Seismic and gravity modelling of crustal structure in the Central Graben, North Sea. Observations along MONA LISA profile 3

High quality wide-angle and deep normal-incidence seismic data sets recorde d during the MONA LISA (Marine and Onshore North Sea Acquisition for Lithos pheric Seismic Analysis) experiment have allowed detailed crustal investiga tions in the area of the Central Graben and south-eastern North Sea to be c onducted. An integrated velocity-density model based on a joint inversion o f traveltimes and gravity data is presented for the crust and uppermost man tle along MONA LISA line 3, which crosses the Central Graben approximately perpendicular to its strike direction. This model shows pronounced crustal thinning and associated Moho uplift beneath the deepest part of the Central Graben. In the graben area, Moho is interpreted to be situated at 25 km de pth and the crystalline crust is about a factor of 2 thinner than in the su rrounding crustal provinces, where Moho is interpreted to reach depths betw een 32 and 36 km. Maximum thickness of the sedimentary column in the Centra l Graben is 10-11 km. The mass deficit of the thick sediments is compensate d by the Moho uplift. The modelled Moho coincides with the base of lower cr ustal reflectivity observed in the coincident normal-incidence seismic sect ion. Good overall agreement between the integrated velocity-density model a nd the normal-incidence data is observed. The migrated deep seismic reflect ion section recorded to 26 s traveltime contains deep west-ward and east-wa rd dipping events symmetric about the deepest part of the Central Graben. T hese dipping upper mantle reflections may be linked to extensional processe s that formed the Central Graben. At least one of the upper mantle dipping reflectors may be a reactivated structure formed originally during Caledoni an collision. (C) 2000 Elsevier Science B.V. All rights reserved.