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

We report compositional, microstructural and seismic properties from 24 samples collected the Middle Allochthon (Seve Nappe) of central Scandinavian Caledonides, its bounding shear zones. The stem both field outcrops continental drilling project COSC-1 include quartzofeldspathic gneisses, hornblende amphibolites, marbles, calc-silicates, quartzites mica schists, medium to high-strain. Seismic velocities anisotropy P (AVp) S (AVs) waves these were calculated using crystal preferred orientation data obtained Electron Backscatter Diffraction analysis (EBSD). Mica-schist exhibits highest (AVP ~ 31%; max AVs ~34%), followed by hornblende-dominated rocks (AVp ~5–13%; 5–10%) ~6.5–10.5%; ~7.5–12%). Lowest is found in calc-silicate ~4%; 3–4%), where symmetry more complex due contribution several phases. Anisotropy attributed to: 1) modal mineral composition, particular amphibole content, 2) CPO intensity, 3) crystallization anisotropic minerals fluids circulating zone (calc-silicates amphibolites), a lesser extent 4) compositional banding with contrasting elastic density. Our results link observed rock composition strain representative section across Central Caledonides indicate that entire Seve Nappe seismically anisotropic. Strain has partitioned on nappe scale, likely scale. High- zones develop at boundaries allochthon internally within show higher than moderately strained interior nappe. may be considered as template for deforming, ductile flowing middle crust, which line general observations mid-crustal settings.