The Skelton Group and the Ross Orogeny : late Neoproterozoic to early Ordovician evolution of South Victoria Land, Antarctica

Cook, Yvonne Anne, 1967-

South Victoria Land basement consists of Neoproterozoic metasediments and metavolcanic rocks which make up the Skelton Group. The Skelton Group was intruded by mafic and felsic plutons and dykes of the Granite Harbour Intrusives during the Ross Orogeny. Ages of plutonic rocks indicate that the Ross orogeny lasted from at least 550Ma (latest Neoproterozoic) to 477Ma (Ordovician) The metasediments of the Skelton Group are now metamorphosed and deformed but originally included intraformational conglomerates, greywacke, siltstone, mudstone, limestone and impure limestone. Coeval with sedimentation was intrusion and extrusion of basaltic plugs, sills and pillow lavas with P- and TMORB chemistry, and trachyte and rhyolite extrusives. These igneous rocks have an origin involving crustal contamination and indicate that the Skelton Group formed in a continental rift environment younger than 680Ma and probably younger than 630Ma. There has been variable deformation and metamorphism of the Skelton Group between lower green schist and upper amphibolite facies. Lithostratigraphic mapping of south Victoria Land is not possible due to extensive disruption and reconstitution of the Skelton Group during the Ross Orogeny. The Lower Baronick Formation (conglomerate and bimodal volcanics) is the only formation that is recognised and is sufficiently undeformed that bedding can be recognised. For mapping purposes, south Victoria Land is sub-divided into fault bounded sub-blocks (c.l-20km in size) each of which has had a relatively coherent geological history during the Ross Orogeny. These sub-blocks are grouped into one of four main structural blocks. Some of these four main blocks contain rocks with a wide range of structures and metamorphic grade. The southernmost block is the Skelton Block which covers 1400km2 and consists of 9 recognisable sub-blocks. Some of the sub-blocks, including Lower Baronick Formation, are relatively undeformed whereas others are totally reconstituted. The Skelton Block contains the least deformed rocks which yield the most information on the origin of the Skelton Group. The Koettlitz Block (290km2) contains two greenschist facies sub-blocks which are relatively unstrained, but very little is known about this block due to limited exposure. The Walcott Block (730km2) is made up of 5 sub-blocks. The sub-blocks include greenschist facies rocks preserving clastic textures, mid to upper amphibolite facies schists and migmatites and there are some high strain gradients between less deformed and more deformed rocks. The Walcott Block underwent extensional tectonics during the Ross Orogeny. The northernmost Dry Valleys Block (4600km2) consists of thoroughly transposed and reconstituted upper amphibolite facies schists and no sub-blocks have been recognised. Dry Valleys Block underwent extensional, compressional and strike slip tectonics during the Ross Orogeny. The boundaries between structural blocks appear to be steeply dipping NW striking faults. Only one of these faults, the Frio Shear Zone, is exposed and forms the boundary between Walcott Block and Dry Valleys Block. Frio Shear Zone has had a complex structural history of reactivation throughout the Ross Orogeny and the latest movement along which uplift of the Dry Valleys Block occurred is sinistralnormal. Some block boundaries are cut by Granite Harbour Intrusives whereas other block boundaries cut Granite Harbour Intrusives. The geological histories of the four main blocks indicate that compressional and extensional tectonics were probably coeval during the Ross Orogeny. The geological histories also reveal that uplift of the four structural blocks may have occurred at different periods during the Ross Orogeny resulting in a complex interrelation between plutonism and deformation of the Skelton Group. Structural blocks of the type mapped in south Victoria Land can be mapped in north Victoria Land and elsewhere in the Transantarctic Mountains. These blocks preserve results of Ross Orogenic processes at different times and places and at different crustal levels throughout the Transantarctic Mountains. A series of NW striking faults are recognised in north Victoria Land which are related to the same tectonic regime as those NW striking faults identified in south Victoria Land and indicate offset of the Ross Orogen in the Victoria L~nd segment of Antarctica. Neoproterozoic continental rifting in south Victoria Land is younger than that of Windermere rift related metasediments in North America and is not a direct temporal correlative has been suggested in association with the SWEAT hypothesis. Correlation with possible rift events near the Cambrian-Precambrian boundary in North America is also plausible.

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

1997Cook

Abstract Only

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