Are The Large Wave-Lenght South American Intraplate Deformation And The Incipient Inversion Of Brazilian Continental Basins Manifestations Of Ongoing Lithospheric/ Crustal Folding?

Abstract

The South American plate (SAP) is now in horizontal compression and shortening. This is shown by stress data compilations, intraplate stress field numerical models and space-based geodetic results. Thrust regimes prevail in SE Brazilian margin, Central Brazil and Amazon region; strike slip regimes prevail in NE Brazilian margin (Assumpcao, 1998). In most Brazilian basins, breakout orientations are usually consistent for the World Stress Map qualities A, B and C, allowing a good estimate of the regional maximum horizontal stress (SHmax), which is in general in good agreement with the available nearby focal mechanisms (Lima et al., 1997). Available stress magnitudes derived from hydrofracturing and leak-off testes have shown that strike-slip stress regimes prevail within the Potiguar and the Campos basin (Lima Neto, 1998 ; Lima Neto & Beneduzzi, 1998). Results from the French geodetic space-based system DORIS are consistent with the observed ongoing compression since all but one of the available base-lines are shortening. Base lines that cross the Andes towards the intraplate are shortening 13-20 mm/yr; intraplate shortening between Cachoeiro Paulista (SE Brazilian coastal ranges) and Kourou (French Guyana) is about 7 mm/yr (L. Soudarin, CNES, Toulouse, written communication). Geodetic results have also confirmed that the Andean belt is still moving eastwards, thrusting the Brazilian craton (Norabuena et al, 1998). The compression / shortening is probably due to the interactions amongst the SAP and its neighborhood (mostly its convergence with the Nazca and the Caribbean plates and its divergence from the African plate) and with the asthenosphere as well (Cobblentz R Silver & Russo, 1996).