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

By Daniel John Tatham

Abstract

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

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