MAJOR LATE CRETACEOUS COOLING EVENTS IN THE EASTERN PENINSULAR RANGES, CALIFORNIA, AND THEIR IMPLICATIONS FOR CORDILLERAN TECTONICS

Fission-track ages from the Peninsular Ranges batholith in the San Jac into Mountains near Palm Springs, California, indicate two major Late Cretaceous cooling events. Concordant sphene and zircon fission-track ages of plutonic rocks (tonalite and low-K granodiorite) range from ca . 85 Ma to 98 Ma, with a mean of 92 Ma. The concordance of these ages over 2.5 km of exposed vertical section, coupled with previous U-Pb zi rcon dating (94-93 Ma), implies major cooling from crystallization tem peratures to below approximately 235-degrees-C between 94 Ma and ca. 9 2 Ma. Cooling is attributed to initial intrusion of the tonalite and g ranodiorite into an evolving brittle-ductile normal fault zone (detach ment) followed by approximately 10-14.5 km of exhumation during contin ued crustal extension along the Santa Rosa mylonite belt. Field relati onships, microstructural fabrics, and amphibole thermobarometry from t he mylonite belt support the notion that extensional faulting was prec eded by west-directed thrusting. This sequence of tectonic thickening followed by extension is similar to other areas in the southwestern Co rdillera and may represent the creation and failure of an unstable reg ional crustal welt in Late Cretaceous time. A second event is evident from concordant fission-track ages and indistinguishable track-length distributions from apatite at all elevations. These data indicate very rapid cooling of the tonalitic plutons through the apatite partial an nealing zone (approximately 130-70-degrees-C) between ca. 74 Ma and 80 Ma (mean age of 76 Ma). The occurrence of Late Cretaceous-age regiona l unconformities throughout Baja and southern California suggests that this cooling event was the result of surface uplift and erosion. By u sing a reasonable range of geothermal gradients (20-40-degrees-C/km), the cooling data imply a minimum of 4-5 km of exhumation in conjunctio n with the ca. 76 Ma event. Surface uplift coincided with several impo rtant changes in the North American and Farallon (or Kula) plate motio ns. These include an increase in convergence rate, an increase in the absolute velocity of the North American plate, and a change in the dir ection of convergence between the plates. The changes in plate motions are thought to have initiated low-angle or flat-slab subduction that, in turn, was responsible for the Laramide deformation of the Cordille ran foreland. Surface uplift, erosion, and associated cooling during e xhumation of the eastern Peninsular Ranges are considered to be an ini tial expression of Laramide tectonism. Phenomena documented here are c onsistent with uplift brought on by tectonic erosion of the North Amer ican lithospheric mantle due to low-angle subduction and by subsequent underplating of lower crustal rocks.