
In the eastern part of the area, where the rocks are not faulted, the joints are for the most part widely spaced. In many exposures in this area, only two sets of joints are present, in some only one, and in a few broad exposures there are none.
In the faulted area to the west, between the Lost Peak and Border fault zones, joints are much more numerous than elsewhere. In nearly every exposure two or more sets are present, and they are generally spaced only a few feet apart. At most places one set is more closely spaced than the others, and this is likely to be one that is widely distributed through the region. Near faults the joints parallel to them are more closely spaced than elsewhere, and the other sets of joints are poorly developed. In the Guadalupe Peak horst, the unfaulted tract that lies in the middle of the faulted belt, both field observations and air photographs indicate that joints are as numerous as in the faulted areas nearby.
Joints are numerous also in the area west of the Border fault zone, and tend to be closely spaced. The number of sets present is greater than to the east, and 3 or more are likely to be found at most exposures.
DIPS OF JOINTS
East of the Border fault zone, the joints commonly stand nearly vertically. This relation is best shown near Guadalupe Peak and El Capitan, where the joints can be traced down through the limestone cliffs for long distances. The rocks east of the Border fault zone are horizontal or gently tilted, so that these vertical joints stand nearly normal to most of the bedding planes. In some of the formations, however, the bedding planes have an original depositional slope, as in the Capitan limestone, or are tilted and contorted as a result of Permian movements, as in the Bone Spring limestone. The joints cut through these rocks without deflection from their vertical position, as shown on plate 11, B. A few joints dipping at angles of 60° or less were noted in the Capitan limestone, but they are minor features. No horizontal or gently dipping joints were observed, either in well-bedded strata or in the massive Capitan limestone.
West of the Border fault zone, both in the Cutoff Mountain section and in the foothill area to the south, the dips of the joints are less than to the east. The rocks of this region dip at angles up to 45°, and so far as observations have been made the sloping joints stand approximately normal to the bedding. Joints trending in the direction of dip are thus vertical, but those parallel to the strike depart from the vertical by the amount of dip of the strata. This relation is barely perceptible in rocks tilted at angles of 10° or less, but is a striking feature in rocks tilted at angles approaching 45°.
On the accompanying maps, plates 20 and 21, observations of the trends of the joints have been summarized by several methods of plotting. On plate 20, the observed joint trends at each station are indicated by radiating lines of equal length. As the joints are shown only where observed, such plotting does not show the actual abundance of the joints of different trends on the ground.
The observations made, however, are sufficiently representative to give a fair sample of the number of joints of each set actually present. The observations can thus be summarized statistically. The area shown on plate 20 is therefore divided into 10 unit areas, and the relative abundance of different joint sets in each unit is plotted as "roses." (Note that the north-south boundaries of the unit areas follow structure lines, and that the east-west boundaries are chosen arbitrarily.) On the regional tectonic map, plate 21, the area of plate 20 has been divided into two large unit areas, one east of the Border fault zone (constituting areas 2, 3, 4, 6, and 10 of pl. 20) and the other west of it (constituting areas 5, 8, and 9). For each of these areas, a more generalized rose has been prepared.
Each rose shows the relative abundance of joints in every 5° of arc, expressed in percentage of the total number observed. Five-degree units were chosen because 5° is the approximate limit of error in the observations, and is the amount of variation which joint sets, or even individual joints show in single exposures. As originally worked out, wide variations were found between percentages at some of the adjacent points on the arcs, which apparently resulted from a personal equation in making the observations. The percentages were therefore evened by means of sliding averages. Each figure used in plotting the roses is thus the average of the original percentage for the point shown and the percentage of the two points lying 5° on each side of it.
Some of the roses, such as those for areas 1, 5, 8, and 9 of plate 20, contain a possible error in that observations were made in part on inclined joints, which are normal to the planes of tilted beds. If such joints were rotated to vertical positions, those in the directions of the dip and strike of the beds would have the same trend as before, but those at intermediate positions would be deflected, and would thus have a new relation to the strike and dip joints. In beds dipping 45° (the maximum observed in the area) joints diverging at an angle of 45° from the strike would, when rotated to vertical, diverge about 55° from the strikea deflection of about 10°. Few observations were made, however, on beds so steeply tilted. More than 90 percent in each unit area were made where the beds dip 10° or less, for which the deflection would be about 1°. This is well within the limits of error for the observations.
The type of rose selected for plotting has the advantage of showing clearly the dominant joint sets for the areas. As the trends plotted extend radially from the center of the rose, the minor joint sets tend to be crowded more closely than the dominant ones. The latter are thereby exaggerated. Some of the minor trends are actually more important than their insignificant appearance on the roses suggests.
As shown by this statistical method, by far the most abundant joints in the area studied trend north-northwest. This set is especially abundant in the region east of the Border fault zone (roses 2, 3, 4, 6, 7, and 10, pl. 20). Air photographs of the region northeast of Guadalupe Peak show innumerable north-northwesterly joints traversing the Capitan limestone on the mountain sides. On the ground, the joints of this set commonly appear as long, parallel, open fractures, and are more prominent than any of the other joints. The set trends in about the same direction nearly everywhere, although near McKittrick Canyon in the northeast part of the area (area 4), joints that may belong to the same system have a northwestward course. The north-northwesterly set is present also, though less abundantly, in the foothill area west of the Border fault zone (roses 5, 8, and 9), and less conspicuously in the Cutoff Mountain section farther north (rose 1).
Associated with the north-northwesterly joint set at most exposures, and particularly east of the Border fault zone, is another at nearly right angles, or with east-northeasterly trend (roses 2, 3, 4, 6, 7, 9, and 10). In places in the eastern part of the area the north-northwesterly and east-northeasterly sets are the only two present. There is a tendency for the second set to trend more nearly eastward in the southeast part of the area than in the northeast part, as may be seen by comparing roses 4 and 10. The set is generally represented by fewer and less-open fractures than the north-northwesterly set. In the extreme northeast part of the area (rose 4), however, its numbers equal or exceed those of the north-northwesterly set. In the field, the east-northeasterly set appears to trend parallel to the face of the Reef Escarpment, but plotting of numerous observations suggests that its members actually diverge from the trend of the escarpment at an acute angle.
The third abundant joint set in the area trends north-northeast, forming an angle of about 50° with the dominant north-northwesterly set. It is most abundant in the foothill area west of the Border fault zone (roses 5 and 8). In the northwestern Patterson Hills it is the dominant fracture in many of the exposures. Traces of it are detected in parts of the area east of the Border zone (roses 6, 7, and especially 10). At many places it is associated with another fairly abundant set, lying nearly at right angles, or trending west-northwest.
In the Cutoff Mountain area observations, which are unfortunately inadequate, suggest that the dominant joints trend northwest, parallel to the faults of the district (rose 1). In addition to the sets described, there are some joints of other trends unrelated to the four dominant ones. These joints occur in various parts of the area, and particularly west of the Border fault zone. In some places there are well-marked north-south and east-west joints. None of these other sets is very common.
There is thus a distinct difference between the joint sets in the east and west parts of the area, the line of demarcation being approximately along the Border fault zone (pl. 20). To the east, the north-northwesterly and east-northeasterly sets are dominant, almost to the exclusion of the others, To the west, the north-northeasterly and west-northwesterly sets are prominent, although the two other sets are present, but less abundantly developed.
RELATION OF JOINTS TO OTHER TECTONIC FEATURES
As shown by the preceding descriptions, the joints seem to be younger than the tectonic features of Permian age in the mountains. They are much more closely related to the younger tectonic features, formed during the uplift of the Guadalupe and Delaware Mountains. Thus, the dominant north-northwesterly joint set is parallel to the dominant north-northwesterly fault system, and its members are much more closely spaced near the faults. The west-northwesterly and north-northeasterly joint sets are likewise followed by some faults, especially west of the Border fault zone, where such joints are abundant. There are, however, no faults parallel to the east-northeasterly set. The three joint sets first named therefore may be of the same age as the faults; or they may be somewhat older and have prepared the way for the later faulting. The absence of faults of east-northeasterly trend may indicate that the joints of this set are of a different age, or that they were tighter than the other sets and hence gave less encouragement to movement along them.
The joints are related also to the form of the Guadalupe and Delaware Mountains as a whole. The dominant, north-northwesterly set trends parallel to the longer axis of the mountains and the east-northeasterly set trends at right angles to it. The other two are diagonal to the axis but are more abundant west of the axis than to the east of it, indicating that they are somehow related to the uplift. The fact that the joint sets extend without deflection across local changes in the dip and strike of the beds indicates that they have originated from regional, rather than from local forces.
The dips of the joints are related in some manner to the tilting of the strata. Where the strata are horizontal the joints are nearly vertical, but where the strata are tilted the joints remain normal to the bedding planes. This condition is most evident west of the Border fault zone, where the tilting has resulted from rotation of the beds during block faulting. The joints may have formed during or after the tilting of the strata, in which case stresses were transmitted along the beds in the same manner as if they had been horizontal. The joints, however, may have been formed before the tilting; if so, they were subsequently rotated to their present positions. The latter interpretation has been adopted by Melton for similar joints normal to tilted beds in Oklahoma.67
The age relations of the different joint sets to one another have not been determined, although evidence on the question might be obtainable by detailed study of their intersections. The close relation of the four dominant joint sets to the Guadalupe and Delaware Mountains uplift and associated features suggests that they were all formed at about the same time. Some of the minor sets that have no clear relation to the uplift were possibly formed at another time, perhaps during the period after the deposition of the Permian and before the mountains were uplifted. It is difficult to believe that jointing of some sort did not take place in the region during this long and probably eventful time interval.
REGIONAL RELATIONS OF THE YOUNGER TECTONIC FEATURES
GUADALUPE AND DELAWARE MOUNTAINS
The Guadalupe and Delaware Mountains, of which the area studied is a part, constitute an uplifted block of the earth's crust more than 100 miles long in a north northwesterly direction and about 50 miles wide (fig. 1). Both in the area studied and elsewhere the uplifted block is asymmetrical. The broad eastern flank dips gently, without folding or faulting, toward the Pecos Valley and the Llano Estacado; the narrow western flank dips steeply toward the Salt Basin. At its south end, in the Apache Mountains, the uplift flattens out and pitches southward toward the lava plateaus of the Davis Mountains; at its north end it fades out on the east slope of the Sacramento Mountains. The Sacramento Mountains form a similar broad uplifted block, but their crest lies west of the crest of the Guadalupe Mountains and en echelon with it.
Both in the area studied and outside the crest and west flank of the uplifted block are much broken by faults, most of which have a north-northwest trend parallel to the axis (fig. 15, A, and pl. 21). Extending irregularly through the faulted belt, but generally flanking the mountain crest on the west, are several major faults, on which the strata are downthrown westward. Within the area studied and southward in the Delaware Mountains they form the Border fault zone. Farther north, in the Guadalupe Mountains of New Mexico, they form the Dog Canyon fault zone, which lies en echelon to the Border zone and about 5 miles east of it. In New Mexico the space between the Border and Dog Canyon fault zones is occupied by the Brokeoff Mountains, which are lower than the crest of the mountains east of Dog Canyon (fig. 2). The minor faults east and west of the major faults are downthrown in such a manner as to somewhat counteract the effects of uplift caused by the major faulting and tilting of the strata.
In parts of the Guadalupe and Delaware Mountains uplift are faults trending in other directions than north-northwest. About 10 miles south of the area studied is a prominent system that trends northeast (pl. 21). Individual members are discontinuous, but the system as a whole extends from the Border fault zone on the west to the outcrops of the Castile formation on the east. These northeasterly faults are prominent features on aerial photographs, and are indicated not only by offsets of the beds, but by numerous straight valleys. Apparently the northeasterly fault system is accompanied by strong jointing, likewise indicated by drainage. They may be related to the east-northeasterly joints within the area studied.
Farther south, in the south part of the Guadalupe and Delaware Mountains uplift, is another system of faults that trends west-northwest. The most prominent members of the system bound the north side of the Apache Mountains, but others are found to the north and south.
The faults on the north side of the Apache Mountains extend diagonally across the south end of the Guadalupe and Delaware Mountains uplift, and raise the strata to the south, contrary to its southward pitch. They cross the belt of north-northwesterly faults near Seven Heart Gap.68 Both systems of faults are apparently of later Cenozoic age, but the west-northwesterly system has the same trend as the reef zone in the Permian rocks of the Apache Mountains. This relation suggests that it was formed by renewed movement on an older tectonic trend (figs. 15, A, and 16, A).
West of the Apache Mountains across the Salt Basin is the Sierra Diablo (fig. 1). Like the Guadalupe and Delaware Mountains, it is a broad, asymmetrical uplift, but its faulted flank is on the east and its tilted flank on the west. Toward the south, its east flank consists of several blocks of gently dipping strata, such as the Baylor Mountains. The blocks stand lower than the main uplift and descend in steps toward the basin. Toward the north, thick alluvial deposits extend up to the main fault at the edge of the uplift, and few remnants of the depressed eastern flank are exposed.
The main faults of the Sierra Diablo have an average northerly trend, but the trend of individual faults is more variable than in the Guadalupe and Delaware Mountains (pl. 21). The group that outlines the east side of the uplift includes a north-northeasterly fault, and a fault made up of several curves, whose average trend is northward. These faults have had much the same history as those of the Border zone in the Guadalupe Mountains. The rims of their escarpments have receded some distance from the fault traces, and are indented by several large canyons, as though the first faulting took place some time ago. Later movements are suggested by the manner in which the bases of the escarpments follow the fault lines, by the well-developed alluvial fans on their downthrown sides, by scarps in the alluvial deposits, and by uplifted terraces in the mountains.
Extending diagonally across the Sierra Diablo in the same manner as in the Apache Mountains is a set of west-northwesterly faults. Most of them are of smaller displacement than the northward trending faults along the eastern border. Larger faults of west-northwesterly trend bound the north and south sides of the uplift. Several of the west-northwesterly faults stand nearly in line with faults of the same trend near Seven Heart Gap in the Apache Mountains and may be continuous be neath the alluvial deposits of the Salt Basin.
The age relations between the northerly and the west-northwesterly fault systems are complex. The last movements on the west-northwesterly faults are older than the last on the northerly faults, for their scarps have been much eroded and show none of the evidences of recent movement seen on the others. They may have formed at about the same time as the older movements on the other set, however, because faults of either trend are likely to terminate against those of the other. Ancient movements, some dating back to Paleozoic time, took place along, or on the same trend as, the west-northwesterly faults (figs. 15, A and 16, A and B). The west-northwesterly faults therefore may have resulted from Cenozoic movements along old trend lines at a time when the forces were such as to produce dominant, northward trending tectonic features.
The Salt Basin, which lies between the Sierra Diablo on the west and the Guadalupe and Delaware Mountains on the east, is a great depression 5 to 15 miles wide and of about the same length as the mountain ranges themselves (fig. 1). Except for outcrops of bedrock along its margins, it is floored entirely by unconsolidated Cenozoic deposits. Well records show that these deposits are hundreds or even thousands of feet thick.70
The basin is a sunken block of the earth's crust. Outcrops along its borders consist of rocks that lie high on the mountains to the east and west, and have been warped down or faulted down to their present positions. At the north and south ends, known as Crow Flat and Ryan Flat, respectively, the basin appears to be a downwarp, but in the longer central section, faulting dominated. The structure of the bedrock beneath the unconsolidated deposits of the basin is unknown but is assumed to be complex.
The high points on the opposite sides of the basin do not correspond in height. The high point on the east side, near Guadalupe Peak, lies opposite a low-lying section of the Diablo Plateau to the west. The high point on the west side, in the Sierra Diablo, lies 30 miles or more to the south, opposite the lower Apache and southern Delaware Mountains.
BASIN AND RANGE PROVINCE IN TEXAS AND NEW MEXICO
The Guadalupe, Delaware, and Apache Mountains, and the Sierra Diablo constitute a small part of the Basin and Range province, which extends far to the west and northwest.71 Nearby parts of the province resemble the area studied in tectonic and geomorphic features. These parts, which include northern trans-Pecos Texas, and that part of New Mexico between the Pecos River and the Rio Grande, are known as the Sacramento section.72 Tectonically, this section could be classed as a fracture belt of low mobility.73 It resembles many other block-faulted regions of the earth, including the rift-valley area of East Africa.74 It differs from the latter mainly in the smaller scale of its features.
The Sacramento section consists of alternating block mountains and desert basins with a general northward alinement (fig. 1). Most of the mountains have a steep escarpment on one side, outlined by faults, and a gentle slope on the other which follows the dip of the beds. The mountains are made up of a plate of Paleozoic and Mesozoic sedimentary rocks several thousand feet thick, and of an underlying basement of pre-Cambrian crystalline rocks. The sedimentary rocks are little disturbed except by the uplift of the ranges themselves, which were raised in Cenozoic time. They appear to have been only lightly affected by earlier movements, such as those of older Cenozoic time. There are, however, some large masses of intrusive igneous rock in the western part of the Sacramento section that are of post-Cretaceous age though probably older than the faulting. In some of the desert basins, thin sheets of basaltic lava are interbedded with or are spread over the surface of the basin deposits. These sheets of lava are mostly younger than the faulting but may be related to it.
Superficially there is an apparent gradation in the Sacramento section from block mountains into fold mountains. The Sierra Diablo, for example, is decidedly blocklike, and any tilting of the strata is the direct result of faulting. The Guadalupe and Delaware Mountains, however, are more archlike, but with the arch greatly modified by faults. The Sacramento and Sandia Mountains farther north have the form of broad domes or arches, faulted on one side and pitching down at their north and south ends.75 The Sangre de Cristo Mountains, which lie next north of the Sandia Mountains, are true folded ranges, and are the southernmost prongs of the Rocky Mountains. Along their axes, a core of pre-Cambrian rocks is exposed, and their sides are broken in places by thrust faults.76
This northward gradation from one sort of tectonic feature into another is more apparent than real, as the folding and block faulting took place at different times. The folding of the southern Rocky Mountains is mainly of late Cretaceous and early Tertiary (Laramide) age, and the block faulting farther south is mainly of later Cenozoic age. Whatever folding there is in the mountains to the south may have been inherited from a deformation that was contemporaneous with the folding of the mountains to the north. At the time of the block faulting of the mountains to the south, the mountains to the north were not only broadly uplifted but locally broken by normal faults such as those that lie between the west side of the Sangre de Cristo Mountains and the Rio Grande depression.77 A similar interpretation for the Sacramento section has been made by Bryan78 and for the Great Basin of Utah, Nevada, and California by Nolan.79
REGIONAL RELATIONS OF JOINTS
Some observations have been made on the regional arrangement and distribution of joints in the area surrounding the southern Guadalupe Mountains. In addition to the field observations in the area studied, extensive field observations have been made by me in the Sierra Diablo.80 Recently I have also studied aerial photographs of the region embracing the Guadalupe and Delaware Mountains and the Sierra Diablo, and from them have obtained information on the regional relations of the joints. The regional relations as now known are summarized on plate 21; on this plate, field observations on joints in the southern Guadalupe Mountains and the Sierra Diablo are summarized on roses.
North-northwesterly joints are probably dominant the entire length of the crest of the Guadalupe and Delaware Mountains uplift, as are the faults of the same trend. They are dominant near the crest in the area studied, and aerial photographs indicate that they are also dominant farther south. East-northeasterly (or northeasterly) joints are not prominently expressed in the aerial photographs except in the region immediately south of the area studied, or about midway along the length of the Guadalupe and Delaware Mountains uplift. They may be present elsewhere, but have little topographic expression. As indicated below (p. 124), these two joint sets are probably closely related to the Cenozoic uplift of the mountains.
Farther east and northeast, on the long, gently tilted east slope of the uplift, other fractures seem to dominate. The east-west linear features (probably fracture zones) in the Castile formation of the Gypsum Plain have already been noted (pp. 90-91). They seem to have formed by readjustments within the Castile which do not influence the overlying and underlying formations. North of the Gypsum Plain, at Carlsbad Cavern (in the Capitan and Carlsbad limestones of the Reef Escarpment), cave openings have been carved along two major joint sets, the dominant one trending east-northeast to east, with the other nearly at right angles.81 Near the Reef Escarpment in this vicinity, as shown on aerial mosaics and the new topographic map of the Carlsbad Cavern quadrangle, ridges and valleys in the limestone have the same east-northeast to east trend, but they pursue a sinuous course, parallel to the curves in the Reef Escarpment and the Capitan reef. This indicates that the joints in this vicinity are more closely related to Permian structural features than to Cenozoic structural features formed during the uplift of the mountains.
Near the south end of the Guadalupe and Delaware Mountains uplift, the north-northwesterly joints are crossed by another set of west-northwest trend, parallel to the faults along the north side of the Apache Mountains. In aerial photographs they are prominently displayed in Permian limestones along the crest of the Apache Mountains, and also in the Cretaceous rocks along the same trend to the southeast. In the Sierra Diablo, across the Salt Basin to the west, similar joints prevail. They are indicated by field observations summarized by the roses on plate 21, and are also prominently displayed on aerial photographs. They are parallel to one of the prominent systems of faults in the Sierra Diablo. Few joints in the Sierra Diablo are parallel to the northerly faults that bound its eastern side.
The west-northwesterly set of joints in the Apache Mountains and the Sierra Diablo is probably related to structural features older than the uplift of the Guadalupe and Delaware Mountains, in part perhaps of Paleozoic age.
Only fragmentary information on the trends of joints is available away from the immediate vicinity of the Guadalupe and Delaware Mountains. To the east, Melton82 has noted joints in the cap rock of the Llano Estacado that trend mainly west-northwest. They are probably unrelated to any of the systems just described in the Guadalupe and Delaware Mountains. To the west, joints have been noted by Richardson83 and Dunham84 in the Franklin and Organ Mountains. The structure here is more complicated than farther east, and there are extensive igneous intrusions. The joints of these mountains therefore may be more of local than regional significance.
HISTORY OF GUADALUPE AND DELAWARE MOUNTAINS UPLIFT
In order to understand the Guadalupe and Delaware Mountains uplift, the time relations as well as the physical features and space relations must be known. Something of its history can be deduced from the features already described, and more can be obtained from the Cenozoic deposits and land forms that are described later. In addition, parts of the history of which there is little record in the mountains themselves can be inferred by comparison with adjacent, similar regions where the record is better known. These lines of evidence and the inferences to be drawn from them are summarized here.
FEATURES OLDER THAN THE UPLIFT
The Guadalupe and Delaware Mountains uplift is of post-Cretaceous age. Evidence from adjacent parts of trans-Pecos Texas and New Mexico indicates that Cretaceous seas covered the entire region. They spread over a nearly level surface, or peneplain, that was formed in older Mesozoic time. The peneplain is now exposed at many places in the region, and the summit peneplain of the Guadalupe Mountains is probably a part of it, although now stripped of its postulated Cretaceous cover. The peneplain bevels Paleozoic features, such as those shown on figure 15, B, and is tilted and faulted by the movements that produced the present ranges. It is, therefore, a convenient datum plane for separating older and younger tectonic features.
Farther south in trans-Pecos Texas there are extensive masses of volcanic rocks of early Cenozoic age. They lie unconformably on deformed Cretaceous rocks, and are themselves folded and faulted. In this area, therefore, movements took place in late Cretaceous or early Tertiary (Laramide) time and after the volcanic epoch, perhaps in Oligocene or Miocene time (p. 108). These movements probably also affected the Guadalupe Mountains area.
The early phases of the uplift of the Guadalupe and Delaware Mountains are imperfectly recorded in the region. The initial uplift, however, may have taken place at the same time as that farther northwest in the Sacramento section, where deposits, land forms, and tectonic features related to it are well exposed and have been studied by Kirk Bryan and his students.85 According to Bryan, the initial uplifts here took place before the deposition of the Santa Fe formation, and hence were probably of Miocene or early Pliocene age. They thus correspond approximately to the post-volcanic deformation in trans-Pecos Texas.
East and west of the Guadalupe Mountains, deposits of about the same age as the Santa Fe formation were formed as a result of erosion that followed the initial uplifts. To the east they form the cap of the Llano Estacado and are a part of the Ogallala formation. To the west, they probably form the main mass of the thick, unconsolidated deposits of the Salt Basin. The nature of the latter deposits is little known, however, because they are everywhere covered by Quaternary deposits.
In the mountains themselves, some indication of the nature of the initial uplift is given by the present stream patterns in the limestone uplands (fig. 19). Some of the streams seem to be unrelated, and hence antecedent, to the fault blocks that they cross; thus, the stream in South McKittrick Canyon crosses from the downthrown to the upthrown side of the Lost Peak fault zone with little or no deflection (pl. 22). Other streams, such as those in the upper courses of Dog and West Dog Canyons, follow depressed fault blocks, and a appear to belong to a later generation. In the parts of the mountains where no faulting has taken place, the two generations of streams cannot be differentiated. It seems plausible, however, that most of them were consequent on the surface of the original uplift, and that in the limestone areas their courses became relatively fixed by incision into the resistant rock.
As indicated by the stream pattern, the initial uplift was a broad arch, not broken by as many faults as at present. The crest of the arch was probably near the present summits of the southern Guadalupe Mountains, for the supposedly consequent streams radiate northeastward, northward, and northwestward from it (fig. 19). The slopes of the arch seem to have been more gentle than the present slopes of the mountains, and its crest may not have stood as high. The incised streams of the limestone areas have distinctive, meandering courses, and join one another at wide angles, forming an open, dendritic pattern, as though they originally flowed down a gentle slope. This pattern, shown in the stippled areas of figure 19, is unlike that shown in the southeast part of figure 19, where the rocks are less resistant, and where the streams could adjust their courses to the steeper gradients of later periods.
Part of the jointing of the rocks of the Guadalupe and Delaware Mountains probably took place during the initial uplift. Fracturing of the rocks near the surface is likely to take place, even under the application of stresses too gentle to produce faults. If the rocks were jointed during the early phases of the uplift, the faults that came into existence later followed the pre-existing fractures.
If it could be proved that the joints normal to tilted beds in the western part of the area were originally formed in a vertical position, and had been rotated along with the beds at the time of block faulting, the suggested conclusion that the joints were older than the faults would be confirmed. It is equally possible, however, that the joints originated in their present attitudes, after the beds had been tilted.
In the northwest part of the Sacramento section, according to Bryan,86 the main block faulting, by which the present basins and mountain ranges were outlined, took place after Santa Fe deposition, and hence in late Pliocene or early Pleistocene time. According to Bryan:
These post-Santa Fe movements appear to be of the same age as the main phase of the uplift of the Guadalupe and Delaware Mountains.
During this phase the mountains were raised nearly to their present height and were given nearly their present form and outlines. The probable archlike form of the initial uplift was at this time broken into many fault blocks, especially near its crest. These blocks gave rise to the second generation of consequent streams, such as those in Dog and West Dog Canyons. The faulting did not result from the collapse of the initial arch, for it is deduced that the arch was not raised as high during the initial phase as it was afterwards, during the main phase. The main phase of the uplift probably resulted from continued application of stresses like those which caused the initial uplift, but of sufficient intensity to cause the mountain area to be further uplifted and to be broken into fault blocks.
Most of the faults in the area probably date from the main phase of the uplift. The scarps that follow faults east and west of the Border zone are eroded to the same degree where they are cut on the same sort of rocks, such as the Capitan limestone. Moreover, the scarp along the Border zone exhibits remnants of a topography equally mature, although most of the present features of the scarp indicate modification by renewed erosion resulting from subsequent movements.
Younger movements in the Guadalupe and Delaware Mountains are indicated by the faulting of deposits of probable older Pleistocene age. The movements took place after an extended period of quiescence, for some of the deposits that are now faulted were laid down on a pediment carved from the disturbed Permian rocks.
Movements appear to have taken place only along faults that were already in existence, and to have resulted in displacements in the same direction as during the main phase. The displacements, however, were only about a tenth as great as the older ones, amounting at most to several hundred feet (p. 113). Movement took place along the faults of the Border zone and those immediately west of it, or in a much narrower belt than during the main phase. Faults to the east and west were undisturbed.
No definite evidence is available as to whether or not the faulting of the later phase was accompanied by further uplift of the mountain area. An increase in the relief of the mountains with respect to the floor of the Salt Basin is indicated not only by the displacements on the faults themselves but also by the dissection of the older, probably early Pleistocene deposits, which was caused by the accelerated activity of streams resulting from a change in base level. This dissection, however, may have been caused by a subsidence of the basin, rather than by an uplift of the mountains. The dissection of similar older deposits on the east flank of the mountains by streams draining into the Pecos River may have resulted in part from actual uplift of the mountains, although it was undoubtedly influenced by other factors.
The later phase of the uplift is younger than the deposits of probable early Pleistocene age and older than deposits of Recent and perhaps later Pleistocene age, which are undisturbed by it. It is, therefore, perhaps of later Pleistocene age. No evidence for any still younger movements has been found in the Guadalupe and Delaware Mountains.
NATURE OF STRUCTURE BENEATH THE SURFACE
Most of the available information on the tectonics of the Guadalupe Mountains and their surroundings relates to features at or near the surface, and very little is known of the structure of the deeper-lying rocks. A little information on the subsurface structure is afforded by wells. Some in the Salt Basin have been drilled in the unconsolidated Cenozoic deposits, and two in the Guadalupe and Delaware Mountains have been drilled through the Permian into the underlying rocks. No geophysical studies have been made in the region.
Some idea of the nature of the structure at depth can be obtained by projecting downward the features seen at the surface. On figure 18 the four structure sections of plate 3 have been redrawn and expanded downward to the top of the basement rocks. The top of the basement is assumed to lie 8,000 feet below the top of the Bone Spring limestone, and faults are assumed to have plane surfaces, dipping at the same angle underground as on the outcrop.
The actual details of the features shown on the expanded sections may be modified by changes in the structure with depth. The conditions assumed in drawing them are obviously too idealized, and may be even unnatural. Thus, the depths to the basement rocks, although based on thicknesses at the nearest outcrops, may not be the true figures for the area, and the depth may change from place to place across the area. Also, the faults may die out with depth or change their dip.
Dying out of the major faults with depth seems unlikely, however, because it would imply a mass of incompetent rocks below the surface, whereas, so far as known from nearby outcrops, the beds between the Bone Spring and the basement rocks are competent limestones and sandstones. Moreover, in nearby mountain ranges, such as the Sierra Diablo and the Sacramento and San Andres Mountains (fig. 1), the basement rocks are broken by faults of the same sort as those in the overlying rocks and as those in the Guadalupe Mountains.
Little can be said as to possible changes in dip of the faults at depth. Within the limits of observation, even where the faults cross areas of high relief, they seem to have plane surfaces. However, some of the major faults have curved traces, concave toward the downthrow, and this may indicate a similar curvature in vertical section. Further, some of the fault blocks west of the Border zone have been rotated, and it has been suggested that "a tilted block can only rotate against a curved surface."87
As shown on the sections of figure 18, the amount of vertical displacement by faulting and tilting is small when compared with the width of the belt of uplifted rocks or the thickness of the sedimentary shell. There appears to be a tendency toward simplification of the structure downward by the joining of closely spaced faults, so that, at the top of the basement rocks, the faulting is concentrated along several large breaks, rather than dispersed along smaller breaks. Details of these conclusions may be modified by the factors just discussed. Thus, if the faults are concave on their downthrown sides, they must intersect at shallower depths than shown on figure 18.
RELATIVE VERSUS ACTUAL MOVEMENTS
In describing the structure of the region, the Guadalupe and Delaware Mountains were said to have been uplifted, and the Salt Basin to have been depressed. These are relative terms. So far as one can tell from the present relations between the fault blocks, their situations might have resulted from differential uplift of the entire area, from differential subsidence, or from a combination of the two.
EVIDENCE FOR ACTUAL UPLIFT
Evidence as to the actual nature of the movements in the Guadalupe and Delaware Mountains is clearer than in the ranges farther west which are surrounded by complex tectonic features. The east flank of the Guadalupe and Delaware Mountains is hinged on the gently dipping rocks of the Pecos Valley and Llano Estacado, which stand at a much lower altitude and remained relatively stable during the late Cenozoic movements. The difference in altitude between the mountains and the Llano Estacado thus furnishes some measure of the actual uplift which has taken place in the mountains.
Before the uplift, which took place after Cretaceous time, the region probably lay near sea level. The present height of the mountains has resulted from uplift above this position, partly by epeirogenic movements which also raised the plains to the east, and partly by more localized disturbances which occurred in several stages. During the first stage, as suggested by the stream patterns, the mountain area did not rise to its present height. A much greater elevation evidently took place during the second stage. Further uplift during the third stage is possible but not proved. Apparently the mountains never stood much higher than they stand today.
EVIDENCE FOR ACTUAL DEPRESSION
The actual movements that have taken place in the Salt Basin are less certain than those in the mountains. The basin has been deeply filled with unconsolidated deposits, but this deep filling is due more to the absence of through-flowing drainage than to any actual elevation or depression. The basin may have subsided while the mountains were being uplifted, it may have been raised to a slighter extent than its surroundings, or it may have remained at about its original position, while the mountains were raised around it.
The depth of the rock floor below the surface of the unconsolidated deposits of the Salt Basin is uncertain, as even the deepest wells drilled in the basin have failed to reach bedrock. One well, drilled near the southwest corner of the area studied, was still in unconsolidated deposits at a depth of 1,620 feet;88 hence the underlying rock floor lies 2,000 feet or less above sea level. This level is lower than the surface of the Llano Estacado east of the mountains, but it may still be higher than the altitude of the region before Cenozoic disturbances.
In nearby basins, scanty well records indicate that in places the rock floor beneath the unconsolidated deposits lies considerably above sea level, and in other places lies at or below sea level.89 These relations suggest that the intermontane basins on the whole were raised above their original positions, but by smaller amounts than the adjacent mountains, and that actual subsidence took place only in a few areas.
ORIGIN OF LATER TECTONIC FEATURES
By what means did the later tectonic features of the Guadalupe and Delaware Mountains come into existence? The features are much simpler than those in regions where folding and overthrusting prevail, yet their origin is elusive because of their very simplicity. In folded and overthrust regions, lateral compression of the crust is an obvious, dominant force. Here, the effects of such compression are not clearly evident, yet the crust has been raised and lowered into mountains and basins, and has been fractured by faults and joints. Is this another manifestation of lateral compression, or have the tectonic features arisen from some other set of forces?
ORIGIN OF JOINTS
The faults and joints that have fractured the rocks of the region are closely related to the formation of the mountains and basins. The manner in which they are arranged may furnish tangible clues to the orientation of the stresses that deformed the region. The joints are more widely distributed and are possibly older than the faults, hence their origin will be considered first.
Final interpretation of the joints probably cannot be made from the study of a small area alone, for there are likely to be significant regional variations in their patterns that can be determined only by a study of a wider area. Such variations are suggested, for example, by comparing the observations in the southern Guadalupe Mountains with those in the Sierra Diablo (pl. 21). In view of the present lack of detailed knowledge of these regional variations, conclusions based on joint studies in the area of this report must be regarded as tentative.
An explanation of the joints in the area must recognize the large number of joint sets present, the parallel and transverse relations of the most abundant pair to the axis of uplift, and the greater development of the next most abundant pair on one side of the axis than on the other. It must recognize also the common habit of joint sets to lie at right angles to one another, the absence of inclined joints except in tilted beds, and the lack of horizontal shift of one part of the area relative to another.
The effects of deformation have been pictured diagrammatically by the figure known as the strain ellipsoid.90 When this figure is compressed, fracturing may take place normal to its long axis (parallel to its short axis) as a result of tension, or diagonally to the axes as a result of shearing. Most joints of regional extent are probably either tension joints or shear joints. In the Guadalupe Mountains, where the joints are dominantly vertical, the long and short axes of an ellipsoid would lie horizontally, and the forces causing the jointing would be directed in a horizontal plane.
The dominant joint set in the area, the one that trends north-northwest parallel to the axis of uplift, is probably of tensional origin, and results from a stretching of the rocks east-northeastward and west-southwestward. This origin is suggested by the great number of faults parallel to it, which implies that it was the most open of all the sets of fracture, and therefore the most subject to movement.91 The pair of joint sets diagonal to the dominant set, trending north-northeast and west-northwest, may be the result of shear. They are less open than the first, and they are followed by fewer faults.
The origin of the east-northeasterly joints at right angles to the dominant set is less easy to explain. If formed at the same time as the others, they should lie normal to the direction of greatest compression, or along planes on which fracturing is not expected to occur. Some difference between them and the others must exist, as they are not followed by any faults. Moreover, they seem to be distributed differently than the dominant set. The latter, and the faults of the same trend, are found in greatest abundance close to the axis of uplift, as shown on plate 21, and apparently are less prominent eastward, away from the axis. The east-northeasterly joints are not only common near the axis, but seem to prevail about halfway between the north and south ends of the uplift (not far south of the area studied, pl. 21), and to extend for some distance east of the axis.
The east-northeasterly joints may be older, more fundamental features than the other joints, formed as a result of tension like the dominant set, as a byproduct of compression at right angles to the axis of uplift, before it was raised to great height. Their abundance about halfway between the north and south ends of the uplift is in harmony with this interpretation. If the east-northeasterly joints are older, they may have originated during the early phase of the uplift of the mountains, of mid-Tertiary and older age (pp. 108 and 120).
Under this explanation, the dominant, north-northwesterly younger joints resulted from a reversal of forces. During the main phase of the uplift of the mountains, compression continued at right angles to the axis of uplift but was transmitted into the superficial rocks in the form of vertically acting movements. As a result of these vertical movements, tension developed in an east-northeast and west-southwest direction, causing the formation of the dominant north-northwest joints along the axis of uplift. This may have taken place at the same time as, or slightly before, the faulting.
ORIGIN OF FAULTS
The faulting of the area may have taken place after the jointing as a result of movements along the fractures thus formed. If of different age, the faults arose from a set of stresses different from that which caused the joints, and merely followed the lines of weakness already created. More probably, they resulted from similar stresses, acting on the rocks with greater force than before.
The faults seem to be tensional features. Wherever observed, their planes dip toward the downthrow, in the usual manner of normal faults in other regions. Study of the structure sections as drawn suggests that some extension of the crust resulted from the faulting (fig. 18). Moreover, the narrow and in part deeply depressed grabens found in many places could result only from an extension of the outer crust. The faults of the Border zone, following a zigzag course across the faulted tract (pl. 21), give the impression of a major tensional break whose trace was determined by already existing lines of weakness. Near the faults there is no crumpling or folding, such as one expects from compression. Further, the faults are in many places filled by veins, which suggests that they were under tension rather than compression.
ORIGIN OF UPLIFT
The uplift of the mountains and the displacement of its rocks by faults are closely related movements, yet they result to a certain extent from opposing forces. Tensional faulting can lower sections of the earth's crust in the direction of gravity; it cannot raise them. Nevertheless, the mountains have been progressively raised against gravity, and from the known geologic history, it would seem that the raising of the mountains went hand in hand with the faulting. The opposition of the two forces is illustrated by sections BB' and CC' of figure 18. If the now disrupted beds in these sections were reconnected by moving each fault block back to its original position, the uplift would be much higher than it now is.
Apparently the uplift and the downfaulting were not caused by isostatic readjustment, resulting from loading of the depressed areas by the deposition of basin deposits, and from unloading of the elevated areas by erosion. This factor has been analysed by Gilluly.92 As he points out, isostasy would not explain the formation of the initial uplifts and basins. From computations based on an area similar to the Guadalupe and Delaware Mountains region, he finds that "local compensation could theoretically account for perhaps one-third to one-half of the observed displacement." He concludes, however, that the factor of isostasy is subordinate and "that the ultimate cause of the first faulting has likewise been the prime factor in continuing the movement."
Tectonic features, such as those in the Guadalupe and Delaware Mountains, which include both normal faults and uplifted areas, have been explained by Bucher93 as resulting from alternations of rather brief, severe times of compression, and of longer periods of relaxation and tension, both of which are of wide areal extent. "Regional tension created the basins and furrows * * * while the epochs of compression forced up the positive units." This implies that the faulting, which is of tensional origin, took place during long intervals between brief times of uplift of the mountains.
This theory explains many features of the region, but it also raises many difficulties. There is no evidence that the times of uplift were distinct from the times of faulting; instead, the two seem to have gone hand in hand. There is also no evidence that the times of uplift were of shorter duration than the times of faulting. The last period of faulting, which displaced the older unconsolidated deposits seems, in fact, to have been relatively brief, and to have been preceded and followed by times of quiescence during which pediment cutting and other erosion processes acted for a long period without interruption.
Moreover, considering the region in its relation to other parts of the southwestern United States it is difficult, by any means of correlation that we now possess, to separate the times of uplift and faulting in such ranges as the Guadalupe and Delaware Mountains from times of compressive deformation elsewhere. Thus, the first uplift suggested for the mountains, possibly of pre-Pliocene age, corresponds closely in time to the period of post-Oligocene folding that is manifested elsewhere in trans-Pecos Texas. The second uplift and faulting, of late Pliocene and early Pleistocene age, took place at about the same time as the broad uplift of the San Juan Mountains in Colorado94 and the strong folding of the Coast Ranges in California.95 This correlation suggests that a single epoch of deformation resulted in one place in block faulting, in another in epirogeny, and in a third in orogeny.
As suggested when interpreting the joints (p. 124) the Guadalupe and Delaware Mountains may have arisen as a result of deep-seated compression, manifested at the surface by essentially vertical uplift, which put the surface rocks under tension, thereby producing along the crest of the uplifted region an extensive system of tension joints and normal faults.
This interpretation closely resembles the early suggestion by Gilbert96
Last Updated: 28-Dec-2007