IS THE NORTHWARD MIGRATION OF THE CHILE TRIPLE JUNCTION “CONTROLLING” THE TECTONIC EVOLUTION OF THE PATAGONIAN CORDILLERA?

One of the foremost tectonic features of the South American margin is the Chile Triple Junction (CTJ). Localized subduction erosion occurs at the CTJ site, where the Taitao Ophiolite and near trench plutons were emplaced at during the Pliocene. Extensive basaltic back arc plateaus erupted as result of the formation of transient asthenospheric windows as different CR segments were subducted when the CTJ migrated northwards during the Miocene and Pliocene. Another, hypothetical, effect of oblique CR subduction refers to the opening the Golfo de Penas Basin, considered as a transtensional basin formed at the trailing edge of a forearc sliver (“Chiloé Block”), supposedly moving northwards and bounded to the east by the Liquiñe-Ofqui Fault which extends along the edge of the Cordillera from 37º to 47ºS. However, as the sedimentary fill of the Golfo de Penas Basin includes thick Cretaceous, Eocene, and Miocene sedimentary sequences its formation undoubtedly predates CR subduction.

Other authors have suggested that, at a broader regional scale, the Neogene tectonic history of the Patagonian Cordillera itself can be closely tied to the northward passage of the CR, but a close examination of the striking geological differences between areas north and south of the CTJ shows that these were established long before the beginning of CR subduction in the Miocene. North of the CTJ, the Patagonian Cordillera resulted from the Late Miocene inversion of an essentially Oligocene highly attenuated, extensional basin filled by the quasi-oceanic lavas and mafic dyke swarms of the Traiguén Formation. South of the CTJ, were no evidences for Oligocene extension has been found, fission track data show that exhumation and uplift have occurred in eastwards progressing steps during the Late Oligocene, Mid Miocene and Late Miocene. Although the structures associated to each one of these pulses of orogenic front migration cannot be clearly identified from the available geological database, syntectonic molasse deposits covered large spans of extra-Andean southern Patagonia during the first two exhumation episodes. Even if CR subduction has produced undeniable effects in the forearc, the abrupt changes in the Neogene tectonic history north and south of the CTJ seems to better trace and follow the Paleozoic and Mesozoic events linked to Gondwana breakup.