The Pacific-North American plate boundary south of Los Angeles provides a unique opportunity to study inception and localization of major continental wrench zones. By ~4.5-6 Ma, the locus of dextral shear was in its modern location in the Gulf of California Extensional Province (GEP), having “jumped” east from a trajectory through the LA basin and continental borderland. Earlier, beginning by ~10-12 Ma, dextral shear in the northern GEP was distributed, and accommodated by ~east-west extension, smaller-magnitude north-south contraction, and clockwise vertical-axis rotations. Generally east-directed oblique extension in the Salton Trough – Laguna Salada area was ongoing by 8-12 Ma or earlier. The oldest clearly extension-related strata in the Salton Trough are ~8.3 Myr old conglomerates that accumulated along a steep N-striking fault (an early breakaway of the west Salton detachment?). Younger deposits record transfer of that fault's footwall into the upper plate of the detachment. Minor(?) tectonism affected older strata (~17-14 Ma) also and preliminary (U-Th)/He data suggest onset of moderately rapid, extension-related(?) cooling by or before 10-12 Ma (Luther et al., this volume). East-trending folds began to form by ~7 Ma and folding probably continued locally or episodically, ultimately outlasting detachment slip. Clockwise vertical-axis rotations began before ~6 Ma.

I hypothesize that the area and magnitude of traction between former Farallón microplates and the basal western edge of North America grew eastward through time, due to heating, increased buoyancy, and dewatering of the of the young slab, as mantle upwelling formerly located below the Pacific-Microplate spreading center migrated eastward to its present, tomographically imaged location under the Gulf of California. This caused bonding of Peninsular Ranges lower crust to the microplates and eastward migration of the plate boundary, beginning with distributed dextral wrenching that evolved to discrete strike-slip faulting once crustal thinning formed a weak zone.