A LITHOSPHERIC-INTERACTION MODEL FOR THE YELLOWSTONE AND NEWBERRY MELTING ANOMALIES

The age-progressive relation between rhyolitic ash-flow tuffs of the Yellowstone Plateau and similar older tuffs along the flanks of the eastern Snake River Plain (ESRP) has led to various tectonomagmatic models. Two general schools of thought interpret Yellowstone as (1) the result of processes originating entirely beneath the lithosphere, most notably by a deep-mantle plume, or (2) as the product of regional tectonic interactions between the lithosphere and sublithospheric upper mantle, locally exploiting lithospheric structural features. Despite the popularity of the deep-mantle plume hypothesis, it fails to address adequately the regional late Cenozoic tectonics and structure of the Intermountain West. The mid-Miocene appearance of the NE-propagating melting anomaly at the SW end of the ESRP coincided spatially and temporally with emergence of the NW-propagating Newberry melting anomaly and with a major tectonic reorganization of the Western U.S. During the mid-Miocene, a transition occurred from localized extension along the Northern Nevada rift zone and regional tholeiitic and bimodal volcanism to accelerated uplift, regionally distributed extension, and time-progressive bilateral migration of volcanism to the east and west. The resulting tectonic regime since the mid-Miocene has involved northward migration of the Mendocino triple junction and progressive outward concentration of basin-range extension toward the east and west margins in response to oblique extension and shear, concomitant with propagation of the Yellowstone and Newberry melting anomalies. Yellowstone and Newberry represent self-sustaining melting anomalies propagating along transform accommodation zones bounding the region of oblique extension.