EXTENSIONAL COLLAPSE OF THE NEOPROTEROZOIC ARAÇUAÍ OROGEN, BRAZIL: THE ROLE OF REGIONAL CRENULATION CLEAVAGE

Orogenic collapse is the process of regional crustal extension, driven by gravity, that occurs during or soon after convergent or collisional orogeny. We have identified a belt of normal-sense shear accommodating Neoproterozoic extensional collapse in the Brasiliano (=Pan African) Araçuaí orogen, on the east side of the São Francisco craton in eastern Brazil. This north-south-trending zone, here called the Chapada Acauã zone, is 15 – 45 km wide, and dips, on average 25° east. It involves phyllitic diamictites and phyllites containing outstanding kinematic indicators. The Chapada Acauã zone lies east of the Espinhaço fold-thrust belt, in which strata were thrust westwards onto the São Francisco craton, and west of a belt of high-grade metamorphic rocks (that also contain west-verging structures) and a belt of granitic intrusions. In many examples worldwide (e.g., the South Tibetan detachment), collapse-related normal-sense shear zones contain thick mylonites or brittle faults that dip toward the collapse region. In contrast, the normal-sense (down-to-the-east) shear of the Chapada Acauã zone occurred primarily by the development of an antithetic (west-dipping) penetrative extensional crenulation cleavage. This cleavage (S2), crenulates the S1 schistosity that had formed as an axial-planar foliation of folds in a preexisting west-verging thrust system. S2 cleavage is itself axial-planar to a set of east-verging folds contained within east-dipping enveloping surfaces. Rotated stretched pebbles of the diamicton, and en echelon extension gashes, also indicate regional normal-sense displacement in the zone. Structural relations in the Chapada Acauã zone emphasize that gravitational loading can generate a regional penetrative asymmetric crenulation and related folds, and that the formation of such crenulation can accommodate regionally significant normal-sense shear during orogenic collapse.