Left-Shear and N-S Contraction In Monterey Formation, Northwestern Santa Barbara Channel, California: Structural Evidence In Support of Neogene Vertical Axis Rotation

The Western Transverse Ranges (WTR) show predominantly east-striking fault and fold trends in contrast to the northwest-striking faults and folds (~San Andreas parallel) in the rest of coastal California. Active structural deformation in the WTR shows a mix of N- or S-vergent contractional faulting and left-lateral strike-slip. In 1980, Luyendyk et al. reported on paleomagnetic data showing 70-90^o clockwise declination anomalies in Neogene rocks of the WTR; an interpretation of rotating panels bounded by left-silp faults was proposed as a kinematic model for the vertical axis rotation. In 1988, Namson and Davis estimated 25-35 km of latest Neogene shortening across the WTR using balanced cross-section construction; an interpretation of detached, dip-slip thrust imbrication was proposed as a kinematic model for the shortening. Can these two contrasting kinematic views be reconciled?

An extensive (12 x 40 km) 3D seismic data set located in the northwestern Santa Barbara Channel provides excellent detail on the deformation of the Miocene Monterey Formation. These data reveal early deformation that supports the clockwise rotation and late deformation that supports the N-S contraction. The top Monterey stratigraphic horizon describes an east-trending, north-vergent anticline. Early deformation is best shown by a corridor of several discrete east-trending, relay overlapping faults that extends as a semi-continuous structural zone along the northern margin of the anticline. Late deformation is best shown by a (contractional) growth syncline. North of the fault corridor, the top Monterey dips steeply (35-55^o) northward. South of the fault corridor, the Monterey dips gently (5-10^o) south-southeastward across a region of distributed small faults and discrete, northwest-trending folds. On the basis of throw, faults can be divided into: 1) northeast-striking normal-separation faults, 2) northwest-striking reverse-separation faults, and 3) east-striking mixed-separation faults. Fault offsets die out up-section in the Mio-Pliocene Sisquoc. This pattern indicates distributed Miocene to Early Pliocene left-shear.