Mesozoic magmatic activity in the North Sea Basin: implications for stretching history

Abstract

Abstract Recent developments in the theory underlying magma generation allow the volumes and compositions of melt generated during extension of the lithosphere to be used to make quantitative statements about basin evolution. In the Mesozoic North Sea Basin there is a well defined relationship between magma chemistry and the degree of lithospheric attenuation. The largest extents of melting (<2%) resulted in the alkali basalts of the Forties province which occur in an area where β (the stretching factor) may be as high as 2, and which erupted within 30 million years of the onset of extension. In off-axis areas of the rift the degrees of melting are smaller and melt compositions are more undersaturated and extreme (nephelinites and ultrapotassic rocks). In the Forties province the observed melt compositions may be reconciled with melting on the dry peridotite solidus at a normal (1280°C) potential temperature ( T p ) assuming a relatively thin mechanical boundary layer (MBL) of 70 km prior to rifting. Alternatively, a T p of 1380°C will produce the alkali basalts from dry periodotite at the observed extension factors with an initial MBL thickness of 100 km. A T p as high as 1480°C, which would give rise to regional pre- and syn-rift uplift, can be ruled out because the observed extent of melting is too small, the amounts of extension too large, and the rift episode too short. Off-axis magmatism may be explained by melting within a MBL with an initial thickness of ∼100 km or more at a T p of 1280°C. Models for extension of the lithosphere by simple shear along an initially planar detachment fault fail to account for either the existence or the location of North Sea magmatism, with or without elevated potential temperatures if the dry solidus governs melting. This conclusion is independent of the initial dip of the detachment and also holds when the initial dip in the lithospheric mantle is double that in the crust. The timing, nature, and location of magmatic activity can be used to provide information about the onset of rifting, the rates of extension, the amount of strain, and the thermal history of a basin. When linked with studies of faulting and subsidence this approach may be of some importance for hydrocarbon exploration.