continental growth spurts were all before 1 ga
DESCRIPTION
Continental growth spurts were all before 1 Ga. This plus ophiolites, kimberlites, komatiites, TTG,UHPT…suggest that modern plate tectonics was not occurring before Late Precambrian Today, delamination equals arc and plateau addition. Can we bridge geophysics,geochemistry & geodynamics?. - PowerPoint PPT PresentationTRANSCRIPT
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Continental growth spurts were all before 1 Ga
This plus ophiolites, kimberlites, komatiites, TTG,UHPT…suggest that modern plate tectonics was
not occurring before Late Precambrian
Today, delamination equals arc and plateau addition
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Can we bridge geophysics,geochemistry & geodynamics?
Continents; the missing link The lower crust is transient It is recycled 6 times faster than upper crust Recent arc growth estimates are 5X previous
estimates! Therefore, a huge previously unaccounted for
flux
Mantle convection controlled from the top; episodicity is plate reorganization, not mantle avalanches!
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TRI-CYCLING THROUGH THE EARTH
RIDGE-TRENCH ‘CYCLE’ (the escalator)
SUPERCONTINENT CYCLE (the trolley)
SLAB-PLUME ‘CYCLE’ (the tube) SUBTERRANEAN CYCLE
(delamination, orogenic cycle; the elevator)
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Modes of advectionTUBES
CIRCUITS,
BOUNDARY
LAYERS
ELEVATOR
HEAT EXCHANGER
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Estimating mantle sources & sinks,continental addition & loss,
through time, depends critically on when deep subduction,
got underway
David Scholl
1 Gyr of oceanic crust subduction=70 km of eclogite
According to Stern, deep subduction started at 1 Ga
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New parameters Scale (size) Homologous temperature Stress Buoyancy parameter Architecture Fertility Entrainability
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What triggered this new understanding?
The upper crust paradox
Recognition of true arc growth rates
The need to get rid of enormous amounts of lower crust
Appreciation of role of Eclogite
Appreciation of SCALE
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Recent workers who are, or are almost onto this, once they leave the crust or MORB
Bob & Suzanne Kay
Roberta Rudnick
Sobolov, Green, Scholl, Elliott, Natland, Presnall, Cin-Ty Lee
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MASS BALANCE km3/yr
TOTAL MIDPLATE MAGMA VOLUMES 3-6 Growth rate of arc garnet pyroxenite cumulates 1.5-6 Underside erosion & delamination of continents 4-6 Production rate of magmas in arcs 4-5
LOWER CRUSTAL LOSS = ‘HOTSPOT’ MAGMATISM CYCLE TIME 40-80 MYR 10-30 KM CHUNKS
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dense roots fall off warm up in
ambient mantle rise
possible mechanism for Atlantic & Indian ocean plateaus & DUPAL anomaly
Delamination cycle