Mantle flow distribution beneath the California margin

Sylvain Barbot

doi:10.1038/s41467-020-18260-8

Abstract

Although the surface deformation of tectonic plate boundaries is well determined by geological and geodetic measurements, the pattern of flow below the lithosphere remains poorly constrained. We use the crustal velocity field of the Plate Boundary Observatory to illuminate the distribution of horizontal flow beneath the California margin. At lower-crustal and upper-mantle depths, the boundary between the Pacific and North American plates is off-centered from the San Andreas fault, concentrated in a region that encompasses the trace of nearby active faults. A major step is associated with return flow below the Eastern California Shear Zone, leading to the extrusion of the Mojave block and a re-distribution of fault activity since the Pleistocene. Major earthquakes in California have occurred above the regions of current plastic strain accumulation. Deformation is mechanically coupled from the crust to the asthenosphere, with mantle flow overlaid by a kinematically consistent network of faults in the brittle crust.

Highlights

The surface deformation of tectonic plate boundaries is well determined by geological and geodetic measurements, the pattern of flow below the lithosphere remains poorly constrained
Contemporary surface deformation at the California margin is captured by a dense continuous geodetic observatory[1,2] and the long-term slip accumulation on numerous faults is well established[3], providing a unique opportunity to study the mechanics of strain accumulation at a continental transform
We devise a mesh of 20 × 20 km semi-infinite volume elements that extend down from 20 km depth along a coordinate system aligned with the plate boundary axis, with x1 and x2 the parallel and perpendicular axes, respectively

Summary

Introduction

The surface deformation of tectonic plate boundaries is well determined by geological and geodetic measurements, the pattern of flow below the lithosphere remains poorly constrained. Most plate boundaries involve either divergence, like at oceanic spreading centers and continental rifts, or convergence, such as subduction and collision zones, but a small fraction involves transform boundaries that facilitate plate kinematics on the global sphere These boundaries accommodate plate-parallel relative plate displacement by strike-slip motion on vertical or steeply dipping faults. Contemporary surface deformation at the California margin is captured by a dense continuous geodetic observatory[1,2] and the long-term slip accumulation on numerous faults is well established[3], providing a unique opportunity to study the mechanics of strain accumulation at a continental transform. Below the seismogenic layer, deformation occurs in localized shear zones that creep steadily down to the brittle-ductile transition, below which the deformation becomes plastic and more distributed, controlled by diffusion creep, grain-boundary sliding, and dislocation creep[18,19]. Geophysical data provide strong evidence for the mobility of the asthenosphere below the California margin, including far-reaching postseismic deformation[27,30], the development of seismic anisotropy in the upper mantle[31], and strong lateral variations of seismic velocity in the lower-crust and upper-mantle below major strike-slip faults in southern California[32]

Methods

Results

Conclusion