Sedimentology, geochronology and provenance of the late Permian and Triassic Mitu Group in Peru—The evolution of continental facies along a transform margin

Abstract

AbstractIn the late Permian and Triassic, the continental Mitu Group formed in extensional basins along the length of the Cordillera Oriental and Altiplano of present‐day Peru. Given the presence of coeval arc systems only in northern Chile and southern Ecuador but not in Peru the tectonic setting of the Mitu basin has been interpreted variably as orthogonal continental rift, sinistral transtensional rift, aulacogen and back‐arc basin. The Mitu Group comprises continental mass flow and alluvial fan, fluvial, aeolian and minor lacustrian facies and hosts thick piles of subalkaline and alkaline intermediate and felsic ignimbrites and mafic lavas. The age of the Mitu Group had originally been established as ranging from the Late Permian to the late Triassic on the basis of structural considerations and scarce biostratigraphic data. Recently, U–Pb zircon ages from ignimbrites and sedimentary rocks have been taken to constrain the Mitu Group to the Middle and Late Triassic. We performed a sedimentological, heavy mineral, and zircon geochronological and Lu‐Hf isotope study of the Mitu Group in 14 sections mainly in southern and central Peru, and one section in northern Peru. Ten new U–Pb concordia ages on ignimbrites intercalated in the Mitu Group successions offer a new robust stratigraphic framework and constrain the stratigraphy of the Mitu Group between 260 and 205 Ma. In combination with maximum likelihood ages of deposition derived from detrital zircon, U–Pb geochronology places the deposition of the Mitu Group between ca. 270 and 194 Ma (lower Guadalupian into the Sinemurian). Detrital zircon U–Pb age distributions and heavy mineral assemblages reflect a strongly recycled Precambrian Amazonian and Palaeozoic proto‐Andean provenance. The Palaeozoic detrital age patterns are highly variable, and temporally and spatially random. A local provenance can generally not be identified. εHf(t) values in zircon obtained from ignimbrites and sedimentary rocks indicate variable degrees of crustal recycling. In the course of the Palaeozoic, εHf(t) values become on average progressively less negative, with a large proportion particularly of Mitu age zircons' εHf(t) values encompassing less evolved and moderately juvenile compositions. Along strike of the basin stratigraphic thicknesses, and rates and times of accumulation vary strongly with larger thicknesses and rates being registered in southern Peru. This suggests that the Mitu basin had been divided into a number of subbasins with individual histories of subsidence, accumulation, and volcanism. Absent a magmatic arc, late Permian–Triassic Peru evolved in a sinistral plate tectonic and regional framework expressed particularly in the sinistral Late Gondwanide orogeny predating the Mitu Group. We interpret that a sinistral transform fault linked the subduction zones in southern Ecuador and northern Chile and that the Mitu Group basin has formed by sinistral transtensional and transpressional movements along a related transcurrent fault inboard of the transform margin. In the broader framework of the accretionary Terra Australis orogen along the western Gondwana margin, this interpretation fits its evolution in the Andean segment in an internally consistent way.