Constraints on the tectonics of the San Andreas Transform system from geodetic and seismotectonic data
Abstract
The San Andreas transform zone includes large regions of transtensional and transpressional deformation, but interpretation is complicated by changes in the geometry of the transform with time, by relative motion of the rocks on either side of the transform, and by tectonic processes unrelated to the transform, such as extension in the Basin and Range province. We analyse the GPS-determined 2-D velocity field in California to determine the distribution of shear strain rate, dilation (which is a proxy for the vertical elongation rate), transform-parallel elongation rate, and transform-normal elongation rate. We also analyse seismicity using Kostrov's method, which allows an independent determination of the dilation field, the orientation of the principle axes of the strain-rate tensor, and the shape of the strain-rate tensor (constriction, plane strain, or flattening). Taken together with geological data on the distribution of active faults, uplift, and subsidence, we map the different styles and orientations of active deformation in California. Three main contributors to the deformation field can be identified. (1) Shear strain related to the transform system follows the San Andreas Fault SAF) and the Eastern California Shear Zone (ECSZ), but the locus of maximum rate of shear strain lies west of the SAF south of the Transverse Ranges, and east of the SAF north of the Bay area. (2) Transform- parallel shortening is a major contributor to deformation in the Transverse Ranges and adjacent regions of coastal southern California, and in the northern part of the ECSZ; transform-parallel extension is dominant in SE California and the southern part of the ECSZ, and in the Coast Ranges from the Bay area northward. This antisymmetric pattern of deformation is a predictable consequence of the strain-rate field around a transform shear zone of finite length. (3) Transform-normal extension in eastern California and Nevada is related to Basin and Range tectonics; the resulting westward motion of the Sierra Nevada / Great Valley block is absorbed by transform-normal shortening in the California Coast Ranges.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.U54A..07P
- Keywords:
-
- 6924 Interferometry (1207;
- 1209;
- 1242);
- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242);
- 7250 Transform faults;
- 8111 Continental tectonics: strike-slip and transform