Topographic reconstructions of the Trans-Gondwanan mountain belt

Abstract

Reconstructing the evolution of the shape of Earth’s surface in deep time has not been previously attempted. How topography changes through time is essential in understanding the controls on ancient Earth systems (e.g. climate, biology and atmosphere/hydrosphere chemistry). The Neoproterozoic to Cambrian East African Orogen amalgamated through an important time for Earth’s climate. Here, I attempt the first reconstruction of the changing topography of the trans-Gondwanan mountain belt as a first step in revealing the significance of the mountain belt on climate throughout this period. The topographic reconstruction was attempted by incorporating inverted metamorphic pressure–time (P–t) data into a compositional isostatic equilibrium equation. This was done to determine an approximate elevation of the mountain belt relative to the modern-day elevation. By georeferencing the P–t data to current geological provinces, and incorporating them into a full plate model, a paleo-geographic topographic reconstruction was developed through the final amalgamation sequence of Central Gondwana. Across the orogen there is a variability in the depths that the rocks were buried and ultimately the elevations the mountain belts reached above sea level. The Arabian Nubian Shield (accretionary orogenesis from ~750–600 Ma) produced elevations of up to ~3 km peak elevation, similar to average heights of the current day European Alps. In the Mozambique and India/Madagascar belts much higher elevations of up to ~8 km, are predicted from ~650 –530 Ma., elevations similar to the current day Himalayas. In addition to developing a methodology to apply topography in deep time, a proof-of-concept study was undertaken to efficiently obtain relevant pressure-time data for future campaign-style topography reconstruction studies. This was done using garnets from a well characterised transect across Southern India, which dated using the novel laser Lu–Hf inductively coupled plasma reaction cell mass spectrometry (LA-ICP-MS/MS) technique. Quartz inclusions within these were then analysed using RAMAN spectroscopy to determine their trapping pressures. These produced results of up to ~12–15 kbar at ages ~600 to 540 Ma (peak conditions) which agree with conventional pressure-time studies and demonstrate the potential of this workflow.