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.