LATE CENOZOIC TECTONICS OF THE CENTRAL AND SOUTHERN COAST RANGES OF CALIFORNIA

The central and southern Coast Ranges of California coincide with the broad Pacific-North American plate boundary. The ranges formed during the transform regime, but show little direct mechanical relation to st rike-slip faulting. After late Miocene deformation, two recent generat ions of range building occurred: (1) folding and thrusting, beginning ca, 3.5 Ma and increasing at 0.4 Ma, and (2) subsequent late Quaternar y uplift of the ranges. The ranges rose synchronously along the centra l California margin and are still rising; their long axes are quasipar allel to the plate boundary and strike-slip faults. The upper crustal internal and marginal structures of the ranges are contractional, domi nated by folds and thrusts resulting from the convergent component of plate motion. New;ly constructed transects using seismic reflection an d refraction, plus gravity and magnetic studies, reveal lower crustal basement(s) at depths of 10-22 km, The upper surface of the basement a nd Moho show no effect of the folding and thrusting observed in the up per dust. We conclude that horizontal shortening is accommodated at de pth by slip on subhorizontal detachments, and by ductile shear and thi ckening, The ranges are marked by high heat flow; weak rocks of the Fr anciscan subduction complex; high fluid pressure; bounding high-angle reverse, strike-slip, or thrust faults; and uplift at a rate of 1 mm/y r beginning about 0.40 Ma, Transverse compression manifested in foldin g within the Coast Ranges is ascribed in large part to the well-establ ished change in plate motions at about 3.5 Ma.