Figure 1. Geologic Framework provinces in the southern California crust. Image courtesy of Andreas Plesch (August, 2020).

Overview

The SCEC CRM provides a three-dimensional description of the ductile rheology of southern California’s lithosphere, based on constraints from seismic, geologic, and experimental rock deformation studies, and several rounds of community input (see list of workshops in sidebar). The September 2020 release comprises two components: a three-dimensional geologic framework model (GFM) of southern California’s crust, and a set of synthetic aggregate ductile flow laws for each GFM rock type. Together with SCEC Community Thermal Model (CTM) temperatures and pressure, this information provides a model for southern California’s ductile rheology. Given strain rate(s), effective viscosity and differential stress may be calculated throughout the region. The GFM may be queried for rock type using the SCEC GFM viewer; please see link in the DATA PRODUCTS box to the right. We anticipate that the CRM will evolve as it is examined by the SCEC community, and all are welcome to participate in the TAG and contribute to future versions.

The preliminary GFM comprises 23 provinces, each with a set of lithologic layers. It is shared as lat-lon files of province boundaries, tables with depth intervals and rock types for each province, and petrological descriptions for each rock type. The CRM rheology information includes a table of preferred mineral flow laws, a description of mixing laws and assumptions, and a table of synthetic whole-rock flow laws for each of the GFM rock types, as well as Matlab codes for calculating effective viscosity profiles. The whole-rock flow laws are analytical expressions, and may be integrated into codes such as RHEOL_GUI (Montesi and Leete, 2018), enabling quick and accurate calculations of effective viscosity and differential stress.

Research Priorities

1. Generate flow laws and/or guidance for the rheology of ductile shear zones.
2. Generate flow laws or guidance for addressing transient rheology 
3. Define a workflow for connecting the results of GF viewer queries to the code RHEOL_GUI (Montesi and Leete, 2018)
4. Add brittle-plastic rheology (for bulk upper crust, sediments and shear zones) to the CRM 

Data Products

Geological Framework

• GF polygons boundaries, lat-lon (various formats, e.g., CRM_polygons.lonlat)
• Crustal column descriptions for each province with depth intervals and major representative rock types, their modal mineralogy and descriptions (CRM_Crustal_Columns_in_Narrative_Format, CRM_slab_format.xlsx)
• README files describing GF province boundary files and lithology/crustal column files

Rheology

File summarizing rheology: CRM_rheologies_writeup.pdf (also CRM_rheologies_writeup.doc). This file explains how the CRM whole-rock aggregate flow laws were developed, including parameters, data sources, assumptions and example calculation results. The flow laws are determined using the MPGe averaging model of Huet et al. (2014). Flow laws and their coefficients are given in this file. A csv file with all CRM rheologies in compact form (GFLitho_MPGe_Wet.csv) is described in section 3 of CRM_rheologies_writeup.pdf.  For each crustal lithology, two sets of parameters are given, one in which water fugacity must be specified by the user, and the other where water-saturated conditions are assumed. The mantle is assumed to be damp with C_{OH}=480 p.p.m.

Matlab codes for calculating rock rheology from mineral flow laws, and for plotting effective viscosity profiles: HuetMPGeWet.m and VizGFMix.m. Also input files required by these codes: rock.mat and CRM_slab_format.xlsx (from the Geological Framework). HuetMPGeWet.m is included strictly for documentation purposes and is not required for users of the CRM. The CRM rheologies are shared in its csv output file, GFLitho_MPGe_Wet.csv. 

README files describing rheology files, codes and code input files, and example viscosity profile output plots.

Figure 2. Effective viscosity profile generated for quartz diorite and a suite of minerals at 850°C and a strain rate of 10^-15 s^-1, using Matlab codes HuetMPGeWet.m and VizGFMix.m (by Laurent Montesi). Water saturation is assumed for granodiorite results (heavy black line). Red dashed line indicates the effect of reducing water fugacity by a factor of ten relative to saturated conditions.

Selected Publications

Overview

Hearn, E., M. Oskin, G. Hirth, L. Montesi, W. Thatcher, W. Behr, and E. Pauk  (2020, 09). The SCEC Community Rheology Model (CRM). Poster Presentation at 2020 SCEC Annual Meeting.  SCEC Contribution #10606.

Relevant Publications

Huet, B., P. Yamato, and B. Grasemann (2014, 04). The Minimized Power Geometric model: An analytical mixing model for calculating polyphase rock viscosities consistent with experimental data. J. Geophys. Res. Solid Earth, 119, 3897–3924, doi:10.1002/2013JB010453.

Montesi, L. G., & Leete, W. (2018, 08). RHEOL_GUI: A Matlab-based graphical user interface for the interactive investigation of strength profiles. Poster Presentation at 2018 SCEC Annual Meeting. SCEC Contribution #8500.