A LARGE, SUBDUCTION GENERATED SYNTAXIS IN THE GONDWANIDES OF SOUTHERN GONDWANA-LAND?

A syntaxis is a megascopic structure formed by a bundle of elongated structures (such as folds, thrust faults), in which the bundle is pinched in the middle to create the image of a bow-tie shaped map-pattern. Eduard Suess introduced it in tle last century (as Schaarung in German) to describe the map patterns of the sinuous mountain ranges and interpreted it as resulting from greater shortening in the pinched centre than along the out-fanning flanks.

The Gondwanides show a pattern of deformation that has been compared with the US Rockies in the Cape Foldbelt of Southern Africa. This pattern continues into South America, but loses intensity in the buried basins and uplifts of Argentinian hinterland (mainly in the Claromecó and the Chaco-Paraná flexural hinterland basins). West and southwest of these basins Palaeozoic orogeny marched consistently away from the continental interior onto newly accumulted subduction-accretion material of early to medial Palaeozoic age. This outward march can be followed by plotting distinct arc magmatic fronts from the early to the latest Palaeozoic-earliest Triassic. When the magmatic fronts are plotted they display a pattern that can most clearly be interpreted as their disruption by a series of at least two, cross-cutting generations of right-lateral strike-slip faults of latest Palaeozoic to earliest Mesozoic age displacing parts of the Palaeozoic subduction-accretion material northwestward with respect to the continental interior. As late Palaeozoic strike-slip disruption sub-parallel with the orogenic trend is also suspected in the Tasmanides of eastern Australia, we interpret the latest Palaeozoic to earliest Mesozoic tectonics of the Gondwanides in terms of the generation of a syntaxis with its pinched centre in South Africa that resulted from the shortening of a promontory of Gondwana-Land on a flattened subduction zone. Material escaping from this zone of shortening is thought to have created the large strike slip-systems in South America and Australia. This is a working hypothesis that brings a considerable degree of order into the otherwise hard-to-understand tectonic trend-lines of southern Gondwana-Land in the latest Palaeozoic and earliest Mesozoic interval and makes testable predictions concerning the structure of a vast and poorly-understood region.