Seismic constraints on a large mafic intrusion with implications for the subsidence history of the Danish Basin

Abstract

Seismic refraction data from the ESTRID‐1 profile are used for seismic velocity modeling along the strike of a large mafic intrusion in the Norwegian‐Danish Basin, central Denmark. The P wave velocity structure identifies a ∼8 km thick sedimentary succession with velocities between 1.8 and 5.7 km/s. The top basement is defined by a step to vp = 6.2 km/s. In the middle to lower crust a high‐velocity body (vp > 6.7 km/s), with its top located at about 10–12 km depth, is interpreted as a high‐velocity and high‐density gabbroic intrusion of Permian age. This high‐velocity body explains the large (∼50 mGal) positive gravity anomaly known as Silkeborg Gravity High. The intrusion has a minimum volume of 40,000 km3, which implies that the magma influx and the consequent cooling of the lithosphere from high temperature could have had profound effects on the subsidence of the Danish Basin, in particular because the magma probably intruded during only a few events and other similar structures cover much of the basin. An anomalously high velocity gradient (from 7.0 km/s in the middle crust to 7.7 km/s at 30–32 km depth) in the central part of the intrusion coincides with an interval without Moho reflections, indicative of a gradual transition zone between the crust and the mantle. This feature may show the location of the feeder dykes of the intrusion.