Structure and chronology of the Oval Peak batholith and adjacent rocks: Implications for the Ross Lake fault zone, North Cascades, Washington ROBERT B. MILLER Department of Geology, San Jose State University, San Jose, California 95192 SAMUEL A. BO WRING Department of Earth and Planetary Sciences, Washington University, St Louis, Missouri 63130 ABSTRACT The Foggy Dew and Twisp River fault this region (for example, Davis and others, zones separate the batholith and its wall 1978). These faults experienced protracted The >200-km-long Ross Lake fault zone rocks on the northeast from weakly meta- movement histories with both strike- and dip- (RLFZ) forms the northeastern border of the morphosed strata of the Methow basin. The slip motion (Misch, 1966,1977; Tabor and oth- crystalline core of the North Cascades. This Foggy Dew fault zone contains mylonitic ers, 1984; Monger, 1986; Ray, 1986). The Ross Late Cretaceous(?) and Paleogene structure is gneiss derived from the Oval Peak batholith, Lake fault zone (RLFZ) is one of these major an ~lS-km-wide system of faults and fault Skagit Gneiss, and tonalitic rock that displays structures. It forms the northeastern border of zones, several of which deform the Paleocene U-Pb zircon systematics suggestive of an Eo- the predominantly Mesozoic crystalline core of (-65 Ma, U-Pb zircon) Oval Peak batholith. cene age. Sillimanite schist and amphibolite the North Cascades, generally separating the The southern segment of the Gabriel Peak are also present. Mylonites record dextral plutonic-metamorphic core from slightly meta- tectonic belt of the RLFZ is defined by strike slip with a component of normal slip, morphosed Jurassic-Cretaceous strata of the strongly deformed rocks of the foliated down to the northeast. Movement in the fault Methow basin (Fig. 1). southwestern margin of the Oval Peak batho- zone continued subsequent to deformation in As originally defined by Misch (1966,1977), lith, structurally underlying pre-Late Cre- the Gabriel Peak tectonic belt and ended by the RLFZ consists of a series of major faults and taceous Twisp Valley Schist, and structurally 48 Ma. Mylonites in the Twisp River fault associated fault zones (Fig. 2). Most are steep highest part of the subjacent Skagit Gneiss, zone also reveal dextral strike slip, but appar- north-northwest- to northwest-striking struc- which here consists of Late Cretaceous ently with less normal slip. tures and include the Ross Lake fault proper (87 Ma) and Paleocene (60 Ma) orthogneiss. The RLFZ experienced a diachronous his- (Misch, 1966), Hozameen-North Creek fault Kinematic indicators in moderately steeply tory from at least 65 to 45 Ma. This history (Misch, 1966; McGroder, 1987), Twisp River northeast-dipping mylonitic orthogneiss re- was dominated by dextral strike slip, but fault (Barksdale, 1975), Foggy Dew fault cord reverse slip. The strong solid-state flat- there were significant components of normal (Barksdale, 1975), and Gabriel Peak tectonic tening fabric in the foliated margin of the slip and reverse slip on some structures. Dur- belt (Misch, 1977; Miller, 1987). Oval Peak batholith formed during or shortly ing the latter part of this interval (-57-45 The RLFZ extends for more than 200 km after emplacement and overprinted a mag- Ma), the RLFZ probably recorded a trans- from near the Columbia River in Washington, matic fabric observed in the core of the batho- tensional tectonic regime transitional between where it is covered by Miocene basalts (Misch, lith. Foliation and associated stretching linea- extension to the east in Omineca metamor- 1966; Raviola, 1988), to southern British Co- tion within the Gabriel Peak tectonic belt are phic core complexes and dextral strike slip to lumbia, where it is truncated by the Fraser- oblique to the regional structural grain but the west related to oblique convergence along Straight Creek fault (Fig. 1) (Monger, 1985). concordant with the contacts of the batholith. the continental margin. Early Cenozoic plu- Several major faults are candidates for the offset These patterns in part record emplacement of tonism, metamorphism, and ductile deforma- Ross Lake fault proper west of the Fraser- the batholith as an expanding diapir into the tion have strongly overprinted any of the Straight Creek fault. Kleinspehn (1985) corre- tectonic belt. Reverse slip in the tectonic belt major Cretaceous deformation postulated in lated the Ross Lake fault with the dextral Yala- occurred before, during(?), and shortly after previous tectonic models for the RLFZ. kom fault, Monger (1986) matched the Ross emplacement but ended by -55-58 Ma. Lake fault with the Kwoiek and Bralorne (Rus- The core of the Oval Peak batholith in- INTRODUCTION more, 1986) faults, and Coleman (1989) corre- truded a large tongue of Twisp Valley Schist lated the Ross Lake and Mission Ridge faults on the north. Porphyroblast microstructures Strike-slip faults played an important role in (Fig. 1). and Ar-Ar and K-Ar geochronology indicate the late Mesozoic(?)-early Cenozoic evolution Although there are numerous conflicting in- that amphibolite-facies dynamothermal met- of the Cordillera of British Columbia and Wash- terpretations of the chronology, magnitude of amorphism of the schist occurred during ington by disrupting and juxtaposing different movement, and overall displacement history of intrusion. parts of a mosaic of small accreted terranes in the RLFZ, most workers (Misch, 1966; Hau- Additional material for this article (an appendix) is available free of charge by requesting Supplementary Data 9019 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 102, p. 1361-1377, 12 figs., 3 tables, October 1990. 1361 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/10/1361/3380696/i0016-7606-102-10-1361.pdf by guest on 26 September 2021 1362 MILLER AND BOWRING 122 120 1 18 51° Figure 1. Simplified geo- logic map showing major early Cenozoic high-angle faults of the North Cascades (N.C.) and southern Cana- dian Coast Mountains and „ their spatial relations to ma- so jor Eocene extensional faults (tick marks on upper plate) along the margins of the Omineca metamorphic core complexes. Several units dis- cussed in the text are also shown on the map. CB, Chil- liwack batholith (Oligocene- 49° Miocene); CM, Cooper Moun- tain batholith (mid-Eocene); K, Kettle dome; MRF, Mis- sion Ridge fault; NWCS, Northwest Cascades thrust system (Cretaceous); OK, Okanogan Complex; RLF, Ross Lake fault; RLFZ, Ross Lake fault zone; V, Valhalla 48° Complex. Modified from Johnson (1985), Parrish and others (1988), Potter (1986), Price and others (1985), and Roddick and others (1979). Coast Plutonic Complex Undifferentiated • (mainly Cretaceous - Eocene) Methow-Tyaughton basin Crystalline Core of the North Cascades (Jurassic - Cretaceous) (Cretaceous - Eocene) Bridge River-Hozameen terrane Omineca Metamorphic Complexes (Permian - Jurassic) (mainly Jurassic - Eocene) gerud, 1985; Miller and others, 1985) consid- marks a suture across which the North Cascades Trexler and Bourgeois (1985) contended that ered the fault zone to have undergone major were accreted, whereas Ray (1986) postulated the fault zone was active during mid-Cretaceous dextral movement. Davis and others (1978) that the crystalline core was thrust eastward deposition in the Methow basin and that the suggested a minimum of 160 km of Late Juras- along a structure that subsequently was steep- basin formed by Cretaceous "oblique-transla- sic or Early Cretaceous dextral movement on a ened and then experienced large dextral move- tional compression." "proto-Ross Lake fault" in order to restore the ments. Kriens and Wernicke (1986) argued that Numerous plutons that range between 90 and North Cascades crystalline core to a position at there is little displacement on the fault zone. 48 Ma occur within the RLFZ (Fig. 2). Some the south end of the Okanogan crystalline belt. The timing of movement on the RLFZ is also plutons have intruded parts of the fault zone but In contrast, Barksdale (1975) emphasized dip problematic. Misch (1966) proposed that dis- were deformed by continued movement. One slip and sinistral slip in the southernmost RLFZ. placement occurred prior to 90 Ma, whereas such pluton, the Oval Peak batholith (Adams, Hamilton (1978) suggested that the RLFZ Haugerud (1985) argued for middle Eocene slip. 1961; Libby, 1964), intruded the southern seg- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/10/1361/3380696/i0016-7606-102-10-1361.pdf by guest on 26 September 2021 OVAL PEAK BATHOLITH, NORTH CASCADES, WASHINGTON 1363 ment of the fault zone. This batholith and its The TVS is predominantly siliceous biotite tilting of originally more-gentle isograds (Hau- wall rocks occupy a critical position, as they schist and impure quartzite (metachert). Am- gerud and others, 1988; Kriens, 1988). have been affected by three major strands of the phibolite, greenschist, calc-silicate rock and RLFZ, the Foggy Dew fault, Twisp River fault, marble (Adams, 1961), and local ultramafite Skagit Gneiss and Gabriel Peak tectonic belt. This report de- form small (<50 m wide) lenses. scribes the emplacement and deformation histo- The TVS displays two generally coaxial The Skagit Gneiss (Misch, 1966,1968) forms ries of the Oval Peak batholith and its wall generations of tight to isoclinal folds (F[, F2) much of the northern crystalline core of the rocks, and the implications of these histories for and axial-planar foliation (Si, S2). Transposition North Cascades. We have subdivided the Skagit the development of the RLFZ. We show that makes it difficult to distinguish Fi from F2, and Gneiss in the map area into three northeast- both strike- and dip-slip displacement have been the predominant foliation (S1.2) is considered to dipping orthogneiss units, in descending order, important and present geochronological data be composite. Mineral lineation (L1.2) is parallel the Battle Mountain gneiss, Tuckaway Lake that bracket the timing of the latest major to FI_2 axes. Abundant open to tight folds (F3) gneiss, and Lake Juanita leucogneiss (Figs.