CLIMATIC AND TECTONIC CONTROL ON EROSION ACROSS THE ALPINE FAULT, SOUTH ISLAND, NEW ZEALAND

 

In the South Island of New Zealand, both tectonics and climate play a role in the rate of erosion. The Alpine Fault is a transpressional fault, crossing the South Island of New Zealand along the Australian-Pacific plate boundary, that also acts as a divide of the four samples collected for the purpose of this study. The Southern Alps run parallel to the Alpine Fault creating drainages and catchments to the east and west of the fault. To the west of the Alpine Fault an extremely wet climate with high rates of physical erosion and uplift exist, while the east is characterized by a drier climate with significantly lower rates of physical erosion and uplift (Jacobson et. al, 2002). Understanding the erosional history of the region is significant for studies on the relationships between climate and tectonics on orogenic evolution. This study uses U-Pb geochronology and zircon fission track analysis (ZFT) thermochronology on four modern river samples from three different catchments to evaluate the respective roles of tectonics and climate on erosion and exhumation. Three of the samples were collected west of the Alpine fault and one sample was collected east of the fault. The sizes of the catchments vary from sample to sample. U-Pb ages of zircons in each sample controlled for variation in source rocks. Zircon fission track analysis was also conducted to determine the thermochronological ages of the four samples. The thermochronological ages of the samples to the west of the Alpine fault are younger (most prominent age peaks ranging from 12.3 Ma-15Ma) than the age of the sample to the east (most prominent age peak of 27.98 ± 0.69). While this suggests that climatic controls do influence erosion rates along the northern Alpine fault, these results are significantly different from the results across the drainage divide to the south. Comparing the data from this study to the data of Tippett and Kamp, 1993, indicates that the Marlborough Fault Zone on the northern end of the South Island is characterized by late Miocene exhumation reflecting an earlier history of the plate boundary with respect to other areas to the south along the Alpine fault and/or lower magnitude exhumation with respect to areas to the south.