Mica, deformation fabrics and the seismic properties of the continental crust

Geoffrey E. Lloyd, Robert W.H. Butler, Martin Casey, David Mainprice

Research output: Contribution to journalArticle

50 Citations (Scopus)

Abstract

Seismic anisotropy originating within the continental crust is used to determine kinematic flow lines within active mountain belts and is widely attributed to regionally aligned mica. However, naturally deformed micaceous rocks commonly show composite (e.g. S-C) fabrics. It is necessary therefore to understand how both varying mica content and differing intensities of multiple foliations impact on seismic interpretations in terms of deformation fields. An outcrop analogue for granitic mid-crustal deformed zones is used here to calibrate the seismic response against both parameters. Seismic responses are modelled using crystallographic preferred orientations for polymineralic, micaceous granitic gneisses, measured using Electron Back-Scatter Diffraction. The sample results are generalised by modelling the effects of variations in modal composition and the relative importance of deformation fabrics of variable orientation, so-called rock and fabric recipes. The maximum P- and S-anisotropy are calculated at 16.6% and 23.9% for single-foliation gneisses but for mixed (i.e. S-C) foliation gneisses these values reduce to 5.8% and 7.5% respectively. Furthermore, mixtures of multiple foliations generate significant variations in the geometry of the seismic anisotropy. This effect, coupled with the geographical orientation of fabrics in nature. can generate substantial variations in the orientation and magnitude of seismic anisotropy (especially for shear waves) as measured for the continental crust using existing receiver function and teleseismic near-vertical incidence methods. Thus, maps of seismic anisotropy varying with depth in deforming continents need not imply necessarily depth-varying deformation kinematics and tectonic decoupling. (C) 2009 Elsevier B.V. All rights reserved.

Original languageEnglish
Pages (from-to)320-328
Number of pages9
JournalEarth and Planetary Science Letters
Volume288
Issue number1-2
Early online date23 Oct 2009
DOIs
Publication statusPublished - 30 Oct 2009

Keywords

  • mica
  • seismic anisotropy
  • continental tectonics
  • S-C fabrics
  • CPO
  • EBSD
  • lattice-preferred orientation
  • velocity anisotropy
  • receiver functions
  • Tibetan Plateau
  • rich rocks
  • shear
  • beneath
  • zone
  • reflectivity
  • elasticity

Cite this

Mica, deformation fabrics and the seismic properties of the continental crust. / Lloyd, Geoffrey E.; Butler, Robert W.H.; Casey, Martin; Mainprice, David.

In: Earth and Planetary Science Letters, Vol. 288, No. 1-2, 30.10.2009, p. 320-328.

Research output: Contribution to journalArticle

Lloyd, Geoffrey E. ; Butler, Robert W.H. ; Casey, Martin ; Mainprice, David. / Mica, deformation fabrics and the seismic properties of the continental crust. In: Earth and Planetary Science Letters. 2009 ; Vol. 288, No. 1-2. pp. 320-328.
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AB - Seismic anisotropy originating within the continental crust is used to determine kinematic flow lines within active mountain belts and is widely attributed to regionally aligned mica. However, naturally deformed micaceous rocks commonly show composite (e.g. S-C) fabrics. It is necessary therefore to understand how both varying mica content and differing intensities of multiple foliations impact on seismic interpretations in terms of deformation fields. An outcrop analogue for granitic mid-crustal deformed zones is used here to calibrate the seismic response against both parameters. Seismic responses are modelled using crystallographic preferred orientations for polymineralic, micaceous granitic gneisses, measured using Electron Back-Scatter Diffraction. The sample results are generalised by modelling the effects of variations in modal composition and the relative importance of deformation fabrics of variable orientation, so-called rock and fabric recipes. The maximum P- and S-anisotropy are calculated at 16.6% and 23.9% for single-foliation gneisses but for mixed (i.e. S-C) foliation gneisses these values reduce to 5.8% and 7.5% respectively. Furthermore, mixtures of multiple foliations generate significant variations in the geometry of the seismic anisotropy. This effect, coupled with the geographical orientation of fabrics in nature. can generate substantial variations in the orientation and magnitude of seismic anisotropy (especially for shear waves) as measured for the continental crust using existing receiver function and teleseismic near-vertical incidence methods. Thus, maps of seismic anisotropy varying with depth in deforming continents need not imply necessarily depth-varying deformation kinematics and tectonic decoupling. (C) 2009 Elsevier B.V. All rights reserved.

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