Extensional collapse in the Neoproterozoic Aracuai orogen, eastern Brazil: a setting for reactivation of asymmetric crenulation cleavage

摘要

The Aracuai orogen of eastern Brazil is one of many Brasiliano/Pan African orogens formed during the Neoproterozoic assembly of Gondwana. Its western edge, bordering the Sao Francisco craton, is the Serra do Espinhaco fold-thrust belt, in which top-up-to-the-west (reverse-sense) faults, west-verging folds (F_1), and east-dipping spaced to phyllitic cleavage (S_1) developed. We have found that the kinematics of deformation changes markedly at the hinterland margin of this fold-thrust belt. Here, beneath a plateau known as the Chapada Acaua, metadiamictite and fine-grained pelitic schist comprise an east-dipping belt that contains an assemblage of structures indicative of top-down-to-the-east (normal-sense) shear. This assemblage includes a cascade of F_2 folds that refold F_1 folds and verge down the dip of the belt's enveloping surfaces, vertical tension gashes, and top-down-to-the-east rotated clasts. Based on the presence of these structures, we propose that the plateau exposes a regional-scale normal-sense shear zone, here called the Chapada Acaua shear zone (CASZ). Because F_2 folds refold F_1 folds, normal-sense shear in the CASZ occurred subsequent to initial west-verging thrusting. Considering this timing of motion in the CASZ, we suggest that the zone accommodated displacement of the internal zone of the Arac,uai orogen down, relative to its foreland fold-thrust belt, and thus played a role in extensional collapse of the orogen. The CASZ trends parallel to preserved thrusts to the west, and thus may represent an inverted thrust fault. Notably, throughout the CASZ, S_1 schistosity has been overprinted by a pervasive, west-dipping asymmetric crenulation cleavage (S_2). The sigmoid shape of S_1 surfaces in S_2 microlithons require that slip on each S_2 surface was top-down-to-the-west. S_2 cleavage is axial-planar to the down-dip verging F_2 folds. Based on its geometry, we suggest that S_2 cleavage initiated either as an antithetic extensional crenulation cleavage during reverse-sense shear, or as a near vertical asymmetric crenulation cleavage formed during east-west shortening of a preexisting east-dipping schistosity. Subsequent normal-sense shear in the CASZ reactivated this cleavage, causing clockwise rotation of S_2 domains (as viewed looking along-strike to the north), in a manner similar to that of rotational 'bookshelf faults'. Such movement could have accommodated concomitant vertical flattening of the CASZ during extensional collapse. aracuai orogen; crenulation cleavage; brasiliano orogeny; orogenic collapse; shear zone; crustal extension;