University of Cambridge

Department of Earth Sciences, Cambridge

• Home
• Admissions
• Teaching
• Research
• Resources
• Contact
• People
• News
• Library
• Museum

• Academic Staff
• Research Staff
• Research Students

People » Research Students » Jonathan Paul

The African Landscape: Surface Manifestations of Sub-Lithospheric Mantle Convection

Supervisor: Nicky White

I am a PhD student at the Bullard Laboratories within the Department of Earth Sciences at Cambridge. Previously I spent four years as an undergraduate at Imperial College in London, graduating in 2010 with a First Class Honours degree in Geophysics. I am a Fellow of the Geological Society of London and a member of Trinity College. 

Research Interests

The transient form of longitudinal river profiles – where height is plotted as a function of distance from the head – is governed by the history of uplift rate (viz., changes in base level) and moderated by the erosional process. A river profile represents an energy balance between discharge and elevation. Pulses of uplift are manifest as broad, convex-upwards knickzones which progressively advect headward along the profile at a rate proportional to the integrated discharge of the river.

We use a simple advective-diffusive formulation to calculate river profiles. Inversion involves a minimisation of misfit between these calculated profiles and observed data, achieved by varying the input uplift rate history. Large sets of river profiles are simultaneously inverted in this way for a smooth spatial and temporal history of uplift.

Our results are validated against independent geological constraints, including uplifted marine fauna and river terraces, which may be radiometrically dated (Ar-Ar and U-Series) to obtain local estimates of uplift rate. To this end, we will undertake four weeks' field work in Madagascar (Summer 2012) to collect rock and fossil samples for subsequent geochemical analysis.

We are also able to predict sediment flux delivered to the deltas of major river catchments through time – an important indirect constraint on the history of vertical motions at the surface. The offshore solid sediment flux history will be calculated by mapping sediment isopachs derived from 2D seismic reflection data, and then compared to our onshore predictions.

I am also interested in the mechanisms driving uplift at the surface. Crustal and lithospheric thicknesses are useful in determining at what depths, and over what wavelengths, surface topography is supported. I model teleseismic receiver functions in order to obtain direct constraints on crustal structure and Moho topography.

Background

Linking the surface with the mantle

The practice of inverting river profiles to give a history of uplift has arisen from the need to have purchase on convection in the sub-lithospheric mantle— a realm we cannot see. A critical limitation of modelling dynamic topography from seismic tomographic imagery is that such observations are strictly limited to the present-day. It is therefore impossible to derive gravity- or seismology-based models which offer constraint on the timing and development of dynamic topography. Judicious interrogation of the stratigraphic record can offer independent estimates of uplift rates, such as measuring the elevation of Cretaceous shoreline deposits to derive upper-bound examples of such figures. Yet the weakness of this method lies in the inability to formalise such sparse and often qualitative geological estimates into self-consistent and quantitative numerical models. Recently, tomographic data have been combined with geomorphically-derived uplift rates from southern Africa, providing useful temporal constraints for dynamical modelling. 

Africa

The morphology of the African landscape contains many unique features found throughout the continent. The basin-and-swell structure of surface topography was recognised nearly a century ago and is subject to continued investigation: this pattern may represent a surface manifestation of sub-lithospheric mantle convection. The underlying drivers behind epeirogeny— that is, long-lived and long wavelength crustal uplifts in intraplate settings— are still poorly understood and remain hard to reconcile with the theory of plate tectonics. That Africa is surrounded by passive margins and has remained relatively stationary with respect to the mantle frame of reference since lower Oligocene times makes it an ideal natural laboratory in which to study epeirogeny. The mechanical coupling between vertical stresses at the base of the lithosphere and vertical motion at the surface is excellent.

Publications and Conference Proceedings

Last updated on 03-May-12 11:41