T63 Focus on IGCP: Modern and Ancient Plate Boundaries and Orogens I: In Memory of Chris McA. Powell

Tuesday, 6 November 2001
8:00 AM - 12:00 PM

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

Paper number: T63-9
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

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.