Many areas of large-scale crustal extension, such as the Basin and Range province of western USA, have exposed inactive detachment fault surfaces with very shallow dips (<10o). However, fault-plane analysis of present-day earthquakes worldwide shows that faults with such shallow dips are not usually seismically active. Thus much controversy has arisen over the original angle of dip of these faults at their initiation. This project will test the hypotheses that (1) the faults initiated at dips of 10o or less and remained in t hat orientation, and (2) they initiated at dips of 30o to 60o and were subsequently tilted. Paleomagnetic analysis of selected rock units from the footwall of one such detachment fault in South Mountains, Arizona, will provide the test. The results will have substantial significance for the tectonic evolution of large areas of extended continental crust.
