The influence of the lithological and geotechnical properties of rocks on the morphology of glacial valleys

• Main Author: Augustinus, Paul Christian
• Other Authors: Selby, M. J.
• Format: Thesis
• Language: English
• Published: University of Waikato 2013
• Subjects: Antarctica; New Zealand; Cross Valley; Antarc*
• Online Access: https://hdl.handle.net/10289/8232

The glacial valley cross-profile has traditionally had its development attrituted to the physical properties of the eroding glacier, with the input of the properties of the eroded rock mass to the development of the valley system considered in a purely qualitative sense. The present study shows that the size of the outlet trough is directly related to the volume of ice discharged through it, estimated from the glacier contributing area. The trough size and morphological variations therein, can be partially attributed to the influence of the bedrock strength properties. Rock intact strength measures showed little relationship to the form of the glacial trough. However, a modified rock mass strength method was developed and applied to a variety of morphological and geological terrains in the vicinity of the Main Divide of the New Zealand Southern Alps. The results indicate a significant correlation between the cross-valley form and mass strength (RMS) properties of the eroded bedrock. The RMS controls on the development of the trough were: joint spacing, joint orientation and joint continuity. The trend suggests that weaker, more densely jointed bedrock tends to develop broader, flatter valleys. RMS with respect to subaerial processed controls the extent of post-glacial/interglacial modification of the trough slope, and development of zones of weakened slope rock that could be preferentially exploited by subsequent glazier re-advances. Due to their position astride the Alpine Fault, the New Zealand Southern Alps are subjected to high levels of shallow crustal horizontal stresses. The PHS directions are indicated by geodetic and earthquake first-motion studies, as well as conjugate shear joint and glacial valley orientations. The in situ stress field may control the location and extent of rock failure, when considered in conjunction with the high gravitational stresses induced by the extreme relief. Finite element models of typical glacial troughs suggest that rock intact strength properties control the likelihood and site of stress-induced bedrock failure. Thus, the shape of a glacial trough depends not only on the physical properties of the glacier, but on the geotechnical propertied of the host rockmass. The stress-induced controls on the site of rick mass failure are important controls on the locus of erosion. Following development of the glacial trough, considerable modification of the size and form of the valley cross-profile may occur depending on the mass strength of the de-buttressed slope rock.