A multidisciplinary investigation into the eruptive style, processes, and duration of a Cascades back-arc tholeiitic basalt: A case study of the Brushy Butte flow field, northern California, United States

The back-arc of the Cascades volcanic arc in northern California is dominated by monogenetic tholeiitic basalts that erupted throughout the Pleistocene. Elucidating the eruptive history and processes of these tholeiitic basalts is important for understanding potential future eruptions here. We focus on the well-exposed Brushy Butte flow field, which constructed a ~150 m tall edifice, has flow lobes up to >10 km long, and in total cover ~150 km2 with an eruptive volume of 3.5 km3. We use a multidisciplinary approach of field mapping, petrography, geochemistry, paleomagnetism, geochronology, and lidar imagery to unravel the eruptive history and processes that emplaced this flow field. Tholeiitic basalts in northern California have various surface morphologies and vegetation cover but similar outcrop, hand sample, and thin section petrographic appearances, and making them hard to distinguish in the field. Geochemistry and paleomagnetism offer an independent means of distinguishing tholeiitic basalts. Lavas of the Brushy Butte flow field are similar in all major-oxide and trace-element abundances but differ from adjacent tholeiitic basalts. This is also apparent in remanent magnetic directions. Additionally, paleomagnetic data indicate that the flow field was emplaced during a geologically brief time interval (10–20 yr), which 36Cl cosmogenic dating puts at 35.7 ± 1.7 ka. The 1 m-resolution lidar imagery shows that these flows erupted from at least 28 vents encompassing multiple scoria cones, spatter cones, and craters. The flows can be grouped into at least four pulses using stratigraphic position and volume. Pulse 1 is the most voluminous, comprising eight eruptions and ~2.3 km3. Each subsequent pulse started rapidly but decayed quickly, and each successive pulse erupted less lava (i.e., 2.3 km3 for pulse 1, 0.6 km3 for pulse 2, 0.3 km3 for pulse 3, and 0.2 km3 for pulse 4). Many of these flows host well-established lava channels and levees (with channel breakouts) that lead to lava fans, with some flows hosting lava ponds. Similar flow features from tholeiitic basalt eruptions elsewhere demonstrate that these sorts of morphologies generally occur in eruptions that have been ongoing for weeks, months, or longer (e.g., Puʻu ʻŌʻō eruption at Kīlauea, Hawaiʻi).