Australia - Plates and Lineaments

Australia: The Land Where Time Began

Australia - Plates and Lineaments

The Australian continent was part of the super-supercontinent Rodinia for about 1 Gy. Following the breakup of Rodinia the drifting apart of its constituent parts the crustal blocks came together again to form the Supercontinent Gondwana, comprised of the continents of the Southern Hemisphere, of which the crustal blocks making up the Australian continent came together and attached to Gondwana. Gondwana joined with Laurasia, the Northern Hemisphere continents, to form Pangaea that existed between about 300-200 Ma. According to plate tectonic theory (Holmes, 1931; Hess, 1962; Vine & Mathews, 1963; Morley & LaRochelle, 1964;) but also (Meyerhoff & Meyerhoff, 1974) and other chapters in Kahle, 1974; also Larin, 1993) about 200 Ma Pangaea and Gondwana began fragmenting. As the component continental crustal plates drifted apart the present day continents of South America, Africa, apart from the younger Atlas Mountains, Antarctica and Peninsula India each moved apart from each other, as they continue to do to the present. When Australia separated from Antarctica it began it move towards the equator, the movement continuing at the present at a rate of 65-70 mm/year, in a direction that is slightly east of north (e.g. Ludbrook, 1980, p. 91; Parker, 1993), and eventually will collide with the Indonesian Archipelago. These lateral migrations are the most recent of a series of movements which profoundly affected the structure of all the continents, including Australia, as they introduced stresses and strains to the crustal rocks and blocks that are most obviously manifested in fracture patterns.

In landscape analysis structure is impotent everywhere. Along the coasts of Australia many sectors are straight or linear, which also applies to rivers and other large-scale topographic features. They are coincident with, and are expressions of, lineaments, which are long, straight or gently arcuate structures that are deep-seated (Driscoll & Campbell, 1997). Many of these lineaments are fault zones (O'Leary et al., 1976), while others are linear zones of strain, that are deep-seated, and are susceptible to weathering (e.g. Russell, 1935; Turner & Verhoogen, 1960, p. 476; Nabarro, 1967, p. 4). Some of these are of great antiquity, though varying in age, having formed as soon as the crust of the Earth had cooled, consolidated and become brittle (see Skobelin, 1992). They resulted from crustal shearing or torsion (e.g. Kalb, 1990) that is related to the migration of plates. They have been, and continue to be active recurrently. They are part of a global network (e.g. Vening Meinesz, 1947) and in Australia (Hills, 1946, 1956), and as elsewhere, they are roughly aligned NE-SW and NW-SE. Also, latitudinal trends, that are associated with conjugate or genetically and geometrically, shears in the basement rock, criss-crossing the landmasses.

For at least the last 90 My the Australian continent has been moving roughly northward and, as it is composed of blocks of different compositions that have different strengths, the jostling resulting from the lateral movement has caused and is still causing shearing, or lateral deformation that results from torsion, or excess transverse stress. This results in many orthogonal fracture patterns that are seen in outcropping brittle rocks (Vening Meinesz, 1947; Kalb, 1990; O'Driscoll, 1986). The continued and continuing migration of blocks as well as distortion within them is attested to by the many earthquakes and tremors recorded, and in some cases felt (e.g. Gordon & Lewis, 1980; McCue, 1990; Bowman, 1992; Greenhalgh et al., 1994; Twidale & Bourne, 2000a; Bourne & Twidale, 2005). The scarps and zones of weakness that result are classified as of tectonic origin, i.e., resulting from deep-seated crustal movements. Neotectonic features are those that occurred after the Miocene.

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