QUANTIFYING MASS REDISTRIBUTIONS IN ARCS FROM ARC TO PLUTON SCALES: AN EXAMPLE IN THE CENTRAL SIERRA NEVADA, CALIFORNIA

Quantifying mass redistributions caused by tectonic and magmatic processes in arcs remains elusive. We combine a digital geologic map compilation covering ~12,000 km² of the central Sierra Nevada (CSN) with structural, geochronologic and geochemical syntheses to evaluate mass transfer through a plane at ~10 km paleo-depth. The total map area of Cretaceous, Jurassic, and Triassic plutons is 6141 (52.4% of area), 847 (7.2%) and 165 km² (1.4%), respectively. Corrections for plutons “hidden” by subsequent flare-ups increase the Jurassic to 1758 km² (15%) and Triassic to 1172.3 km² (10%). Recycling by magmatic addition increases mass movement through a given plane but does not increase mass additions in the crustal column. Flux (km³/km²/m.y) through the 10 km deep plane also includes stalled and erupted magmas at higher levels resulting in a volume of 124,000 km³ added or transported through this part of the crust, more than the entire volume of 10 km thick crust over the mapped area. Extrapolated to the entire crust, these results imply CSN Mesozoic magma volume additions (~600,141 km³) and average Moho fluxes for the Triassic (0.086), Jurassic (0.144) and Cretaceous (1.011) magmatic flare-ups. Thus, the Cretaceous flare-up was volumetrically ~12 and ~6 times larger than the Triassic and Jurassic flare-ups, respectively.

Presentations by Scheland et al. (this session) outline mass redistribution at the pluton scale. Narrow structural aureoles around plutons imply that material was transported vertically during ascent by magmatic and solid-state processes in regions now occupied by the plutons, processes that must dominate mass redistributions in high magma volume arcs.

During magma addition, the original host rock was also redistributed by tectonism. Our compilation of ~680 strain measurements and structural observations indicate that the arc crust typically shortened across-strike and thickened vertically during the three flare-ups with the great majority of this resulting in downward movement of host rock. Arc-wide steep bedding, cleavages and faults suggest that mass redistribution occurred via bedding rotation, internal strain, and downward motion on bedding parallel faults. Significant surface uplift (a few kilometers) is only seen in the Cretaceous, coincident with Moho depth increase of > 30 km.