SHORTENING WITHIN UNDERPLATED OCEANIC-CRUST BENEATH THE CENTRAL CALIFORNIA MARGIN

Structures within a tectonically underplated layer of early Miocene age oceanic crust indicate that the lower crust of offshore central California has been deformed under compression prior to the inception of the San Andreas transform system along the continental margin. Shortening in the lower crust is inferred from the geometry of two structures, the Santa Lucia monocline and the Santa Maria antiform, delineated by an integrated interpretation of multichannel and wide-angle seismic reflection data in conjunction with regional gravity data and teleseismic receiver functions. The 700 km of multichannel seismic data and 300 km of wide-angle seismic data from the 1986 Pacific Gas and Electric/EDGE seismic experiment form the primary data base used to establish the northwesterly trend and extent of the monocline and the WNW trend of the antiform. In cross-sectional view, the Santa Lucia monocline is a 30-degrees kink in the oceanic crust, while the Santa Maria antiform is characterized by local thickening of the oceanic crust. These structures are probably the result of convergence between the Monterey and Arguello microplates that took place as subduction along the margin waned. The multichannel seismic reflection data image two Miocene age shelf basins, the Santa Lucia and offshore Santa Maria basins that formed within a transtensional stress regime. The offshore Santa Maria basin directly overlies the Santa Maria antiform, suggesting that either the upper and lower crustal stress regimes were completely decoupled in Miocene time or that the two features formed in different places. Differential slip between the upper crust and lower crust may have been accommodated along a detachment zone located above oceanic crust. Convergence within the oceanic crust and apparent differential motion between upper and lower crust are indicative of the processes that can affect the transition from subduction to an oceanic-continental transform zone.