THE PERUVIAN FLATSLAB

The modern Peruvian flat-slab, the world's longest, results mainly from 3 factors: subduction of the Nazca aseismic ridge and exhumed Inca Plateau, subduction of young oceanic crust and relatively fast westward absolute motion of South America. These factors can explain the observed 400-500 km length of the flat-slab, a lack of active arc volcanism, and Neogene undersaturated alkaline magmatism with arc affinities in eastern Peru. The impingement of the thick buoyant aseismic ridges changed the crustal state of stress increasing the coupling between the two plates and enhancing seismicity and crustal deformation. Despite the change, the rate of uplift and canyon incision in the flat segment is similar to that in the steeper subducting segment to the south, and there are no obvious differences between the two segments in intensity, timing or structural style of the Neogene Quechua orogenic phases. Both thick and thin-skinned characterize the fold-thrust belt above the flat-slab. The main controls on the structural style are likely lithofacies and inversion of Permian and Jurassic faults. Radiometric ages document several Cenozoic pulses of eastward magmatic migration. The cessation (12 Ma) of magmatism in the northern part of the flat-slab correlates with the complete subduction of the Inca Plateau and the arrival of the Nazca Ridge. The magmatic lull following Nazca Ridge subduction began at the end of the Miocene. Most of the emplacement of the Na-rich delamination-related magmas of the Cordillera Blanca Batholith and coeval ignimbrites along a major crustal lineament took place during the southern swept of the Nazca Ridge. Batholith emplacement involved crustal scale transtensional deformation that caused extensional collapse and lithospheric thinning beneath the Western Cordillera. Due to strain partitioning during oblique convergence, subduction of the Nazca Ridge also enhanced fore-arc extension and locally inverted Paleogene grabens. A long flat-slab running from north of Lima to the Chilean border (7°-18°S) is also postulated during the deposition of Neocomian quartz-rich sandstones in the Morro Solar Group/Hualhuani Fm. Drowning and subsidence of the Jurassic arc, uplift of Paleozoic and Precambrian rocks and widespread extension and segmentation of the coastal ranges in fault-bounded basins accompanied an abrupt change in igneous activity.