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- Almqvist, Bjarne S.G., et al.
(författare)
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Seismic anisotropy of mid crustal orogenic nappes and their bounding structures : An example from the Middle Allochthon (Seve Nappe) of the Central Scandinavian Caledonides
- 2021
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Ingår i: Tectonophysics. - : Elsevier. - 0040-1951 .- 1879-3266. ; 819
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Tidskriftsartikel (refereegranskat)abstract
- We report compositional, microstructural and seismic properties from 24 samples collected from the Middle Allochthon (Seve Nappe) of the central Scandinavian Caledonides, and its bounding shear zones. The samples stem both from field outcrops and the continental drilling project COSC-1 and include quartzofeldspathic gneisses, hornblende gneisses, amphibolites, marbles, calc-silicates, quartzites and mica schists, of medium to high-strain. Seismic velocities and anisotropy of P (AVp) and S (AVs) waves of these samples were calculated using microstructural and crystal preferred orientation data obtained from Electron Backscatter Diffraction analysis (EBSD). Mica-schist exhibits the highest anisotropy (AVp similar to 31%; max AVs similar to 34%), followed by hornblende-dominated rocks (AVp similar to 5-13%; max AVs similar to 5-10%) and quartzites (AVp similar to 6.5-10.5%; max AVs similar to 7.5-12%). Lowest anisotropy is found in calc-silicate rocks (AVp similar to 4%; max AVs similar to 3-4%), where the symmetry of anisotropy is more complex due to the contribution to anisotropy from several phases. Anisotropy is attributed to: 1) modal mineral composition, in particular mica and amphibole content, 2) CPO intensity, 3) crystallization of anisotropic minerals from fluids circulating in the shear zone (calc-silicates and amphibolites), and to a lesser extent 4) compositional banding of minerals with contrasting elastic properties and density. Our results link observed anisotropy to the rock composition and strain in a representative section across the Central Scandinavian Caledonides and indicate that the entire Seve Nappe is seismically anisotropic. Strain has partitioned on the nappe scale, and likely on the microstructural scale. High- strain shear zones that develop at boundaries of the allochthon and internally within the allochthon show higher anisotropy than a more moderately strained interior of the nappe. The Seve Nappe may be considered as a template for deforming, ductile and flowing middle crust, which is in line with general observations of seismic anisotropy in mid-crustal settings.
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| 2. |
- Cyprych, Daria, et al.
(författare)
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Seismic anisotropy from compositional banding in granulites from the deep magmatic arc of Fiordland, New Zealand
- 2017
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 477, s. 156-167
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Tidskriftsartikel (refereegranskat)abstract
- We present calculated seismic velocities and anisotropies of mafic granulites and eclogites from the Cretaceous deep lower crust (similar to 40-65 km) of Fiordland, New Zealand. Both rock types show a distinct foliation defined by cm-scale compositional banding. Seismic properties are estimated using the Asymptotic Expansion Homogenisation - Finite Element (AEH-FE) method that, unlike the commonly used Voigt-Reuss-Hill homogenisation, incorporates the phase boundary network into calculations. The predicted mean P- and S-wave velocities are consistent with previously published data for similar lithologies from other locations (e.g., Kohistan Arc), although we find higher than expected anisotropies (AV(P) similar to 5.0-8.0%, AV(S) similar to 3.0-6.5%) and substantial S-wave splitting along foliation planes in granulites. This seismic signature of granulites results from a density and elasticity contrast between cm-scale pyroxene +/- garnet stringers and, plagioclase matrix rather than from crystallographic orientations alone. Banded eclogites do not show elevated anisotropies as the contrast in density and elastic constants of garnet and pyroxene is too small. The origin of compositional banding in Fiordland granulites is primarily magmatic and structures described here are expected to be typical for the base of present day magmatic arcs. Hence, we identify a new potential source of anisotropy within this geotectonic setting.
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