Rheological Heterogeneity of the Southern California Lithosphere

Laurent G. Montesi

Published September 10, 2023, SCEC Contribution #12900, 2023 SCEC Annual Meeting Poster #204 (PDF)

Southern California remains an area where seismic risk is extremely high. The complex network of faults makes it difficult to predict where future major events will take place. Modeling helps address this issue by revealing the interaction between tectonic blocs and the faults inside and at the boundary of these blocs. To enable such modeling, the Southern California Earthquake Center has developed several community models. Here, I use these products to study the lithosphere-scale strength and rheology varies in Southern California and discuss how tectonic elements appear to interact. These calculations are based on the geological framework (GFM) that defines 19 lithotectonic blocs and the various rock types present at depth. The Community Rheology Model (CRM) assigns flow laws to each rock type based on an averaging of experimentally derived mineral flow laws. The Community Thermal Model (CTM) defines the temperature at each depth in 14 heat flow provinces, which do not necessarily correspond to the GFM blocs. Assuming a strain rate, I calculate the strength envelope for combinations of GFM bloc and CTM provinces and integrate that strength to a depth of 100 km. This process is repeated at various strain rates and generates an effective rheology for each GFM/CTM combination, expressed as integrated strength vs. strain rate. As seen in other tectonic environments, this effective rheology is highly non-linear, with the strength varying only by a factor of 10 as the strain rate changes over ten orders of magnitude. This is because the brittle regime dominates strength in all but the hottest areas. The effect of lithological makeup is secondary to that of the heat flow regime. Thus, strength is minimal in the center of an E-W transept passing through the Salton trough, while it is maximum in the center of a similar transept crossing the Sierra Nevada. In a 1-D strike-slip setting, the integrated shear stress is constant. However, this would lead to dramatic variations in strain rate. As these variations are not observed, stress is likely heterogeneous, with strain rate regulated by geometric incompatibility. Blocs like the Transverse Ranges and the San Gabriel Mountains, which are essentially rigid and poorly aligned with the regional strike-slip regime, are likely the largest impediment to obtaining a uniform shear stress.

Key Words
Rheology, strength

Citation
Montesi, L. G. (2023, 09). Rheological Heterogeneity of the Southern California Lithosphere. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)