Cooling and Exhumation of the Sierra Nevada Batholith in the Mount Whitney Area (California) Based on (U-Th)/He Thermochronometry.

Plutonic rocks of the Mount Whitney area were intruded at about 83-87 Ma during the latest stages of emplacement of the Sierra Nevada Batholith. The region is located in the eastern part of the batholith, and is bounded on the east by the currently active Sierra Nevada Frontal Fault, a normal fault. Here we report new apatite and zircon (U-Th)/He ages on a dense 3.5 km vertical transect 20 km S of Whitney. Published K/Ar and U/Pb ages indicate that this area experienced very rapid cooling, > 100° C/km, from 83 Ma to 79-80 Ma. Our new ages suggest that this period was followed by a progressive decrease of the cooling rate from 79 to 55 Ma. Between 79 and 75 Ma the cooling rate was ∼ 15° C/km, and < 10° C/km from 75 to 55 Ma. Considering the relatively shallow emplacement depth of the Mount Whitney area plutons (2-3 kbar), and the duration of the progressive cooling event ( ∼ 30 Ma) we consider that post-intrusion thermal re-equilibration is achieved in a few million years and that exhumation of the batholith must have also occurred between 83 and 55 Ma. This period was followed by a long episode of very slow apparent exhumation, < 0.03 mm/yr, between 55 and 11 Ma, associated with a very slow apparent cooling rate (< 1° C/Ma). This event, recorded by the apatite (U-Th)/He ages, could either be a period of very slow exhumation, or related to the residence of the dated samples within the zone of partial retention for Helium. While the vertical age profile cannot distinguish between these possibilities, efforts in progress to measure the concentration profiles of these apatites using the ⁴He/³He method should be diagnostic. The occurrence of a period of slow apparent exhumation until as late as 11 Ma implies that the Sierra Nevada Frontal Fault was only active after 11 Ma. This result is in accord with estimates based on tilted lithologies in the Central and Northern part of the Sierra Nevada. New dating in progress will allow better constraints on this initiation age. Considering the other dated normal faults in the vicinity of the Sierra Nevada Frontal Fault, a westward propagation of fault initiation is suggested.