TRANSPRESSION AND DOWNWARD CRUSTAL FLOW DURING THE CRETACEOUS HIGH FLUX MAGMATIC EVENT IN THE CENTRAL SIERRA NEVADA, CALIFORNIA

Periods of high-flux magmatism are increasingly recognized in most arcs. Some authors have postulated that such high-flux events occur during extensional or transtensional tectonic regimes. In the central Sierra Nevada, California, our studies of numerous plutons, and metasedimentary and metavolcanic pendants around the 94-85 Ma Tuolumne batholith (TB) indicate that transpression occurred during the Cretaceous high-flux event, during which a large volume of host rock was displaced downwards relative to the Earth's surface.

Cretaceous transpression caused all metamorphic host rock in this region to have steeply dipping, axial planar foliations, steeply plunging mineral lineations, and strain intensities that range from relatively low values to up to ~70% shortening. Locally meter- to km-scale, oblique, sometimes melt-bearing, shear zones form synchronously with the above structures and typically have dextral, strike-slip component. Local shear zones with reverse kinematics occur adjacent to these oblique, dextral shears, sometimes forming flower structures: the timing of at least some of these reverse shear zones is synchronous with the oblique shear zones. Both the TB and slightly older plutons have steep magmatic foliations and lineations that overprint internal contacts and are continuous with host rock structures and thus are also interpreted to be syn-emplacement regional structures. All of these plutons preserve at least some evidence of stoping.

Thermobarometric data from these plutons indicate emplacement depths of ~2.5-3.5 kbars: these plutons now intrude volcanic rocks with U/Pb zircon ages that overlap within error the U/Pb zircon ages of the plutons. Thus effusive volcanics have been quickly displaced downwards to >10 km depths within 1-2 m.y. The above observations agree well with the interpretation that regional transpression occurred contemporaneously with both magma ascent and dramatic downward flow of host rock by regional ductile flow and rotation of beds to steep dips, and by local stoping. We speculate that this was a Sierran-wide event during the Cretaceous.