SCEC Award Number 18193 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title A community thermal model (CTM) of the southern California lithosphere
Investigator(s)
Name Organization
Wayne Thatcher United States Geological Survey David Chapman University of Utah Amir Allam University of Utah
Other Participants Colin Williams (USGS)
SCEC Priorities 1b, 1e, 2d SCEC Groups SDOT, CXM, CISM
Report Due Date 03/15/2019 Date Report Submitted 04/29/2019
Project Abstract
 We are developing a 3D Community Thermal Model (CTM) to constrain rheology and so better understand deformation processes in SoCal. Work carried out in 2018 has strengthened our conclusion that transient thermo-mechanical processes are at work in SoCal. By obtaining a digital version of SoCal sLAB depth (Fig. 1d) kindly provided to us by Ved Lekic (Univ. of Maryland), we used pixel counting to make more accurate estimates of average LAB depths within each heat flow region (HFR). This shows more clearly than our previous work that sLAB depth is insensitive to SHF, averaging 70 ± 5 km for 11 of our 14 HFRs (Fig. 1b). Internally consistent 1D steady-state geotherms can be computed for 5 of these 11 regions (LA, MD, WTR, WBR, CCR) but is not possible for 6 others (SN, ETR, VB, PR, SG, GV).
More sophisticated thermal models are thus required for these 6 regions. In 2018 we made some preliminary calculations using analytic solutions for both the slab window (Fig. 1f) and Salton Trough extension models with promising results. A common feature of these models is a much warmer lower crust and upper mantle lithosphere than is shown for the steady-state geotherms. If true this suggests that HFRs with low present-day surface heat flow (SHF) reflect a transient condition, with lower crust/upper mantle temperatures being warm or hot beneath much of SoCal. This in turn implies that lateral contrasts in effective viscosity may be much more moderate than the SHF map of Fig. 1a suggests.
Intellectual Merit The Community Rheology Model (CTM) project uses SoCal thermal, seismological, geochemical and petrological constraints to substantially revise its 3D temperature structure. Despite a wide variation in surface heat flow we have found that LAB depth averages a remarkably uniform 70 ± 5 km over much of the region, implying transient thermal conditions and a rather uniformly warm lower crust and upper mantle. Since rheology is very temperature-dependent our results will contribute substantially to SCEC’s Community Rheology Model (CRM) and in turn shape the lithospheric deformation of SoCal.
Broader Impacts The CTM, like other SCEC community models, will enhance the center’s infrastructure and research capabilities. We also suggest that our conceptually simple approach to modeling SoCal’s thermal field (Fig. 1) will make its importance easily comprehensible to scientists and educators in SoCal and elsewhere.
Exemplary Figure Fig. 1: Work flow followed here in constructing southern California geotherms: (a) access SoCal surface heat flow (SHF) database; (b) Subdivide SoCal into 14 heat flow regions (HFRs); (c) compute default 1D steady-state conductive geotherms for each HFR; (d) use seismic estimates of average lithosphere-asthenosphere boundary depth (sLAB) for each HFR and assume it is identical to the thermal LAB depth in next computational steps; either (e) constrain steady-state geotherms using LAB depth, dry and fully hydrated asthenosphere peridotite solidi, and P/T estimates from erupted lavas and xenoliths; or (f) compute transient geotherms in HFRs where steady-state conditions do not apply.
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