Location of the southern edge of the Gorda slab and evidence for an adjacent asthenospheric window:: Results from seismic profiling and gravity

As the Mendocino Triple Junction migrates northward along the California margin it is widely presumed to leave a "slab-free" or "asthenospheric" window in its wake. A 250-km-long south-north seismic refraction-reflection profile crossing the transition from transform to subduction regimes allows us to compare and contrast crust and upper mantle of the North American margin before and after it is modified by passage of the Mendocino Triple Junction. From the seismic data we have determined that (1) the crust is laterally homogeneous in velocity to a depth of 20 km (interpreted by us as Franciscan complex), (2) below 20 km depth the crust is characterized by velocities of greater than or equal to 7.0 km/s for the southern half of the profile and by velocities of less than or equal to 7.0 km/s for the northern half, (3) regions of high reflectivity in the crust occur below similar to 20 km depth throughout the profile, and (4) the North American crust is thickest (similar to 35 km) in the center of the profile and thins to similar to 25 km at either end. From the gravity data we have determined that (1) asthenospheric densities (3.2 g/cm(3)) occur subjacent to the North American crust in the center of the profile, and (2) a wedge of lithospheric mantle density material (greater than or equal to 3.2 g/cm(3)) is required on the southern end of the profile. We interpret these combined results to indicate that our profile crosses the southern edge of the Gorda plate and that directly adjacent to this edge is an asthenospheric window with overlying mafic rocks in the crust. These mafic rocks and a reforming lithospheric mantle increase in thickness southward.