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
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