HR: 16:30h
AN: V44B-03 INVITED     [Abstracts]
TI: Central volcanoes: insights to volcanic plumbing networks from surface geology
AU: * Rowland, J V
EM: j.rowland@auckland.ac.nz
AF: University of Auckland, Private Bag 92019, Auckland, 1004 New Zealand
AU: Ebinger, C J
EM: c.ebinger@gl.rhul.ac.uk
AF: Royal Holloway, University of London, Egham, Surrey, TW20 0EX United Kingdom
AU: Wilson, C J
EM: cj.wilson@auckland.ac.nz
AF: University of Auckland, Private Bag 92019, Auckland, 1004 New Zealand
AB: Central volcanoes are common to magmatic rifts in all extensional tectonic settings, from oceanic ridges (Iceland) and continental rifts (Main Ethiopian Rift, MER) to arcs (Taupo Volcanic Zone, TVZ). Volcanism is typically bimodal, with silicic central volcanoes and intervening regions of basaltic dike swarms reflecting the presence of shallow and deep levels of magma storage, respectively. Central volcanoes are a secondary feature in rift evolution, requiring magma accumulation at a thermal or density barrier to form a shallow chamber that acts as a trap for magma injected from deeper sources. Their structural morphology arises from interactions between magmatic and mechanical processes and provides insights into large-scale volcanic plumbing networks. Extinct volcanic systems exposed in Iceland's Neogene lava pile provide constraints on 3-D structure of active systems. Plutons, <10-25km² in area, are surrounded by swarms of inclined dikes, a consequence of magmatically-induced stress heterogeneity. Extinct regional dike swarms are of similar scale to active fissure swarms. In the MER, central volcanoes developed at <2Ma after mechanical processes had thinned crust sufficiently to allow asthenospheric upwelling and associated thermal weakening. Early (11-2Ma) normal faults >65km long border the rift but are largely abandoned due to Quaternary localisation of extensional strain (c. 6mm/yr) within an en echelon series of 60km long, 20km wide magmatic segments comprising one or more central volcanoes, aligned volcanic vents, fissure and dike swarms and small-offset normal faults. Central volcanoes are in various stages of tectonic dissection, with normal faults and aligned basalt cones overprinting silicic edifices. The intensity of rift overprint increases toward Afar, where oceanic spreading is established. Active TVZ central volcanoes include extremely productive rhyolite calderas, Okataina and Taupo. This rift is unusual; basaltic volcanism comprises <1% total volcanic output although substantial intrusion of mafic magma must occur at depth given the immense heat flux through this region (>750mW/m²). Low density pyroclastic rocks blanket the central TVZ to depths of a few km, precluding fissure development and providing a density barrier to mafic magma ascent. Extremely efficient hydrothermal heat transfer to depths of <6-8km likely also provides a thermal barrier to silicic magma ascent. Nonetheless, the central TVZ is the most prolific rhyolitic centre on Earth; magma throughput is efficient and exploits tectonic assistance. Three intense rifting events have occurred in historic time. One of these accompanied the 1886 Tarawera eruption from Okataina. The two others at Taupo in 1922 and 1983 saw no eruption but were associated with earthquake swarms, preceded by uplift and followed by subsidence. These episodes suggest the presence of magma accumulation and migration. Conversely, fault displacement and extensive ground fissuring are known to accompany eruptions from Taupo Volcano. Rhythmicity of magmatic and tectonic processes interact non-linearly so that rifting events are regularly spaced but eruptive events are chaotic.
DE: 8002 Continental neotectonics (8107)
DE: 8038 Regional crustal structure
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8178 Tectonics and magmatism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005