QUANTIFYING MAGNITUDES AND RATES OF MASS REDISTRIBUTIONS IN ARCS IN THE CENTRAL SIERRA NEVADA, CALIFORNIA

Tectonic and magmatic processes lead to large mass additions and redistributions in arcs, yet quantifying the magnitudes, rates and mechanisms of these 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 these issues for a single plane through the arc at ~10 km paleo-depth. In the CSN map area, the total 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 progressive 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. These results can be extrapolated from surface to Moho depths to calculate 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, indicating that the Cretaceous flare-up was volumetrically ~12 and ~6 times larger than the Triassic and Jurassic flare-ups, respectively.

While these magma additions occurred, the original host rock also underwent redistributions due to tectonism. Our compilation of ~680 strain measurements combined with structural studies 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. Significant surface uplift (a few kilometers) is only seen in the Cretaceous, coincident with Moho depth increase of > 30 km. Narrow structural aureoles near plutons imply that during magma ascent material was transported downwards by a range of magmatic and solid-state mechanisms in regions now occupied by the plutons. Arc wide steep bedding, cleavages and faults suggest that regional mass redistribution occurred via bedding rotation, internal strain, and downward motion on bedding parallel faults.