Project Abstract
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We develop a SCEC “Community Thermal Model” (CTM) to supply a standard reference point from which to make future refinements and corrections and provide a uniform starting point for constructing models that depend on temperature. We constrain this model with observations of surface heat flow, bounds on thermal conductivity and radiogenic heat production in the crust and uppermost mantle and compute a series of 1D conductive geotherms consistent with available data. With 2015 SCEC support we have made preliminary calculations of southern California steady-state geotherms using observational constraints on relevant controlling parameters along the LARSE I seismic transect. The intersection of a geotherm with asthenosphere melting curves is then an estimate of lithospheric thickness and lithosphere-asthenosphere boundary (LAB) temperature Ta. These depths are in rough agreement with the seismic receiver function results of Lekic et al. (2011) that show values of 70-75 km in imaged LAB depth along the LARSE I profile. In our 2016 work we have used 250 surface heat-flow measurements to define 12 geographically distinct heat flow regions (HFRs). Model geotherms within each HFR are constrained by averages and variances of surface heat flow, thermal conductivity, and radiogenic heat production. Even excluding geotherm outliers, Moho temperature differences between some adjacent HFRs exceed 300˚C. Since effective viscosity depends exponentially on temperature, these Moho temperature differences require large (factor of ~1000) lateral variations in effective viscosity. Such lateral and depth-wise variability is surely large enough to have important implications for earthquake cycle and longer-term ductile deformation in southern California. |
