Structure and Chronology of the Oval Peak Batholith and Adjacent Rocks: Implications for the Ross Lake Fault Zone, North Cascades, Washington

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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.

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122 120 1 18

51°

Figure 1. Simplified geologic 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 discussed 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-

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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 deform 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. 3 movements in the fault zone. of S]_2 lack an axial-planar fabric. and 4). These amphibolite-facies rocks were Metamorphism ranges from the biotite zone mapped as granitized metasedimentary rocks by ROCK UNITS' to the sillimanite zone. In general, metamorphic Adams (1961) and Libby (1964), but they dis- grade increases from northeast to southwest, and play relict igneous textures and locally contain The Oval Peak batholith is elongate parallel the envelope is entirely in the amphibolite facies. microgranitoid enclaves. to the RLFZ. The well-foliated southwestern This northeast-southwest gradient is also charac- Battle Mountain Gneiss. Intrusive relations margin of the batholith, structurally underlying teristic of the North Cascades core farther north and age data indicate that the oldest unit is the Twisp Valley Schist, and structurally highest along the RLFZ (Misch, 1966, 1968) and may Late Cretaceous (~87 Ma) Battle Mountain part of the subjacent Skagit Gneiss define the reflect either a thermal dome (Misch, 1968) or gneiss. This hornblende tonalite gneiss ranges southernmost segment of the Gabriel Peak tec- from well-foliated and lineated tectonite to pro- tonic belt. This belt is a broad zone of strong tomylonite. The gneiss forms a narrow (¡s200 m ductile deformation that extends for 50 km to 121 /Hozameen Fit thick) belt that wraps around the western and the northwest of the batholith (Fig. 2). On the southern margins of the Oval Peak batholith and northeast, the batholith and its wall rocks are truncated by mylonites of the Foggy Dew and Twisp River fault zones, that are in turn separated from weakly metamorphosed rocks of the Methow basin by the Foggy Dew and Twisp River faults.

Twisp Valley Schist

The Twisp Valley Schist (TVS) is the major supracrustal unit within the northeastern part of the North Cascades crystalline core. The unit is poorly dated. It is intruded by variably metamorphosed plutons, the oldest of which is the 90 Ma Black Peak batholith. The TVS has been tentatively correlated with the Permian to Juras- sic Hozameen Group (Misch, 1966) and with the Napeequa Schist of the Chelan Mountains terrane farther west within the crystalline core (Tabor and others, 1989). The TVS forms a thin (<300 m) but continuous envelope around the southern, southwestern, and part of the northern margin of the Oval Peak batholith (Fig. 3). Sills and dikes of the batholith and the Skagit Gneiss intrude this envelope. A larger tongue of schist was intruded by, and extends into, the northern part of the batholith. The TVS is also exposed in a narrow belt between the batholith and the Twisp River Figure 2. Simplified map of the southern segment of the Ross Lake fault zone. RC, Ruby fault. Creek heterogeneous plutonic belt, which is at least in part ~48 Ma (Miller and others, 1988); TRF, Twisp River fault; TVS, Twisp Valley Schist. Dots, Twisp Valley Schist (pre-Late Cretaceous); horizontal dashes, Methow basin (Jurassic-Cretaceous); v's, North Creek Volcan-

'Additional rock descriptions are in Appendix 1, ics (pre-Late Cretaceous); blank, Skagit Gneiss (Late Cretaceous and Paleocene); +'s, Late available free of charge by requesting Supplementary Cretaceous and Paleocene plutons; squiggles, Paleogene mylonites; random dashes, Eocene Data 9019 from the GSA Documents Secretary. plutons. Modified from Misch (1966,1977), Barksdale (1975), and Miller (1987).

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Contact

Fault

Dip of fault and/or contact

Foliation

Stretching lineation

U-Pb sample locality

K-Ar sample locality

Figure 3. Geologic map of the Oval Peak batholith and adjacent rocks. Foliation in the core of the batholith is magmatic; foliation in all other units is solid state. Generalized mainly from Miller (1987); area east of Twisp River fault and Foggy Dew fault is modified in part from McGroder and Miller (1989). (Note overlap in center.)

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Figure 4. Cross sections through the Oval Peak batholith and adjacent rocks (no vertical exaggeration). Dashed lines superimposed on the pattern for the Twisp Valley Schist represent the trace of foliation; other patterns are elongate parallel to foliation. Cross-section lines and symbols are shown in Figure 3.

its envelope of TVS (Fig. 3). The Battle Moun- Lake Juanita Leucogneiss. The Lake Juan- protoliths of the leucogneiss intruded. They tain gneiss was intruded by concordant sheets of ita leucogneiss is dominated by leucogranodio- broadly resemble the TVS, but their origin is the Tuckaway Lake gneiss and by leucocratic ritic and trondhjemitic gneisses, some of which unknown. gneiss similar to the Lake Juanita leucogneiss. have a crystallization age of —60 Ma (see These intrusive sheets generally range from 10 below). In the Gabriel Peak tectonic belt, near Oval Peak Batholith cm to 10 m in thickness. In some places, the the tectonically modified intrusive contact with Battle Mountain gneiss occurs as screens within the Tuckaway Lake gneiss, the leucogneiss is The Oval Peak batholith is herein subdivided the Tuckaway Lake gneiss. The Battle Mountain well foliated and in places mylonitic. Elsewhere, into a foliated margin and a weakly foliated to gneiss is distinguished from the latter unit by its most of the gneiss is lineated but only weakly isotropic core (Fig. 3), both of which were coarser grain size, higher color index, and the foliated. Coarse-grained igneous textures are probably emplaced at -65 Ma (see below) as a presence of hornblende and large sphene. generally preserved in the leucogneiss. Solid- single body. Aplite and pegmatite dikes (10-30 The Battle Mountain gneiss is nearly identical state foliation is defined by fine- to medium- cm thick), commonly garnet bearing, occur in age and is similar in mineralogy to the Black grained recrystallized aggregates of biotite and throughout the batholith. Peak batholith, which is exposed to the north- muscovite, elongate quartz, recrystallized quartz Weakly Foliated to Isotropic Core. The west within the RLFZ. The gneiss may repre- lenses, and bands of finely recrystallized mosaic pluton consists mainly of light-colored biotite sent a deformed outlier of the batholith or of quartz, plagioclase, and myrmekite. tonalite that grades locally into biotite trond- possibly was detached from the batholith by The unit is more heterogeneous to the south hjemite (Adams, 1961; Libby, 1964). Primary movements in the fault zone. where leucogneiss intruded numerous bodies of minerals are plagioclase (average An30) (-55%), Tuckaway Lake Gneiss. The Tuckaway tonalitic and granodioritic gneiss. Dikes and quartz (-20%), biotite (10%-15%), epidote Lake unit consists of strongly foliated and lin- other small (<300 m2) bodies of late- and post- (5%-10%), microcline (0%-5%), sphene (1%), eated, medium-grained biotite tonalite gneiss metamorphic granitoids, ranging in composition and apatite (<1%) (Libby, 1964). Hornblende that grades into mylonite. The gneiss occurs as a from gTanite to diorite, have intruded the leu- occurs locally. The presence of magmatic epi- collection of metamorphosed intrusive sheets cogneiss. Some quartz monzonite that contains dote (App. 1) suggests crystallization at >6 kbar that form a narrow zone between the Battle phyric K-feldspar resembles parts of the middle (Zen and Hammarstrom, 1984). Pressures of Mountain and Lake Juanita gneisses. It is in- Eocene Cooper Mountain batholith (Barksdale, 5-7 kbar are indicated by empirical and exper- truded by dikes and sills of the structurally lower 1975) exposed to the south. imental hornblende barometers for four samples Lake Juanita leucogneiss and by undeformed Enclaves of biotite schist, quartzitic schist, and in the core and foliated margin (J. M. Hammar- granitoids. In the south, the volume of younger amphibolite, typically <10 by 25 m, are com- strom, 1989, written commun.). rocks increases, and the Tuckaway Lake gneiss mon within the leucogneiss. These enclaves are The core of the pluton shows weak to moder- is not present as a mappable unit (Fig. 3). remnants of the country rocks into which the ate foliation defined by large euhedral biotite

Figure 5. A. Tonalité from the core of the Oval Peak batholith. Magmatic foliation, trending approximately parallel to base of figure, is defined by large biotite (B) grains. Note the oscillatory zoning and synneusis in plagioclase. Quartz (Q) is undulose but has not changed shape, forming nonaligned interstitial aggregates. Crossed polars; field of view is 12.4 mm in length. B. Tonalité gneiss of the foliated margin of the Oval Peak batholith. Solid-state foliation, trending parallel to base of figure, is defined by aggregates of recrystallized biotite and moderately recrystallized quartz. This rock has undergone considerable recrystallization, but large, relict oscillatory-zoned plagioclase grains are widespread. Crossed polars; field of view is 12.4 mm in length.

books and locally by tabular plagioclase. Mi- large, oscillatory-zoned plagioclase (Fig. 5B). In termediate volcanic strata on the northeast. crostructures indicate that this foliation formed protomylonites, relict plagioclase, epidote, and Metamorphic assemblages in these strata are of by magmatic flow (Fig. 5A). Aligned biotite and quartz porphyroclasts are set in a fine- to the sub- to lower greenschist facies, and ductile plagioclase are euhedral and are not recrystal- medium-grained mosaic of plagioclase, quartz, deformation is generally absent. The mylonites lized. They are enclosed in a groundmass of pla- biotite, and myrmekite. define a continuous, ~1-km-wide fault zone, gioclase, K-feldspar, and quartz crystals that The above microstructures indicate that folia- and Miller (1987) postulated that the Twisp display igneous textures. Quartz is the first min- tion originated by solid-state deformation. Most River fault is a splay of the Foggy Dew fault eral in tonalités to become elongate during plas- convincing is the recrystallization of biotite and (Fig. 3). Despite this continuity, there are signif- tic solid-state flow, and its presence in generally quartz into elongate aggregates; however, the icant differences along strike in lithology and in weakly recrystallized, interstitial, nonaligned ag- gradation from a magmatic foliation in the core orientation of fabrics. Thus, in the following dis- gregates is strong evidence that the foliation is of the pluton to a strong solid-state foliation in cussion, we distinguish the Foggy Dew fault magmatic (for example, Hutton, 1988; Paterson the margin probably indicates that the latter zone from the Twisp River fault zone. and others, 1989). overprinted a magmatic foliation. Foggy Dew Fault Zone (FDFZ). The Foliated Margin. A zone of moderately to Recrystallization may have occurred at higher Foggy Dew fault separates the Jurassic-Creta- strongly foliated tonalité, -1.1 to 2 km wide, temperatures in the southern part of the foliated ceous Methow basin from mylonitic gneiss, am- forms the southern and southwestern margin of margin, because plagioclase in mosaics is more phibolite, biotite schist, and minor greenschist of the batholith. The intensity of foliation increases calcic in the south, and biotite is green-brown in the FDFZ (Barksdale, 1975). Mylonitic folia- gradually from the core toward the marginal the north and red-brown farther south (Libby, tion dips moderately to steeply northeast, and zone, whereas a weak stretching lineation is re- 1964). The Battle Mountain gneiss shows sim- stretching lineation plunges gently southeast. stricted to s; 50 m of the envelope of TVS. ilar relationships (App. 1). The greater width of Mylonitic gneiss in the FDFZ has several pro- Mylonite is confined to an even narrower (®;20 the foliated zone to the south may reflect the toliths. Some mylonites grade southwestward m) zone adjacent to the contact. higher temperatures and/or higher strain. into progressively less strongly foliated tonalites Tonalité gneiss of the foliated margin is re- of the Oval Peak batholith (Fig. 3) and clearly crystallized to plagioclase + quartz + biotite + Mylonitic Rocks in the Foggy Dew and are derived from the batholith. The transition to sphene ± hornblende ± epidote + opaques ± Twisp River Fault Zones nearly isotropic rocks, showing only a magmatic microcline. Foliation is best defined by medium- foliation, is abrupt (<30 m) in some places. grained aggregates of recrystallized biotite (Fig. The Foggy Dew and Twisp River faults form Other mylonites are derived from Lake Juanita 5B). Sphene, hornblende, elongate quartz, and the northeastern boundary of the Oval Peak leucogneiss. Small bodies of granodioritic and recrystallized quartz lenses also define foliation. batholith and its wall rocks (Fig. 3). Both of quartz monzonitic mylonites that have protoliths Lineation is marked by aggregates of quartz and these faults are probably brittle structures that atypical of the Oval Peak batholith may be de- biotite. Most rocks display minor to moderate separate ductilely deformed amphibolite- and rived from small intrusions into the fault zone, reduction in grain size by recrystallization. Relict upper-greenschist-facies mylonites on the south- far-traveled fault slices, or a marginal phase of textures commonly survive, as best shown by west from bedded clastic sedimentary and in- the batholith that was preserved only in the fault

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zone. Medium-grained porphyritic tonalite dikes Relict textures are preserved in one large pretations are supported by the presence of gab- intrude the Oval Peak mylonites and share (>300 m wide), relatively weakly foliated body bro in the Ruby Creek heterogeneous plutonic the same fabric as the latter rocks but generally of metamorphosed gabbro and diabase. Me- belt (Misch, 1966) and Skymo Complex (Wal- are less deformed. They may represent the pro- dium- to coarse-grained, nearly equant, bent lace, 1976) in the RLFZ near 49°N. Finally, the tolith for mylonite, which has U-Pb systematics hornblendes in these rocks may be magmatic or amphibolites possibly represent the metamor- suggestive of an Eocene age (see below). pseudomorphs after augite. Acicular horn- phosed roots of the Triassic Spider Peak Forma- The gneisses are predominantly protomylo- blendes form overgrowths on fractured grains. tion, which forms the basaltic basement to the nites and homoclastic (Hanmer, 1987) mylo- These textures and the assemblage (hornblende Methow basin in British Columbia (Ray, 1986). nites; ultramylonites occur as 10- to 20-cm-thick + plagioclase + biotite + sphene) indicate rela- Most of these correlations are compatible with bands. The gneisses were deformed under lower- tively static metamorphism under temperatures major strike slip in the FDFZ. amphibolite- and upper-greenschist-facies condi- similar to those recorded by the strongly foliated Greenschist-facies mylonites occur locally in tions and have partially recrystallized to biotite + amphibolites. the FDFZ. The Foggy Dew fault juxtaposes the muscovite + epidote + quartz + plagioclase + Zones of well-foliated amphibolite within this largest block of these rocks against massive vol- hornblende. Plagioclase (calcic oligoclase) and body display textures that grade continuously canic rock of the Methow basin, and they are K-feldspar porphyroclasts are bent and micro- from altered gabbro to mylonitic amphibolite. faulted against amphibolite on the southwest. fractured. Strongly bent biotite and lesser mus- This gradation, which occurs down to the scale Some mylonitic greenschist contains plagioclase covite porphyroclasts ("fish") have finely recry- of a thin section, indicates that gabbro and dia- porphyroclasts (relict phenocrysts?) and may stallized tails aligned in C-surfaces. Quartz forms base were the protoliths for at least some of the represent metamorphosed andesite of the fine-grained layers and lenses and displays sub- mylonite. Hornblende porphyroclasts in mylo- pre-Upper Cretaceous North Creek Volcanics grains and recrystallized grains that define an nite may be analogous to the stubby amphiboles (Misch, 1966) or the Upper Jurassic-Lower Cre- oblique foliation. Aggregates of quartz, plagio- in weakly deformed rocks. The textural heter- taceous Newby Group and/or mid-Cretaceous clase, and biotite mark the stretching lineation. ogeneity in the metamorphosed gabbro and Midnight Peak Formation (Barksdale, 1975) of Mylonitic gneiss is interlayered with amphib- diabase probably reflects differences in deforma- the Methow basin. Mylonitic siliceous chlorite olite that typically forms bodies <20 m thick. tion under amphibolite-facies conditions. schist may be derived from sedimentary and/or Contacts are mostly concordant to foliation, but It is difficult to correlate amphibolite in the felsic volcanic rocks of the basin. The schist con- tongues of gneiss, probably derived from the FDFZ with other units in the region. Metaba- tains plagioclase and quartz porphyroclasts that Oval Peak batholith, locally cut amphibolite at a sites are subordinate constituents of the TVS, but are relict clasts or phenocrysts. high angle, indicating a modified intrusive con- they probably represent flows rather than intru- Interlayered mica schist and amphibolite tact. Thin (10-25 cm) boudinaged gneiss layers sive rocks. Gabbros are present in the Hozameen dominate near the intersection with the Twisp within amphibolites were originally dikes or Group, which may represent the northern contin- River fault zone but are subordinate elsewhere. sills. uation of the TVS (Misch, 1966). Trace- Rare mylonitic sillimanite schist (Fig. 7) is inter- The amphibolite contains the assemblage element studies (Geary and Christiansen, 1989) calated with gneiss near Crater Creek. This hornblende + plagioclase ± biotite + epidote ± of metabasites indicate the presence of MORB schist illustrates the metamorphic break across quartz + sphene ± chlorite. Thin layers of garnet and "within-plate tholeiite" components in both the Foggy Dew fault, as it equilibrated at -625 amphibolite occur rarely. There are two genera- the TVS and FDFZ. The FDFZ, however, also °C and 5 kbar (D. L. Whitney and R. B. Miller, tions of hornblende (Fig. 6). The first consists of contains rocks with a primitive island-arc unpub. data). It probably represents mylonitized medium-sized, poorly to well-aligned stubby tholeiite signature not recognized in the TVS. TVS, but correlation with the Little Jack unit grains, many of which are bent porphyroclasts Alternatively, mafic rocks may have intruded (Tabor and others, 1989), which occurs next to and show patchy zoning. Well-aligned, typically the Foggy Dew fault prior to mylonitization and the Ross Lake fault -45 km to the northwest, is acicular hornblendes constitute the second, dom- emplacement of the Oval Peak batholith or may possible. inant generation and define lineation. Very fine- have been transported along the fault during a Mylonitic Rocks in the Twisp River Fault grained acicular hornblende marks C-surfaces. pre-Oval Peak movement phase. These inter- Zone (TRFZ). The Twisp River fault separates mylonite of the TRFZ from andesite and arkose of the pre-Late Cretaceous North Creek Volcan- ics. The latter is bounded to the northeast by the North Creek fault (Fig. 2). The mylonites are

Figure 6. Mylonitic amphibolite in the Foggy Dew fault zone, showing S-C fabric. Fine-grained hornblende defines foliation (S), which trends parallel to base of figure and wraps around larger hornblende porphyroclasts. C surfaces, trending from upper left to lower right, are marked by even finer-grained, acicular hornblende. The sense of shear is dextral. Plane light; field of view is 3.3 mm in length.

Figure 7. Mylonitic sillimanite schist in the Foggy Dew fault zone. $1-2 (parallel to base of figure) is marked by micas and sillimanite and has been deformed into a sigmoidal pattern. Biotite-defined C surfaces trend from upper left to lower right. The large garnet in the center has a tail of ilmenite that occurs only on one side of the porphyroblast. Plane light; field of view is 3.3 mm in length.

derived from a variety of protoliths, including tonalite porphyry, volcaniclastic breccia, schist, metachert, phyllite, sandstone, conglomerate, and minor tonalite gneiss. These protoliths have counterparts on both sides of the TRFZ. Mylonitic metachert and washed in 80 °C 3N HN03 for 30 min prior to tions are approximately equal and are "inter- garnet-bearing schist grade southwestward into dissolution. Zircon dissolution and chemistry nally concordant." Because of the linear nature the TVS. Volcaniclastic rocks are probably use methods modified after Krogh (1973), utiliz- of concordia from 0 to 100 Ma, internal concor- equivalents of the North Creek Volcanics. Cor- ing Teflon microbombs as described in Parrish dance is not sufficient to assign a crystallization relation of other mylonites is less certain. (1987). Zircons were dissolved at 240 °C in 48% age. Thus multiple fractions are required to Metasandstone and metaconglomerate may rep- HF for 4-6 days. Total procedural blanks aver- evaluate the true age of a sample. resent rocks of the mid-Cretaceous Virginian aged 50 pg for Pb and 20 pg for U. Isotopic Oval Peak Batholith. Zircons from both the Ridge Formation of the Methow basin, but shale compositions were measured on a VG 354E isotropic tonalite and well-foliated tonalite of clasts dominate in the mylonites in contrast to mass spectrometer. Corrections for common Pb the Oval Peak batholith are water clear, stubby, chert-rich conglomerates of the Virginian Ridge. utilize measured feldspar Pb's (Table 1). Uncer- and euhedral; very few grains have inclusions of Clastic rocks also occur in the North Creek Vol- tainties are estimated following Ludwig (1980, any kind. Zircons from the isotropic sample canics. Phyllite in the TRFZ may be derived 1988) and on the long-term reproducibility of have a relatively large range in 206Pb/238U ages from the TVS, North Creek Volcanics, or Meth- standards and samples. Decay constants used are (Table 2, Fig. 8). The data from two fractions ow section. Mylonitic gneiss possibly correlates from Steiger and Jäger (1977). (1 and 3) exhibit internal concordance, and with the Oval Peak batholith, but these rocks Pb isotopic composition was measured by the another fraction (2) shows evidence for inheri- are separated by a narrow belt of TVS. Tonalite silica-gel phosphoric acid method and U by the tance of slightly older zircon. Two fractions of porphyry probably was intruded into the TRFZ phosphoric acid-graphite method. Lead data zircon from the foliated sample yield different and subsequently deformed. were corrected for instrumental mass fractiona- results. One fraction (4) is internally concordant 206 238 The mylonites mainly recrystallized in the tion of 0.6 ± 0.02% per amu based on replicate with a Pb/ U age of 61.4 Ma and a upper greenschist facies. Gneiss and porphyry analysis of NBS standards; U was corrected for 207Pb/235U age of616 Ma xhe other (5) has a display the assemblage quartz + plagioclase + 0.1 % per amu mass fractionation based on repli- 206pb/238U age 0f 60 g Ma and a 207pb/235U biotite + muscovite + epidote ± chlorite. Abun- cate analysis of the NBS U-500 standard. age of 62.8 Ma and is interpreted to contain a dant chlorite in some rocks implies lower A general problem in interpreting the age of small inherited component. metamorphic temperatures than in the FDFZ. zircons from young rocks is that discordance can The five fractions of the batholith show evi- be expressed as dispersion of data points parallel dence for both Pb loss and inheritance of older 206 238 GEOCHRONOLOGY to concordia. In this situation, the Pb/ U zircon. We interpret the crystallization age of the age and the 207Pb/235U age of individual frac- Oval Peak as about 65 Ma based on the most U-Pb Zircon

Five samples were dated by the U-Pb method TABLE 1. SAMPLE DESCRIPTIONS AND ALKALI FELDSPAR Pb ISOTOPIC COMPOSITIONS with the goal of bracketing the timing of defor- Sample and unit Latitude Longitude Alkali feldspar* mation in the RLFZ. Units analyzed include the 207 204 208p 204 isotropic core and foliated margin of the Oval ^Pb/^Pb pb/ pb b/ pb

Peak batholith, Lake Juanita leucogneiss, Battle OP-D Oval Peak batholith, 48° 17.97" 120°27.19' 18.868 15.587 38.461 Mountain gneiss, and mylonitic gneiss of the nnnfoliated FDFZ. OP-F Oval Peak batholith. 48°12.76' 120° 17.87' foliated margin Analytical Procedures. Minerals were sepa- CAS-LE Lake Juanita leucogneiss 48°16.38" I20°28.79" 18.936 15.589 38.542

rated from 20-60 kg samples using standard Q MY-1 FDFZ Immilli,' mylonite 48 13.I2' 120-15.77' 18.859 15.588 38.456 crushing, heavy-mineral concentration, and BMG Battle Mountain gneiss 48°I6.23' 120°27.8I' 18.864 15.583 38.387 magnetic techniques. Zircons were selected for analysis by hand picking in alcohol; only •Alkali feldspars were leached in 5% HF in order to obtain the least radiogenic Pb (see Housh and others, 1989, lor details of the experimental procedure). The least radiogenic leach is interpreted as the best approximation of the initial Pb isotopic composition of the sample. inclusion-free grains were selected. Zircons were

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TABLE 2. U-Pb ZIRCON DATA

Sample* Wt. U Pbt Atomic ratios^ Age (Ma) ± error (2a) (mg.) (ppm) (ppm) 206 204 207p 235 238 Pb/ Pb ^'Pb/^Pb ^Pb/^Pb 206pb/238u b/ u ^Pb/ !! 207pb/235u Wpb/^Pb

1 OP-Dd-1 2.780 370.4 3.57 1366 0.047572 0.055037 0.01017 0.06671 65.2 ± 0.3 65.6 ± 0.3 78.0 ± 6 2 OP-D d-2 6.051 379.6 3.45 990 0.048095 0.057365 0.00956 0.06338 61.3 ± 0.3 62.4 ± 0.4 103.9 ± 11 3 OP-D d-2(#2) 3.616 391.9 3.72 1095 0.047563 0.056347 0.00999 0.06555 64.1 ± 0.3 64.5 ± 0.4 77.6 ± 7

4 OP-F d-2 8.717 318.9 2.90 2151 0.047453 0.057831 0.00956 0.06257 61.4 ± 0.3 61.6 ± 0.3 72.1 ± 5 5 OP-Fnml 3.390 267.7 2.44 1250 0.048806 0.071066 0.00948 0.06381 60.8 ± 0.3 62.8 ± 0.5 138.5 ± 13

6 CAS-LE d-2 3.874 515.3 4.57 725 0.047527 0.057023 0.00934 0.06118 59.9 ± 0.3 60.3 ± 0.3 75.8 ± 3 7 CAS-LE nml 4.437 876.9 7.55 1212 0.047846 0.056362 0.00907 0.05981 58.2 ± 0.3 59.0 ± 0.4 91.7 ± 9

8 MY-1 nml 3.036 383.4 2.81 455 0.047925 0.061008 0.00768 0.05074 49.3 ± 0.2 50.3 ± 0.6 95.6 ± 25 9 MY-1 nmlO 4.138 670.0 6.65 3448 0.051304 0.059609 0.01038 0.07345 66.6 ± 0.3 72.0 ± 0.3 254.5 ± 3

10 BMG d-1 7.111 247.3 3.40 739 0.048207 0.126498 0.01360 0.09042 87.1 ± 0.4 87.9 ± 0.5 109.4 ± 9 11 BMG nm4 6.380 195.0 2.67 1202 0.047675 0.120664 0.01363 0.08962 87.3 ± 0.4 87.1 ± 0.5 83.2 ± 10 12 BMG nm4(#2) 10.313 205.1 2.81 552 0.048134 0.125203 0.01356 0.09001 86.8 ± 0.4 87.5 ± 0.5 105.8 ± 6

*d, diamagnetic; nm, nonmagnetic at specified degrees of side tilt. t Radiogenic Pb. §206pb/204pb is the measured ratio corrected for mass fractionation. All other ratios are corrected for mass fractionation, common Pb from Table 1, and the laboratory blank (206Pb/204Pb : 207Pb/204Pb : 208Pb/204Pb = 19.25:15.73:38.78).

concordant zircon fraction. This age is consistent served in the Oval Peak samples cannot be ruled 207Pb/235U age of 50.3 Ma, which nearly over- with the 206Pb/238U sphene age of 65.3 Ma out. We interpret the leucogneiss as being lap within uncertainties. A strongly magnetic reported by Miller and Walker (1987). It is younger than the Oval Peak. fraction has a Pb-Pb age of 254 Ma (Table 2) slightly different from the preliminary age of 61 Battle Mountain Gneiss. Three fractions of and thus shows evidence for inheritance. It is ± 3 Ma reported by Miller and others (1989) clear, inclusion-free zircon from this gneiss yield possible that the least-magnetic fraction is based on slightly discordant data. The inherited nearly concordant analyses with 206Pb/238U strongly discordant and represents mylonitized component is probably older than the oldest Pb- ages of 87.3, 87.1, and 86.8 Ma (Table 2; Fig. Oval Peak or older intrusive rocks. We favor an Pb age of the two fractions showing inheritance 8). We interpret these results to indicate that the interpretation that the crystallization age of the (138 Ma). crystallization age of the tonalitic protolith is tonalitic protolith is distinctly younger than Lake Juanita Leucogneiss. A well-foliated about 87 Ma. those of the Oval Peak batholith and the Lake and lineated leucogneiss was sampled within the Mylonitic Gneiss in FDFZ. A sample of my- Juanita leucogneiss. This is supported by the Gabriel Peak tectonic belt. Two magnetic frac- lonitic gneiss from the FDFZ yielded sparse, K-Ar age (55.8 ± 3.6 Ma) for a nearby tions of zircon are internally concordant (Table brown-colored, magnetic zircons. These zircons amphibolite in the fault zone, which is older 206 238 2; Fig. 8) but are slightly different. The best contain significantly more common Pb than than the Pb/ U age for the least-magnetic estimate of the crystallization age of the protolith do the other samples. The least magnetic frac- fraction. Thus this fraction may provide the best is 60 Ma, although some discordance as ob- tion has a 206Pb/238U age of 49.3 Ma and a control on the last deformation in the FDFZ.

.0104 - OP-D .0140 -

OP-F .0138 - CAS-LE .0100 - 64

.0136 62 .0096 - .0134 60

.0092 - .0132 _ 58.

207Pb/ 235 U .0088 I . i L JL _L .0130 .057 .059 .061 .063 .065 .067 .069 .086 .088 .090 .092 B Figure 8. A. Concordia plot for zircons from the Lake Juanita leucogneiss (CAS-LE), nonfoliated core (OP-D) of the Oval Peak batholith, and foliated margin (OP-F) of the Oval Peak batholith. B. Concordia plot for zircons from the Battle Mountain gneiss.

K-Ar, Ar-Ar, and Fission-Track TABLE 3. K-Ar DATA ON HORNBLENDE Geochronology Sample description Location K 40Ar (rad.) 40Ar (rad.) Date (wt. %) (1010mo!/gr) (%) (Ma) Hornblende from well-recrystallized amphib- Mylonitic amphibolite 120°16.I9" 0.288 0.2823 20.9 55.8 i 3.6 olites in the envelope of TVS and in the FDFZ from Foggy 48°13.32' yields K-Ar ages of 54.0 ± 0.7 and 55.8 + 3.6 Dew fault zone

Ma, respectively (Table 3). An amphibolite in Amphibolite from 120°27.66' 0.46 3.633 45.0 54.0 ± 0.7 envelope of Twisp 48°16.ir the TVS envelope has an Ar-Ar isochron age of Valley Schist 57.8 ± 0.6 Ma (E. E. Geary, D. Christiansen, and R. B. Miller, unpub. data). These data suggest that amphibolite-facies metamorphism was over by about 55 Ma. rich layers, "foliation fish" and rotated lozenges phic break across the fault suggest large dis- An isotropic tonalite from the Oval Peak bath- (Hanmer, 1986), asymmetric plagioclase por- placement, particularly if the implied vertical olith yielded K-Ar biotite and sphene fission- phyroclasts, and local ductile shear zones with offset resulted from oblique movement. Most track ages of 47.4 ± 0.9 Ma (R. W. Tabor, curving foliation trajectories. Asymmetric open correlations of amphibolites in the FDFZ with 1987, written commun.) and 50.3 ± 5.0 Ma to isoclinal mesoscopic folds of mylonitic folia- mafic rocks elsewhere in the region indicate sig- (J. A. Vance, 1986, personal commun.), respec- tion in amphibolite, which have axes oblique to nificant offset. Finally, the possibility that the tively, indicating that the batholith cooled below stretching lineation, are also consistent with dex- Foggy Dew fault is a segment of a >500-km- 250 °C by about 48 Ma. tral shear. long structure, which to the north is marked by The effects of dextral shear are also evident in the North Creek fault, Hozameen fault, and KINEMATICS OF THE FDFZ AND the TVS for > 1 km southwest of the mapped Yalakom fault (Figs. 1 and 2) (McGroder, FOGGY DEW FAULT fault zone. These include S-C fabric, oblique fo- 1987), also suggests major movement, as the liation in quartz ribbons, and asymmetric pres- Yalakom apparently underwent 150 ± 50 km of The kinematics of ductile deformation are sure shadows on garnet. dextral slip (Umhoefer and others, 1989). readily determined in mylonites in the FDFZ. Movement on the Foggy Dew fault is more The orientation of mylonitic foliation and lin- difficult to document. The fault is not exposed, KINEMATICS OF THE TRFZ AND eation indicates oblique slip. Foliation dips but we infer slip in the brittle regime subsequent TWISP RIVER FAULT moderately to steeply northeast (Fig. 9A), and to mylonitization to account for the juxtaposi- lineation plunges 0° to 35° southeast, averaging tion of metamorphic grades across the structure. Mylonites in the TRFZ display steep north- about 22° (Fig. 9B). Well-developed meso- and The map pattern suggests that the Foggy Dew west-striking foliation and subhorizontal stretch- microscopic kinematic indicators record dextral fault dips northeast, and thus a component of ing lineation (Figs. 9C and 9D). Strongly shear. The FDFZ thus experienced oblique normal slip is probable, as has been documented lineated and weakly foliated mylonites compose right-lateral strike slip with a smaller component for ductile deformation in the FDFZ. Such slip significant parts of the fault zone, and constric- of normal dip slip, down to the northeast. The is compatible with the interpretation that tional strain is best shown by stretched clasts in dip-slip component may increase to the south movement in the RLFZ tectonically removed breccia and conglomerate. In metasedimentary where mylonitic foliation is less steep and lin- some of the thick load that buried the crystalline rocks, the stretching lineation defined by clasts eation is closer to a down-dip orientation core (compare with Whitney and McGroder, and mineral aggregates is folded around crenula- (Hopkins, 1987). 1989). tions and larger tight to isoclinal folds, which Asymmetric structures indicative of dextral The magnitude of slip in the FDFZ and on display prominent axial-planar foliation. shear were recognized in mylonites at more than the Foggy Dew fault is unknown because the Shear bands (C' surfaces) and less common 20 localities. These include type I and type II component of dip slip across these structures S-C fabrics in eight oriented thin sections indi- (Lister and Snoke, 1984) S-C fabrics, shear presumably precludes locating piercing points. cate dextral slip. The TRFZ may record a bands (C' surfaces), oblique foliation in quartz- The width of the FDFZ and major metamor- greater component of strike slip relative to dip

Figure 9. Structural data in the FDFZ and TRFZ. A. Poles to foliation in the FDFZ (61 points). B. Lineation in the FDFZ (41 points). C. Poles to foliation in the TRFZ (15 points). D. Lineation in the TRFZ (21 points).

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slip than does the FDFZ, because foliation is Northern Intrusive Contact of the Oval ment of Se. At this time, sillimanite replaced steeper and stretching lineation generally is shal- Peak Batholith andalusite, and garnet rims may have recrystal- lower (Fig. 9). lized; (3) local microfolding of sillimanite. The The kinematics of the poorly exposed Twisp Tonalité of the Oval Peak core intruded the timing of replacement of andalusite by kyanite River fault are difficult to determine. The appar- TVS in the north along a contact that is gener- relative to growth of sillimanite is uncertain. ent straightness of the fault trace suggests that it ally concordant with foliation in the schist and Kyanite grew statically, possibly later than the is steep, as is the foliation in the TRFZ, and the with magmatic foliation in the tonalité. The in- synkinematic sillimanite. Some kyanite is un- fault may also have undergone dextral strike tensity of magmatic foliation increases as the dulose, however, compatible with growth before slip. The juxtaposition of polydeformed TVS contact is approached. Sills and dikes of tonalité sillimanite. against lower-grade rocks east of the fault is and aplite, locally boudinaged, are common for This history records dynamothermal meta- compatible with a component of dip slip. distances as much as 20 m away from the morphism that was at least in part synchronous pluton. Foliation within rare small schist xeno- with intrusion of the Oval Peak batholith. De- EMPLACEMENT OF THE OVAL liths has been rotated, suggesting that the TVS formation probably resulted in part from intru- PEAK BATHOLITH AND contains a pre-Oval Peak fabric. sion. These interpretations are supported by the DEFORMATION IN THE GABRIEL The relationship between intrusion of the absence of static contact metamorphism and by PEAK TECTONIC BELT batholith and metamorphism of the TVS is best geochronologic data that indicate cooling of the recorded by porphyroblasts (mainly garnet and TVS within 7 to 10 Ma after intrusion. Meta- The geometry, contact relations, and strain andalusite) near Scaffold Peak and on the Scaf- morphism initially occurred at low pressure. The patterns of the Oval Peak batholith are impor- fold Creek-Oval Creek divide (Fig. 3). Andalu- replacement of andalusite by kyanite, combined tant for understanding the mechanics of em- site has been partially or completely replaced by with thermobarometric data (D. L. Whitney placement of this pluton and the movement muscovite and locally by biotite, sillimanite, and and R. B. Miller, unpub. data), indicates that history of the Gabriel Peak tectonic belt. kyanite. Some andalusites are bent, undulose, pressure subsequently increased, possibly by tec- The batholith has a complex shape. It is en- and elongate in S|_2. Inclusion trails of quartz tonic loading. The pressures implied by kyanite closed by the TVS, except on the northeast side and opaques define a straight to commonly are compatible with the presence of magmatic where it is truncated by the northeast-dipping weakly crenulated Sj in andalusite and a straight epidote and the hornblende barometry for the FDFZ (Fig. 4B). The pluton structurally overlies S; in garnet cores and locally staurolite. S; is batholith. The inferred metamorphic history the envelope of TVS on the south and southwest commonly oriented at high angles to the exter- may indicate that the batholith was intruded within the Gabriel Peak tectonic belt (Figs. 3 nal mica-defined foliation (Se = S1.2) (Fig. 10), during crustal thickening. and 4). Northwest of Oval Peak (Fig. 3), the which wraps around porphyroblasts. Silliman- contacts of a lobe of the batholith are synformal ite, mostly fibrolite, is aligned in Se and has Foliated Margin of the Oval Peak Batholith (Fig. 4A), whereas contacts and magmatic folia- been openly microfolded. Rare small- to and the Gabriel Peak Tectonic Belt tion in the northernmost "finger-like" projection medium-sized grains of kyanite within andalu- (War Creek area, Fig. 3) of the batholith are site porphyroblasts are randomly oriented and in Structural Patterns. The foliated margin of broadly antiformal. one sample form radiating bundles. the Oval Peak batholith together with the struc- In the following sections, we first describe the The above microstructures suggest the follow- turally underlying TVS and mylonitic, structur- northern contact where initial intrusive relations ing history: (1) growth of garnet cores and ally highest part of the Skagit Gneiss define the have not been obscured by movement in the staurolite, followed by andalusite during or sub- ~2- to 3-km-wide southern segment of the Gabriel Peak tectonic belt or FDFZ. We subse- sequent to formation and, in the case of andalu- Gabriel Peak tectonic belt (Fig. 2). The northern quently discuss deformation in the tectonic belt site, weak crenulation of foliation preserved in segment is delineated by the strongly deformed and conclude that the batholith was emplaced as porphyroblasts; (2) deformation of andalusite, western margin of the Black Peak batholith (Fig. an expanding diapir into this fault zone. garnet, and staurolite with continued develop- 2) (Misch, 1977; Hoppe, 1984) and by mylonitic Skagit Gneiss (Miller, 1987). Contacts and foliation within the southern segment dip mainly between 30° and 70° to the northeast. Sills and dikes of the Oval Peak batholith have intruded the TVS, but in general the

Figure 10. Porphyroblastic schist in the TVS. A deformed andalusite porphyroblast, partially replaced by muscovite, has inclusion trails (Sj) oriented at a high angle to S1.2 (trending parallel to base of figure).

Si_2, which is defined by micas and fibrolite (mineral with high relief), wraps around the porphyroblast. Note the garnet porphyroblast in upper right, with abundant inclusions in its core and a clear rim. Plane light; field of view is 3.3 nun in length.

contact is sharp and has been tectonically modi- the south, it swings into concordance with the northwest, or less commonly southeast, oblique- fied. The TVS-Battle Mountain gneiss contact is regional north-northwest strike (Fig. 11). ly to the down-dip lineation in adjacent units also tectonic, as it is sharp and concordant with Foliation within the tectonic belt is generally (compare Figs. 12C and 12G). F3 folds are foliation, intrusive relations are absent, and the concordant with contacts (Figs. 11, 12A, and markedly noncylindrical, displaying hinge cur- gneiss is mylonitic. Similarly, orthogneiss units 12B), but lineation is less systematically ori- vatures of as much as 90°. of the Skagit Gneiss display tectonically modi- ented. Northwest of Star Peak, a north- to The intensity of deformation and type of fied intrusive contacts. northeast-trending, moderately steep, nearly strain within the southern Gabriel Peak tectonic There are systematic differences in deforma- down-dip lineation is present in mylonitic Skagit belt also vary between rock units In general, tion along strike within the southern Gabriel Gneiss and in the outermost part of the Oval Skagit orthogneiss, particularly the Battle Moun- Peak tectonic belt. The most intense deforma- Peak margin (Figs. 11 and 12C). Southwest of tain gneiss, and TVS are more strongly foliated tion took place in the segment northwest from this part of the tectonic belt (structurally lower), and lineated than is the foliated margin of the Star Peak, as shown by mylonites in the TVS, lineation in weakly deformed Lake Juanita leu- Oval Peak batholith. Flattening fabrics domi- Battle Mountain gneiss, Tuckaway Lake gneiss, cogneiss plunges gently (0°-30°) and is parallel nate in the foliated margin, as evidenced by the and locally the Oval Peak batholith. The Lake to the north-northwest regional trend. The orien- moderate to strong foliation but only weak to Juanita leucogneiss is also mylonitic near its tation of lineation also changes along strike to locally moderate lineation. Amphibolite next to contact with the Tuckaway Lake gneiss. The the southeast within the tectonic belt (Figs. 3 the batholith-TVS contact shows chocolate tab- intensity of deformation in the leucogneiss, and 11). Southeast of Bernice Lake, lineation let boudinage also indicative of flattening. however, drops off to the southwest at structur- generally plunges gently at a small angle to the Farther from this contact, however, the TVS and ally lower levels where there is weak to trends of contacts (Figs. 11 and 12D). The con- Skagit Gneiss have both strong foliation and lin- moderate lineation and weak foliation. Fabric sistent spread from down-dip to strike-parallel eation. Some mylonitic orthogneiss displays development also decreases along strike to the lineation, and the absence of overprinting rela- stronger lineation than foliation. southeast within the tectonic belt, east of Horse- tions, suggests that the differently oriented lin- Kinematic indicators yield consistent results head Pass, where rocks are moderately foliated eations are broadly synchronous. in the segment of the tectonic belt northwest of and lineated, rather than mylonitic. Neverthe- Structural patterns in the TVS envelope (Figs. Star Peak where lineation is close to down dip. less, the foliated margin is wider to the south, 12E-12H) do not consistently match those of S-C fabrics and shear bands in oriented samples and the deformation field is perturbed near the other units in the tectonic belt. Northwest of from 10 localities of Skagit Gneiss and two of southern contact of the batholith. Here, foliation Bernice Lake, mineral lineation (L1.2), sheath the Oval Peak batholith indicate reverse slip, in the Lake Juanita leucogneiss is subparallel to fold axes, and Fi_2 axes vary in orientation but with a small component of dextral strike slip. the contact, but over a distance of 1.5 to 2 km to generally plunge gently to moderately (<30°) The latter component increases to the south. Reverse slip is also recorded in mylonites for — 15 km northwest of the batholith within the tectonic belt. Farther northwest, mylonites record dextral strike slip (Miller, 1988). The magnitude of slip is unknown. Dikes within the Oval Peak batholith and TVS are relevant to the timing of deformation and pluton emplacement. Aplite and pegmatite dikes occur throughout the batholith but are concentrated in the foliated margin. The dikes cut solid-state foliation and in most places lack a fabric. Deformed and recrystallized dikes are also present, however, and at one locality, solid- state foliation is axial planar to a folded pegmatite. These relations indicate that solid-state foliation formed during or shortly after emplacement of the pluton and overlapped in time with emplacement of residual melt. Foliated sills and dikes are more abundant in the TVS envelope, although undeformed bodies are also common. Foliated sills are locally deformed by open to tight F3 folds of the TVS. Some of these bodies presumably are part of the Oval Peak batholith, but leucogneisses may be related to the Lake Juanita leucogneiss, and still other Figure 11. Simplified map showing generalized trends of foliation and stretching lineation in rocks intrusive into the TVS are of unknown the southern part of the Gabriel Peak tectonic belt. The tectonic belt is indicated by the squiggle affinity. Collectively, the relations described pattern. Trends are also shown for structurally lower Skagit Gneiss outside the tectonic belt. above indicate that F3 structures in the TVS and Contacts of the Oval Peak batholith are drawn for reference. Note that down-dip lineation solid-state foliation in the foliated margin are within the Gabriel Peak tectonic belt continues northwest of the batholith, indicating that it is essentially contemporaneous with emplacement not solely related to emplacement of the batholith. See text for further discussion. of the batholith.

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Figure 12. Structural data in the Gabriel Peak tectonic belt. A. Poles to foliation north of Beraice Lake, exclusive of the TVS (67 points); contours begin at 4 sigma, and contour interval = 4 sigma. Contouring method after Kamb (1959). B. Poles to foliation south of Bernice Lake, exclusive of the TVS (91 points); contours begin at 4 sigma, and contour interval = 4 sigma. C. Lineation north of Bernice Lake, exclusive of the TVS (39 points). D. Lineation south of Bernice Lake, exclusive of the TVS (39 points). E. Poles to foliation in the TVS north of Bernice Lake (22 points). F. Poles to foliation in the TVS south of Bernice Lake (16 points). G. Lineation in the TVS north of Bernice Lake (20 points). H. Lineation in the TVS south of Bernice Lake (20 points).

Interpretation of the Southern Segment of The flattening fabric in the foliated margin is gested by the increase in foliation intensity and the Gabriel Peak Tectonic Belt. Structural re- compatible with models of diapirs (for example, the development of strong down-dip stretching lations in the southern segment of the tectonic Dixon, 1975; Cruden, 1988) and contrasts with lineation within the Skagit orthogneiss units as belt are best explained by a combination of the plane strain expected in fault zones. Flatten- the TVS is approached. The presence of this (1) deformation related to emplacement of the ing may be explained by ballooning (for exam- lineation, and asymmetric kinematic indicators Oval Peak batholith as an expanding diapir and ple, Ramsay, 1989) and/or by viscous drag that yield consistent sense of shear, is not entirely (2) reverse dip slip in the belt before, during, and along a rising diapir (Schmeling and others, in accord with the flattening predicted for dia- after intrusion. 1988). Mineral lineation and fold axes (FI_2, F3) pirs. Furthermore, the Lake Juanita leucogneiss Evidence for diapiric intrusion includes the within the TVS envelope have variable orienta- probably is younger than the batholith (see presence of magmatic foliation in the core of the tions that may reflect heterogeneous flattening geochronology), and thus mylonitic leucogneiss pluton that is overprinted in the margin by solid- during pluton emplacement, because in a fault likely records post-Oval Peak reverse slip. state foliation that formed during or shortly after zone, these structures would likely rotate into Moreover, the intensity of deformation in the emplacement (for example, Bateman, 1984). parallelism with the displacement direction. batholith is apparently less than that in the older The synchroneity of deformation and intrusion Furthermore, a fault zone does not readily ac- Battle Mountain gneiss and TVS, suggesting that is indicated by dike relations and the relatively count for curvature of foliation trajectories and there also was movement prior to emplacement small difference (—7-10 Ma) between the crys- shallowing of lineation at the south end of the of the batholith. The decreasing intensity of de- tallization age (U-Pb zircon) of the batholith and batholith, both of which may reflect stretching formation in orthogneiss at the southeast end of the Ar-Ar and K-Ar hornblende ages in the TVS around the pluton during emplacement. the batholith may reflect the dying out of slip in envelope. Mineral assemblages in the foliated Other relationships in the southern segment of the tectonic belt, possibly by transfer to other margin are compatible with submagmatic condi- the Gabriel Peak tectonic belt are more compat- structures within the RLFZ. tions, as is the relatively even development of ible with reverse dip slip in a throughgoing fault In conclusion, the southern Gabriel Peak tec- foliation. Ductile shear zones and heterogene- zone. The strongest such evidence is the conti- tonic belt is interpreted to record the following ously distributed mylonites are expected if the nuity of the tectonic belt along strike to the history: (1) reverse slip of unknown magnitude foliation formed mainly by movements in a fault northwest of the batholith as a reverse-slip shear in a ductile shear zone; (2) emplacement of the zone (compare with Paterson and others, 1989). zone (Fig. 11). A dip-slip shear zone is also sug- Oval Peak batholith, possibly concurrent with

reverse slip. The pluton ascended diapirically between U-Pb ages of plutonic rocks and K-Ar Ma brittle motion also occurred in the northern along the shear zone with late expansion that and Ar-Ar ages of amphibolites reflect moder- part of the Gabriel Peak tectonic belt (Hoppe, deformed its margin and its wall rocks; (3) con- ately fast cooling of the area. K-Ar biotite and 1984). tinued reverse slip in the shear zone. fission-track sphene ages of the Oval Peak bath- The magnitude of pre-65 Ma motion in the olith (-48-50 Ma) may also record cooling or RLFZ is unresolved, despite many interpreta- TIMING OF DEFORMATION, resetting during the thermal event associated tions of large Cretaceous movement. We have PLUTONISM, AND METAMORPHISM with intrusion of the Eocene Cooper Mountain presented evidence for an unknown amount of IN THE SOUTHERN RLFZ and Golden Horn batholiths. pre-Oval Peak deformation in both the FDFZ Movement in the FDFZ continued after ces- and Gabriel Peak tectonic belt. Misch (1966) The ~65 and 60 Ma crystallization ages of the sation of deformation in the southern Gabriel proposed pre-Late Cretaceous displacement on Oval Peak batholith and Lake Juanita leuco- Peak tectonic belt. This is indicated by the age the Ross Lake fault and Twisp River fault, based gneiss, respectively, indicate that a significant data and by the map pattern that shows that the upon their truncation by the 90 Ma Black Peak amount of the mylonitization in the FDFZ was foliated margin of the batholith is truncated by batholith. There are, however, problems with Cenozoic in age. The magnitude of earlier de- the FDFZ. The age data permit the inference this hypothesis. The Ross Lake fault may not cut formation in the FDFZ is unknown. Its only that earlier motion in the FDFZ was synchro- the Black Peak batholith, but it cuts mylonitic record is local discordant intrusive contacts be- nous with movement in the tectonic belt. Both Skagit orthogneiss of the Gabriel Peak tectonic tween Oval Peak rocks and mylonitic amphibo- of these zones do have a component of dextral belt. At least part of this orthogneiss crystallized lites, which suggest that the latter were deformed strike slip, although it may be small in the south- at 68 Ma (Miller and others, 1989). The obser- prior to intrusion. Progressive deformation may ern part of the tectonic belt. The contrasting vation that the Twisp River fault does not cut have occurred during the Paleocene to middle senses of dip slip on these structures served in a the 90 Ma batholith, which has been confirmed Eocene. Recrystallized mylonitic amphibolite relative sense to uplift the batholith shortly after by our mapping, also poses an apparent prob- cooled below the blocking temperature for K-Ar its emplacement. Such uplift may explain the lem. The Foggy Dew fault splays northward hornblende by 55.8 ± 3.6 Ma. U-Pb systematics relatively rapid cooling of the block bounded by into the Twisp River fault and North Creek fault in zircons from tonalitic mylonite suggest that these structures. (Fig. 2), Paleogene mylonites in the FDFZ are ductile deformation may have continued until continuous with mylonites in the TRFZ, and both are dextral shear zones. These relations ~50 Ma (see above). Movement in the fault IMPLICATIONS FOR TECTONIC imply that the FDFZ and TRFZ behaved as a zone and on the Foggy Dew fault ended by the MODELS OF THE ROSS LAKE single Paleogene structure. middle Eocene, because these structures are FAULT ZONE truncated by the Cooper Mountain batholith The relationship between the Twisp River (Barksdale, 1975; Wade, 1988), which yields Tectonic models for the RLFZ have been fault and Black Peak batholith may be explained K-Ar biotite ages of 48 Ma (Tabor and others, hampered by poorly bracketed ages of deforma- if the fault is a pre-Black Peak structure that 1987; V. R. Todd, 1986, written commun.). tion and kinematic histories. The earliest well- became inactive after intrusion of the pluton. In This batholith was emplaced into shallow levels, documented deformation is the pre- and syn- this interpretation, post-Black Peak movement probably cooled quickly, and resembles other Oval Peak (65 Ma) reverse slip within the continued along the FDFZ and stepped east- 45-50 Ma K-feldspar-rich intrusions in the Gabriel Peak tectonic belt. Hoppe (1984) ward to the North Creek fault. The North Creek North Cascades (Wade, 1988). For these rea- interpreted U-Pb zircon and sphene ages to indi- fault is poorly studied, but available data permit sons, the K-Ar biotite ages probably closely ap- cate that mylonitization in the northern segment this hypothesis. Steep foliation and subhorizon- proximate the crystallization of the batholith. of the tectonic belt took place after 68 Ma and tal lineation in mylonites next to the fault indi- Deformation in the southern segment of the possibly at —63-65 Ma. This mylonitization oc- cate strike slip (C. G. DiLeonardo, 1989, Gabriel Peak tectonic belt occurred in the Pa- curred during dextral strike slip (Miller, 1988). personal commun.), and movement is bracketed leocene and early Eocene(?). Involvement of the Reverse slip in the southern segment continued between early Late Cretaceous (Cenomanian) Oval Peak and Lake Juanita intrusions provides until perhaps 55 to 58 Ma. and 50 Ma (McGroder, 1987). There are signifi- a lower limit for latest deformation, and the Ar- Major Paleocene to mid-Eocene deformation cant differences, however, between the FDFZ Ar isochron (57.8 ± 0.6 Ma) and K-Ar horn- in the RLFZ is best documented by the oblique and the fault zone accompanying the North blende (54.0 ± 0.7 Ma) ages of recrystallized dextral slip in the FDFZ. This movement con- Creek fault. The FDFZ is much wider, and there amphibolites in the TVS envelope are an upper tinued after deformation in the southern segment is only a very small metamorphic discontinuity limit. Some of the deformation in this segment of the Gabriel Peak tectonic belt. across the North Creek fault in contrast to the occurred prior to and during intrusion of the Post-50 Ma deformation has been demon- Foggy Dew fault. Thus, a pre-90 Ma Twisp Oval Peak batholith, as discussed above. strated in the northern RLFZ. In British Colum- River fault does not explain the continuity of the The K-Ar and Ar-Ar ages of amphibolites bia, Haugerud (1985) postulated dextral strike FDFZ and TRFZ, or why the Paleogene FDFZ from both the east and west sides of the Oval slip and normal dip slip, down to the northeast, more closely resembles the TRFZ rather than Peak batholith indicate that this pluton and its on the Ross Lake fault partly on the basis of a the North Creek fault. Alternatively, Miller wall rocks had cooled below the blocking stretching lineation in 46 Ma orthogneiss dikes (1988; McGroder and Miller, 1989) suggested temperature for hornblende by about 55 to 58 within the Skagit Gneiss. The Hozameen fault that dextral motion on the Twisp River fault Ma. Microstructures and field relations show may have been active after 46 Ma in Canada was transferred westward to the northern, dex- that dynamothermal metamorphism of the TVS (Ray, 1986), although motion on the Hoza- tral segment of the Gabriel Peak belt. Strike slip was broadly synchronous with intrusion of the meen-North Creek fault ceased by 50 Ma in may have been transferred by widespread batholith. Thus, the relatively small differences Washington (McGroder, 1987). Minor post-50 reverse-slip ductile shear zones in the southern

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part of the Black Peak batholith. The orientation in northern Washington and southern British V. Todd, and J. Vance. We also gratefully of many of these shear zones is consistent with a Columbia, including the Leavenworth, Entiat, acknowledge the late Peter Misch for many compressional stepover zone in a dextral strike- Puget, and Yalakom faults (Fig. 1). Latest mo- helpful discussions of the Ross Lake fault zone slip system. tion in the northern RLFZ may have overlapped and the North Cascades. K-Ar ages were pro- These opposing interpretations emphasize the that of the north-south-striking Fraser-Straight vided by R. Tabor and J. Nakata of the U.S. uncertainties in estimates of the amount of pre- Creek fault, but movement on the latter con- Geological Survey and by the Washington Di- Cenozoic motion in the RLFZ. Nevertheless, tinued into the late Eocene and offset the RLFZ vision of Geology and Earth Resources. Jane our data argue against the model of Kriens and (for example, Vance and Miller, 1981; Monger, Hammarstrom kindly made available her anal- Wernicke (1986) in which the RLFZ represents 1985). Johnson (1984, 1985) contended that yses and interpretation of hornblende barome- a zone of predominantly pure shear with negli- the strike-slip faults reflect a transtensional tec- try. This work was supported by the National gible displacement. The evidence for unidirec- tonic regime responsible for nonmarine basins Science Foundation (Grants EAR87-07956 and tional simple shear in the FDFZ, TRFZ, and that received thick sedimentary deposits from EAR89-03656), Washington Division of Geol- Gabriel Peak belt, and the metamorphic break about 57 to 42 Ma. Oblique slip in the FDFZ ogy and Earth Resources, and U.S. Geological across the Foggy Dew fault, is contrary to their was synchronous with initiation of these basins Survey. hypothesis. Moreover, anastomosing conjugate and also is compatible with transtension. Plate shear zones and other structures characteristic of motion reconstructions further suggest that pure shear in crystalline rocks (Choukroune and oblique convergence dominated along the mar- Gapais, 1983; Gapais and others, 1987) are ab- gin of northwestern North America during the sent. Finally, our evidence for major Cenozoic Paleogene and presumably resulted in intraplate mylonitization contradicts their interpretation dextral strike-slip faulting (for example, Enge- bretson and others, 1985; Beck, 1983). REFERENCES CITED that post-Cretaceous deformation in the RLFZ Adams, J. B., 1961, Petrology and structure of the Stehekin-Twisp Pass area, was insignificant. We thus envision that in the late (?) Paleo- North Cascades, Washington [Ph.D. thesis]: Seattle, Washington, Uni- versity of Washington, 171 p. cene to middle Eocene, the RLFZ occupied the Barksdale, J. D., 1975, Geology of the Methow Valley, Okanogan County, Washington: Washington Division of Geology and Earth Resources SIGNIFICANCE FOR transition zone between an extensional regime to Bulletin 68, 72 p. PALEOGENE EXTENSION AND the east and a strike-slip-dominated regime to Bateman, R., 1984, On the role of diapirism in the segregation, ascent and final emplacement of granitoid magmas: Tectonophysics, v. 110, p. 811-831. TRANSCURRENT MOTION IN the west along the continental margin. The Beck, M. E., 1983, On the mechanism of tectonic transport in zones of oblique FDFZ best illustrates the transition by display- subduction: Tectonophysics, v. 93, p. 1-11. THE PACIFIC NORTHWEST Carr, S. D., Parrish, R. R., and Brown, R. L., 1987, Eocene structural develop- ing components of both dextral strike slip and ment of the Valhalla Complex, southeastern British Columbia: Tec- tonics, v. 6, p. 175-196. Paleocene to mid-Eocene slip in the southern normal slip. Choukroune, P., and Gapais, D., 1983, Strain pattern in the Aar granite (central Alps): Orthogneiss developed by bulk inhomogeneous shortening: RLFZ was coeval with major extension to the Journal of Structural Geology, v. 5, p. 411-418. east in the Omineca metamorphic core com- CONCLUSIONS Coleman, M., 1989, Early Tertiary deformation in the Bridge River terrane near Lillooet, British Columbia: Geological Society of America Ab- plexes and with dextral strike slip elsewhere in stracts with Programs, v. 21, no. 5, p. 68. Cruden, A. R., 1988, Deformation around a rising diapir modeled by creeping the North Cascades and southern Canadian (1) The Oval Peak batholith was emplaced flow pasta sphere: Tectonics, v. 7, p. 1091-1101. Coast Mountains (Fig. 1). Crustal shortening in Davis, G. A., Monger, J.W.H., and Burchfiel, B. C-, 1978, Mesozoic construc- into the RLFZ as an expanding diapir and de- tion of the Cordilleran "collage," central British Columbia to central the eastern Omineca belt ended by 58 Ma and formed its wall rocks within the Gabriel Peak California, in Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western United States: Society of Economic was followed by east-directed extension from 58 tectonic belt. Reverse slip occurred in this belt Paleontologists and Mineralogists, Pacific Coast Paleogeographic to 52 Ma (Carr and others, 1987; Parrish and Symposium, 2nd, p. 1-32. before, during, and after intrusion. Dixon, J. M., 1975, Finite strain and progressive deformation in models of others, 1988). This extension overlapped in time (2) Motion in the FDFZ included compo- diapiric structures: Tectonophysics, v. 28, p. 89-124. Engebretson, D. C., Cox, A., and Gordon, R. G., 1985, Relative motions with mylonitization in the northeast-dipping nents of both dextral strike slip and normal dip between oceanic and continental plates in the Pacific basin: Geological FDFZ, which has a component of normal slip Society of America Special Paper 206,59 p. slip. Gapais, D., Bale, P., Choukroune, P., Cobbold, P. R., Mahjoub, Y„ and bracketed between ~60 and 50 Ma. In the (3) Deformation between 65 and 48 Ma is Marquer, D., 1987, Bulk kinematics from shear zone patterns: Some field examples: Journal of Structural Geology, v. 9, p. 635-646. RLFZ, reverse slip in the southern Gabriel Peak well documented in the southern segment of the Geary, E. E., and Christiansen, D., 1989, Trace element geochemistry and wAr/39Ar geochronology of mafic amphibolites from the Twisp Valley tectonic belt was followed closely by the oblique RLFZ. Schist and Twisp River-Foggy Dew fault zone. North Cascades: Geo- slip in the FDFZ. The near coincidence of this (4) The RLFZ records a transitional tectonic logical Society of America Abstracts with Programs, v. 21, no. 5, p. 83. Hamilton, W., 1978, Mesozoic tectonics of the western United States, in transition with that in the Omineca belt is in- regime during the Paleogene between extension Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western United States: Society of Economic Paleontologists and triguing, although the transition in the RLFZ to the east and transcurrent motion to the west. Mineralogists, Pacific Coast Paleogeographic Symposium, 2nd, may record a localized effect (for example, mi- (5) Early Cenozoic magmatism, metamor- p. 33-70. Hanmer, S., 1986, Asymmetrical pull-aparts and foliation fish as kinematic gration of restraining and releasing bends) of the phism, and ductile deformation strongly over- indicators: Journal of Structural Geology, v. 8, p. 111 -122. 1987, Textural map units in quartzo-feldspathic mylonitic rocks: geometry of the strike-slip system. Latest motion printed any evidence of major Cretaceous Canadian Journal of Earth Sciences, v. 4, p. 2065-2073. on the Foggy Dew fault and FDFZ, and still Haugerud, R. A., 1985, Geology of the Hozameen Group and the Ross Lake movement in the RLFZ proposed by earlier tec- shear zone, Maselpanik area, North Cascades, southwestern British younger normal slip in the RLFZ in Canada tonic models. Columbia [Ph.D. thesis]: Seattle, Washington, University of Washing- ton, 263 p. (Haugerud, 1985), was probably contempo- Haugerud, R. A.. Tabor, R. W„ Stacey, J., and Van der Hayden, P., 1988, raneous with motion on west-dipping normal What is in the core of the North Cascades, Washington? A new look at ACKNOWLEDGMENTS the structure and metamorphic history of the Skagit Gneiss of Misch faults to the east, such as occur on the west side (1966): Geological Society of America Abstracts with Programs, v. 20, of the Okanogan dome (Templeman-Kluit and no. 3, p. 168. We are grateful for discussions and con- Hopkins, W. N., 1987, Geology of the Newby Group and adjacent units in the Parkinson, 1986; Orr and Cheney, 1987). southern Methow trough, northeast Cascades, Washington [M.S. structive criticisms of the manuscript by thesis]: San Jose, California, San Jose State University, 95 p. E. Brown, D. Cowan, R. Haugerud, M. Mc- Hoppe, W. J., 1984, Origin and age of the Gabriel Peak orthogneiss, North Paleocene(?) to middle Eocene dextral strike Cascades, Washington [M.S. thesis]: Lawrence, Kansas, University of slip occurred on several northwest-striking faults Groder, D. Parkinson, S. Paterson, R. Tabor, Kansas, 79 p.

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