FORMATION OF ASYMMETRIC CRENULATION CLEAVAGE AS A MECHANISM TO ACCOMMODATE OROGENIC COLLAPSE: EXAMPLES FROM EASTERN BRAZIL

The Araçuaí orogen of eastern Brazil is one of many Brasiliano/Pan African orogens formed during the Neoproterozoic assembly of Gondwana. Its western edge, bordering the São Francisco craton, is the Serra do Espinhaço fold-thrust belt, in which top-up-to-the-W (reverse-sense) faults, W-verging folds (F1), and E-dipping schistosity (S1) developed. Along the hinterland margin of the fold-thrust belt, beneath a plateau known as the Chapada Acauã, fine-grained pelitic schist and metadiamictite occur in a gently ESE-dipping belt that contains an assemblage of structures indicative of top-down-to-the-ESE (normal-sense) movement. This assemblage includes F2 folds that verge ESE, down the dip of the belt's enveloping surfaces, vertical tension gashes, and rotated clasts. We propose that the plateau exposes a regional-scale normal-sense shear zone, here called the Chapada Acauã shear zone (CASZ). Considering that the CASZ accommodated post-thrusting normal-sense displacement of the internal zone of the Araçuaí orogen with respect to its external zone, the CASZ is an extensional detachment associated with orogenic collapse. In contrast to other examples of regional-scale extensional detachments (e.g., the South Tibet detachment), the CASZ at its present level of exposure does not contain mylonitic foliation. Rather, the zone contains a pervasive asymmetric extensional crenulation cleavage (S2) that is axial-planar to down-dip verging F2 folds. S2 surfaces are west-dipping (antithetic to the enveloping surfaces of the CASZ) and bound microlithons containing sigmoidal S1 surfaces. Sigmoid shapes in microlithons require that slip on each S2 surface was top-down-to-the-west. This slip permitted clockwise rotation of the S2 surfaces (as viewed looking NNE, along strike) in a manner similar to rotational bookshelf faults and, therefore, accommodated down-dip (normal-sense) in the CASZ. This example implies that formation of asymmetric extensional crenulation cleavage can play an active role in accommodating displacement in normal-sense shear zones, can form in response to gravitational loading, and can contribute to regional vertical thinning and horizontal stretching during orogenic collapse.