Internal Deformation of the Southern Sierra Nevada Microplate Associated with Foundering Lower Lithosphere, California

Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California themed issue

Internal deformation of the southern Sierra Nevada microplate associated with foundering lower lithosphere, California

Jeffrey Unruh1, Egill Hauksson2, and Craig H. Jones3 1Lettis Consultants International, Inc., 1981 North Broadway, Suite 330, Walnut Creek, California 94596, USA 2Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, USA 3Department of Geological Sciences and CIRES (Cooperative Institute for Research in Environmental Sciences), CB 399, University of Colorado Boulder, Boulder, Colorado 80309-0399, USA

ABSTRACT here represents westward encroachment of Sierra Nevada east of the Isabella anomaly. The dextral shear into the microplate from the seismicity represents internal deformation of the Quaternary faulting and background eastern California shear zone and southern Sierra Nevada microplate, a large area of central seismicity in the southern Sierra Nevada Walker Lane belt. The strain rotation may and northern California that moves ~13 mm/yr microplate are concentrated east and south refl ect the presence of local stresses associated to the northwest relative to stable North Amer- of the Isabella anomaly, a high-velocity body with relaxation of subsidence in the vicinity ica as an independent and nominally rigid block in the upper mantle interpreted to be lower of the Isabella anomaly. Westward propaga- (Argus and Gordon, 1991, 2001). At the latitude Sierra lithosphere that is foundering into the tion of foundering lithosphere, with spatially of the Isabella anomaly, the majority of micro- astheno sphere. We analyzed seismicity in this associated patterns of upper crustal deforma- plate translation is accommodated by mixed region to evaluate patterns of upper crustal tion similar to those documented herein, can strike-slip and normal faulting in the southern deformation above and adjacent to the Isa- account for observed late Cenozoic time- and Walker Lane belt (Fig. 1), a zone of distributed bella anomaly. Earthquakes in the southern space-transgressive deformation in the south- northwest-directed dextral shear east of the Sierra and San Joaquin Valley were relocated ern Walker Lane belt east of the Isabella Sierra Nevada and north of the Garlock fault. using joint hypocentral inversion and double- anomaly, and is a potentially observable con- Through kinematic analysis of earthquake focal difference techniques, and groups of focal sequence of the foundering process in other mechanisms, Unruh and Hauksson (2009) doc- mechanisms were inverted for the components orogens. umented an east to west transition from north- of a reduced deformation rate tensor. The northwest–directed dextral shear in the southern deformation fi eld derived from this analy sis INTRODUCTION Walker Lane belt to west-northwest extension reveals two distinct departures from horizon- and vertical thinning in the southern High tal plane strain associated with distributed This paper presents a systematic analysis of Sierra, and suggested that internal deformation northwest-directed dextral shear east of the seismogenic deformation above and around the of the Sierra block is driven by a combination of Pacifi c plate: (1) heterogeneous extension and Isabella anomaly, a narrow, vertically elongated distributed plate motion and local forces asso- crustal thinning in the high Sierra and west- zone of anomalous high P-wave speeds in the ciated with removal of lower lithosphere. Our ern foothills east of the Isabella anomaly; and upper mantle beneath the southern San Joa- goal here is to extend the study area (of Unruh (2) pronounced counterclockwise rotation of quin Valley, California (Benz and Zandt, 1993; and Hauksson, 2009) to the west (Fig. 1), and the principal strains from regional trends in Fig. 1). Following Jones et al. (2014), the Isa- compare patterns of upper crustal deformation the southwestern Sierra Nevada and across bella anomaly is centered below lat 36°N, long with new tomography and seismic imaging of the Kern Arch. Based on comparison with a 119.3°W. It is ~100 km in diameter, extends to the lower crust and upper mantle beneath the three-dimensional tomographic model, the depths of ~200–225 km, and is characterized southern Sierra Nevada microplate (Reeg, 2008; extension in the southern Sierra is spatially by 4%–5% increase in P-wave speed relative Frassetto et al., 2011; Jones et al., 2014). associated with relatively thinner crust and to adjacent asthenospheric mantle. The Isabella We also assess regional variations in defor- anomalous low P-wave speeds in the upper anomaly is approximately equant in plan view mation style in the context of late Cenozoic mantle (40–90 km depth range) directly east and appears to plunge ~60° to 70° east in cross- time- and space-transgressive deformation in of the Isabella anomaly. These relations sug- sectional views (Jones et al., 2014). the Walker Lane belt to the east. As discussed gest that seismogenic crustal thinning is local- The Isabella anomaly has been interpreted by Saleeby et al. (2012, 2013), a predicted and ized above upwelling asthenosphere that is to be lower lithosphere that detached from the potentially observable consequence of the foun- replacing foundering lithosphere. Counter- base of the Sierra Nevada and is foundering or dering process is epeirogenic transients. We clockwise rotation of strain trajectories in convectively descending into the asthenosphere propose that kinematic transients also may sys- the southwest Sierra occurs southeast of (Saleeby et al., 2003; Zandt et al., 2004; Jones tematically occur in the crust as the foundering the Isabella anomaly, and is associated with et al., 2004; Boyd et al., 2004). In a previous process propagates laterally, and we develop this seismogenic west-northwest–striking dex- study (Unruh and Hauksson, 2009), we evalu- hypothesis by comparing our results with geo- tral faults. We suggest that the deformation ated background seismicity in the southern logic data from the southern Walker Lane belt.

Geosphere; February 2014; v. 10; no. 1; p. 107–128; doi:10.1130/GES00936.1; 6 ﬁ gures; 1 table; 2 supplemental ﬁ les. Received 7 April 2013 ♦ Revision received 7 November 2013 ♦ Accepted 10 December 2013 ♦ Published online 14 January 2014

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F long 120.7°W to 118.167°W. We collected a 337° u 7° earthquake data from the Southern California l D N t P Seismic Network and Northern California Seis- IIWW mic Network to determine 3-D Vp and Vp/Vs, Gar crustal models of the study region using the lock F methods of Thurber (1993). We used these mod- au MMojaveojave lt els to relocate the background seismicity using bblocklock EEasternastern double differencing techniques (Waldhauser CCaliforniaalifornia and Ellsworth, 2000). In the fi nal step of data sshearhear processing we determined fi rst-motion focal zzoneone mechanisms for ~20,000 events with 12 or more fi rst motions. We used the grid-searching algorithm and computer programs by Reasenberg 115°11 5 and Oppenheimer (1985) to determine the fi rst- ° W motion, lower hemisphere focal mechanisms. In most cases, the fi rst-motion focal mechanisms Figure 1. Oblique Mercator map of the study region using the Pacifi c–Sierra Nevada Euler are well constrained by the combined azimuthal pole (Argus and Gordon, 2001) as a basis for projection. The blue band illustrates distrib- coverage of both networks. uted strike-slip deformation in western California that accommodates ~75% of the motion between the Pacifi c plate and stable North America. The green band encompasses the east- KINEMATIC ANALYSIS ern California shear zone and Walker Lane belt, which in southern California accommodate the remaining 25% of Pacifi c–North America motion. The black box outlines the region We used a micropolar continuum model for covered by the layered geospatial data sets in Figure 2. The dashed line dividing the black distributed brittle deformation (Twiss et al., box indicates the seismicity study area discussed herein (western part), as well as the region 1993; Twiss and Unruh, 1998) as a basis for evaluated in Unruh and Hauksson (2009) (eastern part). O—Owens Valley; S—Saline Val- inverting focal mechanisms from groups of ley; P—Panamint Valley; IW—Indian Wells Valley; D—Death Valley. earthquakes to derive a reduced deformation rate tensor. For a detailed description of the analytical approach, see Unruh and Hauksson PRESENTATION OF SPATIAL DATA P-wave model derived from the ambient noise (2009, and references cited therein). tomography of Moschetti et al. (2010), which The study area in Figure 1 was subdivided Geospatial data sets covering the study area in was held fi xed above 90 km depth for 14 itera- into smaller regions to evaluate the local seis- Figure 1 that are referenced in this paper include tions and then freed. Thus the shallower part of mogenic deformation associated with key struc- a hillshade map of topography; smoothed ele- this particular inversion most closely resembles tures and geophysical features, and to capture vation contours of the Sierra Nevada (1000 ft a smoothed version of the Moschetti et al. (2010) lateral variations in deformation geometry. A intervals, i.e., 304.8 m) and San Joaquin Val- results. The tomography depth sections and spa- map of the polygonal subregions is provided ley (100 ft intervals, i.e., 30.48 m); seismicity tial data sets herein were exported from a fully in the Supplemental File1. Inversions of data recorded from 1981 to 2008 for which focal georeferenced geographic information systems from individual subregions were performed mechanisms are calculated; Quaternary faults (GIS) database and organized for viewing in a using an automated grid search algorithm called (Jennings, 1994; Kelson et al., 2010); depth to layered Acrobat (pdf) fi le (animated version the Moho (Frassetto et al., 2011); 40 km, 70 km, of Fig. 2); individual layers corresponding to 1Supplemental File. Zipped fi le containing ex- and 170 km depth slices through a three-dimen- discrete spatial data sets are labeled A through planatory text, a PDF map of the locations and geom- sional (3-D) P-wave tomographic model (Jones O. The layered pdf format permits selection of etries of individual subregions sampled for kinematic et al., 2014); regional geology; and derivative these data sets for viewing individually or in inversion of focal mechanisms, and two Excel tables of data analysis. If you are viewing the PDF of this maps of the regional deformation fi eld from combination: i.e., to view certain combinations paper or reading it offl ine, please visit http:// dx.doi the results of the seismicity analysis. The tomo- of data, use the layering function of Figure 2. .org/10.1130 /GES00936 .S2 or the full-text article on graphic inversion we chose was started with a Note that when Figure 2 is fi rst opened in Acro- www.gsapubs .org to view the Supplemental File.

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FLTSLP_2K6 (L. Guenther and R. Twiss, Uni- versity of California, Davis; see Appendix D in Guenther, 2004, for user’s manual). The inver- Figure 2 (on following page). Geospatial data for the region outlined by the black box in sion algorithm incorporates standard bootstrap Figure 1. (To view the animated version of Fig. 2, please visit http:// dx .doi .org /10.1130 methods to estimate uncertainties in the best-fi t /GES00936 .S1 or the full-text article at www.gsapubs.org.) The individual data sets are model parameters. The micropolar deforma- organized in a series of georeferenced layers that may be viewed separately or in combi- tion model is parameterized by the following: nation using the capabilities of the Acrobat (pdf) layering function (click “Layers” icon (1) the orientations of the principal strain rates along vertical bar on left side of window for display of available layers; turn layers on or off by clicking the box to the left of the layer name; to view the legend for the fi gures, (d1 > d2 > d3; lengthening positive); (2) a scalar parameter (D) formed by a ratio of the differ- see O). See text for recommendations of specifi c layer combinations as appropriate. Indi- ences in the principal strain rates, that char- vidual layers include the following. (A) Quaternary faults. Faults designated as Quater- acterizes the shape of the strain rate ellipsoid; nary in age from Jennings (1994), modifi ed with additional mapping and data from Brossy (3) a scalar parameter (W ) that characterizes the et al. (2012). (B) Kinematic domains. Dotted lines outline areas with deformation styles relative vorticity of rigid, fault-bounded blocks that are distinctly different from horizontal plane strain associated with northwest dex- about an axis parallel to the intermediate princi- tral shear. The outlined areas without shading and containing arrows are characterized by horizontal extension and vertical thinning; the orientations of the arrows indicate the pal strain rate axis d2; and (4) the ratio V of the vertical deviatoric deformation rate to the maxi- local trend of the maximum extensional principal strain rate d1. The shaded outlined area is mum deformation rate (Unruh et al., 2002). Pos- characterized by horizontal plane strain and a counterclockwise rotation of the maximum itive values of V indicate net crustal thickening, extensional and maximum shortening strains from regional trends, interpreted to refl ect negative values indicate net crustal thinning, and distributed west-northwest–directed dextral shear (arrow). The northwest-elongated area a value of zero indicates horizontal plane strain labeled Transpression is characterized by northwest dextral shear and net crustal thick- (Lewis et al., 2003). ening. See text for further discussion. (C) Cross-section lines. Locations of cross sections Data tables with the inversion results are pro- presented in Figure 4. (D) Geographic features. Labels identify features described in the vided in the Supplemental File (see footnote 1), text. (E) Seismicity. Earthquakes recorded between 1981 and 2008 by the Northern Cali- along with additional details of the analytical fornia Seismic Network and Southern California Seismic Network, for which fi rst-motion approach. focal mechanisms were derived using the methods of Reasenberg and Oppenheimer (1985). The micropolar continuum theory that forms See the Supplemental File (footnote 1) for the location and geometry of subregions sam- the basis for the inversion relates patterns of dis- pled for kinematic inversions. (F) 100 ft (30.48 m) contours, San Joaquin Valley. Contours tributed seismogenic slip to velocity boundary of smoothed topography in the low relief San Joaquin Valley, 100 ft (30.48 m) intervals. conditions (Twiss et al., 1993; Twiss and Unruh, (G) 1000 ft (304.8 m) contours, mountainous topography. Contours of smoothed topography 1998). Because the period of time over which in mountains, 1000 ft (304.8 m) intervals. (H) Strain trajectories. Horizontal trajectories of the earthquakes occurred is very short and maximum extensional strain rate (d1) and maximum shortening strain rate (d3). The tra- essentially instantaneous relative to geologic jectories are drawn parallel to the local orientations of the principal strains derived from time, the incremental strains represented by the inversion of groups of earthquake focal mechanisms (data in Tables A1 and A2 in the Sup- seismicity data are assumed to be equivalent to plemental File [see footnote 1]; see Fig. 3 for locations of subregions sampled for analysis). the instantaneous strain rates that form the basis See text for further discussion. (I) Moho depth. Contours on the depth of the Moho beneath of micropolar theory. For ease of discussion, the the southern Sierra Nevada and Walker Lane belt in kilometers below sea level (data from principal strain rates are referred to in this paper Frassetto et al., 2011). Note that Moho depths under the San Joaquin Valley interpreted by simply as maximum extension and maximum other workers have been interpreted to be ~30 km (e.g., Holbrook and Mooney, 1987; Fuis and Mooney, 1990; Fliedner et al., 1996; Ruppert et al., 1998); we do not attempt to rec- shortening (d1 and d3 of the inversion solution, respectively). oncile these results with the deeper receiver function Moho shown here. (J) Geologic map. The inversion results for individual sub- Geologic map modifi ed from Crafford (2007) in Nevada and from Saucedo et al. (2000) regions are synthesized in a map of the regional in California, which in turn are derived from 1:250,000 county maps in Nevada and Jen- seismogenic deformation fi eld (Figs. 2B, 2H, nings’s (1977) 1:750,000 map in California. (K) 40 km tomography. Horizontal section at 2N). The horizontal components of the defor- 40 km depth through the three-dimensional (3-D) P-wave velocity model of Reeg (2008). See mation are depicted by smooth trajectories explanation layer for key to color ramp used to depict horizontal variation in P-wave speeds. drawn parallel to the local trends of maximum (L) 70 km tomography. Horizontal section at 70 km depth through the 3-D P-wave velocity extension and maximum shortening associated model of Reeg (2008). See explanation layer for key to color ramp. (M) 170 km tomography. with individual inversion results (Figs. 2H, 2N). Horizontal section at 170 km depth through the 3-D P-wave velocity model of Reeg (2008). In cases where the principal strains are steeply See explanation layer for key to color ramp. (N) Hillshade. Hillshade map of topography plunging to subvertical, there is effectively no for the study area outlined in Figure 1. (O) Explanation. Lines, symbols, and colors used resolved horizontal component and their respec- to depict spatial data. See text for more complete descriptions. Also included in this layer tive trajectories are not plotted on the map. The is a graticule with latitude and longitude, a scale bar, and a north arrow. CA—Coalinga interpretation of the absence of one of the princi- anticline; DM—Durrwood Meadows; DR—Diablo Range; IWV—Indian Wells Valley; pal strain trajectories is straightforward. If only LH—Lost Hills; LI—Lake Isabella; KH—Kettleman Hills; AP—Antelope Plain; EH—Elk Hills; WR—Wheeler Ridge; WWF—White Wolf fault; ALFZ—Airport Lake fault zone; d1 trajectories are shown, then the deformation is characterized by horizontal extension paral- OVFZ—Owens Valley fault zone.

lel to d1, and d3 is steeply plunging to subvertical, indicating net crustal thinning. Similarly, if

only d3 trajectories are shown, then d1 is steeply

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plunging to subvertical and the deformation the southern Sierra (Figs. 2A, 2H, 2N), and thus ; V locally is characterized by shortening parallel is optimally oriented to accommodate normal

to d3 with net vertical thickening. In some areas slip in the present seismotectonic regime. The one of the principal strains is subhorizontal and zone of extensional deformation extends south not

the other is moderately plunging. Depending on of Durrwood Meadows to about the latitude of V which of the principal strains is plunging, the northern Lake Isabella, where it terminates and ) associated with s 3 t d associated deformation is transtensional (V < 0; is separated from another elliptical east-west– n e m d3 plunging) or transpressional (V > 0; d1 plung- trending zone of extension by a narrow zone = 0 (horizontal plane strain) m V o

ing). Due to the plunge of one of the principal of dominantly strike-slip faulting and shearing C

strains, the horizontal strain trajectories typi- between Lake Isabella and Walker Basin (Figs. dence interval cally are not orthogonal in areas characterized 2B, 2D, 2H, 2N). As previously noted in Unruh by transpression and transtension. and Hauksson (2009), the Durrwood Meadows

area is characterized by anomalous deformation 1 RESULTS: KINEMATIC DOMAINS where both the d and d principal strains are d of vertical deformation parameter strains from regional orientations seismicity associated with Quaternary-active folds at the 95% conﬁ of releasing stepovers among strike-slip faults 1 2 distinguishable from horizontal and extensional, thus accommodat- The study area in Figure 1 can be divided into ing a ﬂ attening or pancaking of the crust. distinct kinematic domains based on the direc- In Unruh and Hauksson (2009), we previously n

tion of macroscopic shear implied by the orien- inferred that an ~30-km-wide zone of transten- o i t tations of the principal strains, and the relative sional deformation in the eastern Sierra Nevada a m r o

contribution of vertical thickening or thinning to separated regional plane strain and north-north- f e d

the bulk deformation. These kinematic domains west dextral shear in the Walker Lane belt from f o e

are outlined in Figures 2B, 2H, and 2N. The key vertical thinning in the southern High Sierra. l y t characteristics of each domain are summarized The current results do not indicate a smooth s t n

in Table 1; detailed descriptions of the domains east to west kinematic transition or gradient in a n i

are presented in the Supplemental File (see the parameter V between the Walker Lane belt m o footnote 1). and interior of the Sierra microplate. Rather, D Horizontal extension and crustal thinning Systematic north to south counterclockwise rotation Northwest dextral shear dextral shearWest-northwest Northwest dextral shear negative value Possible transtension; however, northwest dextral shear Local counterclockwise rotation of the principal Transpressional Positive values of vertical deformation parameter Sparse seismicity Across most of the study area in Figure 2 the transition between horizontal plane strain North-northwest dextral shear Local extension (i.e., vertical the seismogenic deformation reﬂ ects distrib- and vertical thinning is relatively abrupt and 1

uted northwest-directed dextral shear east of the eastern boundary of thinning is irregular in d , the Paciﬁ c plate. In the Walker Lane belt and plan view and not parallel to the physiographic 1 d t

south-central San Joaquin Valley, the maximum boundary between the Sierra Nevada and south- s a e h

extension and maximum shortening strains ern Walker Lane belt (Figs. 2B, 2D, 2N, 2H). t u , northeast- , northwest- , northeast- o 1 1 1

are subhorizontal and the vertical deforma- Salients of horizontal plane strain extend west- s d d d - t t s s

tion parameter V is zero, indicating horizontal ward from the Walker Lane belt into the east- a a 3 e e d – h t

plane strain. Maximum extension generally ern Sierra Nevada, notably in the southeastern t y , moderately plunging 3 s r 3 u t d e d o e s is oriented west-northwest–east-southeast to corner of the range (Figs. 2B, 2D, 2N, 2H). The w - m h t t s o TABLE 1. SUMMARY OF KINEMATIC DOMAINS OF KINEMATIC 1. SUMMARY TABLE r e northwest-southeast, and maximum shortening northern extent of the Sierra extensional domain e o g n w - t h n t

is oriented north-northeast–south-southwest to is poorly constrained by these data. i s r a e o r t n northeast-southwest, consistent with northwest- w S and subvertical : : 1 n n i d i

directed macroscopic dextral shear. Several Counterclockwise Rotation of the Principal l a a r r a t t t s s ﬁ rst-order variations are superimposed on this Strains across the Kern Arch n o e e 3 3 z 3 n n i d d r d a regional pattern. a l l o p p h l The maximum extension and shortening l y a a l t t l n n a r o Extension and Crustal Thinning in strains in the southwestern Sierra Nevada north o e z z i i n r r southwest southeast southwest north-northeast–south-southwest e o the Southern Sierra Nevada of approximately lat 35°N are subhorizontal o H G and rotated distinctly counterclockwise from H A large contiguous area with values of the regional trends. The zone of strain rotation vertical deformation parameter V ≤ –0.7 (here straddles the east end of the White Wolf fault assumed to indicate dominantly vertical thin- and the southern end of the Breckenridge fault, ning) spans the southern Sierra Nevada and is and encompasses the Kern Arch, an uplifted approximately centered on lat 36°N and long and west-tilted topographic salient in the south- 118.5°W (Figs. 2B, 2H). The Kern Canyon fault, eastern San Joaquin Valley that is mantled by y e t l l a Holocene-active normal fault, is located within Tertiary marine strata and late Cenozoic ﬂ uvial l e a B V a d

this extensional domain. Reconnaissance inves- deposits (Maheo et al., 2009; Saleeby et al., e n i a n u v a q tigations (Nadin and Saleeby, 2010), geologic 2009) (Figs. 2A, 2H, 2N, 2O). The direction e L a N r o e a mapping (Brossy et al., 2012), and paleoseismic of maximum extension in this domain trends J r k l r n a e a trenching (Kelson et al., 2010) indicate that the northeast-southwest and the direction of maxi- i S S W n n n r r Kern Canyon fault primarily accommodates mum shortening trends northwest-southeast, r n i e e e a h h h t t east-down normal slip. The Kern Canyon fault representing an ~40° counterclockwise rotation t m u u u o o o o S S Sierra Foothills north of lat 36.5°N Horizontal plane strain: northwest-southeast Kern Arch and southwestern Sierra NevadaKern Horizontal plane strain: northeast-southwest Southwestern San Joaquin Valley Subhorizontal northeast-southwest S strikes at a high angle to the d1 trajectories in from the orientations of the principal strains D

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in the southern Walker Lane belt and southern For example, a northwest-trending cluster the section of the White Wolf fault between the San Joaquin Valley to the east and west, respec- of events at the southwest end of section Tehachapi Mountains and San Joaquin Valley tively. The maximum extension and shortening A–A′ (Fig. 3B) includes two distinct planar was dominantly reverse with a very minor com- strains are subhorizontal and values of the verti- alignments, which we interpret to be two dis- ponent of right-lateral slip. The dextral compo- cal deformation parameter V are close to zero crete faults. These events are within subregion nent observed in the surface rupture along this (Table A1 in the Supplemental File [see foot- NWW1 (Fig. 3A; Table A1 in the Supplemental reach is anomalous, given the seismogenic and note 1]), indicating that the deformation is char- File [see footnote 1]). The western fault align- geodetic evidence for a dominant left-lateral acterized by horizontal plane strain and crustal ment at horizontal distance ~2.5 m dips steeply component in the coseismic rupture (Bawden, shearing. northeast. The second alignment at horizontal 2001). At the southeastern corner of the San Joa- The counterclockwise rotation of the princi- distance ~3.4 km is subvertical. Steeply dip- quin Valley, where the Sierra Nevada intersects pal strains is superimposed on the regional thin- ping to subvertical seismogenic faults also are the Tehachapi Mountains, the general northeast ning in the southern Sierra described in the pre- present in the 5–8 km depth range between hori- trend of the surface ruptures was crosscut by an vious section. West and northwest of Durrwood zontal distance 7–12 km on section A–A′ (Fig. ~6-km-long north-northeast–striking fault with Meadows maximum extension trends consis- 3B). Inversion results for NWW1 indicate trans- left-lateral offset (Fig. 3A). East of the north- tently west-northwest–east-southeast, similar to tensional dextral shear resolved on west-north- northeast–striking fault segment, the pattern of

d1 trends in the Walker Lane belt to the east. To west–striking faults (Fig. 3B; Table A1 in the northeast-trending fractures along the surface the south, maximum extension trends approxi- Supplemental File [see footnote 1]). Northwest- trace of the White Wolf fault resumed; however, mately east-west between Durrwood Meadows trending alignments of epicenters crossed by many of the ruptures showed evidence for both and Lake Isabella. In a smaller and isolated the northeastern part of section B–B′ (Fig. 3A) extensional and left-lateral displacement (Fig. region of extension and crustal thinning between appear to be associated with subvertical faults 3A). Near the eastern limit of the approximately

Lake Isabella and Walker basin, d1 trends west- in the 7–9 km depth range between horizontal continuous coseismic rupture, the main surface southwest–east-northeast. The ﬂ attening defor- distances 9 and 15 km (Fig. 3C). These events trace turned abruptly north-northeast and exhib- mation at Durrwood Meadows appears to span are within subregion SEW3 (Table A1 in the ited a left-lateral component of slip, similar to or encompass the counterclockwise rotation of Supplemental File [see footnote 1]), and inver- the 6-km-long north-northeast–striking segment

d1 from west-northwest–east-southeast to the sion results indicate distributed dextral shear to the west. Surface deformation progressively north to approximately east-west and northeast- on a west-northwest–striking structural fabric, died out east of this point, accommodated by southwest to the south. It is possible that the local with a possible small component of horizontal short, discontinuous north-south– to north- ﬂ attening deformation at Durrwood Meadows extension and crustal thinning (V = –0.2) that east-striking faults accommodating extension is a combination of the west-northwest–east- cannot be distinguished from horizontal plane (Buwalda and St. Amand, 1955). southeast–directed extension at approximately strain at the 95% conﬁ dence interval (Table The change in character of the 1952 sur- lat 36°N, and the northeast-southwest–directed A1 in the Supplemental File [see footnote 1]). face rupture from dominantly reverse slip on extension to the south in the Lake Isabella area; Planar alignments of hypocenters also are pres- a northeast-striking fault along the base of the that is, rather than a smooth north to south varia- ent in the 5–9 km depth range in section C–C′ western Tehachapi and San Emigdio Mountains

tion in the orientation of d1, there are discrete (Fig. 3D), but are less well expressed than on the (plus a minor, anomalous component of dextral

domains of west-northwest–trending d1 to the other two sections. The clusters highlighted in motion), to a mix of left-lateral slip on north-

north and northeast-southwest–trending d1 to Figure 3D correspond to subregions SWNB and northeast–striking faults and reverse-left slip on the south that overlap or interfere in the vicin- SEW2 (Fig. 3); inversion results indicate dis- northeast-striking faults occurs in the vicinity ity of Durrwood Meadows, resulting in the two tributed dextral shear on west-northwest–strik- of counterclockwise rotation of the principal horizontal principal strains there both being ing faults for both subregions (Table A1 in the strains. Left-lateral slip on the north-north- extensional. Supplemental File [see footnote 1]). east–striking segments of the surface rupture is

Although the strain geometry in the vicinity The principal strains from inversions of earth- consistent with local rotation of d1 to a north-

of the Kern Arch could be accommodated by quakes in this region are oriented ~45° to the east-southwest orientation and d3 to a north- shearing along either west-northwest–striking strike of the steeply dipping to subplanar seis- west-southeast orientation (Fig. 3A). Although dextral faults or north-northeast–striking sinis- mogenic faults, indicating resolved right-lateral the 1952 surface rupture at the eastern end of tral faults, patterns of seismicity between the motion on these and other west-northwest– the White Wolf fault is very complex and likely White Wolf fault and Breckenridge fault suggest striking structures. Despite the proximity to the includes effects of gravitational slope failure that the macroscopic deformation is character- northeast-striking White Wolf fault, west-north- triggered by strong ground shaking (Buwalda ized by distributed west-northwest–directed west–striking planar alignments are dominant and St. Amand, 1955), the fi rst-order change in dextral shear (Fig. 3). Relocated earthquakes in Figure 3A. We thus infer that seismogenic faulting style is consistent with the rupture prop- defi ne a series of west-northwest–striking pla- deformation in this region primarily refl ects dis- agating across, and refl ecting, a counterclock- nar alignments of hypocenters in an ~20 km by tributed west-northwest–directed dextral shear. wise rotation of the principal strains between the 20 km area encompassing the eastern end of the Additional evidence for local counterclock- Tehachapi Mountains and southwestern Sierra White Wolf fault and southern end of the Breck- wise rotation of the principal strains in the Kern Nevada (Figs. 2A, 2H, 2N). enridge fault (Fig. 3A). Cross sections normal to Arch region can be inferred from patterns of the linear trends of the epicenters reveal steeply surface faulting that occurred during the 1952 Transpression in the Southwestern dipping to subvertical planar alignments of M 7.2 Arvin-Tehachapi earthquake on the White San Joaquin Valley hypocenters in the 1–8 km depth range (Figs. Wolf fault, the eastern end of which encroaches 3B, 3C), indicating that the west-northwest– into the domain of west-northwest–directed dex- Positive values of the vertical parameter V trending epicentral lineaments are associated tral shear (Fig. 3A). As documented by Buwalda (Table 1) in the Elk Hills and Antelope Plain with high-angle faults. and St. Amand (1955), surface rupture along regions (ELKH and ANTP, respectively; Table

112 Geosphere, February 2014

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/1/107/3332669/107.pdf by guest on 29 September 2021 Internal deformation of the southern Sierra microplate

!! ! ! ! ! !! ! ! ! ! ! ! ! ! !!! ! ! ! ! ! !! ! ! 118°40′0″W ! 118°35′0″W 118°30′0″W ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! !! ! ! ! !! !!! ! !!!!!! ! ! ! !!!!! ! ! !!!!!! ! ! ! !!! !! !!! ! !! !!! ! !! A ! ! !! !! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! !! ! ! ! ! !! ! !! ! !! ! ! ! ! !!! ! ! ! !!! ! ! ! ! ! ! ! ! ! ! ! ! !! ! !! ! ! ! !!!! ! ! ! !! !!!! ! ! ! !! !!! !!!! ! !! !!! ! ! ! !!!! ! ! ! !! !!! ! ! ! ! !!! ! ! ! !! ! ! ! ! ! !! ! ! ! ! ! ! ! !!!! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Sierra !!! t ! ! ! ! ! ! l ! ! ! !!! ! ! ! !! ! ! Nevada u ! ! !! ! ! ! ! ! ! ! a ! ! ! f ! ! ! ! ! !!!! ! ! ! !! ! ! !! ! ! ! ! !! ! ! e ! ! !N ! ! ! !! ! ! ! ! ! ! ! g ! ! ! ! ! ′0″ ! ! ! ! ! Walker ! d ! ! i ! ! ! ! ! ! ! ! ! ! r ! ! ! ! ! !!!! !A′ Basin ! ! ! ! !! 35°25 ! ! ! !! n ! ! ! ! ! ! ! e !!!! ! ! ! ! ! ! ! ! ! !!!! ! ! ! k ! !! ! ! ! ! !! !! ! !!! ! ! ! ! ! ! ! ! !!! ! c ! ! ! ! ! ! ! ! ! !! ! ! ! ! !!! !! ! e ! ! ! !! ! ! !! ! ! ! !! !! ! r !! ! !!!!!!!!!!! !! !!!! ! ! ! !!!!!!!! ! ! !!!!!! ! ! !!!!!!! ! ! ! !!! ! !! ! ! BBreckenridge fault ! !!!!!!! ! ! ! ! ! ! !!!!!!!! ! ! ! ! !!!!!! ! B′ ! ! ! !!! ! ! NNWW2WW2 ! !!!!!!!! ! !! ! !!! ! ! ! ! ! ! ! ! !! !!!! !! !! ! ! !!! ! !! ! ! !! SSEW3EW! 3! ! ! ! ! !! ! ! !! ! !! ! !!! ! ! ! ! ! ! ! ! !!! !!! ! ! ! ! !! !!!! ! ! ! ! ! !!!!!!!! !! ! ! ! ! !! ! ! ! ! ! ! !!!!!! ! !!!! ! ! ! ! ! ! ! ! ! ! ! !! ! !! ! C′ ! ! ! !!! ! ! ! !! ! !! !!! !! ! ! ! ! ! ! !!! ! !! !! !! ! ! ! ! ! ! ! ! ! ! ! !! !! !! ! ! ! ! ! ! ! ! ! !! ! !! ! !! ! ! ! !! ! !!!! ! ! !!! ! ! ! ! ! ! ! !! ! ! ! !! ! ! !! !!!! ! ! !!! ! ! ! ! ! !! ! ! ! !! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! !! Figure 3 (on this and following ! ! ! !! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! !! ! ! ! ! ! ! ! ! !!! ! ! ! ! ! !! ! ! ! ! ! ! ! !!!! ! !! ! ! ! ! ! ! ! ! !!!!!! !!!! !! !! ! ! ! ! ! ! ! !!!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! !! ! ! ! !! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! !! !!!!!!! ! ! ! ! ! !! three pages). (A) Detail of seis- ! ! !! ! ! ! ! !!!!!!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !!! ! ! ! !! !!!!! !!! !!! ! ! ! !! ! ! !! ! ! !! ! ! ! !! !!!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !!! !! !! !! ! ! ! ! !!! ! ! !! ! ! ! ! ! ! ! ! ! ! !! !! ! !!!! ! ! ! ! ! ! ! ! ! !! ! ! ! !! !!! ! !! ! ! ! !! ! ! ! ! ! ! ! ! ! ! !!!! !! ! ! ! ! ! ! ! ! ! ! ! !!! ! ! ! ! ! ! ! ! ! ! micity in the southwestern ! ! ! ! ! ! ! ! !! ! ! ! ! !! !! ! ! !! !! ! !! ! ! ! ! ! ! ! ! ! ! !! !! ! ! ! !! !!!!! ! !! ! ! ! !! ! ! ! ! ! !!! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! !! !! ! ! ! ! ! ! ! !!!! !! ! !!!! ! ! ! ! !! ! !! ! ! ! !! ! ! ! ! ! ! ! ! !!! !! ! ! ! ! ! !! ! ! !! ! ! !! ! !!! !! ! ! ! ! ! !!! ! ! ! ! ! ! !! !!!!! ! !! !! !!! !! ! ! ! ! ! !! ! !!! !!! ! ! ! ! !! !!!!! ! ! !! !! !! ! ! ! ! ! !!!!! !! ! ! ! ! ! !!! ! ! !!! ! ! ! Sierra Nevada between the east- ! ! ! ! ! ! !!!! !!! ! ! ! ! ! !!!! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! !! !! !! !! ! ! !!! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !!! !!! ! ! ! !!! ! !!! ! !!!! ! ! ! ! ! ! ! !! ! ! !! ! !! ! ! ! ! !! ! ! ! ! !! ! !! ! ! ! ! ! !! ! !!!!!!!! ! !! ! ! ! !! !! ! ! !!! ! !! ! ! ! !!!! ! ! ! ! !!! !! ! ! ! ! ! ! !! !! !!!!! ! !! !!!!!! !! ! !!! !! !! !! ! ! !!! ! !! ! ! ! ! ! ! ! ! ! !!!! ! !! ! ! ! ! !! !!! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! !! !!! !!! !! ! ! !!!!! ! ern end of the White Wolf fault, ! ! !! ! !! !!!! ! ! ! ! ! !! ! !!!!!! !! ! ! ! ! ! ! ! !! ! ! ! ! !! !! ! ! ! !!!!!!! ! !! ! !!!! ! ! ! !!! ! ! ! ! ! ! ! ! ! ! ! ! !!!!!!!!! !! !!!! !!!!!! ! ! !! ! !! ! ! ! !! ! ! ! !!!!!!!!!!! ! ! ! ! !!! ! ! ! ! ! !!! ! ! ! ! ! ! ! ! !!!!!!!!!!! !! !!! ! !!! !! ! ! ! ! ! ! !!! ! ! ! ! ! ! ! ! !!!!!!! !! !!! !! !! !! ! !! ! ! ! ! ! ! ! ! ! ! !! !! ! !!!!!!! !!!! ! !!! ! !! !! ! !! !!! ! ! ! ! !!! ! ! !! !!! ! !!!!! ! ! ! ! ! ! !! ! ! ! ! !!! ! ! ! ! ! ! !!!!!!! !!!!!! ! !!!! !! ! ! !! ! !! ! southern Breckenridge fault, ! ! ! ! !! ! ! !!!!! ! ! ! ! ! !!!!! ! ! ! !! !! ! ! ! ! ! !! ! ! !! ! !!!! !! ! !!! !! ! !! ! ! !!! ! ! ! ! !! !! ! ! ! ! ! !!! ! ! ! !!!! ! !! ! ! ! !! ! ! ! ! ! !! ! ! ! !! !! !! ! !! ! ! ! ! ! ! ! ! !!!!!! ! ! ! ! ! ! ! !! ! ! ! !! ! !! !!!!! ! ! ! !! !! ! !!! !!! ! !! ! ! ! !!!!! ! ! !!! !! ! ! !! ! ! !!!!!! ! !! ! !! !!! ! ! !!!! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! !! !!!!!!!!! ! ! ! ! ! !! ! ! ! ! !! !!!!!!!!!!!!! ! ! !!!! ! ! ! ! ! !! ! ! ! ! !!!!!! !!!! ! ! N !!!!!!!!!!!!!!! !! ! ! ! ! ! ! ! !!! ! ! ! !!! !! ! ! and Walker Basin (for location, !!!!!!!!!!!!! !! ! ! ! ! ! ! ! ! !! ! ! !!! !!! !! ! ! ! ! ! ! ! !!!!! ! ! ! ! ! !! ! ! ! !!!!!!!!!!!!!!! ! ! !!! !!!!!! ! ! ! ! ! ! ! ! ! !!! ! !!!!!! ! ! ! ! ! ! SSEW2EW2 ! !! !!!!!!!!!! ! ! !!! ! ! ! !! ! ! ! ! ! ! ! !! !! ! ! ! ! !!!!!!!!!!!! ! !! ! ! ! ! ! ! !!! ! ! !!!!! ! ! ! ! ! ′0″ !!! !!!!!!!!!!!!!!! ! ! !! ! !! ! ! ! ! !!!!! ! ! ! !!!! !!!!!! ! ! !! !! ! ! ! ! !!!!!!!! ! ! ! !!!!!!!!! !! !!! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! !!!!! ! ! ! ! !!!!!!!!!!!!!!!!!!!!!!! ! !! ! ! ! ! ! !! ! ! ! ! !! ! ! ! !!! !!!!!!!!!!!!!!!!!! !!! ! ! !!!!!!! ! ! !! ! ! ! ! ! ! ! ! ! ! ! see Figs. 2A, 2D, 2E, 2N). Green ! !! !!!!!!! !!!! ! ! ! ! !! !! ! !!! ! ! ! ! ! !! ! ! !! ! ! ! !!! !! !!!! !! ! ! !! !!!!! ! !! ! !!! !! ! !! ! ! ! !!! ! ! ! ! ! ! ! ! ! !! ! ! ! ! !! ! ! ! !! !! !! ! !! ! ! ! ! ! ! ! !!!!!! ! ! !!!! !! ! ! !! ! ! ! ! ! ! !! !! !! ! ! !! !!! ! ! ! ! ! ! ! !! ! !!!!! ! ! !! ! !! ! ! ! ! ! ! ! ! ! ! ! !!! !! !! ! ! !!! ! ! ! ! ! ! ! ! ! ! ! !! ! !! !!! !! ! ! ! !!!! ! ! ! 35°20 ! ! !! ! !!!!!!! !! !!!!! ! !!! ! ! ! ! ! ! ! ! !! !!! ! ! ! ! !!!! !!!! ! ! ! ! ! ! !!! !!! !! ! !! ! ! ! ! ! ! ! !!! ! ! ! ! ! ! ! ! ! lines show locations of seis micity ! ! !!!! !!!!!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! !!! !!!!!!! ! ! !! ! ! !!!!!!! !! !! ! ! !!! !! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! !! !!!!!!!!!!! ! ! ! !! !!!! ! !! ! ! ! ! !! ! ! ! ! ! ! ! ! ! !! ! ! !!!!!!!!!!!!! !!! !! !!! ! ! ! ! ! !! !!!! ! ! ! ! ! ! ! ! ! !! ! ! ! !!!! !!!!!! !!!!!!!!!! ! ! ! !!!! ! ! ! ! !! ! ! ! ! ! ! ! !!! ! ! ! ! !!!! ! ! !! ! ! !!!! ! !! ! ! !! ! !!! ! ! ! ! ! ! !! ! ! !! !! !!! !!!!!!! !!!! !! ! !! ! !! ! ! ! ! ! !! ! ! ! ! ! ! ! ! !!! ! !!!!! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !!! !! ! !! !! ! ! ! ! !!! !! !! !! ! !! !! ! !!!!!!!!! ! ! ! !! !! !! !!! ! ! ! ! ! cross sections (B–D). Boxes sur- !! !!!!! ! !! !! !!! ! ! ! ! ! !! ! !! ! ! ! ! !! !! !! !!! ! ! ! !! ! !! !! ! !!! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! !! !! !! ! ! ! ! ! !!! ! ! ! ! ! ! ! !!!!! ! ! ! ! !!! ! ! !!! ! !!!!!! !!!! ! ! ! ! !!!! ! !! ! ! ! ! !! ! ! !! ! !! !! !! !!!! ! !! !!! !!!!! ! !!! ! ! !!! ! ! ! !! ! !!! ! ! ! !! ! ! ! ! ! !!!!! !! ! ! ! !!!! ! ! ! ! !!! ! !! !!! ! ! ! !! ! ! ! ! ! !! !! ! !!!!! !!!!!!! ! ! ! ! !!! ! !! !! !! ! ! ! ! ! !!!! !!!!!!!!!!!!!!!!! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! !! !!!!! ! ! ! ! !!!!! !! !!!! !!!! ! ! !!! ! ! ! ! ! !! !! !! ! !! ! ! !!! ! !!!!!!! ! ! ! !!!!! !! !!!! !!! ! ! ! ! ! ! ! ! ! !!! !! !! !!! ! !!!!! !! ! ! ! ! ! !! ! rounding the section lines show !!! ! !! !! !!! !! ! !! ! ! ! !! ! ! ! ! !! ! ! !!! !! !!!!!!! ! !!!! ! ! ! ! ! !! ! ! ! ! ! ! !!!! !!!!!! !!!!! !! !! ! ! !! !! ! ! ! ! !!! ! !!!! ! ! ! ! ! ! ! !! !! !! ! !!!!!! ! ! ! ! !! ! ! ! ! ! !! ! !! ! ! ! ! !! ! !! ! ! ! !!!! !!!! !!!! ! !! ! ! !!!! ! ! ! !! !! ! !! ! ! ! ! ! ! ! ! ! !!!! !! ! ! !!! ! ! !! ! ! ! !!! ! ! ! !!! !!!!!!! ! ! ! ! !! !!!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! !! ! ! !!! ! !! A ! ! ! !! !! ! ! !!! ! ! ! ! ! ! !! ! !! ! ! !!! !! ! ! !!!!! ! ! ! ! !!! !!! ! ! ! !!! ! ! ! ! ! ! ! ! ! ! !!! !! ! !! ! !!!! !! !! ! ! !! ! ! !! ! ! ! ! ! ! ! !!! !! ! ! ! ! !!! !! ! ! ! ! ! ! ! several discrete widths that ! !!! !! ! ! ! ! !! ! ! ! !! ! ! !! ! !! !!!!! !! !! ! !!! !!! ! ! !! ! !! ! !! ! ! ! !! ! !! ! ! !! ! !!! ! ! ! ! !! ! ! !! !!!!!!! ! ! ! !! ! ! ! ! !! !!! !!! ! ! ! ! !!!!!!!!!!!! ! ! !! ! ! !!! ! ! !! ! ! ! ! !!!! !!!!!! ! ! ! ! ! ! ! !!!! ! ! ! ! ! ! !! ! !!! !! ! !! ! ! !!!!! ! ! ! ! ! ! ! ! ! ! ! ! ! !!!!!!!! !! ! ! ! ! ! ! ! ! !! !! ! ! ! !! ! ! ! ! ! ! !!! ! !!!! ! ! ! ! ! ! !! !! !! ! ! ! ! !! ! ! ! ! ! ! ! ! ! !! !! ! ! ! ! !!!! ! ! ! ! ! ! !! ! ! were sampled to produce the !! ! ! !!! ! ! !!! ! !! ! ! ! ! ! !!! ! ! ! !! ! ! ! ! !! ! ! ! ! !! !!! ! ! ! ! ! ! ! ! ! ! ! ! ! !! !!! ! ! ! ! ! !! ! !!!! !!!! ! ! ! ! ! ! ! ! ! !! ! ! !!! ! ! ! !! ! ! ! !! !! ! ! ! ! ! ! !! !! ! !!! ! ! ! ! ! ! ! !! !! ! ! ! !! ! cross sections (±500 m, 1000 m, ! ! ! ! ! ! ! ! ! !! ! !! ! !! ! ! ! ! ! ! !!! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! !! ! !! ! !!!! ! ! ! ! ! ! ! ! ! ! ! ! ! NNWW1WW1 ! ! ! ! !! ! ! !! ! ! ! ! ! !! !! ! ! ! ! ! ! ! ! ! ! ! !!!! !! !! ! ! ! ! ! ! ! ! ! 2000 m). Detail of 1952 coseismic ! ! ! !! ! ! ! ! !!! !! ! ! ! ! !! !! ! ! ! ! !! ! ! ! !!! ! ! ! ! ! ! !!! ! !! ! !! ! !! ! ! ! ! !!! ! ! !! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! !! ! ! ! ! ! ! ! ! !! !!! ! ! ! !! ! !! ! ! ! !!! ! !!! ! ! ! ! !! ! ! ! !! ! ! !!! ! surface rupture and kinematics !!!! ! ! !! !! ! ! !! ! ! !!! ! ! !! ! ! ! ! ! ! !! !!! ! !!!! !! ! ! ! !! ! ! ! ! !!! !! !! !! !!!!!! ! ! ! !! ! ! ! ! ! ! ! ! !!!!! ! !!! ! ! !! !! ! ! ! ! !!!! !! ! ! ! ! !! ! !! !! !!!! ! ! !!! ! ! ! ! !! ! ! ! ! ! !!! ! ! ! ! ! ! ! !! ! !!!!!!!! ! ! ! !! ! ! is from Buwalda and St. Amand ! !! !! ! !!!!! !! ! ! !!!! ! ! ! ! ! !! !! ! !!! ! ! ! !!!! ! ! ! ! ! ! !!! ! ! !!! ! !!! !! ! ! ! ! !! !! ! ! ! !!! ! !! ! !!!!! !! ! !!! ! ! ! ! ! !! !! ! ! ! !! !! !! ! !! !! !! !! !! ! !! ! !! !!!! !!! ! ! ! ! ! !! ! ! !!! !! !! ! !! ! !! ! ! ! B ! ! ! !!!! !! !! !!!! !!! ! ! ! ! ! ! !! ! !!! !!!!!! !! !!!! ! ! ! ! ! ! (1955). (B, C, D) Seismicity cross ! ! !!!! !! ! !! ! !! !! !! ! ! ! !!! ! !! !!!!! ! !!! ! ! ! ! ! !! !!!!! !!! !! !! !!!!!!!! !! ! ! ! ! ! ! !!!!! ! !! ! ! ! ! ! ! ! ! ! ! !!!! ! ! ! !! ! !! ! !!!!!!!!!! ! ! !!!!! ! ! !! ! !! ! !! !!!!!! !! ! !! ! ! ! ! ! ! !! ! ! ! ! ! !!!!!! !!!!! sections A, B, and C, respectively. !! ! ! ! !!! !! !!! ! ! ! ! ! ! ! ! ! !!! ! !! ! !!!!! ! ! ! ! ! ! ! ! ! !!! ! ! ! ! ! ! ! !! ! ! ! ! ! !! ! ! ! ! ! ! ! !! !! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! !! ! ! ! ! ! ! N ! ! ! ! ! ! ! !! ! ! See A for location of section lines. ! ! ! ! ! ! !!! ! ! ! !! ! ! ! ! ! !!! ! ! ! ! ! ! ! !! ! ! !!! ! ! ! ! !! ! ! !! ! ! ! ! ! ′0″ ! ! ! !! ! ! !! ! !! ! !! ! ! ! ! t ! !!! !!! ! ! ! !! ! l !! ! ! ! ! ! ! ! !! ! ! ! ! ! Size of hypocenters is scaled to e ! ! ! ! ! ! !! ! ! u !! !! ! ! ! !!! ! ! ! ! ! !! r ! !!! !! ! ! ! ! ! a ! ! !! !! ! ! u !! f ! ! ! !!!! ! ! ! ! t ! !! ! ! !! ! ! ! ! ! 35°15 ! ! ! ! ! ! ! !! magnitude. Colors represent dis- p !! !!! ! ! ! ! ! ! ! ! ! ! f ! ! ! ! ! ! ! u ! ! ! l C ! ! ! r ! !! !!! ! !!!!! o ! ! ! ! !! e ! ! !! !! ! tance that hypocenters were pro- ! ! ! ! !! ! ! !! !! ! c W !!! ! ! ! ! ! ! ! !! ! !! ! a !!!!! ! ! ! ! !!! !! ! ! ! ! f ! ! ! ! !!!! ! ! r !!! ! ! !SSWNBWNB ! !!! !!! ! jected onto the section line: red, !!! ! ! ! u e !! ! t ! ! ! ! s i ! ! !! ! ! ! ! ! ! ! ! ! ! 2 h ! ! ! !! ! !! ! ! ! ! ! !! 0–500 m; yellow, 500–1000 m; 5 W ! !! ! !! ! ! 9 ! ! ! ! !!! ! ! ! !! ! ! ! ! 11952 surface rupture !! ! ! ! ! ! ! ! ! n ! ! green, 1000–2000 m. r ! ! ! ! ! ! ! ! e !! t! ! ! ! !! ! s ! ! ! ! ! ! ! ! ! a ! ! ! ! ! ! easterne White Wolf fault ! ! !!! ! ! ! ! ! ! ! ! ! !! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! !!!!! ! !!!!!!!!! ! !!!! ! 0 2 mmii !!! ! ! !! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! Tehachapi!! Mountains! ! ! ! ! !! ! ! ! !! !! ! ! ! ! ! ! ! ! !! ! !! ! ! ! ! !! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! 0 4 kmkm ! !! ! ! ! ! ! ! ! ! ! ! ! ! !! !!!!!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! !! ! ! Seismicity data from Lin, Shearer, and Hauksson (2007).

Explanation

Earthquake epicenter Area captured on seismicity cross section

Quaternary fault (Buwalda A A′ Seismicity cross section and St. Amand, 1955; Jennings, 1994; Kelson et al., 2010)

Geosphere, February 2014 113

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/1/107/3332669/107.pdf by guest on 29 September 2021 Unruh et al.

A A′ B Southwest Northeast 2 2 Ground surface 1 1

0 !( !( !( !( !( 0 !( !( !( !( !( !( !( !( !( !( !( !( –1 –1 !(!( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !(!(!(!( !( !( !( !( !( !(!( !(!(!(!( !( !(!(!( !( –2 –2 !(!( !( !( !( !( !(!(!(!( !( !(!( !( !(!( !( !(!(!(!(!(!(!(!( !( !( !( !( !( !( !( !( !(!(!(!(!( !( !( !( !(!(!(!(!( !( !(!(!(!( !( !( !( !(!(!( !(!(!( !( !( !(!(!(!(!(!(!( !( !( !( !( !( !(!(!( !( !( !( !(!( !( !(!( !(!(!( !( !( !(!( !(!(!( !( !( !( !( !(!(!( !(!( !( !( !(!(!(!(!(!( –3 –3 !(!(!(!(!( !(!(!( !(!( !( !(!( !(!( !(!(!(!( !(!( !( !( !( !(!( !(!(!( !( !( !( !( !(!(!(!( !(!( !(!( !( !( !( !( !( !(!( !( !(!(!( !( !( !(!(!(!(!(!( !( !(!( NWW1 !(!( !( !( !( !( !( !(!(!(!(!(!( !(!(!(!(!( !( !(!( !(!( !(!(!( !(!(!(!(!(!( !( !(!( !(!(!( !( !( !( !( !(!(!( !(!(!(!(!( !(!( !(!(!( !( !( !( !( !( !(!(!(!(!( !(!( !( !( !(!( !( !(!( !( !(!(!(!( !(!(!(!( !(!( !(!(!(!( !(!( !(!( !( !(!(!(!( !(!(!( !( !(!( !(!(!(!(!(!(!( !( !(!(!( !(!(!( !( !(!( !( !(!(!(!(!(!(!( !(!(!( !( !(!( !(!( !(!(!(!(!( !( !(!(!(!( !(!( !(!( !( !(!( !( !( !( !(!(!( !(!(!(!( !( !(!( !(!(!( !( !( !( !( !( !( !(!( !(!(!( !(!( !(!(!(!(!(!(!(!(!(!(!( !( !( !( !( !( !(!(!(!( !(!( !(!(!(!(!(!( !( !( –4 !(!( !( !( !(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!( !( !(!( –4 !( !(!(!(!( !( !(!(!( !(!(!(!(!(!(!(!(!( !( !(!( !(!( !(!(!(!(!(!(!(!(!( !(!( !( !( !(!(!(!( !( !( !( !(!(!(!( !(!( !( !( !( !(!(!(!(!(!(!( !(!(!(!(!(!(!(!(!(!(!( !( !(!( !( !( !(!(!( !( !(!(!(!(!(!( !(!( !(!(!( !( !(!(!( !( !( !(!(!(!(!(!(!( !(!( !(!(!(!(!( !( !(!(!(!(!(!(!(!(!(!( !( !(!(!(!( !(!(!(!( !(!(!(!(!(!(!(!(!( !( !( !(!( !(!(!(!( !(!(!( !(!(!(!( !(!(!( !(!( !(!( !(!( !( !( !( !(!( !( !( !( !( !( !( !(!(!(!(!(!(!(!( !( !(!( !( !( !( !( !(!(!(!( !(!( !( !( !( !( !( !( !( –5 !( !(!( –5 !( !( !( !(!( !( !( !( !( !(!( !( !( !( !( !(!(!( !( !( !( !( !( !( !( !( !( !( !( !(!( !(!( !( !( !( !( !( !( !( !(!( !( !( !(!( !( !( NWW2 !( !( !(!( !( !(!( !( !( !( !( !(!(!( !( !( !( !( !( !( !( !( !( !( !(!(!( !(!(!( !( !( !( !(!( !(!( !( !( !( !( !( !( !( !( !( !( !(!( !( !(!( !( !(!(!(!( !( !( !( !( !( !( !(!( !( !( !( !(!( !(!( !(!( !(!( !( !( !( !( !( !( !( !( !( !( !(!(!( !( !( !(!( !(!( !( –6 !(!( !( !( !(!( !(!( !( !(!(!( !( !(!( !(!(!(!( !( !( –6 !(!( !( !( !( !( !( !( !( !( !(!(!(!( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !(!( !( !( !(!( !(!( !( !( !( !( !(!( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !(!(!(!( !(!( !(!(!( !( !(!( !( !( !( !(!(!(!( !( !( !( !( !( !(!( !(!( !(!( !( !( !( !(!(!( !( !( !( !( !(!(!( !( !( !( !( !(!(!( !( !( !( !( !(!( !( !(!(!( !(!( !( !( !( !( !( !(!( !( !( !(!(!( !( !( !( !( !( !( !(!( !( !(!( !( !( !(!( !( !(!(!(!( !( !(!(!(!(!( !( !(!(!( !(!( !( !( !( !( !( !(!( !(!( !( !(!(!(!(!( !( !(!(!( !(!( !( !( !( !( !( !( !(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !(!( !(!(!( –7 !( !( !(!(!( !( !( !( !( –7 !( !( !( !( !( !( !( !(!( !( !( Elevation (km) !( !( !( !( !(!( !( !(!( Elevation (km) !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !(!( !( !( !( !!( !( !( ( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!(!(!( !( !( !( !( !( !(!(!( !( !( !( !(!(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( –8 –8 !( !( !( !( !( !(

!( !( !( !( –9 !( –9

!( –10 !( –10 !( !(

–11 Explanation –11 Distance from Symbols Seismicity (Mw) Cross Section Line (meters) –12 !( –12

Transtensional dextral shear ( 0.0–2.0 ! +/– 500 ( –13 2.0–3.0 ! +/– 1000 –13 Distributed dextral shear ( 3.0–4.0 ! ( +/– 2000 –14 4.0–5.0 –14 ( 5.0 and greater No vertical exaggeration –15 –15 012345678 9 10 11 12 13 14 Distance (km)

Figure 3 (continued).

1A in the Supplemental File [see footnote 1]) strain is moderately plunging rather than verti- faulting. Values of V obtained from inversions and the Diablo Range west of the Kettleman cal (Table A1 in the Supplemental File [see foot- of groups of spatially distinct aftershocks below Hills (DIAB subregion; Table 1A in the Supple- note 1]), indicating oblique crustal thickening. 7 km range from 0.7 to 0.85 (Tetreault, 2006), mental File [see footnote 1]) indicate a narrow The zone of transpression east of the San indicating primarily vertical thickening with a zone of transpressional deformation ~15–30 km Andreas fault is inferred to extend to the north- subordinate component of shearing. wide directly east of the San Andreas fault (Fig. west and include the epicentral region of the 2B, kinematic domains). The orientation of 1983 Coalinga earthquake (Figs. 2B, 2D, 2E, INTERPRETATION the maximum shortening strain in this region 2N). Tetreault (2006) performed a detailed is north-northeast–south-southwest (Figs. 2B, analysis of the Coalinga earthquake aftershocks, Extension Above Anomalously Slow 2N), consistent with dextral shear on the San including paleomagnetic tests of folded bedding Asthenosphere Andreas fault and with shortening at a high for vertical axis rotations and kinematic inver- angle to west-northwest–east-southeast–trend- sions of focal mechanisms using a micropolar Comparison of the loci and extent of vertical ing late Cenozoic folds in the western San Joa- approach, and concluded that the Coalinga thinning with horizontal depth slices through a quin Valley such as the Elk Hills and Wheeler anticline is deforming due to a combination 3-D tomographic model of Jones et al. (2014) Ridge anticlines. The maximum extensional of right-lateral shear and thrust and/or reverse reveals a fair to good correlation with anoma-

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C B B′ 2 Southwest Northeast 2 Ground surface

1 1

!( !( !( !(!( !( 0 !( 0

!( !(!( !( !( !(!(!( !( –1 !( !( !( –1 !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !(!(!(!( !( !( !( !( !( –2 !( !( !( !( !( !(!(!( –2 !(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !(!( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( –3 !( !( !( –3 !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( (!!( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( –4 !( !( !( !( !(!( –4 !(!( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!(!( !( !( !( !( !(!( !(!( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !(!( !( !(!( !( !(!(!( !( !(!( !(!( !(!(!( !( !( !( !( !( !( !(!(!( !( !( !( !( !( !(!( !( !( !(!(!(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !(!( !( !( !( !(!( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( –5 !( !( !( !( !( !( !( !( !( !( !( !( !( !( –5 !( !( !(!( !( !( !( !(!( !( !( !( !( !( !( !(!( !( !( !( !(!(!( !( !( !( !( !( !( !(!( !( !( !(!( !( !( !( !(!( !( !( !( !(!( !( !( !( !( !( !(!( !( !( !(!( !( !( !( !( !( !( !( !( !( !(!( !(!( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !(!( !( !(!( !( !( !( !( !( !( !( !(!(!( !(!(!( !( !( !( !( !( !(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !(!(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !(!( !( !( –6 !( !(!(!( !(!( !(!( !( !( !( !( –6 !( !( !( !( !( !( !( !(!(!(!( !( !( !( !(!(!( !(!(!( !( !( !( !( !( !( !( !( !(!( !( !(!( !( !( !( !( !( !( !( !( !(!( !( !(!(!( !( !( !(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( –7 !( !( !( –7 !( !( !( !( !( !( !( Elevation (km) !( !( Elevation (km) !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!(!( !( !( !( !( !( !(!( !(!( !( !( !( !(!( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !(!(!( !( !( !( !( !( !( !( !( !(!( !( !( !(!(!( !( !( !( !( !( !( !( !( !( !(!( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !(!(!( !( !( !( !( !(!( !( !(!( !(!(!(!(!(!(!( !(!( !(!(!(!(!( !( –8 !( !(!( !( !( !(!(!( !(!( !( !( !( !(!(!( !( –8 !( !(!(!( !( !( !( !(!( !(!( !( !( !( !(!(!(!( !( !(!(!(!( !( !( !( !( !( !( !( !(!( !( !(!( !( !(!(!(!( !( !(!(!(!(!( !(!( !( !( !( !(!( !( !(!( !(!( !(!(!(!(!( !(!(!(!( !(!( !(!( !( !(!(!( !( !( !( !(!(!(!(!(!(!(!(!( !(!( !( !( !(!(!(!( !( !( !( !( !( !( !( !( !( !( !(!(!( !( !(!( !( !( !(!( !( !( !( !( !( !( !(!(!(!(!(!(!( !(!(!( !( !( !(!( !(!( !( !( !(!(!(!(!(!(!( !(!(!( !( !( !( !( !( !(!(!(!(!(!(!(!( !(!( !(!( !( !(!( !( !( !(!(!( !(!( !( !( !( !(!(!(!(!(!(!(!(!( !(!( !( !(!( !( !( !( !( !( !( !(!(!(!( !(!( !( !( !( !( !( !( !(!( !( !( !( !(!(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !(!( !(!( !( !( !(!( !( !( !( !( !( !( !(!( !( !(!( !( –9 !( !(!( !( !( –9 !( !( !( !( !( !( !( SEW3 !( !( !( –10 !( –10 !( !(

!( –11 –11 Explanation –12 Distance from –12 Symbols Seismicity (Mw) Cross Section Line (meters)

Distributed dextral shear ( 0.0–2.0 ! +/– 500 –13 –13 with possible extension ( 2.0–3.0 ! +/– 1000 ( 3.0–4.0 ! +/– 2000 –14 ( 4.0–5.0 –14 ( 5.0 and greater –15 –15 012345678 9 10 11 12 13 14 15 16 Distance (km)

Figure 3 (continued).

lous low P-wave speeds (low Vp) at lower extensional domain is associated with low Vp with the zone of upper crustal extension (Figs. crustal and upper mantle depths. We compare extending to depths of 70 –100 km (Figs. 4A, 2B, 2I, 2K). our results with the inversion of Jones et al. 4D–4F; see Figs. 2A–2C, 2N, 2O for locations The spatial relationship between patterns of (2014) shown here that started from (and gener- of section lines). This spatial correlation is dis- upper crustal deformation, relatively shallow ally preserves) the ambient noise tomography of cussed in greater detail in the following. Moho, and velocity variations in the upper man- Moschetti et al. (2010) for the crust. Our inter- In addition to anomalous low Vp at lower tle are further illustrated by a series of cross sec- pretation is informed by uncertainties associated lithospheric depths, upper crustal extension tions through the tomographic model of Jones with tomography in Jones et al. (2014). In the in the southern Sierra Nevada at the latitude et al. (2014; see Figs. 2C and 2N for locations tomographic model we have adopted, the con- of Porterville is associated with a shallower of section lines). Section A (Fig. 4A) shows that tiguous region of vertical thinning centered on Moho than to the north along the range (Fras- extension and thinning in the Sierra upper crust approximately lat 36.2°N and long 118.5°W is setto et al., 2011). Structure contours on Moho occur above low-velocity upper mantle east of similar in size, extent, and shape to the region depth reveal ~10 km of positive relief beneath the steeply east dipping Isabella anomaly. The of low Vp at 70 km depth beneath the southern the southern Sierra Nevada at approximately east to west transition from shearing in the Sierra Nevada (Figs. 2B, 2D, 2L). Cross sec- lat 36°N relative to equivalent positions along Walker Lane belt to extension and thinning in tions through the velocity model show that the the western Sierra slope to the north, coincident the Sierra Nevada is associated with a zone of

Geosphere, February 2014 115

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C C′ D Southwest Northeast 2 2 Ground surface 1 1

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!( !( !( –1 !( –1

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!( !( !( !(!( !( !( !( !( !(!( !( !( !( !( !(!(!(!( !( !( !(!( !( !( !( !(!( !( !( !(!( !( !( !(!( !(!(!( !(!( !( !( !(!(!( !( !(!( !( !(!( !( !( !( !( Elevation (km) !( !(!(!( !(!( !( !( !( !( Elevation (km) !( !( !( !(!( !( !(!(!( !( !(!(!(!(!(!(!( !(!(!( !( !( !( !(!(!(!(!(!( !( !(!( !((!!( !( !( !(!( !(!( !( !( !( !( !(!( !( !( !( !(!( !( !( !( !(!(!( !( !(!(!( !(!( !( !( !( !(!( !( !( !( !( !( !( !(!( !(!( (! !( !( !( !( !( !(!(!( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !(!( !(!( !( !( !(!(!(!( !( !(!( !( !( !(!( !( !( !(!( !( !(!( !( !(!( !( !( !( !( !( !(!( !( !( !( !( !(!( !(!( !( !( !( !(!( !( !( !( !( !( !(!(!( !( !( !( !( !( !( !(!(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!(!( !(!( !( !( !(!(!( !(!(!(!( !( !(!( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !(!( !( !(!(!(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!(!(!(!(!(!(!( !(!( !(!( !( !( !( !( !( !( !(!( !( !(!( !(!( !(!(!( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !(!(!(!(!( !( !(!( !(!( !( !( !( !( !( !(!( !( !(!( !(!( !( !( !( !( !(!( !( !(!( !(!(!( !( !( !( !( !( !(!(!( !(!( !( !(!( !( !( !( !( !(!( !( !( !(!(!(!(!( !( !( !(!(!(!( !( !( !( !( !(!( !( !( !(!( !(!(!(!( !(!( !( !(!( !(!( !(!( !(!(!( !( !( !( !(!( !(!( –8 !( !( !( !( !( !(!(!(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( –8 !( !( !( !( !( !( !(!(!(!(!( !( !( !( !(!(!( !( !( !( !( !( !( !(!( !( !( !( !( !( !(!(!( !( !( !(!(!( !( !( !( !( !( !( !( !(!(!( !( !( !( !( !( !( !( !( !(!( !(!(!(!( !( !(!(!( !( !(!( !( !( !( !(!( !( !( !( !( !(!( !( !( !(!( !( !( !( !( !( !( !( !( !( !(!(!( !( !( !( !(!( !( !( !( !( !( !(!( !( !(!(!( !( !(!( !( !(!( !( !(!( !( !( !( !(!(!(!( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !(!( !(!( !( !( !( !(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !(!( !(!( !( !( !( !(!(!( !( !( !(!(!( !( !(!(!( !( !( !(!( !(!( !( !( !( !( !( !(!( –9 !(!(!(!( !( –9 !( !( !( !( SWNB!( !( !( !( SEW2 !( !( !( !( !(!( !(!(!(!( !( !(!( !(!( !( !( !( !( !( !( !( !( !( !( !( !( !(

!( !( !(!( –10 !( !( !( –10 !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( –11 !( –11 !(

!( Explanation !( –12 !( !( Distance from –12 !( Symbols Seismicity (Mw) Cross Section Line (meters)

!( Distributed dextral shear ( 0.0–2.0 +/– 500 –13 !( ! –13 ( 2.0–3.0 ! +/– 1000 ( 3.0–4.0 –14 ! +/– 2000 –14 ( 4.0–5.0 No vertical exaggeration ( 5.0 and greater –15 –15 012345678 9 10 11 12 13 14 15 16 17 18 19 20 Distance (km)

Figure 3 (continued).

anomalous low upper mantle wave speeds that hills and eastern San Joaquin Valley. Although of net vertical thinning (domain KING; see the extend down to 70–80 km depths. Speciﬁ cally, there is a steeply east dipping zone of relatively Supplemental File [footnote 1]), in contrast to the change in deformation style is associated higher P-wave speeds (~+2% to +3%) in the the dominantly horizontal extension and verti- with deepening of the base of the low-wave 100–250 km depth range beneath the Sierra cal thinning beneath the western Sierra slope to speed material. In contrast, deformation in the Nevada at this latitude, it is not as fast as in the the south. western San Joaquin Valley adjacent to the San feature we identify as the Isabella anomaly to Cross-sections X, Y, and Z (Figs. 4D, 4E, 4F, Andreas fault and Coast Ranges, west of the the south ~+4% to +5%). Low upper mantle respectively) pass through the center of the Isa- Isabella anomaly, is transpressional and accom- velocities are present beneath the Moho in the bella anomaly and span a range of orientations modated by mixed strike-slip and thrust faulting Walker Lane belt along section C, but do not between northeast-southwest and northwest- (Fig. 4A). Similar relationships are visible in abruptly deepen westward to maximum depths southeast (Figs. 2C, 2M). These three cross east-west section B (Fig. 4B), which skirts the of 60–80 km beneath the Sierra, as in sections sections consistently show that upper crustal northern margin of the Isabella anomaly. Section A and B. Kinematic inversion of sparse focal extension is occurring above anomalous low C (Fig. 4C) crosses the Sierra Nevada north of mechanisms above the eastern margin of the Vp upper mantle in the 50–80 km depth range the Isabella anomaly and reveals possibly intact high Vp upper mantle beneath the Sierra foot- directly east of the Isabella anomaly. The transi- high-velocity lower lithosphere directly below hills and San Joaquin Valley indicates shearing tion from crustal thinning in the southern Sierra the crust of the western Sierra slope and foot- deformation with possibly a minor component to dextral shear in the Walker Lane belt is associ-

116 Geosphere, February 2014

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A A A′ 42°N Southwest Northern Northeast projection 5000 of KCF 40°N Western Sierra Nevada Walker Lane 4000 Transverse San Joaquin Belt Ranges 3000 Valley 38°N 2000 WWMFMF A′ Elevation OOVFVF SAF FFLVFLVF 1000 36°N (meters above sea level) 0 A ? ? 34°N ? 122°W 120°W 118°W 116°W –50 Location of Section A–A′

–100 Explanation

8 Oblique-slip fault; arrow indicates sense –150 of separation 6

Depth (km) Thrust fault

4 Seismogenic deformation –200 characterized by horizontal extension and vertical 2 thinning; bounded on east by dextral shear –250 0

–2 Relative P-wave Velocity (percent) –300 –4

Figure 4 (on this and following ﬁ ve pages). (A–F) Cross-sections (below sea level) A, B, C, X, Y, Z (A–F, respectively) through the three- dimensional P-wave tomography model of Jones et al. (2014) that starts from the converted S-wave model of Moschetti et al. (2010) (see Figs. 2C, 2H for locations). Colors reﬂ ect deviation from mean wave speed at each depth. Topography above sea level is vertically exagger- ated (A—20×; B, C—13×; D—15×; E, F—13×); scale below sea level for all sections is 1:1. Annotations to the sections: thickness of the crust is indicated by dashed pattern above Moho (from Frassetto et al., 2011); locations of major active strike-slip, thrust, and normal faults are extended below sea level to the base of seismicity (~15 km); and upper crustal deformation style is from inversions of focal mechanisms (arrows indicate horizontal extension and vertical thinning; away or toward symbols for strike-slip faulting and out-of-plane motion). OVF—Owens Valley fault zone; WMF—White Mountain fault zone; KCF—Kern Canyon fault; SAF—San Andreas fault; SNFF—Sierra Nevada frontal fault; ALFZ—Airport Lake fault zone; DM—Durrwood Meadows.

ated with abrupt eastward increase in Vp in this Isabella anomaly (Fig. 4F), similar to relations slowness) and vertical narrowing of these zones same depth range. Section Y passes along and in sections X and Y. of anomalously low wave speeds. In contrast, across the irregular south margin of the exten- To summarize, cross sections that pass directly section C north of the Isabella anomaly shows sional domain in the Sierra foothills (Fig. 2B, through the Isabella anomaly (A, X, Y, Z; Fig. 4; a deeper Moho (maximum depth ~50 km) and 2C, 2N). Deformation in the foothills directly Figs. 2C, 2I, 2N) consistently show the Moho the +4% to +8% higher wave speeds conﬁ ned east of the Isabella anomaly locally is character- deepening westward from ~35 km beneath the to 40–70 km depths beneath the western Sierra ized by shearing (Fig. 4E). The east end of sec- eastern Sierra Nevada crest to a maximum depth foothills. Seismogenic deformation in the foot- tion Z (Fig. 4F) crosses the Coso Range, which of ~40 km in the eastern and central San Joa- hills is characterized by distributed northwest is within a right-releasing stepover between the quin Valley directly above the Isabella anomaly dextral shear with a possible small component dextral Airport Lake fault zone on the south (Frassetto et al., 2011; Fliedner et al., 1996, of net vertical thinning. and the dextral Owens Valley fault to the north 2000). The Sierra crust east of the Isabella anom- Previously workers have inferred that the (Unruh et al., 2008). The multiple fault branches aly is underlain by upper mantle with anoma- low-velocity mantle to the east of the Isabella composing the releasing stepover are depicted lous low P-wave speeds extending to depths of anomaly in the depth range of 50–100 km rep- as a negative ﬂ ower structure on section Z. ~70–90 km or more. The west to east transition resents upwelling asthenospheric mantle (Jones These structures are above the eastern margin of from extension in the Sierra to shearing in the et al., 1994; Zandt et al., 2004; Boyd et al., 2004; the zone of anomalous low upper mantle veloci- Walker Lane belt generally is associated with Saleeby and Foster, 2004; Frassetto et al., 2011; ties that underlies the Sierra Nevada east of the eastward shallowing (or decreasing integrated Saleeby et al., 2012, 2013). If this is correct,

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B B B′ 42°N West East

5000 40°N 4000 Sierra Nevada WWalkeralker 3000 LaneLane BeltBelt 2000 Elevation OOVFVF San Joaquin Valley KKCFCF 1000 38°N 0 (meters above sea level) B B′ 36°N ? –50

34°N 122°W 120°W 118°W 116°W Location of Section B–B′ –100

Explanation –150 Oblique-slip fault; 8 arrow indicates sense Depth (km) of separation 6 Thrust fault –200 4 Seismogenic deformation characterized by horizontal extension and vertical 2 thinning; bounded on east –250 by dextral shear 0

–2 Relative P-wave Velocity (percent) –300 –4

Figure 4 (continued).

then a substantial part of the high topography of tributed extension (Göğüş and Pysklywec, 2008; Propagation of Dextral Shear into the the southern Sierra Nevada at the latitude of the D’Agostino et al., 2011). In the case of the east- Southeastern Sierra Microplate upper Kern River drainage area is supported by ern Anatolian plateau, extension there is occur- asthenosphere that replaced preexisting lower ring in the broader tectonic context of north-south The domain of counterclockwise rotation of

lithosphere. The steepest topographic gradi- convergence between the Arabian and Eurasian d1 and d3 trends in the southern Sierra Nevada ent along the western Sierra slope and highest plates, indicating that the local buoyancy forces encompasses a network of late Cenozoic faults smoothed elevations are associated with: low within the orogen at least balance, or exceed, far- mapped in the Sierra foothills and across the Vp at lithospheric depths; a more shallow Moho fi eld tractions on the boundaries from collisional Kern Arch near Bakersfi eld (Figs. 2N, 2H, 2A, relative to equivalent positions along the Sierra plate interactions. We infer that the same is true 2D). These faults are on trend with the west- slope to the north and south; and thin crust for areas of the southern Sierra undergoing active northwest–striking seismogenic dextral faults (< 35 km) (Figs. 2G, 2I, 2L). The Sierra crust extension, the difference being that the far-fi eld between the White Wolf and Breckinridge is extending and thinning above the upwelling plate motions giving rise to tractions on the sides faults (Fig. 3A). Although many of the faults asthenosphere. of the Sierra microplate are translational rather in the foothills north and west of Bakersfi eld A similar process of lithospheric foundering than convergent. This inference is consistent with were mapped as Quaternary faults by Jennings accompanied by upwelling asthenosphere has the results of numerical modeling by LePourhiet (1994), evidence for late Quaternary activity been proposed to account for the high topography and Saleeby (2013) that suggest that elastic has not been documented for most of the struc- of the Apennines (Shaw and Pysklywec, 2007) stresses in the crust arising from negative buoy- tures. If these faults are currently active, then the and the east Anatolian high plateau (Şengör ancy of the Isabella anomaly may be comparable general west-northwest trend suggests that they et al., 2003). Like the southern Sierra, both of to elastic stresses associated with shearing along may accommodate distributed dextral shear that these orogens are deforming internally by dis- the San Andreas fault. is beginning to encroach on the Sierra micro-

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C C C′ 42°N West East

5000 4000 40°N San Joaquin Sierra Walker Lane 3000 Valley Nevada Belt 2000 OOVFVF WWMFMF

Elevation 1000 38°N 0 C C′ (meters above sea level) 36°N

–50 34°N 122°W 120°W 118°W 116°W

Location of Section C–C′ –100

Explanation –150 Oblique-slip fault; 8 arrow indicates sense Depth (km) of separation 6 Seismogenic deformation –200 characterized by 4 transtension

2 –250 0

–2 –300 Relative P-wave Velocity (percent) –4

Figure 4 (continued).

plate from the eastern California shear zone ogy. The local strain rotation, and presumably beneath the southern Sierra Nevada, Kern Arch, and Walker Lane belt. Evidence for progressive stress rotation, be may be due to the following. and Tehachapi Mountains. Moho depths (Malin westward expansion of the Walker Lane belt 1. Dextral shear may be deﬂ ected west-south- et al., 1995; Frassetto et al., 2011) indicate that into the Sierra microplate has been summarized west through thin crust and lithosphere that was the crust is between 30 and 35 km thick in this (e.g., Jones et al., 2004), so this may be an incre- stripped of its mantle lid during the Laramide region (Figs. 2I, 2N). Near Porterville north

mental step in that process. The west-northwest orogeny. Based on structural and thermochrono- of the Kern Arch, where d1 trajectories in the direction of macroscopic dextral shear in the logical relations, Saleeby (2003) proposed that western Sierra and eastern San Joaquin Val- southeastern Sierra and across the Kern Arch the mantle lithosphere was sheared off from ley are not rotated signifi cantly relative to the trends more toward the west than either Pacifi c– beneath the southernmost Sierra in the Late northwest-southeast trends in the Walker Lane North America or Sierra–North America plate Cretaceous by a shallow fl at segment of the belt to the east, the crust is ~40–45 km thick and motion, however, which would typically be a Laramide slab. According to this model, the underlain by relatively high velocity lithosphere restraining geometry and be expected to pro- metamorphic Rand Schist was underplated to to depths of 70 km (Figs. 2A, 2H, 2I, 2K, 2L). duce localized transpressional deformation. The the remaining Sierra lithospheric column dur- It is possible that westward-propagating dex- rotation of the strains maintains horizontal plane ing subsequent steepening of the Laramide tral shear may be localized in the southwest- strain as the crust fl ows through this bend. The slab, replacing the mantle lid (Saleeby, 2003). ern Sierra because the thin crust and absence

counterclockwise rotation of the strains implies The zone of rotated d1 and d3 trends, Quaternary of mantle lithosphere result in relatively low that the distributed plate motion locally is inﬂ u- faults, and distributed west-northwest dextral integrated strength there. The counterclockwise enced by local forces and/or variations in rheol- shear are associated with relatively thinner crust strain rotation may thus represent refraction

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D X X′ 42°N Southwest Northeast 5000 WWalkeralker 4000 40°N Sierra Nevada LLaneane 3000 BBeltelt

2000 Elevation SAF San Joaquin Valley KKCFCF OOVFVF 38°N 1000

(meters above sea level) 0 X′

36°N X

? ? 34°N –50 122°W 120°W 118°W 116°W Location of Section X–X′

–100

Explanation

Oblique-slip fault; –150 arrow indicates sense of separation Depth (km)

Thrust fault 8 Seismogenic deformation characterized by horizontal –200 6 extension and vertical thinning; bounded on east by dextral shear 4

2 –250

–2 –300 Relative P-wave Velocity (percent) –4

Figure 4 (continued).

of the strain trajectories (and distributed plate or background state of stress in the crust, then stresses are independent of the viscosity of the motion) through relatively weak lithosphere. the trajectories of the maximum compressive medium. We add to this the background state of 2. The counterclockwise strain rotation may stress should rotate toward parallelism with stress, which is characterized by the maximum σ refl ect the infl uence of local stresses and/or the compressive hoop stresses with proxim- compressive stress ( 1) oriented approximately processes associated with the Isabella anom- ity to the downwelling. For the case of a rising north-northeast–south-southwest and the mini- σ aly. For example, simple models of horizontal mass, the predicted hoop stresses in the overly- mum compressive stress ( 3) is west-northwest– stresses that develop adjacent to thickened litho- ing crust are tensile (analogous to circumferen- east-southeast. We assume that these stresses spheric mantle (e.g., Fleitout and Froix deveaux, tial stress in the walls of a pipe containing gas are equal and opposite in magnitude, consistent 1982) predict inward-directed compression under positive pressure) and the trajectories of with maximum shear stress parallel to north- above the locus of downwelling. Radial hori- the maximum tensile background stress would northwest dextral shear in the Walker Lane belt. zontal fl ow of crust associated with vertical predictably rotate toward parallelism with the We assume a linear constitutive relationship to thickening above a mantle drip will predictably tensile hoop stresses near the upwelling mass. compare the modeled stress trajectories with the generate horizontal compressive circumferential We derive horizontal deviatoric stresses from observed incremental strain trajectories (note σ or hoop stresses that are exerted perpendicular Morgan’s (1965) solution for the velocity fi eld that 1 should parallel d3). to both the axis and radius of the downwelling generated by a descending or rising sphere in a In the fi rst example (Fig. 5A), the Isabella mass. The compressive hoop stresses in the crust Newtonian viscous medium to infer the general anomaly is assumed to be a downwelling mass. are analogous to stresses that would develop horizontal stress trajectory patterns for several The predicted hoop stresses in the crust above within the walls of a pipe that contains a gas scenarios in the southern Sierra Nevada and San the sinking drip are compressive, causing the σ under negative pressure. If there is a regional Joaquin Valley. In this formulation the deviatoric north-northeast–south-southwest 1 trajectories

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E Y Y′ 42°N West northwest East southeast 5000 4000 40°N 3000 San Joaquin Valley Sierra Nevada 2000 Elevation KKCFCF SSNFFNFF ALFZ 38°N 1000 0 (meters above sea level) Y 36°N Y′

? ? –50 34°N 122°W 120°W 118°W 116°W Location of Section Y–Y′

–100

Explanation

Strike-slip fault –150 8 Normal fault Depth (km) Seismogenic deformation 6 characterized by plain strain –200 and distributed shear 4 Seismogenic deformation characterized by horizontal 2 extension and vertical –250 thinning; bounded on east by dextral shear 0

–2 –300 Relative P-wave Velocity (percent) –4

Figure 4 (continued).

σ to rotate toward parallelism with them and thus the 1 trajectories to be defl ected away from it. tioning system data is an expression of local be defl ected around the anomaly. The magnitude The net result is a counterclockwise rotation of asthenospheric upwelling (e.g., Saleeby et al., σ σ of rotation of the stresses depends on the ratio the 1 and 3 trajectories southeast of the Isa- 2012, 2013). Although this model predicts the of the antibuoyancy to the regional deviatoric bella anomaly relative to regional trends, similar strain trajectories to rotate counterclockwise in σ stresses. The 3 trajectories are perpendicular to to the observed counterclockwise rotation of the the southern Sierra Nevada, the displacement σ the 1 trajectories, and thus are defl ected toward d3 and d1 trajectories in this region. The loca- of the locus of upwelling east of the Isabella the anomaly. The net result is a clockwise rotation of this load could represent crustal relax- anomaly produces a worse fi t to the strain tra- σ σ tion of the 1 and 3 trajectories southeast of ation above a detachment of a dense Isabella jectories in the southern San Joaquin Valley. the Isabella anomaly relative to regional trends, anomaly and/or convective fl ow in response to The latter scenario (Fig. 5D) is an attempt to which is opposite to the observed counterclock- delamination. explain the counterclockwise rotation of the

wise rotation of the d3 and d1 trajectories in the We also test scenarios that assume that a locus trajectories in the southern Sierra and San Joa- same region. of upwelling underlies an area on the northeast- quin Valley with a downwelling mass. In order In the second example (Fig. 5B), background ern margin of the Kern Arch inferred to be ris- to fi t the strain trajectories, the geometry of the stress defl ections are the result of local upward ing rapidly (Saleeby et al., 2013) (Fig. 5C) and stress defl ection requires the sinking mass to stresses. The load is placed to best fi t observed a best-fi t locus of downwelling that would be be ~90 km beneath the southwestern end of the strain trajectory defl ections. The predicted hoop located beneath the southwestern San Joaquin San Joaquin Valley; however, the tomography stresses associated with thinning crust above the Valley (Fig. 5D). The former scenario (Fig. reveals slow rather than fast mantle at this depth σ rising mass are tensile, causing the 3 trajec- 5C) assumes that rapid surface uplift northeast (Figs. 2H, 2L, 2O), indicating that this scenario tories to be defl ected toward the anomaly and of the Kern Arch documented by global posi- is unlikely.

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F Z Z′ 42°N West East

5000 WWalkeralker 4000 San Joaquin Valley Sierra Nevada LLaneane 40°N BBeltelt 3000 DDMM Coso Range 2000 Elevation SSAFAF KKCFCF SSNFFNFF 1000 38°N 0 (meters above sea level) Z′ 36°N Z

? ? –50 34°N 122°W 120°W 118°W 116°W Location of Section Z–Z′ –100

Explanation

Oblique-slip fault; –150 arrow indicates sense of separation 8 Depth (km) Negative flower structure 6 –200 Normal fault

4 Thrust fault

2 Seismogenic deformation –250 characterized by horizontal extension and vertical 0 thinning; bounded on east by dextral shear –2 –300 Relative P-wave Velocity (percent) –4

Figure 4 (continued).

The simple models presented here indicate that the observed counterclockwise strain defl ection in the southern Sierra Nevada and San Figure 5 (on following page). Models for defl ection of background stresses (red lines—σ1 Joaquin Valley may be due to horizontal stresses trajectories; blue lines—σ3 trajectories) due to the stresses above rising or sinking spherical associated with buoyancy in the vicinity of the masses in the upper mantle (after Morgan, 1965). In all models, background σ1 and σ3 are surface projection of the Isabella anomaly. If assumed to be +1 MPa and –1 MPa, respectively. Black lines show the observed trajectories this is the correct dynamic interpretation of the of maximum extension (d1, solid lines) and maximum shortening (d3, dashed lines) from strain geometry, then the seismotectonic defor- inversion of earthquake focal mechanisms (see Figs. 2H, 2N, 2O). (A) Scenario assumes a mation may refl ect recent or ongoing detach- downwelling mass coincident with the Isabella anomaly. The radius of the mass is 50 km, the ment of the Isabella anomaly, accompanied depth is 125 km, and its density relative to the surrounding mantle is +50 kg/m3. (B) Scenario by upward fl ow of infi lling asthenosphere or assumes an upwelling mass coincident with the Isabella anomaly. The radius of the mass is relaxation of a previously thickened crust (e.g., 40 km, the depth is 50 km, and its density relative to the surrounding mantle is –50 kg/m3. Hoogenboom and Houseman, 2006). Although (C) Scenario assumes presence of an upwelling mass beneath a region of rapid surface uplift our assumptions are simple and overlook rheo- northeast of the Kern Arch indicated by vertical global positioning system data. The radius logical stratifi cation, power-law rheologies, and of the mass is 40 km, the depth is 50 km, and its density relative to the surrounding mantle is more complex load patterns, the basic defl ec- –35 kg/m3. (D) Scenario assumes a downwelling mass beneath the southwestern San Joaquin tion patterns derived from our models should Valley. The radius of the mass is 50 km, the depth is 95 km, and its density relative to the be valid. Our primary objective in presenting surrounding mantle is +50 kg/m3. these simple models is to begin to address the possible effects of local forces associated with

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B D

37°00″N 36°00″N 35°00″N 37°00″N 36°00″N 35°00″N 121°00″W 120°00″W 119°00″W 118°00″W 117°00″W 121°00″W 120°00″W 119°00″W 118°00″W 117°00″W Figure 5. Figure

A C 37°00″N 36°00″N 35°00″N 37°00″N 36°00″N 35°00″N 121°00″W 120°00″W 119°00″W 118°00″W 117°00″W 121°00″W 120°00″W 119°00″W 118°00″W 117°00″W

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the foundering process on regional patterns of dextral shear through Panamint Valley. Total and western White Mountains (ca.3 Ma; Stockli stress and deformation associated with distrib- late Cenozoic relative northwest displacement et al., 2003) to the north. uted shearing east of the Paciﬁ c plate. is ~17 km, implying a long-term average rate of We interpret the results of our study as evi- To summarize, we suggest that the simplest ~4 mm/yr since 4.2 Ma. dence that east-west extension and vertical thin- interpretation for west-northwest dextral shear West of Panamint Valley and east of the mod- ning in the southern Sierra Nevada are occurring in the southeastern Sierra Nevada is that it rep- ern Sierra Nevada, Indian Wells Valley was an above asthenospheric mantle that is rising in resents westward propagation of dextral motion extensional basin in the hanging wall of a north- response to west-directed removal or founder- from the eastern California shear zone into the south–striking, east-dipping normal fault sys- ing of lower lithosphere (Fig. 4). The zone of southern Sierra microplate. The west-northwest tem that was active between ca. 7.5 and 3.5 Ma asthenospheric upwelling, imaged as anoma- direction of macroscopic dextral shear is oblique (Fig. 6B; Monastero et al., 2002). Subsidence lous low Vp extending to depths of 70–90 km to regional northwest plate motions, however, accompanied by normal faulting also occurred (Fig. 4), and associated upper crustal thinning and thus requires a complimentary rotation of ~35–50 km north of Indian Wells Valley, repre- extends a maximum of ~125 km east of the Isa- the stresses in the southwestern Sierra to accom- sented by deposition of lacustrine facies of the bella anomaly, beyond which modern deforma- modate a horizontal plane strain rather than Coso Formation adjacent to the northwestern tion is dominated by distributed dextral shear in oblique convergence (i.e., transpression). The Coso Range between ca. 4 and 6 Ma (Kamola the Walker Lane belt (Figs. 2B, 2L; cf. 2B, 2M). mechanics of the counterclockwise strain rota- and Walker, 1999). East-west extension in this If it is assumed that the kinematic facies of tion in the southeastern Sierra may involve both region ceased and the Indian Wells Valley gra- upper crustal thinning and dextral shear have laterally varying rheology and local stresses. ben became inactive after ca. 3.5 Ma. The exten- approximately constant positions relative to Dynamic models of the foundering process need sional basin was subsequently deformed by the the locus of active foundering, the pattern of

to account for observed convergence of the d3 Airport Lake fault, Little Lake fault, and other late Cenozoic time- and space-transgressive trajectories toward the Isabella anomaly in the active dextral faults of the western Walker Lane deformation in the southern Walker Lane belt southern San Joaquin Valley, and divergence belt at this latitude (Fig. 6C). The onset of strike- outlined here could be explained by progres-

of d1 trajectories around the eastern margin of slip faulting occurred after 3.5 Ma, and possibly sive westward propagation of lithospheric foun- anomaly in the southern Sierra Nevada. as recently as 2 Ma (Monastero et al., 2002). dering or delamination (Fig. 6). In this model, The Airport Lake fault and associated splays to lower lithosphere detached beneath what is now DISCUSSION: LATE CENOZOIC the north currently are the major active strike- the Panamint Range ca. 15 Ma and progres- MIGRATION OF EXTENSION AND slip faults along the eastern margin of the Sierra sively foundered westward, accompanied by NORTHWEST DEXTRAL SHEAR microplate at this latitude (Unruh et al., 2002). upwelling asthenosphere and east-west exten- WITH PROGRESSIVE WESTWARD The Pliocene–Pleistocene onset of dextral shear sion of the overlying crust at the rate of several FOUNDERING OF LOWER in Indian Wells Valley is similar in timing to the tenths of millimeters per year (Fig. 6A). By LITHOSPHERE? onset of dextral slip farther north in the eastern ca. 7.5 Ma, foundering propagated west into Inyo Mountains (ca. 2.8 Ma; Lee et al., 2009) the region currently occupied by Indian Wells In Unruh and Hauksson (2009), we proposed that the east to west variation in deformation kinematics across the transition from the Walker Lane belt to the Sierra Nevada could be Figure 6 (on following page). Model for late Cenozoic time- and space-transgressive defor- a snapshot of a time- and space-transgressive mation in the southern Walker Lane belt tied to progressive westward foundering of process associated with westward propaga- lower lithosphere. Dashed line with barbs shows inferred locus of foundering at various tion of dextral shear in the Walker Lane belt time intervals. DV—Death Valley; PR—Panamint Range; AR—Argus Range; SR—Slate (see discussion in Jones et al., 2004, and refer- Range; IWV—Indian Wells Valley; KCF—Kern Canyon fault; BF—Breckenridge fault; ences therein). Recent work has reﬁ ned the late CF—Calico fault; BWF—Blackwater fault; LF—Lockhart fault; LEF—Lenwood fault; Cenozoic history of the Walker Lane belt at the SHF—Spring Hills fault; BRF—Bullion Ranch fault; MVF—Mirage Valley fault; ALFZ— latitude of southern Sierra (Fig. 6), and supports Airport Lake fault zone; OVF—Owens Valley fault; IA—Isabella anomaly. (A) Foundering the hypothesis that northwest dextral shear, pre- beneath the PR at 15 Ma and 4 Ma drives east-west extension and separation of the PR from ceded by east-west extension, has propagated the AR (Andrew and Walker, 2009). (B) Foundering propagates west beneath IWV after ca. westward with time. 7.5 Ma. East-west extension and opening of IWV between 7.5 and 3.5 Ma (Monastero et al., Andrew and Walker (2009) found that 2002) occurs above upwelling asthenosphere. Northwest translation of the AR relative to between ca. 15 and 4.2 Ma Panamint Valley the PR begins ca. 4 Ma (Andrew and Walker, 2009) as dextral shear propagates westward opened via east-west extension, progressively in the wake of extension. (C) Foundering propagates westward beneath the Sierra Nevada separating the Argus Range and Panamint in Late Miocene–Pliocene time, producing modern Sierra Nevada mountain range. Onset of Range (Fig. 6A). Based on stratigraphic and strike-slip faulting in IWV after 3.5 Ma, and possibly as recently as 2 Ma (Monastero et al., structural relations, Andrew and Walker (2009) 2002), follows cessation of extension there. (D) Hypothetical scenario for future westward estimated that ~4 km of east-west extension propagation of dextral shear into the southeastern Sierra microplate. Rate of dextral slip occurred between ca.15 and 4.2 Ma, implying increases on west-northwest–striking faults in the western Mojave block such as the LF and an average horizontal extension rate of ~0.3 LEF, and possibly shorter Quaternary-active structures such as the SHF, BRF, and MVF. mm/yr during that period. After 4.2 Ma, the The KCF changes from a dominantly normal fault to a dextral-oblique fault, and with sig- Slate Range and Argus Range began moving niﬁ cant cumulative slip the high Sierra east of the KCF separates from the rest of the micro- northwest with respect to the Panamint Range plate and forms a distinct range in the southwestern Walker Lane belt similar to the PR. (Fig. 6B); Andrew and Walker (2009) inter- With cumulative deformation, faults crossing the Kern Arch may form an organized system preted this to indicate the onset of northwest linking the eastern California shear zone and Walker Lane belt to the San Andreas system.

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Internal deformation of the southern Sierra microplate

37°0′0″N 36°0′0″N 35°0′0″N 37°0′0″N 36°0′0″N 35°0′0″N

? ? ? ? ? ? LEF

? ? ? HF

PR PR

SR SR ? 117°0′0″W 117°0′0″W ?

F a F ? R LF HFH M A AR

SHFSHS F ? a V 5 BRF .

M MMVF

3 ?

? - 4 5 ~ ~4 Ma . 7 ~7.5-3.5 Ma ~ ? 118°0′0″W 118°0′0″W

A N - n N ioon SSN-NAot mmoti 119°0′0″W 119°0′0″W

? 120°0′0″W 120°0′0″W

? D B

121°0′0″W 121°0′0″W

37°0′0″N 36°0′0″N 35°0′0″N 37°0′0″N 36°0′0″N 35°0′0″N Figure 6. Figure

F CFC V

D DV

a R R

SR SR P PR S M

117°0′0″W 117°0′0″W WF

4 BWFBW - 5 1 R ~ ~15-4 Ma A AR

V Z F L W

I IWV AALFZ a M

0 . f e g 2 o 118°0′0″W 118°0′0″W - v

n OVFO i i 5 t s r .

u e

active active a c F C KCF KCF d K ~ ~3.5-2.0 Ma o n L Locus of Locus of u o foundering f a ad ev N ra er SSierrai Nevada 119°0′0″W 119°0′0″W IA

San Joaquin Valley 120°0′0″W 120°0′0″W 02040 mi km 02040 0204060 C A 121°0′0″W 121°0′0″W

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Valley (Fig. 6B), expressed in the upper crust by et al., 2002, 2008). The Airport Lake fault zone SUMMARY AND CONCLUSIONS the onset of east-west extension and subsidence is on strike with the dextral Blackwater fault in of ancestral Indian Wells Valley graben (Monas- the eastern California shear zone south of the Kinematic analysis of earthquake focal mech- tero et al., 2002). By 4.2–4.6 Ma (Andrew and Garlock fault (Fig. 6C), and both are associated anisms reveals horizontal extension and vertical Walker, 2009), east-west extension in the Pana- with a secular velocity gradient that marks the thinning of crust in the southern Sierra Nevada mint Range area was replaced by oblique northwestern margin of the eastern California shear above acoustically slow upper mantle, which west opening of Panamint Valley (Fig. 6B) at the zone (McClusky et al., 2001; Miller et al., 2001). may be upwelling asthenosphere in the wake of rate of several millimeters per year, refl ecting Upper crustal extension in the southern Sierra foundering lower Sierra lithosphere represented passage of the wave of upwelling asthenosphere Nevada is currently localized along the Kern by the Isabella anomaly. The horizontal trajec- and westward propagation of Walker Lane Canyon fault (Fig. 6C). Westward propagation tories of the maximum extensional and maxi- dextral shear in its wake. East-west extension of dextral shear could be accommodated by mum shortening strains are rotated distinctly continued in Indian Wells Valley for ~1–2 m.y. an increase in the slip rate on west-northwest– counterclockwise relative to regional trends after dextral shear began in the Panamint region, striking Quaternary faults in the Mojave block in the southern Sierra and San Joaquin Valley however, because asthenospheric upwelling was west of the Blackwater fault, and by a transition southeast of the Isabella anomaly. Based on lin- still active beneath Indian Wells Valley due to from normal slip on the Kern Canyon fault to ear alignments of earthquake epicenters in the proximity of foundering to the west. Between dextral-normal oblique motion, similar to what southwestern Sierra in the vicinity of the rotated ca. 3.5 and 2 Ma, the foundering process moved is observed on the Owens Valley fault (Fig. 6D). strains, the deformation represented by the west beneath the Sierra foothills and San Joa- In this scenario, the Kern Canyon fault and asso- rotated principal strains is interpreted to be dis- quin Valley and northwest dextral shear propa- ciated structures several million years hence tributed west-northwest–directed dextral shear gated into Indian Wells Valley (Fig. 6C). Dextral would exhibit an older history of accommodat- that is propagating westward from the eastern shear also localized in what is now the Owens ing a low rate of east-west extension, on which a California shear zone. The strain rotations may Valley to the north, forming the modern eastern younger episode of dextral-normal displacement refl ect the presence of local stresses associated margin of the Sierra microplate at this latitude. at higher rates is superimposed. With suffi cient with relaxation of subsidence in the vicinity of The locus of extension directly east of actively cumulative deformation, the southeastern part of the Isabella anomaly. Patterns of late Cenozoic foundering lithosphere currently is the southern the Sierra Nevada east of the Kern Canyon fault time- and space-transgressive deformation in Sierra Nevada, and is accommodated by seis- would be separated from the rest of the intact Panamint Valley and Indian Wells Valley (i.e., mogenic deformation and active normal slip microplate and possibly form a discrete moun- extension followed by northwest dextral shear) on the Kern Canyon fault (Nadin and Saleeby, tain range similar to the modern Panamint and east of the Sierra microplate potentially can be 2010; Kelson et al., 2010). The rate of horizon- Coso Ranges in the Walker Lane belt to the east. explained by westward propagation of litho- tal extension associated with normal slip on It is interesting to observe that the western- spheric foundering and associated kinematic the Kern Canyon normal fault above upwelling most Quaternary faults in the northern Mojave facies similar to those currently observed east asthenosphere is in the very low tenths of milli- block, including the Lockhart fault, Spring fault, of the Isabella anomaly. The active deformation meters per year (Kelson et al., 2010), in contrast and Blake Ranch fault, strike more toward the in the southern Sierra Nevada may be a snap- to the higher rates of dextral shear in the western west than the Helendale and Calico faults in the shot of the process that separated the bedrock Walker Lane belt to the east (i.e., average late central and eastern parts of the Mojave block of the Argus, Panamint, and Coso Ranges from Quaternary slip rate on the Owens Valley fault (Fig. 6D), toward the domain of strain rotation the Sierra batholith and widened the southern determined from paleoseismic studies ranges in the southwestern Sierra, and are more parallel Walker Lane belt in late Cenozoic time (Jones from 0.5 to 3.6 mm/yr; Beanland and Clark, to the direction of macroscopic shear there. Col- et al., 2004). 1994; Lee et al., 2001; Bacon and Pezzopane, lectively, these structures may represent a very ACKNOWLEDGMENTS 2007; a secular rate of ~5–7 mm/yr was inferred youthful stage in the development of an orga- from geodetic data across Owens Valley by Gan nized and interlinked fault system that eventu- We acknowledge support of this research by the et al., 2000; McClusky et al., 2001). In general, ally will transfer dextral shear from the eastern National Science Foundation (grant EAR-0607625 it appears that the rate of the early phases of California shear zone across the southwestern to Unruh, grants EAR-0454535 and EAR-0607831 extension and normal faulting is lower than the Sierra Nevada and into the San Joaquin Valley. to Jones), and the U.S. Army Corps of Engineers. The ideas and interpretations presented herein were rates of subsequent shear and strike-slip fault- The hypothesis outlined in Figure 6 primarily developed in conversations with Jason Saleeby, Zorka ing, the difference ranging from a factor of 3 to focuses on east to west migration of the foun- Saleeby, Peter Molnar, Greg Houseman, Hersh Gil- an order of magnitude. The difference in rates dering process at the latitude of the study area, bert, Anthony Frassetto, Allen Glazner, Frank Mona- may indicate different driving forces for the two which appears to differ from previous models stero, John Dewey, Eugene Humphreys, and Colin Amos. We thank Frank Monastero and Cooper Brossy kinematic facies, i.e., local buoyancy forces ver- that infer lithospheric foundering initiated in an for comments on an early version of the manuscript, sus far-fi eld plate tractions. area near the northern end of the modern Owens and Jason Saleeby, Jeffrey Lee, and an anonymous A hypothetical scenario for future westward Valley and migrated southwest to the current reviewer for Geosphere for constructive reviews, all encroachment of dextral shear into the Sierra location of the Isabella anomaly (Zandt, 2003; of which signifi cantly improved the fi nal paper. Zandt et al., 2004). 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