HR: 0800h
AN: T41A-0357    [Abstracts]
TI: Late Quaternary Faulting along the San Juan de los Planes Fault Zone, Baja California Sur, Mexico
AU: * Busch, M M
EM: melanie.busch@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, United States
AU: Coyan, J A
EM: joshua.coyan@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, United States
AU: Arrowsmith, J
EM: ramon.arrowsmith@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, United States
AU: Maloney, S J
EM: Sara.Maloney@nau.edu
AF: Department of Geology, Northern Arizona University, Flagstaff, AZ 86011, United States
AU: Gutierrez, G
EM: martingg@uabcs.mx
AF: Departamento de Geologia Marina, Universidad Autonoma de Baja California Sur, La Paz, BCS 23080, Mexico
AU: Umhoefer, P J
EM: paul.umhoefer@nau.edu
AF: Department of Geology, Northern Arizona University, Flagstaff, AZ 86011, United States
AB: As a result of continued distributed deformation in the Gulf Extensional Province along an oblique-divergent plate margin, active normal faulting is well manifest in southeastern Baja California. By characterizing normal-fault related deformation along the San Juan de los Planes fault zone (SJPFZ) southwest of La Paz, Baja California Sur we contribute to understanding the patterns and rates of faulting along the southwest gulf-margin fault system. The geometry, history, and rate of faulting provide constraints on the relative significance of gulf-margin deformation as compared to axial system deformation. The SJPFZ is a major north-trending structure in the southern Baja margin along which we focused our field efforts. These investigations included: a detailed strip map of the active fault zone, including delineation of active scarp traces and geomorphic surfaces on the hanging wall and footwall; fault scarp profiles; analysis of bedrock structures to better understand how the pattern and rate of strain varied during the development of this fault zone; and a gravity survey across the San Juan de los Planes basin to determine basin geometry and fault behavior. The map covers a N-S swath from the Gulf of California in the north to San Antonio in the south, an area ~45km long and ~1-4km wide. Bedrock along the SJPFZ varies from Cretaceous Las Cruces Granite in the north to Cretaceous Buena Mujer Tonalite in the south and is scarred by shear zones and brittle faults. The active scarp-forming fault juxtaposes bedrock in the footwall against Late Quaternary sandstone-conglomerate. This ~20m wide zone is highly fractured bedrock infused with carbonate. The northern ~12km of the SJPFZ, trending 200°, preserves discontinuous scarps 1-2km long and 1-3m high in Quaternary units. The scarps are separated by stretches of bedrock embayed by hundreds of meters-wide tongues of Quaternary sandstone-conglomerate, implying low Quaternary slip rate. Further south, ~2 km north of the Los Planes highway, the fault steps to the right 2km with no overlap. The fault is inactive until ~3km south of the Los Planes highway where scarp heights in the Quaternary sediments rise to ~3-11m for ~11km with an average trend of 160°, implying increasing slip rate. The fault then steps left 2km with no overlap, trending 145°. Scarp heights range from 3-6m in the step. The southernmost 9km of the fault zone, trending 200°, is marked by discontinuous scarps and embayed bedrock, reflecting diminished fault activity. The footwall landscape in this area is characterized by a broad, gently-sloping, low-relief pediment surface with thin Quaternary cover, disrupted by inselberg-like hills. The young scarp-forming fault appears to have reactivated older faults to rupture this pediment, reflecting the episodic nature of slip along this fault zone. Preliminary OSL ages of the youngest faulted deposit imply a Late Pleistocene-Holocene slip rate of 0.1-1mm/yr. The SJPFZ is thus characterized by reactivation of pre-existing faults to rupture a pre-existing low relief erosional landscape. Whereas the entire region might have experienced the quiescent period that allowed for development of the low- relief, stable surface along the SJPFZ, we speculate that while the SJPFZ was dormant, other faults within the gulf-margin system were actively accommodating strain.
DE: 8002 Continental neotectonics (8107)
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8107 Continental neotectonics (8002)
DE: 8175 Tectonics and landscape evolution
DE: 9350 North America
SC: Tectonophysics [T]
MN: 2007 Fall Meeting