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

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

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...

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

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

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

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

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

eclogite

granulite

Semprich et al., 2010, IJES

H2O out

= >10%

2900

3200

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)

Craig et al., 2011, GJI, Congo basin

– thick lithosphere and long sedimentary record– in compression and subsiding today– large negative gravity anomaly

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

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

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

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

• 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

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

18.04.23

Vermeesch et al., 2004

D. James, Nature, 2002

Crustal burial and metamorphism

homogeneous thickening

lithospheric folding

lower crustal metamorphism due to burial and heating

Model for the E. Barents Sea (Semprich et al. 2010)

Armitage & Allan, 2010

Typical subsidence in cratonic basins worldwide

Preservation of orogenic rootsFischer (2002)

D. James, Nature, 2002

=300

kgm-3

R = h/m

Fischer, Nature 2002

Fischer, Nature 2002

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

WET DRY

http://www.mantleplumes.org/LowerCrust.html

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

Density evolution of thickened crust (ca. 55-75 km)

Compositional dependence of density evolution

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

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

Interplay of lower crustal metamorphism and continental

lithosphere dynamics

Nina S.C. Simon & Yuri Y. Podladchikov

T. John

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

Preservation of orogenic rootsFischer (2002)

D. James, Nature, 2002

=300

kgm-3

R = h/m

Fischer, Nature 2002

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

DRY

http://www.mantleplumes.org/LowerCrust.html

WET

Our model