Preliminary Observations of the Mantle Transition Zone Beneath South America from Non-Plane Wave Stacking of P-to-S Receiver Functions

Abstract:

We performed a P-to-S receiver function (RF) study of the Mantle Transition Zone (MTZ) beneath South America. The dataset is a combination of all the available broadband seismic stations data of South America stored in three datacenters (IRIS, Universidade de São Paulo, and Rede Sismográfica Brasileira). RF stacking is made using the CCP method, with a nonplane wavefront assumption. RFs from 20803 teleseismic events generated 1861 stacked traces, with at least 4 RFs in 2 degrees radius. The results allow the identification of three major domains with distinct structure. The first domain, along the Cordillera has the 410 km discontinuity (D410) unusually deeper than normal, ~425 km depth, and reduced MTZ thickness, ~235 km. In the central portion of the continent, where the Nazca plate is believed to reach the MTZ, we find an area of about 10 degrees wide with the 660 km discontinuity (D660) depressed to ~675 km depth, and the MTZ is thicker, ~260 km. Lastly, the third domain occurs in the northeast of Brazil, where beneath the Archean cratons it was observed a very shallow D660 (~645 km) and thin MTZ (~235 km). The first domain indicates hotter than normal temperatures around the D410, most likely extending into the upper mantle, and might reflect an upwelling of a convection system behind the subducted plate. The second domain indicates colder than normal temperatures in the D660, and can be related to the emplacement of slab material above this discontinuity. In the third domain, the craton related environment, where colder and less altered lithosphere occurs, two hypotheses are offered to explain the shallow D660 and thin MTZ. Hot material is ascending from the lower mantle as part of the convective system related to the nearby mid-ocean ridge, or the Archean lithosphere acts as a refractory layer, not allowing heat to escape from the lower mantle upwards and being laterally dispersed by the MTZ.