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Toward a Geologic Framework for the Community Rheology Model, with focus on the Mojave region

Michael E. Oskin, Whitney M. Behr, Alex E. Morelan, Andreas Plesch, & John H. Shaw

Published August 12, 2017, SCEC Contribution #7520, 2017 SCEC Annual Meeting Poster #232

Strain accrues heterogeneously across southern California due to the interaction of plate-boundary driving forces with lithospheric structure. To understand this process, we undertake the construction of a geologic framework for the Community Rheology Model (CRM). We begin with defining a set of eleven lithotectonic provinces with boundaries defined by faults and geologic horizons in the SCEC Unified Structural Representation (USR). These regions share common attributes and cover significant portions of southern California. The characteristics and distribution of these provinces reflects inherited geologic history - especially the long and varied record of subduction, followed by Late Cenozoic rifting and translation along the San Andreas fault system. As a pilot study for the CRM, we focus on the lithologic structure of the central Mojave province surrounding the Landers and Hector Mine earthquake ruptures. This area exemplifies how the composition of the lithosphere must be inferred from a variety of sources: surface geology, isotope geochemistry, xenoliths, seismic velocity, potential fields, and tectonic history. The upper crust of the central Mojave resembles the intact eastern Sierra Nevada batholith. However geophysical constraints, and tectonic exhumation of middle to lower crustal rocks in surrounding areas, together indicate that the lithosphere was tectonically modified, probably during Late Cretaceous low-angle subduction. This event resulted in removal and replacement of the mantle lithosphere and lowermost crust from surrounding regions, though geochemical evidence suggests that the Proterozoic mantle lithosphere remains intact beneath the central Mojave. During the Late Cretaceous the central Mojave may have undergone tectonic thinning, acting as the upper plate to crustal-scale detachment-faulting that exhumed surrounding regions. Petrologic and geophysical constraints, integrated with exhumed exposures, support that the middle and upper crust here, and in much of southern California, is dominantly felsic, with a significant fraction of quartz. Mafic to intermediate compositions, evident from xenoliths, and resembling some of the deeper, now exhumed parts of the Sierra Nevada arc, predominate between ~25 km depth and the Moho, at ~30 km depth. The xenoliths also reveal the mantle lithosphere as dominantly spinel lherzolite, with plagioclase replacing spinel above ~40 km depth in regions of significant melt infiltration.

Oskin, M. E., Behr, W. M., Morelan, A. E., Plesch, A., & Shaw, J. H. (2017, 08). Toward a Geologic Framework for the Community Rheology Model, with focus on the Mojave region. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)