NEOTECTONICS AND ACTIVE TECTONICS OF SIERRAN-NORTH AMERICA TRANSTENSION, COSO RANGE, CALIFORNIA

We compare deformation geometries inferred from fault kinematic data and from seismic focal mechanism solutions along the transtensional Sierra-North America margin in the Coso Range of eastern California. We use a micropolar continuum model to invert both data sets for the orientations and relative magnitudes of the principal strains and for the vorticity of fault blocks relative to that of the large-scale continuum deformation.

Shear fractures with kinematic indicators are observed in canyons cutting the late Cenozoic lava flows that underlie Wild Horse Mesa. ⁴⁰Ar/³⁹Ar whole-rock geochronology dates the lavas at <3.5 Ma and paleomagnetic data record finite clockwise rotations of 11 ± 6° (viewed down). The shear fractures are separable into two homogeneous subsets. The first is characterized by subhorizontal WSW-ENE maximum lengthening (d₁), subhorizontal NNW-SSE maximum shortening (d₃) and subvertical intermediate shortening (d₂). The second subset is a plane strain characterized by a similar geometry but with d₁ subvertical instead of d₂. The faults that accommodate these two deformations define distinct preferred orientations that are spatially mixed. Assuming that the faults are coeval, these results suggest a fine scale of partitioning into strike-slip-dominated and thrust-dominated deformations.

Earthquake focal mechanisms likewise separate into two coeval homogeneous subsets, however these occur at distinct depths. The deep subset (5-8 km) has principal axes subparallel to those of the first subset of fault data (i.e., strike-slip). For the shallow subset (<5 km) the d_k axes are subparallel to the deep subset except that d₃ is vertical instead of d₂. For these events the relative 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 (d₂=0).

The results for seismicity are consistent with dextral strike-slip faulting at depth on a NW-striking fault with crustal thinning above. These characteristics are both included in the first subset of the fault data, which indicates strike-slip (subhorizontal d₁ and d₃) as well as a crustal thinning signature (non-plane strain with d₂ shortening). We interpret the thrust-dominated subset of fault data to reflect local accommodation of block rotations.