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Past, Present and Future. What have we learned? Mantle and Plates are an intimately coupled system Deep mantle structure is important for the surface Geological information provides quantitative constraints Mixing is complicated!. Where are we now? -Circulation models

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Past, Present and Future

  • What have we learned?
  • Mantle and Plates are an intimately coupled system
  • Deep mantle structure is important for the surface
  • Geological information provides quantitative constraints
  • Mixing is complicated!

Where are we now?

-Circulation models

-Generation of plates with exotic rheologies

-Making real subduction zones!

-Modeling isotopic and petrological heterogeneity

-Modeling of observations in simple contexts (complications)

Where are we going?

-Self-consistent modeling of mantle flow and lithospheric deformation

-Connection to surface processes (sea-level; climate)

-Understanding deep Earth structure and consequences

(seismology via mineral physics)

-Feedback between geodynamic models and tectonics

plates and subduction

Plates and Subduction

Lecture 5: Geodynamics

Carolina Lithgow-Bertelloni


Plates Mantle Convection

Continuous generation of

dynamical (thermal) +

geochemical (compositional) = seismic heterogeneity

[including phase transitions!]

[Zhao et al., 1997]


What is a plate?

Lithospheric Fragment

Strong non-deforming interior

Diffuse plate boundaries?

Narrow, weak, rapidly deforming boundaries


Subduction zones-asymmetric


Motion described by rotation

Plate motions


Piecewise continuous velocity field in space and time

Hard for fluid dynamics

Significant toroidal motion (I.e transform-like)

Part of convecting system (top thermal boundary layer…)

Continental plates


Toroidal Motions

-Homogeneous convecting

fluid-No toroidal power

-Lateral viscosity variations

i.e. PLATES!

-But why? Dissipates no heat

-Ratio: Plate characteristic

Horizontal divergence


Radial vorticity


[Dumoulin et al., 1998]


Observed P/T Ratios

  • P/T power not equipartitioned
    • Reference Frames!
  • Toroidal power
    • Pacific basin (largely)
    • Oblique subduction

[Lithgow-Bertelloni et al., 1993]

how to treat plates

How to treat plates?

Generating plates self-consistently

“Exotic” Rheologies with a physical basis

Imposing Plate Motions

Investigate scales of flow

Construct mantle circulation models

compare to seismology

History of plate motions

Past plate motions (driving forces)

Plate Rearrangements

imposing plate velocities

Imposing plate velocities

Study scales of flow in the mantle

Do plates organize flow

Suppress smaller scales (capture plumes?)

Influence heat flow at the CMB?

[Zhong et al., 1998]

[Bunge and Grand, 2000]

scales of flow plates organize

Scales of flow: plates organize

Plates + Strong Lower Mantle organize flow

Suppress smaller scales (capture plumes?)

Give rise to large scale heterogeneity

[Bunge and Richards, 1996]

making plates theory

Making plates: theory

Shear-localizing feedback mechanisms required

Broad, strong plate-like regions

Weak, narrow plate boundaries

Toroidal motion (almost transforms)

Ridge localization

Physical basis?

Many characteristics not reproduced

Subduction initation


Temporal evolution and plate rearrangement

[Bercovici, 2003]

making plates advances

Making plates: Advances

Melt viscosity reduction key to

Asthenosphere generation

Localizing ridges

Better plate-like behavior

Stability and no fragmentation

Long-wavelength heterogeneity

[Tackley, 2000]

subduction and slabs

Subduction and Slabs

How do they start?

Asymmetric Downwelling

Seismically active to ~700 km

(phase transitions? Reactivation of faults?)

Cold------> STRONG?


Volatile fluxing

[Zhao et al., 1997]


Initiation of subduction

[Hall et al., 2002]

thermal structure








Thermal structure

Depth (km)

kinematic models

Kinematic Models

[van Keken et al., 2001]

Stress-dependent rheology: focuses flow

-higher interface temperatures

-lower crustal temperatures


-Sediment melting

-Low temperature dehydration

(consistent with trace elements)

-Water to great depths

petrologic structure

Petrologic structure




[van Keken et al., 2001]


What happens to slabs?

Trench Rollback

Heating of the plate

Return flow

Effect of phase transitions

Multicomponent system

Positive vs Negative Clayperon slope

Slab Deformation (Are slabs strong?)

Upper vs Lower Mantle

Delamination of crust from lithosphere?

Importance for seismic/geochemical heterogeneity

Ultimate fate (CMB?)

Seismic evidence Tectonics

Importance for seismic/geochemical heterogeneity

Consequences for mantle convection and core

slabs and trench rollback

Slabs and trench rollback


Fluid velocity magnitude=L/LDUD

[Kincaid and Griffiths, 2003]

effect of phase transformations

[Thorsten Becker, 2003]








[Christensen, 1996; 1997]

  • Buoyancy-thermal, compositional, phase buoyancy
  • Rate of trench rollback

Phases in the slab

Effect of phase transformations

Depth (km)

-200 0 200 400 600 800 1000

Distance (km)

-0.18 -0.08 +0.02 +0.12 +0.22

Density Contrast (Mg m-3)

-0.18 -0.08 +0.02 +0.12 +0.22

Density Contrast (Mg m-3)

slab morphology and strength

Effect of Viscosity

[Tao and O’Connell, 1993]

Trench rollback

phase transitions

[Christensen, 1996]

Slab morphology and Strength


Strength of Slabs

Half of viscous


in bending and


[Conrad and Hager,1999]

slab deformation delamination

Slab Deformation: delamination

[Christensen and Hoffman, 1994]

Competition: density and rheology

-0.18 -0.08 +0.02 +0.12 +0.22

Density Contrast (Mg m-3)

slabs and geochemical heterogeneity

Slabs and geochemical heterogeneity

[Xie and Tackley, PEPI, in press]


Slabs and Seismic Structure

[Grand, 1994]

1300-1450 km

[Jordan & Lynn, 1974]

Caribbean Anomaly/Farallon- Jordan & Lynn (1974)

Marianas- Creager & Jordan (1986)

Farallon-Grand (1987, 1994)

Aegean-Spakman et al. (1993)

Western Pacific Slabs-van der Hilst et al. (various)

[Grand et al., 1997]


Direct Comparisons: Using Past Tectonics

[Replumaz et al., 2004]

[Voo et al., 1999]


Slab dynamics and tectonics

  • Effect of changes in plate motion
  • Alter slab dynamics
    • dynamical (seismic ?)structure in areas of long-lived subduction

[Tan et al., 2002]


Fate of slabs: consequences

  • Depth-dependent properties
  • Perovskite forming reaction at 660 km

[Tan et al., 2002]