Deformation of the NE Basin and Range Province: the response of the lithosphere to the Yellowstone plume?

Abstract

SUMMARY Tectonic deformation in the NE Basin and Range Province of the western United States is concentrated in two zones, the N-S trending northern part of the Intermountain seismic belt in northern Utah, SE Idaho and SW Wyoming and the E-W trending Central Idaho seismic zone, which converge at Yellowstone in NW Wyoming. Accepting the popular view that the Yellowstone volcanic field is above an upwelling plume from the deep mantle, I explore the possibility that deformation of the brittle upper crust in these two seismic zones may be caused by return flow from this plume. This return flow is assumed sheared SW in the direction of motion of the North American plate relative to Yellowstone. The Snake River Plain (SRP), a linear outcrop of Neogene and Quaternary basalt that trends SW from Yellowstone for more than 400 km, bisecting the angle between the two seismic zones, may be a trail formed as the plate moves SW across the upwelling plume. Beyond ∼150 km from Yellowstone, slip vectors on active normal faults in both deforming zones are oriented inwards at ∼45° towards the SW azimuth along the SRP. Normal Slip rates on these faults are typically 1 mm yr−1, implying extensional strain rate ∼10−15 S−1. Proportions of normal slip and strike slip suggest faults in the Intermountain seismic belt are rotating anticlockwise at ∼2° Myr−1 around vertical axes, and faults in the central Idaho seismic zone are rotating clockwise at similar rates. Within ∼100 km of Yellowstone, observed sense of rotation around vertical axes in both zones appears reversed. Tilt rates of hanging wall beds suggest most faults in both zones are rotating at ∼2° Myr−1 (also ∼10−15 S−1) around horizontal axes oriented NW. The relative size of rotation rate around vertical axes and horizontal extensional strain rate is shown to imply that velocity in the underlying deforming ‘fluid’ is small in the direction perpendicular to the axis of the SRP. In an initial model, this velocity is set to zero, enabling the two-dimensional pattern of flow at the horizontal upper boundary of this deforming fluid to be solved straightforwardly. This approximation enables observed rates and senses of horizontal extension and rotation around a vertical axis to be explained as consequences of the pattern of horizontal gradients of velocity parallel to the axis of the SRP in the deforming fluid beneath the brittle layer. A refinement suggests that velocity perpendicular to the SRP near the upper boundary of the deforming ‘fluid’ is small but inward towards the SRP from both sides, and is accompanied by downwelling beneath the SRP beyond ∼100 km SW of Yellowstone. This pattern of flow will act to move magma in the uppermost mantle beneath the SRP from its sides. It may thus reconcile petrological observations that SRP basalts do not show deep mantle characteristics expected had they been extruded from the Yellowstone plume, with the existence of this plume nearby. Conservation of angular momentum around horizontal axes perpendicular to the SRP for the combined system of the plume interacting with the North American plate leads to the deduction that the plume radius is ∼50 km, consistent with results of other investigations, and its upwelling velocity is ∼60 mm yr−1.