Exhumation and Basin Development Related to Formation of the Central Andean Plateau, 21° S

Abstract

Thermochronological (apatite fission track, AFT) and sedimentological data from the central Andean high plateau and its eastern foreland reflect the Tertiary tectonic evolution of this plateau. AFT data define several stages of exhumational cooling in the plateau and its foreland: A first stage (AFT dates of 40 and 36 Ma), restricted to the central Eastern Cordillera, can be attributed to initial thrusting increments following the Incaic Phase. Around 33--30 Ma, cooling occurred along basement highs over the entire plateau; kinematics were thrusting (eastern Cordillera) as well as normal faulting (Altiplano). From 17 Ma onwards, exhumational cooling took place in the eastern foreland (Interandean-Subandean), reflecting the eastward migration of the thrust front into the Chaco foreland basin. Sedimentary data agree with the AFT data. Before 32 Ma, finegrained playa-mudflat deposits indicate tectonic quiescence in most parts of the endorheic basin. Around 32 Ma, a strong change to coarse clastics took place. Between 32 and 18 Ma, alluvial fans and fluvial deposits formed adjacent to fault-bound basement highs. Later growth strata indicate thrusting activity. Since about 10 Ma, flat-lying continental deposits indicate tectonic quiescence from the Altiplano to the Eastern Cordillera, whereas in the Chaco the present foreland was created. The data are discussed in terms of the development of the active continental margin and a possible scenario is suggested: The Incaic Phase (45--38 Ma) might have been linked to a slab breakoff and subsequent flat slab conditions. During the associated gap in volcanism (38--28 Ma), flat slab subduction may have caused hydration of the upper plate mantle and the initial formation of the plateau. Later plateau uplift could have been caused by crustal thickening owing to tectonic shortening, first within the plateau and its foreland and, since about 10 Ma, in the foreland only.