Download Lecture 2.2 - Convergent plate boundaries

Geodynamics
Kinematics of plate tectonics
Lecture 2.2 - Convergent plate boundaries
Lecturer: David Whipp
[email protected]
Geodynamics
www.helsinki.fi/yliopisto
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Goal of this lecture
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Present convergent plate margins
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Plate boundaries
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Three different principal types of plate boundaries
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Divergent (accreting)
Convergent (subduction)
Transform
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Plate boundaries - Convergent (subduction)
Fig. 1.7, Turcotte and Schubert, 2014
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Plate boundary where the sense of
motion is convergent
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Plate boundaries - Convergent (subduction)
Fig. 1.7, Turcotte and Schubert, 2014
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Plate boundary where the sense of
motion is convergent
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Subduction destroys oceanic lithosphere,
recycling it within the Earth
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Plate boundaries - Convergent (subduction)
Fig. 1.7, Turcotte and Schubert, 2014
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Plate boundary where the sense of
motion is convergent
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Subduction destroys oceanic lithosphere,
recycling it within the Earth
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Geometry varies, but most have a trench, accretionary
sedimentary prism, volcanic line (or arc)
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Plate boundaries - Convergent (subduction)
Fig. 1.7, Turcotte and Schubert, 2014
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Why does the plate subduct?
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Plate boundaries - Convergent (subduction)
Fig. 1.7, Turcotte and Schubert, 2014
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Why does the plate subduct?
As the lithosphere cools and contracts, its
density increases, eventually exceeding that of
the underlying asthenosphere
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Plate boundaries - Convergent (subduction)
Fig. 1.7, Turcotte and Schubert, 2014
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Why does the plate subduct?
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This results in a gravitational instability that causes the
oceanic plate to sink into the underlying asthenosphere
As the lithosphere cools and contracts, its
density increases, eventually exceeding that of
the underlying asthenosphere
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Plate boundaries - Convergent (subduction)
Fig. 1.7, Turcotte and Schubert, 2014
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•
Why does the plate subduct?
As the lithosphere cools and contracts, its
density increases, eventually exceeding that of
the underlying asthenosphere
FSP
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This results in a gravitational instability that causes the
oceanic plate to sink into the underlying asthenosphere
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As it sinks, plate motion is driven in part by the dense sinking end of the plate
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pulling it along via the slab pull force 𝐹SP
Plate boundaries - Convergent (subduction)
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The geometry of subducting
plates is quite variable, but typical
subduction angles are ~45°
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If gravity drives subduction, why
don’t slabs subduct with 90° dips?
Fig. 1.9, Turcotte and Schubert, 2014
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Plate boundaries - Convergent (subduction)
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The geometry of subducting
plates is quite variable, but typical
subduction angles are ~45°
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If gravity drives subduction, why
don’t slabs subduct with 90° dip
angles?
Fig. 1.9, Turcotte and Schubert, 2014
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Induced mantle flow and back-arc spreading
Mantle corner flow
Slab roll-back
Fig. 1.11, Turcotte and Schubert, 2014
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Subduction induces flows in the mantle that may control the slab dip angle
These flows may also produce enigmatic back-arc spreading. Why is there
extension at a convergent plate margin?
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Test your might
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You can find a short quiz about this lecture at https://elomake.helsinki.fi/lomakkeet/63002/lomake.html
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Please take the quiz to help me know what you have learned
Your answers are anonymous and will not count in your
course grade
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