The structural evolution of the Fyfe Hills‐Khmara Bay region, Enderby Land, East Antarctica

Abstract

Five generations of structure (D1‐D5) have been recognized in the Early Archaean supracrustal gneisses in the Fyfe Hills‐Khmara Bay region in Enderby Land, East Antarctica. The D1, D2 and D3 events comprise the Napier structural episode. They resulted in pervasive deformation during Archaean granulite facies metamorphism, and predated the intrusion of the Middle Proterozoic Amundsen dykes. The Napier metamorphic culmination was broadly coeval with, but outlasted both, D1 and D2. D3 occurred during waning granulite facies metamorphism. The D4 and D5 events comprise the Rayner structural episode, the effects of which are largely restricted to amphibolite facies retrograde shear zones in which the Amundsen dykes are deformed. The Napier Structural Episode is characterized by: (i) mesoscopic, isoclinal, recumbent, F1 folds which formed during intense deformation accompanying the prograde burial of the gneissic sequence; (ii) large scale, tieht to isoclinal, reclined F2 folds formed at deep crustal levels; and (iii) large scale, non‐cylindrical, asymmetric, upright F3 folds formed in response to differential vertical movements during the final stages of stabilization of the gneissic sequence. The development of a pervasive mesoscopic gneissic layering associated with the Napier structures is attributed to a variety of processes, only some of which are related to deformation mechanisms. The Rayner structures are restricted to amphibolite grade high strain zones which formed in response to displacements between essentially rigid blocks of granulite. Progressive localization of D4 and D5 structures within the retrograde zones, ultimately resulting in the development of pseudotachylite, reflects deformation at successively higher crustal levels during the excavation of the gneissic sequence.