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Cover image for Crustal Shortening in the Australasian Plate, IGC paper
Crustal Shortening in the Australasian Plate, IGC paper
Title:
Crustal Shortening in the Australasian Plate, IGC paper
Author:
LAING, A. C. M
Personal Author:
Publication Information:
Melbourne: Mines Department, Victoria, 1976
Physical Description:
16p. : 2 illus., 1 tables, 1 document.
Series:
Geological Survey of Victoria Unpublished Report 1976/52
General Note:
(Reference Abstract -- Abstract ) In an earlier paper the writer showed that the regional dip of sediments from Proterozoic to Mesozoic age increases regularly in the Australasian plate eastwards towards the Pacific margin; and that the distribution of the Paleozoic basins show a pattern of growth of the Australian continent by accretion round the West Australian Shield. In this paper further analysis is made of this tectonic pattern. In Victorian Ordovician rocks outcrop nearly continuously ( except for overlying Devonian rocks and intruding post Ordovician granites) along the Great Dividing Range separating the Murrray Basin in the north form the Otway and Gippsland Basins in the south. These Ordovician rocks are folded in a series of fairly regular anticlines and synclines with an average dip on the limbs of 60 degrees. This type of folding is best seen in Central Victoria either outcropping as at Castlemaine or in the Bendigo Mines in which anticlinal axes were traced to depths of nearly 1000 metres to mine the crestal saddle quartz reefs for gold. However, similar somewhat steeper folding is seen in the Ordovician in Eastern Victoria. In a number of places the Ordovician sediments are overlain unconformably by relict Upper Devonian Lower Carboniferous synclines which have an average flank dip of about 15 degrees. The Ordovician is intruded by a number of post Ordovician granites. In order to calculate the crustal shortening post Lower Carboniferous it is assumed folding took place to right across Victoria to the same degree as in the relict Upper Devonian Lowere Carboniferous synclines. Therefore tha amount of crustal shortening along an east west line through Victoria from Stawell to Cape Howe is the present distance 650 km x sec 15 minus 650 km equals 26 km. The amount of crustal shortening since Lowere Devonian times excluding thrust faulting is 650 km - 120 km (width of granite intrusion) times sec 60 minus 650 km equals 410 km. In New Zealand the Ordovician is dipping vertically but even if we assume that the degree of post Lower Devonian folding under the Tasman sea is only similar to that across Victoria the crustal shortening is 1670 km (sistance Cape Howe to New Zealand) sec 60 minus 1670 km equals 1670 km. In New Zealand the Devonian is also steeply dipping and assuming an average dip of 40 degrees under the Tasman sea there is 500 km of crustal shortening post Lower Carboniferous. In Australia west of a line from Portland north to Mt Isa both the post Lower Devonian and post Lower Carboniferous crustal shortening is negligible, beds of these ages beinf only gently folded. For some reason the zone of extreme crustal shortening has successively become more localised closer to the Pacific margin and since the Mesozoic has been in the Circum pacific belt through New Zealsand. Ir is concluded that the expansion of the Pacific Basin has been the major factor is folding of rocks in Australasia. It is inferred that a similar reduction in tectonic intensity may be present elsewhere in the world outward from the Pacific margin. It is also inferred that a similar reduction in tectonic intensity may take place south of the Swiss Alps-Himalayan mountain belt. It is possible that the expansion of the Pacific Basin may be caused by a localised rise in the liquid core causing a basinal expansion and also causing a deepening of the Pacific to compensate for the rise in high density core material. It is noted that the circum pacific seismic belt is now 700 km deep and it is inferred that planets with liquid cores are unstable and that the Earth and Mars (obviously the canals are faults) will eventually break up into fragments like the probable former planet now forming the asteroids.