The Generation of Melting Anomalies by Plate Tectonic Processes

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)