Lithospheric instability beneath the Transverse Ranges of California

Recent high-resolution seismic experiments reveal that the crust beneath th e San Gabriel Mountains portion of the Transverse Ranges thickens by 10-15 km (contrary to earlier studies). Associated with the Transverse Ranges, th ere is an anomalous ridge of seismically fast upper mantle material extendi ng at least 200 km into the mantle. This high-velocity anomaly has previous ly been interpreted as a lithospheric downwelling. Both lithospheric downwe lling and crustal thickening are associated with the oblique convergence of Pacific and North America plates across the San Andreas Fault, though it s eems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle. We show by means of numerical experiment that the balance between buoyancy forces that driv e deformation and viscous stresses that resist deformation determines the g eometry of crustal thickening and mantle downwelling. We use a simple two-l ayered lithospheric model in which dense lithospheric mantle overlies relat ively inviscid and less dense asthenosphere and is overlain by buoyant crus t. External plate motion drives convergence, which is constrained by bounda ry conditions to occur within a central convergent zone of specified width. A fundamental transition in the geometry of downwelling is revealed by our experiments. For slow convergence, or low crustal viscosity, downwelling o ccurs as multiple sheets on the margins of the convergent zone. For fast co nvergence or crust that is stronger than mantle lithosphere a single downwe lling occurs beneath the center of the convergent zone. This complexity in the evolution of the system is attributed to the interaction of crustal buo yancy with the evolving gravitational instability. In order for a narrow do wnwelling slab to have formed beneath the Transverse Ranges within the last 5 Myr, the effective lithospheric viscosity of the convergent region is at most about 10(20) Pa s.