PASSIVE EMPLACEMENT OF A COMPOSITE FELSIC PLUTON IN A ZONE OF INTRA-ARC EXTENSION: MODELING DEVELOPMENT OF THE MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

Field relations indicate that the emplacement of the Mount Whitney Intrusive Suite (MWIS) was accommodated, at least in part, by extensional structures associated with a zone of Late Cretaceous dextral shear along the eastern margin of the Sierra Nevada batholith. Granite porphyry dikes that are comagmatic with the suite's youngest member strike northeast and record a component of northwest extension parallel to the suite's long axis. We suggest that these dikes are upper crustal expressions of a mid-crustal pull-apart that developed at a dilatational stepover between the proto-Kern Canyon and ancient Sierran frontal faults, and that this extensional zone served as the feeder for the MWIS plutons. Because the suite is much longer than contemporaneous offset mapped along the proto-Kern Canyon fault it is likely that rising magmas spread out beyond the margins of the extensional zone to form plutons that are elongate parallel to the northwest-trending structural fabric of the eastern Sierra.

We have modeled the MWIS as a tabular intrusion that grew incrementally above its feeder dike as rock was translated upward and outward from its base along paths that approximate displacements around a half-sill growing in an elastic medium. This geometry reproduces the suite's nested structure and balances its rate of emplacement with that at which space is created by extension (10 mm/yr). Finite-element modeling of the intrusion's conductive cooling history indicates that warming of the crust by early batches of magma significantly prolonged the crystallization intervals of later ones. This decrease in cooling rate is consistent with the growth of alkali-feldspar megacrysts in the suite's younger members due to textural coarsening. It also accords with the transition from the compositional heterogeneity of oldest member to the symmetric zonation of the youngest because later magma batches, which remained largely molten between intrusive pulses, accumulated to form a central volume that subsequently differentiated by crystal fractionation.