A deformação das coberturas terciárias do planalto da Borborema (PB-RN) e seu significado tectônico.

Abstract

The occurrence of Tertiary sediments at high elevations on the Borborema plateau, northeastern Brazil, have been viewed as evidence for a Cenozoic uplift pulse affecting the whole northeastern portion of the Brazilian shield. A structural, sedimentological and geochronological study carried out on Cuite, Bom Bocadinho, Araruna, Dona Ines and Solanea-Bananeiras tablelands, Paraiba State, revealed that the Serra dos Martins Formation (continental sandstones and conglomerates of presumable Early Tertiary ages) underwent a major uplift pulse under a compressional stress field. In the studied portion of the Borborema plateau, those slightly assymetrical mesas comprise a large N- and NNE-dipping homocline. The most penetrative structures observed in outcrops are two sets of vertical joints striking NNW-SSE and E-W. SE-dipping thrust faults associated with intraestratal detachments and faults are also very frequent. Low angle, dextral strike-slip faults oriented NW-SE are present in the contact zone between the crystaline basement and the sedimentary cover. Dextral shear fractures trending NE-SW and NW-SE occurr to a lesser extent. In the Solanea-Bananeiras mesa, the most conspicuous structures are NNE-SSW and NW-SE normal faults, as well as subvertical joint sets bearing SSE, WNW-ESE and NE-SW. Fault slip analysis and paleostress determinations indicated a SE-NW oriented compressional stress direction for the nucleation of the most penetrative family of brittle structures affecting the Serra dos Matins Formation, which includes the conjugated sets of joints, the reverse faults, the low angle dextral strike-slip faults and intrastratal slip zones. Furthermore, the acute bissector of the conjugate sets of joints is almost parallel to the principal stress direction obtained from the paleostress analysis. On the Solanea-Bananeiras mesa, the Serra do Martins Formation is unconformably overlain by a package of siliciclastic sediments correlated to the Miocene-Pliocene Barreiras Formation. The Barreiras Formation is affected only by normal faults and vertical joints. The uplift of the Borborema Plateau, at least in the studied area, must had been driven by a combination of mechanisms, such as variation in the in-plane stress field associated with the influence of Tertiary volcanic events in northeastern Brazil.