The structural and metamorphic geology of basement rocks in the McMurdo sound area, Antarctica

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Abstract
The petrography and structure of three small areas, the Nussbaum Riegel and the north and south sides of the Garwood Valley in the “Dry Valleys” region of Victoria Land, Antarctica, are described and discussed. The rocks are mostly pure or impure marbles plus lesser quantities of amphibolite, schist and quartz‐feldspar rocks, and mineral assemblages indicate that they have been metamorphosed under conditions of the amphibolite fades, and possibly, locally under conditions transitional to the granulite facies. In general they have not suffered retrograde metamorphism. Nussbaum Riegel is characterized by isoclinal folds and very well‐developed boudinage structure. Examination of the boudins reveals that the competent beds behaved in a ductile manner when failing parallel to the fold axis and in a brittle manner when failing perpendicular to the fold axis. An explanation for this observation is offered, based on variation in strain rate in various directions during the development of the fold. The Garwood Valley areas are complex and two generations of folds are recognized. The large scale structure of the south side of the Garwood Valley is interpreted as a tight basin resulting from the interference of the two generations of folds. The large scale structure of the north side of the Garwood Valley is believed to be similar in style to that of the south side, but insufficient data are obtainable to allow detailed interpretation.