Quaternary faulting and background seismicity in the southern Sierra Nevada microplate are concentrated east and south of the Isabella anomaly, a high-velocity body in the upper mantle interpreted to be lower Sierra lithosphere that is foundering into the asthenosphere. We analyzed seismicity in this region to evaluate patterns of upper crustal deformation above and adjacent to the Isabella anomaly. Earthquakes in the southern Sierra and San Joaquin Valley were relocated using joint hypocentral inversion and double-difference techniques, and groups of focal mechanisms were inverted for the components of a reduced deformation rate tensor. The deformation field derived from this analysis reveals two distinct departures from horizontal plane strain associated with distributed northwest-directed dextral shear east of the Pacific plate: (1) heterogeneous extension and crustal thinning in the high Sierra and western foothills east of the Isabella anomaly; and (2) pronounced counterclockwise rotation of the principal strains from regional trends in the southwestern Sierra Nevada and across the Kern Arch. Based on comparison with a three-dimensional tomographic model, the extension in the southern Sierra is spatially associated with relatively thinner crust and anomalous low P-wave speeds in the upper mantle (40–90 km depth range) directly east of the Isabella anomaly. These relations suggest that seismogenic crustal thinning is localized above upwelling asthenosphere that is replacing foundering lithosphere. Counterclockwise rotation of strain trajectories in the southwest Sierra occurs southeast of the Isabella anomaly, and is associated with seismogenic west-northwest–striking dextral faults. We suggest that the deformation here represents westward encroachment of dextral shear into the microplate from the eastern California shear zone and southern Walker Lane belt. The strain rotation may reflect the presence of local stresses associated with relaxation of subsidence in the vicinity of the Isabella anomaly. Westward propagation of foundering lithosphere, with spatially associated patterns of upper crustal deformation similar to those documented herein, can account for observed late Cenozoic time- and space-transgressive deformation in the southern Walker Lane belt east of the Isabella anomaly, and is a potentially observable consequence of the foundering process in other orogens.
This paper presents a systematic analysis of seismogenic deformation above and around the Isabella anomaly, a narrow, vertically elongated zone of anomalous high P-wave speeds in the upper mantle beneath the southern San Joaquin Valley, California (Benz and Zandt, 1993; Fig. 1). Following Jones et al. (2014), the Isabella anomaly is centered below lat 36°N, long 119.3°W. It is ∼100 km in diameter, extends to depths of ∼200–225 km, and is characterized by 4%–5% increase in P-wave speed relative to adjacent asthenospheric mantle. The Isabella anomaly is approximately equant in plan view and appears to plunge ∼60° to 70° east in cross-sectional views (Jones et al., 2014).
The Isabella anomaly has been interpreted to be lower lithosphere that detached from the base of the Sierra Nevada and is foundering or convectively descending into the asthenosphere (Saleeby et al., 2003; Zandt et al., 2004; Jones et al., 2004; Boyd et al., 2004). In a previous study (Unruh and Hauksson, 2009), we evaluated background seismicity in the southern Sierra Nevada east of the Isabella anomaly. The seismicity represents internal deformation of the Sierra Nevada microplate, a large area of central and northern California that moves ∼13 mm/yr to the northwest relative to stable North America as an independent and nominally rigid block (Argus and Gordon, 1991, 2001). At the latitude of the Isabella anomaly, the majority of microplate translation is accommodated by mixed strike-slip and normal faulting in the southern Walker Lane belt (Fig. 1), a zone of distributed northwest-directed dextral shear east of the Sierra Nevada and north of the Garlock fault. Through kinematic analysis of earthquake focal mechanisms, Unruh and Hauksson (2009) documented an east to west transition from north-northwest–directed dextral shear in the southern Walker Lane belt to west-northwest extension and vertical thinning in the southern High Sierra, and suggested that internal deformation of the Sierra block is driven by a combination of distributed plate motion and local forces associated with removal of lower lithosphere. Our goal here is to extend the study area (of Unruh and Hauksson, 2009) to the west (Fig. 1), and compare patterns of upper crustal deformation with new tomography and seismic imaging of the lower crust and upper mantle beneath the southern Sierra Nevada microplate (Reeg, 2008; Frassetto et al., 2011; Jones et al., 2014).
We also assess regional variations in deformation style in the context of late Cenozoic time- and space-transgressive deformation in the Walker Lane belt to the east. As discussed by Saleeby et al. (2012, 2013), a predicted and potentially observable consequence of the foundering process is epeirogenic transients. We propose that kinematic transients also may systematically occur in the crust as the foundering process propagates laterally, and we develop this hypothesis by comparing our results with geologic data from the southern Walker Lane belt.
PRESENTATION OF SPATIAL DATA
Geospatial data sets covering the study area in Figure 1 that are referenced in this paper include a hillshade map of topography; smoothed elevation contours of the Sierra Nevada (1000 ft intervals, i.e., 304.8 m) and San Joaquin Valley (100 ft intervals, i.e., 30.48 m); seismicity recorded from 1981 to 2008 for which focal mechanisms are calculated; Quaternary faults (Jennings, 1994; Kelson et al., 2010); depth to the Moho (Frassetto et al., 2011); 40 km, 70 km, and 170 km depth slices through a three-dimensional (3-D) P-wave tomographic model (Jones et al., 2014); regional geology; and derivative maps of the regional deformation field from the results of the seismicity analysis. The tomographic inversion we chose was started with a P-wave model derived from the ambient noise tomography of Moschetti et al. (2010), which was held fixed above 90 km depth for 14 iterations and then freed. Thus the shallower part of this particular inversion most closely resembles a smoothed version of the Moschetti et al. (2010) results. The tomography depth sections and spatial data sets herein were exported from a fully georeferenced geographic information systems (GIS) database and organized for viewing in a layered Acrobat (pdf) file (animated version of Fig. 2); individual layers corresponding to discrete spatial data sets are labeled A through O. The layered pdf format permits selection of these data sets for viewing individually or in combination: i.e., to view certain combinations of data, use the layering function of Figure 2. Note that when Figure 2 is first opened in Acrobat all data layers are displayed, producing the very cluttered view shown in the non-animated version (some non-Adobe pdf viewers will not show the layer controller). We recommend that users open the file, click the layering icon, and initially turn off most of the layers for ease of viewing.
For this study we analyzed the (1981–2008) background seismicity in the southern Sierra Nevada and southern San Joaquin Valley, approximately located in the rectangular search box in Figure 1; lat 34.617°N to 37.0° N and long 120.7°W to 118.167°W. We collected earthquake data from the Southern California Seismic Network and Northern California Seismic Network to determine 3-D Vp and Vp/Vs, crustal models of the study region using the methods of Thurber (1993). We used these models to relocate the background seismicity using double differencing techniques (Waldhauser and Ellsworth, 2000). In the final step of data processing we determined first-motion focal mechanisms for ∼20,000 events with 12 or more first motions. We used the grid-searching algorithm and computer programs by Reasenberg and Oppenheimer (1985) to determine the first-motion, lower hemisphere focal mechanisms. In most cases, the first-motion focal mechanisms are well constrained by the combined azimuthal coverage of both networks.
We used a micropolar continuum model for distributed brittle deformation (Twiss et al., 1993; Twiss and Unruh, 1998) as a basis for inverting focal mechanisms from groups of earthquakes to derive a reduced deformation rate tensor. For a detailed description of the analytical approach, see Unruh and Hauksson (2009, and references cited therein).
The study area in Figure 1 was subdivided into smaller regions to evaluate the local seismogenic deformation associated with key structures and geophysical features, and to capture lateral variations in deformation geometry. A map of the polygonal subregions is provided in the Supplemental File1. Inversions of data from individual subregions were performed using an automated grid search algorithm called FLTSLP_2K6 (L. Guenther and R. Twiss, University of California, Davis; see Appendix D in Guenther, 2004, for user’s manual). The inversion algorithm incorporates standard bootstrap methods to estimate uncertainties in the best-fit model parameters. The micropolar deformation model is parameterized by the following: (1) the orientations of the principal strain rates (d1 > d2 > d3; lengthening positive); (2) a scalar parameter (D) formed by a ratio of the differences in the principal strain rates, that characterizes the shape of the strain rate ellipsoid; (3) a scalar parameter (W) that characterizes the relative vorticity of rigid, fault-bounded blocks about an axis parallel to the intermediate principal strain rate axis d2; and (4) the ratio V of the vertical deviatoric deformation rate to the maximum deformation rate (Unruh et al., 2002). Positive values of V indicate net crustal thickening, negative values indicate net crustal thinning, and a value of zero indicates horizontal plane strain (Lewis et al., 2003).
Data tables with the inversion results are provided in the Supplemental File (see footnote 1), along with additional details of the analytical approach.
The micropolar continuum theory that forms the basis for the inversion relates patterns of distributed seismogenic slip to velocity boundary conditions (Twiss et al., 1993; Twiss and Unruh, 1998). Because the period of time over which the earthquakes occurred is very short and essentially instantaneous relative to geologic time, the incremental strains represented by the seismicity data are assumed to be equivalent to the instantaneous strain rates that form the basis of micropolar theory. For ease of discussion, the principal strain rates are referred to in this paper simply as maximum extension and maximum shortening (d1 and d3 of the inversion solution, respectively).
The inversion results for individual subregions are synthesized in a map of the regional seismogenic deformation field (Figs. 2B, 2H, 2N). The horizontal components of the deformation are depicted by smooth trajectories drawn parallel to the local trends of maximum extension and maximum shortening associated with individual inversion results (Figs. 2H, 2N). In cases where the principal strains are steeply plunging to subvertical, there is effectively no resolved horizontal component and their respective trajectories are not plotted on the map. The interpretation of the absence of one of the principal strain trajectories is straightforward. If only d1 trajectories are shown, then the deformation is characterized by horizontal extension parallel to d1, and d3 is steeply plunging to subvertical, indicating net crustal thinning. Similarly, if only d3 trajectories are shown, then d1 is steeply plunging to subvertical and the deformation locally is characterized by shortening parallel to d3 with net vertical thickening. In some areas one of the principal strains is subhorizontal and the other is moderately plunging. Depending on which of the principal strains is plunging, the associated deformation is transtensional (V < 0; d3 plunging) or transpressional (V > 0; d1 plunging). Due to the plunge of one of the principal strains, the horizontal strain trajectories typically are not orthogonal in areas characterized by transpression and transtension.
RESULTS: KINEMATIC DOMAINS
The study area in Figure 1 can be divided into distinct kinematic domains based on the direction of macroscopic shear implied by the orientations of the principal strains, and the relative contribution of vertical thickening or thinning to the bulk deformation. These kinematic domains are outlined in Figures 2B, 2H, and 2N. The key characteristics of each domain are summarized in Table 1; detailed descriptions of the domains are presented in the Supplemental File (see footnote 1).
Across most of the study area in Figure 2 the seismogenic deformation reflects distributed northwest-directed dextral shear east of the Pacific plate. In the Walker Lane belt and south-central San Joaquin Valley, the maximum extension and maximum shortening strains are subhorizontal and the vertical deformation parameter V is zero, indicating horizontal plane strain. Maximum extension generally is oriented west-northwest–east-southeast to northwest-southeast, and maximum shortening is oriented north-northeast–south-southwest to northeast-southwest, consistent with northwest-directed macroscopic dextral shear. Several first-order variations are superimposed on this regional pattern.
Extension and Crustal Thinning in the Southern Sierra Nevada
A large contiguous area with values of the vertical deformation parameter V ≤ –0.7 (here assumed to indicate dominantly vertical thinning) spans the southern Sierra Nevada and is approximately centered on lat 36°N and long 118.5°W (Figs. 2B, 2H). The Kern Canyon fault, a Holocene-active normal fault, is located within this extensional domain. Reconnaissance investigations (Nadin and Saleeby, 2010), geologic mapping (Brossy et al., 2012), and paleoseismic trenching (Kelson et al., 2010) indicate that the Kern Canyon fault primarily accommodates east-down normal slip. The Kern Canyon fault strikes at a high angle to the d1 trajectories in the southern Sierra (Figs. 2A, 2H, 2N), and thus is optimally oriented to accommodate normal slip in the present seismotectonic regime. The zone of extensional deformation extends south of Durrwood Meadows to about the latitude of northern Lake Isabella, where it terminates and is separated from another elliptical east-west–trending zone of extension by a narrow zone of dominantly strike-slip faulting and shearing between Lake Isabella and Walker Basin (Figs. 2B, 2D, 2H, 2N). As previously noted in Unruh and Hauksson (2009), the Durrwood Meadows area is characterized by anomalous deformation where both the d1 and d2 principal strains are horizontal and extensional, thus accommodating a flattening or pancaking of the crust.
In Unruh and Hauksson (2009), we previously inferred that an ∼30-km-wide zone of transtensional deformation in the eastern Sierra Nevada separated regional plane strain and north-northwest dextral shear in the Walker Lane belt from vertical thinning in the southern High Sierra. The current results do not indicate a smooth east to west kinematic transition or gradient in the parameter V between the Walker Lane belt and interior of the Sierra microplate. Rather, the transition between horizontal plane strain and vertical thinning is relatively abrupt and the eastern boundary of thinning is irregular in plan view and not parallel to the physiographic boundary between the Sierra Nevada and southern Walker Lane belt (Figs. 2B, 2D, 2N, 2H). Salients of horizontal plane strain extend westward from the Walker Lane belt into the eastern Sierra Nevada, notably in the southeastern corner of the range (Figs. 2B, 2D, 2N, 2H). The northern extent of the Sierra extensional domain is poorly constrained by these data.
Counterclockwise Rotation of the Principal Strains across the Kern Arch
The maximum extension and shortening strains in the southwestern Sierra Nevada north of approximately lat 35°N are subhorizontal and rotated distinctly counterclockwise from regional trends. The zone of strain rotation straddles the east end of the White Wolf fault and the southern end of the Breckenridge fault, and encompasses the Kern Arch, an uplifted and west-tilted topographic salient in the southeastern San Joaquin Valley that is mantled by Tertiary marine strata and late Cenozoic fluvial deposits (Maheo et al., 2009; Saleeby et al., 2009) (Figs. 2A, 2H, 2N, 2O). The direction of maximum extension in this domain trends northeast-southwest and the direction of maximum shortening trends northwest-southeast, representing an ∼40° counterclockwise rotation from the orientations of the principal strains in the southern Walker Lane belt and southern San Joaquin Valley to the east and west, respectively. The maximum extension and shortening strains are subhorizontal and values of the vertical deformation parameter V are close to zero (Table A1 in the Supplemental File [see footnote 1]), indicating that the deformation is characterized by horizontal plane strain and crustal shearing.
The counterclockwise rotation of the principal strains is superimposed on the regional thinning in the southern Sierra described in the previous section. West and northwest of Durrwood Meadows maximum extension trends consistently west-northwest–east-southeast, similar to d1 trends in the Walker Lane belt to the east. To the south, maximum extension trends approximately east-west between Durrwood Meadows and Lake Isabella. In a smaller and isolated region of extension and crustal thinning between Lake Isabella and Walker basin, d1 trends west-southwest–east-northeast. The flattening deformation at Durrwood Meadows appears to span or encompass the counterclockwise rotation of d1 from west-northwest–east-southeast to the north to approximately east-west and northeast-southwest to the south. It is possible that the local flattening deformation at Durrwood Meadows is a combination of the west-northwest–east-southeast–directed extension at approximately lat 36°N, and the northeast-southwest–directed extension to the south in the Lake Isabella area; that is, rather than a smooth north to south variation in the orientation of d1, there are discrete domains of west-northwest–trending d1 to the north and northeast-southwest–trending d1 to the south that overlap or interfere in the vicinity of Durrwood Meadows, resulting in the two horizontal principal strains there both being extensional.
Although the strain geometry in the vicinity of the Kern Arch could be accommodated by shearing along either west-northwest–striking dextral faults or north-northeast–striking sinistral faults, patterns of seismicity between the White Wolf fault and Breckenridge fault suggest that the macroscopic deformation is characterized by distributed west-northwest–directed dextral shear (Fig. 3). Relocated earthquakes define a series of west-northwest–striking planar alignments of hypocenters in an ∼20 km by 20 km area encompassing the eastern end of the White Wolf fault and southern end of the Breckenridge fault (Fig. 3A). Cross sections normal to the linear trends of the epicenters reveal steeply dipping to subvertical planar alignments of hypocenters in the 1–8 km depth range (Figs. 3B, 3C), indicating that the west-northwest–trending epicentral lineaments are associated with high-angle faults.
For example, a northwest-trending cluster of events at the southwest end of section A–A′ (Fig. 3B) includes two distinct planar alignments, which we interpret to be two discrete faults. These events are within subregion NWW1 (Fig. 3A; Table A1 in the Supplemental File [see footnote 1]). The western fault alignment at horizontal distance ∼2.5 m dips steeply northeast. The second alignment at horizontal distance ∼3.4 km is subvertical. Steeply dipping to subvertical seismogenic faults also are present in the 5–8 km depth range between horizontal distance 7–12 km on section A–A′ (Fig. 3B). Inversion results for NWW1 indicate transtensional dextral shear resolved on west-northwest–striking faults (Fig. 3B; Table A1 in the Supplemental File [see footnote 1]). Northwest-trending alignments of epicenters crossed by the northeastern part of section B–B′ (Fig. 3A) appear to be associated with subvertical faults in the 7–9 km depth range between horizontal distances 9 and 15 km (Fig. 3C). These events are within subregion SEW3 (Table A1 in the Supplemental File [see footnote 1]), and inversion results indicate distributed dextral shear on a west-northwest–striking structural fabric, with a possible small component of horizontal extension and crustal thinning (V = –0.2) that cannot be distinguished from horizontal plane strain at the 95% confidence interval (Table A1 in the Supplemental File [see footnote 1]). Planar alignments of hypocenters also are present in the 5–9 km depth range in section C–C′ (Fig. 3D), but are less well expressed than on the other two sections. The clusters highlighted in Figure 3D correspond to subregions SWNB and SEW2 (Fig. 3); inversion results indicate distributed dextral shear on west-northwest–striking faults for both subregions (Table A1 in the Supplemental File [see footnote 1]).
The principal strains from inversions of earthquakes in this region are oriented ∼45° to the strike of the steeply dipping to subplanar seismogenic faults, indicating resolved right-lateral motion on these and other west-northwest–striking structures. Despite the proximity to the northeast-striking White Wolf fault, west-northwest–striking planar alignments are dominant in Figure 3A. We thus infer that seismogenic deformation in this region primarily reflects distributed west-northwest–directed dextral shear.
Additional evidence for local counterclockwise rotation of the principal strains in the Kern Arch region can be inferred from patterns of surface faulting that occurred during the 1952 M 7.2 Arvin-Tehachapi earthquake on the White Wolf fault, the eastern end of which encroaches into the domain of west-northwest–directed dextral shear (Fig. 3A). As documented by Buwalda and St. Amand (1955), surface rupture along the section of the White Wolf fault between the Tehachapi Mountains and San Joaquin Valley was dominantly reverse with a very minor component of right-lateral slip. The dextral component observed in the surface rupture along this reach is anomalous, given the seismogenic and geodetic evidence for a dominant left-lateral component in the coseismic rupture (Bawden, 2001). At the southeastern corner of the San Joaquin Valley, where the Sierra Nevada intersects the Tehachapi Mountains, the general northeast trend of the surface ruptures was crosscut by an ∼6-km-long north-northeast–striking fault with left-lateral offset (Fig. 3A). East of the north-northeast–striking fault segment, the pattern of northeast-trending fractures along the surface trace of the White Wolf fault resumed; however, many of the ruptures showed evidence for both extensional and left-lateral displacement (Fig. 3A). Near the eastern limit of the approximately continuous coseismic rupture, the main surface trace turned abruptly north-northeast and exhibited a left-lateral component of slip, similar to the 6-km-long north-northeast–striking segment to the west. Surface deformation progressively died out east of this point, accommodated by short, discontinuous north-south– to northeast-striking faults accommodating extension (Buwalda and St. Amand, 1955).
The change in character of the 1952 surface rupture from dominantly reverse slip on a northeast-striking fault along the base of the western Tehachapi and San Emigdio Mountains (plus a minor, anomalous component of dextral motion), to a mix of left-lateral slip on north-northeast–striking faults and reverse-left slip on northeast-striking faults occurs in the vicinity of counterclockwise rotation of the principal strains. Left-lateral slip on the north-northeast–striking segments of the surface rupture is consistent with local rotation of d1 to a northeast-southwest orientation and d3 to a northwest-southeast orientation (Fig. 3A). Although the 1952 surface rupture at the eastern end of the White Wolf fault is very complex and likely includes effects of gravitational slope failure triggered by strong ground shaking (Buwalda and St. Amand, 1955), the first-order change in faulting style is consistent with the rupture propagating across, and reflecting, a counterclockwise rotation of the principal strains between the Tehachapi Mountains and southwestern Sierra Nevada (Figs. 2A, 2H, 2N).
Transpression in the Southwestern San Joaquin Valley
Positive values of the vertical parameter V (Table 1) in the Elk Hills and Antelope Plain regions (ELKH and ANTP, respectively; Table 1A in the Supplemental File [see footnote 1]) and the Diablo Range west of the Kettleman Hills (DIAB subregion; Table 1A in the Supplemental File [see footnote 1]) indicate a narrow zone of transpressional deformation ∼15–30 km wide directly east of the San Andreas fault (Fig. 2B, kinematic domains). The orientation of the maximum shortening strain in this region is north-northeast–south-southwest (Figs. 2B, 2N), consistent with dextral shear on the San Andreas fault and with shortening at a high angle to west-northwest–east-southeast–trending late Cenozoic folds in the western San Joaquin Valley such as the Elk Hills and Wheeler Ridge anticlines. The maximum extensional strain is moderately plunging rather than vertical (Table A1 in the Supplemental File [see footnote 1]), indicating oblique crustal thickening.
The zone of transpression east of the San Andreas fault is inferred to extend to the northwest and include the epicentral region of the 1983 Coalinga earthquake (Figs. 2B, 2D, 2E, 2N). Tetreault (2006) performed a detailed analysis of the Coalinga earthquake aftershocks, including paleomagnetic tests of folded bedding for vertical axis rotations and kinematic inversions of focal mechanisms using a micropolar approach, and concluded that the Coalinga anticline is deforming due to a combination of right-lateral shear and thrust and/or reverse faulting. Values of V obtained from inversions of groups of spatially distinct aftershocks below 7 km range from 0.7 to 0.85 (Tetreault, 2006), indicating primarily vertical thickening with a subordinate component of shearing.
Extension Above Anomalously Slow Asthenosphere
Comparison of the loci and extent of vertical thinning with horizontal depth slices through a 3-D tomographic model of Jones et al. (2014) reveals a fair to good correlation with anomalous low P-wave speeds (low Vp) at lower crustal and upper mantle depths. We compare our results with the inversion of Jones et al. (2014) shown here that started from (and generally preserves) the ambient noise tomography of Moschetti et al. (2010) for the crust. Our interpretation is informed by uncertainties associated with tomography in Jones et al. (2014). In the tomographic model we have adopted, the contiguous region of vertical thinning centered on approximately lat 36.2°N and long 118.5°W is similar in size, extent, and shape to the region of low Vp at 70 km depth beneath the southern Sierra Nevada (Figs. 2B, 2D, 2L). Cross sections through the velocity model show that the extensional domain is associated with low Vp extending to depths of 70 –100 km (Figs. 4A, 4D–4F; see Figs. 2A–2C, 2N, 2O for locations of section lines). This spatial correlation is discussed in greater detail in the following.
In addition to anomalous low Vp at lower lithospheric depths, upper crustal extension in the southern Sierra Nevada at the latitude of Porterville is associated with a shallower Moho than to the north along the range (Frassetto et al., 2011). Structure contours on Moho depth reveal ∼10 km of positive relief beneath the southern Sierra Nevada at approximately lat 36°N relative to equivalent positions along the western Sierra slope to the north, coincident with the zone of upper crustal extension (Figs. 2B, 2I, 2K).
The spatial relationship between patterns of upper crustal deformation, relatively shallow Moho, and velocity variations in the upper mantle are further illustrated by a series of cross sections through the tomographic model of Jones et al. (2014; see Figs. 2C and 2N for locations of section lines). Section A (Fig. 4A) shows that extension and thinning in the Sierra upper crust occur above low-velocity upper mantle east of the steeply east dipping Isabella anomaly. The east to west transition from shearing in the Walker Lane belt to extension and thinning in the Sierra Nevada is associated with a zone of anomalous low upper mantle wave speeds that extend down to 70–80 km depths. Specifically, the change in deformation style is associated with deepening of the base of the low-wave speed material. In contrast, deformation in the western San Joaquin Valley adjacent to the San Andreas fault and Coast Ranges, west of the Isabella anomaly, is transpressional and accommodated by mixed strike-slip and thrust faulting (Fig. 4A). Similar relationships are visible in east-west section B (Fig. 4B), which skirts the northern margin of the Isabella anomaly. Section C (Fig. 4C) crosses the Sierra Nevada north of the Isabella anomaly and reveals possibly intact high-velocity lower lithosphere directly below the crust of the western Sierra slope and foothills and eastern San Joaquin Valley. Although there is a steeply east dipping zone of relatively higher P-wave speeds (∼+2% to +3%) in the 100–250 km depth range beneath the Sierra Nevada at this latitude, it is not as fast as in the feature we identify as the Isabella anomaly to the south ∼+4% to +5%). Low upper mantle velocities are present beneath the Moho in the Walker Lane belt along section C, but do not abruptly deepen westward to maximum depths of 60–80 km beneath the Sierra, as in sections A and B. Kinematic inversion of sparse focal mechanisms above the eastern margin of the high Vp upper mantle beneath the Sierra foothills and San Joaquin Valley indicates shearing deformation with possibly a minor component of net vertical thinning (domain KING; see the Supplemental File [footnote 1]), in contrast to the dominantly horizontal extension and vertical thinning beneath the western Sierra slope to the south.
Cross-sections X, Y, and Z (Figs. 4D, 4E, 4F, respectively) pass through the center of the Isabella anomaly and span a range of orientations between northeast-southwest and northwest-southeast (Figs. 2C, 2M). These three cross sections consistently show that upper crustal extension is occurring above anomalous low Vp upper mantle in the 50–80 km depth range directly east of the Isabella anomaly. The transition from crustal thinning in the southern Sierra to dextral shear in the Walker Lane belt is associated with abrupt eastward increase in Vp in this same depth range. Section Y passes along and across the irregular south margin of the extensional domain in the Sierra foothills (Fig. 2B, Fig. 2C, Fig. 2N). Deformation in the foothills directly east of the Isabella anomaly locally is characterized by shearing (Fig. 4E). The east end of section Z (Fig. 4F) crosses the Coso Range, which is within a right-releasing stepover between the dextral Airport Lake fault zone on the south and the dextral Owens Valley fault to the north (Unruh et al., 2008). The multiple fault branches composing the releasing stepover are depicted as a negative flower structure on section Z. These structures are above the eastern margin of the zone of anomalous low upper mantle velocities that underlies the Sierra Nevada east of the Isabella anomaly (Fig. 4F), similar to relations in sections X and Y.
To summarize, cross sections that pass directly through the Isabella anomaly (A, X, Y, Z; Fig. 4; Figs. 2C, 2I, 2N) consistently show the Moho deepening westward from ∼35 km beneath the eastern Sierra Nevada crest to a maximum depth of ∼40 km in the eastern and central San Joaquin Valley directly above the Isabella anomaly (Frassetto et al., 2011; Fliedner et al., 1996, 2000). The Sierra crust east of the Isabella anomaly is underlain by upper mantle with anomalous low P-wave speeds extending to depths of ∼70–90 km or more. The west to east transition from extension in the Sierra to shearing in the Walker Lane belt generally is associated with eastward shallowing (or decreasing integrated slowness) and vertical narrowing of these zones of anomalously low wave speeds. In contrast, section C north of the Isabella anomaly shows a deeper Moho (maximum depth ∼50 km) and the +4% to +8% higher wave speeds confined to 40–70 km depths beneath the western Sierra foothills. Seismogenic deformation in the foothills is characterized by distributed northwest dextral shear with a possible small component of net vertical thinning.
Previously workers have inferred that the low-velocity mantle to the east of the Isabella anomaly in the depth range of 50–100 km represents upwelling asthenospheric mantle (Jones et al., 1994; Zandt et al., 2004; Boyd et al., 2004; Saleeby and Foster, 2004; Frassetto et al., 2011; Saleeby et al., 2012, 2013). If this is correct, then a substantial part of the high topography of the southern Sierra Nevada at the latitude of the upper Kern River drainage area is supported by asthenosphere that replaced preexisting lower lithosphere. The steepest topographic gradient along the western Sierra slope and highest smoothed elevations are associated with: low Vp at lithospheric depths; a more shallow Moho relative to equivalent positions along the Sierra slope to the north and south; and thin crust (< 35 km) (Figs. 2G, 2I, 2L). The Sierra crust is extending and thinning above the upwelling asthenosphere.
A similar process of lithospheric foundering accompanied by upwelling asthenosphere has been proposed to account for the high topography of the Apennines (Shaw and Pysklywec, 2007) and the east Anatolian high plateau (Şengör et al., 2003). Like the southern Sierra, both of these orogens are deforming internally by distributed extension (Göğüş and Pysklywec, 2008; D’Agostino et al., 2011). In the case of the eastern Anatolian plateau, extension there is occurring in the broader tectonic context of north-south convergence between the Arabian and Eurasian plates, indicating that the local buoyancy forces within the orogen at least balance, or exceed, far-field tractions on the boundaries from collisional plate interactions. We infer that the same is true for areas of the southern Sierra undergoing active extension, the difference being that the far-field plate motions giving rise to tractions on the sides of the Sierra microplate are translational rather than convergent. This inference is consistent with the results of numerical modeling by LePourhiet and Saleeby (2013) that suggest that elastic stresses in the crust arising from negative buoyancy of the Isabella anomaly may be comparable to elastic stresses associated with shearing along the San Andreas fault.
Propagation of Dextral Shear into the Southeastern Sierra Microplate
The domain of counterclockwise rotation of d1 and d3 trends in the southern Sierra Nevada encompasses a network of late Cenozoic faults mapped in the Sierra foothills and across the Kern Arch near Bakersfield (Figs. 2N, 2H, 2A, 2D). These faults are on trend with the west-northwest–striking seismogenic dextral faults between the White Wolf and Breckinridge faults (Fig. 3A). Although many of the faults in the foothills north and west of Bakersfield were mapped as Quaternary faults by Jennings (1994), evidence for late Quaternary activity has not been documented for most of the structures. If these faults are currently active, then the general west-northwest trend suggests that they may accommodate distributed dextral shear that is beginning to encroach on the Sierra microplate from the eastern California shear zone and Walker Lane belt. Evidence for progressive westward expansion of the Walker Lane belt into the Sierra microplate has been summarized (e.g., Jones et al., 2004), so this may be an incremental step in that process. The west-northwest direction of macroscopic dextral shear in the southeastern Sierra and across the Kern Arch trends more toward the west than either Pacific–North America or Sierra–North America plate motion, however, which would typically be a restraining geometry and be expected to produce localized transpressional deformation. The rotation of the strains maintains horizontal plane strain as the crust flows through this bend. The counterclockwise rotation of the strains implies that the distributed plate motion locally is influenced by local forces and/or variations in rheology. The local strain rotation, and presumably stress rotation, be may be due to the following.
1. Dextral shear may be deflected west-southwest through thin crust and lithosphere that was stripped of its mantle lid during the Laramide orogeny. Based on structural and thermochronological relations, Saleeby (2003) proposed that the mantle lithosphere was sheared off from beneath the southernmost Sierra in the Late Cretaceous by a shallow flat segment of the Laramide slab. According to this model, the metamorphic Rand Schist was underplated to the remaining Sierra lithospheric column during subsequent steepening of the Laramide slab, replacing the mantle lid (Saleeby, 2003). The zone of rotated d1 and d3 trends, Quaternary faults, and distributed west-northwest dextral shear are associated with relatively thinner crust beneath the southern Sierra Nevada, Kern Arch, and Tehachapi Mountains. Moho depths (Malin et al., 1995; Frassetto et al., 2011) indicate that the crust is between 30 and 35 km thick in this region (Figs. 2I, 2N). Near Porterville north of the Kern Arch, where d1 trajectories in the western Sierra and eastern San Joaquin Valley are not rotated significantly relative to the northwest-southeast trends in the Walker Lane belt to the east, the crust is ∼40–45 km thick and underlain by relatively high velocity lithosphere to depths of 70 km (Figs. 2A, 2H, 2I, 2K, 2L). It is possible that westward-propagating dextral shear may be localized in the southwestern Sierra because the thin crust and absence of mantle lithosphere result in relatively low integrated strength there. The counterclockwise strain rotation may thus represent refraction of the strain trajectories (and distributed plate motion) through relatively weak lithosphere.
2. The counterclockwise strain rotation may reflect the influence of local stresses and/or processes associated with the Isabella anomaly. For example, simple models of horizontal stresses that develop adjacent to thickened lithospheric mantle (e.g., Fleitout and Froixdeveaux, 1982) predict inward-directed compression above the locus of downwelling. Radial horizontal flow of crust associated with vertical thickening above a mantle drip will predictably generate horizontal compressive circumferential or hoop stresses that are exerted perpendicular to both the axis and radius of the downwelling mass. The compressive hoop stresses in the crust are analogous to stresses that would develop within the walls of a pipe that contains a gas under negative pressure. If there is a regional or background state of stress in the crust, then the trajectories of the maximum compressive stress should rotate toward parallelism with the compressive hoop stresses with proximity to the downwelling. For the case of a rising mass, the predicted hoop stresses in the overlying crust are tensile (analogous to circumferential stress in the walls of a pipe containing gas under positive pressure) and the trajectories of the maximum tensile background stress would predictably rotate toward parallelism with the tensile hoop stresses near the upwelling mass.
We derive horizontal deviatoric stresses from Morgan’s (1965) solution for the velocity field generated by a descending or rising sphere in a Newtonian viscous medium to infer the general horizontal stress trajectory patterns for several scenarios in the southern Sierra Nevada and San Joaquin Valley. In this formulation the deviatoric stresses are independent of the viscosity of the medium. We add to this the background state of stress, which is characterized by the maximum compressive stress (σ1) oriented approximately north-northeast–south-southwest and the minimum compressive stress (σ3) is west-northwest–east-southeast. We assume that these stresses are equal and opposite in magnitude, consistent with maximum shear stress parallel to north-northwest dextral shear in the Walker Lane belt. We assume a linear constitutive relationship to compare the modeled stress trajectories with the observed incremental strain trajectories (note that σ1 should parallel d3).
In the first example (Fig. 5A), the Isabella anomaly is assumed to be a downwelling mass. The predicted hoop stresses in the crust above the sinking drip are compressive, causing the north-northeast–south-southwest σ1 trajectories to rotate toward parallelism with them and thus be deflected around the anomaly. The magnitude of rotation of the stresses depends on the ratio of the antibuoyancy to the regional deviatoric stresses. The σ3 trajectories are perpendicular to the σ1 trajectories, and thus are deflected toward the anomaly. The net result is a clockwise rotation of the σ1 and σ3 trajectories southeast of the Isabella anomaly relative to regional trends, which is opposite to the observed counterclockwise rotation of the d3 and d1 trajectories in the same region.
In the second example (Fig. 5B), background stress deflections are the result of local upward stresses. The load is placed to best fit observed strain trajectory deflections. The predicted hoop stresses associated with thinning crust above the rising mass are tensile, causing the σ3 trajectories to be deflected toward the anomaly and the σ1 trajectories to be deflected away from it. The net result is a counterclockwise rotation of the σ1 and σ3 trajectories southeast of the Isabella anomaly relative to regional trends, similar to the observed counterclockwise rotation of the d3 and d1 trajectories in this region. The location of this load could represent crustal relaxation above a detachment of a dense Isabella anomaly and/or convective flow in response to delamination.
We also test scenarios that assume that a locus of upwelling underlies an area on the northeastern margin of the Kern Arch inferred to be rising rapidly (Saleeby et al., 2013) (Fig. 5C) and a best-fit locus of downwelling that would be located beneath the southwestern San Joaquin Valley (Fig. 5D). The former scenario (Fig. 5C) assumes that rapid surface uplift northeast of the Kern Arch documented by global positioning system data is an expression of local asthenospheric upwelling (e.g., Saleeby et al., 2012, 2013). Although this model predicts the strain trajectories to rotate counterclockwise in the southern Sierra Nevada, the displacement of the locus of upwelling east of the Isabella anomaly produces a worse fit to the strain trajectories in the southern San Joaquin Valley. The latter scenario (Fig. 5D) is an attempt to explain the counterclockwise rotation of the trajectories in the southern Sierra and San Joaquin Valley with a downwelling mass. In order to fit the strain trajectories, the geometry of the stress deflection requires the sinking mass to be ∼90 km beneath the southwestern end of the San Joaquin Valley; however, the tomography reveals slow rather than fast mantle at this depth (Figs. 2H, 2L, 2O), indicating that this scenario is unlikely.
The simple models presented here indicate that the observed counterclockwise strain deflection in the southern Sierra Nevada and San Joaquin Valley may be due to horizontal stresses associated with buoyancy in the vicinity of the surface projection of the Isabella anomaly. If this is the correct dynamic interpretation of the strain geometry, then the seismotectonic deformation may reflect recent or ongoing detachment of the Isabella anomaly, accompanied by upward flow of infilling asthenosphere or relaxation of a previously thickened crust (e.g., Hoogenboom and Houseman, 2006). Although our assumptions are simple and overlook rheological stratification, power-law rheologies, and more complex load patterns, the basic deflection patterns derived from our models should be valid. Our primary objective in presenting these simple models is to begin to address the possible effects of local forces associated with the foundering process on regional patterns of stress and deformation associated with distributed shearing east of the Pacific plate.
To summarize, we suggest that the simplest interpretation for west-northwest dextral shear in the southeastern Sierra Nevada is that it represents westward propagation of dextral motion from the eastern California shear zone into the southern Sierra microplate. The west-northwest direction of macroscopic dextral shear is oblique to regional northwest plate motions, however, and thus requires a complimentary rotation of the stresses in the southwestern Sierra to accommodate a horizontal plane strain rather than oblique convergence (i.e., transpression). The mechanics of the counterclockwise strain rotation in the southeastern Sierra may involve both laterally varying rheology and local stresses. Dynamic models of the foundering process need to account for observed convergence of the d3 trajectories toward the Isabella anomaly in the southern San Joaquin Valley, and divergence of d1 trajectories around the eastern margin of anomaly in the southern Sierra Nevada.
DISCUSSION: LATE CENOZOIC MIGRATION OF EXTENSION AND NORTHWEST DEXTRAL SHEAR WITH PROGRESSIVE WESTWARD FOUNDERING OF LOWER LITHOSPHERE?
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 a snapshot of a time- and space-transgressive process associated with westward propagation of dextral shear in the Walker Lane belt (see discussion in Jones et al., 2004, and references therein). Recent work has refined the late Cenozoic history of the Walker Lane belt at the latitude of southern Sierra (Fig. 6), and supports the hypothesis that northwest dextral shear, preceded by east-west extension, has propagated westward with time.
Andrew and Walker (2009) found that between ca. 15 and 4.2 Ma Panamint Valley opened via east-west extension, progressively separating the Argus Range and Panamint Range (Fig. 6A). Based on stratigraphic and structural relations, Andrew and Walker (2009) estimated that ∼4 km of east-west extension occurred between ca.15 and 4.2 Ma, implying an average horizontal extension rate of ∼0.3 mm/yr during that period. After 4.2 Ma, the Slate Range and Argus Range began moving northwest with respect to the Panamint Range (Fig. 6B); Andrew and Walker (2009) interpreted this to indicate the onset of northwest dextral shear through Panamint Valley. Total late Cenozoic relative northwest displacement is ∼17 km, implying a long-term average rate of ∼4 mm/yr since 4.2 Ma.
West of Panamint Valley and east of the modern Sierra Nevada, Indian Wells Valley was an extensional basin in the hanging wall of a north-south–striking, east-dipping normal fault system that was active between ca. 7.5 and 3.5 Ma (Fig. 6B; Monastero et al., 2002). Subsidence accompanied by normal faulting also occurred ∼35–50 km north of Indian Wells Valley, represented by deposition of lacustrine facies of the Coso Formation adjacent to the northwestern Coso Range between ca. 4 and 6 Ma (Kamola and Walker, 1999). East-west extension in this region ceased and the Indian Wells Valley graben became inactive after ca. 3.5 Ma. The extensional basin was subsequently deformed by the Airport Lake fault, Little Lake fault, and other active dextral faults of the western Walker Lane belt at this latitude (Fig. 6C). The onset of strike-slip faulting occurred after 3.5 Ma, and possibly as recently as 2 Ma (Monastero et al., 2002). The Airport Lake fault and associated splays to the north currently are the major active strike-slip faults along the eastern margin of the Sierra microplate at this latitude (Unruh et al., 2002). The Pliocene–Pleistocene onset of dextral shear in Indian Wells Valley is similar in timing to the onset of dextral slip farther north in the eastern Inyo Mountains (ca. 2.8 Ma; Lee et al., 2009) and western White Mountains (ca.3 Ma; Stockli et al., 2003) to the north.
We interpret the results of our study as evidence that east-west extension and vertical thinning in the southern Sierra Nevada are occurring above asthenospheric mantle that is rising in response to west-directed removal or foundering of lower lithosphere (Fig. 4). The zone of asthenospheric upwelling, imaged as anomalous low Vp extending to depths of 70–90 km (Fig. 4), and associated upper crustal thinning extends a maximum of ∼125 km east of the Isabella anomaly, beyond which modern deformation is dominated by distributed dextral shear in the Walker Lane belt (Figs. 2B, 2L; cf. 2, 2M).
If it is assumed that the kinematic facies of upper crustal thinning and dextral shear have approximately constant positions relative to the locus of active foundering, the pattern of late Cenozoic time- and space-transgressive deformation in the southern Walker Lane belt outlined here could be explained by progressive westward propagation of lithospheric foundering or delamination (Fig. 6). In this model, lower lithosphere detached beneath what is now the Panamint Range ca. 15 Ma and progressively foundered westward, accompanied by upwelling asthenosphere and east-west extension of the overlying crust at the rate of several tenths of millimeters per year (Fig. 6A). By ca. 7.5 Ma, foundering propagated west into the region currently occupied by Indian Wells Valley (Fig. 6B), expressed in the upper crust by the onset of east-west extension and subsidence of ancestral Indian Wells Valley graben (Monastero et al., 2002). By 4.2–4.6 Ma (Andrew and Walker, 2009), east-west extension in the Panamint Range area was replaced by oblique northwest opening of Panamint Valley (Fig. 6B) at the rate of several millimeters per year, reflecting passage of the wave of upwelling asthenosphere and westward propagation of Walker Lane dextral shear in its wake. East-west extension continued in Indian Wells Valley for ∼1–2 m.y. after dextral shear began in the Panamint region, however, because asthenospheric upwelling was still active beneath Indian Wells Valley due to proximity of foundering to the west. Between ca. 3.5 and 2 Ma, the foundering process moved west beneath the Sierra foothills and San Joaquin Valley and northwest dextral shear propagated into Indian Wells Valley (Fig. 6C). Dextral shear also localized in what is now the Owens Valley to the north, forming the modern eastern margin of the Sierra microplate at this latitude. The locus of extension directly east of actively foundering lithosphere currently is the southern Sierra Nevada, and is accommodated by seismogenic deformation and active normal slip on the Kern Canyon fault (Nadin and Saleeby, 2010; Kelson et al., 2010). The rate of horizontal extension associated with normal slip on the Kern Canyon normal fault above upwelling asthenosphere is in the very low tenths of millimeters per year (Kelson et al., 2010), in contrast to the higher rates of dextral shear in the western Walker Lane belt to the east (i.e., average late Quaternary slip rate on the Owens Valley fault determined from paleoseismic studies ranges from 0.5 to 3.6 mm/yr; Beanland and Clark, 1994; Lee et al., 2001; Bacon and Pezzopane, 2007; a secular rate of ∼5–7 mm/yr was inferred from geodetic data across Owens Valley by Gan et al., 2000; McClusky et al., 2001). In general, it appears that the rate of the early phases of extension and normal faulting is lower than the rates of subsequent shear and strike-slip faulting, the difference ranging from a factor of 3 to an order of magnitude. The difference in rates may indicate different driving forces for the two kinematic facies, i.e., local buoyancy forces versus far-field plate tractions.
A hypothetical scenario for future westward encroachment of dextral shear into the Sierra Nevada similar to that represented in the late Cenozoic geologic histories of Panamint Valley and Indian Wells Valley is shown in Figure 6D. The modern tectonic margin of the Sierra microplate directly north of the Garlock fault is represented by the dextral Airport Lake and Owens Valley faults (Fig. 6C) and their link across the Coso Range (McClusky et al., 2001; Unruh et al., 2002, 2008). The Airport Lake fault zone is on strike with the dextral Blackwater fault in the eastern California shear zone south of the Garlock fault (Fig. 6C), and both are associated with a secular velocity gradient that marks the western margin of the eastern California shear zone (McClusky et al., 2001; Miller et al., 2001). Upper crustal extension in the southern Sierra Nevada is currently localized along the Kern Canyon fault (Fig. 6C). Westward propagation of dextral shear could be accommodated by an increase in the slip rate on west-northwest–striking Quaternary faults in the Mojave block west of the Blackwater fault, and by a transition from normal slip on the Kern Canyon fault to dextral-normal oblique motion, similar to what is observed on the Owens Valley fault (Fig. 6D). In this scenario, the Kern Canyon fault and associated structures several million years hence would exhibit an older history of accommodating a low rate of east-west extension, on which a younger episode of dextral-normal displacement at higher rates is superimposed. With sufficient cumulative deformation, the southeastern part of the Sierra Nevada east of the Kern Canyon fault would be separated from the rest of the intact microplate and possibly form a discrete mountain range similar to the modern Panamint and Coso Ranges in the Walker Lane belt to the east.
It is interesting to observe that the westernmost Quaternary faults in the northern Mojave block, including the Lockhart fault, Spring fault, and Blake Ranch fault, strike more toward the west than the Helendale and Calico faults in the central and eastern parts of the Mojave block (Fig. 6D), toward the domain of strain rotation in the southwestern Sierra, and are more parallel to the direction of macroscopic shear there. Collectively, these structures may represent a very youthful stage in the development of an organized and interlinked fault system that eventually will transfer dextral shear from the eastern California shear zone across the southwestern Sierra Nevada and into the San Joaquin Valley.
The hypothesis outlined in Figure 6 primarily focuses on east to west migration of the foundering process at the latitude of the study area, which appears to differ from previous models that infer lithospheric foundering initiated in an area near the northern end of the modern Owens Valley and migrated southwest to the current location of the Isabella anomaly (Zandt, 2003; Zandt et al., 2004). The two models are not necessarily in conflict, however, if lithospheric foundering began over a broad area in the southern Walker Lane belt to the west in the Late Miocene and progressively converged from both the east and the northeast on a much more limited area beneath the southern Sierra Nevada and southeastern San Joaquin Valley.
SUMMARY AND CONCLUSIONS
Kinematic analysis of earthquake focal mechanisms reveals horizontal extension and vertical thinning of crust in the southern Sierra Nevada above acoustically slow upper mantle, which may be upwelling asthenosphere in the wake of foundering lower Sierra lithosphere represented by the Isabella anomaly. The horizontal trajectories of the maximum extensional and maximum shortening strains are rotated distinctly counterclockwise relative to regional trends in the southern Sierra and San Joaquin Valley southeast of the Isabella anomaly. Based on linear alignments of earthquake epicenters in the southwestern Sierra in the vicinity of the rotated strains, the deformation represented by the rotated principal strains is interpreted to be distributed west-northwest–directed dextral shear that is propagating westward from the eastern California shear zone. The strain rotations may reflect the presence of local stresses associated with relaxation of subsidence in the vicinity of the Isabella anomaly. Patterns of late Cenozoic time- and space-transgressive deformation in Panamint Valley and Indian Wells Valley (i.e., extension followed by northwest dextral shear) east of the Sierra microplate potentially can be explained by westward propagation of lithospheric foundering and associated kinematic facies similar to those currently observed east of the Isabella anomaly. The active deformation in the southern Sierra Nevada may be a snapshot of the process that separated the bedrock of the Argus, Panamint, and Coso Ranges from the Sierra batholith and widened the southern Walker Lane belt in late Cenozoic time (Jones et al., 2004).
We acknowledge support of this research by the National Science Foundation (grant EAR-0607625 to Unruh, grants EAR-0454535 and EAR-0607831 to Jones), and the U.S. Army Corps of Engineers. The ideas and interpretations presented herein were developed in conversations with Jason Saleeby, Zorka Saleeby, Peter Molnar, Greg Houseman, Hersh Gilbert, Anthony Frassetto, Allen Glazner, Frank Monastero, John Dewey, Eugene Humphreys, and Colin Amos. We thank Frank Monastero and Cooper Brossy for comments on an early version of the manuscript, and Jason Saleeby, Jeffrey Lee, and an anonymous reviewer for Geosphere for constructive reviews, all of which significantly improved the final paper.