Temperature exerts the strongest control on the 3D rheology of the southern California lithosphere

Wayne R. Thatcher, David S. Chapman, & Colin Williams

Published August 15, 2016, SCEC Contribution #6987, 2016 SCEC Annual Meeting Poster #011

Lithospheric temperature differences influence rheologic behavior, crustal deformation and earthquake occurrence in southern California. However, geotherms are poorly constrained, with those used in published deformation modeling studies typically differing by ~200˚-300˚C in the crust and upper mantle, leading to factor of ~100 variations in estimates of effective viscosity at these depths. Under SCEC auspices we are developing a 3D Community Thermal Model (CTM) to improve temperature estimates and so refine our understanding of earthquake cycle and long-term deformation processes within this complex but densely monitored and relatively well-understood region.

We use more than 250 surface heat-flow measurements to define 12 geographically distinct heat flow regions (HFRs). Model geotherms within each HFR can be constrained by averages and variances of surface heat flow q0 and thermal conductivity (k) and radiogenic heat production (A). Using generic models for (A, k) versus depth we have constructed preliminary 1D conductive geotherms for each HFR. Regional temperature variations between HFRs are as much as 300˚ in the mid-crust and 700˚ at the Moho, much larger than the uncertainties in these preliminary thermal calculations. Such temperature differences correspond to several orders of magnitude lateral variations in effective viscosity, surely large enough to have important implications for modeling ductile deformation in southern California.

However, these local geotherm estimates and their uncertainties can be improved with better constraints on rock type and thermophysical properties within each HFR. The 1D (A, k) structure can be more precisely estimated using inputs from (1) surface geology and subsurface inferences on average crust/upper mantle lithologic structure; (2) seismic imaging results to determine average wave speed versus depth; (3) empirical relations between seismic wave speed and rock type; and (4) laboratory and modeling methods relating thermophysical properties (A, k) to rock type.

A SCEC Community Velocity Model (CVM) that includes independent estimates of S-wave velocity would permit better correlation between seismic velocities and rock type. Construction of a Geologic Framework Model for southern California--explored in a June 2016 SCEC workshop--would contribute importantly to more tightly constraining lithologic structure within each HFR.

Key Words
rheology, temperature

Citation
Thatcher, W. R., Chapman, D. S., & Williams, C. (2016, 08). Temperature exerts the strongest control on the 3D rheology of the southern California lithosphere. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)