Kinematic analysis of the basement-cover contact of the western margin of the Grong-Olden Culmination, central Norwegian Caledonides

Abstract: In orogenic belts, the relations between basement and cover rocks are important because of the information they give about the nature and evolution of the belt. In the Scandinavian Caledonides, basement-cover contacts can be found in windows where Precambrian basement has been exposed in antiformal structures, due to the deep erosion level of the mountain chain. The Grong-Olden Culmination (NE of Trondheim, central Norway) is one such window, composed of two antiforms: the E-W trending Grong Antiform and the N-S trending Olden Antiform. This microstructural study is focused on the kinematics in the rocks adjacent to a basement-cover contact. Kinematic indicators are structures which indicate the sense of shear in deformed rocks. The types used in this study are augen with asymmetric tails and pressure shadows, rotated garnets, C-S fabric, mica fishes and shear bands; emphasis is laid on augen and C-S fabrics. The basement rocks are in general light grey to pink mylonitic gneisses with a pervasive foliation and a strong feldspar+quartz mineral streaking lineation. The thin cover unit consists of a dark garnet bearing quartz-mica schist with a phyllonitic texture, called phyllonites. The lineation is defined by stretched quartz and mica. The results suggest an over all W-NW direction of movement, with indications of an earlier SE directed movement. As driving mechanism of the W-NW shearing, normal faulting superimposed on eastward thrusting is suggested.