On the crustal framework of the central Alto Pajeú Terrane (Borborema Province, NE Brazil): Geophysical and structural data constrain late Neoproterozoic transpression in Western Gondwana

Abstract

The structural framework of the Alto Pajeú Terrane, Borborema Province (NE Brazil) records a complex tectonic evolution related to the Neoproterozoic amalgamation of Western Gondwana. Combined, multiscale geophysical and litho-structural datasets for the Central Borborema Province constrain the regional crustal architecture. Magnetic and radiometric lineaments mostly follow NE-SW and E-W trends that correlate to strike-slip shear zones and associated subvertical mylonitic foliations. Regional, outcrop-scale and microstructural kinematic indicators consistently assign sinistral and dextral motion sense for NE-SW and E-W shear zones, respectively. Four sectors with contrasting litho-geophysical and structural patterns are juxtaposed across major shear zones: the Alto Pajeú, Alto Moxotó and Piancó-Alto Brígida terranes, plus a small portion of the Northern Borborema Province. Expressive magnetic lineaments highlight the boundary between the Alto Pajeú and Alto Moxotó terranes, represented by a pair of sinistral strike-slip shear zones. At outcrop scale, relic thrust surfaces (top-to-NW vergence) with associated flat-lying foliation and down-dip mineral stretching lineations affecting Tonian orthogneisses and metapelites disclose a contractional episode of uncertain age. Transposition of the shallowly-dipping foliation in highly sheared domains imply it is older than the shear-related deformation. The shear zone system and associated structures record the transpressional regime that characterizes the late stages of the Brasiliano/Pan-African Orogeny in the region (ca. 575–565 Ma). Microstructures of a mylonitic orthogneiss from the Juru-Belém shear zone reveal greenschist to amphibolite facies deformational conditions, with increasing importance of subgrain rotation mechanisms at higher strain rates and/or temperature. Microstructural similarities between this rock and tectonites along the southern Patos lineament suggest that bordering and internal regions of the Alto Pajeú terrane were subject to similar thermodynamic conditions during the late Brasiliano transpression.