NEW OBSERVATIONS ABOUT THE STRUCTURAL ARCHITECTURE OF THE GULF OF CALIFORNIA RIFT MARGIN IN BAJA CALIFORNIA SUR: LESSONS LEARNED FROM PAUL UMHOEFER’S LEGACY

Much of our knowledge about the architecture of the Gulf of California (GOC) oblique rift margins and structural controls on sedimentary basins was generated through significant advances by Paul Umhoefer and teams of colleagues and students he led. Inspired by Paul’s legacy, we use structural and geologic mapping to characterize the structural architecture of the GOC rift margin in eastern Baja California Sur. At the SW margin of the Miocene-Pleistocene Santa Rosalía basin (SRB), bedrock canyons expose structures that are elsewhere concealed by marine sediments and the ~1 Ma Reforma Tuff. In Arroyo Infierno, a pre-basin sequence of tuff breccia and volcaniclastic strata transitions up-section to mafic lava flows and capping felsic flows. Pending geochemistry and Ar/Ar geochronology will correlate these units to either pre-rift Comondú Group (Umhoefer et al., 2001, Sed. Geol.) or younger rift-transition volcanic units, or both. These volcanic rocks and early SRB sediments are deformed by a ~5 km-wide, ~N-striking extensional monocline cut by W- and SW-dipping normal faults. E tilting across the monocline, toward the GOC, was the major local mechanism of subsidence in the adjacent SRB during deposition of upper Miocene Boleo Formation. We infer that the monocline formed above a NE-dipping, upward-propagating normal fault. Tilting toward the GOC was also observed by Paul and his students near Loreto ~180 km to the SE (Willsey et al., 2002, J. Struc. Geol.), similar to reported E tilting of the Concepción Peninsula (100 km to SE) and the Puertecitos region (400 km to NW). In Arroyo de las Palmas, we map an E-dipping normal fault with Cretaceous crystalline rocks in the footwall and marine sediments in the hanging wall that thicken abruptly across the fault. These major basin-bounding rift structures are located 5-10 km NE of the toe of the NE-facing main topographic escarpment, similar to observations by Paul and colleagues near Loreto (Umhoefer et al., 2002, GSAB). Other NW-striking normal and dextral-oblique faults with only 10’s of m of vertical offset are located up to 50 km SW of the topographic escarpment and are not part of the main GOC margin. Spatial separation of major rift structures and the topographic escarpment is the result of headward erosion and lateral escarpment retreat prior to 1-Ma emplacement of the Refor_­ma Tuff.