THRUST SYSTEMS IN TURBIDITE-DOMINATED ACCRETIONARY OROGENS: LACHLAN OROGEN, AUSTRALIA AS EXAMPLE

Thrust systems within turbidite-dominated accretionary orogens occur within turbidite, chert and oceanic crust (meta-basalt, gabbro, ultramafics) successions and have leading-imbricate fan geometry, with oceanic crust duplexes exhumed along the leading and frontal faults. Strain gradients and overall shortening (~60-70% over 800-1000km length scales) necessitate decoupling within the oceanic stratigraphy consisting of a turbidite blanket over oceanic basement. Major faults or décollements occur at the turbidite-metabasalt contact and within the oceanic crust near the Moho transition. The leading, frontal faults contain 'Franciscan-like' blueschist blocks within both mud-matrix and serpentinite-matrix mélange and are up to 5km in width. Estimated pressures are generally less than 7kb. Background metamorphism in the deformed turbidite wedges is greenschist to subgreenschist facies. Illite crystallinity and white mica bo values from metamorphic phengites in the turbidites reflect intermediate pressure and a low geothermal gradient. Structural thickening of turbidite sediment wedges is via chevron-folding and thrusting, causing exhumation, subsequent erosion (sediment cannibilsation) with foredeep deposition ahead of the migrating thrust system. Fault zone style in the turbidites is a polydeformed high strain zone up to 1.5km in width above a brittle fault, with transposition layering, crenulation cleavages, and high non-coaxial strains (X/Z>100:1). Thrust sheets are fold-dominated (chevron-style), have one dominant upright cleavage and background X/Z strains of 5:1. Thrust system geometry requires underthrusting, with overall piggy-back thrusting in the overlying sediment wedge. Diachronous deformation in the wedge migrates in the direction of thrust transport. Time-averaged fault-propagation rates are ~6mm/yr, based on Ar-Ar geochronology of fault zone fabrics (Western Lachlan Orogen) assuming continuous deformation. Accretionary orogens commonly consist of leading-imbricate fan thrust systems that can be linked to underthrusting (subduction) in oceanic environments. The process involves structural thickening of the sediment pile and oceanic lithosphere, causing closure of sediment-filled marginal basins and eventual cratonisation of these oceanic materials.