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Cordilleran Collision |
Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964, U.S.A
The Andean Cordillera is the result of interaction between oceanic lithosphere, forming the floor of the Pacific Ocean Basin, and S American lithosphere during the Mesozoic and Cenozoic. Break up of Gondwanaland was accompanied by widespread extensional tectonics in its S American segment during the Early to Mid-Mesozoic. The trend, dimensions and nature of the resulting basins were to a considerable extent controlled by the structure of the pre-Jurassic basement. Elongate basins formed along virtually the length (c. 7,500 km) of the Pacific margin from Northern Peru (5°S) to the Northern Scotia Ridge (56°S). These basins developed within, or immediately behind, a magmatic arc.
Mid-Cretaceous uplift and compression, contemporaneous with the global increase in seafloor spreading rates, took place along the length of this Pacific margin composite back-arc trough. Horizontal shortening accompanied the uplift, but only S of lat. 50°S did extensive penetrative fabrics develop in the basin called the Magellan Geosyncline by Aubouin (1965) because of its partially ophiolitic floor (the ‘Rocas Verdes’) and infilling of deep-marine volcaniclastic turbidites. This Mid-Cretaceous event may have been critical in initiating the process of Cordilleran orogenesis. Subsequently part or all of the Late Cretaceous batholiths were intruded into the uplifted back-arc trough as the locus of magmatic activity moved eastwards to its present location and dominantly E-verging fold-and-thrust belts developed along the eastern flank of the Cordillera.
Thus in the development of the present Andean Cordillera, S of 5°S, ‘collision’ was confined to the re-welding of a zone of failure between an ensialic magmatic arc and its parent continent. For the most part this involved little horizontal displacement and reflects, therefore, what might be described as ‘accordian tectonics’, a curtailed version of the Wilson cycle. Only in the very far S, in Tierra del Fuego and S Georgia, does crust appear to have been consumed in the arc-continent ‘collision’.
Institute for Geophysics, The University of Texas at Austin, 4920 North IH 35, Austin, Texas 78751, U.S.A.
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