A GEOMORPHIC, STRATIGRAPHIC, AND STRUCTURAL STUDY OF TWO SECONDARY FAULTS IN THE TORLESSE FAULT ZONE

This study investigated two minor faults, believed to be subsidiary to the Torlesse Fault, that run through the Porter River valley in Castle Hill, New Zealand. Analysis of the area has better contextualized the geomorphic features of the surrounding area and refined understanding of the nearby Cenozoic stratigraphy. Detailed field mapping in the valley coupled with unmanned aerial vehicle (UAV) imagery led to the production of two digital maps, one containing geomorphic information (correlated glacio-fluvial terrace surfaces, alluvial fans, fault scarps, etc.) and one structural map (fault activity, strikes and dips, stratigraphy, etc.). In depth study of the Porter Valley’s stratigraphy was used to create multiple stratigraphic columns as well. The UAV imagery collected was used to create a high-resolution Digital Elevation Model (DEM), which was analyzed in ArcGIS to measure deformed geomorphic features and fault displacement. The western fault was determined to be a ~30° thrust fault, with the top moving to the east, and the eastern fault to be a high angle (~70°) normal fault with the footwall rising to the east. The traces of the two faults were located in the bedrock of the valley, and slickensides were identified in fault gouge within the western fault, showing a bimodal slip-distribution, as there is evidence for both strike-slip and dip-slip motion. Estimates of total offset for the western and eastern faults can be constrained to 8-147 m and 10-25 m, respectively, although it is worth noting that the western fault is realistically much closer to 8 m than 147 m. It is hypothesized that these two faults are splay faults of the Torlesse Fault and are breaking up the northern fault block to accommodate both vertical and lateral displacement on the fault zone. If there is a strand of the Torlesse confirmed to the north of the valley, it is also possible that the two subsidiary faults are relay faults in a transpressional system within the Torlesse Fault Zone. These findings signify the potential of splay or relay faults as the key to understanding more about the Torlesse Fault Zone and indicate that with further research, these subsidiary faults could help determine precise locations, slip rates, and displacements of the fault zone.