The Generation of Melting Anomalies by Plate Tectonic Processes

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The Generation of Melting Anomalies by Plate
Tectonic Processes

• Gillian R. Foulger
• University of Durham

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• .... based on ideas developed over several years
  by a working group that includes ....
• Don Anderson, Warren Hamilton, Jerry Winterer,
  Jim Natland, Dean Presnall, Peter Vogt, Anders
  Meibom, Hetu Sheth, Seth Stein, Mike OHara
  Alan Smith .... etc

for more see www.mantleplumes.org
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Two key elements

• Variations in lithosphere stress
• Mantle inhomogeneity
• Simply put
• Stress governs location of volcanism
• Fusibility governs volume of magma

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Mantle dehomogenising

• ridges

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Mantle dehomogenising

• ridges
• subduction zones
• eclogitisation of subducted crust

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Eclogite is fusible
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Mantle dehomogenising

• metasomatism of oceanic and continental mantle
  lithosphere
• delamination of thickened lithosphere, including
  lower crust
• erosion of continental lithosphere during breakup

Cantal basalts model, Massif Central, France
Pilet et al. (2005)
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Mantle dehomogenising

• metasomatism of oceanic and continental mantle
  lithosphere
• delamination of thickened lithosphere, including
  lower crust
• erosion of continental lithosphere during breakup

Schott et al. (2000)
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Mantle dehomogenising

• metasomatism of oceanic and continental mantle
  lithosphere
• delamination of thickened lithosphere, including
  lower crust
• erosion of continental lithosphere during breakup

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Mantle dehomogenising
adapted from Meibom Anderson (2003)
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Variations in stress

• Lithosphere cooling
• Spatial and temporal variations in plate boundary
  type tectonics
• Variations in lithosphere strength

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Variations in stress

• Lithosphere cooling
• Spatial and temporal variations in plate boundary
  type tectonics
• Variations in lithosphere strength

From Natland, 2004
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Variations in stress

• Lithosphere cooling
• Spatial and temporal variations in plate boundary
  type tectonics
• Variations in lithosphere strength

adapted from Lundin Doré (2005)
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Proposal

• Hot spot volcanism occurs where
• stress is extensional
• mantle is highly fusible

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Examples
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Hot spots on MORs

• 1/3 of all hot spots are on or near MORs

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East African Rift

• Afar
• Other EAR hotspots?

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Basin Range Province

• Broad, intraplate extensional region
• Associated with subduction of Farallon slab
• Widespread volcanism

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NAVP Iceland

• Formed when continent rifted along Iapetus suture
• Diverse data suggest not hot
• Recycled Iapetus crust can explain geochemistry
  melt volume

Closure of the Iapetus
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Azores

• Kinematic models
• EQ focal mechanisms
• bathymetry
• suggest Azores branch is
• oblique, ultra-slow spreading (3-4 mm/yr)
• diffuse plate boundary
• dextral differential shear motion

Lourenço et al. (1998)
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Time-progressive volcanism
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Predictions

• Melt volumes can be explained by lithosphere
  extension source fertility
• Vertical motions related to shallow tectonic
  processes
• Upper mantle is inhomogeneous
• Migration of volcanism migration of locus of
  extension

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Predictions

• Seismic tomography anomalies indicate composition
  and melt, not only temperature
• Hot spot lavas not required to be hot
• Geochemistry can be explained by inhomogeneities
  in the shallow mantle

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• Thats all folks!

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Example mantle potential temperature, Iceland
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Temperature (Tp) from petrology

• mid-ocean ridges 1280 - 1400C
• Iceland 1280 - 1460C
• Hawaii 1560C
• The only place on Earth hot enough for a weak
  upper-mantle plume is Hawaii

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Seismology does not reliably detect them in the
lower mantle
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Vertical exaggeration x 10
DT 200C
DT 60C
Ritsema Montagner (2003)
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Examplewhole-mantle tomography Iceland
Ritsema et al. 1999
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Please read our bookPlates, Plumes Paradigms
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Iceland A plume from the core-mantle boundary?
Bijwaard Spakman (1999)
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Resolution of the whole-mantle plume

• The data used by Bijwaard Spakman have no
  resolution in the lower mantle
• (courtesy of Karason van der Hilst)

From Foulger et al. (2001)