Fault textures in volcanic conduits evidence for

Authors: Hugh Tuffen Don Dingwell

Publish Date: 2004/09/22

Volume: 67, Issue: 4, Pages: 370-387

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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 longperiod 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 finegrained 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 frictioncontrolled stickslip 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 microliterich 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 strainrate 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 longperiod and hybrid earthquakes at silicic lava domes reflect frictioncontrolled stickslip movement and eventual healing of fault zones in magma which are an accelerated and smallerscale analogue of tectonic faultsMany thanks to Susan Sturton Yehuda Ben Zion Jürgen Neuberg Oliver Spieler Jacopo Taddeucci Emily Brodsky and Ron Vernon for interesting discussions The paper was much improved by thoughtful reviews from Larry Mastin John Stix and Hiroyuki Kumagai HT was funded by an Alexander von Humboldt Fellowship and the Cumberland Geological Society

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