Mantle flow distribution beneath the California margin

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.

Kinematics and Evolution of the Southern Eastern California Shear Zone, Based on Analysis of Fault Strike, Distribution, Activity, Roughness, and Secondary Deformation

“…This average trend and the lack of an apparent relationship between fault strike and fault activity argue against a model in which faults are gradually rotating counterclockwise out of favorable position, to be replaced by newer, more northerly trending faults (Nur et al, 1993). The observed spatial variation in fault activity (i.e., decrease to the east) does partly match the predictions of the translational model of the SECSZ (Barbot, 2020;Dixon & Xie, 2018;Spotila & Anderson, 2004), although the evidence is not particularly compelling due to lack of information on fault onset in the west. A heterogeneous shear model is also not well supported (Dokka & Travis, 1990a), given the lack of focused secondary deformation on domain boundaries and the preponderance of distributed transpression along and between faults.…”

“…Dokka and Travis (1990a) also suggested the locus of deformation in the SECSZ has migrated westward in the last few million years, without providing a geodynamic explanation of why this may have occurred. More recently, Dixon and Xie (2018) proposed a kinematic model that explains this westward migration of shear as a consequence of eastward translation of the upper Mojave block (due to motion on the southern SAF and Garlock fault) relative to a more deeply seated shear zone (see also Barbot, 2020). This model would explain why faults in the east are now inactive, despite having large net displacements, whereas faults in the central and western SECSZ are newer and have less net slip.…”

Earth’s gradients as the engine of plate tectonics and earthquakes

“…Among them, it was noticed that the basal shear stress and the average crustal earthquake stress drop are compatible with each other [379]. Moreover, a significant correlation exists between the strain release by earthquakes at boundaries and plate margins slip rates, e.g., [383] (compare with Paragraph 5.3).…”

Evidence of Mantle‐Based Deformation Across the Western United States

“…Away from perturbations, anisotropy is expected to rotate progressively toward the direction of simple shear, that is, it should be slightly counter‐clockwise of the SAF. Assuming this behavior, there are two points to be made: (a) the crustal anisotropy is ahead of the mantle (Barbot, 2020), suggesting either that the crust is driving the mantle or the crustal deformation zone is narrower (and more highly strained) than that of the mantle (Figure 4c), and (b) the anisotropy near the negatively buoyant ISA mantle anomaly appears to be a simple ”sinker” perturbation to the SAF‐related anisotropy (Figure 4a).…”