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Masters Thesis

Three-dimensional tectonostratigraphic development of the Los Angeles basin as viewed through the Beta 3D seismic survey

The Beta 3D reflection seismic survey used for this study includes the structurally complex southwestern, or offshore, extension of the Los Angeles basin and includes the Beta oil field. Areally, this survey covers 94 square miles (244 Km2) and extends from south of the Palos Verdes uplift and fault northward to the THUMS-Huntington Beach fault. In addition to this seismic survey, other unpublished Beta oil field data including processed dipmeter data, velocity checkshot surveys, and several wireline logs were used to interpret the tectonostratigraphic evolution of the area. Within the study area, four distinct phases of deformation were identified using 3D time interpretation, 2D depth interpretation, and 2D forward modeling. Pre-Miocene structures within basement rocks represent the oldest phase of deformation, or "So". It is best illustrated in the southeast portion of the study area. This deformation represented by "So" is poorly understood, but appears to represent a period of uplift related to pre-Miocene oblique subduction. The second phase, or transtensional deformation "Sl'', commenced about 24 Ma(� 2Ma) and ended at 12Ma. This phase of "S1" is marked by the development of listric faults with domino-style faults linking to a common detachment. Two-dimensional forward modeling places this detachment at 3.0 miles (5.0 Km). This "Sl'' extension appears to be temporally coincident with Basin and Range extension and is thought to be genetically related to it. The synorogenic sediments associated with "Sl" probably represent the early Miocene San Onofre Breccia. The end of "S1" is contemporaneous with the termination of mid-Miocene calc-alkaline volcanism and the initiation of clockwise rotation of micro-plates within southern California. The third phase "S2", is split into two sub-phases (S2E) and (S2L). Tectonism is dominated by right-lateral strike-slip faulting throughout "S2". The early phase (S2E) is characterized by the initiation of strike-slip (Palos Verdes fault) and normal faulting and is best exhibited in the southern region of the study area. The latter phase of "S2" (S2L) is coeval with the initiation of the southern San Andreas system and the opening of the Gulf of California. Undeforn1ed sands representing early Pliocene Repetto Formation document the end of (S2L) phase. Even though they appear undeformed, large-scale channels as much as 2 km across formed during this time and may have been structurally controlled. These channels are truncated by the northwest-striking Palos Verdes fault. Small antifom1s are found internally within the Palos Verdes fault zone and appear to have fom1ed due to restraining bends along the fault, suggesting that right-lateral movement continued during (S2L). The final tectonic phase "S3" involves compression resulting from the Pasadenan orogeny. This deformation is seen within the area by the inception of the Pliocene/Pleistocene unconformity. To the southeast, this deformational event is seen cross-cutting the Palos Verdes fault, indicating the end of both right-lateral and oblique movement along this fault.

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