Extensional collapse in the Neoproterozoic Arac ¸uaı ´ orogen, eastern Brazil: a setting for reactivation of asymmetric crenulation cleavage Stephen Marshak a, * , Fernando F. Alkmim b , Alan Whittington c , Anto ˆnio Carlos Pedrosa-Soares d a Department of Geology, University of Illinois, Urbana, IL 61801, USA b Departamento de Geologia, Universidade Federal de Ouro Preto, Ouro Preto, MG 35400, Brazil c Department of Geology, University of Missouri, Columbia, MO 65211, USA d CPMTC-Instituto de Geocie ˆncias, Universidade Federal de Minas Gerais, Belo Horizonte, MG 31279, Brazil Received 16 July 2004; received in revised form 19 July 2005; accepted 21 September 2005 Available online 4 November 2005 Abstract The Arac ¸uaı ´ orogen of eastern Brazil is one of many Brasiliano/Pan African orogens formed during the Neoproterozoic assembly of Gondwana. Its western edge, bordering the Sa ˜o Francisco craton, is the Serra do Espinhac ¸o 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 ¸uaı ´ 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. q 2005 Elsevier Ltd. All rights reserved. Keywords: Arac ¸uaı ´ orogen; Crenulation cleavage; Brasiliano orogeny; Orogenic collapse; Shear zone; Crustal extension; Neoproterozoic; Extensional crenulation cleavage; Espinhac ¸o fold-thrust belt; Macau ´bas basin 1. Introduction When continental crust thickens during convergent or collisional orogeny, gravitational loading may trigger extensional collapse (or ‘orogenic collapse’) above a normal-sense shear zone (or ‘extensional detachment’). Extensional collapse has been linked to the rise of post- tectonic granites and to the growth of late- to post-tectonic sedimentary basins in the interior of orogens (Dewey, 1977; Burchfiel and Royden, 1985; Norton, 1986; Me ´nard and Molnar, 1988; Platt and Vissers, 1989; Malavieille et al., 1990; Malavieille and Taboada, 1991; Burchfiel et al., 1992; Malavieille, 1993; Rey et al., 2001). Work by Pedrosa- Soares et al. (2001), among others, demonstrates that the Journal of Structural Geology 28 (2006) 129–147 www.elsevier.com/locate/jsg 0191-8141/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2005.09.006 * Corresponding author. E-mail address: smarshak@uiuc.edu (S. Marshak).