Rock fabrics in palaeoweathering profiles below basement-cover interfaces (AMS-study on drill cores from the Caledonian margin, Central Sweden)

Abstract

Basement-cover-interfaces are important crustal boundaries. In many cases they act as detachment horizons. Criteria like pre-erosional basement characteristics, intensity of palaeoweathering and post-erosional processes during burial stage lead to a huge variety of observable alteration and fabric features of basement-cover-interfaces, which may influence the shear-strength. Unconformity-parallel planar fabrics in the weathering profile were facilitated by palaeo-alteration and later processes (Angerer 2005 unpubl. data). Such fabrics may be a factor for lowering the shear-strength (e.g. Wintsch et al. 1995). The probably ubiquitous existence of those fabrics at basement-coverinterfaces is investigated in case studies by means of AMS-fabric analysis, which is a sensitive indicator of rock fabric changes. The present case study is based on sections from two drill cores across the erosional unconformity between Fennoscandian Granite (Revsund) and Cambrian Gärdsjön Fm. (Långviken SGU 73007 and Hara 79002) (petrographic descriptions in Gee, 1978 and Gee et al. 1982).