HR: 0830h
AN: T31E-0881    [PDF]
TI: San Jos\'{e} Island Accommodation Zone, Baja California Sur, Mexico: A Key to Onshore-Offshore Fault Relationships along the Western Margin of the Southern Gulf of California
AU: * Drake, W R
EM: wrd2@dana.ucc.nau.edu
AF: Northern Arizona University, Department of Geology Box 4099, Flagstaff, AZ 86011 United States
AU: Umhoefer, P J
EM: paul.umhoefer@nau.edu
AF: Northern Arizona University, Department of Geology Box 4099, Flagstaff, AZ 86011 United States
AB: The two-stage evolution of the southern Gulf of California included protogulf orthogonal rifting from $\sim$12 to $\sim$6 Ma, followed by overprinting from $\sim$6 to 0 Ma by the highly oblique-divergent, modern plate boundary. The San Jos\'{e} Island accommodation zone, located $\sim$70 km north-northwest of La Paz, Baja California Sur, represents the transition between strike-slip partitioning to the south and the oblique-divergent structural overprinting to the north. The accommodation zone includes San Jos\'{e} and San Francisquito islands, the San Jos\'{e} Canal, and a 4 to 6 km wide belt along the coast of the Baja California peninsula. The accommodation zone separates the La Paz rift segment to the south from the Timbabichi rift segment to the north. The El Carrizal fault bounds the La Paz rift segment, and likely splays onshore in the southernmost accommodation zone. Kinematic data, fault mapping, and geomorphologic and bathymetric observations along the shoreline suggest the presence of en echelon, offshore faults in the San Jos\'{e} Canal between the Baja peninsula and islands. The main faults in the southern San Jos\'{e} Canal appear to be a series of right-stepping, east-dipping normal faults branching northward from the El Carrizal fault. Based on onshore fault trends in the northern accommodation zone, the northern Canal faults form a left-stepping link to the main bounding fault of the Timbabichi rift segment. The faults bounding the western edge of the islands are likely left-stepping, west-dipping normal faults. Steep, triangular facets and cliffs characterize the western edge of San Jos\'{e} Island and suggest that the western island-bounding faults are active. The Pliocene basin, basin-bounding fault, and line of steep coastal cliffs on the eastern side of San Jos\'{e} Island are likely associated with the northern end of the Esp\'{i}ritu Santo normal fault, which experienced a major earthquake in 1995. The basin and faults also may be the termination of a series of faults related to the fracture zone emanating from the Alarc\'{o}n spreading ridge. A similar relationship between basin and accommodation zone development and the evolution of strike-slip/fracture zone systems has been demonstrated in other rifts. Formation of San Jos\'{e} and San Francisquito islands along normal faults may be a result of (1) the latest Miocene-early Pliocene tectonic reorganization to oblique rifting, and (2) active normal faulting in the San Jos\'{e} Island accommodation zone and southward.
DE: 0905 Continental structures (8109, 8110)
DE: 8109 Continental tectonics--extensional (0905)
DE: 8110 Continental tectonics--general (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting