Inversão simultânea de função do receptor e dispersão de ondas de superfície na Bacia do Pantanal

Abstract

The lithospheric structure of the South American continent has been mainly investigated through continental-scale seismic studies and a number of seismic experiments in the Andean region. As a result, there is still a paucity of detailed seismic studies within the South American platform. With the aim of improving seismic coverage in the continent, the Universidade de São Paulo (USP) is presently leading an international, multiinstitutional effort funded by FAPESP that focuses on investigating the deep structure under the Paraná, Pantanal, and Chaco basins through analysis of seismic data (“3-Basin Project”). To that end, existing stations belonging to the Rede Sismográfica Brasileira have been complemented with a temporary deployment of more than 30 broadband stations throughout W-SW Brazil, Bolivia, Uruguay, and Paraguay. Here, we present results obtained from analysis of data collected at 33 stations in and around the Pantanal basin. Our study consists of: (i) an analysis of Moho P-to-S conversion phases in receiver functions to determine crustal thickness and bulk Vp/Vs ratio; and (ii) the joint inversion of the RFs and surface wave dispersion curves to develop S-wave velocity-depth profiles for the crust and uppermost mantle under each station. Estimates of crustal thickness and bulk Vp/Vs ratio have been obtained through the H-κ stacking technique, which performs a weighted stack of the receiver functions amplitudes along phase-moveout curves through a grid-search in the H-κ parameter space. Development of velocity-depth profiles is based on a linearization of the forward problem followed by minimization of a weighted average of the RMS error for the receiver function and surface-wave dispersion datasets. Analysis of P-to-S conversions has revealed that Moho depths and Vp/Vs ratios vary between 33 and 50 km and between 1.56 and 1.83, respectively. Within the Pantanal basin, however, the crust is thinnest (34-38 km) and displays the smallest Vp/Vs ratios (1.56-1.69). The velocity models reveal the existence of up to four different crustal types that correlate with surface geology: (i) crust of 35 km with VS<4.0 km/s, under the basin and along the SW projection of the Transbrasiliano Lineament (TBL); (ii) crust of 45 km with VS>4.0 km/s below 40 km depth, flanking the basin to the East and West; (iii) crust of 50-55 km with VS>4.0 km/s below 40 km depth, flanking the basin to the North and South and along the TBL; and (iv) crust of 42.5-45.0 km with Vs<4.0 km/s in the neighboring Paraná Basin. Existing geodynamic models propose that the Pantanal Basin formed either in the backbulge of the flexural system or at the top of the forebulge, due to extensional bending stresses that reactivated pre-existing faults in the shallow upper crust. We argue that the Pantanal Basin formed in a structurally weaker portion of the foreland crust that was affected by delamination and enhanced bending of the South American plate. Our findings do not allow for discrimination among the competing models, but suggest the TBL was critical in marking the location, origin and evolution of this basin.