Small‐scale upper mantle convection and crustal dynamics in southern California

Abstract

We present numerical modeling of the forces acting on the base of the crust caused by small‐scale convection of the upper mantle in southern California. Three‐dimensional upper mantle shear wave velocity structure is mapped to three‐dimensional density structure that is used to load a finite element model of instantaneous upper mantle flow with respect to a rigid crust, providing an estimate of the tractions acting on the base of the crust. Upwelling beneath the southern Walker Lane Belt and Salton Trough region and downwelling beneath the southern Great Valley and eastern and western Transverse Ranges dominate the upper mantle flow and resulting crustal tractions. Divergent horizontal and upward directed vertical tractions create a tensional to transtensional crustal stress state in the Walker Lane Belt and Salton Trough, consistent with transtensional tectonics in these areas. Convergent horizontal and downward directed vertical tractions in the Transverse Ranges cause approximately N–S crustal compression, consistent with active shortening and transpressional deformation near the “Big Bend” of the San Andreas fault. Model predictions of crustal dilatation and the forces acting on the Mojave block compare favorably with observations suggesting that small‐scale upper mantle convection provides an important contribution to the sum of forces driving transpressional crustal deformation in southern California. Accordingly, the obliquity of the San Andreas fault with respect to plate motions may be considered a consequence, rather than a cause, of contractional deformation in the Transverse Ranges, itself driven by downwelling in the upper mantle superimposed on shear deformation caused by relative Pacific–North American plate motion.