Fabrics in fossil weathering-zones and their significance for detachment tectonics, case studies from the Swedish Caledonian Margin
Angerer, Thomas O.
Two fossil palaeoweathering sections below the autochthonous erosional unconformity between Fennoscandian Revsund Granite and Cambrian Sedimentswere investigatedin drill cores from the SwedishAlum Shale Project (Li'mgvikenSGU 73007 and Hara SGU 79002). By means of magnetic susceptibility (Kbulk) analyses changes of magnetomineralogyand corresponding magnetic fabrics were tracked. The aim of this study is to gather general constraintson the relationship of palaeoweathering and fabric at basement-cover- interfaces. For comparison, a section of a thin, imbricated slice of basement + cover in the Lower Allochthon of the drill core Langviken SGU 73007 was studied as well. The "Hara" granite is a paramagnetictype with Fe-bearingphyllosilicates. A distinct increase of Kbulk is related to progressive Fe-illite/smectite precipitation. The Li'mgvikenautochthonous granite is a ferromagnetic type, dominated by magnetite. Here, Kbulkdecreases extremely in the weathering-breccia due to magnetite break down. Both basement granites show sub- vertical gneissic (and magnetic) foliations, which are very likely related to the 1.8Ga old Storsjon-Edsbyndeformationzone (SEDZ). An unconformity-related fabric change is traceable by AMS, independent of the petrographic (and magnetomineralogic)type: towards the unconformitythe magnetic lineation (Kj)decreases (abruptly in Li'mgviken, gradually in Hara) its inclination, whereas magnetic foliation stays stable. It is interpreted as a composite AMS fabric resulting from superpositionof flat-lying and vertical sub-fabrics. The flat-lying compaction fabric appears to be an ubiquitous feature in fossil palaeoweathering zones (Angerer 2007), and the overprinting vertical fabric is a Caledonian compressional feature, causing the steep orientation of the magnteic foliation. Generally speaking, the magnetic lineation is a sensitive marker for relative changes of intensity in a system of superimposed petrofabrics with almost orthogonally oriented Kj.In the Li'mgvikenLower Allochthon basement slice,Kbulkand P' are either in the range of the autochthonous weathering- breccias or higher, due to local secondary ferrimagneticpyrrhotite precipitation. Cataclastic to protomylonitic deformation with SC and SCC'-fabric development overprinted the unconformity-relatedAMS. Magnetic foliations gradually parallel the almost flat-lying thrust sheet margins due to compaction and shear sub-fabrics. Magnetic lineation is mostly a composite fabric out of both sub-fabrics. The unconformity-related clayey alteration and flat-lying fabric development facilitated propagation of detachments subparallel with this fabric during Calaedonianfolding and thrusting.