COMPLEX STRUCTURAL ARCHITECTURE OF A LONG-LIVED FAMATINIAN HT-LP CONTINENTAL BACK-ARC, SIERRA DE QUILMES, ARGENTINA

WEINBERG, R.; FARIAS, P.; BECCHIO, R. A.; SOLA, A. M.

Tucumán

Congreso; XX CONGRESO GEOLÓGICO ARGENTINO; 2017

Orogens grow by accretion and burial of material. During orogenesis, rocks move through evolving thermal and structural fields within the orogen. In most cases the models of evolution of orogens are based on numerical and analogue modelling, which are based on first order variables, and limited by certain boundary conditions. In the Sierra de Quilmes, part of the Sierras Pampeanas in NW Argentina, excellent exposures of mid-crustal levels of the back-arc of the ~500-440 Ma Famatinian orogen, making it a perfect natural laboratory to investigate the structural evolution of the back-arc of an orogen. Our field observations suggest that two terranes of contrasting HT-LP metamorphic histories, typically with cordierite-bearing migmatites after meta-turbidtes, were juxtaposed by thrusting on an unusually thick shear zone during the contractional phase of the Famatinian orogenesis. The strain history of the two metamorphic terranes is also different. While the hanging wall records transport to the west through intense thrusting towards the magmatic arc, in the footwall, kinematic indicators suggest transport to the south, parallel to the arc during a strongly constrictional deformation event, only faintly recorded in the hanging wall. Moreover, preliminary monazite and zircon geochronology results suggest that W-thrusting and anatexis in both terranes lasted the 70 m.yr. of arc activity between 510 and 440 Ma, and the possible reactivation of the footwall at 400Ma. Results suggest that the east-west plate convergence was resolved first as shortening perpendicular to the arc (orogen-normal) with top-to-west thrusting. This event evolved into an orogen-parallel stretching or lateral escape, where coaxial, horizontal constrictional tectonics prevailed in the footwall. Constriction in the footwall allowed maintaining the shortening regime in the convergent margin without increasing crustal thickening or topographic elevation. Further work aims to further detail the heterogeneous flow in this orogenic belt.