Geophysical evidence for Early Permian igneous activity in a transtensional environment, Denmark

Integrated interpretation of gravity, magnetic and seismic data in the area of the strong Silkeborg Gravity High in Central Denmark has led to a structural geological model involving a transtensional rift feature which probably formed in connection with Permo-Carboniferous wrench faulting and the development of the Oslo-Horn Graben system. Reinterpretation of the deep seismic data from profile 2 of the EUGENO-S project in the area of the Silkeborg Gravity High detected a new thin high-velocity layer (6.25 km/s) at about 7 km deep within the Palaeozoic sedimentary sequence. In the crystalline basement a separate body of high velocity (6.9-7.0 km/s) was found at 11 km deep. Beneath this body the Moho reaches its shallowest position in the area. The seismic results compare well with an interpretation based on magnetism of a shallow volcanic or subvolcanic layer at a depth of 6-8 km. This layer shows strong remanent and induced magnetization with a southward inclination of about 10°. Gravity reinterpretation of the area allows the separation of an upper body of density contrast 0.1 g/cm³ from a lower body of density contrast 0.2 g/cm³. The upper body roughly coincides with the magnetic body. The lower body, the major one, coincides with the deep seismic body and it is responsible for the main gravity anomaly in the area.