Emergent Bimaterial Slip Zones Control Strain Localization and Off-Fault Deformation along Plate Boundary Faults, Death Valley, California

Folarin Kolawole, Lela Kornfeld, Liang Xue, Christie D. Rowe, & Jamie Kirkpatrick

Submitted September 7, 2025, SCEC Contribution #14899, 2025 SCEC Annual Meeting Poster #TBD

Active extensional tectonic settings serve as natural laboratories for studying how crustal deformation accommodates tectonic strain during the evolution of divergent plate boundaries. Death Valley, situated within the Walker Lane segment of the Pacific–North American plate boundary zone, exemplifies an oblique rift characterized by a combination of normal and strike-slip faulting. Here, we explore the northern Black Mountain border fault, where exceptional exposures enable detailed characterization of fracture and alteration damage through integrated outcrop-scale and microstructural analyses, and Schmidt hammer measurements of rock strength. Our field mapping utilizes an 80-m long scanline and captures deformation distributed across the fault zone, where two overlapping principal slip surfaces (an ‘inner’ basinward fault, and an ‘outer’ fault) and their amalgamated damage zones are hosted within exhumed, lithified Miocene sandstones. The results indicate: 1) significantly more intense fracturing within the footwall block of the fault zone, paired with higher mineral veining (indicative of greater fracture dilatancy) in the hanging wall; 2) substantially higher rebound values in the footwall, suggesting greater rock strength relative to the hanging wall; and 3) the inner principal slip surface developed within the weaker hanging wall block. These mechanical contrasts delineate a bimaterial slip zone, shaped by repeated cycles of distributed brittle damage, hanging wall-limited fluid-rock interactions, and deep fault gouge extrusion. To assess the long-term implications of this bimaterial structure, we incorporate observed fault zone properties into multi-scale geodynamic models of normal faulting using ASPECT. The models reveal that over million-year timescales, while the principal fault continues to accommodate peak strain rates, across-fault strain distribution is asymmetric, with significant off-fault deformation in the mechanically weaker block. However, by ten million-years of tectonic extension, tectonic strain partially or fully migrates onto a newly formed fault within the weaker block. Collectively, these findings illustrate how bimaterial fault properties can influence strain distribution and fault evolution over geologic timescales, offering novel insights into fault system dynamics and long-term seismic hazards.

Key Words
Fault rocks, bimaterial faults, normal faults, rifts

Citation
Kolawole, F., Kornfeld, L., Xue, L., Rowe, C. D., & Kirkpatrick, J. (2025, 09). Emergent Bimaterial Slip Zones Control Strain Localization and Off-Fault Deformation along Plate Boundary Faults, Death Valley, California. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology