HR: 1400h
AN: S33A-03    [Abstracts]
TI: Study of a Buried Graben in the Intracratonic Parana Basin, Brazil, With High Frequency Receiver Functions
AU: Costa, T N
EM: thiago@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao 1226 Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AU: Costa, T N
EM: thiago@iag.usp.br
AF: Petrobras, CENPES Ilha do Fundao, Rio de Janeiro, RJ 21000, Brazil
AU: * Assumpcao, M
EM: marcelo@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao 1226 Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AU: Julia, J
EM: jordi@seis.sc.edu
AF: South Carolina University, 901 Sumter St., Columbia, SC 29208-0001, United States
AB: Teleseismic receiver functions are routinely employed to study the structure of the deep crust and uppermost mantle beneath seismic stations. Receiver functions are built from secondary waves generated by the interaction of a direct P-wave with near-receiver discontinuities. Because of the long trajectory between the source and the receiver, it is necessary to low-pass filter the waveforms at about 1 Hz to observe secondary phases from subsurface discontinuities emerge above the background noise. Here we extend the receiver function method to study the topography of the sediment-bedrock interface beneath five stations in the northern part of the intracratonic Paraná basin. The stations cross two suspected grabens hidden beneath the sediments, as previously revealed by Bouguer gravity anomalies (~16-30 mGal). We utilize nearby Andean earthquakes with good signal-to-noise ratios up to ~5Hz to obtain receiver functions at unusually high frequency contents (Gaussian width ~10 s-1) and to infer the S-velocity variation with depth down to the crystalline basement by jointly modeling the high-frequency receiver functions with low-period Rayleigh-wave group velocities. The 1D models reveal an average regional basement depth of ~3.0 km, in good agreement with basement depth estimates at a nearby well, and the stations located above the gravity lows show deeper basement depths in agreement with the graben hypothesis. Interestingly, our S-velocity models reveal the graben basement is ~4.0 km shallower than predicted from the independent gravity modeling. We show this discrepancy can be explained if sedimentary rocks of lower density, as suggested from an empirical velocity-density relationship applied to our S-velocity models, are assumed. A horst-graben structure beneath the northern part of the Paraná basin integrates both gravity and seismic data sets as long as lower densities (and shallower depths) are assumed.
DE: 0935 Seismic methods (3025, 7294)
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
DE: 7205 Continental crust (1219)
DE: 8169 Sedimentary basin processes
DE: 9360 South America
SC: Seismology [S]
MN: 2007 Joint Assembly