Changing direction of magnetic fabric in a thrust unit: an example from the Karkonosze-Izera massif (SW Poland)

Abstract

The southern and eastern margins of the Karkonosze-Izera massif in the West Sudetes expose a nappe pile overthrust toward the NW onto the pre-Variscan continental basement of the Saxothuringian basin. The uppermost thrust sheet, the Leszczyniec nappe, shows an entirely different structural pattern in comparison to all underlying units. Although this contrast of structural trends has been long recognized, the irregular distribution of structural data prevented its reliable tectonic interpretation. Therefore, the AMS method was applied in the area to collect more structural information from meta-igneous, often massive and weakly foliated and lineated rocks of the Leszczyniec unit. An analysis of the magnetic fabric confirms the general difference of structural plan between the Leszczyniec nappe and the underlying tectonic units, interpreted below to result from the juxtaposition of entirely decoupled crustal segments. Furthermore, the AMS data reveals a discrepancy of the magnetic fabric within the Leszczyniec unit itself, between the metabasites and penetrating them metagranitoids. The latter rocks demonstrate a continuous transition of a structural trend towards that, typical of the underlying nappe pile. This abrupt change probably reflects an uplift of the Leszczyniec unit into the shallower crustal level dominated by a different deformation regime.