Multiscale variations of the stress field across North America: Insights for sources of intraplate stress

A new map of maximum horizontal principal stress orientations (S_Hmax) and relative stress magnitudes (faulting regime) throughout North America reveals coherent variations that occur over many spatial scales. More than 700 recent measurements of stress orientation and almost 2000 indicators of relative stress magnitude from earthquake focal plane mechanisms were used to construct this new map. As seen in previous studies, the stress field is compressive in the eastern USA and Canada (reverse and/or strike-slip faulting), with S_Hmax oriented NE-SW to ENE-WSW. Compression in this region may be due to low gravitational potential energy, as indicated by a strong geoid low. We observe a first-order transition westward to less compressive conditions (strike-slip faulting) in the center of the continent, where the S_Hmax orientation is parallel to the plate motion direction, possibly indicating the importance of resistive drag at the base of the lithosphere. The stress gradients (reverse faulting near Hudson Bay and less compressive conditions to the south and west) are nearly the opposite of those predicted by models of postglacial rebound, and they are also inconsistent with the GPS-measured strain rate field. In the western USA, the faulting regime is extensional (normal and/or strike-slip), and S_Hmax rotates dramatically (but coherently) over short distances in some areas, notably outside the margins of the Rio Grande rift and Basin and Range province. Anomalously compressive areas (strike-slip faulting) in the western and central USA, such as southwest Oklahoma and the Wyoming-Utah-Colorado border region, are generally associated with thick lithospheric roots, which likely impose flexural compression in the upper crust. We show that the previous geodynamic models that best match the observed stress field in the western USA incorporate simple estimates of gravitational potential energy and plate boundary tractions, but not modeled mantle flow.