An Integrated Tectono-Magmatic Model for the Evolution of the Southern Peruvian Andes (13-20°S) since 55 Ma

Abstract

The definition and characterization of the Oligocene-Miocene Crucero Supergroup of the southern Peru Inner Arc domain provide a previously unavailable lithostratigraphic framework for the clarification of the geodynamic context of magmatism across the southern Peruvian transect. We integrate new lithostratigraphic, petrologic, and 40Ar/39Ar geochronologic data for these volcanic and hypabyssal rocks with published information to establish a geotectonic model for the transect since 55 Ma. The early Eocene to Early Miocene tectono-magmatic history of the region was controlled by the evolution of a flat-subduction regime, which was initiated at ∼52 Ma with the onset of rapid convergence, terminating the magmatic and hydrothermal activity of the Toquepala superunit of the Coastal Batholith. Collision of the flat slab with the leading edge of the Brazilian Shield lithospheric mantle at 39-40 Ma resulted in catastrophic failure of the overriding plate and crustal-scale, NE-directed ramping, generating the “Incaic” Zongo-San Gaban tectonothermal zone in the proto-Cordillera Oriental. Failure of the flat slab (∼37 Ma), its progressive foundering into the asthenosphere (from ∼37 to 20 Ma), and the upwelling of asthenosphere in its wake are inferred to have generated an eastward-migrating thermal anomaly that severely weakened the lower crust. This resulted in the resumption of Main Arc magmatism in the proto-Cordillera Occidental (∼32 Ma) in an axial regime of transtension, followed at 29 Ma by quasi-instantaneous broadening of the arc to a width of ∼200 km. Further continentward arc expansion into the Inner Arc at ∼25 Ma, generating the mixed crust- and mantle-derived suites of the Picotani Group, resulted in a 360-km-wide swath of magmatism. Termination of magmatism in the Inner Arc at ∼22 Ma reflected the waning stages of anomalous mantle flow associated with foundering of the slab, and was accompanied by substantial uplift and crustal thickening. The subsequent evolution of the transect was initiated and prompted by the broad zone of thermally weakened lower crust. Ductile shortening of the lower crust from ∼22 to 17 Ma generated widespread crustal thickening and uplift across the orogen, and was accompanied by the antithetic oceanward subduction of Brazilian Shield lithosphere beneath the Inner Arc and the initiation of thrusting and folding along the Sub-Andean zone. Delamination and, probably, detachment of the Brazilian Shield from its lithospheric mantle root, as well as partial melting of this root, yielded enriched mafic melts that ascended into the orogenic crust and contributed both thermally and through the release of volatiles to the generation of the Quenamari Group. We emphasize the spatial association of the Crucero Supergroup and comparable suites of the Bolivian Cordillera Oriental with the Bolivian Orocline, and delimit a ~250-km amagmatic longitudinal zone separating the sites of anatexis in southern Peru and northern Bolivia. This is inferred to represent the zone of maximum orthogonal convergence throughout the later Cenozoic, analogous to a “tectonic indentation” regime. Since ∼22 Ma, magma ascent has been possible only in zones marginal to the indentor.