The crust and uppermost mantle below the large positive gravity anomaly in the central part of the Norwegian-Danish Basin, the Silkeborg Gravity High (SGH), is investigated using controlled source seismic data. A more than 80 km long and ~ 20 km thick intrusion is interpreted. The seismic velocity within the intrusion varies from 6.8 km/s at the top (12 km depth) to 7.2-7.7 km/s at the Moho level (30-31 km depth).

The characteristics of the Moho reflectivity vary along the profile. Below the central part of the intrusion, the Moho reflectivity is very weak, due to a small velocity contrast between the lowermost crust (7.7 km/s) and the uppermost mantle (7.9-8.0 km/s). The seismic data show a "ringing" Moho below the western part of the intrusion. The coda trailing the main P_mP reflection is about 1.0 s long and is composed of 4-5 wavelets. We demonstrate that this feature may be explained by a layered transition zone between 30 and 35 km depth, where high-velocity layers of mantle affinity (7.9-8.05 km/s) alternate with layers of typical lower crustal velocity (6.7-7.3 km/s). The characteristics of this layering, which causes theP_mP coda, are modelled by matching synthetic seismograms to the observed data. Synthetic seismograms, calculated using the reflectivity method for 1D models, constrain the nominal layer thickness to 100-500 m. Waveform modelling in 2D constrains the layered internal structure of the transition zone and the length of the heterogeneous zone along the profile. We interpret the layered crust-mantle transition zone as solidified melts of mantle affinity, which were intruded into the lowermost crust. The layered zone is proposed to be associated with the extensional tectonism that occurred in the area in Late Carboniferous-Early Permian, which also caused the large crustal intrusion. The subsequent cooling may have initiated the formation of the Danish Basin.