Past, Present and FutureWhat 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

Lecture 5: GeodynamicsCarolina Lithgow-Bertelloni

Plates Mantle Convection

[Zhao et al., 1997]

Continuous generation of dynamical (thermal) + geochemical (compositional) = seismic heterogeneity

[including phase transitions!]

What is a plate?

Lithospheric FragmentStrong non-deforming interior

Diffuse plate boundaries?Narrow, weak, rapidly deforming boundaries

Ridges-passiveSubduction zones-asymmetricTransforms?

Motion described by rotation

Plate motionsNon-acceleratingPiecewise continuous velocity field in space and time

Hard for fluid dynamicsSignificant toroidal motion (I.e transform-like)

Part of convecting system (top thermal boundary layer…)

Continental plates

Piecewise Continuity in Space and Time

43-48 Ma

Fluid Dynamics and Plate Tectonics

25-43 Ma

Toroidal Motions

[Dumoulin et al., 1998]

€

∇H •V = Demye

m

m

∑l

∑

∇× v( ) • r = Vemye

m

m

∑l

∑

Horizontal divergence(poloidal)

Radial vorticity(toroidal)

-Homogeneous convectingfluid-No toroidal power

-Lateral viscosity variationsi.e. PLATES!

-But why? Dissipates no heat

-Ratio: Plate characteristic

Observed P/T Ratios

[Lithgow-Bertelloni et al., 1993]

P/T power not equipartitionedReference Frames!

Toroidal powerPacific basin (largely)Oblique subduction

How to treat plates?

Generating plates self-consistently“Exotic” Rheologies with a physical basis

Imposing Plate MotionsInvestigate scales of flowConstruct mantle circulation models

compare to seismology

History of plate motionsPast plate motions (driving forces)Plate Rearrangements

Imposing plate velocities

[Zhong et al., 1998]

[Bunge and Grand, 2000]

Study scales of flow in the mantleDo plates organize flowSuppress smaller scales (capture plumes?)

Influence heat flow at the CMB?

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

[Bercovici, 2003]

Shear-localizing feedback mechanisms required

Broad, strong plate-like regionsWeak, narrow plate boundariesToroidal motion (almost transforms)Ridge localization

Physical basis?Many characteristics not reproduced

Subduction initationAsymmetryTemporal evolution and plate rearrangement

Making plates: Advances

[Tackley, 2000]

Melt viscosity reduction key toAsthenosphere generationLocalizing ridgesBetter plate-like behaviorStability and no fragmentation

Long-wavelength heterogeneity

Subduction and Slabs

[Zhao et al., 1997]How do they start?Asymmetric DownwellingSeismically active to ~700 km

(phase transitions? Reactivation of faults?)

Cold------> STRONG?Long-livedVolatile fluxing

Initiation of subduction

QuickTime™ and aBMP decompressor

are needed to see this picture.

[Hall et al., 2002]

Thermal structure

Dep

th (

km)

100

300

500

700

900

1100

1300

Kinematic Models

[van Keken et al., 2001]

Stress-dependent rheology: focuses flow-higher interface temperatures-lower crustal temperatures

Implications:-Sediment melting-Low temperature dehydration(consistent with trace elements)-Water to great depths

Petrologic structure

[van Keken et al., 2001]

Isoviscous

Non-Newtonian

Isoviscous-PW99

Dynamical Subduction Zones

[Billen, 2004]

Maximum ViscosityMaximum Viscosityx 1000x 100

x 1e5

Complete ErosionT > 1400 C at 50 km

Partial ErosionT < 600 C at 50 km

Need other weakening processes in sub-arc lithosphere/mantleË waterË melt

Newtonian Newtonian vs vs non-Newtonian non-NewtonianNewtonian

Non-Newtonian

What happens to slabs?

Trench RollbackHeating of the plateReturn flow

Effect of phase transitionsMulticomponent systemPositive vs Negative Clayperon slope

Slab Deformation (Are slabs strong?)Upper vs Lower MantleDelamination of crust from lithosphere?Importance for seismic/geochemical heterogeneity

Ultimate fate (CMB?)Seismic evidence TectonicsImportance for seismic/geochemical heterogeneityConsequences for mantle convection and core

Slabs and trench rollback

[Kincaid and Griffiths, 2003]

Fluid velocity magnitude=L/LDUD

LD=UDtp

Effect of phase transformations

Phases in the slab

-200 0 200 400 600 800 1000 Distance (km)

Dep

th (

km)

100

300

500

700

900

1100

1300

-0.18 -0.08 +0.02 +0.12 +0.22 Density Contrast (Mg m-3)

[Christensen, 1996; 1997]

Buoyancy-thermal, compositional, phase buoyancyRate of trench rollback

4.6

4.4

4.2

4.0

3.8

3.6

3.4

3.2

Density (Mg m

-3)

30252015105

Pressure (GPa)

opx->gt

ol->wa

wa->ri

Ca-pv in

cpx out

gt->pv

ri->pv+mw

gt out

ol->ri

ol+ri->wa+ri

wa outst in

Ca-pv inil in

il->pv

ri->pv+mwri out

gt out

1000 K

2000 K

1500 K

[Thorsten Becker, 2003]

Slab morphology and Strength

[Tao and O’Connell, 1993]

Effect of Viscosity

[Christensen, 1996]

Trench rollbackphase transitions

Strength of Slabs

[Conrad and Hager,1999]

Half of viscous dissipation in bending and unbending?

Slab Deformation: delamination

[Christensen and Hoffman, 1994]

Competition: density and rheology

Slabs and geochemical heterogeneity

[Xie and Tackley, PEPI, in press]

6.0

5.5

5.0

4.5

4.0

3.5

3.0

Density (g cm

-3)

25002000150010005000

Depth (km)

1600 KAdiabat

Basalt

Harzburgite

Pyrolite

Slabs and Seismic Structure

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)

[Jordan & Lynn, 1974]

[Grand, 1994]

1300-1450 km

[Grand et al., 1997]

Direct Comparisons: Using Past Tectonics

[Voo et al., 1999]

[Replumaz et al., 2004]

Slab dynamics and tectonics

[Tan et al., 2002]

Effect of changes in plate motion

Alter slab dynamics

dynamical (seismic ?)structure in areas of long-lived subduction

Fate of slabs: consequences

[Tan et al., 2002]

Depth-dependent properties

Perovskite forming reaction at 660 km