HR: 0800h
AN: T11D-0407    [Abstracts]
TI: Geophysical Identification of a Large Mafic Intrusion in Central Denmark and its Possible Effects on the Thermal History of the Danish Basin
AU: * Sandrin, A
EM: ALSA@geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oster Voldgade 10, Copenhagen, DK-1350 Denmark
AU: Nielsen, C A
EM: cn@geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oster Voldgade 10, Copenhagen, DK-1350 Denmark
AU: Thybo, H
EM: thybo@geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oster Voldgade 10, Copenhagen, DK-1350 Denmark
AB: It is known that large intrusions produce significant heat influx that may have a relevant role in basin formation and subsidence. To address these subjects a geophysical study is performed in the Jutland Peninsula at the Silkeborg Gravity High (SGH), Denmark, which is believed being a key area for understanding the mechanisms of origin and evolution of the Danish Basin with its economical hydrocarbon occurrences. A seismic refraction study (ESTRID - Explosion Seismic Transects around a Rift In Denmark) was carried out in the area in 2004 and 2005 to investigate the geometry of a supposed mafic intrusion in the middle to lower crust. The first seismic refraction profile (ESTRID-1) is approximately 140 km long in the W-E direction, from the North Sea cost, to the city of Aarhus, and is covered by 238 stations with average spacing of ~600 m. The second profile ESTRID-2 is around 180 km long and orthogonal to ESTRID-1, with 180 stations located every kilometer. The two profiles cross in proximity of the city of Silkeborg. The presence of the intrusion is indicated by a large positive Bouguer gravity anomaly. The intrusion is believed being of a Late Carboniferous-Early Permian age. The ESTRID data are compared with the EUGENO-2 profile to obtain the depth and extension of the intrusion in different directions. The seismic refraction data along the ESTRID-1 profile clearly show a low velocity sedimentary sequence to a depth of approx. 8-9 km. The P-wave velocities in the sedimentary sequence are estimated to be between 1.9 km/s for quaternary sediments and 5.7-5.8 km/s for Late Paleozoic (Permian) sedimentary rocks. An evident refractor with velocity >6.0 km/s is seen at the bottom of the sedimentary sequence. It represents the top to the basement. Along the EUGENO-2 profile a high velocity unit (~6.25 km/s) is interpreted as supracrustal volcanites, possibly related to the underlying intrusion. The stratigraphic position of this volcanic unit in contact with Late Paleozoic rocks likely indicates a similar age for the intrusion. For the ESTRID data, the refracted waves in the crystalline crust indicate a high velocity zone (>6.5 km/s from tomography and ~6.8 km/s from ray-inversion modeling) with the top located at ~11-12 km depth. Seismic tomography as well as ray-tracing modeling and inversion of the ESTRID-1 data indicate an approximate length of the thickest part of the intrusion, in the W-E direction, of >80 km. The lateral extension in the N-S direction as indicated by ESTRID-2 and EUGENO-2 profiles is ~25 km at the top and ~35 km at the base. The profiles infer a depth to the Moho at 30 km at their cross point, but for the ESTRID-1 profile a deeper but not sharp Moho may be identified below the intrusion. The thickness of ~16 km of the intrusion may therefore be estimated, which gives an approximate minimum volume of the intrusive body of ~40000 km3. Such a large intrusion has certainly affected the temperature field in the area. The heat flow generated has therefore to be taken into account when modeling the Danish Basin.
DE: 0935 Seismic methods (3025, 7294)
DE: 8109 Continental tectonics: extensional (0905)
DE: 9335 Europe
DE: 9609 Mesozoic
DE: 9614 Paleozoic
SC: Tectonophysics [T]
MN: Fall Meeting 2005