Australia: The Land Where Time Began

A biography of the Australian continent 

Australia - Morphotectonics

Lineaments are still expressed in the landscape of the present, in spite of their great antiquity. Morphotectonics is the study of the relationship between structure and the surface at the regional of 'mega' scale (Hills, 1961). The alignment of several major coastal sectors, such as the west coast of the Eyre Peninsula, and the Kimberley Block, the Bonaparte Gulf (Elliott, 1994) and the Gulf of Carpentaria, coincide with major crustal fractures, as are also the linear western margins of 'lakes', that are actually salinas, Torrens and Eyre, and the outlines of the Murray, as well as other structural or framed basins.

The courses and sectors of several other major rivers are also remarkably straight, the Darling River being a prime example, but there are also remarkably linear sectors of the Georgina, Diamantina, Lachlan, Thompson Rivers and Cooper Creek. The rivers in all these instances flow through alluvial sediments that are mostly young and are not brittle and lack regular systems and sets of fractures. Underprinting, or the imposition from below of fractures in the underlying basement onto the overlying sediments, has been suggested as the cause of these strait rivers (Hills, 1961; Firman, 1974; Twidale, 2006). The author¹ suggests this may be accounted for by the joggling of deep faults. An alternative suggestion is that the flow of groundwater may be attracted, leading to weathering and subsidence (e.g. Twidale and Bourne, 2000b). It is clear that whatever the mechanism, ancient structures still find morphological expression, even in terrains that are geologically youthful, at local and site scales, as well as regionally (Kalb, 1990). Certain upstream patterns (Saul, 1976; Woodall, 1994; O'Driscoll & Campbell, 1997) may also be accounted for by underprinting. A suggestion has been that they are linked to meteorite craters in the primordial crust, considered by some to be what is now termed the Mohorovicic discontinuity (Skobelin, 1992; Twidale, 2006).

Many salient features of the Australian landscape are explained by lineaments, though they also pose a problem, as they developed 1 Ga, at least, at a time when was still part of Rodinia. The difficulty becomes apparent when it is realised that in all continents of the present the lineaments form part of a global network. The apparent implication of this is that they have either retained their original orientations or have rotated through 90^o, or possibly multiples of this angle during the process of lateral migration, which according to the author¹ is not likely. He suggests it may be possible that this pattern is more apparent than real, with what are random trends being subconsciously fitted into a preconceived pattern, but adds that this is also unlikely, as the patterns reported are statistically real.

Sources & Further reading

1. Twidale, C.R., 2007, Ancient Australian Landscapes, Rosenberg Publishing Pty. Ltd. , NSW