A Comparison of Two Newly Mapped Parts of the High Cascade-Western Cascade Transition Zone, Central Oregon

Abstract

Detailed mapping in the Willamette Pass area and the McKenzie Bridge, quadrangle has revealed two parallel north south down to the east fault systems. Gravity anomalies associated with the faults continue for many kilometers north and south of both map areas, suggesting that the two parallel fault systems are regional structures in the central Oregon Cascade Range. The western system probably moved chiefly between 13 to 9 m.y. B.P. and has no clearly defined topographic scarps. The eastern system probably moved chiefly between 5 and 4 m.y. B.P. and has, in many places, well defined topographic scarps which form the present physiographic boundary between the High Cascade province and the uplifted Western Cascade block. Both fault systems formed immediately before major pulses of basaltic volcanism. The faults are closely associated with hot springs in the Western Cascades and may strongly influence the circulation pattern of hydrothermal fluids.