DETRITAL ZIRCON RECORD OF CORDILLERAN MAGMATISM AS PRESERVED BY JURASSIC-PALEOCENE FOREARC AND FORELAND BASINS OF THE NORTH AMERICAN CORDILLERA

Compilations of detrital zircon (DZ) data from North American Cordilleran basins effectively subdue variability in DZ distributions that is commonly observed in studies of more limited geographic and/or temporal extent, allowing for the investigation of magmatic and sedimentary processes acting at the orogen-scale. We present a compilation of 830 DZ samples (>100,000 individual ages from 70 studies of forearc and foreland strata) to address the Jurassic-Paleocene history of Cordilleran magmatism, placing emphasis on 251-56 Ma grains that we infer to be uniquely derived from the arc (herein “arc-derived grains”). We compare detrital age distributions against >1,000 igneous zircon U-Pb ages from magmatic rocks. Results show that the forearc and foreland DZ records are excellent proxies for arc magmatism because they provide space- and time-integrated records of crystallization that are commonly obscured in igneous records. The DZ distributions show that arc magmatism was continuous throughout Mesozoic and earliest Cenozoic time, with presumably lower-volume magmatism during the Triassic and Early Jurassic (~251-174 Ma), episodic high-volume magmatism beginning in the Middle Jurassic and continuing through the Late Cretaceous (~174-66 Ma), and low-volume magmatism during the Paleocene (until ~56 Ma). However, DZ distributions are spatially and temporally variable across the basins. The forearc is dominated (>80%) by arc-derived grains, whereas the foreland is dominated (>80%) by grains >251 Ma. The dominance of arc-derived grains in the forearc supports that the arc was its dominant source of sediment, with rapid delivery via fluvial processes. The dominance of >251 Ma grains in the foreland demonstrates that rocks exposed in the Cordilleran hinterland and fold-thrust belt were the primary sources of sediment to the foreland. However, proportions of arc-derived grains in the foreland are highly variable, reflecting variations in the topography separating the arc and foreland and highlighting the important role of ash-fall processes in zircon delivery. Overall, the DZ distributions illustrate that Cordilleran basin-fills are reliable long-term archives of zircon production, and by extension, arc activity.