Heat flow, arc volcanism, and subduction in northern Oregon

Abstract

A total of 75 heat flow measurements are presented for the part of the Oregon Cascade Range and surrounding provinces, including the Willamette Valley, the High Lava Plains, the Deschutes‐Umatilla Plateau, and the Blue Mountains, between 43°30′ and 45°05′N. The data document a major change in regional heat flow from values of 40±2 mW m−2. in the Western Cascade Range and Willamette Valley provinces to 100±5 mW m−2 along the boundary between the High Cascade Range and Western Cascade Range provinces. This variation has a half width of approximately 10 km and appears to be exactly the same along the whole study area (a strike length of over 150 km). This heat flow change coincides with a line of hot springs and occurs approximately 20–30 km west of the axis of the High Cascade Range where the major andesite stratovolcanoes of Mount Jefferson and the Three Sisters occur. The heat flow boundary corresponds to a steep gravity gradient with about the same half width and an amplitude of approximately −25 mGal. Both sets of data can be explained by a hot, low‐density region about 60 km wide in the crust below 7–10 km. This heat source region is probably a zone of temporary residence of magmas melting off the subduction zone. Such crustal staging chambers may be commonly involved in the complex petrologic evolution of andesites and granitic rocks in volcanic arcs. The ratio of apparent intrusive to extrusive rocks ranges from 1:1 counting the total volcanism to 10:1 or 50:1 if only andesitic or rhyolitic volcanics are included. The overall heat flow pattern is consistent with active subduction of the Juan de Fuca plate under the Pacific Northwest. The absorption of heat from the continental outer arc by the sinking plate and the occurrence of the high heat flow associated with the volcanic arc are documented. The lack of earthquakes in the Pacific Northwest is probably related to the proximity to the ridge and the fact that several kilometers of sediment overlie the basaltic basement where it begins to be subducted, so that it is already at a minimum temperature of 100°–200°C. During the Oligocene and Miocene the edge of the volcanic arc was in the Western Cascade Range at least 40–60 km west of its present position. During the last 6–10 m.y. the heat flow has decreased from high volcanic arc values to subnormal outer arc values. The effectiveness of subduction as a heat sink is thus clearly demonstrated. The eastward migration of the volcanic arc axis might be associated with the proposed rotation of the Western Cascade Range due to Basin and Range spreading.