Abstract

Paleontologic and stratigraphic evidence points to the early Paleozoic Precordilleran terrane of western Argentina as being the conjugate rift pair of the Appalachians. Stratigraphic similarities of the Cambrian and early Arenig carbonate series and very strong affinities among trilobite, conodont, and brachiopod faunas show their close relationship. The most probable provenance areas are the Appalachian-Ouachita rifted margin and the Newfoundland Appalachians, although the former fits better with geometric and drifting paths suggested by faunal affinities. Increasing Celtic and Baltic brachiopod genera and divergent stratigraphy since the Arenig indicate the drifting of the Precordilleran terrane. Collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed. The collision of the Precordillera with western Gondwana occurred during the mid-Llanvirn to Llandeilo. A magmatic arc related to eastward subduction (present coordinates) was active in the Famatina Range east of the Precordillera. This region of Celtic affinity shows faunal exchange with the Precordilleran terrane since the late Arenig and may represent accreted intra-Iapetus volcanic island-arc complexes. The rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the Precordilleran terrane from periequatorial to peripolar latitudes between the Cambrian and latest Ordovician.

The deep ocean to the west of the Precordillera started to close by the Late Ordovician with the eastward drift of the Chilenian terrane. Absence of volcanic or pyroclastic arc-derived rocks in the Precordillera indicate west-dipping subduction. As Chilenia approached the continental margin, a new forebulge was established on the former collided Precordilleran terrane, developing an erosional unconformity in central Precordillera (Talacasto-Tambolar arch).

A Gondwanic signature was fully developed by the Middle Silurian when the Malvinokaffric Clarkeia Fauna flourished. Before then, the Late Ordovician glacial record and associated Hirnantia Fauna were the first clear tie to Gondwana. During the Silurian the marginal basin behaved as a foreland, with lithosphere rheology and eustasy governing the sequence stratigraphy. Wrench faulting along its eastern boundary displaced the Precordillera toward the south. Continued shortening during closure with the Chilenian terrane in the mid-Devonian produced thrust loading of the basin and generated a thick graywacke succession. Final accretion of Chilenia (Late Devonian) generated a regional angular unconformity between the lower and upper Paleozoic. New eastward subduction was initiated west of the accreted Chilenian terrane during the Late Permian–Triassic as indicated by the Choiyoi volcanic complex, which presently outcrops in the Frontal Cordillera.

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