The north-south-trending Straight Creek Fault roughly bisects the Sauk River quadrangle and defines the fundamental geologic framework of the area (see generalized geologic map). The fault is considered to have had about 100 km of right-lateral strike-slip offset during Late Cretaceous and early Tertiary time (Misch, 1977a ; Vance and Miller, 1981; Monger in Price and others, 1985), but estimates of offset range from about 70 to 180 km and the timing of movement is controversial (Vance, 1985; Vance and Miller, 1992; Tabor, 1994). Within the Sauk River quadrangle, the Straight Creek Fault mostly separates low-grade metamorphic rocks on the west from medium- to high-grade metamorphic rocks on the east. In places, the fault consists of several strands that bound infaulted slivers of unmetamorphosed Tertiary sandstone. On the west side of the fault, a swarm of north- to north-northwest-trending faults within the lower grade rocks may be subsidiary or sympathetic to the Straight Creek Fault.

ROCKS EAST OF THE STRAIGHT CREEK FAULT

Rocks east of the Straight Creek Fault are interpreted here as tectono-stratigraphic terranes based on their overall distinctive lithologies and possible different ages and structural histories (Tabor and others, 1989; Tabor and others, 1987a, b) although they all were thoroughly metamorphosed in the Late Cretaceous and early Tertiary. In the Sauk River quadrangle, three major terranes crop out east of the fault: the Swakane terrane, the Chelan Mountains terrane, and the Nason terrane.

The Swakane Biotite Gneiss is the sole component of the Swakane terrane. It is probably a metamorphosed sandstone or dacitic volcanic accumulation, notable for its uniformity in structure and lithology. Isotopic and geochemical studies have indicated its derivation from either a Precambrian dacitic volcanic accumulation (Mattinson, 1972; Cater, 1982; Tabor and others, 1987a, b) or a younger detrital rock derived from Early Proterozoic and younger source terranes (Waters, 1932; Cater 1982; Rasbury and Walker, 1992).

Overlying the Swakane terrane, and in probable thrust contact with it, as revealed southeast of the Sauk River quadrangle (Tabor and others, 1987a, b), is the herein-named Napeequa Schist of the Chelan Mountains terrane. Formerly we (Tabor and others, 1987a, b) included the rocks making up the Napeequa Schist in the now-abandoned Mad River terrane. New mapping north of the Sauk River quadrangle suggests that although the Napeequa Schist is generally distinct from other rocks of the Chelan Mountains terrane, namely the herein-restricted Cascade River Schist, it has been thoroughly imbricated or interfolded with them and may not be clearly mappable as a separate terrane unit. We now consider the Napeequa Schist to be a part of the Chelan Mountains terrane. The Chelan Mountains terrane also includes the metaplutonic rocks of the Marblemount-Dumbell belt and the Cascade River Schist.

Most rocks of the Napeequa Schist are micaceous quartzites, fine-grained hornblende schist, and amphibolite derived from a protolith of oceanic chert and basalt. Minor marble and small bodies of meta-ultramafic rocks are also characteristic. Mica schist and hornblende-mica schist probably derived from shale and sandstone are common but not unique to the unit. These rocks have been intruded by large, metamorphosed, granitic to granodioritic plutons that yield discordant U-Th-Pb ages suggesting both Late Cretaceous crystallization and contamination by older crustal rocks.

The Cascade River Schist (herein restricted) originally appears to have been a thick sequence of arc-derived clastic rocks with minor arc volcanic rocks. It is now mostly plagioclase-rich mica schist, metaconglomerate, and amphibolitic schist. Minor constituents are silicic schists (metatuff), marble, and amphibolite. Zircons from a dacitic metatuff yield a Late Triassic U-Pb age (see also Cary, 1990).

The Marblemount pluton (equivalent to the Marblemount Meta Quartz Diorite of Misch, 1966) is also Late Triassic in age based on U-Pb analyses of zircons (Mattinson, 1972). Although the exact nature of the contact between the Marblemount pluton and the Cascade River Schist is enigmatic, the contemporaneity of the plutonic rocks and the overlying metavolcanic rocks suggests deposition of the protolith of the Cascade River Schist in a forearc or intra-arc basin wherein intrusion was followed by rapid uplift of arc plutons and further deposition of arc volcanic rocks. Both the pluton and overlying deposits were metamorphosed in the Late Cretaceous and early Tertiary.

The Nason terrane is composed predominantly of the Chiwaukum Schist, a metapelite, characterized by aluminum silicate minerals, as well as some metasandstone and metabasite. The protolith of the Chiwaukum Schist was predominantly marine clastic materials with minor oceanic basalts, perhaps deposited in a distal arc setting (Magloughlin, 1993). Part of the terrane is underlain by the Nason Ridge Migmatitic Gneiss (named herein) derived from the Chiwaukum Schist by more advanced recrystallization and probable igneous injection. The protolith age of the Chiwaukum is uncertain, but it appears to be pre-Late Jurassic. Some workers have argued on the basis of Rb-Sr data that its age is Triassic (Gabites, 1985; Evans and Berti, 1986; Magloughlin, 1986). The contact between the Nason and Chelan Mountains terranes is marked by a gradual lithologic change from dominantly mica schist to micaceous quartzites and fine-grained amphibolite plus a scattering of small meta-ultramafic bodies in the Chelan Mountains terrane along the transition zone. We interpret the contact to be a metamorphosed fault. The elongate, Late Cretaceous Tenpeak pluton has intruded along part of the contact.

The Napeequa Schist overlies the Swakane Biotite Gneiss along a folded thrust fault (Tabor and others, 1987a, p. 117), and it may also overlie the Chiwaukum Schist as explained below. Thus the Chelan Mountains terrane may have been thrust over both the Swakane and Nason terranes prior to Late Cretaceous metamorphism. We do not know if this thrusting was prior to or after accretion to North America.

Apparently all the terranes had been assembled by the Late Cretaceous when they were intruded by deep-seated, synmetamorphic, mostly tonalite to granodiorite plutons. The largest of these include the Chaval, Tenpeak, Eldorado, Sloan Creek, Hidden Lake, Bench Lake, Sulphur Mountain, Jordan Lakes, and Cyclone Lake plutons. These terrane-overlap or stitching plutons characteristically have both igneous and metamorphic features and most yield U-Th-Pb ages of about 94-89 Ma. Some of the stitching plutons (such as the Tenpeak, Chaval, and Sulphur Mountain) have igneous epidote suggesting their intrusion at depths greater than 25 km (Zen and Hammarstrom, 1984). During the Late Cretaceous metamorphism, thorough recrystallization and injection of tonalitic materials transformed parts of the Chiwaukum Schist and the Napeequa Schist into relatively homogeneous paragneiss and banded gneiss. We consider these gneisses to be terrane stitching units as well because they developed their dominant characteristics during Late Cretaceous metamorphism.

Although in the Sauk River quadrangle the dominant metamorphism appears to be Late Cretaceous, just to the north, the oceanic rocks of the Napeequa River area are a significant constituent of the Skagit Gneiss Complex (equivalent to the Skagit Gneiss of Misch, 1966), which, in its core area, was intruded by 60- and 75-Ma orthogneisses and was still being dynamically metamorphosed in the middle Eocene (about 45 Ma) (Babcock and others, 1985; Haugerud and others, 1991). The 75-Ma Hidden Lake pluton appears to lie on the edge of this Tertiary (Skagit) metamorphism because although it has been metamorphically recrystallized, it is undeformed, whereas just to the north, the 75-Ma Marble Creek pluton is strongly deformed, presumably reflecting the Skagit metamorphic event (Haugerud and others, 1991).

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