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Vertical Movements Geology 1 G. Bertotti

Vertical movement

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Page 1: Vertical movement

Vertical Movements

Geology 1

G. Bertotti

Page 2: Vertical movement

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Lateral deformations can change the thickness of the lithospheric column. This has major implications vertical movements (subsidence and uplift) of the Earth’s surface leading to sedimentary basins and mountains

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Page 3: Vertical movement

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linking weight to topography Linking weight changes to vertical movements

H ρ1 h1

Isostasy is Archimedes’ law: The weight of the floating body must be equal to that of the displaced body ρ2

z

H h1 2 1

2

1 g h1 2 g (h1 H )

1 h1 2 h1 2 H

H is the topography, the number we are interested in

CD

An equivalent expression: the weight of the column above a compensation depth must be the same CD=the depth under which ..nothing happens 2

3

H h1 (2 1

)

(h1 H )2 1

h1

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Lateral changes in topography controlled by changes in the weight of the column overlying the CD

ρ1

ρ2 z

h1 H

CD

http://www.geo.cornell.edu/hawaii/220/PRI/isostasy.html

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H h1 (2 1

) The lower the density of the column, the higher the mountains The higher the thickness of the column, the higher will be the topography

One can change densities (left), thicknesses (right) or both

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What about the Earth: Where is the compensation level? which is the floating block?

The best candidate for the compensation level is the LVZ with the lithosphere as floating block

If (and this seems to be true) it floats, the density of the lithosphere must be less than that of the asthenosphere

lith < asth.

LVZ

Remember that the lithosphere is a two components system, lith. mantle and (oceanic-continental) crust

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Page 6: Vertical movement

Depth of the LAB (here defined as the 1200°C isotherm) in Europe

5{) -

,G.,I 48- 140

: I :!:: 1'":' 46- ·Q ..J

44-

42.-

4 ( ) -

38 -

J 6

10 .5

+ '.lb

2.30 22.0 210 11J

+ !13.

itGO 87

101 + 106 +

+ 200 1

D 1 0 1801

H

+ n T

! E 1-l-3 + u + + '}1 + l<tJ

1,, + + 71 .,,"IP 1701 12. 115 +

7C 81 + HD + 12: l

.... - 1GO + +

io + In'

+ n + 1SO I

... 10:1 '60 1MJ1 ....

+ - 1:30 . H 113

+ 1210· + 1<fa 4t 110· .

1001 ,71b 9{)

80 7() 60

<) 50 U.BDepth

Ucm J 120

"'

D 5 1D 15 Latitude

20 25 30 351

TU Delft 6

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Crustal thickness map of europe

The Moho map of Europe

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(km)

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Changing weights (= vertical movements) in the Earth

lith < asth. This is what we had. Life seems easy If the lithosphere becomes lighter topography increases If lithosphere becomes heavier topography decreases

Geologically not really useful, the lithosphere is a complicated “block”

crust

lith. mantle

asthen.

Moho

LAB

ρ crust = 2.6 g*cm-3

ρ lith.mant = 3.6 g*cm-3

ρ asthen = 3.0 g*cm-3

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crust < asth. < lith mantle

It seems that crust and lithospheric mantle play a different game!

We need to look in more detail processes controlling their thickness changes and add them together (quantitative geology!)

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We make a conceptual experiment (= simpler than nature)

We investigate the vertical movements produced by thinning/thickening of the crust and lithospheric mantle separately

We do this by predicting vertical movements 1) during thickness changes 2) after thickness changes

time

dept

h

old young

defo

rmat

ion

A useful tool to describe vertical movements through time is a subsidence curve

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Page 11: Vertical movement

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Easy: • crustal thinning causes downward movement (=subsidence) • crustal thickening causes uplift

thinning

Effects of changes in crustal thickness

thickening

vertical movements

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Nothing happens, no crust-driven vertical movements

vertical movements vertical movements

After the end of crustal thickness changes….

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More complicated as the base of the lithosphere is a thermal boundary

In terms of mass movements

- During “extension/thinning”, mantle rocks will tend to move upward

- during “contraction/thickening”, mantel rocks will tend to move downward

But

If the mantle changes in thickness or not depends on the competition between advection (movement of mass) and conduction (movement of heat)

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The lithospheric mantle

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The competition between conduction and advection: You better sail fast!

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when thinning is slow conduction>>advection ► little thermal anomaly ► little lithospheric thinning

If conduction prevails isotherms remain flat and the lithosphere does not thin

If advection prevails isotherms are deflected and the lithosphere becomes thinner

when thinning is fast advection>> conduction ► strong thermal anomaly ► strong lithospheric thinning

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In the case of extension

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Back to our cartoons of the lithospheric mantle

Fast deformation No thickness changes = little/no vertical movements driven by the lithospheric mantle

The extension case

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The contraction case

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Important thickness changes

In the case of extension the lithospheric mantle thickens!

In the case of contraction

the lithospheric mantle thins and pushed the Earth surface up!

vertical movements

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The thickness of the lithospheric mantle changes and vertical movements occur

at the end of deformation long time after

Following the end of deformation: Fast deformation

exte

nsio

n

subsidence

shor

teni

ng

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uplift

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exte

nsio

n No changes in the thickness of the lithospheric mantle and no vertical movements occur

Following the end of deformation: slow deformation

at the end of deformation long time after

nothing

shor

teni

ng

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nothing

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The rates of deformation controls the magnitude of the thermal anomaly present at the end of deformation

The magnitude of vertical movements depends on the amplitude of the thermal anomaly

The faster extension/shortening, the stronger will be vertical movements after the end of deformation

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fast slow

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durin

g ex

tens

ion

The real world: combining crust and lithospheric mantle

follo

win

g ex

tens

ion

no subsidence some subsidence

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total

mantle

crust fast

total

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crust

mantle

slow

During contraction/thickening

durin

g co

ntra

ctio

n fo

llow

ing

cont

ract

ion

no uplift some uplift

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What is the meaning of “slow” and “fast”? No general answer is possible without knowledge of the

geometry of the system and numerical modelling

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Recap:

during thickness changes: •the crust simply wants subsidence/uplift •what the lithospheric mantle wants ... depends on the rate of thinning/thickening

Following thickness changes: • the crust has nothing more to say (neglecting erosion) •the lithospheric mantle will impose vertical movements; their magnitude depends on the amplitude of the thermal anomaly present at the end of thinning/thickening

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Processes leading to load changes

Tectonics (thinning)

b) 0

10

20

30

d4e0 pth (km) 0 50 100

distance (km) 150 200 250

? ?

Moho

Intra mantle reflections

High velocity body (+8 km/s)

Where is the site of maximum thinning? Where is the site of maximum subsidence?

Is there correspondence between the two?

Knowing that the green sediments were deposited until 70Ma, what will be the shape of the LAB?

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Tectonics (thickening)

other less common processes can change the density of the rocks. One of them is metamoprhism

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When material is eroded, the surface of the Earth goes down but the lower part of the block (the crust) comes up!

Erosion

ρ1

ρ 2 z

h1

H ρ1

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This leads to less mountain, less erosion and eventually an equilibrium profile

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The rivers “pull up” rocks from below

Valleys are the places where the strongest erosion takes place

An extreme situation

The Indus river valley

Rocks are found along the river which were formed at depth of >10km and have been brought to the surface in very recent times

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