Introduction Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue1-2.2000:

1. Introduction

Over the last 20 years, the seismic reflection method has proven to be a useful tool for studying the structure of crystalline continental crust in Sweden and elsewhere. Particular cases of interest include definitive identification and mapping of fracture zones in central Sweden Juhlin et al., 1991; Juhlin, 1995, identification of dolerite and felsic sills or sheets in the Siljan area of central Sweden Juhlin, 1990, and the identification and mapping of mylonite zones in a number of areas in North America Hurich et al., 1985; Johnson and Smithson, 1985; Ratcliffe et al., 1986. These studies and others have clearly shown that it is possible to obtain good images of the crust in the depth range of a few hundred meters to 10 km. Normally, the seismic reflection method can image discontinuities in the crust with dips in the range 0 – 45°, but re- cent studies show that it is possible to image discontinuities with dips up to 60 – 70° Milkereit et al., 1992a. These images demonstrate a geo- metric picture of boundaries in the crust which then need to be interpreted in relation to surface geology and other geophysical information. In addition to the geometric picture, it is also pos- sible in some cases to obtain information on the physical properties of these discontinuities Goodwin et al., 1989; Juhlin et al., 1991. Much of the recent work in the Sveconorwe- gian orogen of south-western Sweden has fo- cused on detailed kinematic e.g. Larson et al., 1986, 1990; Andre´asson and Rodhe, 1990; Park et al., 1991; Wahlgren et al., 1994; Stephens et al., 1996 and metamorphic e.g. Johansson et al., 1991; Johansson and Kullerud, 1993; Wang and Lindh, 1996; Mo¨ller, 1998, 1999 studies, as well as geochronological work with the aim to constrain the timing of deformation and meta- morphism e.g. Johansson et al., 1991; Jo- hansson and Johansson, 1993; Connelly et al., 1996; Page et al., 1996a,b; Wang et al., 1996; So¨derlund et al., 1999. The results of these studies strongly argue for a collisional tectonic scenario to explain the Sveconorwegian orogenic belt cf. Berthelsen, 1980; Falkum and Petersen, 1980; EUGENO-S Working Group, 1988 which is similar to that suggested for the Grenville orogen e.g. Rivers, 1997. Late-Sveconorwegian extension has been suggested to explain decom- pression and exhumation of eclogites in the southernmost part of the orogen Mo¨ller, 1998, 1999. However, current models and interpreta- tions of the Sveconorwegian orogen are con- strained primarily by surface geological information. A major question concerns the tectonic devel- opment of the frontal part of the orogen, north and east of Lake Va¨nern Fig. 1. In order to add a third data dimension, a c.17 km long reflection seismic profile was shot in this area to better constrain the 3D geometry of the defor- mation zones and their deformational history. The bedrock in and north of the Lake Va¨nern area has previously been explored by seismic methods. These data show that the crust is quite reflective down to Moho depths below Lake Va¨nern Juhlin et al., 1989 and in the upper 5 – 6 km north of the lake Dahl-Jensen et al., 1991. However, these experiments were de- signed to study the deeper portions of the crust and little to no information was obtained shal- lower than 1 – 2 km. Wahlgren et al. 1994 discussed different models-compressional, extensional and various combinations of these-to explain the tectonic evolution in the frontal part of the Sveconorwe- gian orogen, north and east of Lake Va¨nern. A model involving two separate compressional events was favoured, and support for this was obtained in a recent 40 Ar 39 Ar study Page et al., 1996a. Since the different tectonic models show similar features at the surface, but differ at depth, the sub-surface structural geometry is crucial for the discrimination between the mod- els. The purpose of this paper is to integrate the results from the reflection seismic study with the surface structural information presented by Wahlgren et al. 1994. It is anticipated that this will lead to a better understanding of the 3D structural geometry in the frontal part of the orogen, and thus provide tighter constraints on its tectonic evolution. A short comparison with the results of seismic reflection studies and the tectonic evolution in the Grenville orogen is also presented.

2. Geological setting