Thrust systems in the Tamworth Zone, southern New England Orogen, New South Wales

Abstract

The Tamworth Zone thrusts and folds have been mapped and described since the 1930s. They form the western margin of the New England Orogen in New South Wales. Understanding their origin and evolution is key evidence in tying the Late Palaeozoic New England Orogen to Australia. Because the northern Tamworth Zone has such well‐preserved thrust geometries, it may serve as a useful model for the rest of the New England Orogen, and for other arcward‐vergent thrust complexes in marine settings. The Mooki thrust system is typified by large‐amplitude folds that are inferred to be of a ramp‐flat origin. The Mooki thrust has some 45 km of displacement. The northern Campo Santo thrust system has a leading edge marked by a braided imbricate fault pattern where a duplex zone is inferred. The main Campo Santo thrust is breached to form the Barraba window which demonstrates that the Campo Santo thrust system has at least 26 km of displacement. There is additional shortening across the Tamworth Zone on the Namoi and Peel Faults, and beneath the Mooki thrust within the Sydney‐Gunnedah‐Bowen Basin as documented by BMR seismic line 91.G01. These structures add perhaps an additional 10 km of shortening across the western New England Orogen. The Triassic Tamworth Zone thrust shortening obducts the older Peel Fault, the Tablelands Complex and the New England Batholith 75–85 km westward above a mid‐crustal, east‐dipping, décollement surface. I suggest that the west‐vergent thrusts of the Tamworth Thrust Belt may be connected to the east‐vergent faults beneath the outboard accreted terrains of the New England Orogen, and thus the entire orogen was obducted 75–85 km onto the Australian margin in the Triassic. Preliminary interpretations of the BMR91.G01 seismic line across the area have suggested that the Peel Fault may represent a major west‐dipping fault that truncates the Tamworth thrusts at depth. If the suggested west‐dipping crustal fault and Peel Fault predate the major westward thrusting, they cannot be connected to their roots in the lower crust. If the suggested fault is continuous to depth, it must be post‐thrusting, and it does not alter the thrust interpretations presented here, which are preserved across it on both sides.