Structural characteristics of the 'Puquios chaos' and its relationship with the Andean middle Cretaceous extensional tectonics at 27°S, northern Chile

摘要

In the Precordillera of the Atacama region (similar to 27 degrees S), the Puquios chaos corresponds to a singular tectonic unit traditionally ascribed to a mid-Cretaceous extensional stage affecting the Mesozoic formations that were deposited between the arc and the backarc settings of the Andean margin. The melange-like unit consists of blocks of limestones in a mainly volcanic matrix that includes andesitic lavas, volcanic breccias and red vulcarenites of Upper Jurassic to Lower Cretaceous age. These rocks constitute a conformable succession of andesites, limestones and vulcarenites (from the base to the top) in the Cerro Aguila strata, observed to the south of the study area.The Puquios chaos is characterized by often isolated, tight to isoclinal folds and a set of low- and high-angle normal faults that were responsible for the block-in-matrix mesoscopic structure of the tectonic unit. These structures are easily observable in the calcareous rocks as the volcanic matrix shows a tectonically brecciated texture. The detailed structural study presented in this work allows us to propose the deformational mechanisms that finally constituted the apparently chaotic unit, including the restoration of the structures related to late contractive stages. Thus, we suggest that the singularity of the Puquios chaos is associated with a local, wide, extensional shear zone linked to a regional detachment surface. The intimate relation between recumbent folds and low- and high-angle normal faults, besides the similar orientation of fold hinges and fault planes, point to the actuation of a brittle-ductile shear zone where the calcareous competent layers gave rise to decametric-scale folds as the volcanic matrix responded to strain by brecciation. Stretching calculations carried out in a domino fault system yielded horizontal extension of around 30% for the high-angle faults.We obtained a relative age of the Puquios chaos formation according to the field relations and the structural determinations. A NE-directed intense extensional stage promoted a significant crustal thinning that exhumed but not affected the late Paleozoic basement. The Puquios structures and the related regional detachments affected the pre-Barremian arc and back-arc formations and were unconformably covered by the latest Cretaceous and Paleocene thick volcanic and volcanosedimentary deposits.