HAMPTON BUTTES: OLIGOCENE TO PLIOCENE RECORD OF VOLCANISM AT THE NORTHERN MARGIN OF THE HIGH LAVA PLAINS

The High Lava Plains (HLP) form the northern margin of the Basin and Range in Oregon. This interface is part of an ongoing study that focuses on the volcanism and faulting in areas that exemplify their style, age, and spatial distribution since at least 16 Ma. New geologic mapping and 40Ar/39Ar ages from the Hampton Buttes, on the northern margin of the HLP province, provides information about the timing of volcanism and structural development along this margin.

The Hampton Buttes are a circular complex of dacitic to rhyolitic lava domes located along the northern margin of the HLP-Blue Mountains province transition. 40Ar/39Ar geochronology yielded an age of ~30 Ma on one rhyolite dome, Hampton Butte, and ~10.2 Ma on a basalt flow that is banked against the base of the butte. The rhyolite of Hampton Butte does not conform to the westward progression of decreasing ages for HLP rhyolite and the basalt is one of the oldest known to occur on the HLP. Several closely spaced, steeply dipping, WNW-trending normal faults delineate the southwest margin of the study area and mimic the trend of faults within the HLP. The eastern half of the faults display greater offset (~30 m) than the western half (~20 m) and cut likely John Day age alluvial and lacustrine deposits, the 12 Ma basalt, a 6.8 Ma ignimbrite, and. Cougar Butte, a dacite dome of unknown age located ~ 9 km south of Hampton Butte. Physical evidence suggests vertical displacement on the southern face of Cougar Butte after its emplacement. A prominent ~N-S trending volcanic lineament ~4 km east of Cougar Butte bends SW for ~1 km before it intersects the WNW-striking faults at the southern edge of the dome complex. This relationship may indicate right-lateral slip along the WNW-striking faults sometime before emplacement of Cougar Butte. A small-unnamed dome, CB and a vent form a line trending NNE that closely parallels a fault trace that intersects the N/S lineament.

Geologic relationships at Hampton Buttes indicate that volcanic features are aligned along NNE trends. Alignment of volcanic features at Hampton Buttes suggests that they might provide a pre-existing crustal weakness for Basin and Range faulting. NW-striking faults are younger than 6.8 Ma and may have a component of right-lateral shear.