Explosive rhyolitic volcanism in the Jemez Mountains: Vent locations, caldera development and relation to regional structure

Abstract

The Jemez volcanic field straddles the western margin of the Rio Grande rift where the rift is intersected by the Jemez lineament in north central New Mexico. The field has a record of volcanism extending back to before 13 Ma. Initial basaltic activity was related to active rifting, with minor rhyolitic eruptions occurring along N‐S rift‐bounding faults. Between 10 and 7 Ma, voluminous andesitic volcanism took place in the central Jemez Mountains area, overwhelming contemporaneous basaltic and rhyolitic magmatism. An apparent tectonic lull took place from 7 to 4 Ma, accompanied by lower eruption rates. During this interval, dacitic magmas were erupted to form the Tschicoma volcanic center, but mafic and rhyolitic volcanism virtually ceased. Since 4 Ma, accompanying resumption of rifting, a growing silicic magma system has been present under the central part of the Jemez Mountains, ultimately evolving to the magma body that produced the voluminous rhyolitic Bandelier tuffs. Explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 Ma. Early eruptions, 3.6–2.8 Ma, produced high‐silica rhyolite ignimbrites restricted to the southwest part of the Jemez Mountains; formation of these units may have been accompanied by caldera collapse. These events were followed by the two caldera‐forming Bandelier Tuff ignimbrite eruptions, 1.45 and 1.12 Ma. Post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both Bandelier events. Vent and caldera locations for the rhyolitic eruptions during 3–4 Ma have been inferred from grain size characteristics, dispersal patterns, and facies variations in the Plinian deposits and ignimbrites. Lithic breccia zones of the pre‐Bandelier ignimbrites indicate possible caldera sources in the southwest part of the present Valles caldera. During the eruption of both Bandelier tuffs, initial plinian falls and early pyroclastic flows emanated from vents centrally located in the Jemez Mountains. In the lower Bandelier Tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows, but there is no strong evidence to suggest such a transition occurred during the upper Bandelier eruption. Calderas associated with the lower and upper Bandelier tuffs (Toledo and Valles, respectively) are almost identical in location, as are the Plinian vent sites for these two large eruptions. The Toledo embayment, northeast of the Valles caldera, contains lava domes from up to 3.6 Ma and may be a caldera or crater associated with early explosive dacitic volcanism in the Tschicoma volcanic center. Post‐1.4 Ma lava domes also fill this depression. The main volcanic features of the Jemez Mountains field, including the Valles caldera complex, eruption vents, and the apical graben of the post‐Valles‐Redondo resurgent block, appear to be aligned along the NE‐SW trending Jemez fault zone. This zone, the local expression of a Precambrian basement feature (the Jemez lineament), has exerted strong control on the location and style of eruptions from the Bandelier rhyolitic magma system.