Seismic studies of the Brasilia fold belt at the western border of the Sao Francisco Craton, Central Brazil, using receiver function, surface-wave dispersion and teleseismic tomography

摘要

The Tocantins Province in Central Brazil is composed of a series of SSW-NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and Sao Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E-W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33-37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74-1.76) were consistently observed in this unit. The foreland domain of the Brasilia foldbelt, on the other hand, is characterized by thicker crust (42-43 km). Low Vp/Vs ratios (1.70-1.72) were observed in the low-grade foreland fold and thrust zone of the Brasilia belt adjacent to the Sao Francisco craton. Teleseismic P-wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goias Massif compared with the foreland zone of the belt and Sao Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goias Massif and the foreland domain of the Brasilia belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasilia foreland domain and the Sao Francisco craton were part of a larger Sao Francisco-Congo continental plate in the final collision with the Amazon plate. (C) 2004 Elsevier B.V. All rights reserved.