Unravelling the spatiotemporal evolution of the Cenozoic Andean (Altiplano-Puna) plateau has been one of the most intriguing problems of South American geology. Despite a number of investigations, the early deformation and uplift history of this area remained largely enigmatic. This paper analyses the Paleogene tectono-sedimentary history of the Casa Grande Basin, in the present-day transition zone between the northern sector of the Puna Plateau and the northern part of the Argentine Eastern Cordillera. Our detailed mapping of synsedimentary structures records the onset of regional contractional deformation during the middle Eocene, revealing reactivation of Cretaceous extensional structures and the development of doubly vergent thrusts. This is in agreement with records from other southern parts of the Puna Plateau and the Eastern Cordillera. These observations indicate the existence of an Eocene broken foreland setting within the region, characterized by low-lying compressional basins and ranges with spatially disparate sectors of deformation, which was subsequently subjected to regional uplift resulting in the attainment of present-day elevations during the Neogene.
The Vicuña Pampa volcanic complex, at the SE edge of the arid Puna Plateau of\ud
the Central Andes, records the interplay between volcanic construction and degradational\ud
processes. The low-sloping Vicuña Pampa volcanic complex, with a 1200-mdeep,\ud
southeastward-opening depression, was previously interpreted as a collapse caldera\ud
based on morphological considerations. However, characteristic features associated\ud
with collapse calderas do not exist, and close inspection instead suggests that the Vicuña\ud
Pampa volcanic complex is a strongly eroded, broad, massif-type composite volcano of\ud
mainly basaltic to trachyandesitic composition. Construction of the Vicuña Pampa volcanic\ud
complex occurred during two distinct cycles separated by the development of the\ud
depression. The first and main cycle took place at ca. 12 Ma and was dominated by\ud
lava flows and subordinate scoria cones and domes. The second cycle, possibly late Miocene in age, affected the SW portion of the\ud
depression with the emplacement of domes. We interpret the central depression as the\ud
result of a possible sector collapse and subsequent intense fluvial erosion during middle\ud
to late Miocene time, facilitated by faulting, steepened topography, and wetter climate\ud
conditions compared to today. We estimate that ~65% of the initial edifice of ~240 km3\ud
was degraded. The efficiency of degradation processes for removing mass from the Vicuña\ud
Pampa volcanic complex is surprising, considering that today the region is arid, and\ud
the stream channels within the complex are predominantly transport limited, forming\ud
a series of coalesced, aggraded alluvial fans and eolian infill. Hence, the Vicuña Pampa\ud
volcanic complex records the effects of past degradation efficiency that differs substantially\ud
from that of today
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