Controls on Strain Partitioning in the White Horse Creek Mylonite, West Coast, New Zealand Michelle Gavel

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1 Controls on Strain Partitioning in the White Horse Creek Mylonite, West Coast, New Zealand Michelle Gavel Abstract The White Horse Creek Mylonites (WHC) are part of the Paparoa Metamorphic Complex on the Northern West Coast of New Zealand. This unit represents a ductile shear zone underneath the Pike Detachment, a low angle, normal fault mechanism formed as a result of the rising core complex. As early as Ma, these mylonites were uplifted to the surface, where they remain exposed. (Schulte 2014) The WHC is a granitic mylonite, characterized by abundance of quartz and feldspar. Although these mylonites have been studied as part of an ongoing investigation of the PCC, little has been done to understand the controls on small scale strain partitioning in the area. In this ongoing study, petrographic analysis and some basic statistical calculations were applied to microstructures in the WHC in an attempt to understand the effects of phyllosilicate availability and location in the protolith on strain variation. Unique ductile textures exhibited by quartz and brittle kinking and fracturing in feldspar, along with increasing size in clast area with decreased deformation are exciting preliminary results that require further analysis by further textural and microscopic analysis Introduction The Paparoa Metamorphic Complex (PCC) (my location figure) is located on the West Coast of New Zealand (Figure 1). Structures and lithologic units associated with the PCC have been interpreted to represent the breaking away of New Zealand from Gondwana. (Tulloch 1989) The PCC consists of two detachments, the top-to-the-southwest Pike

2 Detachment and the top-to-the-northeast Ohika Detachment (Schulte 2014). The White Horse Creek Mylonite (WHC) constitutes a ductile shear zone underneath the Pike Detachment (Tulloch 1989). The WHC are interpreted to have formed at temperatures around 500 degrees Celsius due to mineral structures present in plagioclase grains. (Schulte 2014) They exhibit non-continuous distribution of deformation grade, which is due in part to strain partitioning. (Figure 2) Although the WHC has been studied as part of the PCC, small scale rheology has not been closely examined. Through petrographic microscope work, we sought to examine the controls on small-scale strain variation in the WHC Background Rheologic Understanding Strain partitioning occurs when materials strain differently in response to the same stress. In this study, the controlling stress is the large-scale normal faulting involved with the formation of the Pike Detachment around 100Ma (Schulte 2014). To understand why strain partitioning occurs in nature, Handy proposed that crystal structure and mineral characteristics of grains that compose a crystalline rock would deform in three broad categories: 1) Where grains of similar size all bear the deformation in a single framework, 2) where two weak minerals dominate the composition and produce boudins in a finer matrix, and 3) where one weak mineral and one strong mineral deform at different rates. (Figure 3) It has been predicted that there would be sharp contrast between case 1 and cases 2 and 3, because the transition between a framework supported structure (i.e., where a strong aggregate forms a load bearing framework, interspersed by a weaker mineral) and a matrix supported structure is very sharp. However, cases 2 and 3 are likely to overlap, and could

3 possibly coexist depending on ductility and overall strength of minerals. The most important evidence for these cases comes from microstructural observations made about foliation formation, mineral structure collapse, and recrystallization. These microstructural processes are among the main causes of strain partition, and have a large part in controlling how deformation will occur. On a broader scale, each category described can be related to specific conditions in the lithosphere. Rocks with framework dominated structures, as described in case 1, are more likely to be found at lower crust or upper mantle depths (such as a pyroxenite). Rocks that exhibit matrix dominated rheology (cases 2 or 3), where specific active minerals are controlling the degree of deformation, are most likely to form in an ultramylonitic zone in the crust. (Handy 1990) Textures in Mylonitic Rocks Simpson (1985) provides an excellent description of differing textures of quartz, feldspars, and biotite in granitic mylonites compared to the metamorphic facies each was exposed to before being deformed. Recrystallized quartz grains increase in size with increase in metamorphic grade. In many cases, quartz grains also exhibit preferred c-axis orientation very closely in line with foliation, (Hippertt 1998). Measurements of grain resizing can be done in thin section using the line intercept method, so long as the measurements are corrected for 2-D truncation. Biotite experiences complete recrystallization at high grade facies (above amphibolite) to form finer grained bands. Depending on the grade of metamorphism, plagioclase will display textures ranging from cataclasis (lower grade) to low level kink band formation. This further confirms the notion that under similar, low-grade metamorphic conditions, quartz will behave ductily, and

4 feldspar will likely experience cataclasis simultaneously when deformation occurs at or near the brittle-ductile transition zone. Pressure and temperature conditions in this area of the crust are known to be favourable for engendering strain partitioning. Another important area to focus on when studying strain partitioning in mylonitic rock are the processes related to the mylonite/ultramylonite zone. These controls are also interpreted from microstructural observations. Most ultramylonites exist as millimetre scale bands, and are largely devoid of porphyroclasts. The transition between these bands of ultramylonite and mylonite are typically very abrupt. Making Large Scale Interpretations from Small Scale Observations Hippertt (1998) also describes a granitic mylonite very similar to the WHC is used to examine the boundary relationship between these two textures, in part using scanning electron microscope (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS). Under the microscope and in hand sample, different compositional bands are visible, which means there are different rheological properties present. The transition zone was characterized by microfolds that appear to mimic shear sense. From textural and structural observations of mica, feldspars, and quartz, it was hypothesized that these folds occur during 'kinks' or pauses in the deformation process. These kinks occur when quartz begins to recrystallize to form ribbons at the beginning of mylonitization. From mylonite to ultramylonite, the structural change is best marked by the point where plagioclase begins to recrystallize to mica due to softening. The difference between these kinks is that the first is dominated by crystal-plastic processes, while in the latter solution transfer reactions are predominant. Petrographic microscopy paves a path towards critical observations about the behaviour of feldspars, quartz, plagioclase, and micas in mylonitic rocks. As illustrated above, observations about these minerals, their structural characteristics, and their relative

5 abundances are very important for determining deformation conditions and timelines. For example, examining the size of recrystallized quartz grains could help determine the metamorphic grade at which mylonitization occurred, and to distinguish recrystallization boundaries from grain boundaries. Overall textural analysis will help determine how minerals are arranged, which can further understanding of the lithospheric setting of mylonitization in the WHC Methods Oriented Thin Sections Using the methods outlined by Hansen (1990), four samples were collected at White Horse Creek and prepared for thin section analysis. Each sample was cut parallel to the direction of lineation observed in the foliation of micas within the outcrop to expose the 'motion plane' of the rock fabric. The motion plane best displays extent of shear and microstructures. Figure 4 illustrates how samples were cut and marked for thin section preparation Thin Section Selection Samples collected from the WHC were selected in order to best represent each package of different deformation. Predictions were made about the deformational grade of each sample as follows: Both WHC-1 and WHC-2 will exhibit matrix dominated rheology, with WHC-1 being a low- to -medium grade mylonite and WHC-1 being ultramylonite. WHC- 3 is clast rich, and thus is predicted to have framework supported rheology and classify as a cataclasite.

6 Petrographic Analysis Using a petrographic microscope, relative abundances of quartz, feldspars, and micas were measured. We have also examined textures of these minerals to answer the question of how these minerals are behaving across samples. Using texture and composition indicators outlined in the Atlas of Mylonites (Trouw, 2001), we also hoped to arrive at a more definite classification of each section of the WHC based on microstructural interpretation Image Processing Using the open source software ImageJ, areas of all porphyroclasts greater than 0.05mm 2 in area were measured. Modelling of clast size distribution across the WHC and between samples was done using R. This was done in order to better quantify the porphyroclast/matrix relationship between samples Results WHC WHC-1 exhibits a clast-matrix relationship where some quartz ribbons have begun to form, but where recrystallization has not eradicated all larger grains. Feldspars show evidence of both brittle and ductile deformation. Kink banding is visible in plagioclase grains distributed throughout the thin section, as in Figure 6.1. Some grains appear to be fused together. For example, plagioclase sigma clasts were found with undulose extinction, and

7 distorted twinning patterns that terminated against one another. (Figure 5) Subgrain growth of biotite is also prevalent in plagioclase grains. Quartz is remarkably different from feldspar in WHC-1. Ribbons of quartz are present, but are not dominating the section. Most are 2-3 mm in length and no more than 1 mm wide. These quartz grains are large in comparison to the sparse, smaller than 1mm mica fish that appear in the matrix. Some quartz grains did maintain their original shape, but exhibit extreme undulose extinction, suggesting that they have begun the ductile deformation process. From textures present, this is classified (by Trouw 2010) as a mediumgrade mylonite WHC WHC-2 is nearly devoid of clasts, and exhibits matrix dominated rheology, where mica and quartz form a fine micaceous matrix and abundant quartz ribbons. Many grains are smaller than 2 mm 2 in area. From top to bottom, the section grades from fine grained mica matrix with smaller quartz and feldspar to clasts, to coarser matrix with long, thin ribbons of quartz and mica fish, then back to the finer grained texture. Quartz clasts are uncommon, but those present are highly deformed, with crystal rims that appear to crumble and fade into matrix. Dispersion was observed in rare feldspar grains. Recrystallization rims are present and some rim grains are visible in ten times magnification. Ribbons of quartz appear to wrap around fractured feldspars and plagioclase grains- a texture unique to this outcrop/ thin section. (Figure 6.2) Feldspars are very small, with many well under 1mm 2. These are rounded with little recrystallization rim visible around any grains. Rare feldspars with fractured twinning

8 patterns were observed, but most had intact twins. Due to overall lack of large clasts, this would likely be classified as a high grade mylonite-ultramylonite WHC Mica behaviour in WHC-3 is drastically different compared to WHC-1 and 2. Matrix is dominated by fractured, stained glass window quartz. (Figure 6.3) Mica fish become more prevalent and greater in size, and biotite and muscovite are distinguishable from one another in much larger quantities. Radiation damage halos were observed in rare biotite grains. Dispersion in feldspar is highly common throughout the thin section along with undulose extinction, especially in matrix grains. Sigma clasts are less prevalent, replaced by large, brittly deformed feldspars Image Processing Results Early quantitative data about the WHC was produced using ImageJ. Table 1 reports our findings on percent matrix and total area occupied by grains. WHC-1 had the largest percent of porphyroclasts, at just over 25%. WHC-3 follows at 14% clast, with WHC-2 barely reaching 2% clasts. Figure 7 depicts clast size distributions across thin sections. When sorted according to size, all appear to share a common large number of smaller grains, but overall grain size increase from top to bottom in the bulk outcrop. Different clast size thresholds appear in each sample Interpretation

9 Rheology of each deformational package in the WHC changes over decimetre to metre scales, which is reflected in the emergence of new clast size thresholds in each package. WHC-2 exhibits boudin-matrix rheology, where most of the clasts have been stretched into quartz ribbons or boudins. This accounts for the overall lack of large porphyroclasts in the top of the outcrop. WHC-1, the medium grade mylonite, has clastmatrix dominated rheology. These two packages appear to grade into each other, much like Handy (1990) predicted with rheology cases 2 and 3 (boudin- and clast- matrix rheologies). WHC-3 has a sharp boundary with overlying WHC-1, and its overall rheology is reflects a load-bearing framework (case 1). The presence of these three rheological domains leads to speculation about potential pre- or syn-deformational processes applicable to the WHC. Geothermal alteration, spatially anomalous variations in gradient, or remote heating from the intrusion of the Buckland Granite are all plausible cases for protolith alteration Conclusion and Next Steps There is a significant difference in microstructures and visible mineral abundance throughout the WHC. The presence of minor ductile and brittle structures in quartz and feldspars, especially in WHC-1 and WHC-3 suggest that the entire WHC mylonite group may have spent considerable time at or near brittle-ductile transition depth. Appearance and disappearance of fine grained micaceous matrix could be linked to availability of mica in the protolith. Further work will be informative in reaching a more concrete conclusion about what process brought about the variations in aggregate rock strength that created the strain variations observed in the WHC. Point counting will better allow for quantifying abundances of specific minerals, which will provide a more concrete understanding of how minerals are distributed throughout the WHC. The line-intercept

10 method will be utilized to measure grain resizing, which will help determine metamorphic grade in each section. SEM analysis will be used to confirm compositions where petrographic analysis has left some room for speculation. EDS would also be highly useful in microstructural studies to inspect how mineral composition is affecting deformation and recrystallization. Acknowledgements I would like to thank the Frontiers Abroad program for giving me the opportunity to study geology in New Zealand, along with Rachel Beane, who provided excellent guidance throughout the research process. I would also like to thank Rob Spiers for preparation of thin sections, and Mike Flaws for SEM assistance. References Bailey, C. M., Simpson, C., & De Paor, D. G. (1994). Volume loss and tectonic flattening strain in granitic mylonites from the Blue Ridge province, central Appalachians. Journal of Structural Geology, 16(10), Behrmann, J. H., & Mainprice, D. (1987). Deformation mechanisms in a high-temperature quartz-feldspar mylonite: evidence for superplastic flow in the lower continental crust. Tectonophysics, 140(2-4), Cooper, F. J., Platt, J. P., Platzman, E. S., Grove, M. J., & Seward, G. (2010). Opposing shear senses in a subdetachment mylonite zone: Implications for core complex mechanics. Tectonics, 29, Handy, M. R. (1990). The solid-state flow of polymineralic rocks. Journal of Geophysical Research, 95(B6), Hippertt, J. F., & Hongn, F. D. (1998). Deformation mechanisms in the mylonite/ultramylonite transition. Journal of Structural Geology, 20(I), Johnson, S. E., Vernon, R. H., & Upton, P. (2004). Foliation development and progressive strain-rate partitioning in the crystallizing carapace of a tonalite pluton: Microstructural evidence and numerical modeling. Journal of Structural Geology, 26,

11 Law, R. D. (2014). Deformation thermometry based on quartz c-axis fabrics and recrystallization microstructures: A review. Journal of Structural Geology, 66, Link, G., Goncalves, P., Lanari, P., & Oliot, E. (2015). Role of mineralogical and chemical changes on shear zone nucleation : an example from the Neves area ( Tauern Window, Eastern Alps, Italy ), 17, Prior, D. J., Knipe, R. J., & Handy, M. R. (1990). Estimates of the rates of microstructural changes in mylonites. Geological Society, London, Special Publications, 54(54), Schulte, D. O., Ring, U., Thomson, S. N., Glodny, J., & Carrad, H. (2014). Two-stage development of the Paparoa Metamorphic Core Complex, West Coast, South Island, New Zealand: Hot continental extension precedes sea-floor spreading by 25 m.y. Lithosphere, 6, Simpson, C. (1985). Deformation of granitic rocks across the brittle--ductile transition INTRODUCTION DEFORMATION of quartzo-feldspathic rocks at the brittle- ductile transition in deep-seated fault zones commonly produces mylonites in which quartz and mica deform in a ducti. Journal of Structural Geology, 7(5). Trouw, R.A., & Passchier, C., et. Al. (2010). Atlas of Mylonites and Related Microstructures. Springer-Berlag Berlin Heidelberg Publishing. Tulloch, A. J., & Kimbrough, D. L. (1989). The Paparoa Metamorphic Core Complex, New Zealand: Cretaceous extension associated with fragmeotation of the Pacific margin of Gondwana. Tectonics.

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16 Sample Total Area of Clasts (mm 2 ) Percent Matrix WHC WHC WHC Table Figure Captions Figure 1: Left: Figure after Schulte et. al. (2011) depicting location of the White Horse Creek Mylonites within the Paparoa Metamorphic Complex. Right: New Zealand. Rectangle denotes regional location of study. Figure 2: Outcrop depicting strain varied texture in the WHC. Deformation packages represented by samples WHC-1, 2, and 3 are represented by zones 1, 2, and 3 respectively. Figure 3: Figure after Handy (1990) showing three domains of rheological behaviour for polyminerallic rocks. Figure 4: Illustration of oriented thin section. Arrows illustrate shear sense and topographic up. Notches are cut on thin section to preserve these directions. Figure 5: Plagioclase with terminating and brittly deformed twinning patterns. Subgrain growth of biotite. Matrix is foliated quartz and mica. Figure 6: Thin section photomicrographs of WHC-1, 2, and 3. Accompanying sketches are provided for clarity. 6.1) Large plagioclase grain showing minor undulose extinction and kink banded twinning. Thin rim of recrystallized mica and quartz. Small anhedral quartz and feldspar sigma clasts show undulose extinction and plagioclase fragments exhibit twinning. 4x. XPL. 6.2) Distinct quartz ribbons/boudins appear to wrap around brittly overprinted plagioclase. Matrix is heavily mica dominated. 4x. XPL 6.3) Quartz dominated matrix exhibiting stained glass window texture. Anhedral feldspar (centre) has experienced subgrain regrowth, along with interfingering quartz and mica growth around the mineral rim. Figure 7: Distribution of clast area for each sample. When sorted in order of increasing size, all samples share a significant number of smaller clasts. These are often fragments of quartz or larger mica clasts. Spatial representation of a general fining upwards can be seen. All have comparable sample sizes, ranging from 106 clasts (WHC-1) to 195 (WHC-2).

Strength variation and deformational behavior in anisotropic granitic mylonites under high-temperature and -pressure conditions An experimental study

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