Very large subsidence, with up to 20 km thick sediment layers, is
observed in the East Barents Sea basin. Subsidence started in early
Paleozoic, accelerated in Permo-Triassic times, finished during the
middle Cretaceous, and was followed by moderate uplift in Cenozoic
times. The observed gravity signal suggests that the East Barents Sea is
at present in isostatic balance and indicates that a mass excess is
required in the lithosphere to produce the observed large subsidence.
Several origins have been proposed for the mass excess. We use 1-D
thermokinematic modeling and 2-D isostatic density models of continental
lithosphere to evaluate these competing hypotheses. The crustal density
in 2-D thermokinematic models resulting from pressure-, temperature-,
and composition-dependent phase change models is computed along
transects crossing the East Barents Sea. The results indicate the
following. (1) Extension can only explain the observed subsidence
provided that a 10 km thick serpentinized mantle lens beneath the basin
center is present. We conclude that this is unlikely given that this
highly serpentinized layer should be formed below a sedimentary basin
with more than 10 km of sediments and crust at least 10 km thick. (2)
Phase changes in a compositionally homogeneous crust do not provide
enough mass excess to explain the present-day basin geometry. (3) Phase
change induced densification of a preexisting lower crustal gabbroic
body, interpreted as a mafic magmatic underplate, can explain the basin
geometry and observed gravity anomalies. The following model is proposed
for the formation of the East Barents Sea basin: (1) Devonian rifting
and extension related magmatism resulted in moderate thinning of the
crust and a mafic underplate below the central basin area explaining
initial late Paleozoic subsidence. (2) East-west shortening during the
Permian and Triassic resulted in densification of the previously
emplaced mafic underplated body and enhanced subsidence dramatically,
explaining the present-day deep basin geometry.