Abstract

Fission-track ages from the Peninsular Ranges batholith in the San Jacinto 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 zircon dating (94-93 Ma), implies major cooling from crystallization temperatures to below ∼235 °C between 94 Ma and ca. 92 Ma. Cooling is attributed to initial intrusion of the tonalite and granodiorite into an evolving brittle-ductile normal fault zone (detachment) followed by ∼10-14.5 km of exhumation during continued crustal extension along the Santa Rosa mylonite belt. Field relationships, microstructural fabrics, and amphibole thermobarometry from the mylonite belt support the notion that extensional faulting was preceded by west-directed thrusting. This sequence of tectonic thickening followed by extension is similar to other areas in the southwestern Cordillera and may represent the creation and failure of an unstable regional crustal welt in Late Cretaceous time.

A second event is evident from concordant fission-track ages and indistinguishable tracklength distributions from apatite at all elevations. These data indicate very rapid cooling of the tonalitic plutons through the apatite partial annealing zone (∼130-70 °C) between ca. 74 Ma and 80 Ma (mean age of 76 Ma). The occurrence of Late Cretaceous-age regional unconformities throughout Baja and southern California suggests that this cooling event was the result of surface uplift and erosion. By using a reasonable range of geothermal gradients (20-40 °C/km), the cooling data imply a minimum of 4-5 km of exhumation in conjunction with the ca. 76 Ma event. Surface uplift coincided with several important changes in the North American and Farallon (or Kula) plate motions. These include an increase in convergence rate, an increase in the absolute velocity of the North American plate, and a change in the direction 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 Cordilleran foreland. Surface uplift, erosion, and associated cooling during exhumation of the eastern Peninsular Ranges are considered to be an initial expression of Laramide tectonism. Phenomena documented here are consistent with uplift brought on by tectonic erosion of the North American lithospheric mantle due to low-angle subduction and by subsequent underplating of lower crustal rocks.

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