Climate-controlled variations in scree production, Southern Alps, New Zealand

The interaction of fluvial, glacial, and hillslope processes controls the development of mountain belts and their response to tectonic and climatic forcing. Studies on the contribution of hillslope processes to mountain erosion have focused on bedrock landslides, as they have a profound and readily observed impact on sediment yield and slope morphology. Despite the ubiquity of scree (or talus) mantled slopes in mountainous terrain, the role of frequent, low-magnitude (< 100 m(3)) rockfall events is seldom addressed in the context of landscape evolution. Here we quantify the contribution of rockfall erosion across an 80 by 40 km transect in the Southern Alps, New Zealand, by analyzing the spatial extent of scree slopes mapped from aerial photographs and estimating long-term (10-15 k.y.) rockfall erosion rates from the accumulation of slope deposits below bedrock headwalls and in debris and alluvial fans. Along the rapidly uplifting, high-rainfall western margin, where high rates of bedrock landsliding have been previously documented, scree-mantled slopes are sparse. Rainfall decreases exponentially east of the Main Divide, and the proportion of slopes mantled by scree increases monotonically, attaining a maximum value of 70%. The systematic distribution of scree deposits cannot be attributed to lithologic variation, seismicity, or the legacy of glaciation. Instead, climate may serve as a primary control on scree production, as nearly 70% of the mapped scree deposits in our transect are confined to a narrow elevation range of 1200-1600 m above sea level (masl). Our analysis of altitudinal controls on annual temperature variations indicates that scree production via frost-cracking processes may be maximized between elevations of 1600 and 2300 masl, as higher elevations are subject to persistent permafrost which obviates the frost-cracking process. Rates of rockfall erosion near the rapidly uplifting Main Divide are low (< 0.1 mm/yr), whereas rates in the scree-dominated eastern areas average 0.6 mm/yr and may approximately balance rock uplift.