Tectonic stress within the New Madrid seismic zone

Refraction data indicate a significant high-density rift pillow beneath the New Madrid seismic zone. We present results of linear and nonlinear viscoelastic finite element modeling to determine whether support of the rift pillow may contribute significantly to the total present-day stress field, and we consider the implications for intraplate seismicity. These models were run for a loading time of 100 m.y. to account for relaxation and transfer of stress since the last reactivation of the rift in the mid-Mesozoic. Results indicate that the nonlinear viscoelastic model with rheological stratification based on composition and temperature agrees well with the observed deformation within the seismic zone and with estimates of regional stress magnitudes. The model predicts a maximum compression of 30-40 MPa above the rift pillow in the center of the rift axis. If the magnitude of local compression predicted by the nonlinear model produces the inferred clockwise rotation of the order of 10 degrees-30 degrees in the direction of S-Hmax (maximum horizontal compression) near the rift axis, the magnitude of regional compression is a factor of 1 to 2 times the magnitude of local compression and consistent with an origin due to ridge push forces. The addition of the local stress associated with the rift pillow, however, results in an approximately 30% reduction in the resolved maximum horizontal shear stress. Thus, while the stress associated with the rift pillow can rotate the stress field into an orientation favorable for failure, reduction in the resolved shear stress requires a separate mechanism for strength reduction. Results of the modeling indicate that stresses from the load of the rift pillow may still be present in the upper crust even after 100 m.y. and may still play a role in present-day deformation and seismicity of the New Madrid seismic zone. Local stress fields of significant tectonic magnitudes may also occur around other ancient rift pillows and help explain the observed correlation between intraplate seismicity and failed rift zones.