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Towards the definition of AMS facies in the deposits of pyroclastic density currents

By
M. H. Ort
M. H. Ort
SESES, Northern Arizona University, Box 4099, Flagstaff, AZ 86011, USA
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T. T. Newkirk
T. T. Newkirk
Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, 2020-2207 Main Mall, Vancouver, BC, Canada V6T 1Z4
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J. F. Vilas
J. F. Vilas
Departamento de Geología, Instituto de Geofísica Daniel A. Valencio, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2, C1428EHA Buenos Aires, Argentina
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J. A. Vazquez
J. A. Vazquez
United States Geological Survey, 345 Middlefield Road, MS910, Menlo Park, CA 94025, USA
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Published:
January 01, 2015

Abstract

Anisotropy of magnetic susceptibility (AMS) provides a statistically robust technique to characterize the fabrics of deposits of pyroclastic density currents (PDCs). AMS fabrics in two types of pyroclastic deposits (small-volume phreatomagmatic currents in the Hopi Buttes volcanic field, Arizona, USA, and large-volume caldera-forming currents, Caviahue Caldera, Neuquén, Argentina) show similar patterns. Near the vent and in areas of high topographical roughness, AMS depositional fabrics are poorly grouped, with weak lineations and foliations. In a densely welded proximal ignimbrite, this fabric is overprinted by a foliation formed as the rock compacted and deformed. Medial deposits have moderate–strong AMS lineations and foliations. The most distal deposits have strong foliations but weak lineations. Based on these facies and existing models for pyroclastic density currents, deposition in the medial areas occurs from the strongly sheared, high-particle-concentration base of a density-stratified current. In proximal areas and where topography mixes this denser base upwards into the current, deposition occurs rapidly from a current with little uniformity to the shear, in which particles fall and collide in a chaotic fashion. Distal deposits are emplaced by a slowing or stalled current so that the dominant particle motion is vertical, leading to weak lineation and strong foliation.

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Contents

Geological Society, London, Special Publications

The Use of Palaeomagnetism and Rock Magnetism to Understand Volcanic Processes

M. H. Ort
M. H. Ort
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M. Porreca
M. Porreca
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J. W. Geissman
J. W. Geissman
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Geological Society of London
Volume
396
ISBN electronic:
9781862396722
Publication date:
January 01, 2015

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