DEVONIAN TECTONIC HISTORY OF THE NORTHWESTERNMOST PRECORDILLERA TERRANE (NW ARGENTINA)
Two adjacent areas in the northernmost Precordillera west of Vinchina contain rocks that were deformed during the closure of the ocean between these two terranes. The Rio Bonete area on the north has a NNW-trending basement complex of foliated mylonites, marbles and amphibolites. A mylonite sample provided a primary zircon U-Pb age of 1118 +/- 17 Ma. The platform rocks are represented by the low-grade marbles and metasandstones. The Late Ordovician consists of 1 km of low-grade black shales and metaturbidites below ~3.5 km of basalts faulted onto the basement. Two major faults (with younger on older relationships) are present. We interpret these as thrusts that facilitated obduction of the upper several kilometers of oceanic crust onto the western passive margin of the Precordillera. Shortening direction, based on the orientation of structural elements, was SW-NE consistent with subduction to the east.
The Sierras de Las Minitas, 30 km to the SW, has a shelf sequence deformed and locally metamorphosed fossil-rich alternating sandstones and shales indicating a Lower Devonian age. Overlying polymictic conglomerates contain clasts of Ordovician basalt and Grenville basement lithologies indicating that the sedimentary succession was part of the upper-plate margin of the Precordillera after the obduction of Ordovician onto the basement. These rocks underwent two periods of folding. Removal of the later (Permian) folding indicates that the pattern of Devonian deformation was a series of tight to isoclinal, NW-SE-trending, SW-verging folds, consistent with the SW-NE shortening direction inferred for Ordovician rocks in the Rio Bonete area. The folds are intruded by dikes of Lower Mississippian age indicating that folding was a Middle to Late Devonian event. This is consistent with the Middle Devonian age (ca. 390 Ma) given by several authors for the collision of the Chilenia terrane.