Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctica - West Antarctica, Major Components

West Antarctica is suggested by crustal structure, morphology and geophysical data to be a continental mosaic that is composed of 4 crustal blocks, that are discrete or semi-discrete - Marie Byrd Land, Ellsworth-Whitmore Mountains, the Antarctic Peninsula and Thurston Island blocks. According to some, the Haag Nunataks represent a 5th crustal block (Storey et al.,1988; Johnson & Smith, 1992), though the exact boundaries and relative behaviour of these blocks are not well known. It has been proposed that these blocks, excepting Marie Byrd Land, should be grouped into a single block to be called Weddellia, based on palaeomagnetic analyses (Dalziel & Pankhurst, 1987; Grunow, Kent & Dalziel, 1987b).

It was proposed that the zone between some of the microcontinental fragments of West Antarctica had undergone extensional tectonism (Davey, 198; Jankowski & Drewry, 1981; Dalziel & Elliot, 1982; Jankowski, Drewry & Behrendt, 1983; Schmidt & Rowley, 1986; Garrett, Herod & Mantripp, 1987). In the Marie Byrd Land sector, West Antarctica, evidence has been found that includes block faulting and an abundance of alkaline volcanic rocks (Jankowski & Drewry, 1981; Le Masurier & Rex, 1983). It is also indicated by the data that the blocks of West Antarctica may have moved relative to each other as well as relative to the East Antarctic Craton after the breakup of Gondwana (Elliot, 1975a; de Wit, 1977; Jankowski & Drewry, 1981; Dalziel & Elliot, 1982; de Wit et al., 1988; Grunow et al., 1992). Some researchers believe the blocks can be restored to their original position within Gondwana without the need for displacement between them, or at least not much displacement (Storey et al., 1988).

A special problem in the geology of Antarctica is represented by the Ellsworth-Whitmore Block (Schopf, 1969; Elliot, 1975a; Clarkson & Brook, 1977; Dalziel & Elliot, 1982, Craddock, 1983). A large rugged highland region, which has relief of more than 2 km, connects the Whitmore Mountains to the Ellsworth Mountains. There appears to be a trough between the high-standing massif on which the Whitmore Mountains are situated and the Ellsworth Mountains. Whether the Whitmore Mountains are structurally separated by the trough from the Ellsworth Mountains is not known. The Whitmore Mountains are suggested by radiometric dating and overall stratigraphic relationships to be geologically related to the Ellsworth Mountains (Webers et al., 1982), which leads to the Whitmore Mountains and the Ellsworth Mountains to be considered to be a single block.

Controversy remains regarding the original position and direction of displacement of the Ellsworth-Whitmore Mountains block. Throughout most of the Transarctic Mountains obvious ties to sequences of Gondwana margins have been found, though the structural grain of the Ellsworth Mountains strikes almost transverse to the structural grain of the TAM and the Cape Fold Belt of Africa, which before the breakup of Gondwana were contiguous. Additionally, evidence of only 1 episode of deformation affects the entire stratigraphic sequence, though in the Pensacola Mountains 3 episodes of deformation are recorded. According to the author1 this has led many researchers to believe the Ellsworth-Whitmore Mountains block is allochthonous and has undergone significant displacement relative to East Antarctica as the breakup and dispersal of Gondwana was under way (Schopf, 1969; Dalziel & Elliot, 1982). It has been argued that strike-slip faulting can account for the 90o counterclockwise rotation of the Ellsworth-Whitmore Mountains block away from East Antarctica.

The Ellsworth-Whitmore Mountains block is separated from the Antarctic Peninsula block, to the northwest, by a series of bedrock highs that are bounded by steep escarpments (Doake, Crabtree and Dalziel, 1983; Drewry, 1983). It is strongly suggested by a coupling of magnetic and gravity models and apparent horst-and-graben morphology, that northwest to southeast extension has occurred between the Ellsworth-Whitmore Mountains block and the Antarctic Peninsular block (Doake, Crabtree and Dalziel, 1983; Garrett, Herod & Mantripp, 1987). It has been suggested that the uplift of the Ellsworth-Whitmore Mountains block is related to this rifting  (Garrett, Herod & Mantripp, 1987). It remains uncertain when the extension between the Antarctic Peninsula block and the Ellsworth-Whitmore Mountains block occurred, though in this region the abundance of alkali basalts from the Cainozoic (Garrett & Storey, 1987), as well as the high relief and rugged topography, support the movement occurring in the Cainozoic.

An extension of the Byrd Subglacial Basin separates the Marie Byrd Land block from the Ellsworth-Whitmore Mountains block, and by a deep , narrow trench from the Thurston Island block that is occupied by the Pine Island Glacier. It appears the Thurston Island block is separated from the Antarctic Peninsula block by a similar trench. The inaccessibility of the Thurston Island block and its poor exposure results in little being known about this block. Evidence has been found that suggests the Thurston Island block is part of a magmatic arc from Gondwana, similar to the Antarctic Peninsula  block (Dalziel & Elliot, 1982; Rowley et al., 1983).

The author1 suggests the Thurston Island block probably originated close to the western end of the Antarctic Peninsula block, a location similar to its present position, though it is indicated by the sub-ice morphology there is a significant structural break.

Sources & Further reading

  1. Anderson, John B., 1999, Antarctic Marine Geology, Cambridge University Press
 
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 29/07/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading