South American Global Geoscience Transect 7: Cordillera Frontal to LlanuraChaco Pampeana, northern Argentina

The South American Global Geoscience Transect 7 is located in northern Arge ntina and traverses, from west to east, the Cordillera Frontal, Puna, Sierr as de Famatina, Sierras Pampeanas, and Llanura Chaco Pampeana. Cenozoic str atovolcanoes that intrude Lower Paleozoic slates and Upper Paleozoic contin ental sedimentary rocks constitute the Cordillera Frontal. Upper Precambria n to lower Paleozoic metamorphic and sedimentary rocks intruded by Paleozoi c granitoids comprise the Sierras de Famatina and Sierras Pampeanas. Restri cted outcrops of Ordovician volcanic rock also appear in the Sierras de Fam atina. Upper Paleozoic, Mesozoic, and Cenozoic continental deposits unconfo rmably overlay the lower Paleozoic units. Tilted blocks bounded by lystric reverse faults that have formed in respons e to subduction of the Nazca plate constitute the main structure of the thr ee ranges. Detailed studies of kinematic markers from mylonitic belts withi n Paleozoic igneous and metamorphic basement indicate shortening and overth rusting of the Sierras Pampeanas onto the eastern border of the Sierras de Famatina. Quaternary sediments cover the Llanura Chaco Pampeana that extend s to the east of 65 degrees W. Magnetotelluric deep soundings were done from 1980 to 1993, between 65 degr ees and 68 degrees W. The results of these soundings suggest a conductive l ayer (electrical resistivity similar to 1 ohm-m) at depths ranging from 8 t o 25 km. Saline fluids produced where there is a large amount of free water released by metamorphic reactions have been proposed as an explanation for lower-crustal conductive anomalies. Beginning 1987, new gravimetric studies were performed between 63 degrees a nd 68 degrees W. The Bouguer anomaly map incorporates preexisting and new d ata (more than 700 gravity observations). The gravity field drops from -0.5 mGal at 64 degrees W in the east to -280 mGal at 68 degrees W and reaches nearly-400 mGal to the west in the Chilean part (similar to 69 degrees W) a t latitude 28 degrees S. This regional low is related to crustal thickening within the Cordillera de los Andes. Active seismicity recorded during 1984 -1994 defines part of the Benioff zone at depth.