THE SOUTHERN SAN ANDREAS FAULT SYSTEM AND NORTHERN GULF OF CALIFORNIA: FORMATION OF NEW CRUST ALONG A TRANSTENSIONAL PLATE BOUNDARY

The Pacific-North America plate boundary , between 23^o N and 40^o N latitude, provides useful examples of a variety of processes that produce new crustal area in regions of transtension. This plate margin changes geometry along strike, going, at a large scale, from the right-lateral strike-slip fault tectonics of the NW-trending San Andreas fault system in the north to the oblique rifting of the Gulf of California in the south. Approximately 300 km of right-lateral strike-slip motion took place between these two plates since 6 Ma. At a smaller scale, regions of localized production or destruction of crustal area are found in each segment of the boundary. In the Gulf of California/Salton Trough region, where considerable new crustal area has been formed, this new area was produced by different mechanisms in different places. The mechanisms include: seafloor spreading in the southernmost basins; continental crustal normal faulting and possible lower crustal flow in the northern basins; extremely heavy sedimentation filling extensional basins in the northern Gulf of California and southernmost San Andreas fault system; and formation of new crustal area by magmatic input into a thick pile of sediments (Imperial Valley) . The geometrical adjustment of the deforming region has additionally been controlled by high-angle or listric normal faults, low-angle detachment faults, active NE-striking left-lateral strike-slip faults, and significant tilting and vertical-axis rotations (up to 30 degrees) of smaller fault blocks. In the northern Gulf of California basins, true seafloor spreading is not yet occurring. Localized magmatism contributes to formation of new crust, but geophysical constraints (gravity, moho depth, and seismic velocities) do not permit the crust to contain a high percentage of mafic rock. Thus, as a rifted margin, this is considered a magma-poor end member. Changes in the age of the continent/ocean transition, thickness of new crust, and width of transitional crust along the strike of the oblique rift do not show a simple spatial pattern. In some cases, these have been attributed to differences in depletion of the underlying asthenosphere, arising from variations in geological history, particularly differences in pre-rift subduction-related magmatism. This presentation will summarize the work of many authors who have constrained the tectonic development and the crustal architecture of this transtensional plate boundary, in order to then compare it to other transtensional plate boundary settings such as the Marmara Sea.