Magnetic fabric development in the Lower Seve thrust from the COSC-1 drilling, Swedish Caledonides
Introduction
Transport and emplacement mechanisms of far-travelled, metamorphic nappes and their geophysical characterization in the continental crust are not only major tectonic issues of regional importance in the Scandinavian Caledonides, but also in orogens worldwide, e.g. the Main Central Thrust Zone of the Himalayas. These issues are addressed by the drilling project Collisional Orogeny in the Scandinavian Caledonides (COSC; Lorenz et al., 2015, Gee et al., 2010) with the overall aim to study mountain building processes. The COSC-1 drilling project was accomplished by the Swedish Scientific Drilling Program (SSDP) with support from the International Continental Scientific Drilling Program (ICDP 5054-1-A) in the year 2014. The 2495.8 m deep, vertical drill core intersected the Lower Seve nappe and revealed a remarkably thick (at least 800 m) basal thrust zone in the lower part of the drillhole, characterized by abundant mylonites (Lorenz et al., 2015). This high strain zone is also characterized by distinct seismic reflectivity, with shallow southeastward dipping reflections (Hedin et al., 2016) and significant magnetic susceptibility highs in borehole logs. Therefore, the COSC-1 drill core provides the unique opportunity to systematically study rock magnetic fabrics of different para- and ferromagnetic lithologies in relation to strain localization in this orogenic, crustal-scale high strain zone. Combination with microstructural studies helps address questions such as how strain and deformation mechanisms are distributed across the different lithologies and nappe boundaries, and how deformation mechanisms change with changing P-T-conditions during nappe transport.
Magnetic fabric investigations are an important tool to estimate strain in geological materials (e.g. Borradaile and Henry, 1997; Ferré et al., 2014; Kligfield et al., 1977, Kligfield et al., 1981) and decipher fabric development in deformed metamorphic rocks from crustal shear zones with different strain gradients (e.g. Kontny et al., 2012). Such investigations contribute to a better understanding of the geology behind seismic reflectors and magnetic anomalies in different segments of collisional orogens (Kontny and Dietze, 2014; Punturo et al., 2017). Large ductile shear zones are characterized by planar domains with strongly localized deformation, where strain rate can vary significantly within a single shear zone (Arbaret et al., 2000; Boutonnet et al., 2013; Fossen and Cavalcante, 2017). In normal magnetic fabrics dominated by multidomain magnetite (Borradaile and Jackson, 2004) and phyllosilicates (Martín-Hernández and Hirt, 2003), the magnetic foliation is parallel to the metamorphic foliation and the magnetic lineation is parallel to the stretching lineation or crenulation axis of minerals (e.g. Balsley and Buddington, 1960; Hrouda, 1982; Kligfield et al., 1977). However, significant deviations between petrographic and magnetic fabric and between different subfabrics can be caused by the presence of i) S- and C-planes (Aranguren et al., 1996), ii) single domain magnetite or other minerals that cause inverse fabrics such as cordierite or tourmaline (Ferré and Améglio, 2000; Potter and Stephenson, 1988; Rochette et al., 1992, Rochette et al., 1994), iii) amphiboles with magnetic lineation perpendicular to mineral lineation while the magnetic and mineral foliation are parallel (Biedermann et al., 2018), iv) mineral formation during progressive deformation and metamorphism also allowing fluids into the shear zone (Ferré et al., 2014; Mertanen and Karell, 2012), and v) a complex tectonic history (e.g. Kligfield et al., 1981). Therefore, petrographic and microstructural analyses are required to identify the minerals that dominate magnetic susceptibility and define the rock fabric.
Several AMS studies suggest a positive correlation between the degree of anisotropy and finite strain (Borradaile and Alford, 1987; Ferré et al., 2003, Ferré et al., 2014; Housen et al., 1995; Kontny et al., 2012; Till et al., 2012), and this method seems to be well-suited for the characterization, i.e. strain localization, in crustal shear zones. Kontny et al. (2012) studied the magnetic fabric of the magnetite-bearing Slipsiken shear zone in the upper Seve Nappe Complex and found very high degrees of AMS, with P′ up to 4.8. The high degrees of anisotropy along with a high magnetic susceptibility is related to the concentration of synkinematically formed elongated magnetite aggregates. Additionally, these authors propose a strain memory of magnetite to further increase the degree of AMS. Mattsson and Elming (2001) also relate high degrees of AMS up to 3.1 in the Storsjön-Edsbyn deformation zone in the Fennoscandian Shield to varying concentrations of magnetite. High-temperature shear experiments of magnetite-plagioclase aggregates by Till and Moskowitz (2014) indicated a correlation between strain (up to 300%) and the degree of magnetic anisotropy (up to 2.5). The high degree of magnetic anisotropy along with shape preferred orientation and intercrystalline microstructures identified by electron backscattered diffraction (EBSD) suggest a ductile deformation mechanism of magnetite (Till and Moskowitz, 2013, Till and Moskowitz, 2014), i.e. strain analysis is influenced by the reaction of the grain shape of magnetite or other minerals to deformation. Another important aspect is the magnetic interaction between magnetite grains, which are the driving mechanism for e.g. distribution anisotropy in magmatic rocks (e.g. Borradaile and Henry, 1997). Depending on the arrangement and grain interspace, the orientation of magnetic lineation can be intensified or rotated and thereby strain magnitude can be intensified or decreased (e.g. Grégoire et al., 1998; Cañón-Tapia, 2001).
Below 1700 m a gradual increase in strain with mica schists and intercalated mylonites increasing in frequency and thickness is reported by Hedin et al. (2016) for the COSC-1 drill site. In addition, this Lower Seve basal shear zone is characterized by high magnetic susceptibility zones between 1910 and 2450 m. We used different types of AMS (low-temperature – room-temperature, field-dependent in phase magnetic susceptibility) in order to differentiate magnetic subfabrics and to compare them with magneto-mineralogical (thermomagnetic and reflected light analysis) and microstructural investigations. The aim was to find evidence for strain partitioning due to different petrographic composition and rheological behaviour of the rocks, and to understand the fabric in terms of local strain variation in this crustal-scale shear zone, which is also seen in seismic reflectivity (Hedin et al., 2016).
Section snippets
Geological setting
The Scandinavian Caledonides are mainly characterized by several allochthonous units - Lower, Middle, Upper and Uppermost Allochthon, thrust east- and southeastwards onto the Baltica terrane or Fennoscandian Shield (Fig. 1a). On top of a Lower Allochthon, the Middle Allochthon comprises the former Baltica passive margin to (Iapetus) ocean-continent transition successions including pre-rift volcano sediments (Corfu et al., 2014). In the Middle Allochthon lower to upper greenschist facies nappes
Methods
For this study we have chosen 36 core pieces from the depth interval 1900 m to 2496 m (labelled A1 to A36; Fig. 2b), which mainly showed high magnetic susceptibilities in the susceptibility borehole log (MSUS Log, pers. comm. J. Kück, Fig. 2). We used transmitted and reflected light microscopy, cathodoluminescence (CL), temperature-dependent magnetic susceptibility and different AMS methods to investigate the microstructures and magnetic fabrics. The reorientation of drill core declination was
Macroscopic and microscopic core description
Macroscopically the cores already show structural variations (Fig. 2c). A1 to A3 from the garnet-mica schist unit from 1910 to 1986 m are folded on a cm- to dm-scale (e.g. A2 in Fig. 2c) with mineralogical separation between more light colored and dark green areas. The cores A4 to A12 in the amphibole gneiss and schist unit from 1991 to 20,159 m have subhorizontal mylonitized, fine grained areas of up to 10 cm width with coarser grained areas with abundant porphyroclasts of light colored
Correlation of magnetic fabrics with petrography and deformation in the COSC-1
A high strain zone from the Lower Seve nappe has been drilled by the COSC-1 drilling in the depth interval 1700 to 2496 m depth. Because we reoriented the drill cores using image logs, we obtained geographically oriented AMS data from 152 core specimen over the depth range 1900 to 2496 m. The magnetic fabric was compared to the petrography and microstructures of different lithologies.
Our petrographic study revealed characteristic differences in mineralogy and microstructures in the mica
Conclusions
Our magnetic fabric and microstructural study of drill cores from the COSC-1 borehole from the basal thrust zone of the Lower Seve unit below 1700 m has shown that the orientation of all magnetic subfabrics are coaxial independent of whether ferri- or paramagnetic susceptibility dominates, and parallel to the petrographic foliations. The shape of the AMS ellipsoids and the degree of AMS is controlled by (a) tectonic deformation and strain, (b) alteration and magnetic grain interaction, and (c)
Declarations of interest
None.
Acknowledgements
We are grateful to ICDP for sharing with us the COSC-1 core material. We want to thank Matthias Siebert for core sampling, sample preparation, and AMS measurements, Henning Lorentz for an introduction to core reorientation to LM, Kirsten Drüppel for discussions on the metamorphic mineral assemblages and Reinhard O. Greiling for discussions on microscopic features of the basal thrusts in the Swedish Caledonides. We acknowledge the very constructive reviews of two anonymous reviewers.
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