Plate tectonics generated by nonlinear rheology in mantle convection models & application to thermo-chemical evolution

Plan

From Published Papers

Viscosity (T) : how much?

Expect rigid lid: Earth unusual ?

Plates and mantle

Rheology

Yield strength of
rocks

Slide 9

Strength profile of oceanic lithosphere

Lithospheric deformation

Model

Equations

Slide 14

Surface Strain Rate and V

RaH=2e7; YS=50 MPa (2e4)

Slide 17

"By S."

Characterizing plate tectonics

"Earth’s Tor/Pol ratio ~..."

Slide 21

Time-Dependence

Scaling of plate diagnostics with Yield Stress

Does low viscosity beneath the lithosphere help?

Slide 25

"Varying yield strength"

"Time evolution"

Slide 28

Greatly improves plate quality

So far…instantaneous rheology

Strain weakening?

Mantle shear zone in Greenland

Slide 33

X-section thru shear zone

Simplified 'Damage' evolution

Comparison of various rheologies

Forms lithospheric shear zones

…works with more realistic viscosity profile

Instantaneous flow with strain rate weakening.

Simple 2-lyr model

Same with bigger box

Add SW to time-dep models

…but doesn’t have a large effect on diagnostics

"Divergence:"

Surface Strain rate

Pol- & Toroidal with depth

Continents aid 1-sided subduction, but add time-depn

Summary

Successes and failures

Things that 'help'

Future directions

Mantle convection models and geochemistry

Integrating Geodynamic and Geochemical Models of Mantle Evolution and Plate Tectonics

TEST cartoon models: which ones “work” both geophysically and geochemically?

Chemical model

Physical model

Geochemical evolution: Focus on 2 cases

Homogeneous start: after 1 and 2 Gyr

Slide 58

Homogeneous start
(after 3.6 Gyr)

Layered start: after 200 Myr and 2.5 Gyr

Slide 61

Layered start: present day (3.6 Gyr)

Noble gas outgassing: too much(?)

Ratios in melted material (last 150 Myr)

Pb ratios in melted material

Thermal evolution strongly affected by magmatism: simple example

Isochemical case

with differentiation

Heat flow is almost constant

Temperature varies much less

THE END

Slide 72

Yielding generates “plate tectonics”

Goals

Rheological Model