A phase transition model for basins Nina 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) crust (Baird et al., 1995) model for Williston basin

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 garnet-spinel transitionplag in  = ~2% Systematic mantle  (P,T,X), calculated with Perple_X

1000 m Simon & Podladchikov (2008); EPSL garnet-spinel sp-plag Change of mean column density during stretching (z lith1 :150 km, z crust1 : 35 km,  crust : 2900 kg/m 3, water-loaded subsidence) for different mantle compositions and TDD (  =  0 (1-  T)). 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 basaltwet pelite kg/m 3 eclogite granulite kg/m 3 Semprich et al., 2010, IJES

Thermal expansion coefficient of hydrous crust, normalized to  = 3x10 -5 Fe-Mg-rich metapelite, water saturated density Fe-Mg-rich metapelite, water saturatedAverage mafic lower crust (R&F), 4 wt% H 2 O

Crustal density variation as a function of P, T and composition eclogite granulite Semprich et al., 2010, IJES H 2 O out  = >10%

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 crust  c1 mantle  m= 3300 crust  c2  c1 P1 P2 P1 = P2 ww mantle  m= 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 km to ca 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 (-  ). Conclusions

Systematic density changes in buried crustal layer (f(composition) Initial layer thickness: 20 km Initial layer depth: km Initial lithosphere thickness: 140 km km ºC Densification by decompression Densification by compression and heating Densification by compression Semprich et al., 2010, IJES

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 roots Fischer (2002) D. James, Nature, 2002  = 300 kgm -3 R = h/m

Fischer, Nature 2002

Cooling vs. heating for crustal densification (  ~300 kgm -3 ) WET DRY 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 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 (-  ). 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 roots Fischer (2002) D. James, Nature, 2002  = 300 kgm -3 R = h/m

Fischer, Nature 2002 Cooling vs. heating for crustal densification (  ~300 kgm -3 ) DRY LowerCrust.html WET Our model