Heat Flow Data and Seismic Imaging Reveal Both Transient and Steady-State Thermo-Mechanical Processes at Work Beneath Southern California

Wayne R. Thatcher, & David S. Chapman

Published August 7, 2018, SCEC Contribution #8318, 2018 SCEC Annual Meeting Talk on Mon 10:30

Analysis of heat flow and seismic data provides glimpses of the dynamical processes shaping the thermal evolution of southern California. The present day thermal field bears an imprint of long-lived subduction prior to 30 Ma and subsequent growth of a continental transform boundary. Post 30 Ma processes include (1) a continental analog of seafloor spreading beneath the Salton Trough, (2) thermal pulses due to slab window effects, (3) mantle lithosphere detachment and sinking beneath late Cretaceous batholiths, and (4) extension and formation of metamorphic core complexes (MCC) in the offshore Inner Borderland.

Over 200 high quality surface heat flow (SHF) measurements define 14 distinct southern California heat flow regions (HFRs) where SHF is relatively constant. Assuming seismic estimates of lithosphere-asthenosphere boundary depth (sLAB) coincide with thermal LAB, two remarkable features are revealed. First, for 11 HFRs with SHF 40-83 mW/m2, sLAB depth is a surprisingly constant 70 ± 5 km. These data points naturally separate into 2 clusters, the first with average SHF of 40-58 mW/m2 (Cluster 1), the second with SHF of 68-83 mW/m2 (Cluster 2).

Simple 1D steady-state thermal conduction models can match the six Cluster 2 HFRs. P/T constraints from mantle xenoliths and erupted lavas considerably narrow the range of acceptable geotherms, with Moho at 700˚- 800˚C and LAB 1200˚- 1300˚C. However, such simple models are inconsistent with the low SHF of Cluster 1. 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 30 to 50 km thick lithosphere exposed to hot asthenosphere 3-10 Ma BP conductively cooled and thickened by mafic underplating. Current temperatures are warm in the lower crust, ~850˚C at the Moho, and ~1200˚C at the LAB.

Salton Trough has the highest SHF (100-140 mW/m2) and thinnest lithosphere (45-55 km) in southern California. A model with steady state crustal thickness of 24 km, constant rates of stretching and sedimentation into the Trough and basaltic under-plating at the Moho matches the data, with a hot lower crust and ~1400˚C asthenosphere at the Moho.

Finally, the Inner Borderland HFR has an unusually thin lithosphere of 49±6 km and SHF of 77±6 mW/m2, consistent with its origin as a Miocene MCC subsequently unroofed and depleted of its upper crustal heat producing elements.

Key Words
CTM, heat flow, seismic imaging, thermo-mechanical model

Citation
Thatcher, W. R., & Chapman, D. S. (2018, 08). Heat Flow Data and Seismic Imaging Reveal Both Transient and Steady-State Thermo-Mechanical Processes at Work Beneath Southern California. Oral Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)