131 Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems

This session explores key structural processes that govern the temporal, mechanical, and rheological evolution of crustal/lithospheric extensional and strike-slip systems with a focus on novel field observations, microstructural and analytical methodologies, and numerical modeling.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 217D

Interaction of Strike-Slip and Thrust Faults in Transpressive Plate Convergence: Deep Insights from the Big-Bend of the San Andreas, California Transverse Ranges

Paper number: 131-9
Program: Technical Sessions
Time: 3:55 PM – 4:10 PM

Authors

Abstract

The existence of regional partitioning of oblique plate convergence onto contemporaneously active wrench and thrust systems is well known, for example from central California, Sumatra, northern Tibet, and south Anatolia. But it is not well constrained observationally how wrench and thrust systems work together regionally to consume plate convergence at a crustal or lithospheric scale. Nevertheless, simple kinematic considerations indicate that a strike-slip fault must be offset by or terminate at a contemporaneous thrust fault or detachment; alternatively, they can be mutually cross cutting, producing wider deformation zones.  Oblique convergence below any regional detachment must be accounted for down to the base of the lithosphere.

We address these issues of the deep architecture of transpressive tectonics in the data-rich Big Bend region of the San Andreas fault where oblique plate convergence has produced the active ~125km wide western Transverse Ranges mountain belt. Here loci of active thrust convergence are separated from the San Andreas by 50-100km south of the fault but are within 25km north of the fault. We present observational constraints from new multiscale crustal transects that combine surface geology and petroleum data in the upper crust with modern anisotropic seismic tomography and seismicity in the lower crust and upper mantle.

These transects show that the San Andreas fault is cut or terminated at a depth of 15-20km by the roof thrust of a ~20° N-dipping low velocity zone of sediments to metasediments that is 15km thick and extends from the surface in the Ventura Basin 50km south of the San Andreas to the base of the crust 30km north of the San Andreas fault. Seismicity near the base of the San Andreas fault shows thrust focal mechanisms that are consistent with cutting of the San Andreas by the roof thrust. Below the Moho there is a high-velocity vertical EW slab-like body under the entire Transverse Ranges extending to a depth of 150-250km. This body appears in tomography to be continuous with upper mantle lithosphere of the Peninsular Ranges south of the Transverse Ranges. Its origin is controversial but has been interpreted as a convective drip of mantle lithosphere, or a delaminated or subducted lithospheric slab. Any sub-Moho component of the transpressive convergence would be contained within this slab.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414092

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