Translation of terranes: Lessons from central Baja California, Mexico

Baja California is one of the best modern examples of a continental block, or terrane, that is translating hundreds of kilometers along a highly obliquely divergent plate. The central domain of Baja California is particularly appropriate as a modern analogue for terrane translation because it has a relatively simple late Neogene history and is a buoyant continental block that is likely to be preserved in any future accretion to the North American continent. In contrast, the deep, dense oceanic crust of the Gulf of California, where the main transform faults of the plate boundary lie, has little preservation potential. It is surprising that there is little strike-slip faulting in the central domain of Baja California, even though the plate boundary has a rift angle (angle between rift trend and azimuth of plate motion), α, of only 20°. Recent field data confirm modeling studies of oblique rifting and show that there is a predictable change in fault type, fault orientation, and extension direction with changing rift angle. That is, from secondary structures alone, one can predict the approximate obliquity of rifting and the orientation of the plate motion. With rift angles of 0°-20°, strike-slip faults at low angles to the rift trend will dominate the secondary structures; with rift angles from -20° to -35°, there are strike-slip faults subparallel to the rift trend and normal faults 20°-40° clockwise from the rift trend in dextral shear; above -35°, normal faults are dominant, and there are few or no strike-slip faults as secondary structures.