Location of Repository

Towards using seismic anisotropy to interpret ductile deformation in mafic lower crust

By Daniel John Tatham


The lower crust forms an important geodynamic control in continental tectonics and the communication and coupling of kinematics between surface and deep-Earth processes. An understanding of the relationship between seismic properties, finite strain and fabric orientation thus provides a useful tool in the remote sensing and\ud interpretation of deformation in the lower crust.\ud \ud This thesis outlines a work-flow model by which the seismic properties of a single and representative lower crustal lithology can be calculated and calibrated against\ud finite strain from petrofabric development across a strain gradient. The work-flow model constitutes a multi-disciplinary approach, incorporating field mapping and\ud sample collection, experimental petrofabric determination, and seismic modelling.\ud \ud A review of compositional estimates of the deep crust, including xenoliths, exposed sections and estimates from wide-angle seismic profiles, indicates the importance\ud of mafic lithologies.\ud \ud The Laxfordian-age high-grade shear zone at Upper Badcall, NW Scotland, exhibits a strain gradient in a deformed doleritic Scourie dyke (Lewisian complex) that intersects the zone at a high angle. From an analysis of field data from detailed mapping, the shear zone is shown to be characterised by generally simple shear, but where the tectonic movement direction varies transversely across the shear zone. Calculation of the strain profile across the deformation zone gives shear strains, y up to 57, but with y < 15 being perhaps more realistic. Cumulative displacements\ud total ~1000m left-laterally, and ~600m vertical displacement, north-side up. Nine samples were collected across the shear zone in the mafic dyke, representing a\ud strain gradient from undeformed protolith to the highest recorded stains.\ud \ud The sample suite is characterised as a hornblende-plagioclase-quartz aggregate that develops macroscopic planar and linear fabrics with strain, from an essentially\ud isotropic protolith. Quantification of the aggregate lattice preferred orientation (LPO) using electron backscatter diffraction (EBSD) showed the dominance of\ud fabric development in the hornblende phase, with (100) poles clustering forming normal to the foliation plane and [001] axes parallel to the tectonic X direction.\ud Plagioclase and quartz retained random fabrics from the wall-rock protolith with increasing finite strain. The hornblende LPO fabric, described by the texture index, J, shows a positive logarithmic relationship with strain, where LPO intensity saturated by y ~10.\ud \ud The strain-calibrated quantitative petrofabric description of each sample is used to calculate their aggregate elasticity tensors (Cij) via a Voigt-Reuss-Bill average,\ud and from which seismic properties are derived using Christoffel's equation. Hence, a framework of petrofabric- and strain-calibrated seismic properties is described\ud for a strain gradient in a representative high-grade mafic lithology. P-wave anisotropies up to ~10% are-recorded in the most deformed samples with Vsmax typically between 6.42-6.63kms/-1. S-wave anisotropies record up to 7.23% AV,\ud in the most deformed samples, with Vpmax ranging between 3.62-3.75kms-1 for all samples. The relationship between petrofabric-derived seismic anisotropy and finite strain across the sample suite show a positive relationship, approximated by a logarithmic function, whereby P- and S-wave anisotropy exhibit a steep positive gradient with strain up to y~10.\ud \ud The sample-wise framework of petrofabric- and strain-calibrated seismic properties is interpolated to estimate the continuum relationship between seismic properties,\ud finite strain and petrofabric orientation. In a move to illustrate the application of results in seismic and structural modelling, case study models of crustal deformation are presented for the eastern Basin and Range province, the North Sea rift, and Tibet. Models are promising in their ability to differentiate between regions of lower crust characterised by a uniform mafic composition but different finite strain state and/or petrofabric geometry, although multiple seismic survey methods may\ud be needed to fully interpret results in terms of strain and fabric orientation.\ud \ud In summary, a multidisciplinary approach combining field mapping and sampling, petrofabric characterisation with EBSD, and seismic modelling provides an efficient and reproducible work-flow for the determination of petrofabric-derived strain-calibrated seismic properties of lower crustal materials.\ud \u

Publisher: School of Earth and Environment (Leeds)
Year: 2008
OAI identifier: oai:etheses.whiterose.ac.uk:592

Suggested articles



  1. (1961). The elastic properties of rock-forming minerals, I: pyroxenes and amphiboles.
  2. (1974). Velocities of elastic waves in minerals at atmospheric pressure and increasing of precision of elastic constants by means of EVM.
  3. (1993). seismic velocities calculated from latticepreferred orientation and reflectivity of a lower crustal section: examples of the Val Scsia section (Ivrea zone, northern Italy). doi
  4. (1996). Seismic anisotropy and shear-wave splitting in lower-crustal and upper-mantle rocks from the lvrea Zone - experimental and calculated data. doi
  5. (1998). An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy. doi
  6. (1996). Deformation mechanisms and reaction of hornblende: examples from the Bcrgell tonalite (Central Alps). doi
  7. (1982). Texture Analysis In Materials Science. doi
  8. (2000). Elastic properties of polycrystals-influence of texture and stereology. doi
  9. (1994). Special Issue: seismic properties of crustal and mantle rocks - laboratory measurements and theoretical calculations.
  10. (1984). The magnitude, symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tcctonites. doi
  11. (1995). Seismic velocity structure and composition of the continental crust: a global review. doi
  12. (1991). Chronology and mechanism of depletion in Lewisian granulites. doi
  13. (1983). Complex strain patterns at the frontal and lateral tips to shear zones and thrust zones. doi
  14. (1981). A review of wave motion in anisotropic and cracked elastic media. doi
  15. (1984). Seismic anisotropy - the state of the art: doi
  16. (2007). Development of lattice preferred orientation in clinoamphiboles deformed under low-pressure metamorphic conditions -a SEM/EBSD study of metabasites from the Aracena metamorphic belt, doi
  17. (1994). Flow laws for rocks containing two non-linear viscous phases: a phenomenological approach. doi
  18. (1952). The elastic behaviour of a crystalline aggregate. doi
  19. (1991). The petrophysical basis for the interpretation of seismological models of the continental lithosphere.
  20. (2002). Handbook of seismic Properties of Minerals, Rocks and Ores.
  21. (2000). Seismic Anisotropy in the boundary layers of the mantle. In: doi
  22. (1996). Fabric-related seismic anisotropy in tipper-mantle xenoliths: evidence from measurements and calculations. doi
  23. (2005). Proposal for a terrane-based nomenclature for the Lewisian Gneiss doi
  24. (2005). Petrofubric Derived Seismic Properties of a Mylonitic Quartz Simple Shear Zone: Implications for Seismic Reflection Profiling. In: doi
  25. (2006). Retrograde mica in deep crustal granulites: implications for crustal seismic anisotropy. doi
  26. (1990). An efficient Fortran program to calculate seismic anisotropy from the lattice prcfari-ed orientation of minerals. doi
  27. (1989). Development of shape and lattice preferred orientations: application to the seismic anisotropy of the lower crust, doi
  28. (1993). Interpretation of SKS-waves using samples from the subcontinental lithosphere. doi
  29. (1994). Methods of calculating petrophysical properties from lattice preferred orientation data. doi
  30. (2000). The Seismic Anisotropy of the Earth's Mantle: from Single Crystal to Polycrystal. In: doi
  31. (1965). Elastic moduli of quartz versus hydrostatic pressure at 25 °C and doi
  32. (2006). Seismic lamination and anisotropy of the lower continental crust. doi
  33. (2001). Nanga Parbat crustal anisotropy: implications for interpretation of crustal velocity structure and shear-wave splitting. doi
  34. (2004). The role of pre-existing mechanical anisotropy on shear zone development within oceanic mantle lithosphere: an example from the Oman ophiolite. doi
  35. (2004). Crustal seismic anisotropy in central Tibet: implications for deformation style and flow in the crust. doi
  36. (1992). Exposed crustal cross sections as windows on the lower crust. In: doi
  37. (1999). Feldspar fabrics in a greenschist facies albite-rich mylonite from electron backscatter diffraction. doi
  38. (1999). The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks.
  39. (1983). The techniques of modem structural geology.
  40. (2000). Characterisation of crack distribution: fabric analysis versus ultrasonic inversion. doi
  41. (1992). Xenoliths samples of the lower continental crust. In: doi
  42. (1995). Nature and composition of the continental crust -a lower crustal perspective. doi
  43. (2003). The Composition of the Continental Crust, In: doi
  44. (2005). Imaging the Indian subcontinent beneath the Himalaya. Nature 435,1222. doi
  45. (2004). Thinning and flow of Tibetan crust constrained by seismic anisotropy. doi
  46. (1990). Velocity anisotropy and shear-wave splitting in rocks from the mylonite belt along the Insubric Line (Ivrca Zone, Italy). Earth Planet. doi
  47. (1989). Anisotropy of Vp and Vs in an amphibolite of the deeper crust and its relationship to the mineralogical, microstructural and textural characteristics of the rock. doi
  48. (2001). Laboratory measurements of elastic anisotropy parameters for the exposed crustal rocks from the Hidaka Metamorphic Belt, doi
  49. (1963). Assynt dykes and their metamorphism. doi
  50. (2007). Inferences from shear zone geometry: an example from the Laxfordian shear zone at Upper Badcall, Lewisian Complex, doi
  51. (1997). Velocity anisotropy in shales: a petrophysical study. doi
  52. (1980). Rare Earth geochemistry of the Lewisian granulitefacies gncisscs, northwest Scotland: Implications for the petrogcnesis of the Archaean lower continental crust. doi

To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.