The Mt. Hood region: Volcanic history, structure, and geothermal energy potential

Abstract

The volcanic history of the Mt. Hood region in the last 15 m.y. shows a slow migration of six to eight volcanic centers from a position about 20 km west of Mt. Hood to the most recent volcanic center in the series. This migration was accompanied by the development of the structural setting visible in the Mt. Hood region today. Mt. Hood stands on a swell which runs north and south along the Cascade Range. Within this swell is a graben, the High Cascades graben, which is parallel to the axis of the swell and has the Hood River Fault as its eastern boundary fault. Superimposed on this graben is the Mt. Hood subsidence block. Subsidence of this block and graben is presumably related to withdrawal of magma from beneath the block and eruption of this material on top of the block to form Mt. Hood. The overall structure is one of broad uplift and narrow subsidence that is at least 5 m.y. old and presumably still active. Cutting across the entire Mt. Hood region and most of the Pacific Northwest is a set of northwest‐southeast right‐lateral strike‐slip faults. These are numerous around Mt. Hood, but the offset on any single fault is small (a few meters) and in some cases vertical. These faults are also young and indicate east‐west extension and north‐south compression. An older and probably inactive set of northeast‐southwest folds and thrust faults is evident in the region, particularly in the Miocene Columbia River Basalt Group. These may be related to a long history of north‐south compression. The volcanic history and structure are presumably a result of subduction of the Juan de Fuca plate beneath the North American plate. This subduction is oblique, leading to a substantial component of right‐lateral motion which, in turn, leads to north‐south compression in the Pacific Northwest. Crustal thinning associated with volcanism has resulted in an elevated geothermal gradient around Mt. Hood. This high gradient makes it possible to tap water in excess of 80°C within 1500 m of the surface. If these thermal waters can be produced in large volumes, they might provide a substantial amount of economically competitive energy for space heating and industrial processes, in the region adjacent to the Cascades. A simple geothermal resource calculation shows that a 1000 km2 area around Mt. Hood could produce beneficial heat which, if supplied by electricity, would require over 1000 MW for 30 years.