Deformation of the brittle crust in transtension: Coso-China Lake region, eastern margin of the Sierra Nevada Microplate, southeastern California
Tatia R. Taylor, John F. Dewey
University of California Davis, Department of Geology,Davis, CA 95616
Transtension is oblique divergence between bounding plates or blocks, and combines a coaxial orthogonal extension with a deformation zone boundary parallel noncoaxial component to generate bulk non-plane (triaxial) constrictional strain. The instantaneous stretching direction bisects the acute angle between the direction of divergence (transport direction, TD) and the zone boundary orthogonal. The angle between the zone boundary (zb) and TD determines the dominance of the coaxial or noncoaxial component of strain. Simultaneous normal and wrench fault arrays accommodate horizontal extension, and vertical and horizontal shortening. Structures and blocks in transtension rotate both with and against vorticity, around both vertical and horizontal axes. As structures rotate to positions no longer favorable for slip, they are superimposed by new structures, resulting in apparent multi-phase deformation that is actually polyphase deformation produced by the same event. 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. These theoretical results are compared to extensive field measurements, supplemented by geophysical and geodetic constraints, from a young active transtensional zone, which describe the 3D shape of the on-going deformation.
The Sierra Nevada Microplate is separating from North America at a rate of ~6 mm/yr between the Argus Range and the southern Sierra Nevada Mountains. Within this zone, the Coso-China Lake region is an area of moderate geodetic strain rate (est. 10-15/s), nearly continuous seismicity, high heatflow, active bimodal volcanism, and geothermal activity. Simultaneous coaxial and noncoaxial strain components result in coaxially dominated bulk constriction, and produces a complex system of normal, strike-slip, and oblique slip faults and associated constrictional folding and crustal thinning. Using straightforward assumptions regarding the constancy of zone boundaries, transport direction, strain rate/duration, and volume, and the retro-deformation of the Sierra Nevada Microplate, the calculated strain rate over time and the overall % extension for the northern Coso region is nearly half that of the southern Coso region. Variations in the width of the transtensional zone result in increasing or decreasing strain rates, vertical shortening, and elevation, and may explain the greater elevation/topography of N. Coso vs. that of the Indian Wells Valley. Variations in the angle between TD and zb through time exert influence over the dominant style of deformation within the transtensional zone. This may partly explain the observed dominance of more coaxial deformation in N. Coso vs. more noncoaxial deformation in S. Coso.
Faults in the region cut all lithologies, including Mesozoic Sierra Nevada batholithic basement, Plio-Pleistocene volcanics, and Quaternary sediments, and are seen to reactivate both preexisting Mesozoic Sierra Nevada basement joints and shear zones, and late Miocene-Pliocene faults which accommodated the uplift and exhumation of the northern Coso range and associated Coso Formation. Younger off-fault brittle fracture and joint geometries kinematically resolve transtension, and occur around both outer block regions and internally throughout fault bounded blocks. An examination of the spatial, geographic, and temporal distributions of faults and brittle features in the Coso-China Lake region demonstrates partitioning of strain and kinematic reactivation of existing structures during regional transtension.
