Geophysical-petrological modelling of the East Greenland Caledonides – Isostatic support from crust and upper mantle

Abstract

Teleseismic receiver function analysis imaged a complex upper mantle structure in the Central Fjord Region of East Greenland, including an east-dipping high velocity layer and a mantle wedge of high crustal or low mantle velocities. This was interpreted as a fossil Caledonian subduction complex, including a slab of eclogitised mafic crust and an overlying wedge of serpentinised mantle. In this paper, we use a multi-disciplinary geophysical and petrological modelling approach to test this proposed fossil subduction model.The consistency of the obtained velocity model with the regional gravity field is tested by forward density modelling and isostatic calculations. The models show that the sub-crustal structure, given by the more buoyant mantle wedge and the dipping high velocity/density layer, yield in a markedly better fit as compared to a homogeneous mantle lithosphere.Petrological-geophysical modelling is performed by testing different upper mantle compositions with regard to topography, gravity and seismic velocities using Litmod2D. This suggests that the observed lower crustal/uppermost mantle bodies could be a combination of mafic intrusions, serpentinised peridotite and metamorphosed mafic crust. The preferred composition for the dipping structure is eclogitised mafic crust, and hydrated peridotite filling the overlying mantle wedge. Models lacking an eclogite layer or a hydrated upper mantle composition show an inferior fit and, therefore, are not favoured representatives. This supports the interpretation as a fossil subduction zone complex. The spatial relations with Caledonian structures suggest an early Caledonian origin.