USING SEISMICITY TO CONSTRAIN THE GEOMETRY OF EXHUMATION IN A YOUNG STRIKE-SLIP FAULT ZONE: INDIAN WELLS VALLEY TO THE COSO RANGE, CALIFORNIA

The geometry of a right-releasing step in the transtensional boundary between the SE margin of the Sierran microplate and the southern basin and range province at ~36° N is elucidated using seismicity data. The form of this stepover is important because it appears to host an evolving metamorphic core complex at a depth of ~5 km. Clusters of seismic events are used to define seismic source zones and the focal mechanisms for these events are used to model seismogenic strain. I use a micropolar continuum model to find the orientations and relative magnitudes of the principal strains, and to determine the vorticity of fault blocks relative to that of the large-scale continuum. Best-fitting strain tensors for hundreds of earthquakes in the eastern Indian Wells Valley and along the SW margin of the Coso Range, located about 30 km to the north, display subhorizontal W-E maximum stretching (d₁) and subhorizontal N-S maximum shortening (d₃). Both of these areas display plane strain (d₂=0) and fault-block vorticity that is not different from that of the large-scale continuum. Earthquakes between these two areas are more sparse, however, two seismicity clusters are apparent at Wild Horse Mesa where well-developed, west-facing normal-fault scarps occur. The focal mechanisms for these events define two coeval homogeneous subsets at distinct depths. The deeper subset (5-8 km) has subhorizontal principal strains with WSW-ENE d₁ and NNW-SSE d₃. The vorticity is consistent with down-viewed clockwise rotation of crustal blocks at a rate slower than that of the large-scale continuum. For the shallower subset (<5 km) the principal axes are similarly oriented except that d₃ is vertical instead of d₂. For these events the vorticity suggests that the fault blocks are rotating clockwise (viewed to the north) more slowly than the large-scale continuum. Both solutions are plane strain. Collectively these events accommodate crustal thinning, horizontal dextral shearing, and block rotation; all of which likely contribute to late Cenozoic high heat flow, magmatic activity and putative core complex localization in the area of Wild Horse Mesa. Comparable strain partitioning is not apparent in the Indian Wells Valley or Coso Range seismicity clusters. Instead these areas accommodate predominantly horizontal dextral shearing.