Geology and geothermal resources of the central Oregon Cascade Range. Special Paper 15

Eruption of voluminous silicic tuffs and lesser volumes of iron-rich basaltic to silicic lavas between about 40 and 18 m.y. B.P. and eruption of intermediate calc-alkaline lavas and subordinate tuffs between about 18 to9 m.y. B.P. formed most of the volcanic pile in the central Western Cascade Range. The axis of volcanism had shifted to the vicinity of the High Cascade province by about 9 m.y. B.P., when eruption of more mafic, alkaline, and iron-rich lavas began. Slight local folding in the north-central Western Cascade Range also ended by about this time. Widespread north-southto north-northwest -trending normal faulting accompanied uplift of the Western Cascade block relative to the High Cascade province between about 5 and 4 m.y. B.P . Voluminous basaltic lavas formed a broad platform in the High Cascades in the early Pliocene, and a north-south-trending chain of composite volcanoes developed along the High Cascade axis in the Quaternary as volcanism became slightly more silicic. The youngest composite cones are chiefly andesitic in composition, with local dacitic to rhyodacitic eruptions , although basaltic eruptions have continued to occur on the surrounding platform into the Holocene. Compositional and textural similarity of some of the 9to 0-m.y. B.P. basalts in the Basin and Range and the Cascades and the contemporaneity of changes in volcano-tectonic events in the Basin and Range and the Cascades suggest that the geologic histories of the two areas are closely related. A major change in the plate-tectonic regime about 10 to 8 m.y. B.P. may have affected both areas, resulting in increased extensional tectonic influence in the central Cascades. An additional change in the interaction of lithospheric plates at the end of the Miocene is probably necessary to account for the pronounced uplift and north-south faulting which occurred 5 to 4 m.y. B.P. in the Cascades. The central Oregon High Cascade Range is viewed as a subduction-related volcanic arc which has been strongly affected by Basin-andRange-type extension. Local subsidence of parts of the High Cascade axis is probably caused by magmatic withdrawal and volcanic loading during regional extension.