Continental mafic dyke swarms as tectonic indicators: an example from the Vestfold Hills, East Antarctica

Abstract

The two main types of magma that were emplaced as mafic dyke swarms in the Vestfold Hills are high-Mg tholeiite (HMT) and Fe-rich tholeiite (FRT). The former magma type is less dense than the Vestfold Hills felsic crust, whereas the latter is more dense at deep to intermediate crustal levels. Therefore, FRT dyke emplacement requires a higher deviatoric extensional stress, as is reflected in a more constant orientation. Palaeoproterozoic HMT-FRT bimodal dyke swarms were emplaced around 2.4 Ga and 2240 Ma. The geometry and orientation of 2240 Ma HMT dykes imply a low deviatoric extensional stress field, inconsistent with a mantle plume origin for magmatism. Current petrogenetic models for HMT magmatism suggest generation at active (convergent) plate boundaries. Although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2.5 Ga, the exact mechanisms for melting and the generation of crustal extensional stress are unknown. A possible scenario involves the sinking of gravitationally unstable oceanic crust. A major Mesoproterozoic magmatic—extensional event in the Vestfold Hills started with normal faulting, followed by lamprophyric dyke emplacement and by two distinct episodes of FRT dyke emplacement at 1380 and 1245 Ma. A consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift. By comparison with other mafic dyke swarms, this is interpreted to be due to a sheet of upwelling asthenosphere (elongated plume) around 1.4 Ga, rather than a low-angle detachment. This upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 Ma. Erosion during a long period of uplift prior to 1380 Ma was followed by sedimentation during thermal subsidence, as suggested by segmentation of 1245 Ma dykes. The lithospheric structure inherited from the Mesoproterozoic magmatic—extensional event was probably reactivated during two high-grade tectono-metamorphic events around 1.0 and 0.5 Ga. The Vestfold Hills formed part of a relatively stable craton, peripheral to the mobile belt that was formed during these events.