SUBNORMAL CENOZOIC GEOTHERMAL GRADIENTS IN THE EXTINCT SIERRA-NEVADA MAGMATIC ARC - CONSEQUENCES OF LARAMIDE AND POSTLARAMIDE SHALLOW-ANGLE SUBDUCTION

From 120 to 80 Ma, the Sierra Nevada of California was the site of a vigorous, subduction-generated magmatic arc. At about 80 Ma, arc magmatism left the Sierra and began to migrate far to the east into the interior of the North American plate, where sporadic magmatism occurred during the latest Cretaceous to early Tertiary Laramide orogeny. This migration of magmatism inland has generally been attributed to a shallowing in the angle of descent of the subducting plate. Apatite and sphene fission track thermochronology data indicate that geothermal gradients in the Sierra remained relatively high for 5-10 m.y. after the cessation of Sierran arc magmatism, then decreased to about one-fifth or less of previous values over about the next 10 m.y. The timing, magnitude, and time scale of this cooling are consistent with conductive cooling of the Sierran lithosphere in response to initiation of unusually shallow underflow of a cold subducting plate below. -from Author