Simon Stephenson: A Cenezoic record of uplift, erosion and dynamic support: examples from Madagascar

PhD Overview:

The growth and decay in Earth’s topography is controlled primarily by tectonic and erosional processes.  Mountains are built where the Earth’s crust is thickened at convergent plate margins, such as at the Alps or Himalayas and destroyed by erosion and where the Earth’s crust extends, in locations such as the East African Rift.  However, another, more enigmatic process exists that creates and destroys topography, and this is the focus of my research.

Convection beneath Earth’s tectonic plates, within the Earth’s mantle induces stresses on the base of the plate.  Where the mantle upwells, the stresses push the topography higher, and where it downwells, it draws it down.  The surface expression of these convective process operates on a large scale – wavelengths of 1000 km or more.  This is known as dynamic topography.

In recent years, much effort has been expended predicting modern and historic dynamic topography.  However, this is set against a paucity of observations constraining modern and historic dynamic topography, particularly on the continents.  Understanding long wavelength uplift and subsidence is vitally important for interpreting palaeo sea level, present and historic heat flow, and making predictions of sediment flux to sedimentary basins.

I use a variety of field and remotely sensed data, combined with modelling techniques to investigate the evolution of dynamic topography through time.  In particular, the observation and dating of uplifted surfaces, the dating of erosional events and the modelling of river profiles, which act as a tape recorder for uplift.  I intend to tie results with estimates of sediment and crustal thickness and
petrological information to improve our understanding of the growth of dynamic topography in Southern Africa and the southern Indian Ocean throughout the Cenozoic.

BGA PGRIP presentation (poster)

The physiography of Madagascar is characterised by high-elevation and low relief topography. Cretaceous limestones at elevations of ~ 300 m above sea level and newly dated emergent ~125 ka coral reefs suggest that Madagascar and its margins have been uplifted during Cenozoic times.  Rivers in Madagascar are often deeply incised and contain steepened reaches, which implies that they are responding to changes in uplift rate.  However, apatite fission track and (U-Th)/He thermochronology, and Be-10 derived erosion rates suggest that both Cenozoic and recent denudation rates have been low.  Extensive laterite-capped flat surfaces also suggest long periods of tectonic quiescence during the Cenozoic.  To bridge the gap between evidence for uplift and quiescence, we inverted 2566 longitudinal river profiles using a damped non-negative, least-squares inversion scheme for histories of uplift rate.  We used a simplified version of the stream power erosional model (∂z/∂t=-KA^m S^n + U, where n=1).  Longitudinal profiles were extracted from the 90 m resolution SRTM digital elevation model. Calibration of the stream power erosional model is based on new radiometric dating of marine terraces and incised lateritic peneplains.  Fits to observed river profiles are excellent.  Results indicate that Madagascar’s topography grew by 2 km during the last 15-20 Ma. Calculated uplift and denudation is consistent with independent observations.  Our results suggest that drainage networks in Madagascar contain coherent signals that record regional uplift.  Admittance calculations and nearby oceanic residual age-depth measurements from the passive margins suggest that as much as 0.8 – 1.1 km of Cenozoic uplift in Madagascar was supported by the mantle.

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