SOUTHERN CALIFORNIA AND NW MEXICO: LITHOSPHERIC EVOLUTION IN THE POST-SUBDUCTION REGIME

In southwestern North America, progressive post-Oligocene separation of the Rivera and Mendocino Triple Junctions lengthened the contact zone accommodating transtension or transpression between the Pacific and North America plates. Lithospheric architecture in the transtensional zone (southern San Andreas fault to southern Gulf of California) is constrained by Earthscope projects (e.g., SSIP) and other earlier major surveys, as well as by interdisciplinary marine and onland studies by US, Mexican, and international researchers (e.g., NARS-Baja, Cortes, PEMEX, Pescador). Resulting constraints on Moho depth, crustal architecture, and fault and basin evolution adjacent to this part of the plate boundary are used to address the following issues: 1) Summed strike-slip displacement on known faults in the plate boundary region is less than the (newly calculated, but similar to previous) plate motion totals, leading to various hypotheses to explain the discrepancy with fault slip between 12 and 6 Ma. 2) Location, amount and timing of lithospheric extension and its regional geodynamic effects show that earlier Basin and Range extension was temporally and spatially distinct from the later Gulf of California transtension in the northern Gulf of California, in contrast to the plate boundary zone to the north and south. 3) Post-subduction volcanism, although volumetrically minor away from the modern plate boundary, provides clues to lithospheric/asthenospheric evolution. 4) Lithospheric modification occurred by sedimentation, first along the eastern margin of the Gulf of California, and then since late Miocene time strongly centered on the Salton Trough and northern Gulf of California. 5) “New” crust and mantle lithosphere at the plate boundary in the Salton Trough and the non-oceanic part of the northern Gulf of California formed by sedimentation and magmatism accompanied by lower crustal and/or mantle lithospheric flow from the adjacent continental regions. Much progress has been made on these issues in the past decade, but major questions remain concerning timing of early basin evolution and strike-slip motion on the eastern side of the plate boundary zone and in the Continental Borderland, crustal thickness under western Sonora, and lithospheric mobility throughout the region after subduction ceased.