Feedbacks between mountain building, erosion and climate

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Eroding landscapes fluvial processes
Feedbacks between mountain building, erosion and
climate
Mikaël ATTAL
Acknowledgements Jérôme Lavé, Peter van der Beek
and other scientists from LGCA (Grenoble) and
CRPG (Nancy)
Marsyandi valley, Himalayas, Nepal
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Lecture overview
I. Introduction mountain building and the
critical taper theory. II. Erosion controls the
geometry of mountains? III. Erosion controls the
structure of mountains? 1) Orographic
effect. 2) The curious case of the Himalayas.
IV. To which extent does erosion affect
deformation in mountains? Revisiting river
anticlines.
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I. Introduction mountain building and the
critical taper theory.
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1. Subduction
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Example Barbados accretionary prism
The Atlantic plate plunges under the Caribbean
plate
Source C. Beck, Chambery
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Example Barbados accretionary prism
Structure of the prism (seismic imagery
bathymetry)
Source C. Beck, Chambery
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2. Collision
Building of the Alps (schematic representation)
Structural map of the Alps The piémontaise
units (dark green) correspond to the relicts of
the Alpine ocean to the west, units are
European to the east, they are African
Source "GEOL-ALP" (http//www.geol-alpes.com),
Maurice GIDON, 1998-2003
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Geological cross-section of the Alps (Schmid,
2000) New tectonic interpretation of the
ECORS-CROP profile
NW
SE
Flexural basin
African Units
Suture zone
European Units
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Geological cross-sections across the Himalayas
Indus-Tsangpo suture is what remains of the
ocean which has been closed due to the
convergence and collision of the Asia and India
plates. To the south, Indian units to the north,
Asian units.
Zhao, 1993
Lavé Avouac, 2001
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The critical taper theory
Mechanics of Fold-and-Thrust Belts and
Accretionary Wedges
Davis, Suppe Dahlen, JGR, 1983
Characteristics of the accretionary wedge
- basal decollement, - important compressive
deformation above decollement, minor deformation
below, - taper shaped.
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Mechanical model
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Mechanical model
Coulomb failure criterion
t shear traction at failure, S0 cohesive
strength, µ coefficient of friction, sn
normal traction, pf pore fluid pressure.
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The critical taper
The Mohr diagram is used to solve the equation
and describe the shape of the taper
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a
ß
where
and µ coefficient of friction (µb basal coef.)
a
Linear relationship between a and ß
ß
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Application to natural objects Taiwan
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Topographic profiles (Western Range of Taiwan)
values of a
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Determination of the parameters producing the
best fit between model and field data
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Other examples values of a and ß
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Constraining the parameters
Linear relationship between a and ß
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Modification of the equilibrium
Example mountain building
A subcritical / stable ? a can increase.
X
C
B critical taper. a cannot increase anymore. If
a gt critical value, the taper becomes
supercritical / unstable and collapses.
B
A
C to carry on growing, the taper cannot steepen
anymore so it has to expand horizontally as
well as vertically.
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II. Erosion controls the geometry of mountains?
D
Steady-state FE FA
C
B
A
A no topography, FE 0.
B mountain grows ? FE increases.
C critical taper stage, slope a cannot increase
anymore.
D FA FE ? steady-state. The topography does
not evolve anymore.
Flux
D
FA
Willett Brandon, Geology, 2002
C
B
FA flux of material accreted, FE flux of
material eroded.
FE
Time
A
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II. Erosion controls the geometry of mountains?
F
D-E
D
Steady-state FE FA
A
D FA FE ? steady-state.
E drop in FE (e.g., climate change with less
rain) ? erosion rate decreases ? the topography
is not at steady-state anymore.
F mountain grows again ? FE increases until a
new steady-state is reached (FA FE)
Flux
D
F
FA
Willett Brandon, Geology, 2002
C
B
E
FA flux of material accreted, FE flux of
material eroded.
FE
Time
A
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II. Erosion controls the geometry of mountains?
Steady-state FE FA
? Erosion controls the GEOMETRY of the mountain
range
Remark real mountains are more complex -
presence of discontinuities (e.g. faults), -
different lithologies (more resistant in the core
of the range), - change in crust rheology (e.g.
lower crust partially molten under Tibet ? no
basal friction).
Willett Brandon, Geology, 2002
FA flux of material accreted, FE flux of
material eroded.
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II. Erosion controls the geometry of mountains?
Evolution of the Alps (Schlunegger et al., 2001,
2002)
Schlunegger et al., 2001
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II. Erosion controls the geometry of mountains?
Evolution of the Alps (Schlunegger et al., 2001,
2002)
Schlunegger et al., 2001
Crystalline rocks exhumed 20 Ma ago ? decrease
in erosion rate ? range grows and widens.
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II. Erosion controls the geometry of mountains?
F
D-E
D
Steady-state FE FA
A
D FA FE ? steady-state.
E drop in FE (e.g., more resistant rocks
exposed) ? erosion rate decreases ? the
topography is not at steady-state anymore.
F mountain grows again ? FE increases until a
new steady-state is reached (FA FE)
Flux
D
F
FA
Willett Brandon, Geology, 2002
C
B
E
FA flux of material accreted, FE flux of
material eroded.
FE
Time
A
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III. Erosion controls the structure of
mountains? 1) Orographic effect.
Willett et al., 1993
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III. Erosion controls the structure of
mountains? 1) Orographic effect.
Dominant wind/rain on retro - side
Dominant wind/rain on pro - side
Willett, JGR, 1999
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III. Erosion controls the structure of
mountains? 1) Orographic effect.
Field data Southern Alps, New Zealand
Rainfall
Metamorphism grade
Willett, JGR, 1999
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III. Erosion controls the structure of
mountains? 1) Orographic effect.
Field data Olympic Mts, NW USA
Metamorphism grade
Rainfall
Laumontite (L), prehnite-pumpellyite (PrPu),
pumpellyite (Pu), and chlorite-epidote (ClEp).
Willett, JGR, 1999
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III. Erosion controls the structure of
mountains? 1) Orographic effect.
Southern Alps, New Zealand erosion on the retro
side
Willett, JGR, 1999
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III. Erosion controls the structure of
mountains? 1) Orographic effect.
Olympic Mts, NW USA erosion on the pro side
Willett, JGR, 1999
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III. Erosion controls the structure of
mountains? 2) The curious case of the Himalayas
Break
NOTE deadlines for essays Monday 23rd March,
1600
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III. Erosion controls the structure of
mountains? 2) The curious case of the Himalayas
(Patriat Achache, 81)
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III. Erosion controls the structure of
mountains? 2) The curious case of the Himalayas
India Asia collision
mm/yr
50 mm/yr at the moment
Convergence rate India / Asia
(Patriat Achache, 81)
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? The highest mountain range the largest
high-altitude plateau on Earth
(Tapponnier et al., 2001)