Intraplate records of a transform margin formation: Brittle deformation in the basement of the basins from the northeastern extremity of the Brazilian equatorial margin

Abstract

The diachronous rifting of the Pangea supercontinent, with the dispersion of the continental masses, resulted in the formation of numerous rift systems, which primarily evolved into passive margins, such as the Brazilian Equatorial Margin (BEM). In the South American counterpart, basement structures serve as the primary markers for direct analysis of the nucleation and development of the BEM since much of the rift record occurs in the subsurface. The geodynamic models that explain the evolution of this margin are largely based on geophysical and numerical modeling data. In this paper, we carried out an onshore, multitool, and multiscale structural analysis of the brittle deformation overprinted on the Precambrian basement of the Potiguar and Ceará basins, which led to the proposition of four superimposed events. The D1 event (Late-Brasiliano) is mainly represented by the NE-SW (dextral) stepped fracture systems and the unfilled and quartz- or chalcedony-filled NE-SW to E-W-trending extensional joints. This event is related to NE-SW intraplate transpression, associated with the progressive exhumation of the Brasiliano-Pan African orogenic belt. The D2 event (Jurassic to early Aptian) represents the intracontinental deformation related to the initial rupture of the Pangea. The brasiliano-age ductile shear zones were reactivated (brittle) along with the development of NE-SW normal faults, NNE-SSW en échelon fractures, NE-SW joints, as well as the emplacement of the basic dykes (NE-SW and E-W) related to the Rio Ceará-Mirim magmatism. This event is associated with the NW-SE extension responsible for the origin of the Cariri-Potiguar trend basins. At the beginning of the installation of the equatorial proto-margin, a significant cataclastic process took place, marking the D3 event (post-early Aptian). This enabled the nucleation of hybrid (WNW-ESE, E-W, and NW-SE), shear fractures (WNW-ESE, NNW-SSE, E-W, NNE-SSW, and ENE-WSW), and NW-SE unfilled or filled (chalcedony, iron oxide and hydroxide, and calcite) dilatational joints, which express an E-W dextral transtensional regime related to this event. E-W and NW-SE pull-apart dilatational jogs are characteristic of this event and may be analog to the E-W Messejana and Jacaúna transtensional grabens (western part of the Potiguar Basin). The younger event (D4) is characterized by N–S normal faults and joints, and dry or filled (calcite and iron oxide/hydroxide) NNW-SSE hybrid shear fractures. A clockwise rotation of the South American plate with respect to the African plate and the paleostress field change to N–S compression and E-W extension, are responsible for the formation of D4 structures. The brittle structures developed in the crystalline basement of the Potiguar and Ceará basins represent the intracontinental record of the deformation related to the formation of the Atlantic equatorial margin under a dextral transtensional regime.