Magnetic fabric, welding texture and strain fabric in the Nuraxi Tuff, Sardinia, Italy |
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Authors: | L Pioli R Lanza M Ort M Rosi |
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Institution: | (1) Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy;(2) Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, 10125 Torino, Italy;(3) Environmental Sciences and Geology, Northern Arizona University, P.O. Box 4099, 86011 Flagstaff, AZ, USA |
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Abstract: | Anisotropy of magnetic susceptibility (AMS) has been used to interpret flow directions in ignimbrites, but no study has demonstrated
that the AMS fabric corresponds to the flow fabric. In this paper, we show that the AMS and strain fabric coincide in a high-grade
ignimbrite, the Nuraxi Tuff, a Miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well-defined
magnetic fabric. Natural remanent magnetization (NRM) data indicate that the magnetization of the tuff is homogeneous and
was acquired at high temperatures by Ti-magnetite crystals. Comparison between the magnetic fabric and the deformation features
along a representative section shows that AMS and anisotropy of isothermal remanent magnetization (AIRM) fabric are coaxial
with and reproduce the shape of the strain ellipsoid. Magnetic tests and scanning electron microscopy observations indicate
that the fabric is due to trails of micrometer-size, pseudo-single domain, magnetically interacting magnetite crystals. Microlites
formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff. The fabric was thus
acquired after deposition, before late rheomorphic processes, and accurately mimics homogeneous deformation features of the
shards during welding processes and mass flow. |
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Keywords: | High-grade tuff Magnetic anisotropy Laminar flow Rheomorphism Welding Sardinia |
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