TOPOGRAPHIC EXPRESSION ON A WEAK CRUST: FIELD OBSERVATIONS AND MECHANICAL CONSTRAINTS

The three-dimensional strain distribution within an orogen reflects imposed boundary conditions as transmitted through the rheological structure of the lithosphere. Using a 3D mechanical framework, in conjunction with geological and geomorphic observations, we can predict the topographic response of an orogen to imposed boundary conditions.

The form, geometry, geological history and geomorphology of the Southern Alps of New Zealand vary markedly along strike. The central Southern Alps are characterised by narrow mountain ranges, high strain rates and a classical fluvial foreland basin on their eastern margin. In contrast the southern Southern Alps (Otago region) are more extensive but lower in elevation and have a complex overlying fluvial system.

We show that these geological and geomorphologic differences are controlled by the underlying crustal strengths of the different blocks. Our multi-disciplinary approach utilizes 3D models to investigate the effect an anomalously weak region has on topographic evolution. We compare model results with tectonic stratigraphy, present day topography and Otago’s evolving fluvial system to gain further insight into the processes and features that control topography.

The plate boundary through the South Island initially formed as an ENE–WSW trending dextral strike-slip system. Our models show that coupling of along strike variations in crustal strength has a major and rather complex effect on topography. Despite the largely strike-slip kinematics, surface uplift in the models occurs along the northern boundary of the rheological weak zone (Otago) concomitant with subsidence along the southern boundary of the weak zone. Geological evidence supports these predictions with the coincident uplift of the Alpine Fault normal Hawkdun Range northeast of a large lake (Lake Manuherikia). Since the beginning of the Pliocene the kinematic regime has become more transpressive. Again, geological evidence supports model predictions that mountain building will be focussed on the central Southern Alps in rheologically strong Canterbury region but will be distributed across the rheological weak Otago region. Most tellingly, the shifting tectonic regime is recorded by the infilling of the lake and major drainage realignment of the South Island’s largest river, the Clutha.