Lithospheric Control on the Initiation of the Yellowstone Hotspot: Chronic Reactivation of Lithospheric Scars

摘要

The Yellowstone hotspot is generally interpreted to have resulted from a mantle plume that initiated beneath the Idaho-Oregon-Nevada region at ~18 Ma. We explore an alternative model in which the initiation of Yellowstone magmatism is a result of lithospheric-scale processes-transtensional reactivation of the western Idaho shear zone-rather than a mantle plume. This model is based on both spatial and temporal correlations with deformation in the U.S. West. The first-order observation is that hotspot-related magmatism occurs as a linear N-S feature, which exploits the mantle portion of the inactive western Idaho shear zone and a deep crustal feature along the future Northern Nevada rift. The location for initiation of the McDermitt caldera, the interpreted initiation site of the Yellowstone hotspot, is spatially coincident with the southernmost known extent of the western Idaho shear zone in the lithospheric mantle. In contrast, the feeder zones of the voluminous magmatism of the Columbia River basalts intruded farther north, facilitated by the better-developed fabrics of the northern segment of the western Idaho shear zone. The timing of initial Yellowstone magmatism and Columbia River basalts is coincident with the coupling of western California and the North American plate to the Pacific plate. The kinematics of this coupling suggest oblique divergence and transtensional deformation. Previous numerical modeling of vertical fabrics in the lithospheric mantle show preferential reactivation by transtensional deformation. Thus, the mantle lithosphere- rather than deeper portions of the mantle- appears to exert a fundamental control on the initiation of the Yellowstone hotspot.