Insights into the Rheology of Rocks under Brittle-Ductile Deformation Conditions from an Exhumed Shear Array in the Southern Alps, New Zealand
Authors:   , Grigull Susanne 
Published: November 2021

<p>A suite of brittle-ductile faults in the central Southern Alps, New Zealand is used as a natural laboratory into the rheology of quartz rocks. The fault array is ~2 km wide and formed in the hanging-wall of the SE-dipping Alpine Fault during the late Cenozoic at >= 25 km depth. It was exhumed in the past few Myr and is now exposed 5-7 km east of the Alpine Fault. The faults are near-vertical, extend laterally and vertically over tens of metres, and strike sub-parallel to the Alpine Fault. They displace quartzofeldspathic Alpine Schist (metagreywacke) in a predominantly brittle way. The faults impinge upon and displace abundant centimetre-thick quartz veins that are discordant to the dominant schist foliation. These quartz veins exhibit a full range of slip from fully brittle to fully ductile. In most quartz veins, a ductile component of slip and a 1-3 cm (n=72) wide ductile shear zone are present. The mean total slip measured in the veins is (7.2 +/- 5.8 ) cm (n=72).  This study first develops a method to determine the true shape and displacement of a geological marker from any outcrop orientation. It then uses a set of geometrical scaling relationships exhibited by the ductilely-to-brittlely sheared quartz veins, and the observed interaction between brittle faults and ductilely deforming quartz veins to develop a series of finite-element models that reproduce the field observations. A flow law of the form de/dt = A*(f_H2O)^m*(sig_d)^n*exp(-Q/(R*T)) is used to model the behaviour of the quartz veins. Flow law parameters for the quartz veins and viscous and frictional strength ratios between quartz and schist are determined from these models. For Q = 135 kJ mol^-1, f_H2O= 200 MPa and m = 1.0, the results show that the scaling relationships in the quartz veins are successfully reproduced for A = 10^(-10 +/- 2) MPa^-n s^-1, and n = 4.  The ratio between ductile-to-total slip (D) were measured for 72 veins throughout the brittle-ductile shear array and are highly variable. In order to understand what has led to this variability, we investigate the following parameters: original vein thickness, deformation temperature, water content, microfracturing, calcite fraction, and total slip. D-ratios appear to scale with original vein thickness, however, significant scattering of the D-values indicates that other factors also control D. The temperature resolution (from Titanium-in-Quartz geothermometry and oxygen isotopy) is not high enough to determine whether temperature influenced the D-values. Fourier Transform Infrared Spectroscopy (FTIR), and optical microscopy reveal that water content, microfracturing, and calcite fraction were very similar from one vein to another and therefore did not control the D-ratios either. Detailed outcrop maps of the brittle-ductile shears and displacement-length profiles along five individual faults indicate that the total slip varied rapidly and on short distances (cm- to m-scale) along the faults. We infer that these varying slip rates led to different flow strain rates in the deforming quartz veins and therefore can explain the variations in D-values.  Optical microscopy reveals abundant fluid inclusions in both the deformed and undeformed parts of the veins. These inclusions indicate that the quartz was ‘wet’ and the veins were weakened with respect to the surrounding schist. We therefore infer that the location of the shear zones was predetermined by the position of the brittle faults propagating through the stronger schist and impinging on the weaker quartz veins.</p>