Late Cenozoic Fold and Thrust Belt of the Southern Coast Ranges and Santa Maria Basin, California (1)

Abstract

Balanced cross sections across the Santa Maria basin and southern Coast Ranges of California show the Pliocene to Quaternary convergence to be a developing basement-involved fold and thrust belt. The fold and thrust belt is seismically active as evidenced by compressive earthquakes, geodetic measurements of present shortening, and folded Quaternary deposits. The Pliocene and Quaternary structure of the southern Coast Ranges is dominated by a series of large anticlinal structures that include the Lompoc-Purisima trend, the Casmalia-Orcutt trend, the Point San Luis anticline, the Santa Lucia Range anticlinorium, and the La Panza Range anticlinorium. The anticlinal trends are interpreted to be fault-bend and fault-propagation folds resulting from thrust ramps off thrust flat and a regional detachment at 11-14 km depth. Most of the thrust faults do not reach the surface (blind thrust). The southwestern range front of the San Rafael Mountains is interpreted to be the result of uplift above a ramp in the Point San Luis thrust. The cross section across the southern Coast Ranges shows 26.8 km of shortening from the edge of the continental margin to the San Andreas fault. Region-wide shortening is interpreted to have begun between 2.0 and 4.0 Ma, which yields an average regional convergence rate of 6.7-13.4 mm/yr. The late Pliocene and Quaternary convergence across the onshore Santa Maria basin is interpreted to be 9.2 km and the average convergence rate is 2.3-4.6 mm/yr. Compressive focal mechanisms and our structural analysis suggest that the ramp parts of the thrust system are the most likely sources for earthquakes. The historic seismic record and length of the anticlinal trends suggest that the thrust ramps probably are capable of generating moderate to large earthquakes (5.0<M[W]<7.5). Major thrust ramps underlie the city of San Luis Obispo and adjacent coastal towns. Cross section restoration reveals early formed hydrocarbon traps and accounts for the abundant hydrocarbon accumulations along the Casmalia-Orcutt anticlinal trend and under the Santa Maria Valley. The cross sections and restorations also illustrate the importance of Miocene and early Pliocene normal faults to oil maturation and trapping. Concealed normal faults along the flanks of the major anticlinal trends and the subthrust area along the north side of the Casmalia-Orcutt anticlinal trend form two relatively untested trap styles.