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 For permission to copy, contact [email protected] 107 © 2014 Geological Society of America 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. 1120°W 20 bat all data layers are displayed, producing the °W CCENTRAL very cluttered view shown in the non-animated NNEVADA VVALLEY SSIERRA E version (some non-Adobe pdf viewers will not A 337°N E 7 I °N N show the layer controller). We recommend that E L V T users open the fi le, click the layering icon, and R L A R initially turn off most of the layers for ease R E D WWalkeralker A PACIFIC PLATE PACIFIC of viewing. A Y A LLaneane L S bbeltelt EARTHQUAKE DATA a n For this study we analyzed the (1981–2008) A n background seismicity in the southern Sierra d IIsabellasabella r Nevada and southern San Joaquin Valley, e aanomalynomaly a FFigure 2 335°N S ig approximately located in the rectangular search s O 5° u N re box in Figure 1; lat 34.617°N to 37.0° N and 2 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, Garlo crustal models of the study region using the ck F methods of Thurber (1993). We used these mod- au MMojaveojave lt els to relocate the background seismicity using blockblock 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 algo- rithm and computer programs by Reasenberg 1115°1 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 accommo- date 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.