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

Late Cretaceous shale and sandstone turbidites of the Point San Luis "slab" are isoclinally folded, locally sheared, and faulted, but their severity o f stratal disruption is relatively mild when compared to adjacent polymicti c melange of the Franciscan Complex. We tested the interpretation of a tren ch-slope basin origin for these strata by documenting their paleothermal st ructure, including contacts between turbidites and melange. Values of mean random vitrinite reflectance (R-m) from turbidites are 0.9-1.7%; estimates of maximum paleotemperature are 135 degrees -200 degreesC. Melange matrix s amples yield R-m values of 1.1-2.5%, with an average of 1.5%, and peak temp eratures between 160 degrees and 240 degreesC. The turbidite-over-melange c ontact is locally "cooler over warmer" and was folded after peak heating. T he relatively high paleotemperatures cast doubt on a shallow slope basin mo del (i.e., 1-2 km burial depth). We suggest, instead, that thermal maturati on of the Point San Luis slab occurred much deeper in an accretionary prism (10-15 km), where offscraped trench wedge deposits were faulted against un derplated melange. The paleothermal structure was offset and tilted after p eak heating by two out-of-sequence faults. Late Oligocene to Pliocene strat a rest unconformably above the Franciscan, and there is a significant gap i n 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 o pposite sides of the San Gregorio-San Simeon-Hosgri fault system also can b e used to test interpretations of strike-slip neotectonics. The failure to match these geometries among suspected piercing points at Point San Luis, C ambria, and Point Sur favors a suggestion that differential, post thermal p eak, dextral offset of Franciscan basement has not exceeded 10-15 km.