The Persistence of Glacial Valleys in the New Zealand Southern Alps

Abstract:

One of the most fundamental insights for understanding how landscapes evolve is based on determining whether topography was modified by glaciers or rivers. Alpine landscapes featuring horns, knife-edged ridges, and U-shaped valleys are commonly associated with glacial sculpting, whereas fluvial erosion is known to produce V-shaped valleys via links between river incision and landsliding. Rivers, landslides, and glaciers are all capable of rapid erosion comparable to the highest rates of rock uplift, and there has been progress in modeling fluvial erosion and hillslope response, as well as understanding how landscapes react to the onset of glaciation. However, the timescale involved in the transition from a glacial to a fluvial landscape is poorly constrained and it is unclear how long glacial morphology can survive following deglaciation.

We tested whether the fluvial and hillslope erosional response to tectonic forcing controls the timescale over which glacial topography persists into interglacial periods. We used digital terrain data to quantify the degree of glacial imprint on topography by geomorphometric analysis of cross-sectional valley shape across a spatial gradient in rock uplift and erosion rates in the New Zealand Southern Alps. Our results show that tectonic forcing is a first-order control on landscape evolution and on the persistence of glacial morphology. In Earth’s most rapidly uplifting mountain ranges the lifespan of glacial topography is on the order of one interglacial period, preventing the development of a cumulative glacial signal from the added erosional impact of subsequent glacial stages. Thus we suggest that the present-day physiographic signature of glaciated landscapes is best expressed in, and limited by the extent of low uplift terrain. In addition, emphasizing that the presence of glacially preconditioned topography greatly influences glacial extent and erosion, our results imply that tectonic forcing governs the impact of climate depressions on active orogens beyond controlling their mere vertical extent.