Paper No. 7
Presentation Time: 10:30 AM


CAO, Wenrong and PATERSON, Scott, Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740,

Our synthesis of multiple new datasets indicate that Mesozoic magmatic flare-ups in the Sierra initiate at ~250, 180, 120 Ma and that host rock deformation, crustal thickening and erosional events are also episodic.

Mesozoic thrusts that tilt strata result in 40-50% arc perpendicular shortening. Thrusting events are followed by erosional and depositional events. The tilting-erosion cycles are recorded by three angular unconformities in the Central Sierra Nevada (CSN), now constrained to 260-300 Ma, 195-185 Ma and 115-132 Ma respectively.

Thrusting and erosion occurred before and during early magmatic surge initiation. We propose that during or shortly after the flare-ups, the arc experienced episodic ductile shortening: the magnitudes of which are constrained by ~650 strain measurements. These strain measurements show that (1) less than10% volume loss and/or arc-parallel extension occurred and average arc­scale ductile strain is ~50% shortening and ~100% vertical extension; (2) ductile strain increases and becomes more constrictional with depth; (3) strain does not dramatically increase towards pluton margins, and (4) strain does not show a strong correlation with rock age. The thrusting and ductile shortening contribute to >150% crustal thickening and shortening of ~75%. Such thickening can be achieved with three 10-20-m.y.-long periods with a strain rate of ~10-15 1/s.

Due to limited exhumation of CSN during the Mesozoic (~7 km), such crustal thickening requires downward transport of host rocks (DTH) into lithospheric mantle. The strain rates during DTH are constrained to 10-15 to 10-14 1/s using U/Pb zircon ages and mineral barometry. The strain rates of DTH and the estimated strain rates of crustal thickening are of the same order of magnitude but 1-2 orders of magnitude higher than the one of the retro-arc deformation.

We propose that the deformational cycles of the arc crust mimic the magmatic tempo and multiple feedbacks exist. The thickening events drive fertile materials into zones of melting. The tilting of strata leads to more rapid magmatic ascent by providing weaker anisotropy and DTH. Heating during magma surges aids in increased strain rates and DTH. The thrust-related and ductile shortening together contribute to the dramatic arc thickening and eventual detachment of arc roots.