Abstract
The Salinian block is a section of the California coast that consists of Mesozoic granitoids, Paleozoic metasediments that represent country rocks that were intruded by the granitoids, Cenozoic sedimentary rocks, and an accreted Schist. The block is bounded by the San Andreas Fault and the Franciscan Complex on the east and by the Sur-Nacimiento Fault and the Franciscan Complex on the west. Previous work has suggested the southern Sierra Nevada as the probable source of this allochthonous composite terrane, with westward transport taking place as the hanging wall of a Late Cretaceous detachment system that was the result of the gravitational collapse of the southern Sierra Nevada after the cessation of subduction of the Farallon Plate. This hypothesis is based largely on thermochronologic and geobarometric data from the southern Salinian block (south of Monterey Bay) and the southern Sierra Nevada. Few data exist in the northern Salinian block (north of Monterey Bay).
To test whether the thermal and exhumation history of the northern Salinian Block is consistent with origin as the upper plate of an extensional detachment system, samples were collected at Bodega Head, Point Reyes, Montara Mountain, and Ben Lomond for (U-Th)/He thermochronology and Al-in-hornblende geobarometry.
Samples yield new (U-Th)/He apatite dates that reveal cooling ages in the northern Salinian block that range from as recently as 10.8 Ma to as early as 51.3 Ma. (U-Th)/He zircon cooling dates generally range from 67.1 Ma to 78.5 Ma, except for two samples from Montara Mountain that appear to have reset cooling ages of 13.3 – 18.2 Ma. U-Pb zircon dates are generally in agreement with previous work, with crystallization occurring as early as 111.7 Ma to as late 90.8 Ma.
Calculated equilibrium emplacement pressures in the northern Salinian block range from 3.5 kbar to 7.7 kbar. Samples collected from Ben Lomond may preserve a rotated fault-block and preserve a gradient of emplacement pressures with equilibration pressures decreasing from south to north.
Combining these new data with thermochronologic and geobarometric data from previous work in the southern Salinian block and southern Sierra Nevada, cooling rates were calculated for each block of granitic exposure. Cooling rates in the Salinian block are nearly equal to those of the southern Sierra Nevada, suggesting that the rocks of the Salinian block underwent tectonic unroofing from crystallization temperatures through the (U-Th)/He zircon closure window of 170 – 190 °C. Therefore, instead of the simple westward transport via single detachment, it seems that the Salinian block was part of a multi-stage detachment demonstrated by rapid cooling that began as early as 90 Ma.