1 juin : Séminaire de Jordi Julià

Rifting, uplift and volcanism in the Borborema Province of NE Brazil: Lessons from passive-source imaging.

Jordi Julià, Department of Geophysics/UFRN - Brasil

le jeudi 1er juin à 14h30, en visio

The Borborema Province of NE Brazil can be regarded as the remnant of a larger Proterozoic mobile belt that formed during the Brasiliano-Pan African orogeny at the end of the Neoproterozoic.

Extensional stresses related to the opening of the South Atlantic Ocean left a number of (now) aborted rift basins that, together with a network of shear zones inherited from the Brasiliano orogeny, scar the Province's continental interior. After separation from Africa, the Province was affected by episodes of intraplate volcanism and uplift that have their main expressions along the Macau-Queimadas magmatic alignment (93-7 Ma) to the east, and the high-standing Borborema Plateau and Araripe Basin (~1000 m) to the east and west, respectively.

A number of models have been proposed to explain Mesozoic rifting and subsequent intraplate activity, which include processes of simple and pure shear deformation (with and without mafic underplate), tectonic uplift of rift basins, and thickening of the crust by mantle plumes, small-scale convection cells and/or lateral crustal flow. With the aim at discriminating among these competing models, the deep structure of the Province has been investigated in the past decade through analysis of passively acquired seismic and magnetotelluric datasets. In the eastern Province, joint receiver function and surface-wave dispersion analysis revealed a 4-5 km thinning of the crust surrounding the southern Plateau, from 36-38 km to 30-32 km, along with the presence of a marked intra-crustal discontinuity accompanying thin crust. Interestingly, the analysis also revealed the northern Plateau is a region of elevated thin crust. In the western Province, joint seismological and magnetotelluric imaging revealed an unexpectedly thick crust under the Araripe Basin (36-38 km) underlain by a thin (< 120 km) lithosphere characterized by high conductivity and slow S-velocities (< 4.3 km/s). At upper mantle levels, seismic anisotropy showed the mantle is surprisingly anisotropic at the heart of the Province, while roughly consistent with lithospheric-scale shear zones and Mesozoic stretching elsewhere, and body-wave tomography demonstrated the lithospheric mantle is fast under the southern Plateu, while slow under the Araripe Basin, Cariri-Potiguar trend and the northeastern corner of the Province.

None of the existing models fully explains our observations, so a new framework is proposed in which topographic variations in the Province are mostly the result of differential stretching during continental breakup, with local modifications driven by tectonic inversion and magmatic intrusions, while magmatism as a whole is related to lateral sublithospheric flow along lithospheric thin spots.