Integrating Seismic and Magnetotelluric Constraints on Lithospheric Properties to Explore the Geodynamic Origin of the Southeastern US Stress Field

Session: Seismicity and Tectonics of Stable Continental Interiors
Type: Oral
Date: 4/30/2020
Time: 10:45 AM
Room: 240
Description: 

Although the intraplate stress field in the southeastern United States (SEUS) has been viewed as part of a broadly uniform stress province controlled dominantly by far-field forces, data from the World Stress Map (WSM) as well as additional earthquake focal mechanisms and the attitude of Quaternary faults indicate substantial stress-field variability throughout this region. For example, in the Central Virginia Seismic Zone, focal mechanisms and faults that show evidence for Pleistocene-aged slip indicate a direction of maximum horizontal compression (SHmax) that is highly oblique to that inferred for Eastern North America from WSM data. Here, we examine the role of body forces in producing this heterogeneous stress field. We use CitcomS, a finite-element geodynamic modeling code, to model local lithospheric stresses based on simple input composition, temperature and viscosity fields. We superpose on this modeled stress field an estimate of the far-field stress contribution from ridge push and lithospheric basal shear. Our composition field largely reflects seismically-imaged crustal thickness variations. We use a one-dimensional depth-dependent thermal field; although seismic studies have argued for substantial variations in thermal lithosphere thickness throughout the SEUS, joint analysis with magnetotelluric (MT) data indicates that present constraints cannot reject a uniformly thick thermal lithosphere throughout this region. For viscosity, we use a one-dimensional depth-dependent field as well as three-dimensional fields that are informed by MT imaging. With these simple models, we are able to reproduce many features of the SEUS stress field, including heterogeneous SHmax. Our results indicate that observed stresses arise from the superposition of far-field forces and local body forces. Crustal compositional buoyancy, controlled predominantly by crustal thickness variations, appears to be most important in reproducing observations, although we slightly better match the observed stress field by including laterally variable crustal viscosity mapped from MT imaging.

Presenting Author: Benjamin S. Murphy

Authors