Seismic anisotropy in mid to lower orogenic crust: Insights from laboratory measurements of Vp and Vs in drill core from central Scandinavian Caledonides

Abstract

Recent drilling of the first Collisional Orogeny in the Scandinavian Caledonides scientific borehole (COSC-1) near Åre, Sweden permitted a laboratory investigation of seismic anisotropy on high metamorphic grade and highly deformed core samples. The 2.5km deep borehole crosscuts the amphibolite-grade Lower Seve Nappe and intersects a high-strain shear zone in the lowermost 800m. Measurements of ultrasonic compressional (Vp) and shear (Vs) velocities are conducted at room temperature and pressures ranging from room conditions up to 260MPa on six core sections that represent the most abundant lithologies in the borehole. The core sections consist of two amphibolites, a calc-silicate gneiss, a felsic gneiss, an amphibole-rich gneiss, and a garnet-bearing micaschist from the shear zone. Three mutually perpendicular samples were taken to characterize the anisotropy induced by the clear foliation and lineation. The intrinsic (crack-free velocities) Vp0 and Vs0 in the direction perpendicular to foliation ranges from 5.51 to 6.67km/s and 3.31 to 4.13km/s, respectively. In the direction parallel to foliation the Vp0 and Vs0 ranges from 6.31 to 7.25km/s and 3.53 to 4.35km/s, respectively. Vp anisotropy ranges from 3% in the calc-silicate gneiss to 19% in the micaschist. In the upper crustal seismic reflection survey around the COSC-1 borehole, reflection coefficient analysis suggests that reflectors occur due to impedance contrast between commonly occurring amphibolites and gneisses in the upper 1800m and the micaschists below. When extrapolated to mid-crustal levels the analysis indicates that both rock types can produce reflection coefficients between adjacent lithologies in excess of 0.1. Similarities in lithologies, shear zone thickness, and reflectivity pattern in the Central Scandinavian Caledonides compared to other orogens (e.g., the Himalaya) demonstrate the importance of these measurements as a proxy for in-situ strongly anisotropic shear zones in the middle crust.