Late Miocene (12–5 Ma) extension around the edges of the Gulf of California has been alternatively attributed to “Basin and Range” extension, back arc extension, or development of the Pacific-North America plate boundary. This extension was ENE directed and similar in structural style to extension in the Basin and Range province. Timing constraints permit nearly synchronous onset of this deformation in a belt extending SSE from northernmost Baja California to the mouth of the gulf. Where this extensional faulting continued through Pliocene time to the present, synchronous with motion on the modern transform plate boundary in the Gulf of California, no change in direction of extension can be resolved. Revised constraints on Pacific-North America plate motion support the development of this late Miocene extension as a component of Pacific-North America displacement that could not be accommodated by strike-slip displacement along the existing plate boundary west of the Baja California peninsula. This scenario implies that transfer of Baja California from the North America plate to the Pacific plate was a gradual process, beginning about 12–10 Ma, when motion of the Pacific plate relative to North America was partitioned into separate regimes of strike-slip and dip-slip displacement on opposite sides of Baja California.
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