The effect of bedrock geology on sediment yield in an alpine area of extreme rainfall

摘要

Catchments in the Cropp River area of Central Westland are subject to very high sediment yield in an area which coincides with near maximum uplift rates and rainfall values for the Southern Alps. In order to investigate the effect of bedrock geology on sediment yields in this area bedrock geology was mapped and a structural and metamorphic history determined, from petrographic and field evidence. Sediment yield was estimated from suspended sediment measurements and geomorphology. The preferred interpretation of structural history is compatible with most other work done on Alpine Schist petrogenesis. It involves: 1. An early large-scale isoclinal folding (F₁) with development of an incipient fabric (S₁) parallel to bedding (S₀); 2. A pre-mid-Cretaceous regional metamorphic event (M₂) along a linear trend parallel to the present trend of the Alpine Fault and associated folding (F₂) and fabric development (S₂); 3. A probable late Cenozoic metamorphic event (M₃) affecting rocks in the western schist belt with development of a schistosity (S₃) sub-parallel to S₂ and F₃ folding; 4. Recent possibly continuing faulting producing minor kink folds (F₄) over the width of the schist belt. The M₂ and M₃ metamorphic events occurred in a narrow zone subjected to lateral compression and probably shear heating, and in the case of M₃ this zone is related to the Alpine Fault. Geomorphological geothermal and geological considerations suggest that uplift is occurring at present, localised on the Alpine Fault in the west and on northeast-trending faults in the east. The Cropp area may be in the zone of maximum uplift in the Southern Alps with uplift of 12±2 mm per year. Sediment yield studies: a) from a one-year suspended sediment record, and b) from reconstruction of a 12,000 year geomorphic surface and calculation of the volume of material eroded, suggests a sediment yield of 35,000 tonnes/km² per year, a denudation rate of 13 mm per year for the upper Cropp. Sub-catchment sediment yield is found to vary reflecting catchment geomorphology and the rate and extent of fluvial downcutting. Bedrock geology affects sediment yield by controlling patterns of sediment supply as a function of rock strength and therefore erodibility and controls the mechanism of sediment transport by determining the grain size of material to be transported.