Coupled effects of impact and orogeny: Is the marine Lockne crater, Sweden, pristine?

Abstract

Abstract-Our current understanding of marine-impact cratering processes is partly inferred from the geological structure of the Lockne crater. We present results of a mapping campaign and structural data indicating that this crater is not pristine. In the western part of the crater, pre-impact, impact, and post-impact rocks are incorporated in Caledonian thrust slices and are subjected to folding and faulting. A nappe outlier in the central crater depression is a relic of the Caledonian nappe cover that reached a thickness of more than 5 km. The overthrusted crater is gently deformed. Strike of strata and trend of fold axes deviate from standard Caledonian directions (northeast-southwest). Radially oriented crater depressions, which were previously regarded as marine resurge gullies formed when resurging seawater erosively cut through the crater brim, are interpreted to be open synclines in which resurge deposits were better preserved. The presence of the impact structure influenced orogenesis due to morphological and lithological anomalies of the crater: i) a raised crater brim zone acted as an obstacle during nappe propagation, (ii) the occurrence of a central crater depression caused downward sagging of nappes, and (iii) the lack of an appropriate detachment horizon (alum shale) within the crater led to an enhanced mechanical coupling and internal deformation of the nappe and the overthrusted foreland. Preliminary results of 3-D-analogue experiments suggest that a circular high-friction zone representing the crater locally hinders nappe propagation and initiates a circumferentially striking ramp fault that delineates the crater. Crustal shortening is also partitioned into the crater basement and decreases laterally outward. Deformation of the foreland affected the geometry of the detachment and could be associated with the activation of a deeper detachment horizon beneath the crater. Strain gradients both vertically and horizontally result in non-plane strain deformation in the vicinity of the crater. The strain tensors in the hanging and foot walls may deviate up to 90° from each other and rotated by up to 45° with respect to the standard regional orientation. The observed deflection of strata and fold axes within the Lockne crater area as revealed by field mapping is in agreement with the pattern of strain partitioning shown in the analogue models.

Relationships among magnetic fabrics, microfractures and shock pressures at an impact crater: A case study from Lockne crater, Sweden

“…The crystalline basement rocks were overlain by few kilometres of Caledonian nappe and several tens of metres of Silurian sediments (Högström et al, 2010;Kenkmann et al, 2007). The fractures, caused by pressure release from exhumation (Bahat, 1986(Bahat, , 1991Leith et al, 2014a), may be confused with those of an impact origin.…”

“…The Lockne impact crater is a very well investigated crater that provides a unique opportunity to study the rocks which have experienced an impact, pre-and post-impact tectonic deformation and burial under few kilometres of Caledonian nappe, several hundred metres Silurian sediments, sea water and ice sheet (Kenkmann et al, 2007). It is an important reference for marine cratering processes and is, therefore, used as a standard for comparison with other "wet target" craters on Earth and Mars (Kenkmann et al, 2007).…”

“…It is an important reference for marine cratering processes and is, therefore, used as a standard for comparison with other "wet target" craters on Earth and Mars (Kenkmann et al, 2007).…”

Facies distribution of post‐impact sediments in the Ordovician Lockne and Tvären impact craters: Indications for unique impact‐generated environments

“…Third, the sheet of crystalline ejecta preserved to the north and south of the Tandsbyn gully has its upper surface constantly at a level about 50 m above the reconstructed peneplain. We assume that the peneplain remained as smooth beneath the Tandsbyn gully as in other parts of the Storsjön area (Sturkell and Lindström 2004) because the crystalline basement was too rigid to yield plastically within the relatively small space beneath the gully and because there is no local fault offset that would have proven the presence of brittle deformation of adequate magnitude (for a different view, see Kenkmann et al 2007). Nevertheless, it is documented that Caledonian tectonic movements have caused folding of sediments within the Tandsbyn gully (Lindström et al 2005a).…”

Concentric impact structures in the Palaeozoic of Sweden – the Lockne and Siljan craters

“…However, a recent contribution to the tectonic structure of the Lockne crater interprets the Tandsbyn Gully as a syncline formed in the Caledonian orogeny and thus as evidence of some severe tectonic deformation of the crater (Kenkmann et al 2007). The Lockne crater is indeed situated on the present erosional eastern front of the Caledonian orogen, and a number of features of ascertained tectonic origin have been described (Lindström et al 2005b).…”