Spatial and temporal trends in pre-caldera Jemez Mountains volcanic and fault activity

Abstract

New 40 Ar/ 39 Ar dates from the Jemez Mountain volcanic fi eld (JMVF) reveal formerly unrecognized shifts in the loci of pre-caldera volcanic centers across the northern Jemez Mountains; these shifts are interpreted to coincide with episodes of Rio Grande rift faulting. Early activity in the fi eld includes two eruptive pulses: 10.8-9.2 Ma basaltic to dacitic volcanism on Lobato Mesa in the northeastern JMVF and 12-9 Ma mafi c to silicic volcanism in the southwestern JMVF. While 9-7 Ma eruptions persisted in the southern JMVF, a new eruptive center developed on the La Grulla Plateau in the northwestern JMVF (8.7-7.2 Ma), corresponding with a period of rift widening caused by re acti va tion of Laramide faults in this area. The older 8.7-7.8 Ma mafi c lavas emitted from Encino Point and the younger 7.7-7.2 Ma trachyandesite and dacite erupted on the La Grulla Plateau are assigned to a new unit called the La Grulla Formation. The chemical composition of a 640 m stack of lava fl ows exposed in the northern margin of the Valles caldera changes from dacite to andesite, then back to dacite upsection, becoming slightly more alkalic upward. The shift to more alkalic compositions occurs across a sedimentary break, marking a subtle change in magma source for the older Paliza Canyon Formation and the younger La Grulla Formation lavas. New age constraints from a rhyolite intrusion in the southern JMVF and pumiceous rhyolite deposits in the northern JMVF suggest an episode of localized, 7.6-7.8 Ma rhyolitic volcanism that occurred in the central part of the JMVF between 12-8 Ma Canovas Canyon Rhyolite and 7-6 Ma peak Bearhead Rhyolite volcanism. Younger Bearhead Rhyolite intrusions (7.1-6.5 Ma) are more widespread than previously documented, extending into the northeastern JMVF. Tschicoma Formation dacite erupted at 5 Ma in the Sierra de los Valles and then erupted throughout the northeastern JMVF 5-2 Ma. The more refi ned geochronology of the JMVF indicates that pre-caldera volcanic centers were characterized by geographically and chemically distinct, relatively short-lived, episodes of activity. Volcanism generally migrated eastward through time in the southern JMVF, but the pattern in the northern JMVF had a more complex east (10-9 Ma) to west (9-7 Ma) to east (5-2 Ma) pattern that refl ects the timing of motion on faults. The new ages, coupled with detailed mapping of both volcanic rocks and the Santa Fe Group, document signifi cant pulses of faulting, erosion, and deposition during middle Miocene time and during late Miocene time across the Cañones fault zone in the northern JMVF.

The eruptive and magmatic history of the youngest pulse of volcanism at the Valles caldera: Implications for successfully dating late Quaternary eruptions

“…16 Ma and continuing to ca. 2 Ma (Gardner and Goff, 1996;Kelley et al, 2013), when magmas evolved to more rhyolitic compositions during the Quaternary (Self et al, 1988;Gardner et al, 2010). The Toledo caldera formed during the 1.651 ± 0.011 Ma 1 eruption of the Otowi Member of the Bandelier Tuff (Izett and Obradovich, 1994).…”

“…(e.g., Spell and Harrison, 1993;Phillips et al, 2007;Kelley et al, 2013). The Valles caldera, located in the Jemez Mountain volcanic field of north-central New Mexico (Fig.…”

Petrogenesis of the El Rechuelos Rhyolite, Jemez Mountains Volcanic Field, New Mexico, Usa

“…Four units have been dated using K/Ar geochronology; one dome at 7.5 Ma, one dome at 5.8 Ma, and two domes at 2.0 Ma (Dalrymple et al, 1967;Loeffler et al, 1988). The fifth unit has been 40 Ar/ 39 Ar dated at 2.0 Ma (Kelley et al, 2013), and is the southernmost of three domes erupted along a fracture system striking north-south to the west of Polvadera Peak. The age of this southern dome is in agreement with the K/Ar ages obtained for the other two domes along the fracture system.…”

“…As the ages of these rhyolites have previously been poorly constrained, 40 Ar/ 39 Ar ages are reported for all of the units examined in this study in an attempt to determine more accurate and precise constraints on eruptive timing. Previously reported ages were primarily obtained using K/Ar geochronology (Dalrymple et al, 1967;Loeffler et al, 1988), aside from the El Rechuelos units which have been inconsistently dated using 40 Ar/ 39 Ar geochronology (Justet, 2003;Kelley et al, 2013). In general, K/Ar ages may be questionable as it is not possible to check for excess argon, argon loss, alteration, or other problems which may affect accuracy (McDougall and Harrison, 1999).…”

Volcanism and sedimentation along the western margin of the Rio Grande rift between caldera-forming eruptions of the Jemez Mountains volcanic field, north-central New Mexico, USA

“…The Jemez MountainsMountains volcanic field (JMVF) represents more than 20 million years of volcanic activity that formed as a result of mantle derived heating initiated by extension of the Rio Grande rift (Kelley et al, 2013). The volcanic field is located at the intersection of the N-S trending rift with the northeast-trending Jemez lineament, a deepseated Proterozoic suture that may have localized melting and magma ascent ( Figure 1, Chapin et al, 2004).…”