MODELING THE GROWTH OF A COMPOSITE FELSIC INTRUSION: INSIGHTS FROM AN ONGOING STUDY OF THE MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

The Mount Whitney Intrusive Suite is a large (1200 km²) composite granitic intrusion that was emplaced during a period of about 7 m.y. (90.5-83.5 Ma) at an extensional stepover along the Sierra Crest shear zone. Systematic compositional and textural differences among the suite’s members suggest that warming of the upper crust during its growth led to a transition from an outer member comprised of discrete emplacements to an inner one in which magmas accumulated and differentiated within a central reservoir. In order to better understand how these petrologic differences may be linked the suite’s thermal history we have developed a 2D finite element model that simulates the coupled deformation and heat transfer that accompany emplacement of a large composite intrusion into an elastic upper crust.

The Mount Whitney Intrusive Suite’s field relations provide key constraints for the model, and indicate that: (1) approximately 15 km of slip occurred along the faults that bound the stepover during the suite’s emplacement; (2) its individual plutons are broadly tabular, and the younger members were emplaced beneath the older ones and domed them up; (3) the suite was emplaced at a depth of 7-8 km and grew to a total thickness of at least 4-5 km; and (4) a vigorously-recharged and differentiated reservoir was largely restricted to the central part of the youngest pluton.

Our model indicates that the intervals during which individual magmatic emplacements cooled to their rigid percolation thresholds (RPTs) increased from 10² to 10⁵ years as the suite grew. This implies that magmas emplaced into center of the body are likely to have remained above their RPTs long enough to have assembled a reservoir and differentiated by compaction and hindered settling over a vertical distance of several hundred meters. Although the model’s results are consistent with the structure and petrology of the suite, its inability to reproduce the stress changes that accompanied faulting during episodic extension within the stepover limit its accuracy and will require that we adopt a more complex crustal rheology in order to improve future simulations.