Implications of volcano-tectonic patterns in the Oregon cascades for geothermal exploration
Basin and Range and subduction-related processes may have combined to produce voluminous mafic volcanism and very high heat flow in the central Oregon High Cascades during the last 9 m.y. This high rate of volcanism and heat flow prevails south of the Clackamas River right lateral wrench fault zone. South of this zone the volume of mafic volcanic rocks less than 9 m.y. old increases abruptly and regional heat flow increases by at least 20 mW/m/sup 2/. A similar, but larger, increase in volcanism and heat flow characterizes the transition from the Blue Mountain block to the Basin and Range Province at the Brothers right lateral wrench fault zone. A significant amount of the 0 to 9 m.y.B.P. mafic volcanic rock in the Cascades is basalt, with one or all of the following attributes of contemporaneous basalts of the Brothers Fault Zone-Basin and Range province: (1) anomalously high alkali and iron content relative to normal calc-alkaline rocks; (2) diktytaxitic texture with titaniferous groundmass clinopyroxene. This suggests a common petrogenesis for these basalts, perhaps from partial melting related to Basin and Range spreading. Potential operation of two partial melting mechanisms in the Oregon High Cascades may cause a high rate of more » magmatic heat transferral from mantle regions relative to transferral in the Basin and Range. This means that the High Cascades probably has the highest geothermal potential of any province in Oregon, especially where intercepted by youthful faults. North-south faults which bound a large area of Pliocene to Pleistocene subsidence along the High Cascade axis are good exploration targets. Silicic volcanism at the Brothers Fault zone intersection makes this the best exploration target in Oregon. « less