The Columbia River Basalt Group in the Cascade Range: A Middle Miocene reference datum for structural analysis

Abstract

The distributional patterns of individual Columbia River Basalt Group (CRBG) unite and structures recorded in the CRBG place the following constraints on Neogene tectonics of the northern Oregon Cascade Range: (1) A north–east trending topographic low of regional extent, the Columbia transarc lowland, transected the Miocene northern Oregon Cascade Range. The Columbia transarc lowland existed prior to the eruption of the CRBG and served to funnel CRBG flows across the Miocene Cascade Range. The lowland was apparently devoid of active volcanic centers from before 17 to ∼14.5 Ma. After 15 Ma, volcanic centers began to develop in the southern and central parts of the lowland. (2) Folds and associated faults of the Yakima Fold Belt extend through the western Columbia Plateau and far into the Cascade Range. These structures were active between 16.5 and S Ma and represent N–S shortening. (3) The Portland Hills‐Clackamas River structural zone is a northwest trending dextral wrench fault zone that became active prior to the incursion of CRBG flows (∼16.5 Ma) and limited the distribution of some CRBG units. This structural zone appears to mark the western termination of the Yakima Fold Belt. Other similarly trending, dextral strike‐slip faults are common in the northern Oregon Cascades but did not affect the distribution of CRBG flows and therefore developed in post‐CRBG time. (4) Mount Hood is located near the axis of a major Yakima Fold Belt syncline in the CRBG, in the general location of earlier major andesitic volcanic activity related to the Rhododendron Formation.