Tectonic Implications of Fission-Track Thermochronology and Amphibole Thermobarometry Studies of the Northern Peninsular Ranges Batholith, Southern California.

Abstract

Fission-track data from the San Jacinto Mountains and the Palomar Mountain-Escondido study area indicate that the Late Cretaceous history of the Peninsular Ranges batholith is marked by a series of major cooling events. Coupled with geobarometry measurements and field observations, these data allow for a greater understanding of the nature and timing of both local and regional tectonic events. In the eastern Peninsular Ranges, west-vergent thrusting along the Santa Rosa mylonite belt at $\sim$99-94 Ma was followed by major extensional faulting between $\sim$94 and 92 Ma. Amphibole geobarometry of mylonites indicate that $\sim$10-18 km of crustal thickening occurred as a consequence of thrusting. Fission-track thermochronology and amphibole-plagioclase thermobarometry from footwall rocks (i.e. tonalitic plutons in the San Jacinto Mountains) indicate that subsequent extension resulted in $\sim$10-14.5 km of exhumation. This was accompanied by synextensional intrusion and concomitant cooling of footwall tonalites from crystallization temperatures (730-760$\sp\circ$C) to below the closure temperature of zircon ($\sim$235$\sp\circ$C) in less than $\sim$1 to 2 m.y. Fission-track data from the Palomar Mountain to Escondido study area indicate that the western Peninsular Ranges underwent several intervals of surface uplift and associated erosion between $\sim$100 and 82 Ma. At $\sim$77-76 Ma both eastern and western parts of the batholith experienced additional surface uplift that was likely brought on by changes in plate motions associated with the beginning of the Laramide orogeny. Surface uplift at $\sim$77-76 Ma may explain the development of regional erosional unconformities that are common throughout Baja and southern California. The repeated intervals of Cretaceous and early Tertiary surface uplift also incrementally tilted the batholith and its fission-track age surfaces to the southwest by an amount that is compatible with estimates based on previous geochronologic and thermobarometric data. Tilting to the southwest brings discordant paleomagnetic directions of plutonic rocks from the batholith into alignment with the expected Cretaceous direction, and thus eliminates the need for long-distance northward transport ($\sim$11$\sp\circ)$ of peninsular California as advocated by some workers.