STRUCTURAL DEVELOPMENT OF THE ABERT RIM FAULT: IMPLICATIONS FOR THE INCEPTION OF BASIN AND RANGE FAULTING IN SOUTHERN OREGON

Late Oligocene magmatism and Neogene inception of modern Basin and Range faulting is recorded by volcanic rocks in SE Oregon. Systems of NW- and NNE-striking normal faults yield northward into NW-striking faults of the Brothers Fault Zone and basaltic volcanism of the High Lava Plains. These two fault sets developed simultaneously in some areas and independently in others. New geologic mapping and ⁴⁰Ar/³⁹Ar age-determinations near Lake Abert demonstrate that extension on NW-striking faults pre-dates extension on NNE-striking faults and that both sets may utilize an Oligocene tectonic fabric.

These data identify 3 principal stages of volcanism and deformation in the region: (1) Oligocene age dikes within the Coleman Hills are oriented N32°W and N05°E, similar to late Miocene faults described in stages 2 and 3. Rhyolite, dacite and andesite from this volcano all possess ~22 Ma ages that overlap within uncertainty. (2) Extensional faulting and basaltic volcanism along NW-striking structures and fissures occurred between ~8.7 and 7.7 Ma. An angular unconformity of ~15° between 8.7 Ma and 7.7 Ma basalt units and faults that cut the older unit indicate ~20 m of normal offset occurred during the interval separating these volcanic events. (3) Extensional faulting along the NNE-striking Abert Rim Fault displaces 7.7 Ma basalt units by at least 300 m, indicating that most of the structural relief on the fault formed after that time. Strips of the Rattlesnake tuff mimic the modern trace of the Abert Rim, which suggests paleotopographic control by the Abert Rim Fault by 7 Ma. Furthermore, balanced cross-sections across the fault reveal a greater degree of separation of the ~16 Ma Steens Basalt compared to the Rattlesnake ignimbrite.

Deformation near Lake Abert records rifting of an Oligocene volcano in response to the development and northward propagation of the Abert Rim Fault since ~9 Ma. Volcanism and deformation along NW-striking features (stage 2) is interpreted to reflect dilatational fracturing ahead of a propagating NNE-striking Basin and Range fault (stage 3). Northward propagation of the Walker Lane, the western boundary of the Basin and Range north of 40°N lat., between 9 and 3 Ma provides a logical tectonic explanation for the progression of faulting observed in southern Oregon.