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People » Research Students » Jonathan Paul

Sub-plate topographic support in Africa and Madagascar

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

Predictive models of dynamic (mantle-supported) topography through space and time:

1. Inversion of large sets of longitudinal river profiles to extract a history of uplift rate. Landscape evolution of Africa and Madagascar in the Cenozoic Era.

2. Calculation of sedimentary flux delivered to major river deltas offshore Africa. Balancing against the offshore sediment flux record obtained from isopach mapping of solid sediment thicknesses (2D seismic data).

3. Fieldwork in Madagascar (25 days: June-July 2012): collection of uplifted fossil corals and differential GPS surveys of onshore marine terraces. Corals will be dated using U-Th disequilibria methods; derived uplift rates are linked to terrace elevation and to the Marine Isotopic Record in order to validate our predictive models.

Determination of the underlying drivers behind vertial motions at the surface (epeirogeny): 

4. Inversion of teleseismic receiver functions for crustal structure of Madagascar.

5. Major- and trace-element geochemical data from Neogene basalts in Madagascar: depth and extent of melting.

Background

Rivers

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 time-integrated discharge of the river. Lithological control is probably exerted over much smaller wavelengths than those we model (<5 km). 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.

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 01-Sep-12 14:38