At convergent plate boundaries, subduction can induce back arc rifting, creating a microplate between the subduction trench and the back arc spreading ridge. If subduction stops while rifting continues, this microplate becomes detached from the over-riding plate and can become part of the formerly subducting plate. In oceanic-continental convergent zones this process can reduce the size of continental lithosphere and can create exotic terrains if the continental microplate is obducted back onto the continent. However, the mechanics of this process are poorly understood because the lithospheric plates involved in the process often have complicated histories.
The Baja California microplate is a continental plate fragment that was originally part of North America but subsequently “captured” by the Pacific plate over a time span of ~10-15 Ma. Baja California is an ideal case to study microplate dynamics because (1) its motion relative to both North America and to the Pacific plate are relatively simple and well characterized since ~12 Ma, (2) much of the Baja California peninsula is a structurally intact block that has experienced only minor late Cenozoic deformation, and (3) it is now completely severed from North America with a distinct plate boundary in the Gulf of California. Hence, its microplate kinematics is well known and it displays relatively rigid “plate-like” behavior. 
Using seismic reflection/refraction data collected in 2001, we will explore the origin of the Baja microplate by constraining the crustal and upper mantle structure of the microplate and underlying slab(s). These datasets are from an experiment designed to examine the structure of the Baja microplate. We have a several hundred km-long multichannel seismic (MCS) reflection profile and a wide-angle seismic refraction profile along the southwestern margin of the Baja Peninsula. Neither dataset has been fully analyzed and may yield exciting results. The wide-angle data will provide constraints on the deep crustal and upper mantle structure. Analysis will include ray-tracing, travel-time inversion and seismic tomography. The MCS data will provide constraints on the upper crustal structure and allow us to image faults along the continental margin. 
With these complementary datasets, we will address the following scientific objectives. (1) Characterize the nature and importance of coupling between Pacific microplates and the Baja California microplate. Two main possibilities exist. The subducted Pacific microplates may still be present the Baja microplate. Alternatively, the Pacific microplates may have broken very near their spreading centers, which lie near the paleo trench, forming "slab windows". Using seismic reflection and wide-angle refraction data we can determine whether or not a stalled Cenozoic slab exists beneath Baja California, or whether it simply ends near the paleo-subduction zone. (2) Describe the evolution of coupling between the Pacific plate and the Baja California microplate as the latter was transferred from the North American to the Pacific plate. We will accomplish this by integrating our seismic observations with geological and geochemical information.