THE DYNAMICS OF SYNCHRONOUS FLARE-UPS, MIGRATION, AND FOCUSING IN THE MESOZOIC SIERRA NEVADA ARC

Arcs exhibit multiple, synchronous behaviors that raise questions about the nature of internal feedbacks between arc processes as their boundary conditions evolve. For example, in the Mesozoic central Sierra Nevada three Mesozoic magmatic/tectonic flare-ups occurred with peaks at 225, 161 and 98 Ma, all with broadly symmetric durations of ~20-30 Ma, separated by ~30-40 Ma lulls. Definitive links between flare-ups to parameters of the continuously active subducting plate have not been recognized. Estimates of magma addition rates during flare-ups range from smallest to largest: 50x10³ km³ Myr^-1 for the Jurassic, 100x10³ km³ Myr^-1 for the Triassic, and 300x10³ km³ Myr^-1 for the Cretaceous, resulting in plutonic/volcanic ratios of 20/1 to 30/1.

Compiled field mapping and geochronology datasets demonstrate that during the Cretaceous flare-up the arc simultaneously thickened based on Sr/Y and La/Yb from ~25 to >60 km at a rate of ~ 2 mm/yr, migrated eastward from ~140 Ma to 85 Ma at a rate of 2.6 mm/yr and locally developed an internal magma focusing center from >105 to 85 Ma. The center is defined by an inwards asymmetrical focusing and younging of plutons at a rate of up to 4 mm/yr, resulting in the increase in the size of intrusive complexes and longevity of magma chambers.

⁸⁷Sr/⁸⁶Sr_i and εNd are widely dispersed, but broadly converge to more crustal values during the Cretaceous arc flare-up. West to east migration is the spatial expression of the flare-up, sharing the same isotopic characteristics. This reflects the transition from oceanic to continental arc basement.⁸⁷Sr/⁸⁶Sr_i and εNd in the focusing zone are initially widespread, converging on average values of 0.7064 and -5.83 respectively, by 86 Ma, a less evolved composition than predicted by west to east migration alone. Spatiotemporal and compositional trends are matched in smaller datasets of contemporaneous hypabyssal intrusions and volcanic rocks.

These arc behaviors are likely controlled at depth by coupled upper plate and mantle processes, increasing the mantle input into the crust, and imposing thermal controls on the organization of the magma plumbing system, due to the varying time and spatial scales of each phenomena. We speculate that crustal modulation is a second-order effect that affects flare-up and migration isotopic trends and upper crustal focusing.