THERMAL EVOLUTION OF THE SIERRA NEVADA BATHOLITH, WITH IMPLICATIONS FOR LOCALIZING SOLID-STATE DEFORMATION

The Sierra Nevada batholith has been divided into northern, central, southern, and southernmost segments for ease of geographical reference and to distinguish their geological histories. For example, the central segment experienced voluminous Miocene volcanism, and the southernmost segment underwent rapid uplift shortly after schist underthrusting ca. 90 Ma. We have assembled extensive thermochronological databases for the entire batholith in order to map out its patterns of crystallization and post-emplacement cooling ages. The thermochronological array also serves to further constrain the timing of solid-state deformation, which is present as numerous Mesozoic shear zones mainly within the southern Sierra. The batholith is known to have a west-to-east younging in crystallization age, but the compilation of almost 500 U-Pb zircon ages also reveals a south-to-north trend. The southernmost segment is in bulk distinctively younger (107 ±13 Ma) than the northern segment (137 ±25 Ma), while the southern and central segments both average ~120 Ma. These segments then follow distinct initial cooling paths, with the northern segment rapidly cooling through 500˚C (hornblende Ar ages) just after emplacement (138 ±8 Ma). Average central and southern Sierra Ar hbl ages are 100 Ma, and the southernmost segment remained at 500˚C until 90 ±5 Ma. The entire batholith cooled through 350˚C (Ar biotite) by the same time, with all regions (except the northern, which lacks data) reaching that state between 88 and 85 Ma.

Solid-state deformation in the southern Sierra Nevada took place largely within the time window of cooling from Ar hornblende to Ar biotite temperatures. This deformation has been attributed to Laramide flat-slab subduction, which increased coupling between upper and lower plates and promoted contractional strain in the cooling plutons. Such deformation has thus been suggested to young progressively eastward. However, there is also a distinctive north-to-south younging. In the northern part of the southern Sierra, the Kaiser Peak and Quartz Mountain shear zones were active until 94 Ma, while in the southern part of the batholith, the proto-Kern Canyon fault may have been active as late as 75 Ma. This suggests a geometric control on deformation localization that further thermochronological data may help constrain.