GEOLOGIC STRUCTURE AND TECTONICS OF THE INNER CONTINENTAL BORDERLAND OFFSHORE NORTHERN BAJA CALIFORNIA, MEXICO (WRENCH FAULTS, SAN DIEGO, ENSENADA)

The Inner Continental Borderland west of northern Baja California, Mexico, is an active part of the geologically complex Pacific-North American lithospheric plate boundary. To further understand the late Cenozoic tectonic style and evolution of this continental margin plate boundary, it is necessary to study the geologic structure and tectonic style of the California Continental Borderland. This dissertation presents a relatively complete, marine geological study of the Inner Continental Borderland. Detailed bathymetric maps prepared from both conventional single beam, and multi-, narrow-beam (Sea Beam) echo sounding data show that the Inner Borderland is a regionally distinct physiographic province. Numerous tectonic landforms commonly associated with recently-active faults onshore are shown by the detailed, Sea Beam bathymetric charts made along the San Clemente fault. High-resolution, single-channel, seismic reflection profiles also show that the region is crossed by numerous, late Cenozoic faults which are associated with four principal wrench fault zones: (1) San Clemente-San Isidro; (2) San Diego Trough-Bahia Soledad; (3) Coronado Bank-Agua Blanca; (4) Rose Canyon and Descanso-Estero. Estimates of average late Quaternary sedimentation rates, based upon published piston core analyses, and seismic stratigraphy are used to infer late Quaternary ages for fault activity and submarine canyon/fan development. Right-slip of several kilometers along the major faults is inferred from postulated offset channels and submarine fan/slope apron deposits. The major fault zones are further grouped into two, major wrench fault systems, i.e., San Clemente and Agua Blanca. Each is similar in configuration to the well-known San Andreas and Alpine-Marlborough faults, respectively. Systematic variations in fault character both along strike and at depth in these fault zones are attributed to changes in fault geometry, and/or reorientation of principal strain and inferred stress directions in the region. These variations are inferred to demonstrate that the Inner Borderland is a regionally distinct structural province and that the entire southern California-northern Baja California region may be considered as a broad shear zone associated with transform fault tectonics of Pacific-North American plate interaction.