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

We present in this study magnetic fabrics and microfractures from crystalline basement rocks of the 455 Ma old Lockne impact crater in Sweden. Known analogues of magnetic fabric data and microfracture distributions are used as proxies to estimate the shock pressure experienced by these rocks. Our results demonstrate that the magnetic fabrics in the target basement are representing a pre-impact tectonic or magmatic emplacement fabric, and therefore do not show any shock related re-orientation of the magnetic axes. However, impact origin of the microfractures is evident from their geometrical relationships with the impact crater (radial, concentric, etc.). The results, when interpreted in light of published experimental and natural analogues of similar rock types, reveal that the rocks experienced shock pressure less than 0.5 GPa but greater than 0.2 GPa, respectively. Moreover, the overprinting relationships among the microfractures suggest formation of radial microfractures during the initial compressive phase and concentric microfractures during the following decompressive phase. This study is a novel approach to estimate the shock pressure in weakly shocked rocks, lacking other shock indicators. The identification of characteristic microfracture trends is helpful in locating new impact craters, especially those which have been eroded and deformed.