Heat flow in the Oregon Cascade Range and its correlation with regional gravity, Curie point depths, and geology

Abstract

New heat flow data for the Oregon Cascade Range are presented and discussed. Heat flow measurements from several deep wells (up to 2500 m deep), as well as extensive new data from industry exploration efforts in the Breitenbush and the Santiam Pass‐Belknap/Foley areas are described. The regional heat flow pattern is similar to that discussed previously. The heat flow is about 100 mW m−2 in the High Cascade Range and at the eastern edge of the Western Cascade Range, It is about 40–50 mW m−2 to the west in the outer arc block of the subduction zone. In the high heat flow zone the heat flow is low at shallow depths in young volcanic rocks due to the high permeability of the rocks and the resultant rapid groundwater flow. Below a depth of 200–400 m much of the area appears to be dominated by conductive heat transfer at least to 2–2.5 km depth. There are perturbations to the regional heat flow in the vicinity of the hot springs where values are up to twice the background. The gravity field in the Cascade Range has characteristics that can be closely related to the heat flow pattern. The relationship may be causal, and to examine the relationship in more detail, earlier two‐dimensional modeling is extended to three dimensions. Consideration of the effects of a midcrustal density anomaly, such as might be associated with a region with at least areas of partial melt, has two major consequences. The first of these is that a high‐frequency gravity gradient near the Western Cascade Range/High Cascade Range boundary is explained. Second, the negative gravity anomaly associated with the north half of the High Cascade Range can be removed, and as a result, the prominent northeast/southwest striking regional Bouguer gravity anomaly associated with the north edge of the Blue Mountains becomes continuous across the Cascade Range with a similar feature along the north side of the Klamath Mountains. Apparently, this zone is a major crustal feature upon which the negative gravity anomaly coincident with the high heat flow is superimposed. The correlation, or lack thereof, of the heat flow, depth to Curie point, gravity field, crustal electrical resistivity, crustal seismic velocity, and geology in the High/Western Cascade Ranges is summarized. Many of the data show aspects that can be interpreted in relation to possible high temperatures in the midcrust of the Cascade Range. The High Cascade Range midcrust has unusually high temperatures and contains a zone of magma staging at 10±2 km depth that can also be identified in subdued form in the Cascade Range in Washington and British Columbia.