Paleothermal structure of the Point San Luis slab of central California: Effects of Late Cretaceous underplating, out‐of‐sequence thrusting, and late Cenozoic dextral offset

Abstract

Late Cretaceous shale and sandstone turbidites of the Point San Luis “slab” are isoclinally folded, locally sheared, and faulted, but their severity of stratal disruption is relatively mild when compared to adjacent polymictic mélange of the Franciscan Complex. We tested the interpretation of a trench‐slope basin origin for these strata by documenting their paleothermal structure, including contacts between turbidites and mélange. Values of mean random vitrinite reflectance (Rm) from turbidites are 0.9–1.7% estimates of maximum paleotemperature are 135°–200°C. Mélange matrix samples yield Rm values of 1.1–2.5%, with an average of 1.5%, and peak temperatures between 160° and 240°C. The turbidite‐over‐mélange contact is locally “cooler over warmer” and was folded after peak heating. The relatively high paleotemperatures cast doubt on a shallow slope basin model (i.e., 1–2 km burial depth). We suggest, instead, that thermal maturation of the Point San Luis slab occurred much deeper in an accretionary prism (10–15 km), where offscraped trench wedge deposits were faulted against underplated mélange. The paleothermal structure was offset and tilted after peak heating by two out‐of‐sequence faults. Late Oligocene to Pliocene strata rest unconformably above the Franciscan, and there is a significant gap in thermal maturity across this unconformity, with no evidence to show that Franciscan rocks were reset thermally following the main episode of uplift and erosion. Three‐dimensional orientations of isoreflectance surfaces on opposite sides of the San Gregorio‐San Simeon‐Hosgri fault system also can be used to test interpretations of strike‐slip neotectonics. The failure to match these geometries among suspected piercing points at Point San Luis, Cambria, and Point Sur favors a suggestion that differential, post thermal peak, dextral offset of Franciscan basement has not exceeded 10–15 km.