Tectonophysics 819 (2021) 229045 Available online 4 September 2021 0040-1951/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Seismic anisotropy of mid crustal orogenic nappes and their bounding structures: An example from the Middle Allochthon (Seve Nappe) of the Central Scandinavian Caledonides Bjarne S.G. Almqvist a, * , Daria Cyprych b , Sandra Piazolo c a Department of Earth Sciences, Uppsala University, Uppsala, Sweden b Australian Research Council of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia c School of Earth and Environment, Leeds University, Leeds, United Kingdom A R T I C L E INFO Keywords: Seismic anisotropy Orogeny Strain Shear zone Scandinavian Caledonides 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 (AV P ~ 31%; max AVs ~34%), followed by hornblende-dominated rocks (AVp ~5–13%; max AVs 5–10%) and quartzites (AVp ~6.5–10.5%; max AVs ~7.5–12%). Lowest anisotropy is found in calc-silicate rocks (AVp ~4%; max AVs 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 Scandi- navian 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. 1. Introduction The understanding of orogeny, or how mountains develop, requires distinguishing between autochthonous and allochthonous tectonic units. For example, tectonic nappes are distinguished in the field based on their distinct composition and the presence of high-strain shear zones bounding them. As seismic data provide key geophysical constraints of the crustal structure at depth, it is important to develop a catalogue of seismic signatures (Vp, Vs, Vp/Vs and anisotropy of Vp and Vs; e.g., Brownlee et al., 2017; Jung, 2017). In particular, seismic anisotropy is a sensitive indicator for flow and deformation in the Earth, which can be linked to features such as rock fabric, structure (e.g., Okaya et al., 2019) and shear zones (e.g., Jones and Nur, 1982). Shear zones with widths ranging from mm up to several 10's to 100's of kilometres accommodate relative movement of comparatively rigid surrounding rocks (e.g. White et al., 1980; Ramsay, 1980; Poirer, 1980; Hobbs et al., 1986; Jiang and Williams, 1998; Frederiksen and Braun, 2001; Gueydan et al., 2014; Fossen and Cavalcante, 2017). In brittle shear zones, or faults, defor- mation is highly localized in distinct narrow zones while ductile crustal shear zones are wider, exhibiting a fabric gradient from host rock to high-strain zone that is, in places, accompanied by strongly developed planar foliation, clear mm to dm scale compositional banding and easy * Corresponding author. E-mail address: [email protected] (B.S.G. Almqvist). Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto https://doi.org/10.1016/j.tecto.2021.229045 Received 1 March 2021; Received in revised form 4 August 2021; Accepted 1 September 2021
Seismic anisotropy of mid crustal orogenic nappes and their bounding structures: An example from the Middle Allochthon (Seve Nappe) of the Central Scandinavian Caledonides
Jun 23, 2023
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