ASYMMETRICAL FEATURES ACROSS THE YELLOWSTONE HOTSPOT TRACK AND THE NORTHWEST INCLINATION OF THE YELLOWSTONE PLUME

A marked topographic and tectonic asymmetry is associated with the younger part of the Yellowstone hotspot track (Pierce and Morgan, 1992). This asymmetry might be related to the tilt of a newly observed Yellowstone plume. Recent tomographic studies indicate a tilted thermal plume extends from below the Yellowstone caldera N65W, is 100 km wide, and is inclined 20^o from vertical to a depth of 500-600km (Yuan and Dueker, 2005; R.B. Smith, oral commun. 2004).

Asymmetry across the Eastern Snake River Plain (ESRP) is shown by map patterns of faulting and uplift that flare out like a bow-wave from the volcanic track of the Yellowstone hotspot. Active faulting is aligned into two bands that extend south and west from Yellowstone. Southward from the ESRP are fault Belts IV (now inactive), III (decelerating), II (most active), and I (new). North of the ESRP, Belts III, II, and I are also defined, but Belt IV is not represented. The flaring of these belts about the volcanic track is asymmetric; Belt II extends ~1.4 times farther outward on the south than north sides. The Yellowstone crescent of high terrain is also asymmetric: the axis of the crescent extends ~1.5 times farther south than north. Asymmetry also is reflected in the young basaltic rift zones that are concentrated along the northern margin of the plain, many of them oriented with a north-northwest trend.

We look for a possible connection between the inclined Yellowstone plume and the asymmetric pattern of faulting, uplift, and recent basaltic volcanism. We note: 1) the North American Plate motion has been at a rate and velocity compatible with that of the volcanic hotspot track progression over the last 10 Ma (N55E, at 2.5 cm/yr) and differs by ~120^o from the rise direction of the inclined plume; 2) The heat from a plume rising from 500 km depth inclined 20^o from vertical and at a position 170 km NW of Yellowstone could heat the rock immediately above the plume on the northern band of the hotspot track and contribute to the basaltic NW-trending rifts, uplift, and active faulting (including the anomalous E-W Centennial fault), but paradoxically did not result in the northern zone being wider than the southern zone. 3) Faulting Belt IV on the southern side of the ESRP may be farther from the thermal influence of the plume and above stagnant restite swept to the far side of the inclined plume.