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

New Zealand's Southern Alps are the surface expression of recent (c.5 Ma) o blique convergence between the Australian and Pacific plates. A broad, 'U s haped', zone of high electrical conductivity (maximum depth c.25 km) underl ies 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 fl uid marking a decollement zone above the root of the orogen. Wear vertical zones of high conductivity extend upwards to the surface on both sides of t he conductive zone. On the outboard side of the orogen, which is a broad zo ne of thrust faulting and folding, the conductive zone reaches the surface at the trace of an active fault. Carbon and oxygen isotopic analysis of mat erial from the faults suggests that mid-crustal fluids have been present. M id-crustal fluid appears to have been transported upwards in both the ducti le and brittle parts of the crust within the 'backthrust' region of the oro gen. (C) 2000 Elsevier Science B.V. All rights reserved.