1. Feedbacks between mountain building, erosion and climate
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

2. 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”.

3. I. Introduction: mountain building and the critical taper theory.

4. Convergence friction accretion
1. Subduction

5. Example : Barbados accretionary prism
The Atlantic plate plunges under the Caribbean plate
Source: C. Beck, Chambery

6. Example : Barbados accretionary prism
Structure of the prism
(seismic imagery + bathymetry)
Folds + thrusts
Source: C. Beck, Chambery

7. Convergence friction accretion
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" ( Maurice GIDON,

8. Geological cross-section of the Alps (Schmid, 2000)
New tectonic interpretation of the ECORS-CROP profile NW SE
Flexural basin
African Units
European Units
Suture
zone

9. 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

10. The critical taper theory Characteristics of the accretionary wedge:
“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.

11. Mechanical model

12. Mechanical model Coulomb failure criterion :
τ = shear traction at failure,
S0 = cohesive strength,
μ = coefficient of friction,
σn = normal traction,
pf = pore fluid pressure.
Tau : shear traction at failure, S0 = cohesive strength, mu = coefficient of friction, sigma_n = normal traction and pf = pore fluid pressure.

13. The critical taper
The Mohr diagram is used to solve the equation and describe the shape of the taper

14. Linear relationship between α and β
where
and μ = coefficient of friction
(μb = basal coef.) α
mu_b = basal coefficient of friction
Linear relationship between α and β β

15. Application to natural objects: Taiwan

16. Topographic profiles (Western Range of Taiwan): values of αβ α

17. Determination of the parameters producing the best fit between model and field dataβ α

18. Other examples:
values of α and β β α

19. Linear relationship between α and β
Constraining the parameters
Linear relationship between α and β β α

20. Modification of the equilibrium
Example: mountain building
A: subcritical / “stable”  α can increase. X C
B: critical taper. α cannot increase anymore. If α > 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. β α A β B α β C α

21. 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 α cannot increase anymore.
D: FA = FE  steady-state. The topography does not evolve anymore.
Flux D FA
Critical taper theory: taper growth is self similar, slopes on both sides are invariant through time
Willett & Brandon, Geology, 2002 C B
FA = flux of material accreted,
FE = flux of material eroded. FE A
Time

22. II. Erosion controls the geometry of mountains?D
D-E
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
Critical taper theory: taper growth is self similar, slopes on both sides are invariant through time
Willett & Brandon, Geology, 2002 C B E
FA = flux of material accreted,
FE = flux of material eroded. FE A
Time

23. 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).
Critical taper theory: taper growth is self similar, slopes on both sides are invariant through time
Willett & Brandon, Geology, 2002
FA = flux of material accreted,
FE = flux of material eroded.

24. II. Erosion controls the geometry of mountains?
Evolution of the Alps (Schlunegger et al., 2001, 2002)
Critical taper theory: taper growth is self similar, slopes on both sides are invariant through time
Schlunegger et al., 2001

25. 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.
Critical taper theory: taper growth is self similar, slopes on both sides are invariant through time

26. II. Erosion controls the geometry of mountains?D
D-E
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
Critical taper theory: taper growth is self similar, slopes on both sides are invariant through time
Willett & Brandon, Geology, 2002 C B E
FA = flux of material accreted,
FE = flux of material eroded. FE A
Time

27. III. Erosion controls the structure of mountains?
1) Orographic effect.
Willett et al., 1993

28. 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

29. III. Erosion controls the structure of mountains?
1) Orographic effect.
Field data: Southern Alps, New Zealand
Metamorphism grade
Rainfall
Willett, JGR, 1999

30. III. Erosion controls the structure of mountains?
1) Orographic effect.
Field data: Olympic Mts, NW USA
Metamorphism grade
Rainfall
Laumontite (L), prehnite-pumpellyite (Pr+Pu), pumpellyite (Pu), and chlorite-epidote (Cl+Ep).
Willett, JGR, 1999

31. III. Erosion controls the structure of mountains?
1) Orographic effect.
Southern Alps, New Zealand: erosion on the retro side
Willett, JGR, 1999

32. III. Erosion controls the structure of mountains?
1) Orographic effect.
Olympic Mts, NW USA: erosion on the pro side
Willett, JGR, 1999

33. NOTE: deadlines for essays = Monday 23rd March, 16:00
III. Erosion controls the structure of mountains?
2) The curious case of the Himalayas
Break
NOTE: deadlines for essays = Monday 23rd March, 16:00

34. III. Erosion controls the structure of mountains?
2) The curious case of the Himalayas
(Patriat & Achache, 81)

35. 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)

36.  The highest mountain range + the largest high-altitude plateau on Earth
(Tapponnier et al., 2001)