Incursion of meteoric waters into the ductile regime in an active orogen

Catriona D Menzies,Damon A.H Teagle,Dave Craw,Simon C Cox,Adrian J Boyce,Craig D Barrie,Stephen Roberts

doi:10.1016/j.epsl.2014.04.046

Abstract

Rapid tectonic uplift on the Alpine Fault, New Zealand, elevates topography, regional geothermal gradients, and the depth to the brittle ductile transition, and drives fluid flow that influences deformation and mineralisation within the orogen. Oxygen and hydrogen stable isotopes, fluid inclusion and Fourier Transform Infrared (FT-IR) analyses of quartz from veins which formed at a wide range of depths, temperatures and deformation regimes identify fluid sources and the depth of penetration of meteoric waters. Most veins formed under brittle conditions and with isotope signatures (δ18OH2O = −9.0 to +8.7‰VSMOW and δD=−73 to −45‰VSMOW) indicative of progressively rock-equilibrated meteoric waters. Two generations of quartz veins that post-date mylonitic foliation but endured further ductile deformation, and hence formation below the brittle to ductile transition zone (>6–8 km depth), preserve included hydrothermal fluids with δD values between −84 and −52‰, indicating formation from meteoric waters. FT-IR analyses of these veins show no evidence of structural hydrogen release, precluding this as a source of low δD values. In contrast, the oxygen isotopic signal of these fluids has almost completely equilibrated with host rocks (δ18OH2O = +2.3 to +8.7‰). These data show that meteoric waters dominate the fluid phase in the rocks, and there is no stable isotopic requirement for the presence of metamorphic fluids during the precipitation of ductilely deformed quartz veins. This requires the penetration during orogenesis of meteoric waters into and possibly below the brittle to ductile transition zone.

Highlights

Fluids play a key role in orogenesis through the transport of heat and mass (Bickle and McKenzie, 1987), by changing the rheological behaviour of rocks and localising deformation (Wintsch et al, 1995), and concentrating valuable mineral resources (Weatherley and Henley, 2013)
Oxygen isotopes of vein-forming fluids at deeper levels are indistinguishable from calculated metamorphic fluid signatures, but δD values of fluid inclusions in these minerals identifies them as originally meteoric waters that have equilibrated oxygen with host rocks
A conservative estimate of meteoric water infiltration indicates that the meteoric water flux above the brittle to ductile transition zone is at least two orders of magnitude greater than the potential metamorphic water production from below

Summary

Introduction

Fluids play a key role in orogenesis through the transport of heat and mass (Bickle and McKenzie, 1987), by changing the rheological behaviour of rocks and localising deformation (Wintsch et al, 1995), and concentrating valuable mineral resources (Weatherley and Henley, 2013). In some orogenic belts mantle derived fluids (Kennedy and van Soest, 2007) or fluids liberated from igneous intrusions (Burrows et al, 1986; Reynolds and Lister, 1987) play key roles and have been inferred as carriers of gold in Archean shear zones (Groves, 1993). The high relief of collisional mountain belts provides strong driving forces for the deep penetration of meteoric fluids (Barker et al, 2000; Chamberlain et al, 1995; Koons and Craw, 1991) and the brittle upper crust is expected to be saturated with surface-derived waters. Whether meteoric fluids can penetrate beyond the brittle regime into ductilely deforming rocks remains controversial and conceptually challenging

Results

Discussion

Conclusion