Geological Survey Bulletin 1359 Geology and Mineral Resources of the Northern Part of the North Cascades National Park, Washington

STRUCTURE

The northern part of the North Cascades National Park has undergone several periods of folding and faulting. Rocks of Cretaceous and Tertiary age, such as the Chuckanut Formation, the Hannegan Volcanics, and the Skagit Volcanics, are gently folded. The older, metamorphosed rocks, such as the Custer Gneiss of McTaggart and Thompson (1967) and the greenschist and phyllite of Mount Shuksan (fig. 10), are tightly folded; crinkles and small drag folds are common. Angular unconformities separate the metamorphic rocks from the younger Chuckanut, Hannegan Volcanics, and Skagit Volcanics.

The structure of the metamorphic rocks is not well known, but the general northwest trend of foliation, lithologic units, and fold axes suggests folding about axes of that trend. The major fold in the area is a large northwest-plunging syncline in the Hozomeen Group of Cairnes (1944). Minor folds and crinkles occur in the chert beds in the Hozomeen, the Custer Gneiss, and the greenschist and phyllite of Rose Lake. Small fold axes in the southeastern part of the Custer Gneiss plunge gently south; those in the northern part plunge gently either north or south (pl. 1).

Structure is simpler, although more diverse, in the unmetamorphosed rocks. The folding is fairly open in the Chuckanut Formation. Near Bacon Peak these folds trend north-northeast, but a couple of miles farther north on Hagan Mountain they trend west-northwest (fig. 9). In the northern part of the area the Chuckanut is tilted, but no folds are seen. Bedding was not well enough developed in the volcanic rocks for their structural pattern to be determined, but they clearly lie with angular unconformity on the metamorphic rocks.

Faulting appears to be of several ages: older faults are cut off by the Chilliwack batholith, and younger faults of two ages cut the batholith.

In the eastern part of this area a set of large northwest-trending steeply dipping faults is exposed between Arctic Creek and Ross Lake. This group of faults, which was named the Ross Lake fault zone by Misch (1966, p. 133-134), is part of a zone of shearing more than a half mile wide. On plate 1 we have shown three faults along most of this zone, but shearing in other parts of the phyllite and schist of Ross Lake and the metagabbro indicates that movement has not been confined to only three major faults. North of Arctic Creek the faults are cut off by the Chilliwack batholith. On the east side of Ross Lake the fault zone was traced approximately 12 miles south of Ruby Creek by Misch (1966, pl. 7-1; p. 133-134), where it is cut off by a large intrusion about 20 miles long. Southeast of the intrusion a major fault zone of similar trend has been noted by Barksdale (1958) and by Huntting, Bennett, Livingston, and Moen (1961) along the Twisp River. Thus, the Ross Lake fault zone was probably more than 55 miles long.

West of Ross Lake the southwest branch of the Ross Lake fault zone separates the Custer Gneiss on the southwest from the phyllite and schist of Ross Lake on the northeast. Displacement has been large, with the southwest side displaced upward. Numerous small intrusions are found along this fault zone both within and southeast of the study area. Metagabbro is intruded along this zone, although at least some of the fault movement occurred after the metagabbro was emplaced. Although the faults within this zone cross valleys several thousand feet deep, the straight surface trace of the faults indicates that they dip near vertically. The continuity of the zone, the steep dip of the individual faults, the presence of numerous intrusions, and the pronounced differences of rock type within and on either side of this zone indicate that it is a major fault system.

Two long northerly trending faults of different ages occur between the Ross Lake fault zone and the crest of the Picket Range. Both faults extend south of the Skagit River, but in the area mapped we have traced the easternmost one for 14 miles and the westernmost one for 9 miles. Both faults are near vertical and are marked by offset beds, fault valleys, and zones of granulation. Although the two faults are similar in some respects, the easternmost fault is cut off to the north by rocks of the Chilliwack batholith, whereas the westernmost fault cuts the granitic rocks; lateral movement on the two faults is in the opposite direction.

Faults of three ages were mapped in the Bacon Peak—Hagan Mountain vicinity in the southwest corner of the study area (pl. 1; fig. 9). The ten faults mapped here have a northerly to northwesterly trend and a near-vertical dip. Most of them formed after the intrusion of the Chilliwack batholith, but one northwesterly trending fault, which passes just southwest of Berdeen Lake, is cut off by quartz diorite of the batholith on the northeast shoulder of Hagan Mountain. Two faults that lie on the north side of Bacon Peak have a more westerly trend than the others. They are younger than the more northerly trending faults. As all the faults cut the Chuckanut Formation of Cretaceous and Paleocene age, they must have formed during the Tertiary.

In the northwestern part of the study area, the pattern of faulting changes. Here most of the faults have a northeast trend, though some have either a north or east trend. Many of the faults in this area are steep, and all cut rocks of the Chilliwack batholith. The longest of these is a curving fault, which we have traced for 20 miles from west of Shuksan Lake northeastward almost to Depot Creek (pl. 1; fig. 10). At different places, this fault displaces the greenschist and phyllite of Mount Shuksan, the Hannegan Volcanics, the Chilliwack batholith, and the Custer Gneiss. In its central part its course is straight and is marked by the north branch of Pass Creek and part of the Chilliwack River valley (pl. 1). The northwest side of this fault is downthrown relative to the southeast side.