2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 5
Presentation Time: 2:45 PM


DRAKE, William R., Department of Geology, Northern Arizona University, Box 4099, Flagstaff, AZ 86011 and UMHOEFER, Paul, Department of Geology, Northern Arizona University, Flagstaff, AZ 86011, wrd2@dana.ucc.nau.edu

The Gulf of California is an active, oblique-divergent plate boundary. New fault kinematic data and mapping in key areas between Loreto and La Paz offer insight into the structural evolution of Baja California Sur and the southern Gulf of California. The eastern margin of Baja California Sur is characterized by structurally simple, NNW-striking rift segments linked by structurally complex accommodation zones. North to south, the Loreto, Timbabichi, and La Paz rift segments are separated by the Agua Verde and San José Island accommodation zones, respectively. The San José Island accommodation zone (SJIaz) has complexly tilted strata and alternating sets of dominantly NE- and NW-striking normal faults with offsets ranging from 10s to 100s of meters. Kinematic analysis of SJIaz suggests the overprinting of orthogonal protogulf rift structures by transtensional structures associated with the modern plate boundary. Similar overprinting has been documented to the north, in the Loreto area. In the SJIaz and near Loreto, faulting has largely ceased onshore and moved to near-offshore locations, and mainly to belts of active faults along and east of the large islands. In contrast, recent seismicity and fault data in the Bay of La Paz suggest little or no transtensional overprinting by transtensional structures south of the SJIaz. Southeast of La Paz is dominated by NNE-striking normal faults and the ~N-S rift escarpment of the Cabo rift segment. Preliminary analysis of new bathymetric data suggests that there is a major boundary running from the SJIaz southeastward to Cerralvo Island and through the Cerralvo trough to the Tamayo fracture zone; the 20-km wide boundary separates the greater La Paz – Cabo block from the region to the north. Fault data and bathymetric and fault patterns suggest that the La Paz and Cabo rift segments have together experienced a structural evolution with regional strain partitioning and related to the East Pacific Rise, while SJIaz northward to Loreto acted as another block with little or no strain partitioning and related to the Alarcón, Pescadero, and Farallon spreading ridges. Therefore, the San José accommodation zone likely represents a transition between 1) contrasting ages and patterns of faulting, 2) contrasting amounts of strain partitioning, and 3) broadly defined tectonic blocks.