In the Myrviken area in central Jämtland (Fig. 2), south of Storsjön, the drilling program (Gee et al., 1982) defined the geometry of an exceptionally thick (up to 180 m) alum shale unit directly overlying the Caledonian sole thrust (here corresponding to the Jämtlandian décollement). Twenty-eight drill holes (all cored) provided the basis for identifying a major low-grade uranium, vanadium, molybdenum, and nickel resource in the organic-rich alum shales. Most of the holes also penetrated a thin sandstone-dominated autochthonous Cambrian sedimentary succession overlying late Paleoproterozoic granites of the crystalline basement. Within the allochthonous units, both quartzites, stratigraphically underlying the alum shales, and limestones overlying them, occur in an imbricate stack that comprises the so-called Jämtlandian Nappes of the Lower Allochthon.

The above-mentioned drill holes allow the décollement surface to be mapped in the Myrviken area (Fig. 2) and it shows the typical character of the Caledonian thrust front throughout most of the mountain belt. Interestingly, the fold axes in the allochthon in this area trend approximately N–S instead of NE–SW, possibly due to an anomalous basement high, ca. 50 km to the northeast in the Lockne area (Fig. 2), the result of a middle Ordovician meteorite impact (Lindström et al., 1996). Cross-sections through the area of southern Storsjön illustrate the structure of the imbricate stack (Andersson et al., 1985). Figure 3 shows a 25 km long profile trending NW, and partly NNW, from the thrust front near Hackås to Marby (Gee et al., 1982), oriented approximately parallel to the dip of the Jämtlandian décollement. This drill-hole-based profile ends about 10 km east of the eastern termination of the Dammån-Hallen (DH) seismic profile. If account is taken of the klippe (tectonic outlier) occurring to the south-southeast of Hackås in the Bingsta area, the Jämtlandian décollement can be inferred to provide a regular surface, dipping about 1∘ west–northwest, over a distance of ca. 40 km.

The exposed and near-surface bedrock between the village of Hallen and Lake Liten is dominated by Ordovician turbidites of the Jämtlandian Nappes. Only in the area of southeastern Liten are younger strata (lower Silurian, including quartzites, limestones, black shales, and, perhaps, turbidites) preserved locally in a shallow NW-trending syncline. Together with thick underlying Ordovician turbidites, this Jämtlandian sedimentary succession is folded, regionally, on approximately N-trending axes and apparently imbricated by thrusting that is best exposed to the south of the CSP in the N-plunging Oviksfjällen Antiform. The latter is inferred to be a southern continuation of the Olden Antiform and, as shown on the Strömberg et al. (1984) map, comprises thrust sheets dominated by early Cambrian (perhaps late Ediacaran) quartzites, minor alum shales, and subordinate slices of basement-derived felsic volcanic rocks, similar to the porphyritic rhyolites outcropping in the Mullfjället Antiform, to the west of the Åre Synform.

Between Liten and Byxtjärn, near Undersåker, the seismic profile crosses the thrust between the Lower and Middle allochthons. The former is composed of low to sub-greenschist facies Ordovician turbidites, locally passing up into early Silurian strata. In the hanging wall, the Seve Nappe Complex of the Middle Allochthon dips gently westwards in the eastern limb of the Åre Synform. It comprises mainly quartzites and subordinate calcsilicate-rich psammitic gneisses and marbles, with abundant amphibolitized dolerites and gabbros and some, usually isolated, ultramafites. These rocks comprise a highly reflective assemblage as found in the seismic investigations over the Åre and Tännfors synforms (Palm et al., 1991) and in the more recent seismic data presented in Hedin et al. (2012). Along the thrust contact between the Seve Nappe Complex and the underlying strongly folded and intensely foliated turbidites of the Lower Allochthon, there occurs a sheet of felsic gneisses, locally underlain by a few tens of meters of ductilely deformed Särv Nappe metasandstones and concordant greenstones. Based on the seismic data acquired over the Åre Synform to date (Palm et al., 1991; Hedin et al., 2012, 2016), prominent reflective units that do not crop out in the eastern limb of the synform are expected to be present at depth further west, beneath its central and western parts. Results from the 2.5 km deep COSC-1 borehole show that the reflectivity of the Seve Nappe Complex is due to the contrast between the high metamorphic-grade gneisses and amphibolites (Hedin et al., 2016). Some of the reflections originating from below the bottom of the borehole, interpreted not to be part of the Seve Nappe Complex, can be traced towards the east, but do not extend to the surface.

In the western limb of the Åre Synform and the axial zone of the Mullfjället Antiform, Tiren (1981) mapped a detachment close above the basement and described relationships similar to those in the Caledonian front, i.e., with most of the quartzites, alum shales, and overlying turbidites being allochthonous in relation to the underlying Precambrian acid volcanic rocks with their thin unconformable veneer of alum shales and limestones.

More than 1800 source points were activated along a 36 km long profile (Fig. 2) using a rock-breaking hydraulic hammer (VIBSIST) mounted on a front end loader. Nominal source and receiver spacing was 20 m and a split spread of 360 active channels using 28 Hz geophones was rolled along with the source. In two locations of greater interest, the source point spacing was decreased to 10 m to increase the local fold. No source points were activated at the first 124 receiver locations (in the terrain), or along a few short parts in the western half (no permission to activate the source), resulting in a decreased fold in these areas. The fold along the entire profile therefore shows significant variation (Hedin et al., 2012).

Acquisition of the Liten-Dammån profile used the same VIBSIST source as for the Byxtjärn-Liten profile. Permission to activate the source and plant receivers was not obtained along a nearly 4.5 km stretch of road close to the beginning of this profile, leaving a gap in the acquisition geometry between the Byxtjärn-Liten and Liten-Dammån profiles (Fig. 2). This was partially bridged by using wireless receivers on the western side of the gap, coinciding with the last 1 km of the Byxtjärn-Liten profile, while wired receivers were placed on the eastern side. Source points were activated on both sides of the gap to undershoot it as much as possible. However, complete undershooting was not obtained.

The main profile of 2014 was the 14 km eastwards extension of the Byxtjärn-Liten and Liten-Dammån profiles, beginning at Dammån and ending south of Hallen (Fig. 2). Acquisition parameters for this profile differed from the Byxtjärn-Liten and Liten-Dammån profiles in that a different source was used, and 28 Hz geophones were used instead of 10 Hz geophones. More importantly, the source was less powerful. A 400 kg weight drop mounted on a small Bobcat excavator replaced the VIBSIST source. Previous studies (Sopher et al., 2014; Place et al., 2015) showed that this source could provide enough energy to image the subsurface to the depths of interest for the project, assuming thin Quaternary cover and shallow depths to bedrock.

To resolve the structures not imaged properly in the 4.5 km gap of the Liten-Dammån profile, especially in the uppermost 2 km, a 16 km long profile was designed to fully bridge this gap. Starting at the same location as the Liten-Dammån profile and overlapping with the last 1 km of the Byxtjärn-Liten profile, the Sällsjö profile took a more southern route via the village of Sällsjö before turning north and merging with the Liten-Dammån profile (Fig. 2). Identical acquisition parameters to the Dammån-Hallen profile were used, that is, the same source, recording system, and spread.

Both the VIBSIST source and weight drop source generated enough energy to allow the seismic waves to penetrate to at least 9 km depth (Fig. 5). A direct comparison of the sources is not possible since the profile locations, acquisition geometries, and the ambient noise conditions were not the same. In general, the VIBSIST source provided higher S/N data than the weight drop source (compare Fig. 5a to Fig. 5b). However, merging of the two data sets generates a section which allows a clear correlation between reflections northwest and southeast of the gap on the Liten-Dammån profile (Fig. 5c). In particular, after merging, it is clear that the subhorizontal reflection at 0.7 s southeast of the gap (to the right of CDP 900 in Fig. 5c) is not connected to the two reflections at 0.4 to 0.6 s northwest of the gap (to the left of CDP 350 in Fig. 5c). Furthermore, the two west-dipping reflections at about 1 and 2 s (at CDP 1100, Fig. 5a), respectively, southeast of the gap appear to be connected to the subhorizontal reflections at 1.8 and 2.6 s northwest of the gap (Fig. 5a). Note that these reflections are better imaged on the Sällsjö profile with the weight drop source than on the Liten-Dammån profile with the VIBSIST source (compare Fig. 5a with Fig. 5b at CDP 100 to 300).

The entire composite profile (Fig. 7) shows generally subhorizontal reflections in the uppermost 2 km. Below this depth the reflections are mainly northwest-dipping, but with some subhorizontal reflections. An exception is the patchy highly reflective zone in the upper 2 km in the CSP interval from CDP 100 to CDP 1200, which characterizes the Seve Nappe Complex. The west-dipping nature of this boundary is clearly defined from CDP 1100 to CDP 900, but the boundary becomes more diffuse below the central parts of the reflective zone. The diffuse nature of this boundary at depth was verified by the drilling of the COSC-1 borehole to 2.5 km (Lorenz et al., 2015) and the limited 3-D seismic survey that was acquired after drilling was completed (Hedin et al., 2016). Between CDP 1100 and 4600 along the CSP, distinct, northwest-dipping reflections are present, some of which can potentially be traced from 7 km depth to the subhorizontal reflections between 1 and 2 km depth. These dipping reflections appear to merge into the overlying shallower subhorizontal reflections. Similar dipping reflections were also observed on the CCT profile (Juhojuntti et al., 2001) and some of them can be correlated to the CSP by their geometrical patterns in spite of the two profiles being separated by about 20 km. The source of these dipping reflections has previously been discussed (Palm et al., 1991; Juhojuntti et al., 2001; Hedin et al., 2012). Deformation zones, dolerite sheets, or a combination of the two were considered likely candidates. At the southeastern end of the CSP (CDP 4600 to 5500) the data quality deteriorates significantly (Fig. 6) due to the presence of an up to 60 m thick sequence of unconsolidated Quaternary sediments, which severely attenuate the signals and make it difficult to track the reflections beneath them. However, the shallowest subhorizontal reflection can be traced to about 0.5 km depth at the southeasternmost end of the profile, as can the northwest-dipping reflection at about 6 km. The lack of clear reflections in between these two is due to poor S/N. This reasoning is verified by a comparison with the CCT profile (Fig. 8) on which there are clear northwest-dipping reflections in the equivalent depth interval and in the same structural position along the profile. Note that the Dammån-Hallen profile was not extended further to the southeast due to permitting issues. Even if it had been possible, the thick sequence of unconsolidated sediments, also partly present to the southeast, would probably have made it difficult to acquire good data.

Two important relationships need to be defined – the depth and character of the Jämtlandian décollement, and also the thickness and character of the underlying basement that has been influenced by Caledonian deformation. The drilling in the Myrviken area, southeast of the CSP, clearly defined the Jämtlandian décollement, where Cambro-Ordovician sedimentary rocks are thrust over a thin autochthonous sedimentary cover and the underlying basement shows no evidence of Caledonian deformation. If the geometry of the Jämtlandian décollement in this area (Figs. 2 and 3) is projected into the southeastern end of the CSP, it would be expected to be found at a depth of about 500 m, coinciding with the subhorizontal reflection found at this depth on the southeastern end of the composite section (Fig. 7). This reflection is not continuous northwestwards to CDP 4800, but rather irregular, perhaps due to the variable quality of the data that was acquired over the thick Quaternary sediments. However, we interpret the reflection at about 0.7 km depth at CDP 4800 along the CSP (Fig. 7) to represent the Jämtlandian décollement that was drilled further southeast in the Myrviken area. This reflection can be fairly reliably traced along the CSP to CDP 3300. Here, it is unclear if the décollement continues along the uppermost reflection at 1.2 km depth to CDP 2900 or along the lower one at 1.7 km depth at CDP 2900. Several lines of evidence indicate that the shallower reflection probably represents the Jämtlandian décollement. On the CCT profile, to the north, the Jämtlandian décollement was interpreted to be at about 1 km depth at an equivalent distance from the Caledonian front. Interpretation of the depth to magnetic basement based on the slope of the magnetic anomalies (using the standard Peter's method; see e.g., Reynolds, 2011) along the composite profile (Fig. 7) gives values of about 1.3 and 1 km at CDPs 3100 and 4100, respectively. Note that an alternative interpretation for the Jämtlandian décollement is that it deepens already at CDP 5200 down to 1 km depth. This alternative will be discussed later in the paper.

The new magnetotelluric (MT) survey along the profile (Yan et al., 2016) provides evidence of a gently undulating surface of the prominent uppermost conductive layer, shown in Fig. 7, this being located at ca. 500 m at the eastern end of the CSP, sinking to 600 m at CDP 3800, rising to 400 m at CDP 3500, sinking again to 1100 m at CDP 2700, and then rising again at CDP 1600 to 600 m, before dipping west again beneath the Seve Nappe Complex, west of Undersåker. This undulation fits well with the inferred location of the axes of the synforms and antiforms that are located in the vicinity to the north and south of the CSP line. The highly conductive layer is interpreted to represent the uppermost alum shales. It is therefore possible that the Jämtlandian décollement could be at a depth of about 1.5 km at CDP 2900 along the CSP and, if so, that it shallows to less than 1 km further west at CDP 1500 (Fig. 7) and then deepens at CDP 1300, below the Seve Nappe Complex.

All alternative interpretations accept the evidence for shallow décollements and require a substantially deeper location for the Caledonian sole thrust (e.g., Hedin et al., 2012). In both the Oviksfjällen and Olden antiforms, located to the south and north of the CSP profile, respectively, and apparently crossing it at ca. CDP 3300–3500, there is evidence of substantial shortening, with a quartzite-dominated thrust stack in Oviksfjällen and much internal basement deformation in Olden. The Olden Antiform is of particular interest because it contains an upper part of allochthonous basement (Gee, 1980; Robinson et al., 2014) thrust over the Cambro-Silurian sedimentary rocks of the Jämtland Supergroup. The extent to which sedimentary rocks of the Lower Allochthon might be represented at deeper structural levels than those exposed in the Olden and Oviksfjällen antiforms is, at present, impossible to say; MT methods have difficulty in detecting any features below a strong conductor like the alum shales that is so well defined in the overlying décollement levels. Furthermore, the Oviksfjällen and Olden antiforms do not have a strong magnetic signature. The depth extent of the basement reflectivity is on the order of 10 km and presumably originates in magnetic basement; therefore, it is not clear how these antiforms can be linked to the origin of the basement reflections.

In the following section we discuss alternative interpretations along the CSP. The first one, based on Hedin et al. (2012), focuses on the sole thrust and considers even deeper structural levels for the Caledonian deformation. The second considers the Jämtlandian décollement in relation to the location of the uppermost alum shales and the underlying flat-lying reflectors in the upper 2 km of the crust, in line with the interpretations presented in Juhojuntti et al. (2001) and Korja et al. (2008). Figure 9 illustrates these interpretations.

In Figure 9a we present the section of Hedin et al. (2012) up to CDP 2900 (the easternmost extent of the Byxtjärn-Liten profile); further east we define a consistent prolongation within the CSP. The sole thrust, in western parts at about 4 km depth, rises eastwards to the flat reflectors at about 2 km depth between CDP 3400 and 4200. It then ramps up to ca. 1.5 km and continues at this level to CDP 5100. Here it ramps up again to ca. 500 m and extends eastwards into the frontal décollement in the Myrviken area. The flat sections between CDP 3400 and 5100 both underlie hanging-wall west-dipping reflections, which suggests imbrication. The interpretation in Fig. 9a is based on the geology of the Oviksfjällen Antiform where the early Cambrian (to Ediacaran) quartzites dominate, but include some slices of Precambrian volcanic rocks, particularly in the eastern limb of the structure. Even deeper Caledonian deformation cannot be excluded. In this case, the sole thrust would extend from the frontal ramp at CDP 5100–5200 via a flat to CDP 4400, and from there downwards along prominent west-dipping reflections to a flat at ca. 5 km depth beneath CDP 3000 where it continues westwards along more gently dipping reflections. This alternative would pass into the flat reflectors beneath the Mullfjället Antiform at ca. 7 km depth (Palm et al., 1991) and then perhaps extend beneath the Skardöra Antiform at similar (Hurich et al., 1989) or even greater depths (Gee, 1988). Both these deep sole thrust interpretations require that the shallow Rätan-type basement beneath CDP 3400 and farther east, as suggested by the magnetic data, is allochthonous on top of a basement with similar characteristics.

The interpretation, presented in Fig. 9b, concentrates on the Jämtlandian décollement beneath the Jämtlandian Nappes and the new geophysical and geological evidence relevant to the uppermost 2 km of the crust. The characteristic feature here is the shallow level of the décollement to the southeast of CDP 1400 and the significant deepening westwards below the Åre Synform. The Jämtlandian décollement is probably accommodated within, or in close proximity to, the highly organic-rich Cambrian alum shales that constitute a weak horizon at, or near, the base of the Early Paleozoic Baltoscandian sedimentary succession. In some areas, up to some tens of meters of Ediacaran to lower Cambrian quartzites separate the alum shales from the underlying basement of the Fennoscandian Shield (Andersson et al., 1985). This unit may be locally absent because of the original basement topography, or stripping by the overlying thrust; alternatively, it may be repeated several times within the décollement zone.

The new magnetotelluric (MT) data indicate the presence of a good conductor at ca. 1000 m at CDP 2200 and just below 500 m at CDP 4100 (Fig. 10). Below these depths, the reflection pattern in the seismic profile indicates imbricate thrusting above a detachment horizon. The latter is interpreted as the original, stratigraphic position of the alum shales, which host the Jämtlandian décollement. Within the imbricates, alum shale is brought to a shallower level as indicated by the MT data. This relationship is similar to that observed close to the present Caledonian front (Fig. 3; Gee et al., 1982; Andersson et al., 1985), where successions of alum shales with overlying Lower Ordovician limestones and shales and, further west, underlying quartzites are stacked to several times the original stratigraphic thickness.

Close to the northwestern end of the profile, in the Åre Synform, the 2.5 km deep COSC-1 drill hole provides control on the Lower Seve Nappe. At ca. 1700 m, the borehole enters a mylonite zone, representing a major thrust at the base of the Seve Nappe Complex. This zone extends to the bottom of the borehole, but a transition to rocks of lower metamorphic grade, possibly from the Särv or Offerdal nappes, occurs at ca. 2350 m (Lorenz et al., 2015). Local 3-D reflection seismics at the drill site (Hedin et al., 2016) and a vertical seismic profile (VSP) survey in the drill hole (Krauß et al., 2015) suggest that the base of the thrust zone is located about 200 m below the bottom of the drill hole. The Särv and Offerdal Nappes are not continuous in the Åre area, but thin and pinch out towards the northeast somewhere below the Åre Synform, as indicated in Fig. 9b. However, farther east the Särv Nappe is present in a klippe (Strömberg et al. 1984). It is also remarkable that east of the Åre Synform, the fault zone that separates the Lower Seve Nappe from the Lower Allochthon is very narrow (north of CDP 1150), i.e., significantly different from the mylonite zone observed in the COSC-1 borehole. The contact observed at the surface is most likely a W-dipping normal fault that places the Lower Seve Nappe against the Lower Allochthon and, thus, cuts out the tectonostratigraphy in between. Similar relationships across faults were reported in the area west of Mullfjället in Sweden, and Skardöra in Norway (Sjöström et al., 1991, Braathen et al., 2000). Below 2 km depth, the normal fault passes into a highly reflective zone above the interpreted Jämtlandian décollement, which it either cuts or merges into. The borders of the mylonite zone below the Åre Synform (dotted white lines in Fig. 9) trace along the reflectivity pattern eastwards towards location (1) in Fig. 9b, where also they merge into the above-mentioned NW-dipping highly reflective zone above the Jämtlandian décollement. East of location (1), a prominent shallow basement reflection can be traced subhorizontally towards location (2), where it offsets the overlying reflections and continues upwards towards the southeast (broken line in Fig. 9b). It is interpreted as a thrust fault that at location (2) cuts upwards through the Jämtlandian décollement into the alum shale and brings basement with overlying rocks closer to the surface. The position, CDP 3100 and 3500, corresponds well with the location of the Oviksfjällen Antiform, which about 10 km south of the seismic profile exposes Ediacaran-Cambrian quartzites in its core (Fig. 2). The nature of the reflectivity above this interpreted thrust fault is ambiguous. Possibly, it is similar to the basement reflections farther down. This would imply that the displacement along this particular reflector is a couple of kilometers, as indicated in Fig. 9b.

For the section of the CSP, we suggest the following geological scenario. The high-grade metamorphic Lower Seve Nappe has a comparatively long tectonothermal history with metamorphism and pegmatite intrusion as early as ca. 470 Ma (Li et al., 2014). Its subsequent emplacement as part of the Seve Nappe Complex has caused the penetrative ductile deformation with high internal strain, including gneisses with mylonitic fabric. Thrusting continuously progressed eastwards with the whole nappe stack, including the underlying units of the Middle Allochthon and Lower Allochthon, translated farther towards the foreland on the Jämtlandian décollement in the alum shales.

After metamorphic conditions in the Lower Seve Nappe had decreased considerably, the ca. 1 km thick mylonite zone began to develop by continued or resumed movement along the Seve-Särv boundary. The age of the movement on the interpreted normal fault that separates the Lower Seve Nappe from the Lower Allochthon, east of the Åre Synform, is probably Early Devonian, as suggested by Gee et al. (1994) for the Röragen detachment where movement was inferred to have occurred while thrusting was still going on at depth beneath the Vigelen Antiform. This could explain why both the reflective pattern, which was related to the Seve mylonite zone in the COSC-1 drill hole, and the trace of the normal fault merge into a highly reflective zone that is directly overlying the Jämtlandian décollement at location (1) in Fig. 9b.

While nappe emplacement during Caledonian orogeny progressed towards the foreland, Baltica was successively underthrusting Laurentia. Thus, it is very likely that also the Baltican basement experienced an eastwards progressing deformation, most likely above a sole thrust and possibly reactivating existing structures in the Proterozoic basement. Major orogen-parallel folding (e.g., Åre Synform and Mullfjället Antiform) occurred above this sole thrust. In the CSP, at least some of the deep reflections (around location 3 in Fig. 9b) are thought to represent this basement deformation.

Additional evidence for some Caledonian deformation is found where reflections present below the interpreted Jämtlandian detachment appear to continue through it and offset the interpreted alum shales. Perhaps the best example of this is between CDPs 2600 and 2800 (Fig. 6) where the “double reflection” may offset the detachment and appears to have disturbed the overlying alum shales.

Scientific drilling at the COSC-2 site to 2.5 km will investigate and test the above scenario down into the shallow basement. It will sample at least one of the deep reflectors at its shallowest level and define its nature.

According to the COSC overall scientific targets, the COSC-2 borehole will investigate the metamorphic and structural evolution from the Lower Allochthon down into the basement of the Fennoscandian Shield. Important questions to be answered by the drilling are as follows. (i) What is the nature of the Jämtlandian décollement and where is it located? (ii) Is the metamorphic grade inverted in the middle- to low-grade greenschist facies sedimentary rocks? (iii) Were they heated from above? (iv) What lithologies and structures generate the reflections in the Precambrian basement? (v) What is the timing of deformation at these structural levels?

To reach these goals, the borehole should first drill the turbidites and limestones of the Lower Allochthon, penetrate the uppermost Cambrian alum shales, and then continue downwards in the zone of high reflectivity, probably with repetition of thin Ediacaran to Ordovician sedimentary cover (quartzites, alum shales, and limestones) and then through the Jämtlandian décollement into the Precambrian crystalline basement, sampling at least a 1 km section of the latter.

Two possible locations for the COSC-2 borehole have been identified on the composite profile (Fig. 10). Option 1 is located along the Byxtjärn-Liten profile at CDP 2200 (Fig. 10a). Assuming that the Jämtlandian décollement has been correctly identified in Fig. 9b, the borehole will penetrate four reflectors in the underlying basement between about 1.5 and 2.2 km depth. A drill hole in this location would investigate the imbricate thrusting above the Jämtlandian décollement, whether the inferred deeper (shallow basement) thrust between CDP 1100 and 3100 is present, and, if not, what then causes the two shallower basement reflections. The two deeper basement reflections can be traced down to about 6 km northwest of the proposed site and appear to offset other reflections on the seismic section (Fig. 7). These two must surely be located in the Precambrian basement. One possible disadvantage with the location is that the separation between these four deeper reflections is small, at least on the present processing, and it may be difficult in the borehole to strictly identify the source to each of the four reflections. However, a combination of new high-resolution seismic data and borehole seismic data should allow the source of the reflections to be determined without ambiguity.

Option 2 (Fig. 10b) is at a location (CDP 4100) where the sole thrust appears to be converging upwards towards the Jämtlandian décollement. The drill hole would penetrate the latter, as defined by a zone of flat-lying reflectivity between CDPs 3100 and 5200, at about 500 m depth; as in the Myrviken drill holes, it would be overlain by the shallowest alum shales, the top of which occur at 400 m, based on the MT data. The Jämtlandian décollement is underlain by a duplex structure, about 1 km thick, characterized by more steeply dipping, shorter reflections representing boundaries between Cambrian strata (quartzites and perhaps subordinate alum shales) and fragments of allochthonous Precambrian basement. The basal thrust of the duplex is a well-defined strong subhorizontal to gently NW-dipping reflection, present across the whole of Fig. 10b, between 1.3 and 1.9 km depth; this probably corresponds to the Caledonian sole thrust. At 2.2 to 2.3 km depth, a basement reflector that appears to extend westwards to depths of greater than 7 km would be penetrated by this hole. The reflection from this structure is rather weak at the proposed site, but clearly present.