SUBMARINE EMPLACEMENT OF A LATE MIOCENE VOLCANIC LANDSLIDE COMPLEX, CENTRAL BAJA CALIFORNIA PENINSULA, MEXICO

Submarine landslides are produced by failure of a submarine slope followed by gravity-driven transport of debris into deeper water. Volcanic landslides occur close to active volcanic centers and form as a direct product of volcanism or by remobilization of recently erupted material on steep volcanic slopes. Initiation, transport, and deposition of submarine landslides and volcanic landslides are well documented independently, but relatively little is known about volcanic landslides that initiate on land and are emplaced into a submarine environment. To help fill this knowledge gap, we document stratigraphic, sedimentological, and structural evidence for emplacement of a large late Miocene volcanic landslide complex into the Pacific Ocean in central Baja California, Mexico. This region has a history of late Cenozoic volcanism and sedimentation during a change from subduction of the Farallon plate beneath North America to transtensional rifting in the Gulf of California between ~ 12-7 Ma. Canyon exposures located ~35-50 km south of San Ignacio reveal a ~ 100-m thick deposit of unsorted, matrix-rich, chaotically deformed diamictite with angular to subrounded clasts of dacite and andesite ranging from pebble size to blocks > 4 m diameter, likely sourced from nearby late Miocene volcanoes in the Sierra Santa Agueda. The diamictite is interbedded with late Miocene marine deposits of the San Ignacio and Atajo formations below the 9-10 Ma Esperanza Basalt. Stratigraphic and sedimentological analyses suggest that the diamictite was catastrophically emplaced en masse onto a prograding submarine fan of uncertain water depth. The rapidly moving landslide loaded and entrained blocks of underlying water-saturated sediment to create a multi-km wide zone of chaotic soft-sediment deformation in distal and lateral portions of the landslide. Undeformed marine sediments onlap and bury irregular hummocky paleobathymetry on the upper surface of the landslide. Preliminary structural analysis of deformed marine sediments suggests a N to NW transport direction, apparently sub-parallel to the trend of the paleo-shoreline. These findings shed new light on emplacement dynamics of large-scale volcanic submarine landslides and provide constraints on late Miocene paleogeographic reconstructions of central Baja California.