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Kinematics of the northern : An incipient transform along the Pacific– boundary

James E. Faulds  Christopher D. Henry  Bureau of Mines and , MS 178, University of Nevada, Reno, Nevada 89557, USA Nicholas H. Hinz 

ABSTRACT GEOLOGIC SETTING In the western of , a system of dextral faults accommodates As western North America has evolved 15%±25% of the Paci®c±North American plate motion. The northern Walker Lane in from a convergent to a transform margin in northwest Nevada and northeast occupies the northern terminus of this system. the past 30 m.y., the northern Walker Lane has This young evolving part of the plate boundary offers insight into how strike-slip fault undergone widespread volcanism and tecto- systems develop and may re¯ect the birth of a . A belt of overlapping, left- nism. Tertiary volcanic strata include 31±23 stepping dextral faults dominates the northern Walker Lane. Offset segments of a W- Ma ash-¯ow tuffs associated with the south- trending Oligocene paleovalley suggest ϳ20±30 km of cumulative dextral slip beginning ward-migrating `` ¯are up,'' 22±5 ca. 9±3 Ma. The inferred long-term slip rate of ϳ2±10 mm/yr is compatible with global Ma calc-alkaline intermediate-composition positioning system observations of the current strain ®eld. We interpret the left-stepping rocks related to the ancestral Cascade arc, and faults as macroscopic Riedel shears developing above a nascent lithospheric-scale trans- 13 Ma to present bimodal rocks linked to Ba- form fault. The strike-slip faults end in arrays of ϳN-striking normal faults, suggesting sin and Range extension (Stewart, 1988; Best that dextral diffuses into extension in the Great Basin. Coeval extension and dextral et al., 1989; Christiansen and Yeats, 1992). In shear have induced slight counterclockwise fault-block rotations, which may ultimately the past 13±3 m.y., coincident with the north- rotate Riedel shears toward the main at depth, thus facilitating development ward migration of the Mendocino triple junc- of a throughgoing strike-slip fault. tion and associated termination of , arc volcanism retreated northwestward, Basin Keywords: Walker Lane, Nevada, transform fault, Riedel shear, paleovalley. and Range normal faulting advanced west- ward (Dilles and Gans, 1995; Henry and Per- INTRODUCTION possibly the least developed and youngest part kins, 2001; Surpless et al., 2002), and strike- The western margin of North America con- of the transform boundary. slip faulting began in the northern Walker tains a broad zone of distributed shear extend- In this paper we utilize the youthfulness of Lane. ing from the system to the the northern Walker Lane to assess how strike- The ignimbrite ¯are up is important to un- (Fig. 1; Wernicke, slip fault systems develop. Our new geologic raveling northern Walker Lane evolution be- 1992; Atwater and Stock, 1998). Global po- mapping, structural analysis, and geochronol- cause thick sequences of ash-¯ow tuffs, erupt- sitioning system (GPS) geodetic data indicate ogy (Faulds et al., 2002; Henry et al., 2003, ed from calderas in central Nevada (Best et that a system of dextral faults in the western 2004) de®ne the geometry and kinematics of al., 1989; John, 1995), were deposited in pa- Great Basin, known as the Walker Lane in the the northern Walker Lane and provide the ®rst leovalleys that extended across western Ne- north (Stewart, 1988) and the eastern Califor- detailed constraints on the timing and magni- vada and California (Lindgren, 1911; Henry nia shear zone in the south (Dokka and Travis, tude of dextral displacement across this re- et al., 2003). Our work has shown that the 1990), accommodates 15%±25% of the dex- gion. We develop a kinematic model for this paleovalleys contain distinctive 31±23 Ma tral motion between the North American and youthful system, which applies previously sections (Fig. 2) that re¯ect the southward Paci®c plates (Thatcher et al., 1999; Bennett published clay models to a macroscopic scale. progression of magmatism and provide pierc- et al., 2003). This fault system merges south- This kinematic model may have implications ing lines with which to gauge dextral offset. ward with the San Andreas fault in southern for incipient intracontinental strike-slip faults The southward migration of volcanism is crit- California and terminates northward near the in many tectonic settings. ical, because ash-¯ow tuffs that ®ll paleoval- south end of the Cascade arc. The Walker Lane essentially accommodates dextral mo- tion of the block relative to the central Great Basin. Because the Walker Lane accommodates a signi®cant fraction of Paci®c±North American plate motion and the San Andreas fault system has been growing northward (Atwater and Stock, 1998), it follows that the Walker Lane may also be propagating northwestward. Es- timates of cumulative dextral offset across the shear zone and Walker Lane since time are ϳ50±100 km in southern California (Dokka and Travis, 1990), 60±75 km in west-central Nevada (Oldow, Figure 1. Cenozoic tectonic setting, western North America (after Atwater and Stock, 1998). 1992), and essentially zero at its northwest ter- San Andreas fault system has progressively lengthened in past 30 m.y., as more of Paci®c minus. Thus, the northern Walker Lane in plate has come into contact with North America. MTJÐMendocino triple junction; F.Z.Ð northwest Nevada and northeast California is zone. In map on right, box surrounds study area.

᭧ 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; June 2005; v. 33; no. 6; p. 505±508; doi: 10.1130/G21274.1; 3 ®gures; Data Repository item 2005093. 505 leys in the northern Walker Lane are distinct from generally younger sections farther south. Thin gravels are commonly sandwiched be- tween tuffs and indicate deposition in a ¯uvial system ¯owing from volcanic highlands in central Nevada westward to the Paci®c Ocean. Uplift of the Sierra Nevada relative to the Great Basin had not yet occurred. In western Nevada and northeast California, ignimbrite volcanism terminated by 22 Ma, prior to Walker Lane development. Thus, offset Oli- gocene paleovalleys record cumulative dis- placement across the northern Walker Lane. The northern Walker Lane consists of ki- nematically linked systems of NW-striking, left-stepping dextral faults, N-striking normal faults, and subordinate ENE-striking sinistral faults. From east to west, major dextral faults are the Pyramid , Warm Springs Valley, Figure 2. A: Shaded boxes indicate presence of tuff in paleovalley segment (see Fig. 3B for Honey Lake, and Mohawk Valley faults (Fig. locations and abbreviation de®nitions; ages are in millions of years). B: Northern margin 3B). In the west, major dextral faults merge (dashed line) of paleovalley, Nightingale Range (looking west). C: Schematic paleovalley ®ll at Dogskin , with ages corresponding to individual tuffs. southward with a system of E-dipping normal faults, which coalesce southward to form the

Figure 3. A: Walker Lane in relation to Sierra Nevada and western Great Basin. Box surrounds study area. Strike-slip fault system is progressively better organized to southeast, concomitant with increasing dextral displacement. B: Northern Walker Lane showing left- stepping, NW-striking dextral faults and offset paleovalley axes. Paleovalleys are grouped into three sets. Individual segments in each set are inferred to represent originally continuous paleovalley. DMÐDiamond ; DSÐDogskin Mountain; FSÐFort Sage Moun- tains; HLFÐHoney Lake fault; HPÐHaskell Peak; LRÐLake Range; MVFÐMohawk Valley fault; NRÐNightingale Range; PLFÐPyramid Lake fault; PMÐPeterson Mountain; PRÐPah Rah Range; SLÐSeven Mountain; SVÐSierra Valley; VMÐVirginia Mountains; WSFÐWarm Springs Valley fault. C: Riedel shear model for northern Walker Lane and analogous clay model from Wilcox et al. (1973). Coeval northwest-directed dextral shear and west-northwest extension may account for slight counterclockwise rotation of fault blocks that collapse in domino-like fashion to accommodate extension.

506 GEOLOGY, June 2005 Sierra Nevada frontal fault system. To the of dextral offset across the Warm Springs Val- approximately east-west extension initiated east, the fault splays northward ley fault. On the northwest ¯ank of Dogskin basin development at 13 Ma (Trexler et al., into the W-dipping Lake and Nightingale Mountain (Fig. 3B; segment 1C), Ͼ500 m of 2000), evidence is lacking for strike-slip fault- Range normal faults. This discrete belt of dex- 31.3±24.9 Ma ash-¯ow tuffs (16 units) pinch ing during early stages of extension. More- tral faults gives way northwestward to a dif- out against the southeast margin of a WSW- over, 3.5 Ma sedimentary rocks along the fuse zone of widely spaced NW-striking dex- trending paleovalley. Also, a distinctive rock- Warm Springs Valley fault are as highly de- tral faults and lineaments that extends to the avalanche megabreccia derived from nearby formed as Oligocene tuffs. Also, clockwise southern Cascade arc (Fig. 3A; Grose, 2000). rhyolite domes overlies a 25.1 Ma tuff in both vertical-axis rotation just west of the Carson Paleomagnetic data indicate slight (ϳ15Њ) the and at Dogskin Sink began ca. 9±5 Ma (Cashman and Fon- counterclockwise rotation in much of the Mountain. taine, 2000). Considering these relationships, northern Walker Lane in contrast to 35Њ±44Њ Oligocene ash-¯ow tuffs are distributed we infer that the onset of strike-slip faulting of clockwise rotation farther south near the over a 20 km north-south width between the propagated northwestward in the northern Carson Sink (Cashman and Fontaine, 2000). Pyramid Lake and Warm Springs Valley faults Walker Lane, beginning as early as 9 Ma in (Fig. 3B; segments 1B, 2B). Mesozoic paleo- the southeast and as late as 3 Ma in the north- CONSTRAINTS ON STRIKE-SLIP ridges in the suggest that this west. Subsequent movement on strike-slip and FAULTING greater width, relative to the Nightingale normal faults has been broadly coeval, as ev- Well-de®ned axes and/or margins of Oli- Range and Dogskin Mountain, results from idenced by Quaternary fault scarps and seis- gocene paleovalleys, extending from the the con¯uence of two major tributaries in the micity (Ichinose et al., 1998). Nightingale Range in Nevada to the Diamond paleovalley system. In summary, inferred offsets of an Oligo- Mountains in California, provide piercing Because of thick Neogene basin ®ll in the cene paleovalley system indicate ϳ20±30 km lines with which to estimate dextral offset Honey Lake basin, displacement on the Honey of cumulative dextral displacement, as mea- 40 39 (Fig. 3B). Petrography and Ar/ Ar geochro- Lake fault is more dif®cult to constrain. How- sured orthogonal to the northern Walker Lane nology demonstrate that all segments of this ever, the southern margin of a paleovalley at (Fig. 3B). The inferred magnitude and timing paleovalley system contain a similar, west- Peterson Mountain correlates with that at of deformation suggest long-term slip rates of ward-thinning sequence of 31.3±23.5 Ma ash- Dogskin Mountain, indicating 3±6 km of dex- ϳ2±10 mm/yr, which is compatible with GPS ¯ow tuffs (Fig. 2A and Data Repository Ap- tral offset along the southeastern Honey Lake data from the northern Walker Lane (e.g., 1 pendix DR1 , which includes analytical data fault. A northern branch of the paleovalley Bennett et al., 2003). and sample locations). This sequence includes system is marked by similar 200±250-m-thick the widespread 25.3 Ma Nine Hill Tuff sections of ash-¯ow tuff in the Fort Sage and DISCUSSION (Deino, 1985). The westward thinning of tuffs north-central Diamond Mountains (Fig. 3B; The en echelon dextral-fault pattern, mini- re¯ects greater distance from source calderas segments 3A and 3B). Although the Honey mal offset (5±15 km) on individual faults, and (Fig. 3A), not relative uplift and of the Lake basin may obscure additional tuff de- relatively small cumulative displacement in- Sierra Nevada and related blocks, because posits, the lack of other signi®cant tuff out- dicate that the northern Walker Lane is an in- Miocene volcanic rocks, which predate uplift crops in the Diamond Mountains suggests a cipient strike-slip fault system. Although at these latitudes (Trexler et al., 2000; Henry correlation between these paleovalley seg- and Perkins, 2001), generally cap the Oligo- small left steps on individual faults have in- ments. If correct, the central part of the Honey duced local shortening, the broad left steps be- cene tuffs. Lake fault accommodated 10±15 km of dex- East of the Pyramid Lake fault, the Night- tween major faults accommodate little, if any, tral offset of a W-trending paleovalley. As the ingale Range contains a WSW-trending, ϳ10- shortening and are unlike typical restraining Honey Lake fault zone dies out to the south- km-wide paleovalley with an abrupt northern bends. The left-stepping, en echelon geometry east, part of the displacement may be taken up margin, where the entire Ͼ700-m-thick se- instead resembles patterns of primary Riedel by folding in a restraining bend at Seven quence of 11 tuffs pinches out (Figs. 2B and shears (Petit, 1987) developed above strike- Lakes Mountain and major E-dipping normal 3B; segment 1A). The southern , slip faults in clay models (Fig. 3C; e.g., Wil- faults (Fig. 3B). The westward continuation of however, contains only a thin veneer (Ͻ30 m cox et al., 1973). The macroscopic Riedel the main-stem paleovalley may underlie Sierra ϳ thick) of Oligocene tuff, indicating that the shears in the Walker Lane terminate in N- Valley (Fig. 3B; segment 1D), as suggested by westward continuation of the paleovalley pro- striking normal fault systems, thereby trans- a 300-m-thick sequence of correlative tuff jects south of the range. West of the Pyramid ferring dextral shear to west-northwest exten- west of Sierra Valley at Haskell Peak (Brooks Lake fault in the Virginia Mountains, the sion in the northern Great Basin. The diffuse northern part of a WSW-trending paleovalley et al., 2003). zone of faulting at the northwest terminus of contains an ϳ600-m-thick sequence of 15 ash- The westernmost strand of the northern the Walker Lane in northeast California mim- ¯ow tuffs (Fig. 3B; segment 1B). Nearly iden- Walker Lane, the Mohawk Valley fault, has ics clay models (e.g., An and Sammis, 1996) tical tuff sequences in the Nightingale Range accommodated little dextral offset. Here, pa- involving broadly distributed shear with no and Virginia Mountains indicate correlation of leovalleys are obscure owing to scant expo- throughgoing fault at depth. In this early the respective paleovalley segments. On the sure of Oligocene tuff. However, a contact be- stage, the minor NW-striking faults are also basis of the westerly trend, this paleovalley tween Cretaceous granite and Paleozoic primary Riedel shears, but shear strain has not Ͻ has been offset 5±10 km across the Pyramid shows little ( 1 km) dex- exceeded a threshold necessary to induce a Lake fault. tral offset across the fault (Fig. 3B; Saucedo continuous fault at depth. Evidence for this Similar relationships indicate ϳ10±15 km and Wagner, 1992). earlier stage abounds in northwest Nevada, as Several features suggest a relatively recent blocks between major strike-slip faults com- 1GSA Data Repository item 2005093, Appendix onset of strike-slip faulting in the northern monly contain minor inactive NW-striking DR1, ages and ash-¯ow tuff correlation, is available Walker Lane. For example, Oligocene tuffs dextral faults (Faulds et al., 2002). We there- online at www.geosociety.org/pubs/ft2005.htm, or and Miocene volcanic rocks are equally tilted, fore conclude that the northern Walker Lane on request from [email protected] or Docu- ments Secretary, GSA, P.O. Box 9140, Boulder, CO suggesting little deformation during the ig- records two early stages in the evolution of an 80301-9140, USA. nimbrite ¯are up and arc volcanism. Although intracontinental strike-slip fault system. Stage

GEOLOGY, June 2005 507 one comprises a broad zone of widely spaced western : An update: Internation- 1998, Moment tensor solutions of the 1994 to Riedel shears in northeast California. Stage al Geology Review, v. 40, p. 375±402. 1996 Double Spring Flat, Nevada, Bayasgalan, A., Jackson, J., Ritz, J.-F., and Carre- sequence and implications for local tectonic two, in northwest Nevada, consists of a nar- tier, S., 1999, Field examples of strike-slip models: Seismological Society of America rower belt of overlapping Riedel shears, which fault terminations in Mongolia and their Bulletin, v. 88, p. 1363±1378. may indicate a throughgoing dextral shear tectonic signi®cance: , v. 18, Jackson, J., Haines, J., and Holt, W., 1995, The ac- zone in the upper and/or lower . p. 394±411. commodation of Arabia- plate conver- Bennett, R.A., Wernicke, B.P., Niemi, N.A., Fried- gence in Iran: Journal of Geophysical Re- This stepwise development may characterize rich, A.M., and Davis, J.L., 2003, Contem- search, v. 100, p. 15,205±15,219. other incipient strike-slip fault systems (e.g., porary strain rates in the northern Basin and John, D.A., 1995, Tilted middle Tertiary ash-¯ow Taymaz et al., 1991; Jackson et al., 1995; Range province from GPS data: Tectonics, calderas and subjacent granitic plutons, south- v. 22, doi: 10.1029/2001TC001355. ern Stillwater Range, Nevada: Cross sections Bayasgalan et al., 1999), including those at of an Oligocene igneous center: Geological transform, oblique, and convergent plate Best, M.G., Christiansen, E.H., Deino, A.L., Grom- meÂ, C.S., McKee, E.H., and Noble, D.C., Society of America Bulletin, v. 107, p. 180±200. boundaries. 1989, through Miocene volcanism in Lindgren, W., 1911, The Tertiary gravels of the Si- The slight counterclockwise rotation in the the Great Basin of the : erra Nevada of California: U.S. Geological northern Walker Lane is opposite the typical New Bureau of Mines and Mineral Survey Professional Paper 73, 226 p. clockwise rotation in dextral shear zones. In Resources Memoir 47, p. 91±133. Oldow, J.S., 1992, Late Cenozoic displacement par- the transtensional setting of the northern Brooks, E.R., Wood, M.M., Boehme, D.R., Potter, titioning in the northwestern Great Basin, in K.L., and Marcus, B.I., 2003, Geologic map Walker Lane, coeval dextral shear in the left- Stewart, J., ed., Structure, tectonics and min- of the Haskell Peak area, Sierra County, Cal- eralization of the Walker Lane: Walker Lane stepping fault system and west-northwest re- ifornia: California Geological Survey Map Symposium proceedings volume: Reno, Geo- gional extension may account for the counter- Sheet 55, scale 1:12,000. logical Society of Nevada, p. 17±52. intuitive counterclockwise rotation (Fig. 3C). Cashman, P.H., and Fontaine, S.A., 2000, Strain Petit, J.P., 1987, Criteria for the sense of movement partitioning in the northern Walker Lane, This sense of rotation would ultimately rotate on fault surfaces in brittle rocks: Journal of western Nevada and northeastern California: , v. 9, p. 597±608. Riedel shears toward the orientation of the Tectonophysics, v. 326, p. 111±130. Saucedo, G.J., and Wagner, S.L., 1992, Geologic main shear zone at depth, thus facilitating de- Christiansen, R.L., and Yeats, R.S., 1992, Post Lar- map of the Chico quadrangle: California Di- velopment of a throughgoing upper-crustal amide geology of the U.S. Cordilleran , vision of Mines and Geology, Regional Geo- strike-slip fault. As such a fault develops and in Burch®el, B.C., et al., eds., The Cordilleran logic Map Series, map no. 7A, scale orogen: Conterminous U.S.: Boulder, Colora- 1:250,000. the strike-slip fault system matures, the sense do, Geological Society of America, Geology Stewart, J.H., 1988, Tectonics of the Walker Lane of vertical-axis rotation may reverse and be- of North America, v. G-3, p. 261±406. belt, western Great Basin: Mesozoic and Ce- come compatible with the overall shear strain. Deino, A.L., 1985, Stratigraphy, chemistry, K-Ar nozoic deformation in a zone of shear, in Ernst, W.G., ed., The geotectonic development The apparent change from counterclockwise dating, and paleomagnetism of the Nine Hill Tuff, California±Nevada: Miocene/Oligocene of California: Englewood Cliffs, New Jersey, to clockwise rotations southeastward in the ash-¯ow tuffs of Seven Lakes Mountain, Prentice-Hall, p. 683±713. northern Walker Lane (Cashman and Fontaine, California±Nevada: Improved calibration Surpless, B.E., Stockli, D.F., Dumitru, T.A., and 2000) may re¯ect such a progression. Such methods and error estimates for potassium- Miller, E.L., 2002, Two-phase westward en- 40±argon-40 dating of young rocks [Ph.D. croachment of Basin and Range extension into complex kinematics may characterize the ear- the northern Sierra Nevada: Tectonics, v. 21, ly evolution of strike-slip fault systems in both thesis]: Berkeley, University of California, 457 p. doi: 10.1029/2000TC001257. transtensional and transpressional settings. Dilles, J.H., and Gans, P.B., 1995, The chronology Taymaz, T.J., Jackson, J., and McKenzie, D., 1991, Active tectonics of the north and central Ae- With the eventual northward migration of of Cenozoic volcanism and deformation in the gean Sea: International Geophysical Journal, Yerington area, western Basin and Range and the Mendocino triple junction to the Oregon v. 106, p. 433±490. Walker Lane: Geological Society of America coast, the Walker Lane may ultimately afford Thatcher, W., Foulger, G.R., Julian, B.R., Svarc, J., Bulletin, v. 107, p. 474±486. Quilty, E., and Bawden, G.W., 1999, Present- a more stable con®guration for the Paci®c± Dokka, R.K., and Travis, C.J., 1990, Late Cenozoic North American plate boundary. Notably, the day deformation across the Basin and Range strike-slip faulting in the , Cal- province, western United States: Science, San Andreas fault has a history of stepping ifornia: Tectonics, v. 9, p. 311±340. v. 283, p. 1714±1718. inland (Atwater and Stock, 1998). The Walker Faulds, J.E., dePolo, C.M., and Henry, C.D., 2002, Trexler, J.H., Cashman, P.H., Henry, C.D., Muntean, Lane may therefore re¯ect the birth of a Preliminary geologic map of the Sutcliffe T., Schwartz, K., TenBrink, A., Faulds, J.E., Quadrangle, Washoe County, Nevada: Nevada lithospheric-scale transform fault and presage Perkins, M., and Kelly, T., 2000, Neogene ba- Bureau of Mines and Geology Open-File Re- sins in western Nevada document the tectonic a signi®cant eastward jump in the plate port 03-17, scale 1:24,000. history of the Sierra Nevada±Basin and Range boundary. Grose, T.L.T., 2000, Volcanoes in the Susanville re- transition zone for the last 12 Ma, in Lageson, gion, Lassen, Modoc, Plumas Counties, north- D.R., et al., eds., Great Basin and Sierra Ne- eastern California: California Geology, v. 53, vada: Boulder, Colorado, Geological Society ACKNOWLEDGMENTS p. 4±23. of America Field Guide 2, p. 97±116. This work was supported by the National Science Henry, C.D., and Perkins, M.E., 2001, Sierra Wernicke, B., 1992, Cenozoic Foundation (grant EAR-0124869) and STATEMAP Nevada±Basin and Range transition near of the U.S. Cordillera, in Burch®el, B.C., et Program of the U.S. Geological Survey. We thank Reno, Nevada: Two-stage development at 12 al., eds., The Cordilleran orogen: Contermi- Joann Stock, Ben van der Pluijm, and an anony- and 3 Ma: Geology, v. 29, p. 719±722. nous U.S.: Boulder, Colorado, Geological So- mous referee for helpful reviews and Patricia Cash- Henry, C.D., Faulds, J.E., Garside, L.J., and Hinz, ciety of America, , man and John Oldow for fruitful discussions. N.H., 2003, Tectonic implications of ash-¯ow v. 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