Refining Southern California Geotherms Using Seismologic, Geologic, and Petrologic Constraints

Wayne R. Thatcher, David S. Chapman, Amir A. Allam, & Colin Williams

Submitted August 15, 2017, SCEC Contribution #7857, 2017 SCEC Annual Meeting Poster #224

Lithospheric deformation in tectonically active regions depends on the 3D distribution of rheology, which is in turn critically controlled by temperature. Under the auspices of the Southern California Earthquake Center (SCEC) we are developing a 3D Community Thermal Model (CTM) to constrain rheology and so better understand deformation processes within this region. Here we present the elements of a Version 1.0 CTM and describe how they are obtained.

The San Andreas transform system has sliced southern California into distinct blocks, each with characteristic lithologies, seismic velocities and thermal structures. Guided by the geometry of these blocks and more than 250 surface heat-flow measurements we define 13 geographically distinct heat flow regions (HFRs) within which surface heat flow is roughly constant.

Model geotherms within each HFR are constrained by averages and variances of surface heat flow q0 and the 1D depth distribution of radiogenic heat production (A) and thermal conductivity (k), which are strongly dependent on rock type. Crustal lithologies are not always well known and we turn to seismic imaging for help. We interrogate the SCEC Community Velocity Model (CVM) to determine averages and variances of Vp, Vs and Vp/Vs versus depth within each HFR. We bound (A, k) versus depth by relying on empirical relations between seismic wave speed and rock type and laboratory and modeling methods relating (A, k) to rock type.

Many 1D conductive geotherms for each HFR are allowed by the variances in surface heat flow and subsurface (A, k). An additional constraint on the lithosphere temperature field is provided by comparing lithosphere-asthenosphere boundary (LAB) depths identified seismologically with those defined thermally as the depth of onset of partial melting. Receiver function studies in Southern California indicate LAB depths that range from 40 km to 90 km. Shallow LAB depths are correlated with high surface heat flow and deep LAB with low heat flow. The much-restricted families of geotherms that intersect peridotite solidi at the seismological LAB depth in each region require that LAB temperatures lie between 1050 to 1250˚ C, a range that is consistent with a hydrous rather than anhydrous mantle below Southern California.

Key Words
thermal model, rheology, southern California

Thatcher, W. R., Chapman, D. S., Allam, A. A., & Williams, C. (2017, 08). Refining Southern California Geotherms Using Seismologic, Geologic, and Petrologic Constraints. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)