Lithospheric evolution in the wake of the Mendocino triple junction: structure of the San Andreas Fault system at 2 Ma

As the Mendocino triple junction (MTJ) moves northwards up the North Americ an margin, the tectonic regime changes from subduction to strike-slip. For the first few million years following triple junction migration, the San An dreas Fault system consists of several strike-slip faults distributing defo rmation over a region similar to 150 km wide. This same region is expected to be affected by a slab gap beneath North America, created by the northwar d removal of the subducting Gorda plate, and into which asthenospheric mant le is thought to rise to crustal depths. The onshore and offshore Mendocino Triple Junction Seismic Experiment (MTJSE) provides a continuous seismic v elocity-reflectivity cross-section across the deforming zone from the Pacif ic ocean basin to the eastern edge of the California Coast Ranges. The accr etionary complex rocks that make up most of the crustal thickness are under lain by a 5-10 km thick high-velocity (6.4-7.2 km s(-1)) layer at the base of the crust that extends from the Pacific to at least 50 km, and probably 90 km east of the San Andreas Fault. The top of the lower crustal layer dee pens from 7 km beneath the Pacific ocean basin at the west end of the profi le to 23 km at the east end by a gentle (5 degrees-10 degrees) eastward dip punctuated by abrupt offsets at the San Andreas and Maacama fault zones. A t each fault the top of the lower crust is offset by up to 4 km, down to th e east. The Moho is similarly deformed beneath the faults, although by only 2 km. Such localized deformation of the Moho implies that these two strike -slip faults penetrate through the entire crust to the upper mantle. Good a greement between seismic velocity and seismic reflectivity in the vicinity of the faults gives confidence in these results, although details of the of fset beneath the San Andreas Fault are better resolved than those under the Maacama Fault. Seismic velocities in the upper mantle show only a small ch ange along the profile, from 8.1 km s(-1) beneath the Pacific to about 7.9 km s(-1) beneath the Coast Ranges. We infer that upwelling of asthenosphere into the slab gap is limited laterally, or a lithospheric lid is present i n the slab gap by 2 Ma. Gravity data and crustal density structure show tha t most of the margin width is in local Airy isostasy with the changes in cr ustal thickness near the strike-slip faults corresponding closely to change s in surface topography. The crustal blocks defined by the strike-slip faul ts appear to be independently in isostatic equilibrium, provided that the m antle beneath the Coast Ranges has a somewhat lower density than that benea th the Pacific plate. The densities in the Coast Range upper mantle are con sistent with limited temperature elevation, suggesting that the asthenosphe ric mantle is present beneath the depth of seismic energy penetration from our survey.