Abstract
AbstractContinental collision causes deformation in the crust along shear zones. However, the physical and chemical conditions at which these zones operate and the deformation processes that enable up to hundreds of km of tectonic transport are still unclear because of the depth at which they occur and the challenges in imaging them. Ancient exhumed collision zones allow us to investigate these processes much better, for example at the COSC‐1 borehole in the central Scandinavian Caledonides. This study combines data from the COSC‐1 borehole with different seismic measurements to provide constraints on the spatial lithological and textural configuration of the Seve Nappe Complex. This is one of the few studies that shows that core‐log‐seismic integration in metamorphic rocks allows to identify the spatial distribution of major lithological units. Especially gamma ray logs in combination with density data are powerful tools to distinguish between mafic and felsic lithologies in log‐core correlation. Our results indicate that reflections along the borehole are primarily caused by compositional rather than textural changes. Reflections in the Seve Nappe Complex are not as distinct as in greater depths but continuous and several of them can be linked 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. Our results suggest that ductile‐deformed middle crustal reflectivity is primarily a function of pre‐orogenic lithological variations which has to be considered when deciphering mountain building processes.
Highlights
Beyond a general scientific curiosity about the nature of the continental crust and its formation, detailed information on its lower and middle parts is a pre-requisite for mineral and geothermal exploration and the assessment of earthquake hazards
Details on the measurement of X-ray fluorescence (XRF) geochemical data can be found in Sjöqvist et al (2015), information on the set up of the downhole logging and the multi-sensor core logger (MSCL) measurements are documented in the COSC-1 reports (Lorenz et al, 2015a, 2019)
The information from 10 different logs from the core and from downhole measurements can be summarized by four different trend curves (TCs), which show the deviation of the containing properties from their mean value
Summary
Beyond a general scientific curiosity about the nature of the continental crust and its formation, detailed information on its lower and middle parts is a pre-requisite for mineral and geothermal exploration and the assessment of earthquake hazards. Most knowledge on the deep sections of nappe emplacement stems from ancient collision zones that have been exhumed (Fowler & Osman, 2009; Matenco et al, 2010; Viola et al, 2008; Vollbrecht et al, 1989) and requires a combination of geological outcrop studies, borehole investigations, and deep geophysical imaging. The main aim was to better understand the very deep crust that experienced deformation at 120 km depth in a continent-continent collision setting (Xu et al, 2009, 2017)
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