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

Geoffery E Lloyd, J Halliday, Robert William Hope Butler, Martin Casey, J-M Kendall, J Wookey, David Mainprice

Research output: Contribution to journalArticle

17 Citations (Scopus)

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.
Original languageEnglish
Pages (from-to)49-78
Number of pages29
JournalGeological Society Special Publications
Volume360
DOIs
Publication statusPublished - 2011

Fingerprint

petrofabric
Tectonics
Anisotropy
anisotropy
preferred orientation
crystal
tectonics
Crystals
Elastic constants
modeling
cataclasite
seismic property
electron
seismic anisotropy
wave splitting
mylonite
crustal thickening
Rocks
Wall rock
wall rock

Keywords

  • seismic anisotropy
  • crustal strain
  • Himalayas

Cite this

From crystal to crustal : petrofabric-derived seismic modelling of regional tectonics. / Lloyd, Geoffery E; Halliday, J; Butler, Robert William Hope; Casey, Martin; Kendall, J-M; Wookey, J; Mainprice, David.

In: Geological Society Special Publications , Vol. 360, 2011, p. 49-78.

Research output: Contribution to journalArticle

Lloyd, Geoffery E ; Halliday, J ; Butler, Robert William Hope ; Casey, Martin ; Kendall, J-M ; Wookey, J ; Mainprice, David. / From crystal to crustal : petrofabric-derived seismic modelling of regional tectonics. In: Geological Society Special Publications . 2011 ; Vol. 360. pp. 49-78.
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