Models for midcontinent tectonism: An Update

During the past decade, significant progress has been made in defining the nature of midcontinent tectonism as a result of an improved geophysical/geologic data base which permits an integration of seismicity data, crustal structures, and current stress directions. Critically placed microseismic networks have shown that many earthquakes with common focal mechanisms occur in specific spatial patterns. The direction of the maximum horizontal compressive stress is generally directed east‐northeast, which supports an origin associated with ridge‐push forces related to movement of the North American plate. Crustal structures have been mapped locally by seismic studies and regionally by extensive gravity and magnetic anomaly data. These data have been used to relate current seismicity and associated tectonism to one of two models. Increasing evidence from across the midcontinent supports reactivation of preexisting zones of crustal weakness appropriately oriented with respect to the prevailing stress field, the ‘zone of weakness model’, as the dominant control on contemporary tectonism and the ‘local basement inhomogeneity model’ as a mechanism for minor, low‐energy release earthquake activity.