EVALUATING ARC PERIODICITY AND GEOCHEMICAL EVOLUTION WITHIN THE CENTRAL NORTH AMERICAN CORDILLERA: CONSTRAINTS FROM THE U-PB AND HF ISOTOPE RECORD OF DETRITAL ZIRCONS IN FORELAND STRATA

The North American Cordillera, an archetypal ocean-continent subduction orogenic system, records a protracted, episodic history of arc magmatism, contractional deformation, and foreland basin subsidence. Magmatic addition rates in Cordilleran systems vary dramatically over time, with periods of high addition (flare-ups) alternating with periods of low addition (lulls). Causes of cyclic arc magmatism are debated, with end-member models including: (i) changes in plate convergence rates that control delivery of fertile asthenosphere; (ii) changes in crustal thickness by deformation and igneous processes that control input of crustal melts; and (iii) delamination of thickened arc crust that leads to thermal perturbations. The Hf isotopic composition of zircon provides a proxy for crustal input and recycling during arc magmatism, with positive εHf indicating depleted mantle and juvenile crust sources, and more negative εHf indicating greater input from older crust. Foreland basin strata, containing young detrital zircon grains, provide an archive to evaluate evolving tectonic regimes and arc magmatic activity. Using a combination of detrital zircon (DZ) U-Pb age and Hf isotopic data for Jurassic to Cretaceous foreland basin strata deposited from Nevada to Montana (spanning ~1500 km), we evaluate the timing of magmatic pulses, relationships of pulses to changing crustal input, relations to retroarc shortening and foreland subsidence, and along-strike variability. U-Pb ages of DZs record flare-ups from ca 250-210 Ma, ca 180-145 Ma, and ca 110-80 Ma, similar to patterns found in other studies of forearc strata and retroarc strata to the south. Distinct pull-downs to more negative εHf, occur from ca 170-150 Ma and ca 110-90 Ma, which correspond with episodes of shortening, decreasing dip of the subducting slab, and increased subsidence in the foreland. Overall lower εHf for DZs deposited in more southern parts of the foreland are consistent with along-strike changes in age of the crust upon which the arc was constructed. These relations highlight the interconnections between igneous, deformation, sedimentary, and plate margin processes.