The Glass Buttes volcanic complex is a cluster of bimodal (basalt-rhyolite), Miocene to Pleistocene age lava flows and domes located in Oregon's High Lava Plains province, a broad region of Cenozoic bimodal volcanism in south-central Oregon. The High Lava Plains is deformed by northwest-striking faults of the Brothers Fault Zone, a diffuse, ~N40°W trending zone of en echelon faults cutting ~250 km obliquely across the High Lava Plains. Individual fault segments within the Brothers Fault Zone are typically <20 km long, strike ~N40°W, have apparent normal separation with 10-100 m throw. A smaller population of ~5-10 km long faults striking ~N30°E exhibits mutually crosscutting relationships with the dominant northwest striking faults. Basaltic volcanic rocks in the Glass Buttes area erupted during the late Miocene and Pleistocene. The oldest and youngest lavas are 6.49±0.03 Ma and 1.39±0.18 Ma, respectively, based on ⁴⁰Ar/³⁹Ar ages of five basaltic units. Numerous small mafic vents both within and around the margins of the main silicic dome complex are commonly localized along northwest-striking faults of the Brothers Fault Zone. These vents erupted a diverse suite of basalt to basaltic andesite lava flows that are here differentiated into 15 stratigraphic units based on hand sample texture and mineralogy as well as major and trace element geochemistry. The structural fabric of the Glass Buttes area is dominated by small displacement, discontinuous, en echelon, northwest-striking fault scarps that result from normal to slightly oblique displacements and are commonly linked by relay ramps. Northwest alignment of basaltic and rhyolitic vents, paleotopography, and cross-cutting relationships suggest these faults have been active since at least 6.49±0.03 Ma, the age of the rhyolite lavas in the eastern Glass Buttes are. Faults displace Quaternary sedimentary deposits indicating these structures continue to be active into the Quaternary. Long-term extension rates across northwest-striking faults calculated from 2-5 km long cross section restorations range from 0.004 – 0.02 mm/yr with an average of 0.12 mm/yr. A subordinate population of discontinuous northeast-striking faults form scarps and exhibit mutually cross-cutting relationships with the dominant northwest-striking population. Cross-cutting relationships indicate faulting on northeast-striking faults ceased sometime between 4.70±0.27 Ma and 1.39±0.18 Ma. Gravity data at Glass Buttes reveals prominent northwest- and northeast-trending gravity gradients that closely parallel the strikes of surface faults. These are interpreted as large, deep-seated, normal faults that express themselves in the young basalts at the surface as the discontinuous, en echelon fault segments seen throughout the study area and BFZ in general. Elevated geothermal gradients are localized along these deep-seated structures at two locations: (1) where northwest- and northeast-striking faults intersect,(2) along a very prominent northwest-striking active normal fault bounding the southwest flank of Glass Butte. High average heat flow and elevated average geothermal gradients across the High Lava Plains, and the presence of hydrothermal alteration motivated geothermal resource exploration at Glass Buttes. Temperature gradient drilling by Phillips Petroleum and others between 1977-1981 to depths of up to 600 m defined a local geothermal anomaly underlying the Glass Buttes volcanic complex with a maximum gradient of 224 °C/km. Stratigraphic constraints indicate that near-surface hydrothermal alteration associated with mercury ores ceased before 4.70±0.27 Ma, and is likely associated with the 6.49±0.03 Ma rhyolite eruptions in the eastern part of Glass Buttes. The modern thermal anomaly is not directly related to the pre-4.70±0.27 Ma hydrothermal system; rather it is likely a result of deep fluid circulation along major extensional faults in the area. / Graduation date: 2013 / Includes accompanying DVD with digital data supplement (8 GB).

This dissertation integrates a process-based hydrological investigation with an ongoing paired-catchment study to better understand how forest harvest impacts catchment function at multiple scales. We do this by addressing fundamental questions related to the stocks, flows and transit times of water. Isotope tracers are used within a top-down catchment intercomparison framework to investigate the role of geology in controlling streamwater mean transit time and their scaling relationships with the surrounding landscape. We found that streams draining catchments with permeable bedrock geology at the Drift Creek watershed in the Oregon Coast Range had longer mean transit times than catchments with poorly permeable bedrock at the HJ Andrews Experimental Forest in the Oregon Cascades. We also found that differences in permeability contrasts within the subsurface controlled whether mean transit time scaled with indices of catchment topography (for the poorly permeable bedrock) or with catchment area (for the permeable bedrock). We then investigated the process-reasons for the observed differences in mean transit time ranges and scaling behavior using a detailed, bottom-up approach to characterize subsurface water stores and fluxes. We found that the mean transit times in catchments underlain by permeable bedrock were influenced by multiple subsurface storage pools with different groundwater ages, whereas storage in the poorly permeable catchments was limited to the soil profile and that resulted in quick routing of excess water to the stream at the soil bedrock interface, leading to mean transit times that were closely related to flowpath lengths and gradients. Finally, we examined how and where forest trees interacted with subsurface storage during the growing season using a forest manipulation experiment, where we tested the null hypothesis that near-stream trees alone influenced daily fluctuations in streamflow. We felled trees within this zone for two 2.5 ha basins and combined this with isotopic tracing of tree xylem water to test if water sources utilized by trees actively contributed to summer streamflow. We rejected our null hypotheses and found that diel fluctuations in streamflow were not generated exclusively in the near-stream zone. We were unable to link, isotopically, the water sources trees were utilizing to water that was contributing to streamflow. Our results provide new process-insights to how water is stored, extracted, and discharged from our forested catchments in Western Oregon that will help better explain how forest removal influences streamflow across multiple scales and geological conditions. / Graduation date: 2012