PRESERVING VARYING DEFORMATION HISTORIES IN ACTIVE ARCS: A CASE STUDY OF THE SIERRA NEVADA BATHOLITH, CALIFORNIA

Magmatic arcs often preserve complex deformation histories imparted by subduction-zone-related stresses. The Sierra Nevada batholith comprises hundreds of plutons intruded over a >150 Myr duration. Notably, Cretaceous plutons preserve deformation fabrics that record a varying strain field during high-flux arc magmatism and subsequent cooling, displaying two distinct kinematic stages: arc-normal shortening followed by a dominant component of arc-parallel shear. The timing of and sense of motion on these shear zones correspond to important changes in both the kinematics and dynamics of Farallon plate subduction beneath the California convergent margin. In the central Sierra Nevada, contractional shear zones that arose during 100-90 Ma arc-perpendicular convergence of the Farallon plate are generally short and occupy the axial zone of the batholith. 90-80 Ma shear zones mark a change in Farallon convergence direction to strongly dextral oblique, accompanied by a velocity drop—these changes are likely the aftermath of the ca. 95 Ma impact and subsequent subduction of the conjugate to the Shatsky Rise beneath much of the California convergent margin. The shear zones that arose in this time frame are 10s to ~100 km long in the central Sierra Nevada, and are preserved at the eastern edge of the batholith between some of the oldest and youngest plutons. These east-central shear zones, which are generally small-displacement structures, may reflect dextral shear strain concentrated within the thermally weakened youngest zones of the batholith. In contrast, the two shear zones of the southernmost Sierra Nevada occur in the axial zone, and reflect localized effects of Shatsky Rise conjugate subduction and its aftermath. Southward-increasing exposure depths and deformation temperatures offer a view into mid- to lower-crustal conditions in actively deforming arc basement, where early oblique thrusting gave way to dextral shear overprinting during rapid exhumation of the southernmost part of the batholith. Preservation of large, through-going structures may require such rapid exhumation.