Speaker: Dr. Mark R. Legg, Legg Geophysical, 16541 Gothard Street, Suite 107, Huntington Beach, CA 92647, mrlegg@verizon.net
Topic: MIOCENE OBLIQUE RIFTING OF THE SAN DIEGO TROUGH REGION, CALIFORNIA CONTINENTAL BORDERLAND, by LEGG, Mark R., Legg Geophysical, 16541 Gothard Street, Suite 107, Huntington Beach, CA 92647, mrlegg@verizon.net, NICHOLSON, Craig, Marine Science Institute, University of California, Santa Barbara, CA 93106-6150, and SORLIEN, Christopher C., Institute for Crustal Studies, University of California Santa Barbara, Santa Barbara, CA 93106
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Abstract:
Northwest oblique rifting and rotation of the Western Transverse Ranges (WTR) crustal block created the Inner-Borderland Rift during Miocene development of the Pacific-North America plate boundary. The San Diego Trough region occupies the southern third of this segmented continental rift, where tilted crustal blocks and volcanic highland rift geometry is still preserved. Two major phases of oblique rifting are recognized. First, subducted lower crust and upper mantle rocks (Catalina Schist) were rapidly exhumed along the Thirtymile Bank and related detachment faults. San Onofre Breccia along the mainland coast and offshore islands records this initial rifting. Mid-Miocene volcanic flows, volcaniclastic and other sedimentary sequences covered the exposed detachment at the seafloor of the widening rift. A granodiorite pluton on Santa Catalina Island, ~19 Ma, records early volcanism within the rift, whereas widespread volcanic rocks, ~16-18 Ma, record a major magmatic episode. A volcanic archipelago developed along the rift axis defined by a suite of major calderas stretching from Santa Catalina Island through Catalina Basin to the Fortymile Bank and Navy Bank volcanic highlands. The Glendora and Conejo volcanic complexes of the WTR were part of this initial Miocene volcanic province, but subsequently transrotated away from the rift axis. A second extensional phase, possibly associated with later oblique rifting of the Outer Borderland block, disrupted the original detachment surfaces within the rift by high-angle faulting to deeper levels of detachment creating horst block ridges like Thirtymile Bank and adjacent half-graben basins like the San Diego Trough. The Thirtymile Bank detachment fault was further segmented by high-angle faulting near the coast as the Coronado Bank block was westward-tilted and separated from the mainland. Mid-Miocene and older rocks on Islas Los Coronados are tilted from 25° to 35° down to the west like correlative strata imaged within the upper Coronado Bank block. Basalt flows that bracket San Onofre Breccia sequences within the Rosarito Beach basin and derived from the Thirtymile Bank uplift, constrain timing of this later stage to post 16.1-15.5 Ma. As rifting ceased, the late Miocene San Diego Trough region was further sheared by major high-angle right-slip fault zones.
Speaker Information:
Dr. Legg received a B.S. in Space Sciences & Mechanical Engineering from the Florida Institute of Technology in 1973. At F.I.T, he also studied Physical Oceanography for the M.S. program. Dr. Legg continued his graduate studies in Oceanography at the Scripps Institution of Oceanography, with a National Science Foundation Graduate Fellowship, where he received his M.S. in 1980. In 1985, Dr. Legg completed his Ph.D., Geological Sciences, at the University of California, Santa Barbara. His post-graduate research focused upon the geology and seismotectonics of the California Continental Borderland, with detailed studies of the inner borderland west of San Diego and northern Baja California. While a graduate student, Dr. Legg conducted earthquake hazard and risk analysis research for the J.H. Wiggins Company, and helped the California Division of Mines & Geology map faults and earthquakes in the borderland. After the Ph.D., Dr. Legg expanded his marine seismic exploration skills at the Amoco Tulsa Research Center. Since 1988, Dr. Legg returned to Southern California to continue his offshore faulting and earthquake hazards studies. Recent offshore faulting and earthquake research efforts have included submersible dives (Alvin, Turtle, and Delta) to study borderland faults directly, multibeam swath bathymetry mapping of seafloor structure, and acquisition, processing, and interpretation of MCS and single-channel seismic profiles throughout the borderland. He continues to educate the larger community regarding the seismic hazards of the borderland including the potential for locally-generated tsunamis through seminars and publication. As President of Legg Geophysical, a small consulting firm that specializes in Earth Sciences and Risk Analysis, Dr. Legg is involved in many diverse projects relating to both natural and man-made hazards funded by both government and commercial agencies. He is also a participating scientist with the Southern California Earthquake Center, and adjunct professor at San Diego State University, and a Visiting Assistant Research Geophysicist at UC Santa Barbara. His collaboration with scientists at Oregon State University and UC Santa Barbara to produce more accurate maps of the Borderland bathymetry for fault and tectonic studies led to the discovery of the large crater structures to be discussed. Most recently, Dr. Legg's work focuses on obtaining high-resolution multichannel seismic reflection images of active tectonic structures in the California Continental Borderland, so that more accurate and precise understanding of the regional tectonic evolution may be achieved.
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