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Fault textures in volcanic conduits: evidence for seismic trigger mechanisms during silicic eruptions.

Tuffen, H and Dingwell, DB (2005) Fault textures in volcanic conduits: evidence for seismic trigger mechanisms during silicic eruptions. Bulletin of Volcanology, 67 (4). pp. 370-387. ISSN 1432-0819

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    Abstract

    It is proposed that fault textures in two dissected rhyolitic conduits in Iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows. Detailed field and petrographic analysis of such textures may shed light on the origin of long-period and hybrid volcanic earthquakes at active volcanoes. There is evidence at each conduit investigated for multiple seismogenic cycles, each of which involved four distinct evolutionary phases. In phase 1, shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow, forming the angular fracture networks that initiated faulting cycles. Transient pressure gradients were generated as the fractures opened, which led to fluidisation and clastic deposition of fine-grained particles that were derived from the fracture walls by abrasion. Fracture networks then progressively coalesced and rotated during subsequent slip (phase 2), developing into cataclasite zones with evidence for multiple localised slip events, fluidisation and grain size reduction. Phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction-controlled stick-slip behaviour. Increasing cohesion of cataclasites then led to aseismic, distributed ductile deformation (phase 3) and generated deformed cataclasite zones, which are enriched in metallic oxide microlites and resemble glassy pseudotachylite. Continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite-rich flow bands in obsidian (phase 4). Overall, the mixed brittle-ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle-ductile transition, with the rheological response mediated by strain-rate variations and frictional heating. Fault processes in highly viscous magma are compared with those elsewhere in the crust, and this comparison is used to appraise existing models of volcano seismic activity. Based on the textures observed, it is suggested that patterns of long-period and hybrid earthquakes at silicic lava domes reflect friction-controlled stick-slip movement and eventual healing of fault zones in magma, which are an accelerated and smaller-scale analogue of tectonic faults.

    Item Type: Article
    Journal or Publication Title: Bulletin of Volcanology
    Additional Information: The original publication is available at www.springerlink.com
    Uncontrolled Keywords: obsidian ; fracture ; faulting ; volcanic earthquake ; trigger ; hybrid earthquake ; Torfajokull ; Montserrat ; friction ; lava ; lava dome ; cataclastite ; flow banding ; rhyolite ; glass transition
    Subjects: Q Science > QE Geology
    Departments: Faculty of Science and Technology > Lancaster Environment Centre
    ID Code: 25
    Deposited By: Dr Hugh Tuffen
    Deposited On: 05 Jun 2005
    Refereed?: Yes
    Published?: Published
    Last Modified: 26 Jul 2012 16:26
    Identification Number:
    URI: http://eprints.lancs.ac.uk/id/eprint/25

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