CONTROLS ON STRAIN PARTITIONING IN THE WHITE HORSE CREEK MYLONITES, WEST COAST, NEW ZEALAND

The White Horse Creek Mylonites (WHC) developed in gneiss of the Paparoa Metamorphic Core Complex (PCC), New Zealand as a result of movement along the Pike Detachment. Previous investigations have delineated the development of the mylonites within the evolution of the regional PCC. This study aims to evaluate controls on observed cm- to m- scale strain variation. Petrographic analysis of microstructures combined with mineral point counts (using open source digital software) were used to document cm-scale variations in rheology. Areas within the WHC with larger percentage micaceous matrix exhibited more ductile behavior and smaller clast sizes, while less micaceous areas contained more brittle structures and larger clasts. Preliminary results suggest that increased phylosilicate availability correlates with the degree of mylonitization. Furthermore, observations of ductile quartz co-existing with brittle feldspar suggest mylonitization occurred near the brittle-ductile transition zone (temperatures around 450° C) , or adjacent to a heat-radiating intrusive body. The observed increase in clast size coupled with a decrease in micaceous matrix suggests that mica-rich protolith, such as a this gneiss, experiences rheology change over small scales. Future work done to more efficiently quantify mineral percentages throughout the WHC may assist in interpreting strain progression in gneiss or other rocks with variable composition.