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SCEC2021 Plenary Talk, SCEC Community Models (CXM)

The SCEC Community Rheology Model: A modular resource for investigating the effective viscosity structure of southern California

Elizabeth H. Hearn, Laurent G. Montesi, Greg Hirth, Michael E. Oskin, Whitney M. Behr, & Wayne R. Thatcher

Oral Presentation

2021 SCEC Annual Meeting, SCEC Contribution #11113 VIEW SLIDES
The SCEC Community Rheology Model (CRM) comprises a 3D geologic framework model (GFM) of southern California’s lithosphere, and ductile flow laws for each GFM rock type. The GFM is shared as coordinates for 23 lithotectonic province boundaries, tables with lithology columns for each province, and petrological descriptions for each lithology. Whole-rock, steady-state flow laws for the GFM lithologies are in the form of analytical expressions, which may be readily coded up by researchers or integrated into existing codes.

Here, we present computed steady-state effective viscosities for low-strain rocks throughout the southern California lithosphere, assuming CTM temperatures and strain rates that are broadly consistent with current surface deformation. We obtain effective viscosities throughout the upper crust (0 to 16 km) exceeding 10^22 Pa s, except in the Salton Sea region where values are as low as 5 x 10^19 Pa s. In the mid- to lower crust and mantle lithosphere, computed viscosities outside the Salton region range from 10^20 - 10^21 Pa s, and in the mantle below 60 km depth, from 10^18 to 5 x 10^19 Pa s. Viscosities in the Salton region decrease from ~10^19 Pa s in the lower crust to ~1 to 5 x 10^18 Pa s in the mantle. 

Mantle viscosities are governed by CTM temperatures and assumed strain rates because the CRM mantle is currently homogeneous. In the crust, lithology also influences the effective viscosity distribution. For example, the Franciscan and Rand Schists cause local areas of relatively low viscosity in the mid-to upper crust.

Viscosities presented here are comparable to local estimates from postseismic deformation models, if allowances are made for transient rheology. They are about a factor of ten lower than an independent estimate of steady-state viscosities in the lower crust based on seismic velocities (Shinevar et al., 2018).

We emphasize that the viscosities shown here are not prescriptive. The CRM is intended as a jumping-off point for exploring southern California’s rheology, and its modularity and simple formats are meant to encourage researchers to substitute in their own interpretations of lithology, temperature, strain rate, and/or ductile flow laws. During the 2022-2024 bridge period we plan to update the CRM by adding ductile flow laws for high-strain (shear zone) rocks, incorporating heterogeneous mantle, and providing guidance on representing elastic and brittle/plastic materials in deformation models.