HR: 08:00h
AN: S31A-01 INVITED     [Abstracts]
TI: The Kinematic Puzzle of the Gulf of California Rift system
AU: * Stock, J M
EM: jstock@gps.caltech.edu
AF: Seismo Lab, Caltech 252-21, Pasadena, CA 91125, United States
AB: Extensive recent studies of the Gulf of California greatly advanced our understanding of the kinematics of this Pacific-North America plate boundary rift and its crustal structure. However, some kinematic discrepancies that need to be resolved, particularly for the time interval 12-6 Ma, after subduction had stopped but possibly before major opening began in the northern Gulf basins. Correlation of pyroclastic flow deposits across the Upper Delfin Basin segment (Oskin et al., 2001 and subsequent papers) indicates that virtually all of the opening between the coastlines in this segment (from San Felipe/Puertecitos in Baja California to the W side of Tiburon Island) occurred since ~6.1 Ma producing ~250 km of opening of the marine basin. This implies that the crust in the basin should have been brought into the region since ca. 6 Ma, perhaps by production of new igneous crust or remobilization of continental crust to fill the ~250-km gap. The total amount of post-6 Ma opening in the rift, including the onland deformation, is consistent with expected Pacific-North America displacement determined from the global plate circuit since 6 Ma, and it is also consistent with the slip history of the southern San Andreas fault. A smaller amount of motion (tens of kms) can be identified geologically post-12.5 Ma and pre-6 Ma. However, this is not sufficient to match the plate circuit results, which seem to require several hundred more km of Pacific-North America plate motion at this latitude between 12 Ma and 6 Ma. This motion has to have been located elsewhere, not between the modern marine basin boundaries. The Pliocene basin history of the Gulf has varied from place to place, as the loci of extension moved around in the rift system Similar variations in late Miocene time could explain this discrepancy, with abandoned extensional or strike-slip fault systems elsewhere, perhaps in the Sonoran coastal plain. However, further geological and geophysical work is needed to characterize this area and see if such a model is feasible and whether faults can be found that will link kinematically northward to structures of the same age, east of the San Andreas fault. A related discrepancy comes from reports of middle Miocene microfossils in marine sediments within these young northern Gulf basins. If these microfossils are primary, then the 250 km of opening would have a longer history, but it would not change the fact that some slip is missing and must be found elsewhere. In addition, the extensive volcanism in the Puertecitos Volcanic Province and Delfin Basin region since 12.5 Ma should have left a record in the marine sediments. Ash and pumice from these eruptions have been found in the onland exposures of the Gulf marine deposits (Puertecitos Fm, San Felipe marine sequence). Similar evidence should be sought in any future drill holes into the marine section.
DE: 7218 Lithosphere (1236)
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8105 Continental margins: divergent (1212, 8124)
SC: Seismology [S]
MN: 2007 Joint Assembly