ORIGIN OF FAULTS, FISSURES AND VOLCANIC VENT ALIGNMENTS AT A STRUCTURAL TRIPLE JUNCTION, NEWBERRY VOLCANO, CENTRAL OREGON

Newberry Volcano in central Oregon formed at the intersection of extensional fault systems defining the Basin and Range extensional province, the Cascades Graben and the Brothers Fault Zone. A unique structural orientation characterizes each fault system. Basin and Range faults trend north, northeast and northwest. The Cascade Graben is defined by north-trending faults. The Brothers Fault Zone is defined by northwest trending faults. In this study we mapped faults, fissures, and vent alignments using LiDAR topographic data to investigate the stress regime of the volcanic edifice and surrounding environs. We seek to better understand the relationship between volcanic and tectonic signatures of stress regime and sites for proposed development of geothermal energy. Stress regime across a volcanic edifice reflects some combination of sources from plate motion, magma chambers at depth, and topography of the edifice itself. Orientation of faults, fissures and volcanic vent (cinder cones and volcanic vents typically parallel fault trend) alignments vary systematically around the volcanic edifice at Newberry. On the north side of the volcano, faults, fissures and volcanic vents generally trend north-northwest. Ring dikes, fractures and vents follow the rim defining the edge of the caldera. South of the caldera, faults, fissures and volcanic vents trend northeast. Whereas the ring dike and fault system around the caldera rim clearly reflect volcanic processes, the faults and volcanic features on the north side of the volcano parallel Basin and Range faults away from the edifice and apparently merge northward with the Cascade Graben fault system. Thus we conclude that these are tectonic faults formed in a stress regime controlled by plate motions. In contrast, the northeast trending faults in the south are enigmatic and unlike any other faults found regionally. One possible explanation is that these faults represent one arm of a radial fault system, the north arm of which has been reactivated as the result of overprinting by regional crustal extension. Geothermal prospects occur in the northern fault system, which reflects a tectonic rather than a volcanically or topographically controlled stress regime.