EARLY MIOCENE VOLCANISM AND THE PRE-VOLCANIC SURFACE OF THE OREGON BASIN AND RANGE PROVINCE: EXPRESSION OF REGIONAL TECTONISM BETWEEN ~25-20 MA

The pre-Miocene geology of much of southern Oregon is obscured by Middle Miocene and younger volcanism. Late Miocene extension has exposed Early Miocene volcanoes and the pre-volcanic surface over a 625² km area of Lake County, Oregon. The oldest of these rocks occur at Grays Butte (lat: 42°59’00”N, long: 119°57’00”W) where rhyolite intrusion at 8.9 Ma tilted a section of clastic and volcaniclastic sediments ~75° to the SE. Mafic lava flows, formed 21.9 Ma, cap the tilted section and together they record fluvial-lacustrine deposition and lava emplacement into soft sediment. Hoodoos and fault surfaces at Grays Butte indicate a small amount of dextral slip occurred along an E-W striking fault during intrusion of the rhyolite. 22 m.y. old basaltic trachyandesite to trachyte composition dike swarms are exposed ~25 km to the south, within the Coleman Hills (lat: 42°48’00”N, long: 120°05’00”W). The dikes strike either N20-45°W or N5-10°E, intrude volcaniclastic sediments and feed lavas that exhibit invasive flow into hydroclastic tuff breccia. Euchre Butte, a subaerial satellite vent located on the SW flank of the volcano, erupted trachyte lavas and pyroclastic tuff 21.7 Ma. A NNE-SSW trending paleochannel cut into the volcano preserves a 21.8 m.y. old rhyolitic block and ash eruption that grades to a mafic lapilli tuff capped by trachyte lavas. Subaerial trachyandesite lavas were emplaced 23.1 Ma at the Rabbit Hills (lat: 42°37’00”N, long: 119°53’30”W), ~25 km southeast of the Coleman Hills. Rhyolite domes intruded the lavas at 22.3 and 20.3 Ma and exhibit E-W alignments. These observations combined provide a glimpse into a volcano-tectonic environment between ~25-20 Ma that was characterized by: (1) sedimentation into a basin, at least partially water-saturated, preceded volcanism at 23.1 to 20.3 Ma, (2) large composite volcanoes are local sources of intermediate composition lavas and silicic pyroclastic eruptions, (3) E-W-striking faults exhibit control on silicic intrusions and intermediate lavas sourced from fissures oriented normal and oblique to the silicic vents. A tectonic model that reconciles these observations involves formation of pull-apart basins, and volcanic centers, within right steps of a segmented, E-W-striking, dextral shear zone.