Physical and chemical processes associated with deformation, South Victoria Land, Antarctica


Cook, Yvonne Anne, 1967-


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Baement rocks forming the subject of this report include a range of variably deformed granitoid mafic plutonic bodies and the Koettlitz Group into which they intrude. Deformation, and intrusion were part of the Cambrian-Ordovician Ross Orogen . . The Precam brian Koettlitz Group is the oldest group of rocks in The Dry Valleys area. The Koettlitz Group can be divided in to two northwest trending belts, one dominated by marble, the other lacking marble and containing considerable amounts of intercalated orthogneiss. Quartzofeldspathic rocks are present in minor amounts, the sequence being dominated by amphibole, diopside and biotite rich lithologies. The exact origin of this group is unknown although it appears to represent a highly altered meta-sedimentary and meta-basite sequence. These rocks were subjected to intense deformation and accompanying pluton intrusion which resulted in inhomogeneous distribution of strain and widespread chemical change on regional and local scales. Deformation was continuous during amphibolite facies metamorphism and is described here in terms of three 'generations', characterised in the Koettlitz Group primarily by folding. The first generation was pervasive and resulted in extensive transposition and destruction of all original structures. The second generation consists of locally developed, open to tight folds and associated structures deforming those of the first generation. The third generation consists of large scale warping.
High strain zones initiated during local and regional deformation preserve evidence for strain induced metamorphism, element mobility and chemical exchange between dissimilar lithologies on the scale of tens of metres. In these high strain zones, deformation of gabbroic material induced the metamorphic reaction hornblende -> biotite + quartz and promoted element exchange between meta-gabbro and the neighbouring Koettlitz Group and granitoids. On a regional scale, calc-silicates within the Koettlitz Group have illustrated the intimate association between deformation, metamorphism and metasomatism and the effects of rheological contrast on strain partitioning. Deformation and associated metamorphism and element mobility resulted in extensive chemical alteration and transposition of the original Koettlitz Group lithologies on a regional scale, producing a tectonostratigraphy bearing little resemblance to the original structure and chemistry. This widespread element mobility and metamorphism is suggested as an alternative explanation for the observed Koettlitz Group chemistry and mineralogy which is typically interpreted as representing isochemical metamorphism. On a local scale the structural response of the Koettlitz Group to deformation was governed to some extent by rock rheology with incompetent marble and schist horizons tending to flow to areas of lower strain.
In response to regional stress, an extensive NW-SE oriented shear zone was initiated and was active throughout deformation governing the location of syntectonic pluton intrusion. Orbicules within syntectonic plutons have indicated that partially molten magma transmits and responds to deviatoric stress by a variety of strain mechanisms including rigid body rotation, magmatic strain solution and fracture. Orbicules also indicate that the strain developed is dependant on the nature and extent of contiguity existing within the pluton and may be influenced by the regional stress rather than stresses arising solely from the inherent motion of the magma.

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207 p. : ill. (some col.), maps ; 30 cm.


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Cook, Yvonne Anne, 1967-, “Physical and chemical processes associated with deformation, South Victoria Land, Antarctica ,” Otago Geology Theses, accessed May 22, 2024,

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