CIDER 2012 Lithosphere Group Presenters: Huaiyu Yuan & Pierre Bouilhol
description
Transcript of CIDER 2012 Lithosphere Group Presenters: Huaiyu Yuan & Pierre Bouilhol
Understanding craton formation through their geochemical and
geophysical characteristicsA Preliminary Report
CIDER 2012 Lithosphere Group
Presenters: Huaiyu Yuan & Pierre Bouilhol
Group: Cathleen Doherty, Erica Emry, Beth Paulson, Mingming Li, Doug Wiens
Getting Started….
• History of the project
• Who’s involved– 8 members of the group– 4 geophysicists, 3 geochemists, 1
geodynamicist
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…
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
Slave
Bostock 1998
Dipping Upper mantle reflector at 100 km indicates fossil subductions
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. 2003
P-wave receiver functions Electrical Resistivity
Griffin et al. Lithos, 2004
SlaveShallow Anisotropy shallow “red” layer = highly depleted chemical layer (Mg# 92%)
Slave Craton
Yuan and Romanowicz 2010
• To add:– Ages– More constraints on error/uncertainty– More geochemical data
Slave
Slave
Slave
van der Velden and Cook JGR 2005
fossil subductions indicated by“dipping upper mantle reflectors”from LithoProbe project in many places
van der Velden and Cook JGR 2005
Subduction trench (suture) parallel = Shallow Anisotropy Direction
Yuan et al. 2011
North Atlantic:
• New data from Receiver Function
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
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
• N. Atlantic Craton seems to show geophysical similarities with Slave craton
Kaapvaal
• Kaapvaal is different from Slave and North Atlantic craton
• No evidence for layering within the lithosphere
Group 1
Group 2
Kaapvaal
Yuan & Romanowicz, AGU 2012 DI21A-2352
Group 1
Group 2
Kaapvaal
Anisotropy Direction
Shear Velocity Variation
Yuan & Romanowicz, AGU 2012 DI21A-2352
Kaapvaal• No obvious conductivity
layering in Kaapvaal
Evans et al, JGR, 2011
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 170-180 km (Hansen et al, 2009; Kind et al, 2012 AGU)
Kaapvaal
• Working hypothesis for Kaapvaal
Slave & North Atlantic? Kaapvaal?
Lee, Annu. Rev. Earth Planet. Sci. 2011
Working hypothesis for SlaveWorking hypothesis for Slave
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.
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.
Stable Craton
Destroyed Craton
Thank you