South American Global Geoscience Transect 7: Cordillera Frontal to Llanura Chaco Pampeana, Northern Argentina

Abstract

The South American Global Geoscience Transect 7 is located in northern Argentina and traverses, from west to east, the Cordillera Frontal, Puna, Sierras de Famatina, Sierras Pampeanas, and Llanura Chaco Pampeana. Cenozoic stratovolcanoes that intrude Lower Paleozoic slates and Upper Paleozoic continental sedimentary rocks constitute the Cordillera Frontal. Upper Precambrian to lower Paleozoic metamorphie and sedimentary rocks intruded by Paleozoic granitoids comprise the Sierras de Famatina and Sierras Pampeanas. Restricted outcrops of Ordovician volcanic rock also appear in the Sierras de Famatina. Upper Paleozoic, Mesozoic, and Cenozoic continental deposits unconformably overlay the lower Paleozoic units. Tilted blocks bounded by lystric reverse faults that have formed in response to subduction of the Nazca plate constitute the main structure of the three ranges. Detailed studies of kinematic; markers from mylonitic belts within Paleozoic igneous and metamorphie basement indicate shortening and overthrusting of the Sierras Pampeanas onto the eastern border of the Sierras de Famatina. Quaternary sediments cover the Llanura Chaco Pampeana that extends to the east of 65° W. Magnetotelluric deep soundings were done from 1980 to 1993, between 65° and 68° W. The results of these soundings suggest a conductive layer (electrical resistivity ∼1 ohmm) at depths ranging from 8 to 25 km. Saline fluids produced where there is a large amount of free water released by metamorphie reactions have been proposed as an explanation for lower-crustal conductive anomalies. Beginning 1987, new gravimetric studies were performed between 63° and 68° W. The Bouguer anomaly map incorporates preexisting and new data (more than 700 gravity observations). The gravity field drops from −0.5 mGal at 64° W in the east to −280 mGal at 68° W and reaches nearly −400 mGal to the west in the Chilean part (˜69° W) at latitude 28° 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.