Gravity analysis of glaciotectonic processes, central Alpine Fault, South Island, New Zealand

New gravity data collected in the central West Coast of New Zealand's South Island have been used to investigate fault kinematics and structure and the effects of erosion. Buried glacial erosion channels are identified in the footwall of the Alpine Fault in the Wanganui, Whataroa and Fox floodplains. Using a modified technique for estimating channel offsets, these buried channels are found to have been displaced along the Alpine Fault by 383 +/- 74 m, 372 +/- 159 m and 450 +/- 121 m, respectively, since the Last Glacial Maximum. Adopting a date for the Last Glacial Maximum of 19 +/- 1 ka, these offsets correspond to strike-slip slip rates of 20 +/- 5 mm a(-1), 20 +/- 9 mm a(-1) and 24 +/- 8 mm a(-1), respectively, which are consistent with previous estimates of slip rates along the central Alpine Fault. The gravity data are also used to delineate and model the South Westland Fault, a thrust fault system, 10-20 km northwest of and striking subparallel to the Alpine Fault.