Contrasts in tectonic evolution of orogenic belts in the south-east pacific

Abstract

Abstract The orogens of southern South America and Antarctic Peninsula, which are of similar age and apparently linked by the Scotia Arc, are shown to have evolved along markedly different patterns of geosynclinal and orogenic history. The Andes of Patagonia and Tierra del Fuego originated from a Late Mesozoic mio-eugeosynclinal couple and volcanic island arc created by subsidence, extension and rifting. The trend of this belt is unrelated to, and across the previously established Gondwanide framework. During the orogenic stage this marginal sea along the borders of an old continent, where apparently new oceanic crust had been formed, was welded to the continental block; the Andes thus constitute an accretionary belt. Orogenic polarity follows geosynclinal polarity; both geosynclinal and orogenic evolution are characterised by progressive outward migration towards the foreland, incorporating successive flysch and molasse basins in the fold belt. Alpino-type tectogenesis is accompanied by high-grade metamorphism and migmatisation. Recent calc-alkaline andesitic-dacitic volcanism in the Andes, their taphrogenic break-down along the Pacific and association with a deep-sea trench indicate a continuing orogenic regime. In the Antarctic Peninsula, no Late Mesozoic geosynclinal orogensis is recognised. I ts history is dominated by Gondwanide post-orogenic consolidation. Subsequent volcanism and localised, partly fault-bounded basins filled with terrestrial to shallowmarine sediments of limited thickness are the characteristic features of JurassicCretaceous time. Mid-Cretaceous large-scale intrusion of granodioritic magmas was accompanied and followed by regional unlift. This orogensis is superimposed upon, and parallel to the Gondwanide orogenic belt. Deformation is germanotype, except at the base of the Peninsula where tight folding occurred prior to the intrusions. Orogenic molasse sequences deposited in external foredeeps remained little disturbed tectonically. Cenozoic rifting and basaltic volcanism of subalkaline composition indicate that no orogenic regime is affecting this region. The Andean orogen of southernmost South America thus is a continental margin feature where accretion occurred, and which is unrelated to Gondwanide tectonics, whereas the Antarctic Peninsula is an intracontinental orogen of essentially Gondwanide inheritance. The contrasting tectonic setting and framework, sedimentary and orogenic history and structure of the two orogenic belts indicate that they have not been an originally continuous and straight mountain belt along the Pacific margin, prior to bending and disruption in the Scotia Arc region. The term “ Antarctandes” therefore is misleading and should not be used. The Scotia Arc appears as a composite feature of much greater complexity, which probably originated by juxtaposition of tectonic elements of entirely different origin and character, and with partial bending only.