STRAIN ACCOMMODATION AT DIFFERENT CRUSTAL LEVELS DURING THE DEVELOPMENT OF THE FAMATINIAN MAGMATIC ARC, ARGENTINA

Tectonic regimes in active magmatic arcs vary between contractional to neutral to extensional with variations in intensity over time and with depth. These changing regimes may influence magma transfer, crustal rheology, mountain building, crustal growth and the seismic structure of the crust. We explore the change in mechanisms and intensity of deformation from ~ 20 km depths to the surface of the Famatinian arc (magmatic activity ~485-465 Ma) in NW Argentina by studying three areas representative of different crustal levels: the upper crustal Sierra de Narváez, the mid-crustal Sierra de Velasco and the upper- to deep-crustal Sierra de Famatinia. Large-scale folding and brittle faulting of volcanic and sedimentary units characterize deformation at the top of the arc. Upper to mid-crustal (~ 3 kbar) levels show steep, regional NW trending magmatic foliation and bedding in host rocks, while at the deepest exposed levels (~ 5 kbar), local, emplacement-related, relatively flat, magmatic foliations are reoriented along with metasedimentary bedding to a steep, NE trending orientation according to regional contraction with min. of ~30 % shortening in plutons and ~50% shortening in metasediments. Upon crystallization of magmatic rocks, strain localizes in smaller solid-state zones superimposed on magmatic fabrics showing the same orientation and steep lineation but no significant displacement. Large, post-magmatic, NNW-SSE striking ductile shear zones cross cut arc rocks, late in the Famatinian orogeny and cause vertical thickening by westward thrusting.

As a first estimate to determine vertical thickening, we assumed plane strain and 32 % bulk shortening for plutonic and metasedimentary rocks (ratio of 9/1) based on observations in the west-central Sierra de Famatinia. A 20 km thick and 250 km wide upper to middle Famatinian arc crust would experience a decrease in width to ~ 170 km and an increase in thickness to ~26.5 km during arc activity. Observations in the upper crust show a shallow marine environment during arc volcanism thus either growth of an arc root was the dominant thickening process or marine sediments are older and were uplifted to higher elevations. More fieldwork, detrital zircon dating and comparison with isostatic mass balance modeling (abstract # 274293) will constrain strain accommodation further.