EXTRACTING TECTONIC AND CLIMATIC SIGNALS FROM RIVER AND TERRACE LONG PROFILES IN ACTIVE OROGENS, TAIWAN AND NEW ZEALAND

Long profiles of alluvial rivers have long been used to evaluate active tectonism. Deviations from “ideal” shapes—commonly a logarithmic-decay function—are used to infer active uplift or subsidence. Less attention has been paid to what might be the ideal long profile for bedrock rivers. Yet rivers that drain active mountain belts must be responding to the distribution of active uplift along their courses, while simultaneously adjusting to variations in bedrock erodibility and spatial/temporal variations in discharge and sediment load at tributary junctions as well as climate change. Examination of long profiles of rivers draining two active orogens, the Southern Alps of New Zealand and the Central Range of Taiwan, illustrates how the effects of climate changes–glacial erosion and sedimentation in the case of New Zealand, climate-triggered fill events in the case of Taiwan–can overprint and obscure tectonic signals, even in orogens with very high uplift rates (order 10 m/ka). Long profiles of major rivers in these ranges may approximate exponential-, logarithmic-, or power-law decay functions, which are controlled by bedrock erodibility, sediment load from tributaries, and climate superimposing on uplift. Large New Zealand rivers, with long profiles affected by Pleistocene glaciation, have exponential-decay or power-law shapes. Oversteepening of headwaters due to glaciation and flat lower valleys controlled by glaciofluvial deposition strongly constrain profile shape. Tectonic interpretation of uplift and incision therefore is difficult, whether from modern long profiles or from latest Pleistocene terraces. Taiwan rivers, with long profiles unaffected by glaciation, are most closely approximated by exponential-decay functions. Tectonic signal is strongly preserved, particularly where uplift rates increase in the upstream direction. Front-draining rivers in eastern Taiwan generally adjust to the lower frontal uplift rates. The implications are that river profiles in glaciated regions inherit their fundamental shape from glacial and glaciofluvial processes, even in areas of high uplift and denudation rates. On the other hand, river profiles unaffected by glaciation more respond more to rate and style of uplift and thus should be more useful indicators of tectonic processes (style and rate of uplift from long-profile shape and incision rate).