DEFORMATION OF THE UPPERMOST FAMATINIAN OROGEN: MAPPING AND STRUCTURAL ANALYSIS FROM THE SIERRA DE NARVáEZ AND SIERRA DE LAS PLANCHADAS, NW ARGENTINA

The ca. 495 to 450 Ma Famatinian hot, magma-rich, orogen (FO), exposed throughout central and NW Argentina, formed as a result of E-directed subduction and possible collision of the Precordillera Terrane under the Gondwanan margin. The uppermost parts of the FO are exposed in the S. de Famatina, Precordillera de Jagüé, S. de Toro Negro, S. de Narváez, and the S. de Las Planchadas. We present new mapping and structural analysis from the S. de Narváez and S. de Las Planchadas to better understand the tectonic setting and deformational history of the upper crustal levels in the FO.

The study area exposes Ordovician medium- to coarse-grained volcanoclastic rocks interbedded with finer-grained fossiliferous shallow marine sediments. This sequence is intruded by rhyolitic to basaltic Ordovician hypabyssal dikes, plugs, and plutons. Molasse sequences of Carboniferous, Permian, and Tertiary bedded units of red sandstones, shales, and conglomerates unconformably and tectonically overlie the Ordovician units.

Meter- to km-scale, open, upright, gently S-plunging folds with a pressure solution, axial-planar cleavage are discordantly intruded by Ordovician intrusive units indicating deformation synchronous with Ordovician magmatism; this is further supported by magmatic mullions along rhyolite plug margins with axes subparallel to Ordovician host rock fold axes. Folds with similar orientations and styles, but no cleavage, deform the Permo-Carboniferous unconformities and stratigraphic units indicating additional post-Permian folding, possibly related to Andean tectonics. Andean shortening is further expressed by N-S striking, E- and less commonly W-directed thrust faults throughout the area. Rare N-S striking normal faults occur but their timing relative to Andean structures remains uncertain.

The presence of shallow marine sedimentary structures and fossil assemblages indicate that the top of the FO was initially below sea level in the Ordovician. Both pluton emplacement and contractional deformation, although small relative to lower crustal levels of the FO, began to drive surface uplift. Unlike the Famatinian lower crust, where widespread ductile deformation and unusually wide shear zones are ubiquitous, shortening here is accommodated by low-T folding, pressure solution, and brittle slip.