A 2D KINEMATIC-THERMAL MODEL OF FLAT SUBDUCTION INITIATION AND ACCRETION OF THE PELONA AND RELATED SCHISTS OF SOUTHERN CALIFORNIA

The Pelona and related schists of Southern California represent continental detritus deposited, subducted to depths of ~30 km, and accreted during a major Late Cretaceous-Early Tertiary collisional event. Emplacement of the schists coincided with the termination of magmatism along a ~500 km segment of the California continental arc, the disappearance and probable subduction of the forearc and western portions of the arc, and the loss of the mafic lower crust and root of eastern portions of the arc. Conceptual models of schist emplacement differ in detail, but generally involve a dramatic flattening of subduction. In order to better understand the stresses, accretion rates, and emplacement histories of the schists, we constructed a two-dimensional, finite element, kinematic-thermal model of flat subduction initiation using the program Pecube (Braun, 2003). The model is able to satisfy geologic constraints available from the literature on cooling rates (15°/m.y.), peak temperatures (635°C), and steep inverted metamorphic gradient (240°/km) in the Pelona schist with an accretion rate of .5-1 mm/yr. It is likely that accretion rates varied significantly between schist bodies.

At a constant accretion rate of .5-1 mm/yr, the thickest exposures of schist were built over 4-8 m.y. as material was scraped off 400-800 km of lower plate. In two less widely accepted alternative tectonic models, the schists represent either transpressional basin or forearc Great Valley deposits. Both alternative basin types are too narrow (<100 km) to produce the observed inverted gradients.