SUBSIDENCE OF A VOLCANIC-BASIN BY FLEXURE AND LOWER CRUSTAL FLOW - THE EASTERN SNAKE RIVER PLAIN, IDAHO

The Eastern Snake River Plain (ESRP) is a linear volcanic basin interp reted by many workers to reflect late Cenozoic migration of North Amer ica over the Yellowstone hotspot. Thermal subsidence of this volcanic province with respect to Yellowstone has been documented by several wo rkers, but no one has characterized subsidence with respect to the adj acent Basin and Range Province. This paper documents crustal flexure a long the northwest edge of the ESRP, uses flexure to model the dimensi ons of a dense load beneath the basin, and presents evidence in suppor t of density-driven subsidence and lower crustal flow away from the ba sin. Crustal flexure adjacent to the ESRP is reflected by the attitude s of Mesozoic fold hinges and Neogene volcanic rocks. Fold hinges form ed with a subhorizontal plunge and a trend perpendicular to the ESRP b ut now show a southward plunge near the ESRP of as much as 20 degrees- 25 degrees. We present a contour map of equal fold plunges proximal to the ESRP that shows flexure is roughly parallel to and extends 10-20 km north of the average edge of the ESRP. Flexural profiles indicate t he minimum amount of ESRP subsidence, with respect to the Basin and Ra nge; subsidence ranges from 4.5 to 8.5 km. The structural contour map and published seismic and gravity data were used to develop and constr ain flexural subsidence models. These models indicate the flexed crust is very weak (flexural parameter of 4-10 km), interpreted to be a res ult of the high heat flow of the ESRP. Assuming subsidence was induced by emplacement of a dense crustal layer beneath the ESRP, a midcrusta l ''sill'' identified in previous seismic surveys is too wide and prob ably too thin to produce the measured flexure. New dimensions include a thickness of 17-25 km and a half width of 40-50 km, which place the edge of the sill beneath the edge of the ESRP. The dimensions of the E SRP sill are based on isostatic compensation in the lower crust becaus e compensation in the asthenosphere requires an unreasonable sill thic kness of 30+ km and because ESRP seismic, gravity, and heat flow data support lower crustal compensation. Density-driven lower crustal flow away from the ESRP is proposed to accommodate subsidence and maintain isostatic equilibrium. Timing of subsidence is constrained by ESRP exp loratory wells, where 6.6 Ma rhyolites at a depth of 1.5 km indicate m ost subsidence occurred prior to their emplacement, and by strong spat ial correlations between plunge contours and Quaternary volcanic rift zones. Two processes interpreted to contribute to the load include an extensive midcrustal mafic load emplaced at similar to 10 Ma, which pr ovided the heat source for the initial rhyolitic volcanism on the ESRP , and continuing, localized loads from dikes and sills associated with Quaternary basalts. Widespread similar to 10 Ma magmatism and subside nce conflicts with simple time-transgressive migration of the Yellowst one hotspot, indicating a need for revision of the hotspot paradigm.