Date post: | 22-May-2015 |
Category: |
Technology |
Upload: | tu-delft-opencourseware |
View: | 644 times |
Download: | 1 times |
Vertical Movements
Geology 1
G. Bertotti
4 ‒
vert
ical
mov
emen
ts 4 – 11 September
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
2
4 ‒
vert
ical
mov
emen
ts Isostasy:
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
4 ‒
vert
ical
mov
emen
ts
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
2
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
4
4 ‒
vert
ical
mov
emen
ts
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
5
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
4 ‒
vert
ical
mov
emen
ts
Crustal thickness map of europe
The Moho map of Europe
7
(km)
4 ‒
vert
ical
mov
emen
ts
8
4 ‒
vert
ical
mov
emen
ts
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
9
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!)
4 ‒
vert
ical
mov
emen
ts
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
10
4 ‒
vert
ical
mov
emen
ts
Easy: • crustal thinning causes downward movement (=subsidence) • crustal thickening causes uplift
thinning
Effects of changes in crustal thickness
thickening
vertical movements
11
4 ‒
vert
ical
mov
emen
ts
Nothing happens, no crust-driven vertical movements
vertical movements vertical movements
After the end of crustal thickness changes….
12
4 ‒
vert
ical
mov
emen
ts
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)
13
The lithospheric mantle
4 ‒
vert
ical
mov
emen
ts
The competition between conduction and advection: You better sail fast!
14
4 ‒
vert
ical
mov
emen
ts
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
15
In the case of extension
4 ‒
vert
ical
mov
emen
ts
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
16
The contraction case
4 ‒
vert
ical
mov
emen
ts The lithospheric mantle during slow deformation:
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
17
4 ‒
vert
ical
mov
emen
ts
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
18
uplift
4 ‒
vert
ical
mov
emen
ts
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
19
nothing
4 ‒
vert
ical
mov
emen
ts
20
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
4 ‒
vert
ical
mov
emen
ts
fast slow
21
durin
g ex
tens
ion
The real world: combining crust and lithospheric mantle
follo
win
g ex
tens
ion
no subsidence some subsidence
4 ‒
vert
ical
mov
emen
ts
total
mantle
crust fast
total
22
crust
mantle
slow
During contraction/thickening
durin
g co
ntra
ctio
n fo
llow
ing
cont
ract
ion
no uplift some uplift
4 ‒
vert
ical
mov
emen
ts
What is the meaning of “slow” and “fast”? No general answer is possible without knowledge of the
geometry of the system and numerical modelling
23
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
4 ‒
vert
ical
mov
emen
ts
24
4 ‒
vert
ical
mov
emen
ts
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?
25
4 ‒
vert
ical
mov
emen
ts
Tectonics (thickening)
other less common processes can change the density of the rocks. One of them is metamoprhism
26
4 ‒
vert
ical
mov
emen
ts
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
27
This leads to less mountain, less erosion and eventually an equilibrium profile
4 ‒
vert
ical
mov
emen
ts
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
28
Sources of figures http://tsjok45.wordpress.com/2013/06/24/geologie-trefwoord-c/ http://bc.outcrop.org/GEOL_B10/Dlecture10.html http://dc401.4shared.com/doc/Ch0Ecp8M/preview.html http://www.wired.com/wiredscience/2008/04/how-deep-is-eur/ http://www.ees.lehigh.edu/groups/corners/galleries/mapdata/pages/nanga_map.html