Although the Great Basin region has been characterized (29, p. 346) as a "fracture belt of low mobility," the evidence now available indicates that on the contrary the province has been orogenically active throughout a large part of its history. Indeed, the Paleozoic and later history that can be deciphered suggests an interpretation of mountain building that is at variance with the commonly accepted belief that major diastrophic revolutions are relatively brief events marking the end of one geologic era and the beginning of the next.
The pre-Cambrian structural history is little known at present, as the scattered regions in which the pre-Cambrian rocks are exposed have been inadequately studied for purposes of correlation. It would appear, however, that the Great Basin region was active orogenically during at least three epochs in pre-Cambrian time.
The record beginning with Cambrian time is much more complete. The geosynclinal sea, which in the early part of the Paleozoic era covered most of the province and in which many thousands of feet of sediments were deposited, was divided in late Devonian time by a rising arch or geanticline in western Nevada. Locally there was moderate deformation during the uplift, but by Permian time elevation had ceased and most of the positive area had been covered by marine sediments. Coincidentally with the degradation of this geanticline during the Permian epoch a similar uplift began in eastern Nevada and a land mass between two seas persisted there until early Jurassic time. Although an unconformity is recognized between Permian and Triassic sedimentary beds in both the eastern and western parts of the province, it appears to be relatively slight except on the borders of the geanticline; over most of the province the angular discordance between the Permian and Triassic is very small, and the sedimentary beds of the two units have essentially the same geographic distribution.
This younger geanticlinal area, instead of subsiding, like the earlier one, became the site of intense diastrophism in early Jurassic time. To judge from the relatively meager evidence, similar deformation continued recurrently into the early part of the Tertiary period, affecting an area that considerably exceeded that of the Great Basin. No regular pattern in space or time can yet be discerned for these epochs of deformation within the province; in one area there may be evidence of only one episode, but in another nearby area, there may have been several. East of the province, however, there appear to be good grounds for inferring that during successively younger epochs, deformation affected regions farther and farther east. There is no evidence of a similar westward migration unless it may be provided by the early Jurassic orogeny in western Nevada and the late Jurassic folding in the Sierra Nevada. These epochs of deformation can be approximately dated at only a few localities in the province, but at none of these places, strangely enough, has it been shown that deformation took place either during late Jurassic time or during the transition from Cretaceous to Eocene time.
The Mesozoic and early Tertiary folding and thrusting were succeeded closely by the initiation of the block faulting that is the cause, directly or indirectly, of the present relief of the province. The faulting appears to have begun at least by early Oligocene time and to have continued up to the present day. For several reasons, especially because of the apparent contemporaneity of block faulting with some late Tertiary overthrusting on each side of a major tear fault, the block faulting is here regarded as an end stage of the more intense deformation that preceded it.
This concept of an orogenic cycle in which a protracted epoch of spasmodic folding and thrusting was preceded by gentle upwarping and followed by block faulting is in many respects similar to one posthumously proposed by Woodworth (264) for the Appalachian Mountains. The Great Basin region, however, appears to provide a considerably more complete and in some respects a more readily authenticated picture than is available in the older mountain system.
Uncertainties regarding the pre-Cambrian structural history of the province are even greater than those concerning the correlation of the pre-Cambrian formations. The probable existence of three series of formations showing differing metamorphism both in the Wasatch Range and in the Death Valley region suggests that there were two epochs of major diastrophism during pre-Cambrian time, in addition to the orogeny that resulted in the unconformity at the base of the Cambrian. We have, however, little information regarding the nature or extent of these disturbances beyond Blackwelder's suggestion (21) that the folding trended northeastward.
The youngest of the three epochs of orogeny has some features worthy of brief comment. Perhaps the most striking is the absence of any pre-Cambrian granitic intrusive masses that are clearly younger than the sedimentary rocks involved in this deformation. Secondly, in the Inyo Mountains and possibly in the Death Valley region as well several unconformities have been recognized that separate the later pre-Cambrian formations, all of which appear to have undergone relatively little metamorphism. Finally, several observers regard at least part of these sedimentary beds as of terrestrial origin. These three features together seem to warrant the suggestion that in the part of the Great Basin where these rocks are now exposed this youngest pre-Cambrian orogeny was subsidiary and marginal to a more intense deformation in an area as yet unknown. The orogeny may thus have been similar to the younger revolution described below in having a central belt of more intense deformation and intrusion in which spasmodic activity continued over a considerable length of time.
During Paleozoic and early Mesozoic time the Great Basin was dominantly a region of sedimentation. In large part the structural disturbances during this time must be inferred from evidence provided by the lithology and distribution of the sedimentary depositsdata that are summarized on preceding pages. The chief events thus determined include two broad upwarps that formed geanticlines whose axes appear to be parallel to the axes of the great geosynclines in which they were developed. The deformation of the strata involved in the upwarps is negligible compared with the folds and faults that were formed in later Mesozoic and Tertiary time, and these movements may be classed as epeirogenic, but they are regarded as genetically related to and the harbingers of the revolutionary events that followed them.
The earlier geanticline (185) was formed in western Nevada and appears to have come into existence in late Devonian time. Although the involved area appears to have continued to rise until Permian time, the aggregate uplift was not great. Marked angular unconformities reflecting more intense movement have thus far been reported only from the Hawthorne and Tonopah quadrangles, in west-central Nevada (70), where Permian sandstones rest with angular discordance on the Ordovician. There appears to be no evidence that the uplift within the Great Basin was accompanied by other than minor volcanic activity, although the final disappearance of the geanticline may have been roughly contemporaneous with the extrusion of the Permian volcanic rocks that are known in its axial region. West of the province, however, deposition of marine sediments in northern California during both Mississippian and Pennsylvanian time appears to have been accompanied by abundant volcanic activity (51). This asymmetric localization of volcanism with respect to the uplift was repeated during the later upwarp
The second geanticline first emerged as a persistent land mass in the Permian epoch, coincidentally with the disappearance of the older one, whose axis had been about 100 miles to the west. It continued in existence as a relatively narrow barrier throughout early Mesozoic time and was then greatly enlarged to form Schuchert's Cordilleran Intermontane geanticline (211, p. 187). The initial uplift in the Permian epoch appears to have been largely restricted to southern Nevada and southeastern California, approximately on the site of a region of instability in Pennsylvanian and later Mississippian time. Such stratigraphic evidence as is available suggests that the upwarping progressively extended northward, until in late Triassic time the land mass reached the northern boundary of the province.
Like the earlier geanticline, the uplift appears to have been on the whole relatively gentle, but epochs of more vigorous movement appear to have been somewhat more frequent, culminating finally in the profound folding and faulting of the later Mesozoic beds. Thus, in both eastern and western seaways the Permian-Triassic contact in areas close to the geanticline is marked by a fairly pronounced erosional unconformity; furthermore, the Upper Triassic sedimentary series, especially in western Nevada, rests with angular discordance on the older Triassic strata. The clastic and nonmarine sedimentary beds that were deposited so abundantly in the eastern seaway probably have a similar import.
This uplift also resembled the older one in that extensive contemporaneous surface volcanism was concentrated in the trough or sea that was formed west of the geanticline. The fact that the two uplifts had this feature in common is probably significant, but its interpretation is at present uncertain. A possible explanation might be that the active force causing the upwarping lay to the west.
A long period of intense diastrophism that eventually affected all of the Great Basin province began in middle Mesozoic time and continued into the early part of the Tertiary period. The folds and thrust faults that were formed during this time of crustal unrest transformed the province from a region that had been in large part a basin of sedimentation since later pre-Cambrian time into a highland area that was henceforth to be essentially free from marine invasions. It was thus revolutionary in results, although the deformation appears to have been simply an intensification of the broad uplift that began in the southeastern part of the Great Basin in Permian time.
NATURE OF THE DEFORMATION
Our knowledge of the number and relative importance of the different structural features developed during the later Mesozoic and early Tertiary diastrophism is sketchy, being based in large part upon a few detailed studies of rather widely separated areas. So far as this information goes, it appears to indicate, first, that major folds and thrust faults of moderate to large displacement were formed over most of the province: second, that normal faults were formed only locally, although the tendency of much younger faults to follow old structural lines makes this belief somewhat uncertain; and third, that there were developed several extensive belts having nearly east-west trends, at right angles to the strike of the folding and thrusting in the rest of the province. These east-west trends, in part at least, have been developed as a result of dominantly horizontal movement along major transverse faults. Along most of the thrust faults so far described the upper plate has clearly moved eastward relative to the lower plate, and this fact, combined with the apparent westward increase in the intensity of folding, suggests that the active force causing the deformation came from the west.
FOLDS AND THRUSTS
Folding of the pre-Tertiary rocks throughout the Great Basin was recognized in the earliest geologic surveys of the province; indeed, it was so prominent that King (124) at first attributed to folding the form and location of the individual mountain ranges, comparing them with the folded Appalachians. Thrust faults, on the other hand, were not recognized until 1900, but since that time a large number have been described.
In spite of the wide extent of the folding, relatively few individual folds can be traced for any great distance, partly at least because major faults formed during a period of later block faulting commonly transect the trend of the folds. As the down-faulted portion of a fold is commonly concealed beneath younger deposits, only relatively short segments of the faulted folds are now exposed. Other ranges that apparently have monoclinal structure have been interpreted as the strike-faulted limbs of synclines or anticlines, and in several places this interpretation has been confirmed by the discovery of the fold axis where the fault has died out or where the strike of the fold diverges from that of the fault.
Two anticlines and two synclines in the Oquirrh Range, in central Utah (82), have been traced for as much as 15 miles before being terminated by faults along the range front, and if some of the monoclines, such as that found in the House Range, in western Utah, are faulted folds, even greater dimensions may be inferred. In many of the ranges, however, the continuity of the strata is interrupted either by large thrusts or normal faults or by intrusive masses, so that the province shows no such regular sequence of persistent folds as is found in the Appalachians, the middle Rocky Mountains, or even in the Coast Ranges.
King (125, p. 734) many years ago called attention to the westward increase in the intensity of folding, and more recent work, although disclosing local close folding in the east, seems on the whole to indicate that the generalization is valid. Thus the Paleozoic strata in western Utah and eastern Nevada commonly exhibit low to moderate dips, and overturned strata are exceptional, though reported at several localities, such as Goodsprings, Nev. (104), Gold Hill, Utah (187), the Oquirrh Range, Utah (82), and the southern part of the Wasatch Range, Utah (58). In contrast, Ferguson (63, 66, 70) has described the occurrence of widespread close folding, essentially isoclinal in places, at several localities in west-central Nevada, and this condition appears to be rather general throughout the western part of the State.
Although Spurr (225, p. 177) recognized a thrust fault in the southern Great Basin in 1900, Blackwelder's studies in the northern Wasatch Mountains, near Ogden, Utah, which were published in 1910 (14), were the first to establish thrust faulting as a major structural feature in the province. Most of the thrusts found in the next few years were either in the central or southern part of the Wasatch Range or close to the eastern border of the province and led to the rather commonly accepted belief that such faults were characteristic of and limited to a rather narrow zone that trends north-northeast and joins the zone that includes the Bannock and other thrusts in southeastern Idaho and western Wyoming. In recent years, however, thrust faults have been found at several localities that are considerably west of the eastern margin of the basin, and the validity of the earlier generalization now seems somewhat questionable. The distribution of the thrusts so far discovered is shown in figure 12, but this is far from being a final record. Whether or not there are differences in character between the eastern and western thrusts, comparable to the distinction that apparently holds for the folds in the two regions, is at present uncertain. Ferguson (67) believes that at least the great majority of the thrusts mapped by him and Muller in west-central Nevada have smaller displacements and that their relations are more erratic than those of the persistent low-angle or nearly horizontal thrusts that appear to be characteristic of the eastern part of the province. Such a difference in the type of thrusts might be expected in view of the supposed differences in the folds.
The thrusts in the northern and central parts of the Wasatch Range dip eastward, and in the earlier descriptions (14, 155, 112) the thrust plate was considered to have moved toward the west. Calkins (36), however, found that the drag folds indicated an eastward thrust, a conclusion later reached by Blackwelder (15) for the northern Wasatch and in harmony with Eardley's observations in the southern Wasatch (58, pp. 381-385), where an anticline overturned toward the east passes into a thrust. The thrust found by Loughlin (32, pp. 424-425, 438-439) in the Sheeprock Mountains, Utah, also appears to have been deformed and locally shows an easterly dip. Should the contact of the Cambrian limestone and the overlying quartzite in the Frisco district, Utah, prove to be a thrust (see p. 151), still another eastward-dipping thrust would be recorded.
The best-known series of major thrusts is in southern Nevada, near the eastern border of the province. It has been described by Longwell (144, 145) and Hewett (104) and comprises a belt of eight or more thrusts with variable but commonly low westerly dips. Some of the thrusts are regarded as having definitely moved over an old erosion surface (144, p. 570; 85). The absence of basement rocks in the overriding block, together with the observed changes in dip, suggests that the thrusts pass downward into a nearly horizontal sole (186, 147). Hewett (104, pp. 53-54) regards the thrusts in the Goodsprings area of Nevada as being successively younger westward and cites evidence indicative of an erosion interval between two of them. In this region the belt of thrust faults has been traced about 100 miles along the strike.
Of the more westerly thrusts, those at Pioche, Nev. (254, pp. 42-43), although extensive, are not well exposed. They have apparently been warped by later folding or faulting and in one place appear to be younger than some lavas of supposed early Tertiary age. The thrusts at Gold Hill, Utah, on the other hand, are exposed in a relatively small area, but seem to provide a record of prolonged orogeny, in which folding or thrust faulting occurred at several times, separated by epochs of normal faulting and erosion (187). The thrusts at Eureka, Nev., may be of minor extent but have not been adequately studied. That in the Sonoma Range, Nev., according to Muller and Barksdale (176), dips eastward.
In west-central Nevada Ferguson and Muller (69, 70) have recently found an extensive series of thrusts associated with the folded Mesozoic sedimentary rocks. This region is unique among those so far described, not only because it is possible to date the thrusting rather accurately, but because of the complexity and relationships of the fault pattern. The thrusts appear to be intimately related to marginal troughs of the late Triassic seaway, and the forces that caused the thrusting appear to have continued over a time long enough to deform some of the earlier thrusts considerably. Thrust faults had earlier been recognized in the lower Paleozoic sedimentary rocks at Manhattan, Nev. (63), a short distance to the east.
Although all the areas that have been carefully studied exhibit moderate to large numbers of normal faults, it is commonly difficult to determine how many of these are genetically related to the later Mesozoic and early Tertiary epoch of diastrophism, as many of them are obviously either much younger block faults (see pp. 178-184) or are clearly the result of igneous intrusion or extrusion. In a few places, however, there is rather good evidence that normal faulting occurred on a fairly large scale during the period of folding and thrust faulting.
The best example of this relation is at Gold Hill, Utah (187) where several stages of normal faulting can be recognized. Each stage was preceded and followed by folding or thrusting, as is shown either by the termination of the normal faults against the younger thrusts or against transverse faults associated with the thrusts, or, in the numerous examples of recurrent movement along the normal faults, by notable differences in the displacement along the normal faults above and below the thrust planes. The pre-thrust normal faults at Pioche, Nev. (254, pp. 43-44), are possibly additional examples.
TRANSVERSE STRUCTURAL FEATURES
Faults which trend normal to the strike of the folds and thrusts and along which the movement has been largely horizontal occur in many places throughout the province. They are believed to be the product of the same orogenic forces that caused the folds and thrusts. Several varieties of these faults, which have been variously termed "flaws," "tear faults," "cross faults," and "transverse faults," may be distinguished. There is also some evidence suggesting the existence of nearly province-wide east-west belts along which the structural trends are notably at variance with those throughout the rest of the Great Basin. Their trend and the fact that there has been relatively recent horizontal movement of considerable extent along at least one of them suggest that the origin of these belts is similar to that of the simple flaws.
Although these transverse faults have been definitely identified in only a few places, published geologic maps suggest that they are fairly widespread. Where such faults have been recognized, they appear, in part at least, to be rather intimately related to thrusts. Thus several of the smaller transverse faults in the Tintic (142) and Gold Hill (187) districts of Utah and in west-central Nevada (70) may be traced into minor thrusts. The steep dips with strikes normal to the structural trend of these transverse faults change rather abruptly to the low dips and concordant strikes of the thrusts. Locally, as at Goodsprings, Nev. (104), and at Gold Hill, Utah, fairly large thrusts are terminated laterally by contemporaneous transverse faults of this sort. One of the best examples of such a relation, involving a thrust of considerable displacement, is found in the southern part of the Goodsprings district, where one of the major thrust plates, the Sultan, is bounded on the south by the Tam O'Shanter transverse fault.
Another variety of transverse fault is illustrated by the Ironsides fault in the Goodsprings district. It is marked by notable horizontal shifts in the rocks above the Keystone thrust and also offsets the outcrop of the thrust itself for about half a mile. It cannot, however, be traced into the block below the thrust. Hewett (104, p. 49) considers that the Ironsides fault was developed during the epoch of thrusting. Ferguson and Muller (70) have mapped similar faults in west-central Nevada, but in some of these neither the upper nor lower plate is greatly affected, and the faults are best interpreted as steeply dipping portions of the thrust that strike nearly at right angles to the remainder of the thrust outcrops.
The Arrowhead fault, in the Muddy Mountains, Nev. (145, pp. 110-111), and an unnamed transverse fault in the Spring Mountains, to the southwest (144, pp. 572-573), are both major cross faults that differ from those so far described in that they cannot be correlated with a single thrust. Both are interpreted by Longwell as being major "flaws," along which horizontal movement during the epoch of thrusting permitted the development of notably different types of folds and thrusts on the two sides of the faults. He suggests that a still more pronounced transverse fault of this type may extend for many miles beneath the alluvium of Las Vegas Valley.
There are, of course, numerous transverse faults along which horizontal movement can be proved but which cannot be directly connected with thrust faults. It is believed, however, that for many of them this failure is the result of the vagaries of exposure.
Scattered observations suggest the possible existence of three or four transverse structural features of still greater magnitude. Their nature, extent, and relations to the provincial structure are at present uncertain, as there has been relatively little detailed mapping along any of them.
The most southerly of these east-west belts is thought to lie along the northern border of the Mojave Desert region of southern California. The Garlock fault in this region is known to have been the site of profound horizontal movement in relatively recent time (see p. 186), but Noble (180, p. 425) has suggested that it was probably also active in pre-Tertiary time. A strong argument in favor of Noble's view is provided by the differences in the exposures of Paleozoic and early Mesozoic rocks on each side of the fault. To the north there is a thick section of these rocks, considerably folded and faulted but not greatly metamorphosed; south of the fault, however, the pre-Tertiary sedimentary rocks are highly metamorphosed. Recognizable fossils have been found at only a few places, and the bulk of the schists and gneisses of sedimentary origin have been classed on rather inadequate grounds as pre-Cambrian. Regardless, however, of the age assignment of these metamorphic rocks, it seems clear that the regions on the two sides of the Garlock fault have had considerably different structural histories, and the difference is too great to be attributed to the relatively recent movement along the fault.
A second belt is suggested by the recent work of Ferguson and Muller (70) in west-central Nevada. They found a pronounced offset in the outcrops of sedimentary beds deposited along the eastern border of the early Mesozoic seaway north and south of a zone in which east-west structural trends were dominant. A possible third belt lies along the line of the Western Pacific Railroad in western Utah. This belt roughly coincides with the westerly projection of the Uinta axis and is marked by a zone of east-west strikes. A few observations near Wendover, Utah, close to the Nevada boundary, suggest that major transverse faults may also occur in this belt. The most northerly belt lies along the northern boundary of the province and extends from northwestern Utah into northern Nevada. Butler (32) called attention to the east-west strikes in the Raft River Mountains in Utah, and a similar trend in the pre-Tertiary sedimentary beds appears to extend as far west as Mountain City, Nev. The small amount of field work that has been done along this zone, however, has not yet disclosed evidence of faulting with profound horizontal movement.
DURATION AND AGE OF OROGENY
There is little direct evidence on the age of the major folds and thrusts over much of the Great Basin. In most places it is possible to determine only that they are later than the late Paleozoic or early Mesozoic and earlier than the middle or late Tertiary. A large proportion of the interested geologists have assumed that these structural features were formed during a single short orogenic epoch, which was either contemporaneous with the late Jurassic deformation of California or with the Laramide of the Rocky Mountains. Relatively recent geologic work in the province now seems to indicate that these assumptions have doubtful validity and that orogenic activity has probably persisted in one place or another over a considerable length of time, starting as early as the middle of the Lower Jurassic and continuing into the Eocene. Five areas have supplied the evidence for this belief.
In one of these areas, west-central Nevada, Ferguson and Muller (69, 70) have found that the major folding and thrusting began in early Jurassic time and reached a maximum at about the end of the early Jurassic. There must also have been some still earlier though less intense deformation, as there is a marked unconformity between lavas and tuffs assigned to the Middle Triassic and sedimentary rocks assigned to the Upper Triassic. In the second area, the Spring and Muddy Mountains farther south in Nevada, deep erosion and transverse faulting that intervened between epochs of thrust faulting have been recognized by Hewett (104, p. 53) and Glock (85); furthermore, the occurrence of a thick fanglomerate of Upper Cretaceous age (107) suggests strongly that deformation was going on at that time. In the third locality, Gold Hill in western Utah, long continued orogeny has been inferred and was marked by four or five epochs of folding or major thrusting separated by a like number of intervals of normal faulting and erosion. One of the more recent of these thrust epochs involved beds of probable Eocene age (187). The notably different trends of the Wasatch and Uinta folds also imply recurrent orogeny, although where they intersect in the Cottonwood district, Utah (36), it is not possible to assign definite times to the two epochs because of the absence of any post-Jurassic sedimentary rocks; the Uinta deformation, however, is regarded as the younger. Finally, Spieker and Schoff (222) have recently been able to date rather accurately two epochs of orogeny in central Utah a short distance east of the Great Basin, the older one post-Jurassic and earlier than Upper Cretaceous and the younger one in the early part of the Upper Cretaceous.
SIGNIFICANCE OF OROGENY
Even with the meager evidence at present available regarding the age of the folds and thrusts of the Great Basin, it no longer seems possible to regard the orogeny that produced them as simply an extension of either the late Jurassic deformation of California or the Cretaceous-Eocene (Laramide) deformation of the Rocky Mountains. On the contrary, these two better-known events now appear to have been episodes in a long epoch of crustal activity that spasmodically affected the Great Basin throughout later Mesozoic and early Tertiary time.
It is possibly significant that the region for which this recurrent orogeny is suggested includes and borders upon the geanticlinal belt that persisted in the Great Basin from Permian to early Jurassic time (pp. 172-173), especially when it is considered that the beginning of marked orogeny coincided with the disappearance of the western of the two seaways that flanked the geanticline. This coincidence in place and time suggests the hypothesis that there may be a close relation between the geanticline and the orogeny; indeed, it appears possible to consider that the individual spasms of thrusting or folding may represent only local climaxes near the end of a long epoch of increasingly intense deformation that started much earlier as a broad, relatively gentle upwarping. This conception, should it be borne out by future work, should prevent such debates as have occurred in the past as to the exact position and extent of the Laramide and Sierra Nevada orogenies. These terms, together with some new ones, would simply designate subdivisions of a major Cordilleran revolution that began in the Paleozoic era and may, as noted on a succeeding page, be still in progress. The hypothesis also implies that the granitic intrusive masses, whose emplacement everywhere appears to have marked the end of major diastrophism, are of many different ages rather than essentially contemporaneous over wide areas.
The dominant process in the structural history of the Great Basin in middle and late Tertiary time was the block faulting that produced the characteristic basin and range topography of the province. The origin and age of the block faulting and at times even its existence have been the subjects of a very considerable body of controversial literature. Although there is still a lack of agreement on many features of the faulting, most geologists now working in the region seem to agree that at least the greater number of the Basin Ranges are bounded on either one or both sides by a fault or fault zone, that the faults are normal rather than reverse, and that the faulting has occurred from late Oligocene to the present time, although some of the scarps may be fault-line scarps rather than primary scarps.
The folds and overthrusts formed during this time appear to be largely restricted to a relatively narrow east-west belt in southern California and southern Nevada. These structural features appear to have been formed at a somewhat later time than at least some of the block faults in the same region.
The origin of the individual mountain ranges in the Great Basin became a matter of speculation with the early geologic surveys in the sixties and seventies of the last century. The first theory published was that of King (124), who considered the ranges to be a series of eroded folds, similar to those of the better-known Appalachians. Shortly thereafter Gilbert (77, p. 50; 78, p. 41) presented the thesis that the mountain blocks were bordered on one or both sides by profound faults along which elevation had taken place by vertical movements. This initial statement of the block-faulting hypothesis was quickly accepted and corroborated by Powell (195); and King (125, pp. 742-743) also soon agreed to the occurrence of extensive faulting, though he emphasized his belief that the faulting had been superposed on an earlier intense folding. Dutton (54, pp. 47-48) accepted the idea of an earlier period of folding and a later one of faulting, but added the concept that the relief due to the folding had been nearly obliterated by erosion before the faulting occurred. Russell (207) and LeConte (137) also accepted the hypothesis and contributed to the speculations on the origin of the faulting.
The first notable dissent was expressed by Spurr (224) in 1901, as a result of an extended reconnaissance through the central and southern parts of the region. He reported that faults along the range fronts were exceedingly rare; that in some places unfaulted Tertiary beds in the valleys abutted against the older rocks in the mountains; and that the numerous faults within the mountain blocks were not reflected in the topography. Spurr concluded, therefore, that the present arrangement of mountains and valleys was due almost entirely to erosion.
This challenge brought forth replies from proponents of the faulting hypothesis in the succeeding few years. Davis (44, 45) presented a deductive statement of the physiographic features that should characterize a youthful fault block and found that they were present in several ranges which he examined in western Utah and northern Nevada. Louderback (151) made a detailed section of a region in western Nevada, where a basalt flow erupted on a peneplaned surface was faulted and tilted in accordance with the block-faulting hypothesis. Waring (251, 252) and Russell (209) recorded the beautifully simple fault blocks of volcanic rocks of southern Oregon, and Reid (203), on the basis of a study of the region near Lake Tahoe, in western Nevada, reached the extreme conclusion that almost every linear element of the topography was controlled by relatively recent faulting.
Baker (10) in 1913 appears to have been the first to suggest that the limiting faults of some of the ranges were reverse and therefore produced by compression; earlier writers had implied that they were all normal faults. Baker's observations were made in southern California and southern Nevada, where there was clear evidence that the fault planes were vertical or even dipped into the ranges and that the sedimentary rocks on the valley sides of the faults were closely folded and locally overturned,7 also raised objections to the hypothesis outlined by Gilbert; he believed that the valleys had largely been excavated by wind erosion, although he conceded that the deflation was accomplished largely by the removal of soft material that had filled depressions, which might have been formed as a result of faulting.
Louderback, in response to the stimulus provided by Keyes' papers, reviewed the evidence of faulting present in the Sierra Nevada and adjacent ranges in western Nevada (152) and concluded that these ranges owed their present relief to relatively recent faulting. In later papers (153, 154) he proposed a late Pliocene or post-Pliocene age for the beginning of faulting and described various features of the fault scarps and of the fault blocks themselves.
Ferguson (63, 64) meanwhile found that over a large area in west-central Nevada block faulting was initiated before late Miocene time and recognized at least four stages of faulting, only the last two of which are reflected in the present topography. This was the first definite statement that the block faults were not all essentially contemporaneous. Davis (47) at about the same time implied that differences in the dates of faulting were the primary factor in the notably different physiographic stages shown by various ranges. In the same paper Davis accepted McGee's suggestion (163) that the fault planes were curved surfaces that flattened downward, and he postulated the idea that the slope (30°-40°) of the facets of the frontal scarps was approximately that of the deeper portions of fault planes that were nearly vertical where they intersected the pre-fault surface.
Since 1928 there have been several reports on detailed work in various parts of the Great Basin which have shed considerable light on different features of the block faulting. A posthumous paper by Gilbert (80) presented evidence, chiefly physiographic, indicative of normal faulting along the Wasatch and House Ranges of Utah and in the Klamath Lake region of Oregon. Russell (210) and Fuller and Waters (72) described the faulting in the Tertiary volcanic region of southern Oregon and northeastern California, both papers showing clearly that there was no basis for the conclusion reached by Smith (218) that the faults in this region were reverse Gilluly (81), on the basis of work in central Utah, found that the dips of the faults were steeper than had been assumed by Davis and Gilbert and that they were formed at different times, not only throughout the Great Basin but in individual ranges. He considered that the faults were formed between Oligocene and late Pliocene time and that isostasy was inadequate to account for the observed displacements. Longwell (148) found that the early Pliocene (?) block faults in the Boulder Dam region probably flattened downward.
Dissent from the Gilbert hypothesis has, however, continued. Both Willis (256) and Lawson (136) have suggested that the east front of the Sierra, commonly considered the most westerly of the block faults, is rather the site of a steep thrust fault, but neither author has attempted to extend this explanation to the many other ranges in the Great Basin. Bucher (29) classifies the Great Basin region as one of his "fracture belts of low mobility" and believes that the fault pattern was developed as a result of regional tension, which preceded the uplift of individual ranges during a period of compression. Blackwelder (17) revived the Spurr concept that the intermontane valleys have been formed by erosion, but with the important modifications (16) that such eroded valleys were cut in weak rocks and that some of the boundaries between the valleys and the adjacent ranges, which are made up of more resistant rocks, may be old faults; Blackwelder terms such exhumed scarps "fault-line scarps."
The preceding review, though necessarily incomplete, reveals the three main explanations that have been advanced to account for the present topographic relief in the Great Basin. These are:
1. The ranges and valleys are limited by normal faults that are due to tensional forces.
2. They are limited by reverse faults, or by superficial normal faults caused by regional compression.
3. The valleys have been formed by erosion.
In addition some of the observers have postulated a combination of these explanations, Bucher favoring 1 and 2; Blackwelder, 1 and 3; Keyes, in part at least, 2 and 3. In the following pages the available data regarding the block faulting are summarized.
EVIDENCE INDICATIVE OF FAULTING
Four types of evidence have been advanced to prove that the individual ranges in the Great Basin are bordered by block faultsphysiographic evidence, stratigraphic evidence, exposure of a fault plane, and presence of recent fault scarps along the range fronts. As the boundary between mountain and valley blocks is commonly concealed by the gravel accumulating in one or more closed basins, the second and third types of evidence are rarely found; for most places physiographic evidence has been called upon to determine the existence of a fault block.
Gilbert's original statement of the block-fault hypothesis was based almost entirely on physiographic evidence. In particular he emphasized that if the front of a mountain range is linear and straight and cuts indiscriminately across the rock structure, it must be limited by a fault. Many other physiographic features of the ranges have since been cited as evidence of faults, chiefly by Davis (44, 45) and Louderback (151). These include the abrupt rise of the ranges from waste-filled valleys; steep, narrow V-shaped ravines, which open abruptly onto the gravel fans of the valleys and flatten in grade in the central part of the range; triangular facets alined along the mountain front on inter-stream areas; absence of branches of the major valleys cutting through the ranges; mature topography or a thin capping of volcanic rocks on summits or back slopes of ranges; landslides along the range fronts; hanging valleys on range fronts (119); lowest point in adjoining valley close to scarp along the range front (130).
Blackwelder (16) has reviewed these and other proposed criteria and has pointed out that several of them are equally applicable to exhumed or "fault-line" scarps. He regards the following features as positive evidence of true fault scarps: (1) Lack of correlation between rock resistance and surface form; (2) rift features; (3) alluvial deposits on the downthrown block thickest near the fault line; (4) lake or sink close to the scarp base; (5) alluvial fans abnormally small; (6) frequent severe earthquake; (7) displacement of an older topographic surface; (8) dislocation of Recent or late Pleistocene formations; (9) basal scarplets; (10) warped terraces in the canyons; and (11) the fault plane identified as forming part of the scarp face. Some of these features are of relatively little value because of their infrequent occurrence (No. 10, for example) or because of the absence of adequate information (No. 3); and others, such as item 6, are of questionable dependability. Other observers would probably regard additional features as equally valid evidence.
Except for a few of the more carefully studied ranges in the Great Basin, one or more of the criteria above listed have been relied upon by many as indicating a fault-block origin for the Basin Ranges. When critically used (as in 45), there is little doubt that physiographic evidence alone is adequate and diagnostic. In many places, however, the use of evidence of this type has resulted in a failure to distinguish between fault scarps and fault-line scarps; and there has even been a tendency to consider that any elevated block with a more or less linear trend is necessarily a fault block.
Stratigraphic evidence of faulting along the borders of ranges has been recognized at relatively few places because valley fill commonly conceals the downthrown block. Locally, however, the downthrown blocks are exposed, either near the ends of en échelon border faults or by local stripping of the valley fill as a result of the capture of one basin by another or of cutting by such through drainage channels as the Colorado River. Eardley (57) suggests that hard-rock exposures in the valley block may also represent as yet unburied summits on a prefaulting submature topography. Stratigraphic proof of faulting has been found in the Humboldt Lake and adjoining ranges, Nevada (151); the Lake Tahoe region, California-Nevada (203); the Oquirrh Range, Utah (81); the Warner Range, Calif. (210); the Wasatch Range, Utah (58); the Deep Creek Range, Utah (187); the Boulder Dam region, Nevada (148); and the Comstock Lode, Nev. (74). In other places faulting along the range front has been inferred from the presence of parallel step faults within the range (72).
In a few places Spurr (224) and Westgate (254) have found no evidence of faulting at the contact between the rocks that form the ridges and the Tertiary sedimentary beds that underlie the valleys. Ferguson and Cathcart (68), however, have interpreted similar occurrences in central Nevada as the result of sedimentation in the downthrown block, which overlapped the outcrop of the fault.
Several observers have described exposures of the faults bordering the ranges. These have commonly been made accessible by artificial excavations, but in a few places they have been revealed by erosion. The Wasatch fault has been located by Pack (191) and Eardley (58), several faults along the west edge of the Oquirrh Range have been located by Gilluly (81), several Pliocene faults in southern Nevada have been located by Longwell (148), and additional faults in central Nevada have been located by Ferguson (67). In the region studied by Longwell a considerable vertical extent of the fault was revealed, and here at least the dip of the fault steepened upward; at the other localities fairly steep valleyward dips prevailed, ranging from 50° to 72°.
The close correlation between small scarps formed by recent faultingcalled "piedmont" scarps by Gilbert (80) or "fan" scarps by Longwell (146)and the scarps bordering the Basin Ranges was first pointed out by Russell (207), in 1884, and since that time these recent scarps have been commonly considered to indicate the presence of persistent faults. Many of them have been recognized throughout the Great Basinthose in the Lahontan and Bonneville Basins by Russell (208) and Gilbert (79, 80), those along the Sierra Nevada by Hobbs (113), Lawson (134), and Knopf (130); those in central Nevada by Jones (121), Page (193), Gianella and Callaghan (75, 76), those in southern Nevada by Longwell (146), and those in southern Oregon and northeastern California by I. C. Russell (207) and R. J. Russell (210). In some places these scarps have clearly been developed along older border faults and extend along the contact between the hard rocks of the range and the gravel of the valley. Commonly, however, they are found in the gravel some distance from the range front, and tend to be more irregular than the front in plan. Although most of the recent scarps lie at or close to range fronts, some are also found in the intervening valleys (75, 76) and within the mountain ranges (37). Many of them are accompanied by hot springs (207) or are coincident with volcanic cones (130), and some observers therefore consider that hot springs and volcanic cones are also suggestive evidence of block faulting.
The several types of evidence presented by many geologists for such widely separated regions in the Great Basin appear to warrant the statement that most if not all of the major ranges in the province are bounded by faults on either one or both sides.
NATURE OF THE BLOCK FAULTS
Several interpretations of the character of the movement along the Basin Range faults have been offered. Some observers have considered that reverse faults are dominant, but the general opinion has favored normal faults. Adherents to the idea of normal faults differ among themselves as to the active block: some favor absolute elevation of the ranges; others postulate depression of the intermontane valleys; and still others believe that both blocks have been active. In part these differences of opinion appear to reflect differences in character of the faults, but they also arise from the scarcity of detailed studies along the strikes of individual faults. Several discussions of the process are based largely on the assumption that a single fault found in a section normal to the range is persistent throughout the length of the range front.
The three areas illustrated in figure 13 have been mapped in detail and have yielded considerable definite information regarding the nature of the faults and the movement along them. Although this information largely confirms previous beliefs regarding the faults, in some respects it is quite different.
Some of the conclusions resulting from these detailed studies may be summarized as follows: The range front is bordered by a fault zone in which individual faults may be relatively short, and both en échelon relations and step or distributive faults are commonly found within this zone. Although marked changes in strike may be present along individual faults, it would appear that many of the major offsets and irregularities along a range front are the result of such en échelon faulting. Movement along individual faults is normal, and the faults dip at moderate to steep angles (50° to 80°) toward the valley or relatively depressed side. The movement has not been one of simple elevation or depression, however, as both walls of the block have been active, as is shown by the tilting they have undergone. There is a suggestion that the amount or degree of tilting has reached a maximum close to the fault. Some of the faults clearly have been localized by pre-existent structural features, especially older faults; and recurrent movements along the faults appear to be fairly common. Not all the faults are of the same age, and even in a single range the maximum movement along an individual fault may have been much earlier or later than that on a neighboring fault.
It is of course questionable whether these conclusions may be applied widely. The three districts illustrated in figure 13 are representative of much of the Utah portion of the Great Basin, but observations by Geological Survey parties in several parts of Nevada indicate that many of these conclusions are valid in that region also. On the other hand, there is a region in southern California and southern Nevada (10) where they do not apply. This exceptional area, however, appears to have been subjected to deformation unlike that affecting the rest of the province. (See p. 186.)
The observed dips of the faults are uniformly greater than the dips of the facets along the range fronts. Davis (47) and Gilbert (80) interpreted these slopes as being the same as the dips of the fault planes and therefore considered that there had been relatively little erosion of the scarps. However, a considerable prism of rock must have been eroded to account for the difference in dips that is now known to exist. Pack (191) has noted this discordance along the Wasatch Range and has suggested that the observed relations indicate that movement along the fault plane was intermittent and continued over a long time and that in general erosion of the scarp facets to their slopes of 15° to 35° was concomitant with the successive uplifts.
Superficially, the dips of the faults toward the valleys and the local stratigraphic evidence that the valley blocks have moved down with respect to the mountain blocks imply that the faulting was tensional; however, the tilting of both the valley and the mountain blocks and the clear evidence in places that the mountain block has moved upward make this interpretation somewhat less attractive. If the fault blocks that make up the ridges and valleys of the province are considered independently of the surrounding regions as a group of prisms free to move in an inert medium, it may be assumed that fracturing and concomitant tilting may have occurred solely as a result of tensional forces. As the fault blocks are an integral part of the earth's crust, however, tilting such as that whose effects have been observed in the region appears impossible of accomplishment unless accompanied by either plastic flow or widespread shearing at relatively slight depths. It it difficult, therefore, to believe that either fracturing or tilting could have developed as a result of simple tension. Similarly, elevation of the mountain block cannot have been accomplished by regional tensional forces.
The assumed elongation in the crust as a result of the block faulting is likewise not necessarily indicative of tension. The data available are not adequate to prove that elongation in the crust has occurred in the region. The suggestion of Davis (47) that the fault planes are curved and flatten in depth, if borne out by future work, would indicate that the tilting could have been caused by rotation of the blocks on such curved planes, and it is possible that the shortening due to tilting may be of greater magnitude than the extension resulting from normal faulting (206a, p. 768). Simple experiments made with scissors and cards, for example, indicate that shortening of the crust may be a result of normal faulting if the radius of curvature of the fault plane is less than the width of the tilted block.
The present state of our knowledge, however, appears only to warrant the statement that simple tensional forces alone are not adequate to explain the block faults observed in the Great Basin province, although it is possible that the faults may reflect surficial tension, dependent upon and caused by a deeper and dominant tangential pressure. Further discussion of this problem may be found on pages 181-186.
AGE OF THE BLOCK FAULTING
The period during which the block faulting occurred is difficult to establish with certainty because of the scarcity of well-dated Tertiary and Quaternary formations in the province. The problem is further complicated by the failure of many authors to distinguish between the initiation of block faulting as a structural type and the activity along individual faults that have strongly affected the present topography. An additional source of confusion is the belief held by some that most, if not all, of the present scarps along range fronts are not true fault scarps but are fault-line scarps.
It seems clear, however, that the faulting cannot be assigned to a single relatively short period. Aside from the known intermittent character of the movement along many of the faults, Davis (45) long ago recognized different stages in physiographic development shown by adjacent ranges. Later he discussed this subject again (47) and implied that the major cause of the differences lay in the different dates of the faulting. Although in places there may be some question as to the reliability of physiographic criteria as an indication of the time of faulting, owing to the possibility that step or distributive faulting may have prevented the development of linear scarps (118, 130, pp. 78-79) or to the probability that rocks of greatly different resistance to erosion will produce different topographic forms, several observers agree that the present topography of the ranges shows conclusively that the block faulting has not been contemporaneous throughout the province (for example, 81, pp. 1119-1120; 68, p. 378).
Ferguson (64) and Ferguson and Cathcart (68), in addition to presenting physiographic evidence that the block faulting occurred at different times, found that similar faults, though without present topographic expression, both preceded and followed the deposition of sediments belonging to the Esmeralda formation (late Miocene and early Pliocene). The conclusion that these earlier faults were of the same character as the later block faults is based on the fact that the Esmeralda, adjacent to the pre-Esmeralda faults, is composed of material similar to that now being deposited in the fans along range-front scarps, and further, that at least some of the topographically expressed faults have followed the lines of these early faults (67). Westgate (254, p. 44) has also found evidence for block faulting of pre-Pliocene (?) age in southeastern Nevada, and Longwell (148, pp. 1456-1470) has described block faulting in the Boulder Dam region that preceded, accompanied, and followed the deposition of his Muddy Creek formation, of questionable Pliocene age. Gianella (74, pp. 85 87), similarly, has distinguished two major epochs of movement at the Comstock Lode. Finally, Stock and Bode (235, p. 578) consider that their Titus Canyon formation of the Death Valley region, which is of lower Oligocene age, accumulated under topographic conditions similar to those existing at present in the Great Basin and imply that block faulting may well have been active at that time.
At several other localities, notably in the Oquirrh Range, Utah (81, pp. 1116-1118), faults that now have topographic expression are known to have been active at an earlier time, inasmuch as they are premineral, but for at least some of these it is uncertain whether this earlier movement was part of the province-wide block faulting or was related to local igneous phenomena. The evidence provided by the Tertiary sedimentary rocks, however, appears to be conclusive that the block-faulting movements started early in the Tertiary period, probably in late Eocene or early Oligocene time, and were widespread through the later Miocene and early Pliocene.
Louderback (153) has placed the period of topographically expressed faulting as late Pliocene or post-Pliocene and considers that the greater part of the movement was completed before late Pleistocene time. The wide range in physiographic development of the ranges makes it improbable that the bulk of the faulting throughout the province occurred at any one time, and Pack (191, pp. 406-409) believes that movement along the Wasatch fault zone has proceeded intermittently up to the present time. Gilluly (82) also considers that faulting along the Oquirrh Range has continued to recent times.
The validity of this conclusion as to relatively recent faulting has been questioned by Blackwelder (17), largely because of the fact that middle or late Tertiary sedimentary beds are exposed in many of the intermontane valleys, and little material that can be attributed to erosion following late Pliocene or Pleistocene faulting can be found. He considers that the present ranges are the result of erosion of the soft and nonresistant Tertiary beds. Longwell (148) has provided clear evidence that exhumation of old fault scarps has taken place along the Colorado River, and the process may have gone on in other parts of the province as a result of the capture of one basin by another. The determination of its extent must await more detailed topographic and geologic studies in the valleys; but the major displacements of basalt and other lavas known to be younger than the late Tertiary sedimentary rocks, together with the numerous recent scarps, prove that a fair proportion of the range-front scarps are not the result of exhumation. Blackwelder (16) has suggested either through-flowing drainage to the sea, or wholesale reduction of fluviatile gravel to dust and its subsequent removal by winds, as alternative methods by which the vast amounts of debris resulting from widespread exhumation could be removed. Neither suggestion can at present be proved, though the apparent absence of Pliocene sedimentary material over much of the province (106) and the existence of considerable areas of subdued or postmature topography near the crests of many ranges (67) have been proposed as indicative of exterior drainage during Pliocene time.
The best conclusion that may be reached from present information therefore is that block faulting as a process probably began in early Oligocene time and has been more or less continuous ever since. Topographically expressed faults, however, probably date back only to late Pliocene or early Pleistocene time, though there may have been still earlier movement along such faults. There are two possible qualifications to the conclusion of intermittently continuous faulting. One of these results from the apparent widespread distribution of upper Miocene sedimentary beds, many of which were deposited in closed basins, for this suggests that block faulting with concomitant formation of such basins was especially widespread before and during late Miocene time. Increasing knowledge of the meager faunas present in these sedimentary beds and new finds in beds hitherto regarded as unfossiliferous seem, however, to weaken this suggestion. The second qualification is the apparent greater age of the faulting in the southern portion of the province, chiefly southeastern California and southwestern Arizona. Gilluly (83, p. 327) is one of the few who have mentioned this feature in print, but most workers in the province have recognized the notably more advanced physiographic stage of development shown by the southern mountains. Rock plains or pediments surrounding the ranges are widely distributed in this region, and their origin has been the subject of a considerable literature (summarized in 83).
ORIGIN OF THE BLOCK FAULTING
The ultimate cause of the block faulting in the Great Basin has been the subject of speculation for many years by numerous American geologists. Doubtless it will continue to be debated until the far distant time when detailed geologic mapping of the whole province has been completed.
Most of the theories advanced have been affected by the beliefs of their authors as to the nature of the faultingwhether it is normal or reverse. They fall into three general groups(1) theories postulating simple tensional stress, (2) those postulating simple compressive stress, and (3) those requiring a combination of tensional and compressive forces, which either acted at different times or comprised the simultaneous action of a deep-seated regional compressive force and superficial local tension.
Leconte's name is commonly associated with the tensional theory (137), although his theory is based on the possibility of a sort of isostatic adjustment (either up or down) along more or less parallel fissures that developed as a result of an arching of the earth's crust. The arching he conceives to have been caused by "intumescence of the subcrustal liquid." Ransome (197) and Butler (32) adopted modified versions of Leconte's theory; they believed it was unnecessary to postulate a preliminary arching, but considered that the block faulting resulted from a deep foundering of the earth's crust, during which the region was depressed with reference to the adjoining provinces. Somewhat similar is Willis' suggestion (256), that the block faulting has resulted from expansion in the roof of a magma basin underlying the Great Basin; a concomitant effect would be compression in the bordering regions of the Coast Ranges and the Rocky Mountains.
There are relatively few advocates of a strictly compressional origin for the block faults throughout the province; tangential compression, presumably of the same nature as that which caused the earlier folding, is considered by Baker (10) to have been the activating stress. Other geologists, however, have postulated thrust faults bordering individual ranges and possibly would extend this interpretation over the whole province. The east front of the Sierra Nevada, for example, has been interpreted as a thrust by several geologists, most recently by Lawson (136). A modification of the compression theories is that presented by Gianella and Callaghan (76) as a result of their study of some recent rifts showing horizontal movement. They suggest that "the underlying causes of movement in at least the western part of the Great Basin may be related to those in California and * * * horizontal movements must be considered in future studies of Basin Range structure."
The oldest of the theories in which both tensional and compressive forces are assumed is that of Gilbert (78, p. 62), in which the block faulting is considered a surficial, more or less tensional expression of deep folding. This theory has still many adherents. Gilluly (82, p. 88), for example, considers it favorably, as a result of his determination that purely isostatic adjustments are not adequate to account for the observed relative elevation of individual ranges. Other explanations on this order have been proposed by Russell, Davis, and Bucher. Russell's theory (207, p. 453) is similar in some respects to those of Willis, Butler, and Ransome; it stipulates that "if the elevation of the Cordillera system as a whole is the result of tangential strain, may we not consider the force as acting on the borders of the Great Basin, while the interior region subsided and was fractured pari passu as the rim was elevated." Davis' suggestion (46, pp. 93-94), which was probably not very seriously considered even by himself, correlated the block faults directly with major overthrustseach fault having been formed as a result of tension back of the thrust in the boundary region between the moving block and the part of the crust not involved in the thrusting. Bucher (29, p. 346) has given the most recent explanation of the origin of the faulting. He believes that two independent processes are involvedan earlier fracturing resulting from regional tension and a later uplift of the present ridges by "orogenic stresses" along the older tensional fractures.
Correlation of the block faulting, which in many places at least was clearly of the normal type, with at least mild compressive forces is suggested by several features brought out by this review of the Basin and Range province. One of the most persuasive is the areal restriction of the block faults to the region of folded rocks. This is most evident along the eastern border of the province in Utah, where the eastern limit of block faulting rather closely coincides with the western limit of the Plateau region. A similar correlation exists along the eastern border of the province in Arizona and New Mexico, and similar faults have been reported by Mansfield (156, p. 170) and Rubey (206) from the folded region in southeastern Idaho and western Wyoming, northeast of the region here considered. Along most of the western border the essentially undeformed Sierra Nevada batholith forms a somewhat different but nevertheless rigid and massive boundary, the dominantly northward-trending block faults of the Basin and Range province dying out southward as they penetrate into the northwestward-trending batholith. The intrusive mass thus limits the faulted region, though it does not determine the location or trend of individual faults. South of the Sierra Nevada the San Andreas rift appears to act as a comparable surface of discontinuity.
Also significant is the apparent necessity for some sort of plastic deformation to account for the fact that both mountain and valley blocks have been active during the faulting (p. 182). Uplift of the mountain block, together with plastic flow at depth, accords ill with a purely tensional origin for the faulting.
More tangible evidence of the connection between block faulting and compressive forces appears to be provided by the regions on both sides of the Garlock fault (p. 186). North of this transverse fault typical Basin and Range topography, presumably the result of block faulting, is magnificently displayed, but south of the fault the subdued topography shows clearly that there has been essentially no recent fracturing of this kind. The block south of the Garlock fault has, however, been moved eastward in relatively recent time both along the fault itself and along the thrust into which the fault probably changes, and the implication seems strong that the ultimate force causing the horizontal shift may well have been the same as that causing the block faulting.
Finally, there appears to be a suggestive connection in time between the block faulting and the preceding folding and thrusting. Thus, block faulting is believed to have been in progress in early Oligocene time (p. 183), and thrusting seems to have continued well into the Eocene epoch (p. 177), a relation that may be interpreted as sequential and implying continuity in action of the same causative forces.
If it is granted that these features indicate a causal connection between the block faulting and compressive forces, Gilbert's theory that the faults are surficial tensional features resulting from deep-seated folding appears best to fit the known or supposed relations. Moreover, it provides a basis for a rational explanation of the block faulting as an integral part of the diastrophism that started as a broad upwarping in late Paleozoic and early Mesozoic time and reached its climax in the major thrusts, folds, and transverse faults of later Mesozoic and early Tertiary time. The block faults thus represent the final and declining stages of this revolution.
At present it is not at all clear why the deep-seated folding postulated by Gilbert should be reflected surficially by faults and not, as in the Plateau province to the east, by broad warpings and asymmetric folds or monoclines (5). It may well be, however, that the difference is directly or indirectly the result of the prior deformation undergone by the rocks of the Basin and Range provincedirectly, by reason of the greater fracturing in the deformed rocks and the heterogeneity of adjacent rock masses, consequent upon extensive faulting, which made them incompetent to withstand surficial tension; or indirectly, because of the restriction of large masses of igneous rocks to such deformed areas.
A connection between the block faulting and igneous activity has long been considered, especially because of the possibility that large-scale extrusion of volcanic material would be accompanied by the extensive subsidence of individual blocks along faults. This simple explanation is probably not tenable, however, not only because volcanic rocks are absent over considerable areas that now show faulted blocks, but also because of the local evidence of uplift of individual blocks and the province-wide correlation, noted above, of faulting and compressive forces. If we may accept the numerous and extensive exposures of igneous rocks in this province and the steep geothermal gradient that has been found in all the deeper mining districts of the province as evidences of the presence of a layer containing numerous magma reservoirs a relatively short distance below the present surface, mild compression of such a mobile zone into a series of gentle folds must have resulted in extensive fracturing and relative displacement of the fragments of the brittle and thin surface layer. Surface volcanism would thus be a result and not a cause of the fracturing.
It is difficult to choose between the alternate hypotheses of a heterogeneous fractured crust or a mobile magmatic subcrustal layer as the controlling factor in the block faulting. It so happens that both east and west of the province, as well as throughout the Mojave Desert region within the province, where block faulting is relatively minor, there was scant Tertiary volcanic activity and little reason for believing that the exposed surficial igneous rocks originated within the faulted areas. There is also ample evidence of an extensive platform of relatively homogeneous rigid rocks at or near the surface. Possibly these two factors are themselves manifestations of a single major influence, but such speculation is far beyond the scope of this paper.
FOLDING AND THRUSTING
Folding and thrusting of late Tertiary or Quaternary age appears to have been confined chiefly to southeastern California but may possibly have extended into southern Nevada. Baker (8) was one of the first to recognize folds in the later Tertiary sedimentary beds in this region: He also found that the beds were cut by steep reverse faults along the bases of the mountain areas; and his theory that the Basin Ranges were formed as a result of reverse faulting (10) is founded largely on this discovery.
Subsequent exploration in the Mojave Desert region of southern California and its bordering areas has disclosed many localities where disturbance of late Tertiary or Quaternary formations by folding and reverse faulting has occurred. It appears to be significant, however, that these phenomena are in most places rather closely associated, either in location or in trend, with two major fracture zones that bound the Mojave Desert on the north and the southwest. These zones are the Garlock fault and the San Andreas rift. This localization of the compressional features, together with the much greater maturity in topographic expression of the ranges in this area than of those in the larger region to the north, seriously weakens Baker's theory of origin for the province as a whole.
The Garlock fault was first recognized and named by Hess (101) north of Randsburg, in northeastern Kern County, Calif. He described the prominent scarp with its local evidence of very recent movement and stated that the fault could be traced for many miles to both the east and west. Hulin (116) studied the fault in the same region as Hess, and his description emphasizes its rift characternearly vertical dip, branching habit, and a very large horizontal component in its displacement, which he suggests may be 5 miles, with the south side moving east. Hulin also considered that activity along the fault had been in progress for a long time, possibly dating back to the Jurassic. A nearly horizontal thrust along which Mesozoic granitic rock has overriden the later Tertiary rocks is truncated at an acute angle by the rift.
Noble (180) traced the Garlock fault from its junction with the San Andreas rift near the west corner of the Mojave Desert for a distance of 200 miles to the Avawatz Mountains, south of Death Valley. Throughout this distance its rift character is maintained, and the fault has many features in common with the better-known San Andreas rift. Noble considered that the fracture dates back to pre-Tertiary time, and he was the first to call attention to the significance of the fault as a boundary between the youthful and linear mountain ranges in the region to the north and the irregular, shapeless ranges to the south, which exhibit a "stage of erosion nearer the hypothetical end of the arid cycle."
Not much is known about the west end of the fault. It has not been recognized west of the San Andreas rift, against which it apparently terminates. Simpson (213) has mapped the Garlock fault less than 20 miles east of the junction, where it follows the east base of the Tehachapi Range. In this region the alluvium has been folded into a dome 5 miles long and has been steeply tilted near the fault, with dips of as much as 87°. Well logs suggest a vertical component of 4,000 feet in the movement of the fault.
The Garlock fault may be traced to the east along the north side of the Avawatz Mountains, Calif. It turns to the south along the east face of the mountains, and recent work has shown that here it has the relations of a reverse fault, with metamorphic rocks of probable pre-Cambrian age faulted against Tertiary beds (106). In the Ivanpah quadrangle, a few miles farther east, Hewett (103) has found remnants of a large horizontal thrust extending over an area of 30 square miles, along which pre-Cambrian and lower Paleozoic rocks have overriden late Tertiary (Miocene?) sedimentary beds. The eastward movement of the upper plate of the thrust is estimated as at least 10 miles and may be as much as 20 or 25 miles. The relations of this thrust to the southward-trending portion of the Garlock fault are not definitely known, but it seems probable that both must be the product of the same orogenic forces and may very well be two parts of a single fracture system.
The Garlock fault thus appears to be a major fault, probably of considerable antiquity, extending eastward from the San Andreas rift for some 200 miles; the late Tertiary movement along it is dominantly horizontal, with the south side moving eastward for a distance of several miles, possibly as much as 25 miles.
SAN ANDREAS RIFT
Only a small part of the known length of the San Andreas rift (best known for its part in the San Francisco earthquake of 1906) impinges upon the Basin and Range province, although the rift coincides with the southwestern border of the province from the Tehachapi Mountains, at the south end of the Sierra Nevada, southward nearly to the Mexican border (27). Noble (180, 182) has studied a section of this portion of the rift and has described it as a "fault mosaic of sliverlike blocks whose longer axes trend parallel with the strike of the main fault," and he considers that it has possibly been active from late Mesozoic up to Recent time. The earliest datable movement in the region near San Bernardino, Calif., is believed to be of late Pliocene or Pleistocene age.
The rift is regarded as a northwestward-trending, nearly vertical shear zone, along which the movement has been predominantly horizontal. In southern California the offset of topographic features indicates that the recent movement has resulted in a relative shift of the northeast side to the southeast, and some features of the stratigraphy suggests a similar movement amounting to 24 miles. The evidence as to the direction of the major movement is not wholly satisfactory, however, and both Noble (182) and Simpson (213) consider that the net displacement may include a northwest shift of the northeast side.
Both steep reverse faults and low-angle thrusts are found along the rift zone in the San Gabriel and San Bernardino Mountains and have been described by Noble, Simpson, Woodford and Harriss (260) and by Hill (110). The low-angle thrusts, in particular, commonly have more easterly strikes than the rift itself, but they curve into the rift and are closely related to it in their areal distribution. They appear to be somewhat older than the latest horizontal movement along the rift, but, as they involve late Tertiary sedimentary beds, they appear to be of at least approximately the same age as the more recent periods of activity.
There are no detailed geologic maps of the region where the San Andreas and Garlock faults meet, but a recent geologic map of southern California (199) shows that the Garlock fault apparently ends at its junction with the San Andreas rift, for no comparable fracture has been recognized in the block on the southwest side of the San Andreas, which is in large part underlain by Cretaceous, Tertiary, and Quaternary sedimentary rocks. The later Tertiary and Quaternary movement along the Garlock fault therefore appears to have been confined to the Great Basin province, although the termination of the fault at the San Andreas rift suggests strongly that it must at an earlier time have extended farther west. There is, however, no indication of any old east-west structure on the southwest side of the San Andreas rift, northwest of the junction pointthe direction in which the westerly extension of the Garlock should have been shifted, to judge from Noble's observations of the recent topographic offsets. Southeast of the junction, though, there is a pronounced easterly trend in the transverse ranges and in the Ventura Basin of sedimentation. This feature, in connection with Simpson's observations in the San Gabriel Range (213), may be indicative of an older episode of movement along the San Andreas rift in a direction opposite to that of the recent shifts.
Late Tertiary folds or thrusts have been recognized at a few other localities more or less remote from either the Garlock fault or the San Andreas rift. The best known are in the Mojave Desert region and are grouped in a belt parallel to and about 20 miles south of the Garlock fault. At Calico, 10 miles northeast of Barstow, Baker (10) and Lindgren (140) found folded and locally overturned Tertiary sedimentary beds on the under side of an east-west fault that appears to dip beneath the hills to the north. North of Barstow similar Tertiary sedimentary beds of late Miocene age are warped into a syncline that trends east-west. Schaller (107) found a similar synclinal warp in the borate-bearing strata north of Kramer, 30 miles west of Barstow, and steep dips in other saliniferous strata are known at Bissell, still farther west, and at Afton, nearly 40 miles east of Calico. The alinement of these localities suggests relationship in originpossibly a control by an old east-west shear zone. Pack (192), however, considered that the folding in the region was on a small scale and discontinuous and believed that it was only a minor structural feature.
Minor folds and reverse faults that are possibly of late Tertiary age have also been recognized at two localities in southern Nevada (186, 148), northeast of the east end of the Garlock fault, but their correlation with the California structural features is uncertain.
Last Updated: 28-Nov-2007