Download Continental growth spurts were all before 1 Ga

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Transcript
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?
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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!
TRI-CYCLING THROUGH THE EARTH
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RIDGE-TRENCH ‘CYCLE’ (the
escalator)
SUPERCONTINENT CYCLE (the
trolley)
SLAB-PLUME ‘CYCLE’ (the tube)
SUBTERRANEAN CYCLE
(delamination, orogenic cycle; the
elevator)
Modes of advection
TUBES
ELEVATOR
CIRCUITS,
BOUNDARY
LAYERS
HEAT EXCHANGER
According to Stern, deep subduction started at 1 Ga
Estimating mantle sources & sinks,
continental addition & loss,
through time, depends
critically on when deep subduction,
got underway
David Scholl
New parameters
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Scale (size)
Homologous temperature
Stress
Buoyancy parameter
Architecture
Fertility
Entrainability
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
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
MASS BALANCE
km3/yr
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TOTAL MIDPLATE MAGMA VOLUMES

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Growth rate of arc garnet pyroxenite cumulates
Underside erosion & delamination of continents
Production rate of magmas in arcs

LOWER CRUSTAL LOSS = ‘HOTSPOT’ MAGMATISM

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CYCLE TIME 40-80 MYR
10-30 KM CHUNKS
3-6
1.5-6
4-6
4-5
ROOT FORMATION
Delamination
cycle

dense roots

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

fall off
warm up in
ambient mantle
rise
possible mechanism
for Atlantic & Indian
ocean plateaus &
DUPAL anomaly
1
DELAMINATION
2
SPREADING
3
heating
UPWELLING
4
SPREADING
5
ridge