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 thermochrono- 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. Finally, we interpret the and interpreted here to indicate late Quaternary actively extending Basin and Range province. reactivation of this fault system in the context of deformation. The high relief of the Kern Can- In addition to being geologically signifi cant, delamination of mantle lithosphere beneath the yon fault scarps (despite high precipitation in the potential continued activity of the Kern southernmost Sierra Nevada. the region), as well as a 3.3 ka charcoal layer Canyon fault is of societal importance: the U.S. in ruptured Quaternary alluvium just south of Army Corps of Engineers is currently assessing GEOLOGIC SETTING the Isabella basin (U.S. Army Corps of Engi- the need to reconstruct a dam that crosses the neers, Lake Isabella Dam Situation Report, fault at Engineer Point at the southern end of Several styles and stages of fault-related January 9, 2009; Hunter et al., 2009), places the Isabella basin (location 3 in Fig. 2). Should deformation overprint one another in the Kern the Kern Canyon fault activity described in this the dam fail, it is projected that Bakersfi eld Canyon fault damage zone. A key to under- paper well into the Holocene epoch. (Figs. 1 and 2), a city of 800,000 people that lies standing the geologic history of the fault system Punctuated fault activity over 100 m.y. and the 80 km downstream of Lake Isabella, would be is to distinguish the structures that are spatially Neogene and Quaternary remobilizations high- fl ooded within three hours. coincident but differ temporally and kinemati- light the importance of the Kern Canyon fault In this paper, we present new evidence cally. The Kern Canyon fault is a structural zone system in the tectonic evolution of the south- documenting late Quaternary scarps along the in which granitic rocks and metasedimentary western United States. The “Isabella anomaly” Kern Canyon fault system. These scarps show wall rocks consisting of marble, amphibolite, is a high-velocity body in the upper mantle west-side-up displacements along steeply east- quartzite, and phyllite are highly fractured,

1672 Geological Society of America Bulletin, September/October 2010 Quaternary reactivation of the Kern Canyon fault system, southern Sierra Nevada, California

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Bakersfield KCF scarps on downthrown side mapped in field Little Kern basalt flow KCF trace mapped from aerial images Faults Southern SNB WW = White Wolf N = 30 lineaments mapped B = Breckenridge C from aerial images and E = Edison Outer circle = 33% low-T age offsests Fracture trends KG = Kern Gorge USGS mapped WBM = West Breckenridge Mtn. Quaternary faults PKC = Proto-Kern Canyon

Figure 2. Digital elevation model (DEM, A), fault and lineament map (B), and rose diagram of faults and lineaments (C) of the southern Sierra Nevada. Both the DEM and the fault map show numbered locations of scarps mapped in the fi eld and described in Table 1, as well as the outline of the surface projection of the “Isabella anomaly,” discussed in detail in the text. (A) The DEM is labeled with sparse geo- graphic information so that the reader can examine its topography. (B) The 30 faults and lineaments shown here were mapped in the fi eld and from aerial and advanced spaceborne thermal emission and refl ection radiometer images, and are represented in the rose diagram of (C). USGS—U.S. Geological Survey.

sheared, and altered over a width of ~150 m. For subduction trajectory of the Farallon plate be- 2008). It is ~150 km long and up to 2 km wide. 60 km north of the Isabella basin (Fig. 2, at lati- neath the California region (Busby-Spera and Foliation within the shear zone dips steeply tude 35.7°N), it is a north-south–striking brittle Saleeby, 1990; Tikoff and Greene, 1994; Greene to the west and to the east. North of the Isa- structure that overprints a mylonite zone termed and Schweickert, 1995; Tobisch et al., 1995; bella basin to latitude ~36.1°N, ductile fabrics the proto–Kern Canyon fault (Figs. 1 and 2B; Tikoff and Saint Blanquat, 1997). The proto– strike northward. South of the basin, they fol- Busby-Spera and Saleeby, 1990). Kern Canyon fault is a dextral reverse-slip fault low a slightly sinuous southeastward trajectory The proto–Kern Canyon fault is a Late Cre- exposed along an oblique crustal section that (Fig. 2B) and eventually root into lower-crust taceous structure that, along with several other traverses ~25 km of batholithic emplacement tectonites of the Rand thrust system, exposed ductile shear zones in the central to southern depths from its northern to its southern end immediately north of the Garlock fault (Nadin Sierra Nevada, arose in response to changes in (Pickett and Saleeby, 1993; Nadin and Saleeby, and Saleeby, 2008).

Geological Society of America Bulletin, September/October 2010 1673 Nadin and Saleeby

Saleeby, 2008). In this region, the 2-km-wide SNB Inset zone of mylonitized batholithic and metasedi- map

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Slowly denud Slowly S 2008). Dextral slip continued in the ductile re- KCF gime along the entire length of the proto–Kern Canyon fault until ca. 86 Ma. At this time, exhu- mation of the southernmost end of the batholith Bena arly E brought rocks south of the Isabella basin into the

submarine EF extension brittle regime progressively with fault motion fan Ricardo Group through time (Nadin and Saleeby, 2008). The Kern Canyon fault arose in the southwest Sierra Nevada, along the branch south of the Maricopa WWF

Extent of slab (ca. 16–10 Ma) (ca. subbasin Isabella basin, in response to ca. 86 Ma rapid

window migration k Fault initiation unroofi ng of the batholith. Brittle fabrics of the Tejon Garloc Kern Canyon fault merged with and overprinted platform the fabrics of the proto–Kern Canyon fault north of the Isabella basin ca. 6 Ma later, when that Current distribution of batholith Sinistral strike-slip fault KCF Kern Canyon fault region was exhumed into the brittle regime. WWF White Wolf fault Metamorphic rocks Normal faults; The predominantly brittle Kern Canyon fault is barb on hanging wall EF Edison fault Kern River narrower than the proto–Kern Canyon fault mylonite zone, with ductile-brittle dextral shear localized across a zone of ~150 m width. About Figure 3. Miocene (ca. 16–10 Ma) reconstruction of the southern Sierra Nevada batholith 12 km of brittle dextral slip along the Kern Can- (SNB). As discussed in the text, northward migration of a slab window gave rise to several yon fault is recorded in offset pluton contacts in north-south extensional features within and east of the central and southern SNB. the area of the Isabella basin (Ross, 1986; Nadin and Saleeby, 2008).

The town of Kernville, on the north shore of discrete, meters-wide breaks. The three stages Oligocene–Miocene Dextral Lake Isabella, is the dividing point of the proto– of deformation are particularly well expressed Strike-Slip Reactivation Kern Canyon fault and the Kern Canyon fault at Engineer Point, a peninsula at the southern structural zones. South of latitude 35.7°N, duc- end of the Isabella basin (location 3 in Fig. 2). The Kern Canyon fault system was reacti- tile and brittle fabrics of the Kern Canyon fault Early ductile shear is preserved in a zone of vated during Miocene time after a period of follow a southwesterly route, where they merge S–C mylonites and phyllonites; a later phase early Cenozoic quiescence. Regional evidence with the Breckenridge scarp and, farther south, of brittle shear led to through-going cataclasis; suggests there was broad, continuous extension the White Wolf fault (Figs. 1 and 2B; Ross, and late-stage faulting resulted in thin, hematitic throughout the southern Sierra Nevada and the 1986). North of the basin, the Kern Canyon fault gouge zones (Fig. 4; Chester, 2001). westward-adjacent San Joaquin basin (Fig. 2) continues past the proto–Kern Canyon fault to Where the proto–Kern Canyon fault and the since as early as Oligocene time. Late Oligocene latitude 36.7°N, where it dies out by branching Kern Canyon fault diverge south of the Isa- to early Miocene basin subsidence is associated into several faults with displacements of only bella basin, brittle overprint along the southern with deep marine deposits, northwest-southeast – 0.2–1 km (Moore and du Bray, 1978). branch of the proto–Kern Canyon fault is rare, oriented normal faults, and syntectonic volcanic Along the northern segment of the Kern and early-phase ductile deformation along the fl ows (Bartow, 1991; Goodman and Malin, Canyon fault, north of the Isabella basin where Kern Canyon fault is less pronounced. 1992; Mahéo et al., 2009). In early to middle it coincides with the proto–Kern Canyon fault Miocene time, a distinct phase of growth faulting (Figs. 1 and 2B), deformation appears to have Mesozoic Inception and associated volcanism took place in the Mari- been progressive. Stages of deformation are copa subbasin at the southeast edge of the San seen in an overprinting of the early 2-km-wide In Late Cretaceous time, the proto–Kern Can- Joaquin basin (Fig. 2; MacPherson, 1978; Hirst, mylonite zone fi rst by pervasive brittle fault- yon fault developed along the axial zone of the 1986; Goodman and Malin, 1992; Mahéo et al., ing associated with dextral offset in a fracture southern Sierra Nevada batholith in response to 2009). The southeast Sierra extensional domain, zone that is typically ~150 m wide, followed a change in Farallon plate subduction trajectory stretching from just north of the Isabella basin to by localized fracture and vertical offset along (Busby-Spera and Saleeby, 1990; Nadin and the Tehachapi Mountains (Fig. 2A) also records

1674 Geological Society of America Bulletin, September/October 2010 Quaternary reactivation of the Kern Canyon fault system, southern Sierra Nevada, California

Figure 4. Photograph (A) and line sketch (B) looking down on Miocene–Holocene brittle features overprinting Creta- ceous mylonitic fabric along the Kern Canyon fault at Engi- neer Point (see Fig. 2A, point 3 for the location, and Fig. 6C for an oblique aerial view of the scarp). Mylonitic fabrics of the proto–Kern Canyon fault, as indicated by stretched, aligned mica grains in granodiorite of N (A) and light-gray lines in (B), A B are transposed from an original north-south strike by brittle deformation and rotation as rigid, broken clasts along the Kern Canyon fault. (B) Brittle overprint is characterized here by through-going cataclasis outlined in light gray, a thinner gouge zone shown in dark gray, and hematite-lined fractures shown as black lines. early Miocene extension-related volcanism and sistent with Miocene reactivation. Clark et al. the most pronounced (Fig. 2B). The graben is associated faulting along northwest-striking nor- (2005) and Clark and Farley (2007) determined fi lled with ~2.5 km of coarse Oligocene and mal faults (Saleeby et al., 2009; Mahéo et al., that there were two discrete episodes of acceler- Miocene fanglomerates and basal ashes of the 2009). Motion along the Walker basin fault ated basement uplift and associated river inci- Cache Peak volcanic center (Goodman and zone was accompanied by the emplacement of sion in the southern Sierra Nevada since 20 Ma. Malin , 1992; Mahéo et al., 2009). On going closely spaced dikes sourced from the adjacent mapping has revealed that pieces of Edison Cache Peak volcanic center, whose K/Ar and Pliocene Extension and Westward Tilt graben sediment fi ll are broken and displaced Ar/Ar ages are 16.5–21.4 Ma (Evernden et al., on Pliocene–Quaternary faults that are on strike 1964; Bartow and McDougall, 1984; Coles We mapped 30 north-south–trending linea- with the Kern Gorge fault. There has been et al., 1997). (U-Th)/He apatite ages as young ments in the southern Sierra Nevada from fi eld ~700 m of Quaternary displacement on the as 14.8 Ma from the Walker basin refl ect mid- studies and from examination of aerial imagery. Kern Gorge fault (Mahéo et al., 2009). Miocene resetting due to burial, followed by These are (excluding the White Wolf fault) paral- The Kern Gorge and West Breckenridge possible rapid exhumation from beneath vol- lel to the strike of Mesozoic foliations (Figs. 2B Mountain faults are the most pronounced of canic strata (Mahéo et al., 2009). and 2C). Geomorphic and low-temperature the northwest-striking faults in the southwest The Kern Canyon fault is positioned between thermochronometric data resolve more than 20 Sierra Nevada (Fig. 2B). Between these and the Maricopa subbasin and the southeast Sierra west-side-up faults of ~400 m to >2 km in close the north-south–striking Kern Canyon fault extensional domain, and was fi rst reactivated proximity and parallel to scarps of the Kern Can- system is the northward-widening, wedge- in the brittle regime within a dextral strike-slip yon fault system (Mahéo et al., 2009). These shaped Breckenridge-Greenhorn horst (Fig. 2), transfer zone between these two differentially west-side-up normal displacements impart an which was uplifted in Pliocene–Quaternary extending domains. East-side-up vertical motion ~9° westward tilt to the Sierra Nevada basement time (Mahéo et al., 2009). Uplift of the along the White Wolf segment of the Kern Can- that continues as a comparable westward-tilted Breckenridge-Greenhorn horst and the entire yon fault resulted in the ponding of more than homocline in Tertiary strata of the adjacent east- southeast Sierra Nevada resulted in the termina- 12 km of Neogene sediments within the Mari- ern San Joaquin basin. Because of this temporal tion of growth of a >75 km2 submarine fan, the copa subbasin (Goodman and Malin, 1992). A and structural relation, Mahéo et al. (2009) inter- Bena fan (e.g., MacPherson, 1978; Webb, 1981; (U-Th)/He apatite study of exhumed Walker preted that the offsets occurred on related faults Dibblee and Warne, 1988), which was depos- basin rocks (Mahéo et al., 2009) indicates west- of the Kern Canyon fault system that accommo- ited by the ancestral Bena channel (Fig. 2A). side-up motion along the Breckenridge segment dated a late Pliocene (?)–Pleistocene contribu- The Bena fan was choked and overlapped with of the Kern Canyon fault led to ponding of tion to westward tilting in the southern part of a fl ood of terrestrial sediments that resulted in a ~2.5 km of sediments in the Walker basin. The the Sierra Nevada microplate. delta and fl uvial system. zero-offset crossover of scissors-like vertical West tilt related to the Kern Canyon fault Further evidence suggesting regional exten- motions on the oblique-transfer Kern Canyon system appears to die out northwards just as the sion in the southern Sierra Nevada batholith fault system was the Edison graben (Fig. 2B), Kern Canyon fault scarps die out northwards. is a pulse of Pliocene volcanism centered at at the northwest tip of the modern-day White In the southeast San Joaquin basin, the tilted ca. 3.5 Ma (Farmer et al., 2002). Pliocene vol- Wolf fault. The Edison graben is bounded on region is bounded by a set of late Paleogene to canism in the region is observed to be spatially its southern edge by the Edison fault (Fig. 2B), Quaternary northwest-striking normal faults. related to regional extension (e.g., Farmer et al., a southwest-side-up normal fault that initiated These faults controlled the structural evolution 2002). The Pliocene volcanic episode in the in early to middle Miocene time (Dibblee and of the Edison graben and the Maricopa sub- study area includes deposition of the Little Kern Warne, 1988) and that merges with the northeast basin (Fig. 2; Mahéo et al., 2009). The Edison basalt, a lava fl ow at the northern end of end of the White Wolf fault (Fig. 2B). graben lies between the northeast-side-down the Kern Canyon fault (Fig. 2B; described Data from another (U-Th)/He apatite study, Edison fault and a set of southwest-side-down in detail in the next section) dated at 3.5 Ma of basement uplift and river incision, is con- normal faults of which the Kern Gorge fault is by Dalrymple (1963). North-south–oriented,

Geological Society of America Bulletin, September/October 2010 1675 Nadin and Saleeby

ductile-brittle shears in the basalt and a west- side-up scarp within this fl ow (Fig. 5) lie di- rectly along the trace of the Kern Canyon fault. 20 m 2 m N FIELD EVIDENCE FOR QUATERNARY ACTIVITY N The Kern Canyon fault was fi rst recognized t ul by Lawson (1904), who suggested that the Kern fa n Canyon was a graben between two or more yo B an C normal faults. More recently, slope breaks and rn KernKe Canyon fault aligned topographic notches along strike of the Kern Canyon fault were identifi ed through A a study of aerial photos by Ross (1986), who called some of these notches fault-line scarps. Figure 5. Photographs of the Kern Canyon fault in the Little Kern lava fl ow (Fig. 2 location However, despite numerous earthquake epi- 7, and mapped in yellow on Fig. 2B), from the air (A) and from the ground (B). (A) A 20-m centers “in the vicinity” of the fault (Moore and topographic notch appears west of the Kern Canyon fault. Note also the pervasive shearing du Bray, 1978), activity had been dismissed by of basalt in the left foreground. (B) On the ground, blocky basalt outcrops of the intact foot- those authors and others since mapping by Webb wall end suddenly along the trace of the fault, leaving an ~2-m-high scarp, and basalt east of (1946) defi ned a single trace overlain by un- the fault is sheared and eroded from the fault to ~50 m east of the hanging wall. faulted basalt that was later dated as 3.5 m.y. old (Dalrymple, 1963). Overturning this long-held view, we present here details of prominent geo- into granitic and metamorphic basement rocks graphs taken during a helicopter survey of the morphic features along the Kern Canyon fault in a manner typical of southwestern Sierra Kern Canyon fault (Fig. 6), are also keyed to that are consistent with late Quaternary, and in Nevada gorges that were incised since 2.7 Ma Figure 2. Some prominent geomorphic features some instances Holocene, normal displacement. (Stock et al., 2004). This suggests relatively ac- indicative of recent Kern Canyon fault activity celerated uplift of the mountainous terrain west are briefl y described here, starting in the north Scarps of the Kern Canyon Fault System of the Isabella basin. and proceeding south. Most of these features are Meter-scale prominent scarps and Quater- also easily discernable in Google Earth images. The geomorphology of the Isabella basin nary alluvium-fi lled basins run the length of the At 36.6°N, the Kern Canyon fault terminates (Fig. 2A), which was partially fi lled by Lake Kern Canyon fault. These and related promi- in a 1-km-wide, U-shaped trough. Proceeding Isabella after the Kern River was dammed in nent linear topographic breaks (lineaments) of southward, the west side of the Kern Canyon 1952, is a focal point for investigations of late the southern Sierra Nevada batholith (Fig. 2B) is bounded by a 400-m-high cliff that is con- Quaternary activity of the Kern Canyon fault were mapped from fi eld studies, aerial photog- tinuous (except where it is carved by east-west– system. As mentioned earlier, at the Isabella raphy, and digital elevation model (DEM) and running stream channels) for ~26 km along basin the Kern Canyon fault turns from its 025 advanced spaceborne thermal emission and strike of the Kern Canyon fault. Discontinu- northeastward strike to a north-south strike refl ection radiometer (ASTER) images. Their ous scarps at the base of the cliff are typically that follows the proto–Kern Canyon fault. Ac- locations are differentiated with color in Fig- ≥20 m high and trap patches of alluvium east of tive alluvia tion of the Isabella basin is burying ure 2B by the method used to identify them, the fault. Moore and du Bray (1978) noted the mountainous topography east of the Kern Can- and they are listed and described in Table 1. presence of a hot spring and a mineral spring yon fault, but west of the fault, the Kern River The locations of documented Quaternary fea- near latitude 36.5°N, both close to the fault, and Walker Creek canyons are deeply incised tures listed in Table 1, and selected aerial photo- but neither of these has been described in the

TABLE 1. SUMMARY OF SCARPS MAPPED AND KEYED TO LOCATIONS SHOWN IN FIGURE 2 Keyed location Length of Estimated Quaternary (see Fig. 2) Description feature normal offset 1—Walker basin Breckenridge scarp forms west side of basin; K granitic basement to west traps sediment in 8 km ~1300 m since Late Miocene basin. 700 m to Miocene strata. (Mahéo et al., 2010) 2—Havilah Valley Linear normal fault scarp bounds western side of Qal-fi lled valley. Streams bisect scarp here. 1 km ~10 m based on scarp height 3—Engineer Point and Engineer scarp is a normal fault scarp. Basin fi ll is Qal, up to 30 m thick at Engineer Point and 9 km 30 m based on depth of Qal fi ll Isabella basin trapped against K granitic basement to west. Some Qal overlaps Kern Canyon fault (KCF). 4—Cannell Creek Prominent scarp; same rock type on both sides. 2 km Minimum 7 m based on scarp height 5—Brush Creek Prominent scarp; same rock type on both sides. 15 km Minimum 7 m based on scarp height 6—Rincon Spring K basement in fault contact with Q debris fl ow. Abandoned stream channels in footwall block. 0.3 km Minimum 4 m based on scarp height 7—Little Kern basalt Tertiary basalt deposit, sheared and fractured along trace of KCF. Sharp topographic notch along ~0.5 km Minimum 20 m based on height fl o w KCF trace. of topographic notch. 8—Trout Meadows to N–S elongate linear valleys, with Qal valley fill trapped east of KCF. >2 km ? Willow Meadows 9—Little Kern lake and Lake formed by damming of Kern River during earthquake-triggered rockfall of 1868. N/A N/A rockfall

1676 Geological Society of America Bulletin, September/October 2010 Quaternary reactivation of the Kern Canyon fault system, southern Sierra Nevada, California

Figure 6. Photographs of some key scarps discussed in the text, with the Kern Canyon fault sketched in as a dashed line for reference. (A) Oblique aerial photograph of an eroded scarp at Rincon Spring (location 6 on Fig. 2) is ~4 m high. Stream channels are deeply incised into the uplifted footwall but do not cross the ponded Quaternary N alluvium of the hanging wall. Letter B indicates the loca- tion of (B) Cretaceous granitic basement in fault contact with channels Stream incised into footwall incised into Quaternary glacial debris. B The fractures in the granitic N basement strike northward, parallel to the strike of the Kern Canyon fault, shown as A C a white line. (C) Oblique aerial photograph of Engineer Point (location 3 on Fig. 2), a focus of U.S. Army Corps of Engi- neers (USACE) investigations because it is the site of a dam that crosses the Kern Canyon west fault. It is also the fault contact between Cretaceous basement and Quaternary alluvium at the western edge of the Isabella basin, and this fault contact is continuous for 9 km south- wards through the town of N Lake Isabella and into Havilah B D Valley. (D) A short scarp (10 m high) forms a basement-alluvium fault contact at the west edge of Havilah Valley (location 2 on Fig. 2). USACE trenching through the hang- ing wall alluvium in Havilah Valley reveals sediment deformed by liquefaction, and another trench just south of Havilah Valley yielded a 3.3 ka charcoal age in ruptured alluvium (USACE Lake Isabella Dam Situation Report, January 9, 2009; Hunter et al., 2009). geological literature. The Kern Canyon widens sible via lengthy, rugged trails or by helicopter, (e.g., Jones and Dollar, 1986; Busby-Spera and at Little Kern Lake, which formed in 1868 on fi eld observations come only from the original Saleeby, 1990), despite the seismic activity in the west bank of the Kern River when ~0.2 km3 reports by Webb (1936, 1946). In these reports, the area. of rock dammed the river during earthquake- Webb simply stated that a series of lava fl ows We report here evidence gathered from fi eld triggered failure (Townley and Allen, 1939; covered the trace of the fault, and that there investigations facilitated by the U.S. Army Ross, 1986) of the canyon’s eastern slope. was no evidence with which to ascertain the Corps of Engineers that the Kern Canyon fault South of the lake, more elongate, fl at, alluvium- nature of movement on the Kern Canyon fault. displaces the basalt cap. As seen from the air, fi lled meadows line the Kern Canyon fault in [Moore and du Bray (1978) subsequently veri- the lava surface is broken by a sharp topo- the midst of rugged topography. fi ed Hill’s (1955) mapping of the Kern Canyon graphic notch of ~20 m height on the west side At latitude ~36.2°N, the Kern Canyon fault fault as dextral and estimated offset at 7–13 km, of the Kern Canyon fault trace (Fig. 5A). On the leaves the Kern River channel and cuts through increasing from north to south.] Despite numer- ground in this location, generally coherent out- a steep, ~450-m-high cliff capped by a 3.5 Ma ous earthquake epicenters in the vicinity of the crops west of the fault end abruptly in a gully basalt (Fig. 5A; Webb, 1936, 1946; Dalrymple, fault (described in detail in the next section), where deeply weathered basalt is pervasively 1963; Moore and du Bray, 1978) that is on Moore and du Bray (1978) concluded that there sheared and fractured parallel to the trace of average ~100 m thick but locally thickens in was no measurable offset of the basalt since the Kern Canyon fault, resulting in an apparent paleochannels to ~300 m (Dalrymple, 1963). its deposition. However, they were looking for west-side-up scarp of ~2 m height (Fig. 5B). The basalt cap is the most commonly cited dextral offset consistent with their observations Because the topographic break along the trace evidence for lengthy quiescence of the Kern of offset pluton contacts. Post–3.5 Ma activity of the fault is deeply weathered, the fault here is Canyon fault. Probably because it is only acces- along the Kern Canyon fault was thus dismissed expressed as a zone of pervasive fracture ~50 m

Geological Society of America Bulletin, September/October 2010 1677 Nadin and Saleeby wide rather than as a single, discrete fault plane active faulting. Another scarp emerges on the terrain east of the fault, and is trapped against the with confi dently measurable vertical displace- west side of the Havilah Valley (Fig. 6D). This footwall. Finally, the 3.5 Ma lava fl ow is perva- ment. In normally faulted bedrock, slip often scarp is an ~10-m-high linear ridge of shattered sively sheared and fractured exclusively along takes place both along normal faults and on basement that forms the western boundary of the trace of the Kern Canyon fault, and the shear- fractures around the fault, leading to degraded a fl at alluvium-fi lled valley. Another trench ing coincides with an ~20-m-high, west-side-up, or irregular scarps at the surface whose ages and through the hanging wall alluvium here reveals topographic break through compositionally uni- slip amounts can be misinterpreted (Hilley et al., an isotropic sediment mass with fl oating bed form material (Fig. 5). West-side-up faulting was 2001). Petrographic analysis of basalt from both remnants, suggesting deformation by liquefac- therefore contemporaneous with or postdated the sides of the Kern Canyon fault shows that the tion. Scarps of the Kern Canyon fault system lava fl ow. Pervasive tensile and shear fracturing capping fl ow is compositionally uniform, thus continue as the Breckenridge fault at latitude is consistent with deformation of weak material supporting the presence of a true fault scarp. 35.4°N (Fig. 2B). Apatite He data and geomor- in the hanging wall of a normal fault. Further For much of its length from the lava fl ow to phic evidence indicate ~2.1 km of west-side- evidence supporting normal offsets of the Kern 36°N, the main Kern Canyon fault trace forms a up displacement on the Breckenridge segment Canyon fault system in Quaternary time comes sharp linear feature on aerial images. At this lati- since Miocene time (Mahéo et al., 2009). from geodetic and seismic studies. tude, we document (1) a subdued, ~4-m-high, Several lines of fi eld evidence indicate that north-south–striking scarp on whose footwall the Quaternary scarps resulted from west-side- RECENT GEODETIC AND east-west–oriented stream channels are cut off up, normal offset on the Kern Canyon fault. SEISMIC ACTIVITY (Fig. 6A) and (2) Late Cretaceous granitic base- First, all of the north-south–elongate depressions ment rocks in fault contact with a Quaternary fi lled with Quaternary alluvium along the Kern Analysis of seismicity within and along the debris fl ow deposit (Fig. 6B) whose clasts are Canyon fault trace are bounded on their west- southern Sierra Nevada batholith shows that sheared and fragmented. Nadin and Saleeby ern sides by basement rocks, suggesting that earthquakes along the Kern Canyon fault sys- fi rst reported this fault contact (2001) and re- basement forms the footwall of a normal fault. tem occur on shallow normal faults (Fig. 7). corded pervasive fractures in the basement rock Second, although eroded scarps are commonly In the Sierra Nevada east of the Kern Canyon that strike 005°/185° and dip 80°–85° eastward, crossed by stream channels (as in the Havilah fault, earthquakes occur on a combination of concordant with the strike of the scarp. Pro- Valley, Fig. 6D), in several locations east-west– dextral strike-slip and normal faults, and to the ceeding south, west-side-up scarps are several running stream channels of the Kern Canyon southeast and southwest, they occur on sinistral meters in height and show disruption of the fault footwall end abruptly at a scarp (Fig. 6A). strike-slip and thrust faults. Strain-meter and same rock type on both sides of the fault or they Because lateral stream offsets are absent, this global positioning system (GPS) measurements juxtapose similar rock types (e.g., granodiorite suggests that the channels are steeply incising further indicate that the Kern Canyon fault sys- and mica-rich quartzite). The scarps are discon- into the rapidly uplifted western footwall of the tem is undergoing horizontal extension and that tinuous here, in that they are covered in places system. Sediment that ponds above the hanging the southern Sierra Nevada is uplifting relative by alluvial fi ll whose upper surface is probably wall originates from the generally more rugged to adjacent basins. less than 5000 yr old (Page, 2005). Engineer Point is a basement-rock scarp that projects into Isabella basin and is ductilely 119.5° 119° W118.5° 118° W to brittlely sheared across its ~100-m width (Figs. 4 and 6C). It has been well studied in the context of earthquake safety because an earthen Depth (km) dam that traps Kern River water for fl ood con- 0 Figure 7. Seismicity map of the 36.5° trol and irrigation was constructed across this 5 peninsula in 1952. As part of their efforts to study area, showing all earth- quakes of M >3 since 1981, 10 assess the vulnerability of the Engineer Point KCF dam, the U.S. Army Corps of Engineers under- colored according to depth. 15 Focal mechanisms are for took coring, trenching, and a seismic refraction 20 well-constrained M >4.0 events DM study that was reported by Page (2005). High 36° N P-wave velocities in Quaternary alluvium east since 1999. The regions of the of the Kern Canyon fault indicate ~30 m of Durrwood Meadows (DM) and sediment is trapped against uplifted Cretaceous Walker Pass (WP) earthquake granitic basement to the west, suggesting at swarms are outlined. Relo- WP least this much vertical displacement along the cated focal mechanisms and earthquake locations are from fault at this latitude. The Engineer Point scarp 35.5° Clinton et al. (2006), Tan (2006), continues for ~9 km south of the Isabella basin BF but is obscured by development in the town of and Lin et al. (2007). KCF = Lake Isabella. Just south of Lake Isabella, a Kern Canyon fault, BF = Breck- U.S. Army Corps of Engineers trench through en ridge fault, and WWF = White Wolf fault. ruptured alluvium yielded a 3.3 ka charcoal age WWF (U.S. Army Corps of Engineers, Lake Isabella 35° N Dam Situation Report, January 9, 2009; Hunter et al., 2009). About 2 km south of this trench site, the presence of hot springs also indicates

1678 Geological Society of America Bulletin, September/October 2010 Quaternary reactivation of the Kern Canyon fault system, southern Sierra Nevada, California

Seismicity While focal mechanisms near the north- Nevada microplate. This regional strain fi eld is south–trending segment of the Kern Canyon punctuated locally by (1) horizontal extension In the past 35 years, there have been more fault show exclusively normal slip on north- with vertical crustal thinning within the south- than 200 earthquakes greater than M = 3 and striking faults, with the majority of seismic ac- ern Sierra Nevada (Unruh and Hauksson, 2009), at depths less than 10 km in the vicinity of tivity occurring less than 10 km deep, seismicity and (2) shortening across the southwest tip of the Kern Canyon fault—between latitudes to the east and to the southwest is markedly dif- the Sierra Nevada, where measurements from 35°N and 37°N and longitudes 118.3°W and ferent. In the eastern Sierra range front, earth- a geodetic array spaced between Bakersfi eld 118.85°W. The focal mechanisms for these quake focal planes show predominantly dextral and the Garlock fault (Figs. 1 and 2) indicate earthquakes yield further insight into the nature slip on north-northwest–striking faults, and a maximum principal strain azimuth oriented of active faulting in the southern Sierra Nevada most of the focal depths are 5–15 km (Fig. 7). 300° and strain rates of ~0.2 μstrain/yr that in- (Fig. 7). The Durrwood Meadows earthquake This is in good agreement with measurements creased 300% just before the 1952 Kern County swarm, which lasted from October 1983 to of offset and calculations of slip rate from the earthquake (Bawden et al., 1997). P- and T-axes May 1984 (Fig. 7), is the most pronounced re- Sierra Nevada frontal fault zone, which show computed by Bawden et al. (1999) are consis- cent seismic activity in the vicinity of the Kern dominantly dextral slip with subordinate nor- tent with two distinct styles of strain: east-west– Canyon fault. This swarm comprised >2000 mal slip (e.g., Le et al., 2007). In the south- oriented sinistral strike-slip motion between the measured earthquakes, many of which were west Sierra, seismicity in the region of the Walker Pass and the region of the White Wolf M ≥3.0 and the largest of which was M = 4.9 Breckenridge-Greenhorn horst is consistent fault–Breckenridge transition (Fig. 7), and a (Jones and Dollar, 1986). It was localized with active uplift of the horst (Mahéo et al., northward-trending zone of east-west extension ~10 km east of the Kern Canyon fault and occu- 2009). As shown in Figure 7, focal mechanisms aligned with and close to the trace of the Kern pied a discrete, ~100-km-long, north-south ver- in the area where the White Wolf fault meets Canyon fault. tical plane (Lin et al., 2007) with no evidence the southern tip of the Breckenridge fault show of ground breakage. All calculated earthquake sinistral slip along northeast-striking fault planes Global Positioning System Velocities depths were 0–7 km, all but one M >3.0 oc- parallel to the trace of the White Wolf fault, curred between 4.0 and 5.9 km, and all resolved and focal depths are mainly 5–10 km. Where To assess the contemporary horizontal and focal mechanisms were consistent with pure the White Wolf fault reaches into the southern vertical velocities of the southern Sierra Nevada west-side-up, dip-slip displacement (Jones and San Joaquin basin, abundant focal mechanisms batholith, we concentrate here on GPS data, Dollar, 1986; Clinton et al., 2006). These obser- all show northward thrusting along east-west– publicly available on the Southern California vations, coupled with the lack of coincidence of striking fault planes, again parallel to the trace Integrated GPS Network (SCIGN) Data Portal the Kern Canyon fault with the swarm, exclude of the White Wolf fault, and much deeper focal (http://reason.scign.org), from 32 stations in and the possibility that the earthquakes occurred on mechanisms of up to 20 km. The 1946 Walker around the southern half of the batholith (Fig. 8 the main Kern Canyon fault trace. It suggests, Pass earthquake swarm (Fig. 7) presents yet an- and Table 2). Many of these stations have been however, creep is taking place on a nascent other type of seismic activity, of sinistral strike- installed only in the past few years, but they still fault that may be part of the Kern Canyon fault slip shear along northeast-striking planes (e.g., provide valuable insight into relative motion in system, or is at least the easternmost expres- Bawden et al., 1999). this part of the range. sion of southern Sierra Nevada normal faulting Horizontal velocities, shown by the vectors that began in late Cenozoic time. We empha- Strain on Figure 8, are considered relative to station size here that the Quaternary scarps of the Kern LNCO (Lind Cove) because it has been actively Canyon fault comprise the easternmost surface Short-term strain studies across the Kern recording data the longest (since 1999). Relative breaks in a kilometers-wide zone comprising Canyon fault led Vali et al. (1968) to suggest to LNCO and at latitudes where the Kern Can- several late Cenozoic, near-vertical, parallel that ground motions recorded by strain meter yon fault is a north-south–striking lineament, fault strands. as “spikes in strain” could be caused by “small 10 of 11 stations west of the fault are moving Seismic activity of note that probably took slippage” along the fault. Slade et al. (1970) con- westward at rates between 0.6 and 6.8 mm/yr place on the Kern Canyon fault proper includes ducted further analyses of the relative motion (Table 2). In contrast, stations east of the Kern a two-week-long series of earthquakes with between the two arms of the strain meter, which Canyon fault at these latitudes show predomi- more than 500 aftershocks that occurred near were oriented parallel and perpendicular to the nantly eastward motion relative to LNCO. The Kernville in 1868 (Treasher, 1949; Ross, 1986) fault. They found that over one two-day interval, southwesternmost part of the batholith shows a and triggered the rockfall that led to the creation bedrock on the western wall of the fault moved more complicated pattern, probably caused by of the Little Kern Lake. The largest of these was ~0.5 mm in a vertical direction, and suggested: Garlock fault sinistral motion, sinistral slip along later assigned M = 6.5 (Barosh, 1969). More “the motion observed at this site arises from the White Wolf fault, and northward thrusting recently, several M ≥3.0 earthquakes have oc- forces perpendicular to the fault as a whole.” along the San Emigdio–Tehachapi fold-thrust curred along the Kern Canyon fault. Two exam- This implies east-west extension that could re- belt (Figs. 2A and 8; Nilsen et al., 1973; Good- ples, from July 1999 and April 2001, are very sult in west-side-up motion on the Kern Canyon man and Malin, 1992; Mahéo et al., 2009). well constrained, less than 10 km deep, M = 4.2 fault. While the Slade et al. (1970) study was Vertical velocity values with error estimates and M = 4.1 earthquakes, respectively (Fig. 7; inconclusive with regard to permanent offset, it for stations in the southern Sierra Nevada re- Tan, 2006). The 1999 event occurred ~1.8 km did show that strain across the fault is 25 times gion are shown in Figure 8 and listed in Table 2. west of the Kern Canyon fault at the latitude of that of the adjacent bedrock. These velocities are inherently less precise than the Isabella basin, and the 2001 event occurred Regional geodetic and seismologic analyses the horizontal components for several reasons, ~10 km east of the Kern Canyon fault at latitude indicate overall northwest-southeast extension including hydrologic and atmospheric infl u- ~36°N. The focal mechanisms for both indicate (Bennett et al., 2003) and horizontal plane strain ences (Blewitt et al., 2001). However, general normal offset along north-striking planes. (Unruh and Hauksson, 2009) across the Sierra information can be gained from the regional

Geological Society of America Bulletin, September/October 2010 1679 Nadin and Saleeby

N

P572 Owens Valley P467

36.5° Figure 8. Global positioning system stations of the south- LNCO ern Sierra Nevada, westward P566 0.7 –10.2 (0.2) adjacent San Joaquin Valley, (0.5) P571 and eastward adjacent Owens 2.4 Valley. Lines indicate hori- (0.6) zontal motion with respect to station LNCO (in bold print, ARGU in the northwest corner of the P056 Kern Canyon fault –22.2 0.0 map), which has been collecting COSO (0.09) (0.8) –3.6 36° N data the longest (since 1999). (0.2) Numbers indicate vertical ve- BEPK 0.42 locities (excluding stations col- (0.09) lecting only since 2006), with P565 italicized numbers highlighting –12.3 upward motion. All numbers (1.1) are Combine fi ltered velocities ISLK P570 P564 0.45 from the National Aeronautics (0.09) CCCC and Space Administration’s –0.66 Research, Education and Ap- San (0.12) 35.5° plications Solutions Network P567 P563 Joaquin P616 (REASoN) website (see Table 2 2.3 –4.5 Valley (0.4) P569 for velocities and Global Posi- (0.7) RAMT tioning System Velocities sec- –0.79 tion for explanation of REASoN P568 (0.12)

Combine fi lter), except for the THCP BVPP ARM2 numbers shown in bold, which –15.0 P558 0.53 P591 –0.7 (0.09) 0.5 are high-confi dence determina- (0.2) (0.4) (0.3) st White Wolf fault tions from Fay et al. (2008) that ge thru Rid P579 are calculated with respect to Wheeler WGPP Garlock fault P562 –2.0 P557 35° N station ARGU, the easternmost (0.2) EDPP 2.2 station on this fi gure. 1.8 (0.4) 5 mm San Andreas(0.2) fault P559 P560 1.7 0.2 (0.2) (0.5)

119° W118.5° 118° W pattern , and the vertical motions of six stations removal; e.g., Lofgren and Klausing, 1969). The DISCUSSION in this area are described by Fay et al. (2008). precise determinations of relative vertical mo- We chose the combination velocity solu- tions for six of the stations shown in Figure 8 The Kern Canyon fault zone is the only ac- tion from the National Aeronautics and Space (highlighted in bold print) indicate the same tive structural zone that breaks the interior of the Admini stration’s Research, Education, and overall pattern, with the southern Sierra Nevada Sierra Nevada batholith, disrupting the struc- Applications Solutions Network (REASoN; stations moving up at rates of 0.42 to 0.53 mm/yr tural coherency of the batholith. Its longevity Fig. 8 and Table 2) primarily because these val- and the Owens Valley stations moving down at and geometry make it well positioned to ac- ues agree better with the precise values of the six rates of ~0.66 to 0.79 mm/yr relative to station commodate the present regional east-west ex- stations analyzed by Fay et al. (2008). The pat- ARGU in the Argus Range of eastern Califor- tensional stress fi eld. Structural, geomorphic, tern that emerges is one of uplift of all stations nia (Fig. 8; Fay et al., 2008). Because these and geodetic, and seismic observations presented within the Sierra Nevada batholith at rates of further detailed studies of vertical velocities here indicate that the Kern Canyon fault sys- 0.4 to 2.4 mm/yr. In contrast, all stations in the in the region are consistent with the REASoN tem has undergone Quaternary reactivation as a Owens Valley to the east are moving downwards values (R. Bennett, 2008, personal commun.), series of west-side-up normal fault scarps along at rates of up to 3.6 mm/yr, and those in the San we feel confi dent that the pattern is real and, its 130-km length. This remobilization is consis- Joaquin basin to the west are moving down- furthermore, that vertical velocities west of tent with structural and stratigraphic data from wards at much higher rates of 0.7 to 22.2 mm/yr the Kern Canyon fault are generally higher the adjacent southeastern San Joaquin basin (probably refl ecting widespread groundwater than those east of the fault. and from low-temperature, thermochronometric

1680 Geological Society of America Bulletin, September/October 2010 Quaternary reactivation of the Kern Canyon fault system, southern Sierra Nevada, California

TABLE 2. VERTICAL VELOCITIES OF GLOBAL POSITIONING SYSTEM STATIONS AS REPORTED ON THE REASoN WEBSITE Name Latitude Longitude Data since Combine fi ltered Error JPL fi ltered Error SOPAC fi ltered Error PBO offi cial Error LNCO (LIND) 36.3603 –119.0576 1999 0.7 0.2 1.3 0.2 0.4 0.2 N/A P566 36.324454 –119.229286 2005 –10.2 0.5 –8.8 0.1 –10.7 0.4 –10.1 0.2 P056 36.027437 –119.062869 2005 –22.2 0.8 –21.5 0.1 –23.4 0.7 –22.2 0.3 P571 36.231366 –118.766715 2005 2.4 0.6 4.3 0.1 2.4 0.5 4.7 0.2 P565 35.743893 –119.236652 2005 –12.3 1.1 –15.5 0.1 –14.9 1 –16.1 0.1 ISLK 35.662271 –118.4743 1999 0.7 0.2 1.7 0.2 0.3 0.2 1.3 3.5 P567 35.420946 –118.753576 2005 2.3 0.4 3.4 0 1.8 0.4 2.1 2.7 P558 35.138605 –118.611654 2006 2.6 0.5 5.2 0.1 2.1 0.4 3.5 0.2 THCP 35.158182 –118.414571 2000 0.8 0.1 2 0.1 0.1 0.1 –1.2 –3.8 ARM2 35.20119 –118.90962 2000 –15 0.4 –14 0.3 –12.9 0.4 5.1 1.2 P557 34.944385 –118.655586 2005 2.2 0.4 3.2 0.1 2 0.4 3.7 0.5 P559 34.838909 –118.617597 2005 1.7 0.2 2.8 0.1 2 0.1 2.5 0.2 P465 36.466829 –118.132419 2007 –45 3.6 10.4 0.5 4.7 0.7 13.2 2.5 P573 36.0930943 –118.260498 2008 BEPK 35.878388 –118.074092 2000 0.7 0.2 1.8 0.2 0.2 0.2 –1 3.9 P570 35.667348 –118.260038 2006 –0.4 1.4 1.5 0.1 –1.4 1.3 2.5 1 COSO 35.98231 –117.80797 2000 –3.6 0.2 –3 0.2 –4 0.2 7.9 0.9 ARGU 36.05 –117.521944 2000 –0.9 0.3 0.4 0.3 –1.2 0.3 N/A P569 35.377964 –118.123752 2007 0.7 1.2 3.5 0.2 2.8 1 7.5 0.5 P616 35.42454 –117.89328 2007 –3.4 0.5 2.3 0.2 0 0.4 7.6 0.6 CCCC 35.56531 –117.67117 2000 –0.3 0.2 1 0.1 –1 0.2 –3.1 3.7 RAMT 35.338714 –117.683349 2000 –0.6 0.2 0.5 0.1 –1 0.2 –1.5 3.6 P568 35.254302 –118.126498 2007 2.6 1.1 4.4 0.2 4 0.9 9.6 0.4 P591 35.152424 –118.016474 2005 0.5 0.3 1.8 0.1 0.1 0.3 0.8 0.3 P579 35.038762 –118.005764 2006 1.2 0.2 2.3 0.1 0.5 0.1 1.6 0.2 P562 34.982133 –118.188744 2004 –0.1 0.3 1.4 0.1 –0.4 0.3 –0.4 3.7 P560 34.82181 –118.540866 2005 0.2 0.5 2.3 0.1 –0.9 0.5 –2.5 4.7 EDPP 34.946194 –118.830408 2000 1.8 0.2 3.3 0.1 1 0.2 –1.4 4.2 WGPP 35.010848 –118.983687 2000 –2 0.2 –0.9 0.1 –2.4 0.2 –4 3.7 P467 36.570202 –118.090623 2006 5.7 1 4.2 0.1 –0.2 0.9 4 0.2 P572 36.585511 –118.954581 2006 3.7 0.7 5.1 0.1 3.5 0.5 3.2 1.3 P564 35.62291 –119.34938 2006 –59.7 2 –56.1 0.1 –58.9 0.7 –53.9 0.3 P563 35.418669 –119.421165 2005 –4.5 0.7 –4.7 0.1 –5.4 0.6 –4.8 0.2 BVPP 35.157276 –119.347506 2000 –0.7 0.2 0.8 0.2 –1.2 0.2 –3 4.9 Note: Combine fi ltered data are shown in Figure 8, with the exception of stations ARGU, CCC, RAMT, THCP, BEPK, and ISLK. Stations in bold have only been collecting data since 2006 and were discounted from Figure 8. REASoN—Research, Education, and Applications Solutions Network (National Aeronautics and Space Administration [NASA]); JPL—Jet Propulsion Laboratory (NASA); SOPAC—Scripps Orbit and Permanent Array Center (Scripps Institute of Oceanography); PBO—Plate Boundary Observatory (National Science Foundation Earthscope Program).

determinations (Mahéo et al., 2009) that link the normal faults of the southeast Sierra extensional east-west extension ca. 10 Ma is also recorded Kern Canyon fault system with active uplift of domain. Westward tilt and uplift of the horst re- in the central Sierra Nevada by volcaniclastic the Breckenridge-Greenhorn horst (as also sug- moved from the basement the Tertiary strata that rocks of the Carson Pass, ~10 km south of Lake gested by vertical displacement of GPS station had been continuous with westward-dipping Tahoe (Fig. 1). Volcanic sequences that were ex- P567, Fig. 8). The southwest edge of the horst Tertiary sediments in the adjacent eastern San truded 14 m.y. ago were deposited in Cretaceous is defi ned by northwest-striking, principally Joaquin basin. paleochannels and reincised 4 Ma later and cut southwest-side-down, normal faults, some with Evidence throughout the eastern margin of off from sources to the east; this late Miocene reported Quaternary scarps (Fig. 2B; Mahéo the Sierra Nevada microplate indicates that, reincision was ascribed to uplift attendant with et al., 2009). by middle Miocene time, it lay in a generally passage of the triple junction through the lati- After early Cenozoic quiescence, the Kern extensional tectonic setting (Fig. 3), moving tude of the central Sierra Nevada (Busby et al., Canyon fault system was fi rst reactivated in ~15 mm/yr to the northwest relative to North 2008). The emergence of the southeastern Sierra early Miocene time, corresponding to a period America (Wernicke and Snow, 1998). Such ex- Nevada as a sediment source and topographic of regional uplift. (U-Th)/He apatite ages and tension, as expressed by normal faulting along the upland by 8 Ma is also recorded in Ricardo geomorphic analyses indicate two discrete epi- current eastern edge of the Sierra Nevada, likely Group sediments (Loomis and Burbank, 1988). sodes of accelerated river incision in the south- marks the inception of the Sierra Nevada micro- This is consistent with the Miocene uplift and ern Sierra Nevada since 20 Ma (Clark et al., plate (Argus and Gordon, 1991). The Ricardo normal faulting reported by Mahéo et al. (2009). 2005; Clark and Farley, 2007). We suggest that Group, a 1700-m-thick sequence of Miocene vol- It also coincides temporally with the 10–9 Ma the fi rst is related to ca. 10 Ma regional exten- canic and lacustrine sediments deposited between Stanislaus Group of voluminous volcanic de- sion. Mahéo et al. (2009) and Saleeby et al. ca. 19 and 7 Ma in the El Paso basin southeast posits in the central Sierra Nevada, which was (2009) document early to middle Miocene ex- of the Sierra Nevada batholith (Fig. 3), docu- proposed to have erupted at a releasing stepover tension in the southern Sierra Nevada and the ments a change from north-south extension at on dextral transtensional faults that created the southeast San Joaquin basin that suggests struc- 17–15 Ma (attributed to northward migration of largest Miocene volcanic center in the Sierra tural continuity of the White Wolf fault with the a slab window beneath western North America; Nevada (Putirka and Busby, 2007). Kern Canyon fault at that time. The integrated see also Cox and Diggles, 1986; Monastero The second episode of post–20 Ma acceler- system partitioned differential extension across et al., 1997) to east-west extension north of the ated river incision in the southern Sierra Nevada northwest-striking normal faults in the Mari- Garlock fault in the El Paso basin at 10–9 Ma noted by Clark et al. (2005) and Clark and copa subbasin (Fig. 2) from northwest-striking (Loomis and Burbank, 1988). The transition to Farley (2007) initiated ca. 3.5 Ma. This timing

Geological Society of America Bulletin, September/October 2010 1681 Nadin and Saleeby is supported by pre–2.7 Ma amplifi ed bedrock Kern Canyon fault began in mid-Quaternary uplift that drove accelerated river incision for time, as suggested by the contiguously tilted west-draining rivers located between latitudes relict landscape surface and early to mid- 36.5°N and 38°N (Stock et al., 2004). It is also Quaternary lacustrine deposits (Saleeby et al., in good agreement with a central Sierra Nevada 2009; Mahéo et al., 2009). Mid-Quaternary in- study suggesting that topography in the range ception suggests a slip rate of 3.0 mm/yr at the resulted from two periods of uplift, including Isabella basin, which is higher than our previ- stream incision of up to 1 km since ca. 5 Ma ously mentioned estimate. The 3.3 ka charcoal (Wakabayashi and Sawyer, 2001). layer in ruptured Quaternary alluvium just The disrupted Little Kern lava fl ow and Qua- south of the Isabella basin (U.S. Army Corps ternary scarps described here (Fig. 2 and Table 1) of Engineers, Lake Isabella Dam Situation are consistent with the most recent phase of Kern Report, January 9, 2009; Hunter et al., 2009) Canyon normal faulting beginning ca. 3.5 Ma. places Kern Canyon fault activity at this lati- Given the spacing between scarps along the tude well into the Holocene epoch. Kern Canyon fault, it is likely that the fault has While placing slip-rate bounds of 0.01 to ruptured discontinuously along its length since 3.0 mm/yr on the scarp at Isabella basin is a Figure 9. Oblique aerial view of two paral- that time. The lowest slip-rate limit for the Kern useful guide, we reiterate that the abundant and lel scarps of the Kern Canyon system, just Canyon fault comes from the scarp in the basalt often parallel scarps along the Kern Canyon fault northeast of the town of Kernville, Califor- fl ow at the fault’s north end: if 20 m of offset oc- (Fig. 9) suggest it is inaccurate to assign a slip- nia. White dashed lines indicate fault traces. curred over the past 3.5 Ma, this yields a slip rate rate estimate for a single scarp as representative (For scale, longer dashed line is ~4 km.) of 0.006 mm/yr. Until more accurate determina- of vertical displacements for the whole Kern tions of the total slip and the timing over which Canyon fault system. As mentioned earlier, per- it took place can be made for the northern part vasive fracturing common in normal-fault zones of the Kern Canyon fault, this slip rate should be can lead to underestimates of total slip. radius of the Kern Canyon fault zone (Fig. 2B), considered a lower limit. The basalt in this loca- Late Cenozoic fault reactivation in the south- but the main Kern Canyon fault trace is the only tion is pervasively fractured over a 50-m-wide ern Sierra Nevada confi rms the tendency for through-going structure with Quaternary offsets zone, and it is likely that the fractures near the mylonitic foliation to be reactivated as normal that, although discontinuous, are distributed main fault trace also accommodated offset, as faults, even when the regional fl ow fi eld is along its entire 130-km length. The other faults suggested by Hilley’s et al. (2001) study of dis- oblique to foliation (e.g., Giorgis et al., 2006). are generally less than 20 km long and their off- tributed offset in fracture zones around normal Kern Canyon fault brittle offset localized along sets are small or have not yet been measured. faults. Furthermore, fi eld mapping and apatite preexisting Late Cretaceous ductile shear zones The southern Sierra Nevada south of lati- He thermochronologic data indicate that addi- that are oriented north-south (Nadin and Saleeby, tude 36.5°N, and the adjacent San Joaquin tional west-side-up normal scarps lie west of the 2008; Mahéo et al., 2009). The overprinting of basin, are characterized by crust that is thin Kern Canyon fault trace and the lava fl ow cap cataclastic and gouge zones on mylonitized gra- (20–30 km) relative to the ~40-km-thick crust (Mahéo et al., 2009). Indeed, there are often par- nitic and metamorphic basement rocks (Fig. 4) of the central Sierra Nevada (Fliedner et al., allel scarps along the Kern Canyon fault system indicates that the mylonitic foliation was re- 2000). Swarms of extensional earthquakes in the (Fig. 9). Slip rates on all these faults must be in- activated in the brittle regime along the length southern Sierra Nevada reveal modern defor- cluded in the rate along the Kern Canyon fault in of the Kern Canyon fault. Geomorphology mation within the Sierra Nevada microplate order to more thoroughly assess the Quaternary and seismicity indicate normal-fault reactiva- (Jones and Dollar, 1986; Saltus and Lachen- kinematics of the system. tion. Other normal faults in the southern Sierra bruch, 1991; Unruh and Hauksson, 2009). Mod- In the Isabella basin, a 30-m-high scarp of Nevada accommodated Miocene north-south ern deformation has been ascribed to (1) the Cretaceous granitic basement traps Quaternary extension attributed to northward migration of a westward migration of Basin and Range exten- alluvium east of the fault. If 30 m of slip in this slab window, but these brittle faults are oriented sion (Lockwood and Moore, 1979; Jones and location also occurred over the past 3.5 Ma, northwest-southeast (see Fig. 1 of Mahéo et al., Dollar, 1986; Saltus and Lachenbruch, 1991; which is an early date for inception of pure nor- 2009). Some of these appear to have localized Surpless et al., 2002; Niemi, 2003), (2) a shift mal faulting as documented by the sheared lava on preexisting Late Cretaceous extensional frac- in transform slip from the San Andreas fault fl ow, this suggests a slip rate of ~0.01 mm/yr. tures, but the recent activity of the Kern Canyon to the Eastern California Shear Zone (Jones Differential vertical motions of well-constrained fault system localized along the northern half et al., 2004), or (3) delamination of the high- GPS stations ISLK and BEPK (Fay et al., 2008), of a pervasive ductile shear zone, the proto– density mantle lithosphere from beneath the which are on either side of the Kern Canyon Kern Canyon fault. As discussed above, batholith (Saleeby and Foster, 2004; Unruh and fault (Fig. 8), suggest a comparable slip rate of the Kern Canyon fault system was most likely Hauksson , 2009). We fi rst compare the nature 0.03 mm/yr. However, this is an order of magni- reactivated during regional Miocene exten- of Kern Canyon faulting with patterns related to tude lower than Holocene vertical slip rates es- sion, even though the fault is oriented north- Basin and Range extension. timated for the Sierra Nevada frontal fault (e.g., northeast, obliquely to the north-south–directed The timing of extension in the southern Le et al., 2007), and up to two orders of mag- Miocene extension. The modern-day southern Sierra Nevada is protracted and irregular: nitude less than the uplift of the Sierra Nevada Sierra Nevada seismogenic fi eld is consistent early Cenozoic quiescence was followed by a batholith relative to Owens Valley as estimated with east-northeast–west-southwest horizontal transition from north-south extension to east- by Fay et al. (2008). extension and vertical crustal thinning (Unruh west extension in Miocene time and a second It is likely that the most recent phase of nor- and Hauksson, 2009). Many north-south– episode of east-west extension beginning in mal fault activity along the southern part of the oriented Quaternary faults lie within a 75-km Pliocene time. Together with discontinuous,

1682 Geological Society of America Bulletin, September/October 2010 Quaternary reactivation of the Kern Canyon fault system, southern Sierra Nevada, California meter-scale Quaternary offsets along the Kern cause of recent, west-side-up vertical motion (Ducea and Saleeby, 1998; Jones et al., 2004; Canyon fault system, this signals that activity along scarps of the Kern Canyon fault. Exten- Saleeby and Foster, 2004; Clark et al., 2005). along the high-angle Kern Canyon fault sys- sion is commonly linked with basaltic vol- Depositional patterns in the western Sierra tem was punctuated, with two or three discrete canism in general, and this is seen in the 3.5 Ma Nevada–San Joaquin basin transition zone indi- episodes of activity over the past ca. 20 Ma. In fractured lava fl ow in the study area. A Pliocene cate that a region of accelerated subsidence over- contrast, Basin and Range–type extension has volcanic pulse in the southern Sierra Nevada lies the drip (Saleeby and Foster, 2004; Mahéo been migrating steadily northwest along low- has been ascribed to the initiation of mantle et al., 2009). Dynamic models of convective angle normal faults since 36 Ma (e.g., Wernicke lithosphere removal and upwelling of astheno- instabilities predict such subsidence, as well as and Snow, 1998). sphere (e.g., Farmer et al., 2002; Jones et al., coupled uplift marginal to the drip, above the area The mapped southwestern limit of Basin and 2004). This would be a mantle-driven event that vacated by the high-density material that sourced Range extension also coincides with two zones would have thinned the lithosphere and which the drip (e.g., Pysklywec and Cruden, 2004; of shear that separate it kinematically from has invoked predictions of related uplift and ex- Le Pourhiet et al., 2006). These models further the Sierra Nevada microplate to the west. The tension (e.g., Jones et al., 2004). The semicircu- predict substantial basement warping along a Eastern California Shear Zone–Walker Lane lar outline of Pliocene volcanism in the Sierra rolling hinge across the subsidence-marginal up- belt (Fig. 1) is dominated by dextral strike-slip Nevada covers an area from latitudes ~36.6°N lift zone, thereby posing a widespread response faulting. The Walker Lane seismogenic fi eld to ~38°N, and longitudes ~117.8°W to ~120°W to mantle processes as an alternative mechanism indicates northwest-directed dextral strike-slip (Manley et al., 2000). This circle is northward of for the Pliocene–Quaternary phase of activity shear (Unruh and Hauksson, 2009), consistent but slightly overlaps the current map projection along the Kern Canyon fault and neighboring with faults that accommodate northwest transla- of the high-velocity anomaly (Fig. 2) that sig- structures. The punctuated activity along the tion of the Sierra Nevada microplate relative to nifi es the descending upper-mantle lithosphere Kern Canyon fault could indicate relatively low stable North America (e.g., Unruh et al., 2003). at 70–90-km depth (see Fig. 1 of Gilbert et al., overall strain that reappeared over a lengthy time Previous studies also suggest that the eastern 2007). The map patterns suggest that mantle interval and localized deformation along a major Sierra Nevada and the adjacent Walker Lane are lithosphere descent began northeast of its cur- preexisting structural zone. structurally and kinematically distinct from the rent position. Its descent created a zone of the Basin and Range to the east (e.g., Wright, 1976; batholith where the crust is now only ~30 km CONCLUSIONS Stewart, 1988; Unruh et al., 2003). thick because of mountain root removal, and Within the Sierra Nevada frontal fault zone where asthenosphere has upwelled to replace The Kern Canyon fault localized along a Cre- (Fig. 1), Holocene vertical slip rates are 0.2 the delaminated material. taceous ductile, transpressional shear zone in to 0.3 mm/yr (Le et al., 2007), but dextral slip Movement within the southern Sierra Nevada the southern Sierra Nevada. It was reactivated dominates at ~1.8 mm/yr (Lee et al., 2001). microplate is dominated by horizontal exten- fi rst during regional early Miocene extension Consistent with mapping and slip-rate analyses sion and broad uplift. The local phenomenon of as a transfer structure between two northeast- of Le et al. (2007), the modern-day strain fi eld upper-mantle horizontal fl ow during the process southwest–extending domains. Normal dip-slip is characterized by transtension (Unruh and of mantle lithosphere delamination could pro- displacement initiated along the Kern Canyon Hauksson, 2009). Our estimates of vertical slip duce horizontal extension and vertical thinning fault in mid-Quaternary time, following locally rate along the Kern Canyon fault have a wide in regions adjacent to the descending material driven extension, uplift, and volcanism in the spread at 0.01 to 3.0 mm/yr, but fi eld evidence (Le Pourhiet et al., 2006). The relatively thin southern Sierra Nevada. Several other Quater- indicates little to no component of dextral strike- lithosphere (Fliedner et al., 2000) with higher nary normal faults of shorter length and smaller slip displacements along the Quaternary scarps, heat fl ow (Saltus and Lachenbruch, 1991) in displacement developed at the same time, and and seismic evidence also supports exclusively the southeastern Sierra Nevada may produce accommodated westward tilt of the southern normal dip-slip faulting. Deformation on the asymmetric fl ow that concentrates deformation part of the batholith. Slip-rate estimates based Kern Canyon fault thus appears to be decoupled on the eastern margin of the Isabella anomaly. on scarp-height measurements and GPS verti- from dextral shear in the Eastern California The zone over which extensional deformation cal movements range from ~0.01 to 3.0 mm/yr. Shear Zone and transtensional shear along the has been resolved lies directly over the Isabella The lower estimate places initiation of recent eastern Sierra range front. anomaly, the high-velocity anomaly in the upper faulting immediately following deposition of a Finally, we consider the geographic and tem- mantle interpreted to result from convective normal-faulted 3.5 Ma lava fl ow at the northern poral distribution of faulting in the southern descent of the Sierra Nevada mantle lithosphere end of the Kern Canyon fault. Field evidence Sierra Nevada. Miocene transfer faulting deeper into the mantle (Fig. 2; Benz and Zandt, indicates that greater and more frequent dis- along the Kern Canyon fault system in the 1993; Jones et al., 1994; Saleeby et al., 2003; placements occurred along the southern part of southwestern Sierra Nevada was followed by Boyd et al., 2004; Jones et al., 2004; Zandt the Kern Canyon fault, near the Isabella basin, Pliocene–Quaternary uplift of the Breckenridge- et al., 2004). This high-velocity domain has where rupture along the fault has occurred in the Greenhorn horst and Quaternary reactivation of been referred to as a “drip” structure because past 3.3 ka, and seismic activity continues today. the Kern Canyon fault system, which was fol- it is cylindrical in shape, ~100 km in diameter, We favor a higher slip rate for the southern part lowed by the 1983–1984 Durrwood Meadows and extends from near the base of the crust to of the Kern Canyon fault, and caution against extensional swarm. These extensional features a depth of ~225 km (see Fig. 2 of Saleeby and assigning a single slip rate to the entire length of young to the east. This suggests that deforma- Foster, 2004). Southern Sierra Nevada topogra- the fault. Quaternary scarps emerge discontinu- tion is migrating eastward across this part of the phy suggests that the most recent uplift is related ously or are heavily modifi ed by erosion along batholith, which contrasts with the westward to the ascent of buoyant asthenosphere into the its length, suggesting discontinuous rupture. migration of Basin and Range faulting. region vacated by the drip as it descends into Normal faulting in the southern Sierra We argue here that delamination and surface the deeper mantle beneath the southwestern Nevada batholith has migrated eastward since extension resulting from it is the most likely Sierra Nevada and adjacent San Joaquin basin Pliocene time. Modern seismicity segregates

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For these reasons, we suggest that the Chester, J.S., 2001, Structure and petrology of the Kern Can- extension of the Western Idaho shear zone, McCall, Kern Canyon fault is responding to local yon Fault, California: A deeply exhumed strike-slip Idaho: Geological Society of America Bulletin, v. 118, fault: Texas A&M University, Department of Geology p. 259–273, doi: 10.1130/B25382.1. horizontal extension imposed from below by and Geophysics. Goodman, E.D., and Malin, P.E., 1992, Evolution of the convective removal of the mantle lithosphere Clark, M.K., and Farley, K.A., 2007, Sierra Nevada river in- southern San Joaquin Basin and mid-Tertiary “transi- 4 3 beneath the southern Sierra Nevada. cision from apatite He/ He thermochronometry: Eos tional” tectonics, central California: Tectonics, v. 11, (Transactions, American Geophysical Union), v. 88, p. 478–498, doi: 10.1029/91TC02871. p. F2186. Greene, D.C., and Schweickert, R.A., 1995, The Gem Lake ACKNOWLEDGMENTS Clark, M.K., Mahéo, G., Saleeby, J., and Farley, K.A., 2005, shear zone; Cretaceous dextral transpression in the The non-equilibrium landscape of the southern Sierra northern Ritter Range pendant, eastern Sierra Nevada, E.N. thanks Ronn Rose of USACE, David Simpson Nevada, California: GSA Today, v. 15, p. 4–10, doi: California: Tectonics, v. 14, p. 945–961. of URS Corporation, and William Page for the heli- 10.1130/1052-5173(2005)015[4:TNLOTS]2.0.CO;2. Hammond, W.C., and Thatcher, W., 2004, Contemporary Clinton, J.F., Hauksson, E., and Solanki, K., 2006, An evalu- tectonic deformation of the Basin and Range Province, copter survey. We thank Carl Tape for seismic data ation of the SCSN moment tensor solutions: Robust- western United States: Ten years of observation with analysis, Nathan Niemi and Ken Hudnut for guidance ness of the Mw magnitude scale, style of faulting, and the global positioning system: Journal of Geophysical with GPS data, and Joann Stock for comments. We automation of the method: Bulletin of the Seismo- Research, v. 109, 21 p. acknowledge the Southern California Integrated GPS logical Society of America, v. 96, p. 1689–1705, doi: Hill, M.L., 1955, Nature of movements on active faults on Network and its sponsors, the W.M. 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