The Central Oregon High Cascade Graben: What? Where? When?

Abstract

Study of late Miocene recent volcanism in the High Cascades indicated that regional extension culminated in the formation of the Central Oregon portion of the High Cascade graben about 4.5 m.y.b.p. Fault scarps on the east flank of the graben are discontinuous and probably reflect differential subsidence within the depression. Subsidence may have been greater where large volcanic edifices provided a load on thermally weakened crust. The graben is mostly filled by basalts erupted during the extensional episode, but some permeable volcaniclastic debris may be locally ponded against the Green Ridge fault. Silicic volcanism in the highland west of Bend was probably initiated prior to the Pleistocene and may obscure graben bounding faults.