Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 1
Presentation Time: 1:10 PM


BONNICHSEN, Bill, 927 East 7th St, Moscow, ID 83843,

Large, explosive to effusive eruptions of high-temperature rhyolitic magma formed numerous rheomorphic welded-tuff units and many widespread clastigenic lava flows in the Bruneau-Jarbidge, Twin Falls, and western Snake River Plain (SRP) regions of SW Idaho during the late Miocene. The most intense activity was during the ignimbrite flare-up, between about 11.7 and 10.0 Ma. These eruptions led to the collapse of the Bruneau-Jarbidge and Twin Falls eruptive centers and coincided with the beginning of western SRP rifting. Silicic volcanism continued at a reduced rate in those regions until about 6.1 Ma ago. During the flare-up more than 50 large ignimbrite and clastigenic lava units, many more than 20 cu km in volume and some exceeding 100 cu km, were erupted as the eruptive centers developed by piecemeal, but large-scale, collapse. These size estimates exclude much larger rhyolite volumes believed to be buried in the SRP interior. Including their buried parts, the rhyolite units erupted during the flare-up likely had an aggregate volume exceeding 10,000 cu km, which is more than two-thirds of the rhyolite erupted from these three contiguous regions during their 12.7-6.1 Ma activity span. Following the ignimbrite flare-up, NE-SW- directed rifting widened the western SRP, and concomitantly pulled apart the zone from which the rhyolite units had erupted during the flare-up. This NE-SW extension led to the illusion that the Bruneau- Jarbidge and Twin Falls eruptive centers formed as separate entities; in reality they formed during collapse of a continuous crustal zone. This zone of collapse now extends northeastward along the eastern SRP to the Heise region. In the Bruneau-Jarbidge and Twin Falls regions the rhyolite units became hotter and lower in silica through time. In that region, the rhyolitic magmas probably were derived by wholesale melting of part of the Idaho batholith and other crustal materials by repeated injections of large basaltic magma batches. The ignimbrite flare-up represents the time during which the injected basalt was able to melt the largest available amount of fusible crustal material.