COUPLING BETWEEN MAGMATISM, RETRO-ARC CONTRACTION, FORELAND BASIN ARCHITECTURE AND PLATE CONVERGENCE: EVIDENCE FROM DETRITAL ZIRCON GEO-THERMOCHRONOLOGY IN THE CORDILLERAN RETROARC BASIN

In Cordilleran orogenic systems, the temporal and causal linkages between retroarc deformation, voluminous magmatism and plate convergence have remained elusive. This is largely due to the lack of temporal resolution in reconstructing the retroarc deformational history and uncertainties surrounding arc magmatism and plate motion reconstructions. This study compiles high-density detrital zircon (DZ) U-Pb and (U-Th)/He (He) geo-thermochronometric data from the Cordilleran Sevier foreland basin strata to investigate the temporal relationship between arc volcanism and exhumation in the retroarc Sevier thrust belt. The foreland basin DZ U-Pb ages (n=779) record four major volcanic zircon generation events at ~165, 118, 97, and 77 Ma that agree in age with proposed large-volume magmatism in the arc. DZHe ages (n=453) from the foreland basin document intervals of large-magnitude thrust belt exhumation and provide evidence for several episodes of retroarc cooling and tectonic unroofing at ~177, 149, 133, 121, 97, and 77 Ma. These data establish synchronicity between retroarc shortening and voluminous arc magmatism, challenging conceptual models that invoke temporal delays between retroarc shortening and voluminous arc magma generation. The most compelling evidence for their synchronicity comes from bedrock and detrital He constraints on mid-Cretaceous thrust belt activity and the large generation and dispersion of volcanic zircons into the foreland basin around the same time, indicating coeval shortening and voluminous volcanism. Forearc and foreland basin architecture and the high abundance of ~100 Ma DZHe cooling ages in the basin further support major shortening and exhumation in the mid-Cretaceous. The temporal synchronicity in both thrust belt activity and arc magmatism correlates with increased plate convergence rates between the Farallon and North America plate during the Late Jurassic and Cretaceous, implying that both are driven by plate boundary convergence. The temporal coincidence between plate convergence rates, episodes of retroarc shortening (deduced from DZHe ages), and increases in magma production raises the prospect for a potential causal relationship.