PAMPEAN (CHILEAN) FLAT SLAB: GEOPHYSICAL VIEW

The subduction of Nazca plate beneath South America is characterized by variability in its shape along the Andean margin. Below central Chile and western Argentina (27°-33°S), the distribution of earthquake hypocenters clearly defines a flat subduction zone that extends on more than 300 km in east-west direction, at depths around 100 km. Near 33°S, where the Juan Fernandez Ridge (JFR) is subducting over the Nazca plate, a sharp transition from a flat slab to a steep slab geometry is observed. To better know the characteristics of the flat slab zone, geophysical studies have been carried out between 29° to 36°S, including seismology, gravity, magnetotelluric observations, among others. Seismological data have been obtained from global and local networks to obtain 2D and 3D models of the zone. The seismicity at depths greater than 100 km and anisotropy observations favor a slab bending rather than tears to accommodate the sharp change in the slab dip. Relative to the flat slab segment, the main results are: - There is a high seismic activity and more seismic moment release associated to the JFR subduction; - Crustal shallow seismicity is mainly located at the back-arc, in western Argentina; - A double seismic zone is observed north of 33°S, down to depths of 100 km; - Anisotropy shows a continued eastward mantle flow; - Models for the Moho indicate a 65 km thick crustal root beneath the high Andes, a 60 km thick crust beneath the Precordillera, and a gradual shallowing of Moho to 40 km beneath the Sierras Pampeanas; - The mantle above the flat slab does not appear to be hydrated, explaining the absence of active volcanism in the zone. By other hand, in the steep subduction zone south of 33°S, results indicate: - Crustal seismicity is mainly located in the forearc, at the Andes Cordillera in Chile; - It is not clear that a double seismic zone exists in this region; - Anisotropy shows a trench-parallel mantle flow; - The Moho depths are about 50 km beneath the Andes, and decreases to 40 km to the east; - High Vp/Vs velocity ratio are observed at about 100 km depth along the slab and below the active volcanoes at shallow depths, that may be related with hydrated rocks and partial melting. These results shed new light on some characteristics of the Pampean (Chilean) flat slab zone, but can they explain why the slab is flat?