Regional implications of heat flow of the Snake River Plain, Northwestern United States

Abstract The Snake River Plain is a major topographic feature of the Northwestern United States. It marks the track of an upper mantle and crustal melting event that propagated across the area from southwest to northeast at a velocity of about 3.5 cm/yr. The melting event has the same energetics as a large oceanic hotspot or plume and so the area is the continental analog of an oceanic hotspot track such as the Hawaiian Island-Emperor Seamount chain. Thus, the unique features of the area reflect the response of a continental lithosphere to a very energetic hotspot. The crust is extensively modified by basalt magma emplacement into the crust and by the resulting massive rhyolite volcanism from melted crustal material, presently occurring at Yellowstone National Park. The volcanism is associated with little crustal extension. Heat flow values are high along the margins of the Eastern and Western Snake River Plains and there is abundant evidence for low-grade geothermal resources associated with regional groundwater systems. The regional heat flow pattern in the Western Snake River Plains reflects the influence of crustal-scale thermal refraction associated with the large sedimentary basin that has formed there. Heat flow values in shallow holes in the Eastern Snake River Plains are low due to the Snake River Plains aquifer, an extensive basalt aquifer where water flow rates approach 1 km/yr. Below the aquifer, conductive heat flow values are about 100 mW m −2 . Deep holes in the region suggest a systematic eastward increase in heat flow in the Snake River Plains from about 75–90 mW m −2 to 90–110 mW m −2 . Temperatures in the upper crust do not behave similarly because the thermal conductivity of the Plio-Pleistocene sedimentary rocks in the west is lower than that in the volcanic rocks characteristic of the Eastern Snake River Plains. Extremely high heat loss values (averaging 2500 mW m −2 ) and upper crustal temperatures are characteristic of the Yellowstone caldera.

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