Download Lecture 10.5 - Slab pull II

Geodynamics
Plate-driving forces
Lecture 10.5 - Slab pull II
Lecturer: David Whipp
[email protected]
Geodynamics
www.helsinki.fi/yliopisto
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Goals of this lecture
•
Present the part of the slab pull force owing to the olivine to
spinel phase transition
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Slab (or trench) pull
FSP
Forsyth and Uyeda, 1975
•
Slab pull results from the gravitational body force acting on the dense, sinking
oceanic lithosphere.
•
It can be divided into two components:
Downloaded from http://gji.oxfordjournals.org/ at Dalhousie University on September 1, 2013
•
•
•
Fb1, the force resulting from the slab being colder than the surrounding mantle
Fb2, the force resulting from the elevation of the olivine→spinel phase change
Mathematically, we can say
FSP = Fb1 + Fb2
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Slab (or trench) pull
FSP
Heat Transfer
Forsyth and Uyeda, 1975
Fig. 4.59, Turcotte and Schubert, 2014
6.22 The Forces that Drive Plate Tectonics
Downloaded from http://gji.oxfordjournals.org/ at Dalhousie University on September 1, 2013
•
Theortransition
fromseparating
olivine to
e Clapeyron
equilibrium curve
twospinel
phases of
al.
in the mantle generally occurs at
400-500 km depth, but the transition
e 4–58. Since
the subducted
depends
on bothlithosphere
pressure was
andformed on the
thermal structure upon subduction is given by Equation
temperature
ndence of temperature upon depth prior to subduction is
erm given in Figure 4–56. As the subducted lithosphere
Because the descending slab is
mantle, frictional heating occurs at its upper boundary.
relatively cold, the olivine-spinel phase
ional heating were studied in Section 4–26. As discussed
transition
at lower
pressure
ture distribution
dueoccurs
to frictional
heating
– Equation (4–
(shallower
depth)
erimposed
on the initial
temperature
distribution
to give
Figure
6.43 Elevation
he slab. The result is shown in Figure 4–58.
•
Turcotte and Schubert, 2002
of the olivine–spinel phase change4 in
Slab (or trench) pull
FSP
Heat Transfer
Forsyth and Uyeda, 1975
6.22 The Forces that Drive Plate Tectonics
Warm slab
Downloaded from http://gji.oxfordjournals.org/ at Dalhousie University on September 1, 2013
Fig. 4.59, Turcotte and Schubert, 2014
Cold slab
•
Theortransition
fromseparating
olivine to
e Clapeyron
equilibrium curve
twospinel
phases of
al.
in the mantle generally occurs at
400-500 km depth, but the transition
e 4–58. Since
the subducted
depends
on bothlithosphere
pressure was
andformed on the
thermal structure upon subduction is given by Equation
temperature
ndence of temperature upon depth prior to subduction is
erm given in Figure 4–56. As the subducted lithosphere
Because the descending slab is
mantle, frictional heating occurs at its upper boundary.
relatively cold, the olivine-spinel phase
ional heating were studied in Section 4–26. As discussed
transition
at lower
pressure
ture distribution
dueoccurs
to frictional
heating
– Equation (4–
(shallower
depth)
erimposed
on the initial
temperature
distribution
to give
Figure
6.43 Elevation
he slab. The result is shown in Figure 4–58.
•
Turcotte and Schubert, 2002
of the olivine–spinel phase change5 in
Slab (or trench) pull
FSP
Heat Transfer
Forsyth and Uyeda, 1975
6.22 The Forces that Drive Plate Tectonics
Warm slab
Downloaded from http://gji.oxfordjournals.org/ at Dalhousie University on September 1, 2013
Fig. 4.59, Turcotte and Schubert, 2014
Cold slab
•
Theortransition
fromseparating
olivine to
e Clapeyron
equilibrium curve
twospinel
phases of
al.
in the mantle generally occurs at
400-500 km depth, but the transition
e 4–58. Since
the subducted
depends
on bothlithosphere
pressure was
andformed on the
thermal structure upon subduction is given by Equation
temperature
ndence of temperature upon depth prior to subduction is
erm given in Figure 4–56. As the subducted lithosphere
Because the descending slab is
mantle, frictional heating occurs at its upper boundary.
relatively cold, the olivine-spinel phase
ional heating were studied in Section 4–26. As discussed
transition
at lower
pressure
ture distribution
dueoccurs
to frictional
heating
– Equation (4–
(shallower
depth)
erimposed
on the initial
temperature
distribution
to give
Figure
6.43 Elevation
he slab. The result is shown in Figure 4–58.
•
Fig. 6.43, Turcotte and Schubert, 2014
of the olivine–spinel phase change6 in
Slab (or trench) pull
FSP
Forsyth and Uyeda, 1975
•
6.22 The Forces that Drive Plate Tectonics
Thus, the driving force from the phase
transition depends on the location of
the phase transition isotherm 𝑇os in
the sinking slab
Downloaded from http://gji.oxfordjournals.org/ at Dalhousie University on September 1, 2013
•
•
The driving force from the phase
transition is
✓
◆1/2
2(Tc T0 ) ⇢os

Fb2 =
⇢0
2⇡u0
The olivine-spinel phase transition
increases density by ~270 kg m-3
𝛾
Slope of Clapeyron curve
Fig. 6.43, Turcotte and Schubert, 2014
𝛥𝜌os Density difference
ol →6.43
sp Elevation of the olivine–spinel phase change7 in
Figure
Slab (or trench) pull
FSP
Forsyth and Uyeda, 1975
Using typical values for a sinking slab,
the gravitational body force due to
the olivine-spinel phase transition in
the slab 𝐹𝑏2 is ~1.5×1013 N m-1, or
about half of that from the difference
in temperature alone
•
Going back to our original equation
for the slab pull force,
Downloaded from http://gji.oxfordjournals.org/ at Dalhousie University on September 1, 2013
•
6.22 The Forces that Drive Plate Tectonics
FSP = Fb1 + Fb2
we find a total slab pull force of
𝐹𝑆𝑃 = ~4.5×1013 N m-1
Fig. 6.43, Turcotte and Schubert, 2014
Figure 6.43 Elevation of the olivine–spinel phase change8 in
Let’s see what you’ve learned…
•
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•
Reference(s):
Forsyth, D., & Uyeda, S. (1975). On the Relative Importance of the Driving Forces of Plate Motion*.
Geophysical Journal International, 43(1), 163–200. doi:10.1111/j.1365-246X.1975.tb00631.x
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