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

The Marlborough Fault System (MFS), New Zealand is composed of a set of four parallel strike-slip faults that decrease in age from north to south (~8Ma- 1Ma). The rate of deformation and the seismic hazard of this fault zone are unclear. New studies of the MFS aim to identify how fault movement affects the distribution of river patterns and hillslopes and then will extrapolate this understanding to other similar fault systems. Low-temperature thermochronology, which requires the presence of apatites and zircons (target minerals), is one of the key methods for investigating this problem. However, the presence of these target minerals in MFS rocks is not yet known. The purpose of this research was to characterize the abundance and quality of apatite and zircon crystals in a reconnaissance suite of five samples (3 greywackes, 1 shale, and 1 sandstone) recently collected by University of Colorado researchers from across the Marlborough field site to enable better targeting of samples for thermochronological analysis in the future. For this search, 1) thin sections were prepared and 2) rock crushing and mineral separation were performed to compare microscope observations of rock mineralogies with the apatite and zircon yield by mineral separation. Zircon was found in only one thin section (the sandstone) and apatite was not identified in any. Evaluation of the crushed mineral separate under the microscope, however, confirms that these rocks do contain the target minerals. Zircons and apatites of the sandstones are euhedral and characterized by a variety of sizes (from 70 to 40 μm). In the greywackes, the zircons and apatites are scarcer, more rounded, mostly broken, and have the same grain-size range as the sandstone. No zircons or apatites are present in the shale sample. The better yield of the sandstone sample is likely related to its larger grain size (tiny minerals are difficult to recover during mineral separation) or a different provenance than the other units. The key result of this study is that thermochronology will be a useful tool in the MFS, but some units, such as the sandstone, will likely yield better results than others.