Gabriel Carlier

Late Permian–Middle Jurassic lithospheric thinning in Peru and Bolivia, and its bearing on Andean-age tectonics

Abstract Integrated studies and revisions of sedimentary basins and associated magmatism in Peru and Bolivia (8–22°S) show that this part of western Gondwana underwent rifting during the Late Permian–Middle Jurassic interval. Rifting started in central Peru in the Late Permian and propagated southwards into Bolivia until the Liassic/Dogger, along an axis that coincides with the present Eastern Cordillera. Southwest of this region, lithospheric thinning developed in the Early Jurassic and culminated in the Middle Jurassic, producing considerable subsidence in the Arequipa basin of southern Peru. This ∼110-Ma-long interval of lithospheric thinning ended ∼160 Ma with the onset of Malm–earliest Cretaceous partial rift inversion in the Eastern Cordillera area. The lithospheric heterogeneities inherited from these processes are likely to have largely influenced the distribution and features of younger compressional and/or transpressional deformations. In particular, the Altiplano plateau corresponds to a paleotectonic domain of “normal” lithospheric thickness that was bounded by two elongated areas underlain by thinned lithosphere. The high Eastern Cordillera of Peru and Bolivia results from Late Oligocene–Neogene intense inversion of the easternmost thinned area.

Geodynamic evolution of the northern and central Andes during early to middle Mesozoic times : a Tethyan model

The early to middle Mesozoic sedimentary and magmatic history of the northern and central Andes (i.e. the NNE-trending Colombian-Ecuadorian segment, and the NW-trending Peruvian segment) exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings. The late Triassic to late Liassic period began with a widespread marine transgression. From the latest Triassic, the Colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in Peru. During this time, no significant magmatic activity is recorded except in the emerging Colombian area. This period is interpreted as the result of rifting of a Tethyan oceanic arm which separated the Colombian and palaeo-Mexican margins. From latest Liassic, an important calc-alkaline magmatic arc developed along the emergent Colombian segment. Further south, the north-Peruvian shelf probably emerged, but marine sedimentation continued in southern Peru. During middle and early Late Jurassic times, the Colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation. Along the Peruvian segment, a turbiditic trough, emergent areas, and continental basins were created, and the scarcity of calc-alkaline magmatism suggests that only very local subduction took place. This period is regarded as one of the southeastward subduction beneath the Colombian segment, and of Tethyan oceanic crust originating in the newly formed ‘Colombian’ oceanic arm. This pattern would have induced a chiefly left-lateral transform motion along the Peruvian segment. By Kimmeridgian-Tithonian times, the palaeogeographic framework had drastically changed. Along the Colombian segment, magmatic activity ceased, and continental accretions occurred along dextral strike-slip sutures. In Peru, tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation. In the coastal area, arc-related volcanism indicated that subduction took place beneath this segment. This geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the Tethyan ridges and replacement by Pacific spreading centres inducing a roughly northeastward convergence direction.