Quantitative relationships between uplift and relief parameters for the Southern Alps, New Zealand, as determined by fission track analysis

Abstract

AbstractThe Southern Alps mountain chain, New Zealand, has formed as a consequence of late Cenozoic collision of the continental parts of the Pacific and Australia plates. Fission track analysis has yielded estimates of the amount, age of initiation, and rate of late Cenozoic rock uplift for 82 surface samples taken from transects across the Southern Alps. The mean surface, summit and valley elevations in the vicinity of each of the rock sample sites have also been measured. Regression of the geomorphic variables on the uplift variables has been used to establish quantitative relationships between uplift and geomorphology. There are strong and consistent linear associations between uplift and the elevations of the mean surface, summits and valleys. The preferred regression models have uniform slope but varying elevation response between transects. Substitution of space for time has allowed the evolution of landforms to be studied.To the east of the Main Divide, elevation and relief are proportional to, and closely related to, the age of initiation of rock uplift (‘uplift age’) and the amount of rock uplift (r2 > 0·8). Mean surface uplift was delayed for ∼2 Ma after the start of rock uplift, a result of the stripping of a soft cover rock succession that, prior to rock uplift, overlaid the harder greywacke basement. Inter‐transect variations in regression response and x‐intercept are inferred, therefore, to reflect the variable preuplift thickness of cover rocks. However, the regular regression slope for the transects reflects the consistent nature of the interaction between uplift and the erodibility of greywacke basement. Uplift of the mean surface proceeded at 0·4 km/km and 0·4 km/Ma of rock uplift, while the rock uplift rate was 0·8 km/Ma. Summit elevations have increased at a rate of 0·6 km/Ma and valley elevations have increased at 0·2 km/Ma. Regression lines relating mean surface, summit and valley elevations to rock uplift and uplift age diverge from common intercepts; it is concluded, therefore, that the mountains east of the Main Divide have continued to increase in elevation and relief and change in form over time since the start of mean surface uplift. Mountain elevation has little relationship with late Cenozoic mean rock uplift rates of 0·8–1·0 km/Ma or inferred contemporary rock uplift rates (r2 ∼ 0·3).In contrast, to the west of the Main Divide, elevation is shown to be closely related to rock uplift rate (r2 > 0·3).In contrast, to the west of the Main Divide, elevation is shown to be closely related to rock uplift rate (r2 > 0·8). Transects with higher rock uplift rates support higher topography. Landforms are therefore in a stable equilibrium with rock uplift rate, and the landscape contains no residual evidence of the total amount of rock uplift, or the age of uplift. Lithological variation appears to have no relationship with elevation.