Laminar flow in shear zones: the Pernambuco Shear Zone, NE-Brazil

Abstract

The Pernambuco Shear Zone (PSZ) is a 700 km long branch of one of the largest and best-exposed Precambrian strike-slip shear zone systems in the world. The shear zones are part of the eroded record of a large-scale collision zone. Foliation and lineation orientations along the shear zone indicate that local deformation perturbations have produced variable fabric orientations, with up to 40 ° change in dip and strike. However, the dominant flow pattern is consistently horizontal strike-slip across a vertical shear plane. Well-developed dextral shear sense criteria are observed at most localities, although oblate strain fabrics are common and lineations are frequently absent. Very few small folds were found to refold the shear fabric developed in Late Precambrian granites which intrude along most of the shear zone, but sheath folds are common in mylonitized pre-shear gneisses. This geometrical evidence indicates that steady laminar shear flow took place in the initially isotropic granites, and more irregular, but not turbulent, flow preferentially developed in rocks with a pre-existing fabric. The deformation pattern is similar to many other deep crustal shear zones. Laminar shear flow is thought to be common in granites and coarse-crystalline gneisses deformed in upper-amphibolite to granulite facies conditions. The dramatic weakening of the middle crust produced by syn-shear injection of granites and mafic intrusions into vertical shear zones may cause the predominant movement pattern in collision zones to be strike-slip lateral extrusion, rather than compressional thrusting.