[Niedermann] Geochemistry of gases in spring waters along the Alpine Fault, South Island of New Zealand
German Title: Geochemistry of gases in spring waters along the Alpine Fault, South Island of New Zealand
Current Status: completed
Main Applicant:Dr. Samuel Niedermann
Prof. Dr. Joerg Erzinger
The Alpine Fault on the South Island of New Zealand is one of the longest, straightest, and fastest-moving transform faults, with oblique-slip at rates that accommodate over half of the Australia-Pacific plate motion and cause rapid uplift of the Southern Alps. No major earthquakes have occurred on the fault in historic time, but it last ruptured around 1717 AD and is thought to fail in large moment magnitude (Mw>7) to possibly great (Mw>8) earthquakes at ~330 yr recurrence intervals. Rocks southeast of the Alpine Fault are exhumed at rates faster than they can cool, resulting in a 63°C/km geothermal gradient, convective circulation and warm springs without related volcanic activity. Because the seismic cycle is fundamentally controlled by fluids, improved knowledge of fluid circulation in the shallow crust is paramount to understanding earthquake processes on the Alpine Fault. In the context of the Deep Fault Drilling Project (DFDP), we have investigated gas compositions and noble gas isotopic abundances in spring waters from the vicinity of the fault. Free gas was sampled at four springs, while nine other springs provided water samples that were degassed in the lab. N2 is commonly the most abundant gas, CO2 concentrations vary widely from <0.1% to >96%, and CH4 contributes up to 13%. He concentrations are up to ~500 ppm. 3He/4He ratios are generally highest (up to 0.81 Ra) close to the fault and decrease to radiogenic values (<0.04 Ra) towards the southeast at distances of a few kilometers from the fault, with two exceptions: Copland spring (0.42 Ra) is located ~11 km from the fault and exhibits the highest gas flow and the highest CO2 content of all springs studied in this work. Kotuku well, the only location available for sampling on the Australian plate, yields a particularly high 3He/4He ratio of ~3.1 Ra despite low gas flow and ~17 km distance from the fault. Obviously, mantle fluids can penetrate the thick crust beneath the Southern Alps directly at the fault and may be diverted away from it only where major passageways through the crust exist. It remains to be seen whether the high 3He/4He ratio of Kotuku indicates a distinct fluid origin on the northwest side of the fault in general or is just a local feature.
Menzies, Catriona D., Teagle, Damon A. H., Niedermann, Samuel, Cox, Simon C., Craw, Dave, Zimmer, Martin, Cooper, Matthew J., Erzinger, Jörg (2016). "The fluid budget of a continental plate boundary fault: Quantification from the Alpine Fault, New Zealand" Earth and Planetary Science Letters 445 p125-135