Geology of the Skallen Region, Lutzow-Holmbukta, East Antarctica (Earth Sciences)

Abstract

Results of the field surveys in the Skallen region during 1969-1970 are summarized with descriptions of stratigraphy and considerations on geotectonic history of the region. Field and aerial photographs, route maps, and field descriptions are also given. Basement rocks of the Skallen region are metamorphic rocks consisting of paragneisses, brown gneisses, marble, quartzite, and metabasite, and plutonic rocks consisting of charnockitic rocks, garnet gneissose granite, and pink granites. The metamorphic rocks comprise four stratigraphic units, namely, Skallen brown gneiss formation, Skallen lower calcareous formation, Skallen siliceous formation, and Skallen upper calcareous formation, in ascending order. These formations belong to the Skallen group, the total thickness of which, including most of the intercalated plutonic rocks which are transformed in situ from the metamorphites, is over 1500 meters. All the formations are conformable to each other, except the lowermost Skallen brown gneiss formation which is tectonically separated from other formations. The fundamental geologic structure of the region is nappe and associated isoclinal folds with gently and easterly plunging axes and axial planes dipping north ; hence a part of the stratigraphic column of the present region is doubled. Some other folds or fractures were found, being overprinted by or superposing on the main nappe structure. The geotectonic history of the Skallen region is as follows from older to younger : moderately plunging recumbent anticline (nappe) -easterly and gently plunging recumbent anticline (nappe) -east-northeasterly or west-southwesterly and gently plunging open folding-east-southeasterly plunging minor folding-easterly gentle folding and northeasterly open synform-conjugate set of west-northwesterly and northeasterly fractures. The change of stress field from older N-S compressional to younger E-W compressional is estimated. A granulite facies metamorphism resulting in the formation of the metamorphites is considered to be synchronous with both or either of the recumbent anticlines, a dynamic metamorphism to take place during the earlier nappe, formation of charnockite to be synchronous with or slightly later than the subhorizontal recumbent anticline and partly to prolong until the completion of the plunging open folding, intrusion of pink granites, which indicate amphibolite facies conditions and the radiometric age of which is already obtained as about 500 m. y., to be around the stage of the gentle folding. Neither metamorphic nor plutonic effect was found in relation to the fractures