Please use this identifier to cite or link to this item: http://hdl.handle.net/2381/32381
Title: Magnetic anisotropy in rhyolitic ignimbrite, Snake River Plain: implications for using remanant magnetism of volcanic rocks for correlation, paleomagnetic studies and geological reconstructions
Authors: Finn, D. R.
Coe, R. S.
Kelly, H.
Branney, Michael
Knott, Thomas
Reichow, Marc
First Published: 16-Jun-2015
Publisher: American Geophysical Union (AGU), Wiley
Citation: Journal of Geophysical Research: Solid Earth 120
Abstract: Individual ignimbrite cooling units in southern Idaho display significant variation of magnetic remanence directions and other magnetic properties. This complicates paleomagnetic correlation. The ignimbrites are intensely welded and exhibit mylonite-like flow banding produced by rheomorphic ductile shear during emplacement, prior to cooling below magnetic blocking temperatures. Glassy vitrophyric lithologies commonly have discrepantly shallow remanence directions rotated closer to the orientation of the subhorizontal shear fabric when compared to the microcrystalline center of the same cooling unit. To investigate this problem, we conducted a detailed paleomagnetic and rock magnetic study of a vertical profile through a single ignimbrite cooling unit and its underlying baked soil. The results demonstrate that large anisotropy of thermal remanent magnetization correlates with large (up to 38°) deflections of the stable remanence direction. Anisotropy of magnetic susceptibility revealed no strong anisotropy. A strong lineation and deflection of the remanence declination suggest that rheomorphic shear above magnetic blocking temperatures is the dominant mechanism controlling the formation of the magnetic fabric, with compaction contributing to a lesser extent. Nucleation and growth of anisotropic fine-grained magnetite in volcanic glass at high temperatures after, and perhaps also during, emplacement is indicated by systematic variation of magnetic properties from the quickly chilled ignimbrite base to the interior. These properties include remanence directions, anisotropy, coercivity, susceptibility, strength of natural remanent magnetization, and dominant unblocking temperature. The microcrystalline ignimbrite center has a magnetic direction that is the same as the underlying baked soil and, therefore, is a more reliable recorder of the paleofield direction than the glassy margins of highly welded ignimbrites.
DOI Link: 10.1002/2014JB011868
ISSN: 0148-0227
eISSN: 2156-2202
Links: http://onlinelibrary.wiley.com/doi/10.1002/2014JB011868/abstract
http://hdl.handle.net/2381/32381
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Archived with reference to SHERPA/RoMEO and publisher website. ©2015. American Geophysical Union. All Rights Reserved. Version of record: http://onlinelibrary.wiley.com/doi/10.1002/2014JB011868/abstract
Appears in Collections:Published Articles, Dept. of Geology

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