The applications of remote sensing to geomorphological neotectonic mapping in North Canterbury, New Zealand.

Abstract

Techniques of remote sensing and air photo interpretation have been applied in an investigation of the influence of neotectonism on geomorphic development within the Waipara Region, North Canterbury, New Zealand. The present physiography is viewed as the product of a series of interactions between fluvial and marine processes operating on the underlying geology that has been subjected to both past and ongoing structural deformation by folding and faulting. The principal landforms can be divided into smaller units controlled by rock induration, modes and rates of weathering, and changing climatic and tectonic regimes. Lithology and bedrock structure influence geomorphology and drainage patterns by controlling relative erodibility of rocks and stability of slopes. Tectonic aspects of the late Quaternary geomorphology of the Waipara region have been studied by mapping the distribution of the late Quaternary surfaces formed by both marine and fluvial processes. Sequences of marine terraces, that have at most five distinct levels, are well preserved along the eastern margin of the study area. The height and aerial distribution of former shoreline features vary markedly. Both rates and amounts of uplift of individual terraces are variable. Consequently, the present heights of the marine terraces are regarded as the superimposed combination of eustatic sea level change and tectonic movement. The origin of the lower Waipara gorge is interpreted in terms of a hypothesis of antecedence, linked to at least seven periods of tectonic and climatic base level change. Significant recognizable geomorphic features include: fluvial aggradation (Teviotdale and Canterbury Surfaces), last Glacial and Interglacial marine terraces, lateral channel migration (ancestral and present Waipara River), and the modern flood and coastal plains. The same applies to the smaller adjacent Carrington, Yellow Rose and Kate stream systems and the Teviotdale River. Terrace lines, resulting from changes in mode of stream operation, provide isochronous features that can be applied to tectonic analysis of the study region. Surveying of tilted straths and warped aggradation terraces, together with other geomorphic evidence, indicate not only active growth of the main Cass Anticline during the late Quaternary, but also that the sites of most active deformation have migrated more recently to the younger flanking structures of the Kate and Black Anticlines. New Zealand rivers provide good examples of the nature of channel response to actively growing folds and faults and despite differences in size and erosive power, they all show features in common. The evolution of their fluvial systems is detailed in the geomorphic maps prepared from aerial photographs and Landsat imagery at various scales. The main features of channel response to uplift are summarized below: 1. Profiles of tilted straths show conclusive reversals of tilt on the upstream side and an over steepened gradient on the downstream side of the uplift. 2. The upstream side of the uplift has a higher sinuosity index than on the side immediately downstream. 3. The meandering valley topography has been accentuated by incision of the channel pattern leaving relict straths surfaces preserved at higher levels. 4. A significant rotation in the general alignment of the valley axes relative to their present positions has taken place, typically switching abruptly from synclinal troughs across adjacent antic lines. 5. In uplifted areas that have been affected by accelerated uplift and tilting, cross valley morphology consists of two distinct parts. An inner (topographically lower) steep sided, V-shaped profile is incised on the older, much more broadly U-shaped valley morphology. 6. Aggradation has occurred in the reaches upstream and downstream of the uplift. 7. The evolution of slopes and drainage and the hydrologic character of individual rivers appear more sensitive to the relatively slow processes of tectonic uplift and tilt than has been generally realized. 8. Irregularities in the shape of longitudinal river profiles reflect major active geologic structures, indicating recurring uplift. These observations have been used to develop a model for the evolution of drainage within a belt of folding induced by deep seated shear. This model is used to develop general principles for the production of neotectonic maps in other areas of only strike slip tectonics.