Extensional deformation and paleomagnetism at the western margin of the Gulf extension province, Puertecitos Volcanic Province, northeastern Baja California, Mexico

During the late Miocene east-northeast-directed extension in the Gulf of Ca lifornia extensional province, the western rift margin in northeastern Baja California, Mesico, was segmented at the northwest-striking Matomi accommo dation zone, The accommodation zone passed through the northern Puertecitos Volcanic Province and separated pre-6 Ma extension to the north from the u nextended region to the south, Pre-6 Ma northeast-sidedown displacement is documented across the accommodation zone, which may have undergone dextral oblique-slip motion. The rift margin migrated westward during the latest Mi ocene or Pliocene, bypassing accommodation-zone structures and incorporatin g the Puertecitos Volcanic Province into the region of Gulf extensional pro vince deformation. East-northeast- to east-directed extensional deformation is at least post-6 Ma in the northern Puertecitos Volcanic Province and de formation currently continues. Pliocene changes in the Gulf of California s preading center system may have triggered incorporation of the Puertecitos Volcanic Province into the Gulf extensional province as recently as 2-3 Ma, Paleomagnetic analyses of 6.3-6.6 Ma pyroelastic flow deposits show no con sistent evidence for rotational deformation, although minor (similar to 10 degrees-15 degrees) clockwise rotation is possible given anomalous declinat ion directions recorded at some sites, Comparisons with paleosecular variat ion models and the late Miocene paleopole for stable North America imply no statistically significant rotation relative to geomagnetic north, although minor (similar to 10 degrees) clockwise rotation is permissible given unce rtainties, Geologic relationships show that pre-h Ma accommodation-zone str uctures identified in this study did not mark the southern boundary of late r rotational deformation documented to the north, The boundary of Pliocene to Holocene rotations may he a broad, diffuse zone of extensional shear enc ompassing the northeastern Puertecitos Volcanic Province, which accommodate d small rotations.