Abstract
Palaeomagnetic determinations for Jurassic volcanics and a Cretaceous quartz—dolerite sill at three localities around the arcuate Patagonia'n Cordillera show that a bending of 40° to 27° has occurred. Since the structural bend is 90° it is probable that the Mesozoic arc was originally curved. Cainozoic dykes and sills at Lago San Martin in the northwardtrending belt and Sierra Valdivieso in the eastward-trending belt have the same magnetic directions and were intruded after bending. These results essentially confirm the palaeomagnetic determinations of Dalziel et al., (1973a) and further suggest that the collapse of the marginal sea during the Late Cretaceous orogenesis resulted in greater bending in the outer arc than in the inner arc. The orocline must have formed during the Andean orogeny between 80 m.y. and 21 m.y. ago and was accompanied by folding and thrusting. New K—Ar dates for the Dientes de Navarino Sill (105 m.y.), basalt at Isla Carlos in the Straits of Magellan (21 m.y.) and diorite and andesite sills at Lago San Martin (14 m.y.) are also presented.
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