Late Neoproterozoic extensional detachment in eastern Sør Rondane Mountains, East Antarctica: Implications for the collapse of the East African Antarctic Orogen

A geological study of the Balchenfjella area of the eastern Sør Rondane Mountains forms the basis of a revised tectonic history for eastern Dronning Maud Land in Antarctica. The Balchenfjella area can be structurally divided into two metamorphic units that are separated by a SE-dipping ductile extensional shear zone (the Balchen detachment fault). The mafic- and ultramafic-dominant part of the footwall side is exposed as a tectonic window adjacent to the detachment. Metamorphic and structural data indicate that the eastern Sør Rondane Mountains were affected by four phases of deformation (D1-D4). Phases D1 and D2 are developed in the footwall side of the shear zone, whereas D3 and D4 are recorded in both the footwall and hangingwall sides. Mineral assemblages in mafic lithologies are characteristic of granulite facies (D1 and D2) to amphibolite facies (D2 and D3) metamorphism. D1 is characterized by recumbent folds and this phase is interpreted as a compressional tectonic setting coincident with granulite facies metamorphism. D2 represents a transpressional deformation phase characterized by upright folds and a large-scale dextral shear zone. The Balchen detachment fault was formed during the extensional tectonic setting of D3. The timing of D3 extensional deformation is bracketed by the age of D2-related metamorphism (at c. 600 Ma) and the timing of post-kinematic intrusion of granites (at c. 550 Ma). The present results demonstrate that the eastern Sør Rondane Mountains underwent Late Neoproterozoic extensional collapse similar to that observed in other parts of the East African-Antarctic Orogen including the Arabian-Nubian Shield and Madagascar.