RECONNAISSANCE SUMMARY OF HIGH CASCADES GRABEN STRUCTURES IN CENTRAL AND NORTHERN OREGON

The High Cascades intra-arc graben extends from central Oregon to just north of the Columbia River in southern Washington. Despite decades of mapping, many graben structures are known only in reconnaissance, and lack detailed knowledge of offset and age, thus this contribution describes the state of current knowledge. The graben is segmented into three parts, with segment boundaries marked by the Mt. Jefferson and Mt. Hood andesitic stratocones. Older rocks, which along strike are foundered within the graben, crop out near the range crest at the latitude of the two cones, thus precluding the presence of significant through-going faulting. Subsidence developed along each graben segment as eruptions declined following approximately 2 my of elevated eruption rates. Pre-graben eruptions are notable for the presence of MORB-like low-K tholeiites, Fe-rich intermediate lavas, and ash-flow tuffs. The graben is time transgressive, and oldest on the southern end, where the greatest volume of pre-subsidence eruptions took place. The southern segment is the deepest (perhaps as much as ~3 km) and widest (~30 km), subsidence began there ~5 Ma. Continued subsidence in that segment is evidenced by the presence of normal faults cutting and tilting sections of Brunhes age lavas, and several faulted benches of 1-3 Ma lavas just inside the graben margins. LiDAR imagery has revealed many small normal faults cutting very young lavas within the graben. The middle segment is a ~1 km deep graben based chiefly upon a well dated drill hole. Subsidence there began ~4 Ma on the south end of the segment but developed later on the northern end, perhaps as young as ~2 Ma. North of Mt. Hood the structure is a complex half graben offsetting chiefly Plio-Pleistocene lavas dated at 4.4-1.7 Ma. The half graben shallows to the north, with subsidence of at least 600 m on the south, in contrast to ~130 m nearest the Columbia River. Subsidence may be time transgressive along the graben, chiefly post ~3.7 Ma on the south end versus <3.05 Ma on the north.

All segments of the graben are marked by asymmetric uplift of the western graben margin. Tilted fault blocks invariably dip eastward off of the structural high. The width of the uplifted zone west of the graben is on the order of the width of the graben. Paleo-drainages west of the graben are elevated 600-800 m above modern base levels.