Magnetic fabric constraints on oroclinal bending of the Texas and Coffs Harbour blocks: New England Orogen, eastern Australia
Charles Aubourg, Chris Klootwijk, Russell J. Korsch, 2004. "Magnetic fabric constraints on oroclinal bending of the Texas and Coffs Harbour blocks: New England Orogen, eastern Australia", Magnetic Fabric: Methods and Applications, F. Martín-Hernández, C. M. Lüneburg, C. Aubourg, M. Jackson
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We have carried out a magnetic fabric study on deformed, clay-rich rocks from the Carboniferous Texas beds and Coffs Harbour Association in order to test the hypothesis of oroclinal bending of the Texas and Coffs Harbour blocks of the southern New England Orogen, eastern Australia. The magnetic susceptibility is dominated by paramagnetic phyllosilicates, with site-dependent contributions from the ferromagnetic minerals pyrrhotite, magnetite and hematite. Pyrrhotite is ubiquitous in the Texas block and in the central part of the Coffs Harbour block, and may be important as an indicator of the grade of low-temperature metamorphism. The magnetic fabric results, in general, show good agreement with structural observations in the Texas and Coffs Harbour blocks. The magnetic foliation is related to the pervasive cleavage associated with accretionary deformation prior to oroclinal bending. The magnitudes of the anisotropy parameters and the plunge of the magnetic lineation indicate an increase in the intensity of deformation from east of the Coffs Harbour orocline towards the Texas orocline. An imprint from oroclinal deformation is suggested by a significant increase in the anisotropy parameter and by the development of steeply plunging magnetic lineations towards the hinges of the Texas and Coffs Harbour oroclines. The Terrica beds from the Early Permian (Allandale) Terrica inlier in the Texas orocline also show another, pre-tilt induced, tectonic imprint. Remanence data from volcanics in the Alum Rock inlier (293 Ma) and magnetic fabric data from this inlier tentatively constrain the onset of oroclinal bending as prior to extrusion of the Alum Rocks, and the completion of bending as post-Terrica beds deposition and pre-Illawarra Reversal (est. 265 Ma).
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Fabric is a ubiquitous and significant feature of geological materials. The processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains. Petrofabrics thus provide essential keys to understanding the history of geological materials. Magnetic anisotropy is directly related to petrofabric, and has become one of the most rapid, sensitive and widely used tools for its characterization. The relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition, concentration and grain size of mineral grains. Ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved.
The papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations, mineral fabric development, technical advances and rock-magnetic properties.