MIDDLE MIOCENE HALF GRABEN DEVELOPMENT PREDATES WIDESPREAD EXTENSION-RELATED MAGMATISM IN THE NW BASIN AND RANGE, OREGON

Processes controlling the northward propagation of the Basin and Range province in Oregon are poorly understood. Competing models of magmatically-driven faulting and tectonically-driven volcanism have yet to be validated. Differentiating between these end members depends on when extension initiated relative to widespread volcanism in the High Lava Plains (HLP), which cuts obliquely across the northern margin of the Northwestern Basin and Range (NWBR). To the southeast of Burns, OR a depositional basin formed by a family of N-S trending normal faults north of the Brothers fault zone, the Proto Harney Basin (PHB), constrains the onset of extension in the northernmost NWBR. New geologic mapping (1:30,000), measured stratigraphic sections, chemical analysis of volcanic units, and 40Ar/39Ar dating of the PHB rocks provide new insight on the timing of extension relative to magmatism. The PHB formed on the 16.5 Ma Steens Mountain basalt and is filled by fluvial, lacustrine, playa, airfall-ash, and airflow-ash sediments and two intra-basin lava flows. Onlap of the 9.68 Ma Devine Canyon ash-flow tuff onto the distal margin of half grabens demonstrates that extension-related subsidence of the PHB initiated in the Middle Miocene. Cross-sections were constructed to understand the geometric and temporal relationships of fault development, deposition and volcanism. A regional depocenter associated with a large central half-graben demonstrates that normal faulting was well established by ~10 Ma. Numerous smaller N-S oriented half-graben valleys synchronously developed to the west and an axial drainage network flowed eastward into the depocenter. HLP magmatism, in contrast, did not commence until ~10 Ma, as documented by rhyodacite domes, flows, and the Devine Canyon ash-flow tuff. Whereas the HLP magmatism apparently post-dates the onset of extension, a regional reorganization of the loci of subsidence and drainage network does broadly coincide with the magmatism. The western margin of the PHB shifted from aggradation to degradation and major subsidence shifted west to the site of the modern Harney Basin. This reorganization likely reflects the increasing influence of distributed magmatism on crustal deformation, basin development, and sedimentary processes.