An Updated Crustal Thickness Map of Central South America Based on Receiver Function Measurements in the Region of the Chaco, Pantanal, and Paraná Basins, Southwestern Brazil

Abstract

Previous compilation of crustal structure in South America had large unsampled areas including the thin crust in the Sub‐Andean lowlands, largely estimated by gravity data, and the sparsely sampled Amazon Craton. A deployment of 35 seismic stations in Brazil, Bolivia, Paraguay, Argentina, and Uruguay improved the coverage of the Pantanal Basin in Western Brazil, the intracratonic Paraná and the Chaco Basins. Crustal thicknesses and Vp/Vs ratios were estimated with a modified H‐k method by producing three stacked traces to enhance the three Moho conversions (the direct Ps and the two multiples Ppps and Ppss). This modified method gives lower uncertainties than previous studies and shows more regional consistency between nearby stations. The temporary stations and the Brazilian Network (RSBR) have characterized the crustal structure as follows. The Paraná Basin has a thick crust 40–45 km and average Vp/Vs ratio (1.71–1.77), while the Chaco Basin has a slightly thinner crust (35–40 km) and higher Vp/Vs ratio (1.75–1.79). This confirms the lack of widespread magmatic underplating in the Paraná Basin that could be related to the origin of the flood basalts during the South Atlantic opening. A belt of thin crust (30–35 km) with low Vp/Vs (<1.74) is confined to the eastern edge of the Pantanal Basin. Normal crust (38–43 km) is observed along the western edge of the Pantanal, from the southern part of the Amazon craton to the Rio Apa cratonic block. This study, combined with other published data, provides an updated crustal thickness map of South America.

Joint Inversion of Receiver Functions and Surface‐Wave Dispersion in the Pantanal Wetlands: Implications for Basin Formation

“…From 0 to 3.5 km depth, the velocity ranges from ~3.1 to ~3.4 km/s; the upper crust is 3.5 km/s down to 12.5 km depth; the middle crust is 3.7–3.8 km/s down to 30 km; and the lower crust starts with ~4.2 km/s down to 40 km to gradually increases to 4.5 km/s at 50 km depth. Rivadeneyra‐Vera et al () reported a crustal thickness of 45.0 km for this station from H‐κ stacking analysis, a value somewhat smaller than our joint inversion estimate.…”

“…In any case, our crustal thickness estimate is robust, as the later receiver function amplitudes and longer‐period dispersion are correctly predicted. Rivadeneyra‐Vera et al (), using the same data set, reported a crustal thickness over 40 km for station PANT, probably reflecting waveform complexity rather than true crustal thickness. Anomalously, large amplitudes and a nonzero lag time for the leading peak suggest that reverberations from energy trapped in the sedimentary layer dominate the receiver function waveform (e.g., Zelt & Ellis, ).…”

“…Sediment reverberations display very large amplitudes and may contaminate or even mask P‐S conversions from deeper discontinuities (e.g., Julià et al, ; Zelt & Ellis, ). In that case, the H‐κ staking procedure followed by Rivadeneyra‐Vera et al () could have mistaken reverberation amplitudes as Moho‐generated amplitudes and reported erroneous estimates for crustal thickness and bulk V P /V S ratio under this station.…”

“…Most recently, Rivadeneyra‐Vera et al () updated the crustal thickness map of central South America trough H‐κ stacking analysis (Zhu & Kanamori, ) of receiver functions. It is important to realize that, in the Pantanal and Chaco region, they utilized the same receiver function data set considered in our study.…”

“…Indeed, most of the studies that sample the Pantanal region have been conducted on a continental scale (Assumpção et al, ; Chulick et al, ; Feng et al, , ; Lloyd et al, ; van der Lee et al, ; van der Meijde et al, ), with only sparse coverage in this part of the continent. Perhaps the most detailed study is that of Rivadeneyra‐Vera et al (), which incorporated our data set in a continental‐scale study of the South American crust. That study only reported on crustal thickness and bulk VP/VS ratio variation, however, with no detail on internal crustal structure.…”