Oral Presentation

A lithospheric temperature model for Southern California is developed. Surface heat flow measurements at 237 sites are used to define 14 distinct heat flow regions where heat flow is relatively constant; the regions are chosen to conform to physiographic and tectonic provinces. We compute 1D steady-state conductive geotherms for each region using standard methods. The intersection of geotherms with mantle solidi identify the base of the thermal lithosphere boundary which we term tLAB. Predicted thermal lithospheric-asthenosphere boundary (tLAB) depths range from 22 km in the inner Salton Trough to more than 100 km in four of the lowest heat flow regions; several regions cluster between 50 and 70 km. Because geotherms in general are poorly constrained in the deeper lithosphere we sought additional constraints from seismic estimates of the lithosphere asthenosphere boundary termed the sLAB. Partial melting that causes a reduction of seismic wave velocity in the asthenosphere should coincide with the tLAB but they do not in SoCal. The tLAB-sLab comparison separated the heat flow regions into three groups: Group 1, with high average heat flow (79-140 mW/m2) and relatively shallow sLAB depths (45-55 km) located within 3 magmatic regions; Group 2, with moderate heat flow of 68-82 mW/m2; and Group 3, with heat flow of 40-58 mW/m2. For the 11 regions with heat flow 40-82 mW/m2), sLAB depth is a surprisingly constant 70 ± 5 km. We subsequently modified our 1D steady state thermal models to accommodate dynamical processes that have shaped the thermal evolution of southern California since 30 Ma BP: (1) a continental analog of seafloor spreading beneath the Salton Trough and lithospheric stretching in the inner Continental Borderland; (2) steady-state conduction indicative of relative thermal stability in certain regions; and (3) thermal pulses due to detachment or foundering of oceanic lithosphere (Farallon slab) beneath continental crust and/or mantle lithosphere detachment and sinking beneath late Cretaceous batholiths. These modifications resulted in a set of refined geotherms for all of SoCal that satisfy constraints of sLAB depth, {P,T} conditions from mantle xenoliths, and partial melting of a slightly hydrous mantle at the base of the lithosphere. Present day temperatures are warm in the lower crust, ~750˚-850˚C at the ~25-40 km depth Moho, and ~1350˚C at the LAB.