THE PUNA PLATEAU - GEOLOGICAL OVERVIEW

THE CENTRAL ANDES

The Andean orogenic belt is a 7000 km long topographic feature located along the western margin of South America. The highest and widest portion of the belt is located within the Central Andes of southern Peru, Bolivia, and northern Argentina where the mountain belt is over 3 km high by ~800 km wide.The Central Andes consist of:

• Tertiary age arc volcanic rocks of the Western Cordillera;

• Cretaceous and Tertiary age sedimentary rocks and Quaternary alluvium of the Puna-Altiplano plateau;

• deformed Paleozoic sedimentary rocks of the Eastern Cordillera;

• the actively deforming Subandean Zone which comprises the modern Andean fold-thrust belt.

• Adjacent and east of the Subandean Zone are the Chaco and Beni Plains, a flat lowland region that makes up the modern Andean foreland basin and overlies the basement of the Brazilian Shield.

The crust beneath the central Andes is up to ~70 km thick (James, 1971; Wigger et al., 1994; Zandt et al., 1994; Beck et al., 1996) and has been widely attributed to crustal shortening that occurred during Tertiary orogenesis (Jordan and Alonso, 1987; Allmendinger et al., 1997; Sempere et al., 1990, 1997; Kennan et al., 1995; Lamb and Hoke, 1997; Horton and DeCelles, 1997; Horton, 1998). The distribution of these tectonogeomorphic zones throughout the central Andes and the subduction geometry of the Nazca plate beneath the South American plate has led to general acceptance of the Andes as a type example of an orogenic belt associated with oceanic-continental plate convergence (Dewey and Bird, 1970; James, 1971).

GEOLOGIC HISTORY OF THE CENTRAL ANDEAN PUNA PLATEAU

The tectonic evolution of the Andean mountain belt began during Late Jurassic time (Jordan et al., 1983; Sempere et al., 1997) and was characterized by episodes of compression, extension, transtension, and periods of tectonic quiescence (Sempere et al.,1997). During Late Jurassic to Early Cretaceous time, tectonic conditions throughout the central Andes consisted of extension and transtension resulting in sedimentary basins formed by rifting and postrift thermal sag. Widespread contraction began during the Paleogene and is attributed to sustained periods of oblique and normal convergence between the Farallon (Nazca) and South American plates (Pilger, 1981, 1984; Pardo-Casas and Molnar, 1987).

Late Jurassic to Cretaceous (Extension and Transtension)

The initiation of Andean mountain building began during Late Jurassic time (Jordan et al., 1983; Sempere et al., 1997) resulting from the segmentation of Gondwana during the late stages of Pangaea breakup and subsequent western migration of South America across the Pacific Ocean basin (Coney and Evenchick, 1994; Sempere, 1995). Rifting between South America and Africa (opening of the South Atlantic) was underway by the Early Jurassic and continued through Early Cretaceous time. During Late Jurassic to Cretaceous time, the oceanic Phoenix plate traveled southward along the western margin of the South American plate resulting in a passive Andean margin and alternate periods of extension and rifting, and tectonic quiescence throughout the central Andes (Coney and Evenchick, 1994; Sempere, 1995 Sempere et al., 1997).

Late Jurassic to Early Cretaceous stratigraphy in the central Andes consists primarily of nonmarine redbed deposits, alkaline volcanic rocks, and thin successions of marine carbonates (Coney and Evenchick, 1994; Sempere, 1995). Late Cretaceous (Peruvian Compressional Phase). By the beginning of the Late Cretaceous, the spreading ridge between the Phoenix and Farallon plates had migrated southward past the Andean margin and the westward migrating South American plate began to override the oceanic Farallon plate. Periods of oblique and normal convergence between these two plates are widely accepted as the first phase of contractional deformation associated with subduction and crustal shortening in the central Andes.

Paleogene (Eocene Incaic Phase)

Tectonic activity throughout Paleocene to middle Eocene time remained dormant possibly associated with a decrease in average convergence rates (Sempere et al., 1997). However, by late Eocene time rapid normal convergence resulted in some of the fastest recorded convergence rates between the Farallon and South American plates (Pardo-Casas and Molnar, 1987). Paleocene to middle Eocene stratigraphy is characterized by thin (<300 m thick) successions of nonmarine redbed lacustrine deposits and paleosol deposits typically <100 m thick (Horton et al. 2001). Although the Oligocene is often considered a period of tectonic quiescence associated with slow rates of convergence (Pardo-Casas and Molnar, 1987; Sempere et al., 1997), some of the thickest successions of nonmarine strata (up to ~7000 m thick) are represented in late Eocene to Oligocene stratigraphy.These thick successions of mid-Tertiary strata occur throughout the Altiplano plateau and mark a distinct change in the thickness and lithology of Andean stratigraphy from thin successions of marginal marine, lacustrine, and paleosol deposits (<900 m thick) to thick fluvial successions.

Neogene -Present (Miocene - Pliocene Quechua Phase)

The fragmentation of the Farallon plate into the Nazca and Cocos plates began near the end of late Oligocene time (~25 m.y.) (Figure 1-4) and was accompanied by an increased and relatively steady rate of convergence between the Nazca and South American plates that prevailed throughout Cenozoic time (Pilger, 1981, 1984; Pardo-Casas and Molnar, 1987). Rapid convergence during Neogene time resulted in thick nonmarine successions (up to ~5000 m thick) throughout the central Andes that consist of fluvial and alluvial fan deposits and volcanic tuffs.

EVOLUTION OF THE PUNA BASINS

The present endorheic basins of the Central Andean Puna plateau (NW´Argentina) exhibit the timing and character of deformation during the last 40 Ma. Basin-forming processes were related to contractional tectonism due to the build-up of the Central Andean mountain range. The sedimentary record documents the incorporation of the Early Tertiary Andean foreland into the magmatic arc whereby the relatively uniform Puna basin, a retroarc foreland basin, was segmented into several isolated intra-arc basins.

Horizontal shortening started during the Late (or Middle) Eocene (Incaic deformation). It affected the eastern and western border of the Puna basin. Facies distribution patterns, paleocurrent measurements and the composition of synorogenic sediments document the uplift of an extended highland (Proto-Eastern Cordillera) in the east. During the Late Oligocene renewed tectonic shortening led to uplift of basement blocks and initial segmentation of the Puna. Late Oligocene shortening also can be observed in the Northern Altiplano, the Bolivian and Argentine Eastern Cordillera and the Chilean Precordillera. We presume that the widespread occurrence of a Late Oligocene deformation is directly related to a concurrent increase of the arc-normal convergence. Further contractional movements occurred on several pulses (20-17 Ma, 12-9 Ma, 5-2 Ma). They led to the further development of large reverse fault systems segmenting the Puna basin into numerous endorheic basins.