SEARCHING FOR APATITE AND ZIRCON IN ROCKS OF THE MARLBOROUGH FAULT SYSTEM, SOUTH ISLAND, NEW ZEALAND

The nature, history, and geometry of strike-slip faults, and their influence on landscape evolution, are subjects of intensive study owing to the seismic hazards that such faults pose. The Marlborough Fault System (MFS) in New Zealand is composed of four parallel strike-slip faults that are inferred to decrease in age from north to south (~8 Ma- 1Ma). The history of unroofing and uplift associated with oblique-slip motion, the effect on the landscape, and the seismic hazard associated with these faults are not well-constrained. New studies in the MFS aim to identify how the faulting influenced the distribution of river patterns and hillslopes, and then use this information to help understand other similar strike-slip fault systems. One of the key methods for investigating this problem is low-temperature thermochronology, a technique that can be used to decipher bedrock unroofing patterns, which requires the presence of apatite and zircon. In the rocks of the MFS, the presence of these minerals is not yet known. The purpose of this research is to search for apatite and zircon and characterize their abundance and quality in five samples (3 greywackes, 1 sandstone, 1 shale) that were recently collected by University of Colorado researchers in the Marlborough site to enable better targeting of samples for thermochronological work in the future. In this research, 1) thin sections were made and 2) rock crushing and mineral separation were performed to compare microscope observations of rock mineralogies with the results of the mineral separation. Zircon was only found in one of the thin sections (sandstone), but apatite was not identified in any of them. However, analysis under the microscope of the separated minerals confirmed the presence of apatite and zircons in MFS rocks. The sandstone sample contains euhedral zircons that range in size from 70 to 40 μm. In the greywackes, the zircons and apatites are scarce, more rounded, and mostly broken, but exhibit the same grain-size range as in the sandstone. Apatite and zircon were not found in the shale sample. The better yield of zircon and apatite crystals in the sandstone is likely due to its larger grain size or different provenance than the other units. The key result of this research is that thermochronology can be carried out in the MFS, but some units will yield better results than others.