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Understanding craton formation through their geochemical and geophysical characteristics A Preliminary Report CIDER 2012 Lithosphere Group Presenters:

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Presentation on theme: "Understanding craton formation through their geochemical and geophysical characteristics A Preliminary Report CIDER 2012 Lithosphere Group Presenters:"— Presentation transcript:

1 Understanding craton formation through their geochemical and geophysical characteristics A Preliminary Report CIDER 2012 Lithosphere Group Presenters: Huaiyu Yuan & Pierre Bouilhol Group: Cathleen Doherty, Erica Emry, Beth Paulson, Mingming Li, Doug Wiens

2 Getting Started…. History of the project Who’s involved –8 members of the group –4 geophysicists, 3 geochemists, 1 geodynamicist

3 Goals of Project First order observations: –Layered vs. non-layered, corresponding to differences in composition. –Why are these cratons are so different? but still all cratons? 3 Cratons –try to bring together existing geophysical observations –new geophysical evidence where needed (Beth SRF study) –try to add existing geochemical database of xenoliths and crustal rocks. Main goal: understand the differences between cratons, which would ultimately help us to better understand their formation…

4 Focus Sites Three cratons: –Slave –Kaapvaal –North Atlantic Slave & Kaapvaal are both well-studied, lots of geophysical & geochemical datasets North Atlantic has some xenolith data, but few geophysical experiments

5 Slave Bostock 1998 Dipping Upper mantle reflector at 100 km indicates fossil subductions

6 Slave o Spatial overlap of velocity discontinuity (left; from receiver functions) and conductive anomaly (right; from Magnitotelluric studies) indicates paleo- subduction interface o Other receiver functions see the boundary too (et. Abt et al. 2010; Miller et al 2011; Yuan et al 2006) Chen et al. 2009; Jones et al P-wave receiver functions Electrical Resistivity

7 Griffin et al. Lithos, 2004 Slave Shallow Anisotropy shallow “red” layer = highly depleted chemical layer (Mg# 92%) Slave Craton Yuan and Romanowicz 2010

8 To add: –Ages –More constraints on error/uncertainty –More geochemical data Slave

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10 van der Velden and Cook JGR 2005 fossil subductions indicated by “dipping upper mantle reflectors” from LithoProbe project in many places

11 van der Velden and Cook JGR 2005 Subduction trench (suture) parallel = Shallow Anisotropy Direction Yuan et al. 2011

12 North Atlantic: New data from Receiver Function

13 North Atlantic Craton Limited # of SRFs: indicating presence of layering in the shallow upper mantle around 100 km depth. Shear-wave receiver functions in the North Atlantic craton

14 North Atlantic Craton Chemical Layering from olivine Mg #: shallow, highly depleted ver. bottom less depleted North Atlantic craton is consistent with North American craton in general

15 N. Atlantic Craton seems to show geophysical similarities with Slave craton

16 Kaapvaal Kaapvaal is different from Slave and North Atlantic craton No evidence for layering within the lithosphere

17 Group 1 Group 2 Kaapvaal Yuan & Romanowicz, AGU 2012 DI21A-2352

18 Group 1 Group 2 Kaapvaal Anisotropy Direction Shear Velocity Variation Yuan & Romanowicz, AGU 2012 DI21A-2352

19 Kaapvaal No obvious conductivity layering in Kaapvaal Evans et al, JGR, 2011

20 Kaapval Shear velocity w/ depth, no indication of layering MT also no indication of layering (not pictured…yet) Mg # changes significantly at ~175 km Receiver function at km (Hansen et al, 2009; Kind et al, 2012 AGU)

21 Kaapvaal

22 Working hypothesis for Kaapvaal Slave & North Atlantic?Kaapvaal? Lee, Annu. Rev. Earth Planet. Sci. 2011

23 Working hypothesis for Slave

24 Ongoing Work…. Refine Geophysical data, include similar datasets/observations for the three cratons more receiver functions can be added Significant work for geochemistry – Compile Re/Os for the lithosphere, compare it with TDM from Sm/Nd and Hf of the crust. –A closer look to the C and O isotopic composition of the diamonds.

25 Numerical modeling: how does subduction influence stability of Craton roots? Motivation: dehydration of slabs releases water, which is carried further away with regional convection and influences the rheology and composition of Craton roots.

26 Stable Craton Destroyed Craton

27 Thank you

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