TIMING OF OBLIQUE MOTION OF THE LA PUNTILLA – LA FALDA SHEAR ZONE IN NORTHWEST ARGENTINA FROM MONAZITE U-PB GEOCHRONOLOGY

The Valle Fertil fault zone in the Western Sierras Pampeanas of northwest Argentina separates the continental Famatina arc from outboard accreted terranes. The previously identified La Puntilla - La Falda shear zone occurs west of the northern continuation of the Valle Fertil fault zone within the Sierra de Umango. The Sierra de Umango is bounded by the allochthonous Precordillera terrane to the west and crystalline rocks of the Sierras de Maz and Espinal to the east. The Tambillo unit within the Sierra de Umango preserves metapelitic Kfs-Grt-Bt-Sil-Hc migmatites, intermediate-felsic orthogneiss, Grt-amphibolite, and marble and metamorphosed calc-silicate. U-Pb ages from monazite inclusions in garnet and patchy zoned grains in the matrix of migmatitic melansomes date peak metamorphism at 461-453 Ma.

The Tambillo unit is deformed along its western boundary by the La Puntilla – La Falda shear zone, a 4 km wide, NNE-striking and moderate to steeply E-dipping mylonite zone. Previous workers suggested that moderately plunging mineral lineations and kinematic indicators in outcrop and thin section record dominantly dextral, east-side-up displacement across the shear zone. Monazite U-Pb ages from rims on Ordovician monazite cores and unzoned monazite grains within the mylonitic foliation date the timing of deformation 440-430 Ma. Isoclinal folding of the shear zone fabric post-dated 430 Ma and several young monazite ages between 418-390 Ma likely record later deformation.

The timing and type of displacement on the La Puntilla – La Falda shear zone is very similar to amphibolite-granulite facies shear zones within the margin of the Famatina arc and within the Western Sierras Pampeanas that post-date the timing of terrane accretion. Subsequent Devonian shear zones in the region have a sinistral-oblique history. This series of mylonite zones accommodated highly-oblique convergence along the Famatina margin from the Silurian into the Devonian.