Seismic imaging in the frontal part of the Sveconorwegian orogen, south-western Sweden

Abstract

Reflection seismic data have been acquired along a 17 km profile in the eastern frontal part of the Sveconorwegian orogen, south-western Sweden. Receiver spacing was 25 m and nominal shot spacing 100 m. Shots ranging in size from 0.5 to 1.0 kg were fired in 3 m deep shot holes. The data image the structure of the upper part of the crust from depths of a few 100 m down to depths of approximately 10 km. The profile crosses the central axis of an inferred fan-like structure, indicated from earlier, detailed surface mapping results. Deformational fabrics within this structure dip approximately 15–40°E in the western part of the profile and 60–80°W in its eastern part. The seismic data display a bivergent geometry with the central axis situated approximately 8 km east of a major fault (Protogine Zone in earlier studies). Surface structural data predict the hinge axis to be located about 3 km farther east. Despite this discrepancy, there is good general agreement with the structure inferred from the surface data. The image obtained from the reflection seismic data establishes that the fan-like structure is a major, upper crustal feature. The present data in combination with previous deep seismic data sets exclude a single-stage extensional model, with an east-dipping extensional deformation zone protracting eastwards from the western part of the profile. The combined seismic data provide support for a two-stage compressional model for the development of the fan-like structure, with crustal thickening and stacking prior to 955 Ma and later compression with a stronger horizontal component of displacement during 930–905 Ma. The latter gave rise to the development of the complex system of retrogressive deformation zones referred to as the Sveconorwegian Frontal Deformation Zone (SFDZ), a tectonic analogy to the Grenville Front in the eastern part of North America. The SFDZ dips westwards, may become listric at depth and possibly sole in a zone of lower crustal reflectivity west of the present profile. The kinematics and geometry of the SFDZ suggest that reverse displacements (thrusting) may be responsible, at least in part, for the exhumation of the medium- to high-grade rocks in the western part of the fan-like structure. Both the seismic data and the surface structural geology show many similarities to the Grenville Front.