LITHOSPHERIC SHEAR ZONES AND MANTLE-CRUST CONNECTIONS

A crustal-scale ductile shear zone network in the Precambrian granulit e-facies crust of Madagascar is examined to determine the nature of th e connections between the mantle and lower crust. Based on three indep endent data sets - field and satellite mapping, C- and O-isotope geoch emistry and gravimetry - this crust is divided into three zones: (1) o utside 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 100 0 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 in ferred to be controlled by the mantle; they directly tapped mantle-der ived CO2. The small-scale minor shear zones were controlled by crustal processes and focused crustally derived H2O-rich+/-CO2 fluids. The re gular distribution of the shear zones on a crustal scale is in agreeme nt with models of buckling of the continental lithosphere in a compres sional context. The propagation of these mechanical instabilities prom oted and channelled fluid flow. These major Pan-African shear zones th inned the crust and were reactivated during the subsequent drifting of Madagascar and opening of the Indian Ocean during Jurassic to Cretace ous times. They also controlled many of the brittle fault zones in the overlying sedimentary basins. Mantle rooted large-scale shear zones a re inferred to be a general feature of cratonic areas reactivated by s hear zone systems.