MIOCENE-RECENT TECTONO-MAGMATIC EVOLUTION OF SOUTHEAST OREGON: IMPLICATIONS FOR GEOTHERMAL EXPLORATION

Volcanism in the northwest margin of the Basin and Range province started in the early Eocene, and after a short hiatus, started again in the late Oligocene (28 Ma) early Miocene and continued through the mid Miocene (17 Ma). This second volcanic event is thought to have occurred because of hinge rollback of the shallowly subducting Farallon plate in the Oligocene. After volcanism ceased, tectonic extension offset these early-mid Cenozoic units. This extensional event is younger than the central and southern Basin and Range province, which is mostly Eocene in age (~55-33 Ma). The structural framework of the area provides a unique view into young extensional terranes, where high angle normal faults and crustal blocks predominate. The result of volcanism followed by regional extension has led to numerous geothermal anomalies in southern Oregon, northern California, northern Nevada, and western Idaho. Geologic mapping near Paisley, Oregon has shown that faults created during mid Miocene extension have offset volcanic units of late Oligocene-early Miocene age. 2 meter survey data also shows high heat flow near the surface, ranging from 55-80 °F. The range and temporal change in the 2 m data suggests there is a flux of heat flow in the subsurface. A preliminary Simple Bouguer Gravity Anomaly map has also been created for the study area. A range of +24 to +35 mgals has been observed, with higher gravity values attributed to denser bedrock, adjacent to unconsolidated basin fill. Preliminary stable isotope data of water from the potential production well shows that it may be from meteoric sources, with mean δD and δ¹⁸O values of -120.066 and -14.307 (n=3), respectively.