a phase transition model for basins nina simon main colaborators: yuri podladchikov, julia semprich...
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A phase transition model for basinsNina Simon
Main colaborators: Yuri Podladchikov, Julia Semprich
T. John
Blueschist to eclogite transition
Chazot et al., 2005, J.Pet. 46, 2527
Spinel- to plagioclase-peridotite transition
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P-T changes cause reactions and density changes in the mantle and crust
mantle (Kaus et al. 2005)
18.04.23
crust (Baird et al., 1995)model for Williston basin
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18.04.23
Example of rifting with mantle and crustal phase transitions in Tecmod (D. Schmid) Problem solved!
Next: Application time! Fitting of real data...
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Simon & Podladchikov, EPSL, 2008
Al2O3
Na2O
T [°C]
0.25 0.450.05
0.5
2.5
4.5
refractory
fertile
meltin
g trend
P [G
Pa]
garnet-spinel transition plag in
3306
3370
= ~2%
Systematic mantle (P,T,X), calculated with Perple_X
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1 1.5 2 2.5 3 3.5 4 4.5 5
-30
-25
-20
-15
-10
-5
0
4.5 wt% Al O2 3
2.5 wt% Al O2 3
0.5 wt% Al O2 3
RondaTDD
0.45 wt% Na O4.5 wt% Al O
2
2 3 0.05 wt% Na O 4.5 wt% Al O
2
2 3
b
2000
1600
1200
800
400
0
subsid
ence
[m]
R893
R123
1000 mSimon & Podladchikov (2008); EPSL
garnet-spinel
sp-plag
Change of mean column density during stretching (z lith1:150 km, zcrust1: 35 km, crust: 2900 kg/m3, water-loaded subsidence) for different mantle compositions and TDD (= 0(1-T)).
P [
GP
a]
T [ C]°
Petrological densities (P,T)
1. Mantle phase transitions produce density changes on the same order of magnitude than thermal expansion, and with the same sign.
2. Mantle phase transitions produce uplift in strongly stretched continental margins, without additional heating.
3. Phase transition uplift is equivalent to 700 ºC heating using = 0(1-T).
1% density decrease
(stretching factor)
Mantle densities and subsidence in thinned lithosphere
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Crustal densities: important reactions and variations with P-T
density varies non-linearly with P, T: grt-in, plag-out and dehydration reaction produce large density changes dehydration reactions are mainly T-dependent and can cause densification upon heating if water is released wet and dry rocks have fundamentally different P-T dependence of density
dry MOR basalt wet pelitekg/m3
eclogite
granulite
kg/m3
Semprich et al., 2010, IJES
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Thermal expansion coefficient of hydrous crust, normalized to = 3x10-5
Fe-Mg-rich metapelite, water saturated
density density
Fe-Mg-rich metapelite, water saturated
pT
1
Average mafic lower crust (R&F), 4 wt% H2O
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Crustal density variation as a function of P, T and composition
eclogite
granulite
Semprich et al., 2010, IJES
H2O out
= >10%
2900
3200
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Applications
Areas of relatively thick crust:
1. Compressional basins
a) Intra-cratonic basins
b) Foreland basins
(2. Preservation of orogenic roots vs. delamination)
(3. Subduction of hydrated oceanic crust)
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Craig et al., 2011, GJI, Congo basin
– thick lithosphere and long sedimentary record– in compression and subsiding today– large negative gravity anomaly
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Simple modeling of density/isostasy in compressed crust
1. Instantaneous pressure increase due to
1. far field stresses or/and
2. loading by sediments and/or thrusts (foreland basins)
2. Slow thermal re-equilibration
assuming perfect isostasy
crustc1
mantlem=3300
crustc2c1
P1 P2P1 = P2
w
mantlem=3300
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Subsidence due to compression in intra-cratonic basins
Armitage & Allan, 2010
Typical subsidence pattern in cratonic basins worldwide
Small pressure increase followed by conductive thermal re-equilibration
Semprich et al., IJES, accepted
dry compositions produce uplift
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Density and subsidence for large crustal burial/pressure increase
Large pressure increase (equivalent to burial from 20-40 km to ca. 55-75 km) followed by conductive thermal re-equilibration
- foreland basins or buckeled lithosphere- orogenic roots
Vermeesch et al., 2004
Semprich et al., 2010, IJES
dry
wet
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Comparison of crust and mantle densities
Largest is for restitic meta-pelite (not for dry MORB) – at least in an equilibrium world…
Density of dry meta-basalt exceeds mantle densities at sub-Moho depths
mantle (1 GPa)
Semprich et al. (2010), IJES
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• Variations in mantle compositions can cause 1-2 % of density difference, as can P-T variations. Mantle phase transitions enhance the effect of temperature increase (up to 100%) if the crust is thin.
• Crustal densities vary by >10% due to composition and >10% due to P-T in the same rock. Dehydration reactions cause massif densification upon heating and therefore counteract thermal expansion during T increase. Re-hydration will lower density without any increase in temperature.
• Restitic wet meta-pelites have comparable to wet meta-mafic crust. Absolute densities of sub-Moho meta-mafic crust exceed mantle densities whereas more pelitic compositions approach mantle densities.
• Intra-cratonic basins: response to episodic compression will be stepwise subsidence. Compressional events are preserved in the sedimentary record due to phase transitions and densification of the lower crust.
• Lower crustal metamorphism due to heating can account for the extra mass needed to explain the preservation of orogenic roots and foreland basins after the end of compression.
Remarks: Dehydration reactions are less inhibited by kinetics compared to dry reactions. But: Dehydration usually only happens once. The models proposed here require efficient drainage of fluids. Mafic rocks may also dehydrate and densify during decompression (-b).
Conclusions
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Systematic density changes in buried crustal layer (f(composition)
• Initial layer thickness: 20 km
• Initial layer depth: 20-40 km
• Initial lithosphere thickness: 140 km
20-40 km
300-476 ºC
Densification by decompression
Densification by compression and heating
Densification by compression
Semprich et al., 2010, IJES
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18.04.23
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Vermeesch et al., 2004
D. James, Nature, 2002
Crustal burial and metamorphism
homogeneous thickening
lithospheric folding
lower crustal metamorphism due to burial and heating
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Model for the E. Barents Sea (Semprich et al. 2010)
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Armitage & Allan, 2010
Typical subsidence in cratonic basins worldwide
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Preservation of orogenic rootsFischer (2002)
D. James, Nature, 2002
=300
kgm-3
R = h/m
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Fischer, Nature 2002
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Fischer, Nature 2002
Cooling vs. heating for crustal densification (~300 kgm-3)
WET DRY
http://www.mantleplumes.org/LowerCrust.html
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England & Thompson 1984
P-T evolution of thickened crust in mountains (conservative)
• crustal thickening deepens and pressurizes the lower crust (fast process)
• heating of the buried lower crust (slow, 100’ Ma)
dehydration due to heating leads to densification and prevents complete rebound and flattening of root
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Density evolution of thickened crust (ca. 55-75 km)
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Compositional dependence of density evolution
- Only hydrated compositions produce dense root at quite high pressures.
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Dehydration reactions cause strong densification upon heating under certain P-T condition (> -10*) and therefore counteract thermal expansion during T increase. Mafic rocks may also dehydrate and densify during decompression (-b).
• Intra-cratonic basins: response to episodic compression will be stepwise subsidence. Compressional events are preserved in the sedimentary record due to phase transitions and densification of the lower crust.
• Lower crustal metamorphism can account for extra mass needed to explain the preservation of orogenic roots and foreland basins after the end of compression.
• Dehydration reactions are less inhibited by kinetics compared to dry reactions.
• But: Dehydration game can usually only be played once…
• Note: All my models require efficient drainage of fluids…
Conclusions
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Interplay of lower crustal metamorphism and continental
lithosphere dynamics
Nina S.C. Simon & Yuri Y. Podladchikov
T. John
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Vermeesch et al., 2004
D. James, Nature, 2002
Compression and metamorphism in basins and orogens
homogeneous thickening
lithospheric folding
lower crustal metamorphism in thickened crust due to burial and heating
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Preservation of orogenic rootsFischer (2002)
D. James, Nature, 2002
=300
kgm-3
R = h/m
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Fischer, Nature 2002
Cooling vs. heating for crustal densification (~300 kgm-3)
DRY
http://www.mantleplumes.org/LowerCrust.html
WET
Our model