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A phase transition model for basins. Nina Simon Main colaborators: Yuri Podladchikov, Julia Semprich. Spinel- to plagioclase-peridotite transition. Blueschist to eclogite transition. T. John. Chazot et al., 2005, J.Pet. 46, 2527.

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A phase transition model for basins

Nina Simon

Main colaborators: Yuri Podladchikov, Julia Semprich

Spinel- to plagioclase-peridotite transition

Blueschist to eclogite transition

T. John

Chazot et al., 2005, J.Pet. 46, 2527


P t changes cause reactions and density changes in the mantle and crust
P-T changes cause reactions and density changes in the mantle and crust

crust (Baird et al., 1995)

model for Williston basin

mantle (Kaus et al. 2005)


Example of rifting with mantle and crustal phase transitions in Tecmod (D. Schmid) Problem solved!

Next: Application time! Fitting of real data...


3370 in Tecmod (D. Schmid)

Dr = ~2%

3306

Systematic mantle r(P,T,X), calculated with Perple_X

garnet-spinel transition

plag in

Simon & Podladchikov, EPSL, 2008


Petrological densities (P,T) in Tecmod (D. Schmid)

1000 m

Mantle densities and subsidence in thinned lithosphere

Change of mean column density during stretching (zlith1:150 km, zcrust1: 35 km, rcrust: 2900 kg/m3, water-loaded subsidence) for different mantle compositions and TDD (r = r0(1-aT)).

sp-plag

garnet-spinel

Simon & Podladchikov (2008); EPSL

1% density decrease

(stretching factor)

  • Mantle phase transitions produce density changes on the same order of magnitude than thermal expansion, and with the same sign.

  • Mantle phase transitions produce uplift in strongly stretched continental margins, without additional heating.

  • Phase transition uplift is equivalent to 700 ºC heating using r = r0(1-aT).


Crustal densities: important reactions and variations with P-T

dry MOR basalt

wet pelite

kg/m3

kg/m3

eclogite

granulite

  •  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

Semprich et al., 2010, IJES


Thermal expansion coefficient of hydrous crust, normalized to a = 3x10-5

Fe-Mg-rich metapelite, water saturated

Average mafic lower crust (R&F), 4 wt% H2O

Fe-Mg-rich metapelite, water saturated

density

density


Crustal density variation as a function of P, T and composition

Semprich et al., 2010, IJES

H2O out

3200

Dr = >10%

eclogite

2900

granulite


Applications composition

  • Areas of relatively thick crust:

  • Compressional basins

    • Intra-cratonic basins

    • Foreland basins

  • (2. Preservation of orogenic roots vs. delamination)

  • (3. Subduction of hydrated oceanic crust)


Craig et al., 2011, GJI, Congo basin composition

  • thick lithosphere and long sedimentary record

  • in compression and subsiding today

  • large negative gravity anomaly


Simple modeling of density/isostasy in compressed crust composition

  • Instantaneous pressure increase due to

    • far field stresses or/and

    • loading by sediments and/or thrusts (foreland basins)

  • Slow thermal re-equilibration

  • assuming perfect isostasy

rw

crust

rc1

crust

rc2 >rc1

mantle

rm=3300

mantle

rm=3300

P2

P1

P1 = P2


Typical subsidence pattern in cratonic basins worldwide composition

Armitage & Allan, 2010

Subsidence due to compression in intra-cratonic basins

Small pressure increase followed by conductive thermal re-equilibration

dry compositions produce uplift

Semprich et al., IJES, accepted


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

dry

Vermeesch et al., 2004

wet

Semprich et al., 2010, IJES


Comparison of crust and mantle densities increase

mantle (1 GPa)

  • Largest Dr 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

Semprich et al. (2010), IJES


Conclusions increase

  • 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 Dr 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).


Densification by compression and heating increase

Densification by decompression

20-40 km

300-476 ºC

Densification by compression

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

Semprich et al., 2010, IJES


lower crustal metamorphism due to burial and heating increase

Crustal burial and metamorphism

homogeneous thickening

lithospheric folding

Vermeesch et al., 2004

D. James, Nature, 2002



Typical subsidence in cratonic basins worldwide increase

Armitage & Allan, 2010


Preservation of orogenic roots increase

Fischer (2002)

R = h/m

Dr =

300 kgm-3

D. James, Nature, 2002



Fischer, Nature 2002 increase

Cooling vs. heating for crustal densification (Dr~300 kgm-3)

http://www.mantleplumes.org/

LowerCrust.html

DRY

WET


England & Thompson 1984 increase

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



Compositional dependence of density evolution increase

- Only hydrated compositions produce dense root at quite high pressures.


Conclusions increase

  • Dehydration reactions cause strong densification upon heating under certain P-T condition (> -10*a) 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…


Interplay of lower crustal metamorphism and continental lithosphere dynamics

Nina S.C. Simon & Yuri Y. Podladchikov

T. John


lower crustal metamorphism in thickened crust due to burial and heating

Compression and metamorphism in basins and orogens

homogeneous thickening

lithospheric folding

Vermeesch et al., 2004

D. James, Nature, 2002


Preservation of orogenic roots and heating

Fischer (2002)

R = h/m

Dr =

300 kgm-3

D. James, Nature, 2002


Fischer, Nature 2002 and heating

Our model

WET

Cooling vs. heating for crustal densification (Dr~300 kgm-3)

http://www.mantleplumes.org/

LowerCrust.html

DRY


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