EXTENSIONAL THINNING OF ROOF ROCKS ABOVE A PLUTON GROWING IN A ZONE OF INTRA-ARC EXTENSION: MODELING DEVELOPMENT OF THE MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

Field relations indicate that emplacement of the Mount Whitney Intrusive Suite (MWIS) was facilitated, 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. We suggest that this extensional zone, which developed at a dilatational jog between the proto-Kern Canyon fault and the Sierra Crest shear zone, served as the feeder for the MWIS plutons. The suite's length, which is 6 to 7 times as great as contemporaneous offset mapped along the proto-Kern Canyon fault, suggests that rising magmas spread out beyond the margins of the extensional zone to form tabular plutons that are elongate parallel to the northwest-trending structural fabric of the eastern Sierra.

We have modeled the MWIS in two-dimensions as a sheet-like intrusion that grew incrementally above its feeder dike as rock was translated vertically and laterally away from the body's horizontal midline along paths that approximate displacements around a half-sill growing in an elastic medium. This geometry reproduces the suite's nested structure, and predicts doming of roof rocks near the feeder zone due to predominantly vertical displacements adjacent to this part of the intrusion. Extension of the intrusion's roof rocks at same rate as underlying crust significantly thins its roof, however, so that the distal parts of the intrusion crystallize at greater depths than central part. This accords with hornblende barometry, which indicates that rock in the outer member of the MWIS crystallized at a pressure 0.10 to 0.15 GPa greater than that in the innermost member after accounting for present differences in levels of exposure.

Finite-element modeling of the intrusion's conductive cooling history indicates that warming of the crust by early batches of magma more that compensated for increased heat loss due to thinning of the roof, and so significantly prolonged the crystallization intervals of later batches relative to early 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, and with the symmetric zonation of the youngest member due to fractionation of magma that accumulated near its center.