CCB2026

CBX

Structural studies of the California Continental Borderlands revealed a complex tectonic terrane associated with development of the modern Pacific-North America plate boundary (e.g., Lonsdale, 1991). However, detailed studies of the submarine volcanic rocks that crop out throughout the region are limited. Many have ages constrained only broadly to the Miocene (e.g., Vedder et al., 1974). Whether these lavas originated chiefly as melts of subduction-modified mantle (Day et al., 2019) or of depleted Pacific oceanic mantle (Davis et al., 2010) is unclear. Other Borderlands topographic highs are pop-up structures (Legg et al., 2007; Rivero and Shaw, 2011). Alternately, therefore, the volcanics may be uplifted exposures of older regional strata. Evaluating these hypotheses requires new precisely located samples for petrological study. This cruise will conduct sampling across a transect of the Inner Borderlands sub-terrane, nearest the coast. High-resolution seafloor surveys of these sites while underway will establish their spatial (and therefore genetic) associations with tectonic structures. Together, these datasets will elucidate the extent to which deformation in the Borderlands during plate boundary evolution manifested magmatically. Finally, the expedition is designed as an at-sea field trip for UCSD students; it will thus also serve educational goals.