A phase transition model for basins

1. 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

2. 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)

3. Example of rifting with mantle and crustal phase transitions in Tecmod (D. Schmid) Problem solved! Next: Application time! Fitting of real data...

4. 3370 Dr = ~2% 3306 Systematic mantle r(P,T,X), calculated with Perple_X garnet-spinel transition plag in Simon & Podladchikov, EPSL, 2008

5. Petrological densities (P,T) 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).

6. 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

7. 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

8. Crustal density variation as a function of P, T and composition Semprich et al., 2010, IJES H2O out 3200 Dr = >10% eclogite 2900 granulite

9. Applications • 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)

10. Craig et al., 2011, GJI, Congo basin • thick lithosphere and long sedimentary record • in compression and subsiding today • large negative gravity anomaly

11. Simple modeling of density/isostasy in compressed crust • 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

12. Typical subsidence pattern in cratonic basins worldwide 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

13. 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

14. Comparison of crust and mantle densities 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

15. Conclusions • 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).

16. Densification by compression and heating 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

18. lower crustal metamorphism due to burial and heating Crustal burial and metamorphism homogeneous thickening lithospheric folding Vermeesch et al., 2004 D. James, Nature, 2002

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

20. Typical subsidence in cratonic basins worldwide Armitage & Allan, 2010

21. Preservation of orogenic roots Fischer (2002) R = h/m Dr = 300 kgm-3 D. James, Nature, 2002

22. Fischer, Nature 2002

23. Fischer, Nature 2002 Cooling vs. heating for crustal densification (Dr~300 kgm-3) http://www.mantleplumes.org/ LowerCrust.html DRY WET

24. 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

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

26. Compositional dependence of density evolution - Only hydrated compositions produce dense root at quite high pressures.

27. Conclusions • 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…

29. Interplay of lower crustal metamorphism and continental lithosphere dynamics Nina S.C. Simon & Yuri Y. Podladchikov T. John

30. 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

31. Preservation of orogenic roots Fischer (2002) R = h/m Dr = 300 kgm-3 D. James, Nature, 2002

32. Fischer, Nature 2002 Our model WET Cooling vs. heating for crustal densification (Dr~300 kgm-3) http://www.mantleplumes.org/ LowerCrust.html DRY