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.

Description: Inclination shallowing of detrital remanent magnetization in sedimentary strata has solely been constrained for the mechanical processes associated with mud deposition and shallow compaction of clay-rich sediment, even though a significant part of mud diagenesis involves chemical compaction. Here we report, for the first time, on the laboratory simulation of magnetic assemblage development in a chemically compacting illite shale powder of natural origin. The experimental procedure comprised three compaction stages that, when combined, simulate the diagenesis and low-grade metamorphism of illite mud. First, the full extent of load-sensitive mechanical compaction is simulated by room temperature dry axial compression. Subsequently, temperature controlled chemical compaction is initiated by exposing the sample in two stages to amphibolite or granulite facies conditions (temperature is 490 to 750°C and confining pressure is 170 or 300 MPa) both in the absence (confining pressure only) and presence of a deformation stress field (axial compression or confined torsion). Thermodynamic equilibrium in the last two compaction stages was not reached, but illite and mica dehydroxylation initiated, thus providing a wet environment. Magnetic properties were characterized by magnetic susceptibility and its anisotropy (AMS) in both high- and low-applied field. Acquisition of isothermal remanent magnetization (IRM), stepwise three-component thermal de-magnetization of IRM and first-order reversal curves were used to characterize the remanence-bearing minerals. During the chemical compaction experiments ferrimagnetic iron-sulphides formed after reduction of magnetite and detrital pyrite in a low sulphur fugacity environment. The degree of low-field AMS is unaffected by porosity reduction from 15 to ~1 per cent, regardless of operating conditions and compaction history. High-field paramagnetic AMS increases with compaction for all employed stress regimes and conditions, and is attributed to illite transformation to iron-bearing mica. AMS of authigenic iron-sulphide minerals remained constant during compaction indicating an independence of ferrimagnetic fabric development to chemical compaction in illite shale powder. The decoupling of paramagnetic and ferrimagnetic AMS development during chemical compaction of pelite contrasts with findings from mechanical compaction studies.

Description: The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project focuses on mountain building processes in a major mid-Paleozoic orogen in western Scandinavia and its comparison with modern analogues. The transport and emplacement of subduction-related highgrade continent-ocean transition (COT) complexes onto the Baltoscandian platform and their influence on the underlying allochthons and basement will be studied in a section provided by two fully cored 2.5 km deep drill holes. This operational report concerns the first drill hole, COSC-1 (ICDP 5054-1-A), drilled from early May to late August 2014. COSC-1 is located in the vicinity of the abandoned Fröå mine, close to the town of Åre in Jämtland, Sweden and was planned to sample a thick section of the Seve Nappe and to penetrate its basal thrust zone into the underlying lower grade metamorphosed allochthon. Despite substantial technical problems, the drill hole reached 2495.8 m driller's depth and nearly 100 % core recovery was achieved. Surprising was the homogeneity of the Seve Nappe rocks, the unexpected thickness of its basal thrust zone (〉 500 m) and that the drill hole, therefore, did not penetrate the bottom of the thrust zone. However, lower grade metasedimentary rocks were encountered in the lowermost part of the drill hole together with tens of metres thick mylonites that are, unexpectedly, rich in large garnets. The drill core was documented on-site and XRF scanned off site. During various stages of the drilling, the borehole was documented by comprehensive downhole logging. This operational report provides an overview over the COSC-1 operations from drilling preparations to the sampling party and describes the available datasets and sample material.

Description: As support for the COSC drilling project (Collisional Orogeny in the Scandinavian Caledonides), an extensiveseismic survey took place during September and October 2014 in and around the newly drilled 2.5 km deepCOSC-1 borehole. The main aim of the COSC project is to better understand orogenic processes in past andrecently active mountain belts. For this, the Scandinavian Caledonides provide a well preserved case of Paleozoiccollision of the Laurentia and Baltica continental plates. Surface geology and geophysical data provide knowledgeabout the geometry of the Caledonian structure. The reflectivity geometry of the upper crust was imaged byregional seismic data and the resistivity structure by magnetotelluric methods. The crustal model was refined byseismic pre-site surveys in 2010 and 2011 to define the exact position of the first borehole, COSC-1.The completely cored COSC-1 borehole was drilled in Central Sweden through the Seve Nappe Complex, a partof the Middle Allochthon of the Scandinavian Caledonides that comprises units originating from the outer marginof Baltica. The upper 2350 m consist of alternating layers of highly strained felsic and calc-silicate gneissesand amphibolites. Below 1710 m the mylonite content increases successively and indicates a high strain zoneof at least 800 m thickness. At ca. 2350 m, the borehole leaves the Seve Nappe Complex and enters underlyingmylonitised lower grade metasedimentary units of unknown tectonostratigraphic position.The seismic survey consisted of three parts: a limited 3D-survey, a high resolution zero-offset VSP (verticalseismic profile) and a multi-azimuthal walkaway VSP (MSP) experiment with sources and receivers along threesurface profiles and receivers at seven different depth levels of the borehole. For the zero-offset VSP (ZVSP) ahydraulic hammer source was used and activated over a period of 20 s as a sequence of impacts with increasinghit frequency. The wave field was recorded with 3-component geophones and a receiver spacing of 2 m over thewhole borehole length.As first pre-processing steps, the three component VSP data were decoded and vertically stacked. Afterwards, theshots were merged to get a continuous shot gather. A horizontal rotation was performed, based on the S-wavearrivals.The rotated ZVSP-data show a high signal-to-noise ratio and good data quality. Signal frequencies up to 150 Hzwere observed. On the vertical component, clear direct P-wave arrivals are visible. Several P-wave reflectionsoccur below 1600 m logging depth. On both horizontal components, clear direct S-wave arrivals are visible afterrotation what suggests that the penetrated rock is anisotropic. In addition, several PS-converted waves can beidentified.In order to integrate the borehole data into the 3D surface seismic data, further processing concentrated only on theP-waves. First, deconvolution was applied to sharpen the signals and to suppress multiples. Then the wave field wasseparated into upgoing and downgoing components by median filtering. Finally, a corridor stack was generated us-ing the upgoing wave field in order to allow correlation with the borehole logging data and the surface seismic data.