The Generation of Melting Anomalies by Plate Tectonic Processes Gillian R. Foulger University of Durham.

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Presentation transcript:

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

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 MELT experiment 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)

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 of the 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 motion Lourenço 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:~ ˚C Iceland:~ ˚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)