Oblique lineations in orthogneisses and supracrustal rocks: vertical partitioning of strain in a hot crust (eastern Borborema Province, NE Brazil)

Abstract

Detailed structural work conducted at the eastern area of the Neoproterozoic Brasiliano (=Pan-African) Borborema Province (northeastern Brazil) has shown two orientations of stretching lineations with ESE trend in supracrustal rocks and NE trend in underlying orthogneisses. In the metasedimentary sequence, numerous kinematic indicators showing a top-to-the-northwest sense of shear denote a well-developed non-coaxial deformation. In the orthogneisses, lineations formed dominantly during coaxial deformation, although a component of NE-directed shear is locally observed. The two lineations were produced under similar high-temperature metamorphic conditions and are interpreted as the result of a protracted NW–SE contractional strain regime where (i) subhorizontal non-coaxial shear with superimposed flattening led to an initial phase of NW-directed thrusting, (ii) flattening strains mainly accumulated in the orthogneisses with progressive deformation, leading to a lineation oblique to the transport direction, (iii) the subhorizontal fabric in basement and cover rocks was refolded by overturned folds, and, then (iv) cross-cut by conjugate ENE-striking dextral and NNE-striking sinistral shear zones contemporaneous with NE-trending upright folds. It is proposed that vertical partitioning of strain between basement and cover may explain the presence of oblique lineations in this orogenic belt that did not go through a final stage of extensional collapse.