Late Miocene-early Pliocene onset of N-S extension along the southern margin of the Central Andean Puna Plateau: Evidence from magmatic, geochronological and structural observations

MONTERO LÓPEZ, C., HONGN, F., STRECKER, M., MARRETT, R., SEGGIARO, R. Y SUDO, M.,

TECTONOPHYSICS

ELSEVIER SCIENCE BV

Año: 2010 vol. 494 p. 48 - 63

0040-1951

Extensional faulting typifies many of the Earth's Cenozoic orogenic plateaus. The Altiplano–Puna Plateau in the southern central Andes is the world's second largest orogenic plateau; it contains a rich array of extensional fractures and normal faults with closely associated mafic volcanic flows, dykes, and cones. This is in stark contrast to the foreland areas, which record protracted shortening and continuous eastward growth of the mountain belt. Within the Puna area a kinematic change-over from shortening to extension has previously been inferred to have occurred during the Quaternary. However, our research in the southern Puna, together with earlier petrologic investigations in the south-central part of the plateau, indicates an earlier onset for the extensional tectonism. Our analysis has focused on the eastern portion of the Cordillera de San Buenaventura in the southernmost part of the plateau, where we mapped two Upper Miocene–Pliocene volcanic units associated with extensional faulting — the Aguada Alumbrera Ignimbrite and the La HoyadaVolcanic Complex. The structural inventory of these units records N–S extensional deformation in the Puna associated with the development of E–W oriented fractures, dykes, normal and strike–slip faults, and folds. New 40Ar/39Ar dating of magmatic rocks together with their relationships with the mapped structures reveal that widespread extension in the southern Puna had already commenced by ~5 Ma, and possibly as early as 7 Ma. Although the extensional structures on the Puna are typically associated with mafic volcanism, the Upper Miocene–Pliocene La Hoyada Volcanic Complex hosts dykes of intermediate compositions. Mafic to intermediate magmatism and extensional tectonism in this region is thus contemporaneous with, and structurally akin to, that in the central and northern parts of the plateau. The ubiquitous cogenetic relationship between mafic volcanism and extensional tectonism, both within the Puna Plateau and along itseastern flanks, clearly indicates that the normal faulting is a regional phenomenon that is broadly coeval with an inferred rapid and large-scale uplift of the Altiplano–Puna Plateau and a decrease in plate convergence rates. These observations are compatible with a mantle-delamination scenario, in which the delamination of lithospheric mantle may have produced an ensuing weakening of the crust and regional uplift, which may in turn have generated the conditions necessary to trigger gravitational extensional collapse, beginning in the late Miocene to early Pliocene.