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Brittle-ductile Transition Depth in the Mojave Desert Region, Southern California

Megan Perry, Laurent G. Montesi, Suerken C. Matsuyama, & Kali L. Allison

Submitted September 10, 2023, SCEC Contribution #12945, 2023 SCEC Annual Meeting Poster #095

In active continental plate boundaries, earthquakes typically occur only at depths shallower than 12 to 15 km. The maximum depth of seismicity may be related to a critical temperature or a transition from brittle to ductile behavior. In the Mojave Desert region, the maximum earthquake depth appears to vary with earthquake activity, which may correlate with strain rate. This observation is most compatible with the hypothesis that the brittle-ductile transition depth (BDT) controls the depth of seismicity. Here, we use the RHEOL_GUI app to model the strength profile of the Mojave Desert region and report how the BDT is expected to vary with strain rate. RHEOL_GUI incorporates community models developed by the Southern California Earthquake Center. The Geologic Framework Model (GFM) defines 19 litho-tectonic blocks, including “Mojave” and the various rock types present at depth. The Community Thermal Model defines the temperature profile in 14 heat flow provinces, including “Mojave Desert”. The Community Rheology Model assigns flow laws to each rock type by averaging experimentally derived mineral flow laws. We use the flow laws for Granodiorite05, Quartz Diorite, Rand Schist, Gabbro, and Mantle and Byerlee’s friction with hydrostatic fluid pressure in the crust. The app calculates a strength envelope by breaking each stratigraphic layer into sublayers in which a specific rheology controls strength, i.e., requires the lowest stress to deform at the specified strain rate. The model predicts BDTs that are systematically deeper than observed. There are several ways of reconciling this apparent discrepancy. If seismicity bounded by a critical temperature of ~400°C, then surface heat flow needs to be 25% higher than observed across the Mojave. Assuming that brittle strength is high due to low fluid pressure cannot explain the observed depths of seismicity. Ductile strength may be decreased to the point where the predicted BDT is comparable with earthquake observations if the grain size is reduced to 20 microns or ductile creep is dominated by quartz rather than the volumetrically dominant feldspar. Both scenarios are consistent with observations of some exhumed ductile shear zones. The relatively shallow depth of seismicity observed at plate boundaries implies that actively deforming faults and shear zones are weaker than the surrounding areas by a factor of ~0.8.

Perry, M., Montesi, L. G., Matsuyama, S. C., & Allison, K. L. (2023, 09). Brittle-ductile Transition Depth in the Mojave Desert Region, Southern California. Poster Presentation at 2023 SCEC Annual Meeting.

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