STRESS ORIENTATION AND RELATIVE STRESS MAGNITUDES THROUGHOUT THE NORTH AMERICAN PLATE

We have compiled hundreds of new orientations of the maximum horizontal stress (S_Hmax) and, using thousands of well-constrained earthquake focal plane mechanisms, developed the first quantitative map of stress directions and relative stress magnitudes throughout North America. We present the new map and discuss the insights it provides for the causes of crustal deformation. Across central and eastern North America, S_Hmax rotates gradually clockwise to the west, from ~NE–SW along the Eastern Seaboard and southern Canada to nearly E–W in the western U.S. Great Plains. Over much of the same area, a continent-scale transition occurs from strike-slip and/or reverse faulting in eastern North America to strike-slip faulting in the mid-continent to normal and/or strike-slip faulting in western intraplate North America. This transition is likely due (at least in part) to drag at the base of the lithosphere and is almost opposite the pattern predicted by models of glacial isostatic adjustment, indicating that the ambient tectonic stresses in the crust are much greater than those from postglacial rebound. In the generally extensional western U.S., the orientation of S_Hmax rotates over remarkably short distances slightly outside the boundaries of the Rio Grande Rift and Basin and Range extensional domains. Published geodynamic models for the western U.S., incorporating gravitational potential energy and tractions from relative mantle flow, successfully predict most large-wavelength rotations but not the shorter-wavelength (<~200 km) rotations that we observe in the detailed new S_Hmax orientation data. Finally, further west strike-slip and/or reverse faulting are active in a narrow band along the San Andreas plate boundary and in a wide zone further north that covers the Canadian Rockies and southern Alaska. This new-generation stress map will assist identification of potentially active faults and advance our understanding of active tectonic processes.