Punctuated upper-crustal shortening, exhumation, and basin subsidence during flat-slab subduction in southern Peru

Abstract:

New geophysical data help define modern flat-slab subduction in the central Andes, but the geologic consequences of slab shallowing in Peru remain poorly resolved. In better-documented regions of North and South America, changes in subduction architecture have been linked to arc migration, orogenic advance, and shifts in deformation style and kinematics. New structural, sedimentary, and geo/thermochronologic results for southern Peru support a reconstruction of Oligocene-Miocene deformation and sedimentation across the Western Cordillera magmatic arc, Altiplano hinterland plateau, Eastern Cordillera fold-thrust belt, and Subandean foreland basin. The spatial and temporal distribution of Andean shortening and basin subsidence in this region encompass a major phase of arc migration that has been attributed to shallowing of the subducting Nazca slab. Detrital zircon U-Pb ages for syndeformational growth strata pinpoint the timing of key forethrust and backthrust systems (Tinajani and Ayaviri faults, respectively) that partitioned the early foreland basin into a smaller, rapidly subsiding hinterland basin (Ayaviri basin) in the Altiplano plateau. A roughly 28 Ma activation of the basement-involved Ayaviri backthrust along the Altiplano-Eastern Cordillera boundary matches the mid-Oligocene activation age reported for the entire central Andean backthrust belt, which persists for >500 km along strike southward into Bolivia. The Ayaviri structure potentially represents a reactivated deep-seated Triassic normal fault and/or an inherited crustal-scale boundary. Subsequent Miocene activation of the Tinajani forethrust and coeval thrust-related exhumation within the Eastern Cordillera are suggestive of distributed regional shortening during flat-slab subduction. Available data suggest a spatiotemporal coincidence of shallow subduction in southern Peru with inboard arc migration, basement-involved shortening, and focused rapid subsidence in the Altiplano plateau.