Gravity modelling of Lockne, a marine impact structure in Jamtland, central Sweden

The Lockne impact structure in Jamtland (63 degrees 00'20 " N, 14 degrees 4 9'30 " E) formed in the middle Ordovician at approximately 455 Ma. The stru cture is interpreted as a concentric crater with a total diameter of 13.5 k m. The impact took place in a marine environment with a water depth exceedi ng 200 m. The gravity survey of the structure and its surroundings comprise s 467 measurements. The residual gravity map reveals a negative anomaly rea ching 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 zo ne grades out and diminishes at an approximate depth of 2 km. The model als o shows a slight northwestward tilt (1-2 degrees) of the structure. The ano maly 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 targe t normally favour larger negative anomalies than occur in eroded structures of the same size in sedimentary strata. The Lockne structure occurs in cry stalline target rock, and is well preserved. Therefore a small density cont rast of rocks in the Lockne structure is the main reason for the small anom aly. Ln Lockne the impact breccia has only 1/10 of the density contrast com pared to the average supra-aquatic impact structure. Furthermore the fractu red 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 fr om the impact gave the opportunity for the hydrothermal system to develop. (C) 1998 Elsevier Science B.V. All rights reserved.