TRANSLATION OF TERRANES - LESSONS FROM CENTRAL BAJA-CALIFORNIA, MEXICO

Baja California is one of the best modern examples of a continental bl ock, or terrane, that is translating hundreds of kilometers along a hi ghly obliquely divergent plate. The central domain of Baja California is particularly appropriate as a modern analogue for terrane translati on because it has a relatively simple late Neogene history and is a bu oyant continental block that is likely to be preserved in any future a ccretion to the North American continent. In contrast, the deep, dense oceanic crust of the Gulf of California, where the main transform fau lts of the plate boundary lie, has little preservation potential. It i s surprising that there is little strike-slip faulting in the central domain of Baja California, even though the plate boundary has a rift a ngle (angle between rift trend and azimuth of plate motion), alpha of only 20 degrees. Recent field data confirm modeling studies of oblique rifting and show that there is a predictable change in fault type, fa ult orientation, and extension direction with changing rift angle. Tha t is, from secondary structures alone, one can predict the approximate obliquity of rifting and the orientation of the plate motion. With ri ft angles of 0 degrees-20 degrees, strike-slip faults at low angles to the rift trend will dominate the secondary structures; with rift angl es from similar to-20 degrees to similar to 35 degrees, there are stri ke-slip faults subparallel to the rift trend and normal faults 20 degr ees-40 degrees clockwise from the rift trend in dextral shear; above s imilar to 35 degrees, normal faults are dominant, and there are few or no strike-slip faults as secondary structures.