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Open-File Report O-11-12, Geologic Map of the Hawks Valley-Lone Mountain Region, Harney County, Oregon, scale 1:36,000, by Alicja Wypych, William K. Hart, Kaleb C. Scarberry, Kelly C. McHugh, Stephen A. Pasquale, and Paul W. Legge.
Lidar-based map; actual size 46 x 38 inches, scale 1:24,000.
Order publication CD, $15, CD-ROM, from Nature of the Northwest. CD-ROM includes PDF map plate and 28 p. report text.
New geologic mapping and supporting analytical work combined with previous observations and data reveal a complex interplay between regional mafic magmatism, regional fault patterns, and local silicic magma generation and evolution in the Hawks Valley‐Lone Mountain area of southeastern Oregon. This portion of Oregon provides an ideal natural laboratory for field‐based studies of mid‐ Miocene and younger extensional tectonism and magmatism due to well‐developed fault patterns and near continuous volcanism since ~16.6 Ma. Our observations and data indicate that the Hawks Valley‐ Lone Mountain (HVLM) area is characterized by a NW‐striking structural valley that cuts an ~150 km2 mid‐Miocene, trachyte‐trachydacite‐rhyolite volcanic complex. New radiometric ages and stratigraphic relationships indicate that the bulk of HVLM volcanism occurred during a <1 m.y. window synchronous with or shortly following eruption of the Steens Basalt from fissures in the Steens‐Pueblo Mountains located less than 20 km to the east.
Numerous 16.3±0.3 Ma silicic units, most emanating from at least eight local vents, are distinguished based on field relationships, petrography, and major/trace element geochemistry. Resorbed alkali feldspar and resorbed and complexly‐zoned plagioclase indicate that magma mixing and/or crustal assimilation were important petrogenetic processes. Geochemical parameters support these observations and require a major role for crystal fractionation in the evolution of the HVLM silicic materials. The HVLM's structural complexities and its proximity to exposures of temporally equivalent Steens flood basalt eruptive loci suggest that basaltic input into the crust stimulated local melt production, the establishment of multiple small upper‐level magmatic systems along regional lithospheric weaknesses, and open system differentiation leading to the silicic suite. After a more than 15 m.y. hiatus local HVLM volcanism resumed with the eruption of 0.8‐0.5 Ma low‐K, high‐Al olivine tholeiite (HAOT) lavas characteristic of the High Lava Plains region and of relatively shallow basalt melt generation processes.
The Quaternary HVLM magmatism produced small volumes of basalt that were erupted along and offset by reactivated NNE‐striking structures. In contrast, NW‐striking faults, with rare exception, displace only the mid‐Miocene silicic lavas and are in turn cut by NNE‐striking structures. Additionally, there is a strong association of silicic vent locations and these structural trends. Numerous 16.3‐16.5 Ma rhyolite/trachyte vents are situated at or near the intersection of younger NNE‐ and NW‐striking structures indicating that both structural trends were active and available for magma passage in the mid‐Miocene.