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 ~ 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.
Highlights
The understanding of orogeny, or how mountains develop, requires distinguishing between autochthonous and allochthonous tectonic units
We present the results of microstructure and texture derived seismic properties of rocks of the Middle Allochthon (Seve Nappe) from the Collisional Orogeny in the Scandinavian Caledonides (COSC)-1 drill core and field samples from selected field sites within the central Scandinavian Caledonides
Medium-strain samples are internal to the nappes and high-strain samples originate dominantly from shear zones that bound the Seve Nappe some are internal to the nappe itself, originating from a nappe internal shear zone
Summary
The understanding of orogeny, or how mountains develop, requires distinguishing between autochthonous and allochthonous tectonic units. Tectonic nappes are distinguished in the field based on their distinct composition and the presence of high-strain shear zones bounding them. Faults, deformation 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 to recognize object lineations (Piazolo & Passchier, 2002), syntectonic metamorphic reactions, development of shape and crystallographic preferred orientations and most commonly finer grain size relative to adjacent rocks Platt & Behrmann, 1986; Selverstone et al 1991; Raimondo et al 2011) and biotite rich shear zones developed in response to syntectonic melt-rock interaction (e.g., Piazolo et al 2020). At higher grade and in mafic rocks, such zones may be amphibole-rich (e.g., Kruse and Stünitz, 1999)
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