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From crystal to crustal: petrofabric-derived seismic modelling of regional tectonics

By
G. E. Lloyd
G. E. Lloyd
Institute of Geophysics and Tectonics, School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK
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J. M. Halliday
J. M. Halliday
Institute of Geophysics and Tectonics, School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK
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R. W. H. Butler
R. W. H. Butler
Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Meston Building, King's College, Aberdeen AB24 3UE, UK
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M. Casey
M. Casey
Institute of Geophysics and Tectonics, School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK

Deceased.

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J.-M. Kendall
J.-M. Kendall
Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
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J. Wookey
J. Wookey
Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
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D. Mainprice
D. Mainprice
Géosciences Montpellier, CNRS & Université Montpellier 2, 34095 Montpellier, France
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Published:
January 01, 2011

Abstract

The Nanga Parbat Massif (NPM), Pakistan Himalaya, is an exhumed tract of Indian continental crust and represents an area of active crustal thickening and exhumation. While the most effective way to study the NPM at depth is through seismic imaging, interpretation depends upon knowledge of the seismic properties of the rocks. Gneissic, ‘mylonitic’ and cataclastic rocks emplaced at the surface were sampled as proxies for lithologies and fabrics currently accommodating deformation at depth. Mineral crystallographic preferred orientations (CPO) were measured via scanning electron microscope (SEM)/electron backscatter diffraction (EBSD), from which three-dimensional (3D) elastic constants, seismic velocities and anisotropies were predicted. Micas make the main contribution to sample anisotropy. Background gneisses have highest anisotropy (up to 10.4% shear-wave splitting, AVs) compared with samples exhibiting localized deformations (e.g. ‘mylonite’, 4.7% AVs; cataclasite, 1% AVs). Thus, mylonitic shear zones may be characterized by regions of low anisotropy compared to their wall rocks. CPO-derived sample elastic constants were used to construct seismic models of NPM tectonics, through which P-, S- and converted waves were ray-traced. Foliation orientation has dramatic effects on these waves. The seismic models suggest dominantly pure-shear tectonics for the NPM involving horizontal compression and vertical stretching, modified by localized ductile and brittle (‘simple’) shear deformations.

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Contents

Geological Society, London, Special Publications

Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy

David J. Prior
David J. Prior
University of Otago, New Zealand
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Ernest H. Rutter
Ernest H. Rutter
University of Manchester, UK
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Daniel J. Tatham
Daniel J. Tatham
University of Liverpool, UK
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Geological Society of London
Volume
360
ISBN electronic:
9781862394483
Publication date:
January 01, 2011

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