EXPLORING THE 3-D THERMAL STRUCTURE AND RHEOLOGY OF CALIFORNIA: CONSTRAINTS FROM INTEGRATED HEATFLOW AND SEISMIC OBSERVATIONS

The thermo-mechanical conditions of the crust impact how it deforms, with a first-order control on viscosity, strain rate and crustal strength. In tectonically active regions, such as California, which hosts the San Andreas fault, Walker Lane-Eastern California Shear Zone, and Basin and Range province, knowledge of the thermo-mechanical nature of the crust can be used to better evaluate seismic hazard, crustal rheology, and the location of energy resources (e.g., geothermal systems). Here we apply a novel method of integrating surface heatflow with geophysical observations for the base of the seismogenic zone (with an approximate temperature of ~350 °C corresponding to the onset of quartz plasticity and ductile flow) and the Moho depth and temperature to construct a 3D thermal model of California. We use a Monte Carlo-type approach to fit steady-state conductive geotherms to this data within fixed-width bins to evaluate various thermal parameters (e.g., radiogenic heating, diffusivity) and test the degree to which conductive thermal profiles fit the data. Our resultant 3D thermal model of California provides direct constraints on crustal strength and viscosity. Most regions are well described by a conductive geotherm, where surface heatflow correlates with Moho temperature and inversely correlates with seismogenic thickness. We find that the Sierra Nevada batholith is cold and strong, whereas the San Andreas fault system is relatively hot and weak. There are some regions where extremely hot surface heatflow values are not consistent with deeper crustal parameters, which suggests convective/advective processes, such as hydrothermal fluid upwellings and/or volcanism in the Salton Trough and Long Valley caldera regions. Our approach can be used to better evaluate crustal strength and seismic hazard, and it offers new constraints on input thermal parameters for modeling studies.