Fluids in a backthrust regime (Southern Alps, New Zealand)

Abstract

New Zealand's Southern Alps are the surface expression of recent (c. 5Ma) oblique convergence between the Australian and Pacific plates. A broad, ‘U shaped’, zone of high electrical conductivity (maximum depth c. 25km) underlies this orogen. In the ductile region of the crust, the high conductivity zone is sub-horizontal and is interpreted to be a zone of interconnected fluid marking a decollement zone above the root of the orogen. Near vertical zones of high conductivity extend upwards to the surface on both sides of the conductive zone. On the outboard side of the orogen, which is a broad zone of thrust faulting and folding, the conductive zone reaches the surface at the trace of an active fault. Carbon and oxygen isotopic analysis of material from the faults suggests that mid-crustal fluids have been present. Mid-crustal fluid appears to have been transported upwards in both the ductile and brittle parts of the crust within the ‘backthrust’ region of the orogen.