scholarly journals Insights into the Rheology of Rocks under Brittle-Ductile  Deformation Conditions from an Exhumed Shear Array in  the Southern Alps, New Zealand

2021 ◽  
Author(s):  
◽  
Susanne Grigull
Keyword(s):  
New Zealand ◽  
Optical Microscopy ◽  
Water Content ◽  
Southern Alps ◽  
Quartz Veins ◽  
Scaling Relationships ◽  
Alpine Fault ◽  
Flow Law ◽  
Ductile Shear ◽  
D Values

<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>

scholarly journals Insights into the Rheology of Rocks under Brittle-Ductile  Deformation Conditions from an Exhumed Shear Array in  the Southern Alps, New Zealand

2021 ◽  
Author(s):  
◽  
Susanne Grigull
Keyword(s):  
New Zealand ◽  
Optical Microscopy ◽  
Water Content ◽  
Southern Alps ◽  
Quartz Veins ◽  
Scaling Relationships ◽  
Alpine Fault ◽  
Flow Law ◽  
Ductile Shear ◽  
D Values

<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>

Erosion rates of the New Zealand Southern Alps reflect long-term tectonics and transient climate

2021 ◽  
Author(s):  
Duna Roda-Boluda ◽  
Taylor Schildgen ◽  
Hella Wittmann-Oelze ◽  
Stefanie Tofelde ◽  
Aaron Bufe ◽  
...  
Keyword(s):  
New Zealand ◽  
Strike Slip ◽  
Southern Alps ◽  
Fault System ◽  
Tectonic Deformation ◽  
Seismic Cycle ◽  
Erosion Rates ◽  
Alpine Fault ◽  
Oblique Convergence

&lt;p&gt;The Southern Alps of New Zealand are the expression of the oblique convergence between the Pacific and Australian plates, which move at a relative velocity of nearly 40 mm/yr. This convergence is accommodated by the range-bounding Alpine Fault, with a strike-slip component of ~30-40 mm/yr, and a shortening component normal to the fault of ~8-10 mm/yr. While strike-slip rates seem to be fairly constant along the Alpine Fault, throw rates appear to vary considerably, and whether the locus of maximum exhumation is located near the fault, at the main drainage divide, or part-way between, is still debated. These uncertainties stem from very limited data characterizing vertical deformation rates along and across the Southern Alps. Thermochronology has constrained the Southern Alps exhumation history since the Miocene, but Quaternary exhumation is hard to resolve precisely due to the very high exhumation rates. Likewise, GPS surveys estimate a vertical uplift of ~5 mm/yr, but integrate only over ~10 yr timescales and are restricted to one transect across the range.&lt;/p&gt;&lt;p&gt;To obtain insights into the Quaternary distribution and rates of exhumation of the western Southern Alps, we use new &lt;sup&gt;10&lt;/sup&gt;Be catchment-averaged erosion rates from 20 catchments along the western side of the range. Catchment-averaged erosion rates span an order of magnitude, between ~0.8 and &gt;10 mm/yr, but we find that erosion rates of &gt;10 mm/yr, a value often quoted in the literature as representative for the entire range, are very localized. Moreover, erosion rates decrease sharply north of the intersection with the Marlborough Fault System, suggesting substantial slip partitioning. These &lt;sup&gt;10&lt;/sup&gt;Be catchment-averaged erosion rates integrate, on average, over the last ~300 yrs. Considering that the last earthquake on the Alpine Fault was in 1717, these rates are representative of inter-seismic erosion. Lake sedimentation rates and coseismic landslide modelling suggest that long-term (~10&lt;sup&gt;3&lt;/sup&gt; yrs) erosion rates over a full seismic cycle could be ~40% greater than our inter-seismic erosion rates. If we assume steady state topography, such a scaling of our &lt;sup&gt;10&lt;/sup&gt;Be erosion rate estimates can be used to estimate rock uplift rates in the Southern Alps. Finally, we find that erosion, and hence potentially exhumation, does not seem to be localized at a particular distance from the fault, as some tectonic and provenance studies have suggested. Instead, we find that superimposed on the primary tectonic control, there is an elevation/temperature control on erosion rates, which is probably transient and related to frost-cracking and glacial retreat.&lt;/p&gt;&lt;p&gt;Our results highlight the potential for &lt;sup&gt;10&lt;/sup&gt;Be catchment-averaged erosion rates to provide insights into the magnitude and distribution of tectonic deformation rates, and the limitations that arise from transient erosion controls related to the seismic cycle and climate-modulated surface processes.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

scholarly journals Coseismic landsliding estimates for an Alpine Fault earthquake and the consequences for erosion of the Southern Alps, New Zealand

Geomorphology ◽  
2016 ◽  
Vol 263 ◽  
pp. 71-86 ◽  
Author(s):  
T.R. Robinson ◽  
T.R.H. Davies ◽  
T.M. Wilson ◽  
C. Orchiston
Keyword(s):  
New Zealand ◽  
Southern Alps ◽  
Alpine Fault

Variations in exhumation level and uplift rate along the obliqu-slip Alpine fault, central Southern Alps, New Zealand

2005 ◽  
Vol 117 (5) ◽  
pp. 707 ◽  
Author(s):  
Timothy A. Little ◽  
Simon Cox ◽  
Julie K. Vry ◽  
Geoffrey Batt
Keyword(s):  
New Zealand ◽  
Southern Alps ◽  
Uplift Rate ◽  
Alpine Fault

scholarly journals Metamorphism and the P-T History of Alpine Schist from the Newton Range, Southern Alps, New Zealand

2021 ◽  
Author(s):  
◽  
Dave B Murphy
Keyword(s):  
New Zealand ◽  
Southern Alps ◽  
Greenschist Facies ◽  
Garnet Growth ◽  
Compositional Zoning ◽  
Tectonic Events ◽  
Alpine Fault ◽  
Mylonite Zone ◽  
Mineral Assemblages ◽  
T Path

<p>Metamorphic rocks have the potential to record in their mineral assemblages, mineral compositional zoning, and textures, information about geological changes and processes that occur during tectonic events. Interpretations of metamorphic pressure-temperature (P-T) records have traditionally relied on results of geothermobarometry studies, but that approach is not suitable in every case. Metamorphosed greywacke, which makes up ~95% of the New Zealand Southern Alps, has long proven problematic for traditional geothermobarometry because it develops intractable mineral compositions and/or assemblages, especially at relatively low temperature (greenschist facies) conditions. An alternative forward modelling approach using the computer program THERMOCALC was recently used to extract the first detailed P-T history (P-T path) from such previously intractably difficult "greyschist" rocks from a single site in the New Zealand Southern Alps. The present study is the first attempt to apply those new methods to rocks from another study area, and is the first detailed geological study of the Newton Range in the New Zealand Southern Alps. The Newton Range is a ~15 km-long, east-west trending range located ~30 km southeast of the town of Hokitika, ~110 km northeast of the Franz Josef-Fox Glacier region, and immediately to the east of the Alpine Fault in the Southern Alps, South Island, New Zealand. The rocks in the Newton Range are mainly derived from Torlesse Terrane accretionary prism greywacke and argillite (Alpine Schist, greyschist), together with a large pods of ultramafic rock (part of the Pounamu Ultramafic Belt (PUB)) and minor associated metabasic layers (greenschist), all metamorphosed to greenschist facies conditions. The dominant mineral assemblage in the greyschist (Qtz + Ms+ Bt ± Chl ± Ep ± Pl ± Ilm ± Ttn ± Grt ± Zrn ± Tur ± Ap ± Cal), much like that found elsewhere in the Southern Alps. As elsewhere in the Southern Alps, the dominant high-grade metamorphic mineral assemblages in the Alpine Schist in the Newton Range are inherited. The mineral assemblages, compositions, and some textures thus record evidence of processes that took place during tectonic events, presumably mainly in Cretaceous time, prior to the formation of the modern Southern Alps, which are forming today by the ongoing oblique continent-continent collision of the Pacific Plate against the Australian Plate at the Alpine Fault. Compositional zoning in garnet from the greyschist is an important record of the metamorphic P-T path traversed by the host rock as the garnet grew. Occasionally, garnet from the study area contains an inmost core (stage 0) of unusual (anomalously high- or low-MnO) composition. The cores with extremely low MnO are possibly detrital in origin, and those with extremely high MnO may perhaps have grown in the early tectonic episode that formed the Otago Schist. Typically, garnet shows the following core- to rim zoning sequence. Stages 1 & 2 show a progressive decrease in MnO and increase in FeO from core to rim, with higher MnO cores present in rocks with higher whole-rock MnO compositions. Stage 3 is characterised by a gradual decrease in CaO and signifies the growth of Ca-bearing oligoclase late in the garnet growth history. Stage 4 is a discontinuous overgrowth characterised by an abrupt increase in CaO. Such overgrowths have in the past been attributed to garnet growth accompanying the development of the Alpine Fault mylonite zone in the late Cenozoic. In the Newton Range they were only observed on garnet adjacent to the main outcrop of the PUB at ~4.5km from the Alpine Fault, far from the mylonite zone, so local element availability during decompression (and possibly fluid flow and/or metasomatism) may have played a part in the growth of these rims. A P-T path for Alpine Schist from the Newton Range has been estimated using detailed garnet composition data measured along core-to-rim transects across individual garnets, together with predicted garnet compositions and P-T pseudosection results calculated using THERMOCALC. The P-T path starts at ~3.5kbar/400°C, where both garnet and albite coexist, and increases in pressure and temperature to ~6.5bar/500°C where garnet coexists with both albite and oligoclase. The estimated peak metamorphic conditions probably correspond to peak metamorphic pressures, unlike in the Franz Josef-Fox Glacier region where peak conditions (~9.2kbar and 620°C) probably coincided with peak metamorphic temperatures.</p>

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