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

The Use of Palaeomagnetism and Rock Magnetism to Understand Volcanic Processes

By:
, , ,
Edited by:
M.H. Ort, Massimiliano Porreca, J.W. Geissman
DOI: 10.1144/SP396.8

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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.

Geospatial Extents

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Towards the definition of AMS facies in the deposits of pyroclastic density currents
Series title:
The Use of Palaeomagnetism and Rock Magnetism to Understand Volcanic Processes
DOI:
10.1144/SP396.8
Volume
396
Year Published:
2014
Language:
English
Publisher:
Geological Society of London
Contributing office(s):
Volcano Science Center
Description:
22 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
The Use of Palaeomagnetism and Rock Magnetism to Understand Volcanic Processes
Number of Pages:
22
Country:
Argentina;United States
State:
Arizona;Neuqu�n
Other Geospatial:
Caviahue Caldera;Hopi Buttes Volcanic Field