ALONG-STRIKE SEGMENTATION OF THE SOUTHERN ALPS, NEW ZEALAND, FROM GEOMORPHIC ANALYSIS

Analysis of river long profiles, drainage basins, and range-front morphology of the western Southern Alps and comparison with the morphology of the eastern Southern Alps reveal along-strike variations in the nature of the fault-bounded range, which likely reflect structural segmentation along this portion of the Australia-Pacific plate boundary. The study is limited to that part of the range bounded by the right-reverse Alpine fault from the Cascade River, south of which a change in uplift direction results in a poorly defined range front, to Crooked Mary Creek, north of which increased branching of the Alpine fault results in loss of a distinct range front. Scale-space filtering of continuous and windowed data (such as range relief and mountain-front sinuosity) and optimum partitioning of discrete data (such as drainage-basin relief and river gradient) allow identification of five geomorphic domains in the western Southern Alps that have consistent and distinct morphology. The boundary between the north and north-central domains coincides with the branching of the Hope fault. The north-central - central boundary is associated with a decrease in basin area in the eastern Southern Alps and is closely associated with the junction of another branch of the Alpine fault. The boundary between the central and south-central domains is coincident with the northern extent of the Haast-Landsborough basin, the largest in the western Southern Alps and previously suggested as a locus of an eastward migration of the main drainage divide. The boundary between the south and south-central segments is associated with the initial decrease in the distinctiveness of the western range front. Thus, the morphologic segmentation of the Southern Alps is related in large part to the behavior of the Alpine fault, especially its uplift pattern and its branching into major faults to the north.