THE YELLOWSTONE PLUMEHEAD, MID-SIZED IGNEOUS PROVINCES, AND CENOZOIC STRUCTURAL DEFORMATION OF THE NORTH AMERICAN CORDILLERA: A SINGLE, PROLONGED TECTONIC EVENT

Several features of the Yellowstone magmatic system appear to contradict a thermal plume model for its origin. These include eruptions of basalt and rhyolite in and around the hotspot track that do not fit the spatial and temporal progression expected for an upwelling plume conduit preceded by a starting plumehead. We show that the unusual features in and around the Yellowstone hotspot track are, in fact, unusual features observed throughout the entire North American Cordillera (NAC). When viewed collectively, these features are best explained as resulting from a single, prolonged tectonic event that impacted the NAC throughout the Cenozoic Era. Our ideas are motivated by consistent geochemical variations of Tertiary mafic NAC magmas that show involvement of two mantle source regions: ancient, isotopically evolved, sub-continental lithosphere (SCL) and the ocean island basalt (OIB) source region found in every oceanic hotspot track. Convergent margin magmas are not dominated by either of these sources, and the presence of OIB argues strongly for contributions from a thermal plume. All NAC magmas with OIB signature have radiogenic isotopic compositions that lay on a binary linear mixing array, a defining characteristic of nearly every hotspot system. A transition from predominantly SCL to OIB source has been documented throughout the NAC, and is consistent with progressive uplift, deformation, and melting of the lithosphere followed by penetration of plume-derived magmas. We cite emergence of five distinct massive outpourings of mafic lava within the NAC during the Tertiary that are similar in character, if not scale, to the huge outpourings of flood basalt observed at the inception of several classic hotspots. We postulate that magma from these provinces was derived from a single starting plumehead that impinged upon the descending Farallon slab in the Late Cretaceous. The prolonged duration and wide spatial distribution of the magmas reflect the time involved in uplifting the cold slab while displacing material in the mantle wedge and the overlying lithosphere. The Yellowstone plumehead and its interaction with the Farallon Plate is thus viewed as the cause of the current topographic high, the current elevated thermal state, and the atypical geologic activity that occurred within the NAC throughout the Cenozoic.