3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, central Swedish Caledonides

Abstract

The scientific drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides), designed to study key questions concerning orogenic processes, aims to drill two fully cored boreholes to depths of c. 2.5 km each at carefully selected locations in west-central Sweden. The first of these, COSC-1, is scheduled for start late spring 2014 and will target the Seve Nappe Complex, characterized by inverted metamorphism and with parts that have evidently been subjected to hot ductile extrusion. In this study available seismic sections have been combined with surface geology to produce a 3D interpretation of the tectonic structures in the vicinity of the COSC-1 borehole. Constrained 3D inverse gravity modelling over the same area supports the interpretation, and the high-density Seve Nappe Complex stands out clearly in the model. Interpretation and models show that the maximum depth extent of the Seve Nappe Complex is less than 2.5 km, consistent with reflection seismic data. The gravity modelling also requires underlying units to comprise low-density material, consistent with the Lower Allochthon, but the modelling is unable to discern the décollement separating the allochthons from the crystalline Precambrian basement.

COSC-1 – drilling of a subduction-related allochthon in the Palaeozoic Caledonide orogen of Scandinavia

“…The uppermost 500 m was poorly imaged due to the acquisition geometry. Potential field modelling (Hedin et al, 2014) also indicated the base of the Seve Nappe Complex to be at about 2 km depth at the COSC-1 site, consistent with the seismic data.…”

A magnetotelluric investigation of the Scandinavian Caledonides in western Jämtland, Sweden, using the COSC borehole logs as prior information

“…In the present summary, we focus on the area from Are toÖstersund in western Jämtland (yellow square in Figs 1 and 2), where we collected MT data. The detailed geology of this area has been previously described by Gee et al (2010), Hedin et al (2012Hedin et al ( , 2014 and Juhlin et al (2016).…”

“…The location of the first borehole, COSC-1, targeting the Seve Nappe Complex was proposed mainly based on the seismic results. A 3-D interpretation of the Seve Nappe Complex presented by Hedin et al (2014) was based on surface geology, available seismic sections and 3-D inverse modelling of more than 300 ground gravity measurements from SGU. A maximum depth extent of 2.5 km of the Seve Nappe Complex was supported by the inversion models, an interpretation generally consistent with the reflection seismic image.…”

“…New highresolution aeromagnetic data, consisting of 130 acquisition lines with a line and point spacing of 200 and 7 m, respectively, were acquired by SGU in 2011. These were modelled and interpreted by Hedin et al (2014) and used by Juhlin et al (2016) to constrain the interpretation of the complete seismic profile. Furthermore, 3-D seismic data were acquired in 2014 around the COSC-1 borehole, which provide more precise information on the 3-D geometry of the structures around it .…”

“…Before inversion, strike and dimensionality analyses were conducted to determine to what extent the 3-D resistivity distribution can be approximated in 2-D. For the western part of the investigation area, the aeromagnetic map and the regional gravity map (Hedin et al 2014) and the seismic data (Juhlin et al 2016) suggest that the structure is 3-D. In contrast, in the central and eastern parts, the reflection seismic data support the idea that the structure at depth can be approximated as 2-D. Korja et al (2008) found a predominant geological strike direction of N40 • E for the eastern part of their MT profile recorded between the Norwegian border and the Caledonian front.…”