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

Abstract

High quality wide-angle and deep normal-incidence seismic data sets recorded during the MONA LISA (Marine and Onshore North Sea Acquisition for Lithospheric Seismic Analysis) experiment have allowed detailed crustal investigations in the area of the Central Graben and south-eastern North Sea to be conducted. An integrated velocity–density model based on a joint inversion of traveltimes and gravity data is presented for the crust and uppermost mantle 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 25km depth and the crystalline crust is about a factor of 2 thinner than in the surrounding crustal provinces, where Moho is interpreted to reach depths between 32 and 36km. Maximum thickness of the sedimentary column in the Central Graben is 10–11km. The mass deficit of the thick sediments is compensated by the Moho uplift. The modelled Moho coincides with the base of lower crustal reflectivity observed in the coincident normal-incidence seismic section. Good overall agreement between the integrated velocity–density model and the normal-incidence data is observed. The migrated deep seismic reflection section recorded to 26s traveltime contains deep west-ward and east-ward dipping events symmetric about the deepest part of the Central Graben. These dipping upper mantle reflections may be linked to extensional processes that formed the Central Graben. At least one of the upper mantle dipping reflectors may be a reactivated structure formed originally during Caledonian collision.