Heat flow in the state of washington and thermal conditions in the Cascade Range

Abstract

Heat flow data for the state of Washington are presented and discussed. The heat flow in the Okanogan Highland averages 75 mW m−2, and the gradient averages 25°C km−1. The heat flow in the Columbia Basin averages 62 mW m−2, and the mean gradient is 41°C km−1. Both of these provinces are interpreted to have a mantle heat flow of about 55–60 mW m−2, the same value as in the Basin and Range province to the south and the intermountain region of Canada to the north. These areas comprise the high heat flow, back arc region of the Cordillera. In the coastal provinces and the western part of the southern Washington Cascade Range the heat flow is below normal and averages 40 mW m−2 with an average gradient of 26°C km−1. This low heat flow is related to the absorption of heat by the subducting slab, part of the Juan de Fuca plate, that is beneath the Pacific Northwest. Thus the low heat flow area represents the outer arc part of the subduction zone. Within the volcanic arc, the Cascade Range, the heat flow pattern is complicated. The heat flow is best characterized in the southern Washington Cascade Range. The heat flow there averages 75 mW m−2 and the gradient averages 45°C km−1. The heat flow peaks at over 80 mW m−2 along the axial region that coincides with the Indian Heaven, Mount Adams, and Goat Rocks centers of Quaternary volcanism. As is the case in northern Oregon and southern British Columbia, the western edge of the region of high heat flow has a half width of 10 km, implying a heat source no deeper than about 10 km. In the northern Washington Cascade Range the data are too sparse to determine the average heat flow. There are two saddles in the heat flow pattern in Washington, along the Columbia River and in central Washington. The origin for the contrasting heat flow may be segmentation of the heat source or some more local effect. The heat flow of the Cascade Range is well characterized in several locations, and the pattern is similar at all localities. The most striking feature is the 10 km half width of the western side of the high heat flow region where it abuts the low heat flow outer arc region. The axial heat flow ranges from about 80 to greater than 100 mW m−2. The midcrustal temperatures are interpreted to range from about 400°C to 800°C. The source of these high temperatures is interpreted to be a long‐lived midcrustal zone of magma residence that is characteristic of the Cascade Range regardless of erustal type, rate of volcanism, or composition of volcanism. For example, in southern Washington the region of high heat flow spans the width of the Quaternary zone of volcanism with Mount St. Helens and Mount Adams at the west and east edges, respectively. On the other hand, most of the Oregon stratovolcanoes are near the center of anomalous region.