NEW INSIGHTS INTO MIGRATION OF THE CRETACEOUS SIERRAN ARC USING HIGH-PRECISION U/PB ZIRCON GEOCHRONOLOGY
Before ~100 Ma, Cretaceous magmatism in the central SNB was focused near the modern western margin of the range. Near the end of this magmatic period, a system of dextral reverse- (±normal-)sense shear zones formed in Jurassic and Cretaceous plutons along the eastern margin of the Cretaceous plutonic belt. Published mapping suggests that the shear zones are discontinuous but new reconnaissance indicates that the shear zones are more extensive. Published geochronologic data indicate that the shearing began by the onset of the Sierra Crest magmatic event (SCME) at 98 Ma. New geochronologic data from the Mount Givens pluton suggest that movement persisted at least until 91 Ma. We hypothesize that the shear zones both localized and were dissected by plutons of the SCME between ~98 and 85 Ma. The SCME marks an abrupt shift of arc magmatism to the east. No regional migration pattern is observed among SCME plutons but individual plutons and intrusive suites define local age progressions. For example, the Tuolumne Intrusive Suite is concentrically zoned and youngest in the center whereas plutons in the John Muir Intrusive Suite decrease in age from south to north.
Biotite K/Ar and Ar/Ar ages from rocks in the central SNB generally fall between 90 and 80 Ma and do not definite a clear spatial pattern. These dates may reflect regional cooling caused by exhumation or the end of the SCME. Biotite ages of rocks in the John Muir Intrusive Suite cluster near the youngest zircon ages from the suite and are interpreted to record post-magmatic cooling. If post-magmatic cooling governs biotite cooling ages elsewhere in the SNB, biotite Ar ages should increase westward. The few available ages support this possibility, but it should be tested more rigorously with additional data.