Importance of the temperature field and its uncertainties in modeling ductile deformation of the southern California lithosphere

Temperature is arguably the most important parameter controlling ductile deformation in tectonically active regions. Laboratory measurements at lower crust and upper mantle conditions define the mechanisms controlling ductile deformation and constrain quantitative rules relating stress and strain rate. Exhumed ductily deformed rocks reveal the micromechanics of deformation, supplying ground truth that can be compared with lab results. However, even if the mechanism and ductile deformation rules are accepted at face value, strain rates are exquisitely dependent on temperature. Here we critically assess observational data relevant to constraining the southern California lithospheric temperature field. Our goal is to improve estimates of the 3D temperature field and its real uncertainties and apply them to regional deformation modeling. We use a phased approach to estimating geotherms, beginning with simple 1D steady state conductive models. We identify the most important parameters and disaggregate them, separately examining the effects of varying radiogenic heat source concentration, rock type, crust and lithosphere thickness and asthenosphere solidus. We assess geotherm uncertainties by assigning realistic error bounds on all input quantities, propagate these uncertainties by Monte Carlo sampling and determine probability density functions for the geotherm. We find that although other parameter uncertainties contribute, variability in heat sources produces the largest variation in model-predicted geotherms. Because heat production depends strongly on rock type, better characterization of crustal lithology using refined seismic imaging results now becoming available beneath southern California is likely to produce the largest improvements in thermal models. Nonetheless, substantial uncertainty will remain, arguing for adoption of one or a few standard thermal models as common starting points for regional deformation modeling in southern California and elsewhere.