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Géosciences Rennes
UMR 6118
Université de Rennes1
Campus de Beaulieu
35042 Rennes Cedex

02 23 23 60 76


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Workshop TOPOAFRICA

TOPOAFRICA



Convenors : Jean Braun, Olivier Dauteuil, Kerry Gallagher, François Guillocheau, Cécile Robin, Delphine Rouby, Martine Simoes, Jean-Jacques Tiercelin

contact : delphine.rouby@univ-rennes1.fr

tel : 33 2 23 23 57 71, fax : 33 2 23 23 61 00

http://www.geosciences.univ-rennes1.fr/




Two years ago, our group initiated an original project aimed at the reconstitution and the understanding of the topographic evolution of a continent over the last 250 My. Our goal is to quantify the growth of long wavelength (1000 km) topography and the associated rates of uplift over the last 250 My at the scale of a continent - Africa - and to understand their relationship with underlying mantle dynamics and surface processes such as erosion and sedimentation. Our approach involves the joint acquisition of relevant geological data (from sedimentology, stratigraphy, structural geology, hermochronology, etc..) and use of state of-the-art numerical modelling tools.

The evolution of the Earth’s topography over geological time scales is indeed very poorly known. However, it is critical to understanding surface processes such as climate change, sea level variations, global geochemical cycles, biodiversity evolution, but also the nature of the mantle and lithospheric processes that control the topography.

The topography of the Earth is the product, and thus the record, of the interactions between the solid Earth and its surficial envelopes occurring through geological time. However, the few available reconstructions are mainly qualitative, and usually correspond to “rules of thumb†deduced from present-day relationships between relief and tectonic setting. Defining a more rigorous method for a quantification of paleotopography is clearly lacking and is yet of primary importance.

Marginal basin 3D Geometry
Marginal basin 3D Geometry


Most studies of past topography reconstruction have focused on orogenic areas. Few efforts have been devoted to characterizing the more subtle long wavelength topography, such as the doming or plateau uplift of continental areas at the 1000 km wavelength. The amplitude of uplift and subsidence of large regions of the African continent during the Meso-Cenozoic is substantial, yet poorly understood. However, for a variety of reasons, it offers an ideal opportunity to quantify its evolution through geological times : Africa is surrounded by passive margins and bears several intracratonic basins providing an excellent preservation of the product of continental erosion. Furthermore the continent has remained relatively fixed with respect to the underlying mantle at least since the end of the Paleozoic. It is commonly agreed that the long wavelength pattern in the topography reflects mantle dynamics. However, the exact nature of these mantle processes remains unclear (dynamic topography, delamination of the mantle lithosphere, thermal expansion). Key in this debate are the age and the dynamic of the topography.

Existing comprehensive low-T thermochronological database
Existing comprehensive low-T thermochronological database


Our project is subdivided into two parts. In a first step, we have tried to quantify the Meso-Cenozoic surface uplift rates and topographies of the African continent on the basis of (i) new-style paleogeographic reconstructions including, for example, the geometry of aleocatchments, (ii) measures of terrigenous sedimentary fluxes and (iii) thermochronological data. We use a new numerical model of sediment production and transport at the continental scale currently developed at Géosciences-Rennes to perform the quantitative inversion of the observations to yield estimates of past topography. In a second step we aim to determine the geodynamical processes responsible for the anomalous topography of the African continent and its evolution through time.

One expected deliverable of this project will be a unique set of continental-scale, paleotopographic maps based on a large, novel paleogeographical dataset of Africa over the last 250 My made available to the entire community as an GIS (ArcGIS) database.

New style paleogeographic map
New style paleogeographic map


One of the challenges and strengths of this project is to get geologists and modellers to work together on a well-defined objective. This symbiosis, initiated in eosciences Rennes has rapidly reached limitations, our group not covering the whole disciplinary and regional nowledge required. For these reasons, we have decided to organize an international workshop whose objectives are to (1) present our approach, (2) confront our preliminary results to the best regional expertise, (3) discuss the geodynamic scenarii to be tested, (4) improve our ethodological approach, and last but not least, (5) invite scientists interested to join the project in one way or another.

Reconstructed paleotomography beneath Africa
Reconstructed paleotomography beneath Africa (Conrad and Gurnis,2003)




PROGRAMME



Day 1 : Tuesday November 13th 2007 @ Universite de Rennes 1, campus de Beaulieu

10h00 to 12h00 Welcome

in : Hall of CAREN, bâtiment 14b

12h30 Onsite lunch

14h00 : J. Braun & F. Guillocheau “The TopoAfrica projectâ€

in : Amphihéatre Louis Antoine, bâtiment 2

14h30 : M. Brunet “On the track of a new paradigm for the cradle of mankind...in Chad, Central Africaâ€

16h30 “Ice Breakerâ€

in lobby of Amphihéatre Louis Antoine, bâtiment 2

18h00 Departure for downtown Rennes (bus)

18h30 Visit of Espace des Sciences and Planetarium (downtown)

20h30 Diner Restaurant LA CHOPE



Day 2 : Wednesday November 14th 2007 @ Maison Internationale de Rennes (downtown)

SESSION 1 : Mantle and Lithosphere Dynamics chairman : K. Gallagher

8h30 Introduction : K. Gallagher / R. Brown

8h45 S. Fishwick “Seismic studies of the African continent and a new surface wave model of the uppermost mantle†.

9h45 A. Forte “Tomography based convection modelling of mantle dynamics below the African plate : implication for time dependent dynamic surface topography†.

10h45 Coffee Break

11h15 D. Bell “Post-Gondwana evolution of the lithospheric mantle beneath southern Africaâ€

12h15 D. Rowley “Mid-Ocean Ridge Motions in the Indo-Atlantic Frame of Reference from 0 to 80 Maâ€

13h15 Onsite lunch

14hO0 Discussion

SESSION 2 : Volcanism in Africa over the last 250 Myr chairman : F. Guillocheau

15h00 Introduction : F. Guillocheau

15h15 H. Bertrand “CAMP and Karoo : the two largest igneous provinces of the African plateâ€

16h15 Coffee Break

16h45 D. Jerram “Major Volcanic Events in Africa in the last 250Maâ€

17h45 Discussion

free evening



Day 3 : Thursday November 15th 2007 @ Maison Internationale de Rennes (dowtown)

SESSION 3 : Climatic and geological evolution of Africa over the last 250 My chairman : J. Braun

8h30 F. Guillocheau “Palaegeography of Africa through Meso-Cenozoic times : a focus on the continental domain evolutionâ€

9h30 M. de Wit “Tracking the Kalahari epeirogeny : from fission tracks and cosmogenic dating to molecular clocksâ€

10h30 Coffee Break

11h00 Y. Donnadieu “A GEOCLIM simulation of climatic and biogeochemical consequences of Pangea break upâ€

12h00 Discussion

12h30 Onsite lunch

SESSION 4 : Relief dynamics at continental scale chairman : D. Rouby

13h30 D. Rouby “The sedimentary supply of African sedimentary basins over the last 250 Maâ€

14h15 M. Summerfield “Empirical constraints on uplift and denudation in southern Africaâ€

15h15 T. Partridge “The tectonics and geomorphology of Africa : an overviewâ€

16h15 Coffee Break

16h45 N. White “Scales of transient convective support beneath Africaâ€

17h45 Discussion

20h00 Diner Restaurant L’OUVREE



Day 4 : Friday November 16th 2007 @ Maison Internationale de Rennes (downtown)

SESSION 5 : Alteration and transport dynamics at the continental scale chairman : M. Simoes

08h30 M. Simoes “Providing tools to quantify the kinematics of uplift of Africa over the last 200 Myr.â€

09h30 A. Beauvais “Age and nature of lateritic weathering, erosion rates and long-term morphogenesisâ€

10h30 Coffee Break

11h00 J. Gaillardet “Rates and styles of continental denudation : Clues from river geochemistryâ€

12h00 Discussion

13h00 Onsite lunch

14h00 bus to the Mont Saint Michel

16h30 Visit of the Mont Saint Michel Abbaye

20h00 Diner in Cancale



LIST OF PARTICIPANTS

D. Bell Arizona State University

R. Brown University of Glasgow

G. Bertotti Vriej University Amsterdam

M. De Wit University of Cape Town

J. Decker University of Cape Town

K. Dobson University of Glasgow

N. Downey Caltech

D. Jerram University of Durham

S. Nixon Cambridge University

A. Forte Université du Québec àMontréal

S. Fishwick Leicester University

M. Heinz Cambridge University

T. Partridge University of the Witwatersrand

D. Rowley University of Chicago

M. Summerfield University of Edinburgh

R. Walcott University of Edinburgh

N. White Cambridge University

A. Beauvais IRD Nouvelle-Calédonie

H. Bertrand Ecole Normale Supérieure de Lyon

M. Brunet Université de Poitiers

D. Chardon Observatoire Midi-Pyrénées

G. Choblet Université de Nantes

P.Y. Donnadieu LSCE

D. Frizon de la Motte Université de Cergy-Pontoise

P. Leturmy Université de Cergy-Pontoise

J. Gaillardet Institut de Physique du Globe de Paris

M. Schuster Université de Poitiers

Géosciences Rennes

S. Bonnet

S. Bourquin

P. Cobbold

L. Husson

D. Lague

F. Deschamps

P. Davy

P. Jermannaud

C. Helm

E. Le Breton

E. Le Guerroué

C. Le Carlier

C. Loiselet

F. Mvondo

A. Liget

Administration and organization : Marie-Paule Bertrand

Web Site : Alain-Hervé Le Gall

Abstract Book : Catherine Bertin



DIRECTIONS

Tuesday November 13th 2007

Meeting in Géosciences Rennes,

Unoiversité de Rennes 1

Campus de Beaulieu

Hall of CAREN building (b. 14b)

CAREN building (Rennes, France)

Beaulieu campus map small

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CAREN building


Wednesday November 14th to Friday November 16th 2007

Meeting at the MIR (Maison Internationale de Rennes)

7 Quai Châteaubriant, 35000 Rennes

Rennes downtown accommodation small

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ACCOMMODATION

Hotel Résidence Citéa

35 rue d’Antrain, 35700 Rennes

Tel.+33 2 23 21 26 00

Fax. +33 2 23 21 26 01

rennes@citea.com

Rennes downtown map small

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- bus map