EVIDENCE OF A TRANSFORM TYPE MARGIN DURING THE PALEOGENE FROM 38° S TO 44°S LATITUDE IN THE SOUTHERN ANDES

In contrast to its Cretaceous and Neogene location at the continental margin, magmatic activity in the Paleogene of Patagonia becomes unusually voluminous and widespread at the Somon Cura Massif. A large plateau of rhyolitic ignimbrite and alkaline basalts is developed at the southwestern margin of the massif in Paleocene-Eocene times along an old NW-SE transcurrent fault system. In the Oligocene, this activity is manifested as a subalkaline andesite-rhyolite- alkaline basalt belt to the west and an alkaline basalt plateau to the east.

The motion of the Farallon plate relative to South America for Paleogene times was roughly N10ºE, with a southward migrating ocean plate. Previous Paleogene reconstruction considers that the Antarctica plate is fixed and that Patagonia was a solid block. But Patagonia is deeply cross-cut by the NW-SE transcurrent Gastre fault system. Therefore, the southern motion of the Farallon plate at a very low angle would have caused the old Gastre fault system to become extensional. This evidences suggests that the subalkaline-alkaline magmatic association far away from the plate boundary is more likely an intraplate type related to extension, and that the plate margin became a transform type margin. The Liquiñe-Ofqui fault along southern Chile played the role of the transform fault that allowed plates to slide by each other, during which time several rifts developed within the continental plate. Present day plates configurations show a similar situation between the Pacific plate and the North American plate that slide by each other along the San Andreas transform fault, or even earlier in time with the development of the ‘ancestral' San Andreas fault and the Mojave Desert Province within the continental plate. If this is so, then the transform margin between the Pacific plate and North American plate would be at an early stage of what happened at Patagonia during paleogene times.