Article

Structure and petrology of newly discovered volcanic centers in the northern Kermadec–southern Tofua arc, South Pacific Ocean

Journal of Geophysical Research Atmospheres (Impact Factor: 3.44). 01/2008; 113. DOI: 10.1029/2007JB005453

ABSTRACT The NZAPLUME III expedition of September-October 2004 to the northern
Kermadec-southern Tofua (NKST) arc, between 28°52'S and 25°07'S,
resulted in the discovery of at least seven new submarine volcanic
centers and a substantial caldera complex adjacent to the previously
known Monowai Seamount. The volcanic centers form a sublinear chain that
coincides with the Kermadec Ridge crest in the south (Hinetapeka) and
diverges ˜45 km westward of the ridge crest in the north ("V")
just to the south of where the Louisville Ridge intersects with the arc.
All of the centers contain calderas or caldera-like structures, as well
as multiple cones, domes, fissure ridges, and vent fields. All show
signs of recent eruptive and current hydrothermal activity. There are
strong structural controls on edifice location, with cones and fissure
ridges typically associated with faulting parallel to the regional
˜12° strike of the arc front. Several of the calderas are
ellipsoidal, orientated northwest-southeast in the general direction of
least compressive stress. Sampled volcanic rocks, representing the most
recently erupted lavas, are all low-K tholeiites. Two of the centers,
Gamble and Rakahore, yielded only high-silica dacite to rhyolite (69-74
wt% silica), whereas two others, Monowai and "V," yielded only basalt to
andesite (48-63 wt% silica). Mineral assemblages are
plagioclase-pyroxene dominated, with accessory Fe-Ti oxides, apatite,
olivine, and quartz/tridymite/cristobalite, typical of dry volcanic arc
systems. Hornblende occurs only in a felsitic rhyolite from Hinepuia
volcanic center, and zircon is absent. Glass contents range to 57% in
basalts-andesites (mean 20%), and 97% in andesites-rhyolites (mean 59%)
and other quench textures, including swallow-tailed, plumose, or
dendritic crystal forms and crystallites, are common. Most lavas are
highly vesicular (≤63%; mean 28%) and have low volatile contents
(mostly <2 wt%) which, together with the occurrence of tridymite or
cristobalite, indicates explosive eruption and rapid cooling. Exceptions
are rocks from "U" volcanic center, which have low vesicularity and low
glass contents across a wide compositional range, indicating effusive
eruption. Disequilibrium mineral textures, the frequent occurrence of
xenoliths and xenocrysts, and macroscopic evidence for magma mingling
indicate that many of the lavas are hybrids, having resided only a short
time in upper crustal reservoirs prior to eruption. Silicic magmas are
major components of NKST arc volcanism and caldera formation is the
dominant eruptive style. The scale of silicic magmatism is in marked
contrast to the dominant basaltic-andesitic magmatism in the southern
Kermadec arc. With evidence from other arcs, silicic magmatism is now
recognized as a major feature of intraoceanic arcs globally.

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    ABSTRACT: The Monowai volcanic centre, located on the Tonga-Kermadec Arc, consists of a basaltic–andesitic submarine stratovolcano and adjacent caldera. Recent surveys have shown that it is active, but little is known about its structure and evolution. Here we present a combined analysis of swath bathymetry and potential field data from Monowai, acquired during cruise SO215 on the R/V Sonne, in 2011 April–June. The Monowai caldera is associated with a 20-25 mGal Bouguer gravity anomaly high and a broad positive magnetic anomaly. Short-wavelength magnetic anomalies of up to +1400 and -800 nT are observed along the caldera rim and on the summit of the stratovolcano. Inversion of the Bouguer gravity anomaly data shows that the caldera high is caused by a buried dense body consisting of a main unit extending from 3 to 6 km depth, and a shallower ring structure that underlies the rim of Monowai caldera and extends from the seafloor to the top of the main unit. We estimate the average density contrast to be about +450 kgm−3, corresponding to a density of 2650–2850 kgm−3, suggesting a mafic composition. The ring structure is interpreted as set of ring dykes and the main unit as a solidified or partly solidified magma chamber system (the Monowai pluton). The volume of the main unit is estimated to be 200–300 km3. The observed magnetic anomalies are consistent with magmatic intrusion within the Bruhnes magnetic polarity. Analysis of the swath bathymetry data shows that Monowai is located inside a 20 km- wide graben, part of an en-echelon, left-stepping horst and graben system that spans the length of the arc and back-arc. The Monowai caldera is elongated perpendicular to the direction of rifting, suggesting that it is affected by the regional stress regime. Ge- omorphological analysis suggests that the caldera consists of a single collapse structure and that its complex shape and multiple ring faults can be attributed to prolonged ac- tivity and multiple collapse episodes. It remains unclear whether the current eruption at the Monowai cone is fed directly from a small underlying magma chamber, or laterally from a magma body adjacent to the Monowai pluton. However, observation of radial fissure ridges on its flanks suggests that a shallow magma body may have been recently emplaced beneath the summit causing inflation and the formation of extensional cracks.
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    ABSTRACT: References: Lescinsky, D., 1990, Geology, volcanology and petrology of Cerro Bravo, a young dacitic stratovolcano in West-Central Colombia [Master's Thesis]: Hanover, NH, Dartmouth College, 244 pp. Monsalve, M.L., 1991. Mapa preliminar de amenaza volcánica del Volcán Cerro Bravo, INGEOMINAS, Prepared for the Government of Tolima and CRE-Tolima. Thouret, J.C., Cantagrel, J-M., Robin, C., Murcia, A., Salinas, R., and Cepeda, H., 1995, Quaternary eruptive history and hazard-zone model at Nevado del Tolima and Cerro Machin volcanoes, Colombia. Journal of Volcanology Geothermal Research, 66 (1-4):397-426. Thouret, J.C., Murcia, A., Salinas, R., Parra, E., Cepeda, H., and Cantagrel, J-M., Stratigraphy and quaternary eruptive history of the Ruiz-Tolima volcanic massif (Colombia): Implications for assessment of volcanic hazards. Symposium International Géodynamique Andine: Résumés des communications. 15-17 May 1990, Grenoble, France. p. 391-393. Information Contacts: María Luisa Monsalve, Gloria Patricia Cortés, and Cristian Mauricio López, Servicio Geológico Colombiano (SGC), Volcanological and Seismological Observatory, Avenida 12 Octubre 15-47, Manizales, Colombia (URL: http://www.ingeominas.gov.co/Manizales.aspx ). Previous Bulletin report: None!!
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