Desarrollo de la Antártica

EL DESARROLLO DE LA ANTÁRTICA 3 X 10 9 years according to the radio-isotope measurements. On the other hand, West Antarctica comprises mostly Cenozoic volcanics, Mesozoic geosync1ines and \Late iP'aleozoic orogenic zones. The Antarc– tic geology is very much characterized by the Transantarctic mountain range, wthich is the ioss orogenic zone, forming the the west boundary zone of East Antarctica. Geologic dharacteristics of these three ,Ceo 10- gical formations will be briefly described in the followings: i. Precambrian Shield (platform) of lEast Antarctica Tihe ,Precambrian metamorphic rocks of íEast Antarctic shield comprise mostly gneisses and charnokites, which can be identified to the product of volcanic breccÍas by the regional metamorphism and the ultra-metamorphism in Precambrian. A number of geological evidence have been found for indicating that the East Antarctic shield was subjected to frequent regional metamorphísms during the Precam– brian. These metamorphisms can be c1assified into three stages sudh as: l. Granulite facies - Archean era, 2. Amp:hibolite facies - Old lProterozoic era, 3. Green srust facies - Middle Proterozoice era. The radio-isotope ages of these metarmorphic rooks should therefore represent t::he ages of their respective last metamorphisms. ii. Transantarctic Mountains - Ross orogenic zone Transantarctic mountains are formed with large complicated geosyncIine systems of Late Proterozoic --. Old Paleozoico The oldest metamorphic rock in this zone :has age of about 1 x 10 9 ¡years. The geological íhistory of this zone may be characterized by Ross orogeny which took place in Ordovician period ([450 ,...., 500] x !CYl years ago) to finally form the present mountain range after repeated complex geological activities of alternate sedimentation, volcanism, metamorphism and intrusions of granites. The mountain range was subjected rather to the abrasion during Silurian period, and the characteristic Beacon sandstone sedimentation was extensively con– tinued from Devonian to jurassic, resulting in the present Beacon supergroup of about 2,500 m in thickness as illustrated in Fig. 8. Tthe dolerite intrusions in Beacon supergroup are petrologically very similar to dolerites in Tasmania and in South Africa. Further fossil flora, fossil fauna and tillites (glacial deposits) discovered from 96