Description: Driven bysignificant economic benefits, the hydrocarbon industry developed sophisticated methods for the integration of geophysical and geochemical measurements with direct core observations. However, these techniques were developed primarily for sedimentary settings and have been applied only seldom in metamorphic settings. One outstanding example for comprehensive geophysical and borehole data integration is the COSC-1 borehole in the central Scandinavian Caledonides. It was drilled in 2014 and resulted in an extensive dataset to shed light on deformation during continental collision. Our study combines data from downhole logging and zero-offset vertical seismic profiling at COSC-1, with 2D and 3D seismic measurements to provide constraints on the spatial lithological and textural configuration of the Seve Nappe Complex. We show that there are powerful tools to distinguish between mafic and felsic lithologies in log-core correlation but that metamorphic settings bear special challenges for core-log-seismic integration. In contrast to sedimentary basins, reflections in the Seve Nappe Complex are not as distinct but we can link several of them to magmatic intrusions, which have been metamorphically overprinted. Their setting indicates that the Seve Nappe Complex consists of the remnants of a volcanic continental margin. It appears that in spite of the metamorphic overprint around 417+/-9 Ma, the original configuration of the volcanic passive margin is partly preserved in the Seve Nappe Complex and it thus outlasted continent-continent collision including the nappe emplacement. Integration of borehole and three-dimensional geophysical data reveals lithological changes that can then be extrapolated in three dimensions to arrive at a better understanding of the compositionand geometry at mid-crustal levels. Furthermore, our results suggest that mid-crustal reflectivity at COSC-1 is primarily a function of pre-orogenic lithological variations which has to be considered when deciphering mountain building processes.

Description: The COSC-1 project drilled the several hundred meters thick basal shear zone of the Lower Seve nappe with mylonites in mica schists, amphibole schists and gneisses. In zones of high magnetic susceptibility from 1910 to 2450 m we studied magnetic and petrographic fabrics, and magnetic mineralogy. Borehole imaging allowed for geographic reorientation of the samples and offered the opportunity to study anisotropy of magnetic susceptibility (AMS) in relation to tectonic evolution of the Seve thrust. We measured AMS at room temperature, added low-temperature and field-dependent AMS for a subset of samples, and compared magnetic with petrographic fabrics. Triaxial and prolate magnetic fabrics with degree of anisotropy (P′) up to 3.2 together with abundant S-C fabrics and strain partitioning around porphyroclasts indicate dominant simple shear until 2300 m. Magnetite and ilmenohematite mimic the rock fabric due to fabric parallel alignment and/or magnetic interaction and either contribute to increase or decrease of P′, depending on the dominating rock fabric elements. Field-dependency of pyrrhotite and magnetite in kmax-direction further increases P′. Homogeneous and oblate petrographic and magnetic fabrics in the greenschist-grade overprinted rocks below 2300 m with subhorizontal kmax-kint-girdle distributions indicate dominant flattening. AMS depicts shear fabrics including magnetite and ilmenohematite, and is additionally increased by retrograde magnetite-rutile intergrowth in ilmenohematites. We interpret that shape and degree of AMS are controlled by (a) tectonic deformation and strain, (b) alteration and magnetic grain interaction, and (c) field-dependency of deformed pyrrhotite and/or magnetite. We observed that all petrographic and magnetic subfabrics are coaxial, and lineations are mainly E-W to SE-NW directed confirming the transport direction of the Caledonian allochthonous. From our microstructural and AMS results we suggest that thrusting of the Lower Seve unit commenced under simple shear conditions at higher metamorphic grades and subsequently switched to more pure shear under greenschist-grade conditions.