Younger Tectonic Structures

The sub-Cambrian peneplain can be followed eastwards from lake Näkten to near the southwestern margin of the inner crater of Lockne. It remains a relatively flat surface that rises from about 325 m in the west to somewhat over 435 m in the east, over a distance of 9.5 km. This corresponds to the rate that is normal in the region (Sturkell and Lindström, in press in Meteoritics and Planetary Science). The 435 m-plus level is reached at the top of a hill 1.7 km southwest of Hällnäset (National Grid coordinates 698400/145040). 1.1 km to the northeast of this hilltop the level of the surface identified as the sub-Cambrian peneplain has dropped to 340 m. The drop is effected mainly along a fault that can be followed for 6.5 km to the southeast and 2.5 km to the northwest, beyond which we have not been able to follow it.

A fault zone with the same strike and about half as large throw forms the northeast flank of Nordanbergsberget (Fig. 2) at the north margin of the inner crater. In this case the peneplain can be identified at 385 m in the southwest and drops to about 340 m in the northeast. This fault zone can be followed about 4 km along the strike north of Nordanbergsberget. A continuation toward the southeast at Rossbol on the other side of Locknesjön would explain the crater-related distribution of rocks there and finds support in the topography. It would extend the fault zone at least 7 km toward southeast.

The circumstance that the sub-Cambrian peneplain occurs at somewhat below 345 m at Haga (see the preceding chapter on the sub-Cambrian peneplain) is evidence of a drop of about 100 m on the east side of lake Locknesjön. In this case the most likely position of the fault is along the northwest-southeast striking depth axis of Locknesjön. The combined effect of the three cases of vertical relative movement referred to in the preceding paragraphs is that the eastern segment of the Lockne crater has been lowered at least 100 m relative to the rest of the crater. The evidence that this tectonic subsidence has taken place is the preservation of the eastern crater rim at Haga and well-preserved resurge deposits resting on the intact, crystalline basement outside the eastern rim of the crater,

These arguments conflict with the hitherto held view (Lindström et al. 1996) that the eastern segment of the Lockne crater must have been eroded to some depth because of the general north-westward inclination of the regional structure. Having been downfaulted about 100 m, it could, however, be equally well preserved as the western part. It has been impossible to verify this possibility because of an omnipresent, if mostly

Fig. 14. Principal structures of the Lockne area. 1: Boundary between the basement and the autochthonous Palaeozoic. 2: Boundary of impact-related rocks. 3: Boundary of inner crater. 4: Fault; the arrow indicates the downthrown side. 5: Imbricated thrust-faults. 6: Anticline. 7: Boundary of overthrust nappe.

Fig. 14. Principal structures of the Lockne area. 1: Boundary between the basement and the autochthonous Palaeozoic. 2: Boundary of impact-related rocks. 3: Boundary of inner crater. 4: Fault; the arrow indicates the downthrown side. 5: Imbricated thrust-faults. 6: Anticline. 7: Boundary of overthrust nappe.

thin, earth cover. The new situation is that construction of lumbering tracks has created numerous exposures of the sub-Cambrian peneplain with a sporadic cover of resurge sediments. Because the peneplain is preserved, the same should be true for the crater.

The Lockne crater is situated at the southeast front of Caledonian overthrusts. An advanced outlier of the basal overthrust is preserved in the midst of the crater. The overthrusts originally reached far to the east of the present front, and the buildup of nappes by far exceeded the thickness of those remains that have survived erosion. The nappe outlier within the crater consists of Lower and Middle Ordovician rocks that show no signs of having been stirred by the impact. Therefore its roots must be sought at least 9 km northwest of the centre of the crater.

In previous publications we have assumed that the Cambrian and Ordovician sedimentary rocks adjacent to the west side of the crater were allochthonous, although the distance they were transported eastwards might have been modest (Lindström et al. 1996). The principal reasons for this assumption were that the Cambrian black shale at the base of the succession is sheared and evidently served as a detachment zone, and that Ordovician limestones are folded, which requires movement relative to the basement. However, the observed spread of very large-sized ejecta across the concerned terrain would have been impossible if the terrain had travelled from a distance great enough to make it useful to speak of allochthony (Shuvalov et al., this volume). Instead, the mapped structures must be examined in order to find their least, arguable distance of movement, which in this case must be taken to be the most likely one.

The Ordovician terrain west of the brim exhibits five principal, northwest-southeast striking anticlines with wavelengths of 0.5-1 km and amplitudes not over 50 m (Fig. 14). The northeast flanks are steep but not overturned. In this case there is no evidence for a southwest-northeast total shortening of over 200 m. Shortening of a similar order in the northwest-southeast direction occurred normal to two anticlines that strike tangentially to the northwest margin of the brim. These anticlines are strongly southeast-vergent and pass laterally into thrust faults (Fig. 3, section A-B). Tectonic shortening of a slightly different style occurs in the sedimentary and impact-related succession near Ynntjärnen (Fig. 11). The transport involved in this case is of interest, because the tectonized succession includes major bodies of crystalline ejecta (see the chapter on the Ynntjärnen Breccia). A shortening of about 300 m along a 700 m long northwest-southeast transect can be derived for this outcrop area. The discussed deformations do not concern the basement, which was rigid at this scale (Fig. 11). The sum of these arguments is that the nappe boundary should be removed that hitherto was drawn close to the brim (Lindström et al. 1996). A thrust-plane with significant transport occurs 1-4 km farther to the west. No impact-related lithologies have been observed above this nappe boundary.

The arrangement of Caledonian fold axes to the northwest and southwest of the brim suggests that the brim presented some degree of obstacle to tectonic transport in the immediately overlying beds. This was probably because it replaces the weak Cambrian shale that is preserved and serves as level of detachment between the basement and the Ordovician limestones in areas untouched by the impact.

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