Published online May 20, 2010; doi:10.1130/B30009.1 Quaternary reactivation of the Kern Canyon fault system, southern Sierra Nevada, California Elisabeth S. Nadin† and Jason B. Saleeby Division of Geological and Planetary Sciences, California Institute of Technology, MS 100-23, Pasadena, California 91125, USA ABSTRACT INTRODUCTION the Isabella basin (Fig. 2), is a geographically signifi cant location for defi ning Kern Canyon The Kern Canyon fault, the longest fault The Sierra Nevada batholith of California is fault geometry. South of the Isabella basin, the in the southern Sierra Nevada, is an active one of the world’s largest batholiths, and was Kern Canyon fault has been interpreted to con- structure and has been reactivated at discrete assembled by multiple intrusive events largely nect with the Breckenridge scarp and the White times over the past ~100 m.y. in response to during Cretaceous time. The mountain range Wolf fault (Fig. 1; e.g., Ross, 1986). Across this changing lithospheric stresses. After initiation and westward-adjacent Great Valley, fi lled northeast-striking segment of the fault zone, as a Cretaceous transpressional structure, largely with sediments eroded from the batho- brittle deformation is localized along discrete the Kern Canyon fault transitioned into a lith, together constitute a semi-rigid crustal fractures across an ~150-m-wide zone at the dextral strike-slip shear zone that remained block termed the Sierra Nevada microplate, currently exposed surface. This segment devel- active as it was exhumed into the brittle re- whose velocity and rotation vectors differ from oped ca. 86 Ma as a plastic-to-brittle structure gime during regional Late Cretaceous uplift those of the rest of North America (Argus and during exhumation of the southernmost part of of the Sierra Nevada batholith. The Kern Gordon, 1991; Dixon et al., 2000; Sella et al., the batholith (Nadin and Saleeby, 2008; Saleeby Canyon fault was reactivated during Miocene 2002). This “microplate” is bounded to the et al., 2009). North of the Isabella basin, brittle regional extension as part of a transfer zone west by the San Andreas transpressive plate structures of the Kern Canyon fault overprint between two differentially extending domains junction. To the east are the Eastern Califor- the proto–Kern Canyon fault (labeled PKCF in in the southern Sierra Nevada. Subsequent nia Shear Zone and its northward continuation, Fig. 1), a 2-km-wide zone of pervasively duc- normal displacement along the fault began in the Walker Lane (Fig. 1), which together form tilely sheared igneous and metamorphic rocks. Pliocene time. New evidence for fault activity, the western boundary of the Basin and Range The proto–Kern Canyon fault was a transpres- which continued into late Quaternary time, extensional province (e.g., Jones et al., 2004). sive structure that accommodated arc-normal includes its current geomorphic expression Geodetic data show that the Eastern California shortening and dextral offset from 100 to 80 Ma as a series of meters-high, west-side-up scarps Shear Zone–Walker Lane belt is undergoing (Nadin and Saleeby, 2008). that crop out discontinuously along the fault’s dextral shear that accounts for ~20% of total Part of the Kern Canyon fault system near the 130-km length. Relocated focal mechanisms relative motion between the North American latitude of the Isabella basin was remobilized as of modern earthquakes confirm ongoing and Pacifi c plates (Dokka and Travis, 1990; an early to middle Miocene oblique-slip transfer normal faulting, and geodetic measure ments Thatcher et al., 1999; Dixon et al., 2000; Gans zone between two extensional domains, one at suggest that the Sierra Nevada is uplifting et al., 2000; Bennett et al., 2003; Oldow et al., the southernmost end of the San Joaquin basin relative to the adjacent valleys. This evidence 2008; Hammond and Thatcher, 2007). and the other just east of the Isabella basin (see for recent activity overturns a long-held Many studies have shown that there is active Fig. 2 for landmarks; see fi g. 8 of Mahéo et al., view that the Kern Canyon fault has been faulting in the eastern Sierra Nevada range front 2009, for extensional domains). The Brecken- in active for more than 3.5 m.y. Its reactiva- (recent selected references: Le et al., 2007; ridge scarp and White Wolf fault segments of tion indicates that deformation repeatedly Surpless , 2008; Taylor et al., 2008). There have the Kern Canyon fault system lie between these localized along a preexisting crustal weak- also been predictions of extensional defor- extensional domains and are undergoing Qua- ness, a Cretaceous shear zone. We propose mation within the southern part of the Sierra ternary offset, with the notable 1952 M = 7.3 that a system of inter related normal faults, Nevada batholith (Jones et al., 2004) and, more Kern County earthquake attributed to the White including the Kern Canyon fault, is respond- recently, evidence for uplift of the southern Wolf fault. However, focal mechanisms along ing to mantle lithosphere removal beneath Sierra Nevada relative to the Owens Valley to the Breckenridge scarp, the White Wolf fault, the southern Sierra Nevada region. The loca- the east (Fay et al., 2008; see Fig. 1 for location). and the Kern Canyon fault segment north of tion of the active Kern Canyon fault within This study presents the fi rst documentation of the Isabella basin indicate substantially differ- the Sierra Nevada–Great Valley microplate active extensional structures within the range ent styles of modern deformation, which we de- indicates that deformation is occurring within that are consistent with the predicted uplift. scribe in this paper. the microplate. We defi ne the Kern Canyon fault system as The Kern Canyon fault has long been consid- an ~130-km-long zone of faulted basement, ered inactive because a 3.5 Ma lava fl ow (Dal- with several parallel, near-vertical fault strands rymple, 1963) was reported to cap its northern † E-mail: [email protected]; Current address: end (Webb, 1936). Our recent fi eld investiga- Department of Geology and Geophysics, University that strike generally northward from the south- of Alaska Fairbanks, Fairbanks, Alaska 99775-5780, ern edge of the Sierra Nevada along the batho- tions reveal that the basalt is pervasively frac- USA. lith’s long axis (Fig. 1). Latitude 35.7°N, at tured and measurably displaced along the trace GSA Bulletin; September/October 2010; v. 122; no. 9/10; p. 1671–1685; doi: 10.1130/B30009.1; 9 fi gures; 2 tables. For permission to copy, contact [email protected] 1671 © 2010 Geological Society of America Nadin and Saleeby Lake Tahoe CA SSierra Nevada batholith i WalkerW Lane e r a NV r l a k e Pacific Ocean N r e Mono Lake v Figure 1. Regional tectonic a L d a framework, with features dis- a n e cussed in the text. The southern b a t Sierra Nevada batholith and h o westward adjacent San Joaquin San Joaquin Valle l EasternE California Shear Zo 37° 38° 39° N i t a Valley host the main features h s t e discussed: the proto–Kern Can- San r Mesozoic OV n yon, Kern Canyon, Brecken- C intrusive rocks a l ridge, and White Wolf faults. i f Also of note are the Walker Paleozoic and Mesozoic SNFF o (P)KCF IWV r metasedimentary- n Lane–Eastern California Shear y i Andreas a metavolcanic rocks KCF Zone belt of seismicity and the S Bakersfield h BF F e Sierra Nevada frontal fault. OV - Owens Valley a PKC r The box outlines the region IWV - Indian Wells Valley WWF 35° 36° Z o shown in Figures 2, 3, 7, and 8. Garlock Mojavefault Desert nne (P)KCF - (Proto) Kern Canyon fault e KCF - Kern Canyon fault fault BF - Breckenridge fault WWF - White Wolf fault 34° SNFF- Sierra Nevada frontal fault 125° W124° 123° 122° 121° 120° 119° 118° 117° 116° of the fault. Our geomorphic and structural stud- northwest of the Isabella basin (Jones et al., dipping normal faults that (1) place Cretaceous ies, presented here, are used to estimate Quater- 1994; surface projection outlined in Fig. 2) granitic bedrock of the footwall against Quater- nary offset along the Kern Canyon fault system. that has been interpreted to result from the re- nary alluvium of the hanging wall, and (2) cut These are distinguished from late Cenozoic ver- moval of ~106 km3 of dense eclogitic material and offset a 3.5 Ma lava fl ow. The scarps off- tical offsets within the southernmost 200 km of from beneath the southern Sierra Nevada batho- set Pliocene volcanic deposits and Quaternary the Sierra Nevada batholith (Fig. 3), which were lith (e.g., Ducea and Saleeby, 1998; Jones et al., sedimentary deposits and are classifi ed here as revealed by low-temperature thermo chrono- 2004). The presence of extensional structures in young and potentially active structures of the metric studies by Mahéo et al. (2009). Persis- the vicinity of the Isabella anomaly supports pre- Kern Canyon fault system. We distinguish them tent seismicity associated with the Kern Canyon dictions that Pliocene removal of dense eclogitic from the ca. 100 Ma damage zone along which fault system and the contiguous Breckenridge material should increase extensional strain in the they localized, and assess their potential geo- scarp and White Wolf fault (Ross, 1986), as well area (e.g., Jones et al., 2004). This paper docu- dynamic signifi cance. We also present a synthe- as contemporary strain-fi eld models derived in ments fault activity in what was considered a sis of seismic studies to show that normal-fault part from seismic studies (Bawden et al., 1997; tectonically quiescent region between the active earthquakes are associated with continued activ- Unruh and Hauksson, 2009), are also presented strike-slip San Andreas plate boundary and the ity along these scarps.