The SCEC Community Thermal Model (CTM) Version 1.2

Wayne R. Thatcher

Submitted August 15, 2020, SCEC Contribution #10724, 2020 SCEC Annual Meeting Poster #180

Analysis of heat flow and seismic data provides glimpses of the dynamical processes shaping the thermal evolution of southern California since 30 Ma BP. Processes include (1) a continental analog of seafloor spreading beneath the Salton Trough and the inner Continental Borderland; (2) steady-state conduction indictive of relative thermal stability; (3) thermal pulses due to detachment or foundering of oceanic lithosphere (Farallon slab) beneath continental crust and/or mantle lithosphere detachment and sinking beneath late Cretaceous batholiths.

Over 200 high quality surface heat flow (SHF) measurements define 14 distinct SoCal heat flow regions (HFRs) where SHF is relatively constant. As a point of departure we compute 1D steady-state geotherms for all HFRs using standard methods. We assess the consistency of these models with other data and find significant disagreements. In particular, predicted thermal lithospheric-asthenosphere boundary (tLAB) depths are 95-220 km beneath SoCal’s Late Cretaceous batholiths. Seismic estimates of LAB depth (sLAB) provide an independent dataset relevant to our thermal modeling. There are strong reasons to believe sLAB = tLAB) beneath SoCal and much of the active western US. If so two remarkable features of SHF-sLAB systematics jump out. First, for 11 HFRs with SHF 40-82 mW/m2, sLAB depth is a surprisingly constant 70 ± 5 km. Furthermore, these data points naturally separate into 3 clusters, the first with high average SHF (79-140 mW/m2) above 3 magmatic HFRs (Cluster 1); the second with moderate SHF of 68-82 mW/m2 (Cluster 2); the third with SHF of 40-58 mW/m2 (Cluster 3).

Models with active extension, sedimentation, and magmatic underplating match the Cluster 1 geotherms. Simple 1D steady-state thermal conduction models can match the 5 Cluster 2 HFRs. P/T constraints from mantle xenoliths and young lavas considerably narrow the range of acceptable geotherms. However, such steady-state models are inconsistent with the low SHF and ~ 70 km LAB depth of Cluster 3. In each of these 5 HFRs there is geologic and/or seismic evidence for Late Cenozoic detachment and sinking of mantle lithosphere. A transient 1D conduction model is used that includes an initially 50 km thick lithosphere exposed to hot asthenosphere 3-10 Ma BP and then conductively cooled and thickened by mafic underplating. Present day temperatures are warm in the lower crust, ~750˚- 850˚C at the ~ 40 km depth Moho, and ~1400˚C at the LAB.

Key Words
geotherms, temperature field, rheology, crustal deformation

Thatcher, W. R. (2020, 08). The SCEC Community Thermal Model (CTM) Version 1.2. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)