ON RECURRENT RIFTING AND SYNPLUTONIC HEALING OF THE CHILEAN-ARGENTINIAN ANDEAN BACKBONE

Several rifting events are recognized in the southern half of South America starting in the Cambrian, when small plates were either accreted (much of Patagonia) or drifted towards Laurasia (the missing Cuyania).

During the Permian and up to the Late Jurassic NNW-SSE oriented rifts developed sub-parallel to the sutures of the Paleozoic accretionary and magmatic complexes. The biggest of these volcanic-poor depocenters, the Jurassic to Early Cretaceous Neuquén basin, has, however, a composite origin and its southern margin is E-W rifted against Paleozoic Patagonia. The Tarapacá Basin is the NNW extension of the volcanic-poor Aconcagua-Neuquén basins. Extensive volcanism occurs only in the Late Jurassic, as recorded in the Chilean Coastal Range north of 27^os.l., made up mainly of bimodal volcanics built on top of a late Paleozoic accretionary complex and emplaced in a hybrid retreating subduction boundary - “marginal rift” setting. Inbord of this La Negra Formation a continental rift developed during the Late Cretaceous.

Intra-arc, also roll-back related, mostly pyroclastic-infilled fault-bounded basins were developed from 33^o to 39 ^o s.l.) during the late Eocene to earliest Miocene. They strand the western margin of the Neuquén Basin and underlie thick piles of Miocene volcanics extruded during their inversion, the Farellones Formation.

It is suggested that the master faults controlling deposition of the over 10 km thick late Jurassic northern volcanics, though related to a steep slab-pull environment, had the same NNW trend recognized southwards and were later sealed by syntectonic to late arc-tectonic plutons. This healing event prevented inversion during the Late Cretaceous to Miocene compressional events. The bounding normal faults of the northern Late Cretaceous and the southern mainly Oligocene basins, deprived of such plutonism, underwent tectonic inversion, proportional to both Miocene crustal thickening and related amount of calk-alkaline volcanism.

Tectonic inversion hindering related to plutonism emplaced along boundary faults is a mechanism that should be considered when analyzing along strike changes in the tectonic evolution of an active margin orogen. At least not all Andean vertebral spinal disks seem to age equally.