ROLE OF EARLY CENOZOIC FLAT SLAB SUBDUCTION OF THE NAZCA PLATE IN NORTHERN CENTRAL ANDEAN PLATEAU EVOLUTION

The subduction of the Nazca plate has produced one of the highest plateaus within a convergent setting – the Central Andean plateau (CAP), and the nature of subduction acts as one of the main controls on deformation and magmatism in the CAP. The type of subduction varied from normal to flat slab, both spatially and temporally from early Cenozoic to recent. Six formations deposited in the foreland basin of CAP: Quilque, Chilca, Kayra, Soncco, Paruro and Punacancha extended along the Western Cordillera – Altiplano plateau boundary capture the ~52-43 Ma normal, ~43-30 Ma flat slab and ~30-24 Ma transition to normal subduction. We examined deformation, magamatism and elevation history of these basins to better characterize the early evolution of the CAP.

Minimal deposition and deformation mark Quilque and Chilca formations deposited in backbulge and distal foredeep settings during normal subduction. Deposition increased significantly towards the end of normal subduction marked by Eocene proximal foredeep deposits of the Kayra Formation. Maximum deposition and deformation took place during flat slab subduction, as evidenced by the more than 2.5 km stratigraphic thickness of the wedgetop Soncco Formation and progressive unconformities and growth strata produced by increased activity and in the Cusco-Lagunillas (C-L) thrust system. Strike slip activity in the C-L and Urcos-Ayaviri fault system in the late stage of the flat slab subduction created Punacancha basin with several kilometers of stratigraphic thickness. Normal subduction resumed during deposition of the Paruro basin, which also formed in a transpressional setting with a stratigraphic thickness of ~1 km.

Emplacement of a batholith marked the early magmatic activity in CAP. Magmatism in the Andahuaylas-Yauri batholith was characterized by mafic composition with calc-alkaline cumulates during the ~52-43 Ma of normal subduction and by more intermediate nature during the ~43-30 Ma flat slab subduction.

The δ¹⁸O values of meteoric water reconstructed from sedimentary carbonates do not show any major isotopic shifts, but more positive signatures than modern meteroric water, with an average of -8.1±2.5‰ from the Paruro Formation suggests that the elevation of CAP only reached ~1.1±1.0 km by the middle Miocene.