HR: 0800h
AN: T41A-0358    [Abstracts]
TI: Late Pleistocene-Holocene Faulting History Along the Northern El Carrizal Fault, Baja California Sur, Mexico: Earthquake Recurrence at a Persistently Active Rifted Margin
AU: * Maloney, S J
EM: sjm96@nau.edu
AF: Department of Geology, Northern Arizona University, Flagstaff, AZ 86001, United States
AU: Umhoefer, P J
EM: paul.umhoefer@nau.edu
AF: Department of Geology, Northern Arizona University, Flagstaff, AZ 86001, United States
AU: Arrowsmith, J R
EM: ramon.arrowsmith@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States
AU: Gutiérrez, G M
EM: martingg@uabcs.mx
AF: Departamento de Geología Marina, Universidad Autónoma de Baja California Sur, La Paz, BCS 23080, Mexico
AU: Santillanez, A U
EM: martingg@uabcs.mx
AF: Departamento de Geología Marina, Universidad Autónoma de Baja California Sur, La Paz, BCS 23080, Mexico
AU: Rittenour, T R
EM: tammyr@cc.usu.edu
AF: Department of Geology, Utah State University, Logan, UT 84322, United States
AB: The El Carrizal fault is a NW striking, east dipping normal fault located 25 km west of the city of La Paz, Baja California Sur, Mexico and is the westernmost bounding fault of the gulf-margin system at this latitude. The fault is ~70 km long onshore and ~50 km long offshore to the north in La Paz Bay. As many as three Quaternary geomorphic surfaces formed on the footwall and were identified on the basis of mapping and topographic profiling. In the north, the El Carrizal fault splays into multiple strands and exhibits a pattern of alternating N-S and NW-trending segments. Results from geologic mapping, paleoseismic investigations, and preliminary optically stimulated luminescence (OSL) geochronology provide some of the first numerical constraints on late Pleistocene-Holocene faulting along the El Carrizal fault. A 20 m long, 2-3 m deep trench (Trench 28) was excavated across the fault 23 km south of La Paz Bay. The trench was photographed, hand logged, and sampled for OSL dating. The trench revealed a succession of fluvial and channel deposits of sands, gravels, and cobbles. The main fault zone is manifested by a 0.5 m thick wedge-shaped deposit that consists of silty-sand and also contains rotated blocks of caliche- cemented gravels. Preliminary OSL ages from a silty-sand unit offset 2 m by the fault average latest Pleistocene. A trench 4 km south of Trench 28 (Cuadradito Trench) was also documented and sampled for OSL analysis. Preliminary OSL ages from a fluvial sand unit deposited against faulted bedrock range from mid to late Holocene. Sedimentary comparisons and surficial mapping suggest that the Holocene unit at Cuadradito Trench may be correlative to sediment that overlies faulted units from Trench 28. Such a correlation would constrain the timing of the 2 m offset at Trench 28 to be between latest Pleistocene and mid Holocene. A quarry 10 km north of Trench 28 exposes Quaternary sand and gravels buttressed against a 5-10 m wide bedrock shear zone. Here, the sediments appear to be faulted by 3 to 4 earthquakes. The two most recent events may have experienced 1-2 m of offset each. Map relationships suggest that this faulted unit is correlative to a fluvial unit near Bonfil that yields an optical age of early Holocene. This would imply that the two most recent events at the quarry are Holocene in age. Based on preliminary OSL data and total measured offset at trench sites, the slip rates are estimated to be from 0.1 to 0.5 mm/yr. In summary, earthquake recurrence and slip rate may increase towards the north along the El Carrizal fault, consistent with footwall geologic mapping. If the El Carrizal fault abides by the rules of normal fault geometry and fault offset, it is likely that the fault is at a maximum total offset offshore of San Juan de la Costa, a prediction to be tested in the summer of 2008 by offshore imaging using Compressed High Intensity Radar Pulse (CHIRP). Improving the earthquake record along the El Carrizal fault will prove beneficial for understanding the seismic hazards to the city of La Paz. Furthermore, quantifying faulting rates throughout the southwestern margin of the Gulf of California will improve our understanding of the rift-to-drift process and how that process has operated along an oblique-divergent plate margin.
DE: 8000 STRUCTURAL GEOLOGY
DE: 8036 Paleoseismology (7221)
DE: 8107 Continental neotectonics (8002)
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting