Fluid generation and pathways beneath an active compressional orogen, the New Zealand Southern Alps, inferred from magnetotelluric data

Abstract

Forty‐one wideband magnetotelluric (MT) soundings were collected in a 150‐km‐long transect across the Southern Alps of the central South Island of New Zealand, an active compressional orogen. Decomposed MT impedance tensors, vertical magnetic field relations, and reconnaissance soundings at two locations off line imply an approximately two‐dimensional geometry here with average regional geoelectric strike of ∼N40°E, similar to surface geologic trends. Two independent, two‐dimensional inversion algorithms were applied to the MT data, and both imply a concave‐upward (U‐shaped), middle to lower crustal conductive zone beneath the west central portion of the island. The average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along‐strike conductive strands narrower than the transverse magnetic (cross‐strike) mode MT data can resolve. The deep crustal conductor under the Southern Alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust. Fluid interconnection and electrical conduction are promoted by shear deformation. The conductor rises northwestward toward the trace of the Alpine Fault but attains a near‐vertical configuration at a depth of ∼10 km and reaches close to the surface 5–10 km inland of the fault trace itself. The transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle‐ductile transition in uplifting schist and approach the surface through induced hydrofractures. The high‐grade schist becomes resistive after depletion of fluids and continues to extrude toward the Alpine Fault. Shallow extensions of the deep high conductivity are coincident with modern, hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance. To the southeast, high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent Southern Alps orogen, which also exhibits evidence for expulsion of high‐temperature fluids. The higher conductivity inferred along strike (possible anisotropy) could reflect more efficient fluid interconnection in this higher‐strain direction, as well as possible contributions by sheared, fluid‐deposited graphite. Conductivity of the uppermost mantle of the South Island is low, consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by P wave delay studies.