THE HOOD RIVER GRABEN: A LATE PLIOCENE AND QUATERNARY INTRA-ARC HALF GRABEN IN THE NORTHERN OREGON CASCADE RANGE

The High Cascade Range of northern Oregon occupies a segmented and structurally discontinuous, ~30-km-wide, arc-parallel graben that has propagated northward from central Oregon (Green Ridge) since 5.3 Ma at an overall rate of 4 cm/yr. Between Mount Hood and Mount Adams the High Cascade graben coincides with the Hood River graben, a late Pliocene and Quaternary half-graben that forms the Hood River Valley. The Hood River graben is defined by an ~20- to 25-km-wide zone of distributed east-west extension. The north-trending Hood River fault zone, extending from just north of Underwood Mountain to the east flank of Mount Hood (~50 km), defines the eastern boundary fault system of the graben. The fault zone may extend further to the southeast of Mount Hood where it joins more southern strands of the eastern boundary fault of the High Cascade graben, but its precise location in that area is not yet well constrained. The Hood River fault zone became active during the late Pliocene, and has accommodated the greatest amount of graben subsidence with at least 600 m of structural and topographic relief; south of Hood River lavas as young as 1.7 Ma are offset by at least 400 m along a strand of the Hood River fault. Cumulative displacement progressively decreases northward to ~130 m near the Columbia River. The western edge of the graben is topographically indistinct, but may be defined in part by a wide zone of parallel, north-northwest-trending, Quaternary extensional faults forming the Mount Hood fault zone. The Mount Hood fault zone extends at least 45 km from the northwest flank of Mount Hood and progressively down-drops a fold-warped, but generally eastward-dipping stratigraphic section from east to west. North-trending faults associated with the development of the Hood River graben have served as conduits for a number of late Pliocene and Quaternary volcanic vents that occur either within or adjacent to the structural basin. The timing of initial graben formation was contemporaneous with or closely followed a major pulse of mafic vol­canism in the northern Oregon Cascades between 4.4 and 2.1 Ma. This pulse of mafic volcanism was distinctly younger than a similar episode that had culminated in central Oregon by ca. 5 Ma and contrasts with the preceding ten million years, when andesitic eruptions dominated the northern Oregon Cascade Range.