GRAVITY MODELLING OF LOCKNE, A MARINE IMPACT STRUCTURE IN JAMTLAND, CENTRAL SWEDEN

Abstract The Lockne impact structure in Jamtland (63°00′20″N, 14°49′30″E) formed in the middle Ordovician at approximately 455 Ma. The structure is interpreted as a concentric crater with a total diameter of 13.5 km. The impact took place in a marine environment with a water depth exceeding 200 m. The gravity survey of the structure and its surroundings comprises 467 measurements. The residual gravity map reveals a negative anomaly reaching a maximum of 22 g.u. The gravity modelling of the structure revealed its morphology, with a central uplift surrounded by a moat. The fracture zone grades out and diminishes at an approximate depth of 2 km. The model also shows a slight northwestward tilt (1–2°) of the structure. The anomaly of 22 g.u. is relatively small compared to the ones occurring in other craters of similar size. A good preservation state and a crystalline target normally favour larger negative anomalies than occur in eroded structures of the same size in sedimentary strata. The Lockne structure occurs in crystalline target rock, and is well preserved. Therefore a small density contrast of rocks in the Lockne structure is the main reason for the small anomaly. In Lockne the impact breccia has only 1/10 of the density contrast compared to the average supra-aquatic impact structure. Furthermore the fractured target rock shows 1/4 of the density contrast that one finds in average supra-aquatic impact structures. The interpretation is that the cavity and fractures were filled mainly by calcite, precipitated by the hydrothermal system acting after the impact. The abundant water and the residual heat from the impact gave the opportunity for the hydrothermal system to develop.