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Evidences for Subduction in the

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Evidences for Subduction in the Powered By Docstoc
					Outline
 Before starting…


 Indicators of subduction
- Ophiolites
- Accretionary prism
- Isotopic evidences
- Seismic imaging evidences


 Conclusion
The Archean: 4.3 to
2.5 billion years



              antilight.net/science/earth/timeline_ultra.png
                  Modern style plate tectonics

Focus on the most reliable indicators of subduction zone to track the
processes into geological past




                 Cross section by Jose’F. Vigil, from This dynamic planet (USGS), 1999
Indicators of subduction
 Ophiolites
- Fragments of oceanic lithosphere tectonically
  emplaced on continental crust
 Hard to preserve
Stern, 2008
-Preserved in “nappe”, which is easily eroded
 Counter Argument
Condie and Kroner, 2008
-Thickness favorable to preservation




 McBirney, A.R., 1985, Igneous petrology: Freeman
 Cooper & Co.                                       R. Higgs, Search and discovery, 2008
 Hard to form
Condie and Been, 2006
-Oceanic crust thicker (Sleep and
   Windley, 1982)
-Deep layers of ophiolite recycled in
   mantle

Hoffman and Ranalli, 1988
-Obduction resulted in delamination

Furnes et al., 2007
 3.8 Ga Isua Greenstone Belt, Greenland
- Sheeted dikes and pillow basalts
 Oldest well-preserved complete or almost complete
-1.95 Ga Jormua ophiolite, Finland (Peltonen et al., 1996)
-2.0 Ga Purtuniq ophiolite, Canada (Scott et al., 1992)
-1.73 Ga Payson ophiolite, US (Dann, 1997)

    Stern, 2005
    Realistically, “ophiolite do not become widespread in
    the geologic record until 1Ga
Indicators of subduction
 Accretionary prism
- Composed of mélange complexes
  Result of tectonic transfer of oceanic crust and pelagic
  sediments to an arc or active continental margin
 Hamilton, 1998
- Absence of such mélange complexes in the Archean

 Stern, 2008
- Intraoceanic system

 Counter Arguments
Komiya et al., 1999; Kitajima et al. 2001; Shervais, 2006
- Described mélange-like terranes , identified in Archean , as
  accretionary prisms
Isotopic evidences
 Boyet and Carlson, 2005
-Isotopic ratios of 142Nd/ 144Nd

 Farquhar et al., 2002
- 33S isotope anomalies in diamonds
   from Kaapvaal-Zimbabwe craton.
 Counter argument
Shirey et al., 2008
Other processes can descend material into the mantle
-proto-plate tectonics
-delamination of an unstable lid
-injection by meteoric impact
Seismic evidences
 -D.J White, 2003
 Archean western Superior province, Canada
Conclusion
 Two camps
 Other indicators
-Geochemical composition
-Paired metamorphism
-Foreland basin
-Orogens

				
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posted:5/13/2010
language:English
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