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The Generation of Melting Anomalies by Plate Tectonic Processes. Gillian R. Foulger University of Durham. .... based on ideas developed over several years by a working group that includes .... - PowerPoint PPT Presentation
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The Generation of Melting Anomalies by Plate Tectonic
Processes
Gillian R. Foulger
University of Durham
.... 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 O’Hara & Alan Smith .... etc
for more see www.mantleplumes.org
Two key elements:
1. Variations in lithosphere stress
2. Mantle inhomogeneity
Simply put
• Stress governs location of volcanism
• Fusibility governs volume of magma
Mantle dehomogenising
• ridges
MELTexperiment
EPR
Mantle dehomogenising
• ridges• subduction zones• eclogitisation of
subducted crust
Eclogite is fusible
Pyrolite
Eclogite
Yaxley (2000)
A 30/70 eclogite-peridotite mixture can generate several
times as much melt as peridotite
Mantle dehomogenising
• metasomatism of oceanic and continental mantle lithosphere
• delamination of thickened lithosphere, including lower crust
• erosion of continental lithosphere during breakup
Pilet et al. (2005)
Cantal basalts model, Massif Central, France
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)
QuickTime™ and aGIF decompressorare needed to see this picture.
Mantle dehomogenising
• metasomatism of oceanic and continental mantle lithosphere
• delamination of thickened lithosphere, including lower crust
• erosion of continental lithosphere during breakup
Mantle dehomogenising
adapted from Meibom & Anderson (2003)
Variations in stress
• Lithosphere cooling• Spatial and temporal
variations in plate boundary type & tectonics
• Variations in lithosphere strength
Variations in stress
• Lithosphere cooling• Spatial and temporal
variations in plate boundary type & tectonics
• Variations in lithosphere strength
From Natland, 2004
Variations in stress
• Lithosphere cooling• Spatial and temporal
variations in plate boundary type & tectonics
• Variations in lithosphere strength
adapted from Lundin & Doré (2005)
Proposal
“Hot spot” volcanism occurs where– stress is extensional– mantle is highly fusible
Examples
“Hot spots” on MORs
• 1/3 of all “hot spots” are on or near MORs
East African Rift
• Afar
• Other EAR “hotspots”?
Basin & Range Province
• Broad, intraplate extensional region
• Associated with subduction of “Farallon slab”
• Widespread volcanism
NAVP & Iceland
• Formed when continent rifted along Iapetus suture
• Diverse data suggest not hot
• Recycled Iapetus crust can explain geochemistry & melt volume
Closure ofthe Iapetus
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 motionLourenço et al. (1998)
Time-progressive volcanism
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
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
That’s all folks!
Example: mantle potential temperature, Iceland
Temperature (Tp) from petrology
• mid-ocean ridges: ~1280 - 1400˚C
• Iceland: ~1280 - 1460˚C
• Hawaii: ~1560˚C
The only place on Earth hot enough for a weak upper-mantle plume is Hawaii
Seismology does not reliably detect them in the lower mantle
Vertical exaggeration x 10
Iceland
Ritsema & Montagner (2003)
T ~ 200˚C
T ~ 60˚C
Example:whole-mantle tomography:
Iceland
Ritsema et al. 1999
Please read our book:Plates, Plumes & Paradigms
Iceland: A plume from the core-mantle boundary?
Bijwaard & Spakman (1999)Hudson Bay plume?
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)