Structural analysis of the Queenie structure, a topographically prominent, southwest vergent asymmetric fold located 35 km west of Point Sal, constrains the timing and nature of late Neogene deformation in a transect across the central offshore Santa Maria Basin. Analysis of post-Miocene sediments mantling the fold indicates that the Queenie structure was formed in a relatively brief episode of NE-SW directed shortening between 5 and 3 Ma, apparently in response to the onset of compression normal to the North American/Pacific plate boundary. Retrodeformation of depth-corrected cross sections constructed from an extensive set of common depth point seismic data demonstrates that the Queenie structure overprints a Miocene extensional basin and is a fault propagation fold that formed as compression changed the normal slip on the basin margin shear zone to reverse slip. Geometric constraints and seismic data suggest that the controlling shear zone dips moderately (30¿ to 50¿) to the northeast and is essentially nonlistric to the base of the brittle crust at a depth of about 12 km.

Structural analysis of cross sections further shows that average total post-Miocene NE-SW shortening across the Queenie structure is about 3% (0.39 km across the 13-km-wide zone of folding) and that related shortening across the 40-km-wide central reach of the offshore Santa Maria Basin between the Hosgri fault and the Santa Lucia Bank fault during the same period is roughly 1--2% (0.4--0.8 km). High-resolution seismic reflection data show small folds (structural relief less than 10% of the primary Queenie structure) in post-early/late Pliocene sediments on the southwestern flank of the Queenie structure. These folds exhibit minor coaxial deformation along the flank of the structure, suggesting that very low rates of tectonic folding (of the order of 0.005 mm shortening per year) may continue to the present. The transient nature of the Pliocene compressional event that formed the primary Queenie structure and the relatively low rates of recent seismic activity in the central offshore Santa Maria Basin suggest that post-early/late Pliocene interplate shortening at this latitude has been largely accommodated outside the basin.