Transform faulting and plate boundary extension in the Gulf of California were not well-established until early Pliocene time, however, earlier extensional structures attest to consid- erable extension prior to the modern Gulf. This extension has been attributed either to Basin and Range extension, which was unrelated to motion along the Pacific-North American plate boundary, or to an earlier phase of Pacific-North America defor- mation within the Gulf of California region. To distinguish between these possibilities, this project will study the detailed kinematics of both Miocene and Pliocene extensional faulting near the Gulf, in the Puertecitos volcanic province of NE Baja California. Tuffs in the area will provide stratigraphic markers and timing constraints for fault displacement determinations. The results will help determine the amount and timing of exten- sion, constrain the structural configuration of the normal faults controlling the margin of the extensional province and evaluate the evidence for a change in extension direction related to the imitation of more rapid spreading in the Gulf of California in latest Miocene time.
