The San Luis Range, a prominent west-northwest-trending topographic and structural high along the coast of south-central California, is one of a series of elongated structural blocks in the Los Osos/Santa Maria (LOSM) domain. The range is uplifting as a relatively rigid crustal block along bordering northwest-striking reverse faults. Altitudes and ages of marine terraces show that the range is uplifting at rates of between 0.12 and 0.23 m/kyr, with little or no internal deformation. Major geologic structures within the range, including the Pismo syncline and the San Miguelito, Edna, and Pismo faults, do not deform Quaternary deposits or landforms and are not active structures in the contemporary tectonic setting.
The northeastern margin of the range is bordered by the Los Osos fault zone, a southwest-dipping reverse fault that separates the uplifting San Luis Range from the subsiding or southwest-tilting Cambria block to the northeast. The fault zone has had recurrent late Pleistocene and Holocene displacement at a long-term slip rate of 0.2 to 0.7 mm/yr. Uplift of the range is accommodated, entirely or in part, by displacement along this fault zone.
The southwest margin of the San Luis Range is bordered by a complex system of late Quaternary reverse faults that separates the range from the subsiding Santa Maria Basin to the southwest. The fault system includes the Wilmar Avenue, San Luis Bay, Olson, Pecho, and Oceano faults, all of which dip moderately to steeply to the northeast. The cumulative net dip-slip rate of displacement for this system of faults ranges from about 0.16 to about 0.30 mm/yr. Slip rates on individual faults generally range from 0.04 to 0.11 mm/yr.
We infer that the style and rates of deformation occurring within and bordering the San Luis Range are representative of the style and rates of deformation occurring elsewhere in the LOSM domain. Crustal shortening in the domain is accommodated primarily by reverse faulting along the margins of structural blocks and by uplift, subsidence, or tilting of the blocks. In the southern and southeastern parts of the domain, crustal shortening also may be accommodated by active folding and thrust faulting. The west-northwest structural grain and tectonic style within the LOSM domain is unique in the south-central coastal California region, and is transitional between the west-trending structural grain of the western Transverse Ranges and the north-northwest-trending grain of the Santa Lucia and San Rafael Ranges. We interpret that Quaternary deformation within the domain is related to transpression along the North America/Pacific plate margin, renewed late Cenozoic clockwise rotation of the western Transverse Ranges, and convergence of the domain against the relatively stable Salinian crust that underlies much of the Santa Lucia and San Rafael Ranges to the northeast.
