Uplifted marine terraces and active faulting along southwestern La Paz Bay, Baja California Sur, Mexico
Buchanan, Beverly Regina
Northern Arizona University, Flagstaff, AZ, United States (USA)
The Baja California microplate is translating northwestward, in the same direction as the Pacific plate, with respect to the stable North American plate. Plate motion between these plates is distributed across three spatially and kinematically distinct fault systems: 1) the gulf axis, consisting of oceanic spreading centers and transform faults in the center of the Gulf of California, 2) the gulf margin, consisting of normal faults along the southwestern margin of the Gulf of California, and 3) the continental borderland, consisting of strike dip faults offshore along the western Baja California peninsula. Important to this study is the gulf-margin system, where seismic activity has been recorded with earthquake source mechanisms, fault scraps, and uplifted marine terraces. The purpose of this study is to use tectonic and geomorphologic data on uplifted marine terraces to constrain Quaternary slip history along the Carrizal fault footwall, enhance local-and regional-scale studies on strain partitioning, and improve the understanding of rift-to-drift transition along the oblique-divergent plate margin. The Carrizal fault is the westernmost border fault of the gulf-margin system near La Paz. It forms the main gulf escarpment in the La Paz rift segment, and separates the Baja California microplate and North American plate. The Carrizal fault trends N15 degrees W from onshore to offshore into the La Paz bay, dips east, and displays east-down displacement. Offshore, it can be traced about 60 km from the Isla San Jose accommodation zone southward to Playa el Cajete by uplifted marine terraces along the western coastline of La Paz Bay. Onshore, it can traced by a fault scarp more than 70 km from Playa el Cajete southward to Canon las Tinajas. This study focuses on the narrow, 50 km coastal strip of the Carrizal fault footwall that begins near Playa Cajete, and continues northward past San Juan de la Costa, ending near Punta Coyote. One hundred fifty-three remnants of uplifted marine terraces that cluster into four elevations are distributed along two N-S transects. The terraces are isolated, elongated blocks about 0.05-0.30 km in length, and are in sharp contrast to the Tertiary volcanic and sedimentary rocks of the Comondu Group that dominate bedrock of the study area. In addition to the four marine terraces (QIV, QIII, QII, QI), a stripped terrace immediately south and closest to the study area was surveyed. The average elevations of the terraces are 57.1, 41.0, 22.6, 16.6, and 7.2 m respectively. The terraces were dated and correlated using amino acid racemization and U-Th techniques. When dating was not possible, elevations served as the basis of differentiation. Ages were determined to be 420, 320, 128, 105, and 80 ka, and indicate a long term slip rate of 0.1 to 0.2 m/ka along the offshore Carrizal fault since 420 ka. Elevations of marine terrace deposits and the San Juan tuff marker bed displays typical topographic and structural, arch-style geometry of a normal fault, where the center of the fault offshore San Juan de la Costa is older and more uplifted than the younger outer lengths expressed to the north and south away from San Juan de la Costa. The overall faulting patterns and rates in the La Paz region indicate that slip may be transferring eastward along the Partida-Espiritu Santo and Cerralvo faults (as determined by earthquake source mechanisms) toward the main plate boundary.