TRANSTENSIONAL DEFORMATION OF THE BRITTLE CRUST: FIELD OBSERVATIONS AND THEORETICAL APPLICATIONS IN THE COSO-CHINA LAKE REGION, EASTERN MARGIN OF THE SIERRA NEVADA MICROPLATE, SOUTHEASTERN CALIFORNIA

The Sierra Nevada Microplate is separating obliquely from North America at a rate of ~6 mm/yr across the Coso-China Lake region (Coso), located between the Argus Range and the southern Sierra Nevada Mountains. Strain is accommodated by a complex system of normal, strike-slip, and oblique slip faults, associated constrictional folding, and crustal thinning. Our work in Coso addresses how rocks of the brittle upper crust accommodate non-plane strain at an actively deforming obliquely divergent plate boundary, and whether applied transtensional theory can describe and/or predict this deformation.

Transtension combines a coaxial orthogonal extension with a deformation zone boundary parallel noncoaxial component to generate bulk triaxial constrictional strain. The instantaneous stretching direction bisects the acute angle between the direction of divergence (transport direction) and the zone boundary orthogonal. Thus given the geometry of a transtensional zone, the orientations of the instantaneous strain axes can be derived, and orientations of expected associated structures can be predicted. We use straightforward assumptions and geometric parameters derived from retro-deformation of the Sierra Nevada Microplate to calculate the strain rate over time and the overall percent extension for the Coso region. The initial and final geometry of a transtensional zone also allows us to calculate rotation and elongation for any line or plane within the zone. Using these geometric parameters for Coso, we derive both initial and final orientations of structures, and the amounts and rates of shortening, elongation, and rotation of structures in the transtensional zone. This further allows the calculation of the instantaneous and finite strain ellipsoids, and k-values, which describe the overall shape of the deformation and relative degree of prolateness. These theoretical results are compared to extensive structural measurements and field observations from the Coso region. Structural data from Coso demonstrate a generally constrictional strain pattern which corresponds to existing geodetic and geophysical constraints and to the results of theoretical applications. The correspondence of applied theory to field observations implies that additional derived strain characteristics are probably reasonable.