Structure of the Central Scandinavian Caledonides and the underlying Precambrian basement, new constraints from magnetotellurics

Abstract

S U M M A R YWe investigate the structure of the accretionary wedge of the Caledonian orogen, the underlying autochthonous/parautochthonous carbonaceous alum shales and the Precambrian basement. We have conducted 60 broad-band magnetotelluric soundings along a 180 km long profile in Jämtland, Sweden, across the eastern section of the Central Scandinavian Caledonides. Dimensionality analysis and regional strike estimates indicate that the conductivity structure can be approximated by a 2-D model having a N40 • E strike direction, consistent with the dominant geological strike. The determinant average of the impedance tensor, together with the tipper transfer function from the best 34 sites, were inverted by the REBOCC 2-D inversion code. An electrically highly conducting layer beneath the Caledonides images alum shales, the autochthonous Cambrian carbon-bearing black shales on top of the Precambrian basement. Based on the comparison of electrical conductivity and seismic reflectivity models, we suggest that the Caledonian accretionary wedge thickens in a step-wise manner from ca. 1 to 5-6 km towards the west. In the east, the wedge is composed of the lower allochthon. In the west, the wedge reaches the thickness of 15 km and is composed of the lower allochthon at the bottom, the middle/upper allochthons at the top and resistive allochthonous basement slices. The upper crust of the autochthonous Precambrian basement is homogeneous and resistive from surface down to 15 km and can be associated with the Revsund and Rätan granites. The border between the eastern Revsund-and western Rätan-type granites coincides in the Jämtland region, with the boundary between the northern Central Svecofennian Province and the southern Svecofennian volcanic belt and is marked by a subvertical conductor associated with a steeply dipping band of reflectors. The lower crust and uppermost mantle in the easternmost part of the profile are very resistive, whereas in west, they are 2-3 orders of magnitude more conductive. The increase of average crustal conductivity is related to the Caledonian processes or later opening of the Atlantic Ocean that have affected also the lower crust.

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

“…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.…”

“…Since the x-direction was oriented along magnetic north during the field measurements, the regional strike direction is N34 • E with regard to geographic north (accounting for ∼4 • E magnetic declination in the measurement area). Since the strike direction is mostly determined by the longer-period data, the difference of 6 • with regard to the strike direction of N40 • E found by Korja et al (2008) for the MT data recorded along the CCT reflection seismic profile may be related to gradual changes in Precambrian bedrock geology.…”

“…The previous MT research along the CCT reflection seismic profile conducted by Korja et al (2008) covered a region adjacent to ours and was located 5-30 km farther to the north (Fig. 2).…”

“…The VLF method is a useful EM tool for mapping nearsurface structures in the uppermost hundred meters (McNeill & Figure 2. Geologic map of the study area with locations of new broad-band MT stations (blue and purple dots), borehole COSC-1 (yellow star, Lorenz et al 2015a,b), previous MT stations (green diamonds, Korja et al 2008), recent reflection seismics along CSP (continuous yellow lines; Hedin et al 2012;Juhlin et al 2016) and previous reflection seismics along the CCT profile (dashed yellow line, Juhojuntti et al 2001). Based on the bedrock geological map of Sweden, C Geological Survey of Sweden [I2014/00601] and Strömberg et al (1984), and modified after Juhlin et al (2016).…”

“…This westward-dipping reflection, although not continuous, was interpreted as a detachment surface (Palm et al 1991). Korja et al (2008) deployed 60 broadband magnetotelluric (MT) stations along the CCT reflection seismic profile in western Jämtland (note that Figure 1. Geological map of Scandinavian Orogeny and cross-section through the Central Scandes between points W and E (modified after Gee et al 2010).…”