Deep seismic investigation of crustal extensional structures in the Danish Basin along the ESTRID-2 profile

Abstract

SUMMARY The crust and uppermost mantle in the Danish Basin are investigated by modelling the P-wave velocity distribution along the north–south trending seismic profile ESTRID-2. Seismic tomography and ray inversion modelling demonstrate a variable depth to the top of the crystalline crust, from ∼10 km in the northern part of the profile, to ∼2 km depth in the southernmost part. The crystalline crust shows very high P-wave velocity in the central part of the profile, with ∼6.7 km s−1 at depths as shallow as 12 km, and ∼7.3–7.5 km s−1 in the lowermost crust. These values confirm previous results obtained along the orthogonal ESTRID-1 profile and the Eugeno-S profile 2. This high velocity zone in the middle to lower crust is interpreted as a mafic intrusion, which explains a positive gravity anomaly of ∼50 mGal (Silkeborg Gravity High). The total length of the intrusion is at least 80 km in the east–west direction and ∼25– 35 km in the north–south direction. The estimated thickness of the intrusion, from its top to the Moho level is ∼18–20 km, which gives a total minimum volume of ∼40–50 000 km3. The reflectivity properties of the Moho discontinuity are variable along the profile. Below the intrusion, the PmP signal is very weak, due to the small velocity contrast between the lowermost crust (∼7.4 km s−1) and uppermost mantle (∼7.6–7.7 km s−1). The main Moho reflection has a ‘reverberative’ character to the south of the intrusion. This feature is interpreted by layering at the Moho level, possibly due to magmatic underplating. The occurrence of a large crustal mafic intrusion associated with magmatic underplating may be related to extensional/transtensional tectonism in the Tornquist Fan area in the Late Palaeozoic. The extensional event probably caused the opening of a plumbing system for intrusion of mantle derived magma into the crust. The ascending magma may have been injected at upper-middle crustal levels and, during the late phases of the development, ‘squeezed’ laterally along the Moho.