33 Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems (Posters)

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.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Investigating Brittle Faults as Records of Deformation in the Eastern California Shear Zone, Southeastern California

Paper number: 33-4
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Most of the relative movement between the Pacific and North American plates is absorbed by the San Andreas fault system, but approximately 25% is distributed across a wider region of deformation called the Eastern California Shear Zone (ECSZ). The ECSZ developed after a period of regional northeast-southwest extension across southwestern North America which was likely driven by the gravitational collapse of thick crust. It includes an active region (modern ECSZ) and an inactive segment that has been abandoned (paleo-ECSZ). In contrast to earlier NE-SW extension, the ECSZ evolved within a strain field characterized by north-south contraction and east-west extension, though the exact timing of its development and when the paleo-ECSZ was abandoned remain uncertain. This later tectonic regime is kinematically different from the earlier phase of northeast-southwest extension and should be distinguishable using rocks of suitable age and position. The study site is situated in the vicinity of Picacho State Recreation Area in southeastern California. The metamorphic basement rocks, arranged from structurally lowest to highest, include the Orocopia schist, a mafic orthogneiss unit, and the Winterhaven Formation. These units are in turn overlain by 23 Ma volcanic rocks and the <23 Ma to <9.8 Ma Bear Canyon conglomerate. Collectively these units were emplaced and deformed during evolution and development of the ECSZ. This work documents strain variations associated with the evolution of the ECSZ throughout time using a mix of mapping and fault kinematic analysis. Fault kinematic analysis focuses on the well-exposed Bear Canyon conglomerate. Preliminary results indicate that the Bear Canyon conglomerate is cut by dextral, sinistral, reverse, and normal-sense faults, and that the oldest exposures of conglomerate are compatible with deformation characterized by north-south contraction and east-west extension that is similar to the strain field of the ECSZ. These results indicate that development of the paleo-ECSZ occurred >10 Ma and at that time the plate boundary was wider than it is today. Future research will focus on additional fault locations and will seek to refine the dating of the Bear Canyon conglomerate using a combination of U-Pb dating of fault calcite and sanidine geochronology.

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

doi: 10.1130/abs/2025AM-413769

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