Seismic anisotropy of mid crustal orogenic nappes and their bounding structures: An example from the Middle Allochthon (Seve Nappe) of the Central Scandinavian Caledonides

• Published in: Tectonophysics 2021-11, Vol.819, p.229045, Article 229045
• Main Authors: Almqvist, Bjarne S.G., Cyprych, Daria, Piazolo, Sandra
• Format: Article
• Language: English
• Subjects: Amphibolites; Anisotropy; Backscatter; Composition; Crystal structure; Crystallization; Elastic anisotropy; Elastic properties; Electron backscatter diffraction; Fluids; Isotopes; Mica; Mineral composition; Minerals; Nappes; Orogeny; Outcrops; Preferred orientation; Properties; Quartzite; Rock; Rocks; Scandinavian Caledonides; Schist; Schists; Seismic activity; Seismic anisotropy; Seismic properties; Seismic velocities; Shear zone; Silicates; Strain
• ISSN: 0040-1951; 1879-3266; 1879-3266
• DOI: 10.1016/j.tecto.2021.229045

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 ~ 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 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. •Seismic properties inferred for the Middle Allochthon rocks, Scandinavian Caledonides•Mineral modal composition and degree of strain control seismic anisotropy•The entire allochthonous nappe unit is predicted to be seismically anisotropic•AVp ranges from 7 to 31% and max AVs from 7 to 34% in high-strain samples•AVp ranges from 4 to 11% and max AVs from 3 to 13% in medium-strain samples