A kinematic model for the Late Cenozoic development of southern California crust and upper mantle

Abstract

A model is presented for the young and ongoing kinematic deformation of southern California crust and upper mantle that accounts for both the known late Cenozoic geologic record and upper mantle structure. In this model, the high‐velocity upper mantle Transverse Ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere. Using typical scaling relations, we estimate this anomaly to be ∼500°C colder and 1% more dense than average southern California mantle of the same depth. We infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly. The low‐velocity upper mantle anomaly beneath the Salton Trough region is attributed to high temperatures and 1–4% partial melt related to adiabatic decompression during mantle upwelling. Our model has crustal motions in the central Transverse Ranges region differing from mantle motions: While the subcrustal lithosphere south of the Transverse Ranges moves toward and sinks beneath the Transverse Ranges, the corresponding crust largely avoids convergence by adopting a more westerly direction of motion. The differing velocity fields necessitates of a decoupling zone immediately south of the Transverse Ranges near the Moho. We interpret the upper mantle anomalies as results of small‐scale convection beneath southern California, to which we attribute much of the recent tectonic activity in the region.