New Mexico Geological Society Annual Spring Meeting — Abstracts

Explosive rhyolitic eruptions from a large compositionally-zoned magma body have occurred in the Jemez Mountains volcanic center of New Mexico for the past 3+1 Ma. Grain-size characteristics, dispersal patterns and facies variations in plinian pumice and pyroclastic flow deposits around the Jemez Mountains indicate vent locations and their relationship to caldera sites. Early eruptions (c. 3 Ma ago) from the magma body produced at least two high-silica rhyolitic ignimbrites that are restricted to the SW part of the Jemez , Mountains. Studies of lithic breccia zones in the ignimbrites indicate that they probably came from calderas in the SW part of the present Valles caldera. Each of the Bandelier Tuff ignimbrite eruptions (1.45 and 1.12 Ma ago) began with a plinian phase from vent(s) located in the central part of the Jemez Mountains. Early pyroclastic flows issued from the same sites, then a transition to vents located along incipient caldera ring-fractures took place. Ring fracture vents cannot be accurately located, but they may have lain where the ring fracture system crossed the regional Jemez Fault, Zone trend. The location of both plinian vent sites and both calderas (Toledo and Valles) of the Bandelier eruptions were remarkably similar. Another, more conjectural, caldera lies in the site of the Toledo embayment, NE of the Valles caldera: it may be a caldera associated with eruptions from the Tschicoma Mountain volcanic center. Eruptive vents and loci of the calderas in the Jemez Mountains caldera complex fall along a zone that coincides with the Jemez fault zone. This zone, the local expression of a Precambrian basement feature known as the Jemez Lineament, has exerted a strong influence on eruptions from the Jemez Mountains rhyolitic magma system.