Lithospheric shear zones and mantle-crust connections

Abstract

A crustal-scale ductile shear zone network in the Precambrian granulite-facies crust of Madagascar is examined to determine the nature of the connections between the mantle and lower crust. Based on three independent data sets — field and satellite mapping, C- and O-isotope geochemistry and gravimetry — this crust is divided into three zones: (1) outside of shear zones; (2) minor shear zones that are <140 km long and 7 km wide; and (3) major shear zones that are >350 km long (up to 1000 km) and 20–35 km wide. The mantle is uplifted by about 10 km beneath the major shear zones. The major shear zones are rooted in and are inferred to be controlled by the mantle; they directly tapped mantle-derived CO 2. The small-scale minor shear zones were controlled by crustal processes and focused crustally derived H 2O-rich±CO 2 fluids. The regular distribution of the shear zones on a crustal scale is in agreement with models of buckling of the continental lithosphere in a compressional context. The propagation of these mechanical instabilities promoted and channelled fluid flow. These major Pan-African shear zones thinned the crust and were reactivated during the subsequent drifting of Madagascar and opening of the Indian Ocean during Jurassic to Cretaceous times. They also controlled many of the brittle fault zones in the overlying sedimentary basins. Mantle-rooted large-scale shear zones are inferred to be a general feature of cratonic areas reactivated by shear zone systems.