Feedbacks between mountain building, erosion and climate Mikaël ATTAL Marsyandi valley, Himalayas,...

Feedbacks between Feedbacks between mountain building, mountain building, erosion and climateerosion and climate

Mikaël ATTALMikaël ATTAL

Marsyandi valley, Himalayas, Nepal

Acknowledgements: Acknowledgements: Jérôme Lavé, Jérôme Lavé, Peter van der BeePeter van der Beek and other k and other

scientists from LGCA (Grenoble) scientists from LGCA (Grenoble) and CRPG (Nancy)and CRPG (Nancy)

Eroding landscapes: Eroding landscapes: fluvial processesfluvial processes

Lecture overviewLecture overview

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

II. Erosion controls the geometry of mountainsII. Erosion controls the geometry of mountains??

III. Erosion controls the structure of mountainsIII. Erosion controls the structure of mountains??1) Orographic effect.1) Orographic effect.2) The curious case of the Himalayas. 2) The curious case of the Himalayas.

IV. To which extent does erosion affect deformation in mountains? IV. To which extent does erosion affect deformation in mountains? “Revisiting river anticlines”.“Revisiting river anticlines”.

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

1. Subduction

Convergence friction accretion

Example : Barbados accretionary prism

The Atlantic plate plunges under the Caribbean plate

Source: C. Beck, Chambery

Structure of the prism

(seismic imagery + bathymetry)

Source: C. Beck, Chambery

Example : Barbados accretionary prism

2. Collision

Convergence friction accretion

Source: "GEOL-ALP" (http://www.geol-alpes.com), Maurice GIDON, 1998-2003

Building of the Alps (schematic representation):

Structural map of the AlpsThe “piémontaise” units (dark green) correspond to the relicts of the Alpine ocean: to the west, units are European; to the east, they are African

Geological cross-section of the Alps (Schmid, 2000)New tectonic interpretation of the ECORS-CROP profile

Suturezone

European Units

African Units

Flexural basinNW SE

Geological cross-sections across the Himalayas

Zhao, 1993

Lavé & Avouac, 2001

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.

Davis, Suppe & Dahlen, JGR, 1983

“Mechanics of Fold-and-Thrust Belts and Accretionary Wedges”The critical taper theoryThe critical taper theory

Characteristics of the accretionary wedge:

- basal decollement,

- important compressive deformation above decollement, minor deformation below,

- taper shaped.

Mechanical model

Coulomb failure criterion :

SO n pf

Mechanical model

τ = shear traction at failure,

S0 = cohesive strength,

μ = coefficient of friction,

σn = normal traction,

pf = pore fluid pressure.

The critical taper

gHsin w gDsin( ) b d

dx xdz 0

O

H

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

f (, b, )

p f w gD

z w gD

R F

where

Linear relationship between α and β

and μ = coefficient of friction(μb = basal coef.)

β

α

α

β

Application to natural objects: Taiwan

Topographic profiles

(Western Range of Taiwan):

values of α

f (, b, )β

α

Determination of the

parameters producing the

best fit between

model and field data

f (, b, )β

α

Other examples:values of α and β

f (, b, )β

α

Constraining the parameters

β

α

R FLinear

relationship between α and β

f (, b, )

Modification of the equilibrium

A

A: subcritical / “stable” α can increase.

Example: mountain building

B: critical taper. α cannot increase anymore. If α > critical value, the taper becomes supercritical / unstable and collapses.

β

α

A

β

B

α

B

X

C: to carry on growing, the taper cannot steepen anymore so it has to “expand” horizontally as well as vertically.

β

C

α

C

Willett & Brandon, Geology, 2002

Steady-state: FE = FA

II. Erosion controls the geometry of mountains?II. Erosion controls the geometry of mountains?

FA = flux of material accreted,

FE = flux of material eroded.

Flux

Time

FA

FE

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.

A

B C

D

AB

CD

D: FA = FE steady-state.





Flux

Time

FA

FE

F: mountain grows again FE increases until a new steady-state is reached (FA = FE)

E: drop in FE (e.g., climate change with less rain) erosion rate decreases the topography is not at steady-state anymore.

A

B C

D

A

D-E

E

F

FD







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).


Evolution of the Alps (Schlunegger et al., 2001, 2002)

Schlunegger et al., 2001


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.

D: FA = FE steady-state.





Flux

Time

FA

FE

F: mountain grows again FE increases until a new steady-state is reached (FA = FE)

E: drop in FE (e.g., more resistant rocks exposed) erosion rate decreases the topography is not at steady-state anymore.

A

B C

D

A

D-E

E

F

FD


Willett et al., 1993

III. Erosion controls the structure of mountains?III. Erosion controls the structure of mountains?1) Orographic effect.1) Orographic effect.

Willett, JGR, 1999

Dominant wind/rain on retro - side Dominant wind/rain on pro - side


Field data: Southern Alps, New Zealand

Metamorphism grade Rainfall


Willett, JGR, 1999

Metamorphism grade Rainfall

Laumontite (L), prehnite-pumpellyite (Pr+Pu), pumpellyite (Pu), and chlorite-epidote (Cl+Ep).

Field data: Olympic Mts, NW USA


Willett, JGR, 1999


Willett, JGR, 1999

Southern Alps, New Zealand: erosion on the retro side


Willett, JGR, 1999

Olympic Mts, NW USA: erosion on the pro side

III. Erosion controls the structure of mountains?III. Erosion controls the structure of mountains?2) The curious case of the Himalayas2) The curious case of the Himalayas

NOTE: deadlines for essays = Monday 23rd March, 16:00

Break

III. Erosion controls the structure of mountains?III. Erosion controls the structure of mountains?2) The curious case of the Himalayas2) The curious case of the Himalayas

(Pat

riat

& A

chac

he, 8

1)

(Pat

riat

& A

chac

he, 8

1)III. Erosion controls the structure of mountains?III. Erosion controls the structure of mountains?

2) The curious case of the Himalayas2) The curious case of the Himalayas

India – Asia collision

Convergence rate India / Asia

mm/yr

~50 mm/yr at the moment

(Tap

ponn

ier

et a

l., 2

001)

The highest mountain range + the largest high-altitude The highest mountain range + the largest high-altitude plateau on Earthplateau on Earth

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