GEOLOGIC MAP OF THE HAWKS VALLEY-LONE MOUNTAIN REGION, HARNEY COUNTY, OREGON, SCALE 1:24,000

The Hawks Valley - Lone Mountain region (HVLM) of southeastern Oregon is located at the transition between the Basin & Range and High Lava Plains provinces and exemplifies relationships between extensional tectonism and magmatism. Recent geologic mapping at the 1:24,000 scale sponsored by the USGS EDMAP Program plus supporting analytical work build on earlier observations and data to reveal a complex interplay between regional mafic magmatism, regional fault patterns, and local silicic magma generation and evolution. The HVLM is characterized by a NW‐striking structural valley that cuts an ~150 km², trachyte‐trachydacite‐rhyolite volcanic complex. Ar-Ar and K-Ar radiometric ages and stratigraphic relationships indicate that the bulk of HVLM volcanism occurred during a <1 m.y. window synchronous with or shortly following eruptions of the Steens Basalt from vents less than 20 km to the east.

Numerous 16.3±0.3 Ma silicic units derive from up to eight local vents and from others external to the map area. Most of the HVLM silicic vents are situated at or near the intersection of younger NNE‐ and NNW‐striking structures indicating that both structural trends were active and available for magma passage in the mid‐Miocene. The silicic units are distinguished based on field relationships, petrography, and major/trace element geochemistry and frequently preserve clear evidence for complex, open system differentiation processes in the form of resorbed alkali feldspar and resorbed and complexly-zoned plagioclase crystals. Between 0.8 to 0.5 Ma, small volume low-K, high-Al olivine tholeiite (HAOT) eruptions emanated from fissures along reactivated NNE-striking structures and were subsequently displaced by NNE-striking faults. These young NNE-striking faults also displace the older silicic units and NNW-striking structures.

Proximity to temporally equivalent Steens flood basalt eruptive loci and the structural complexities suggest that mid-Miocene 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 HVLM silicic suite. The Quaternary basalts are products of regional extension and relatively shallow basalt melt generation processes.