GLOBAL TOPOGRAPHY

CONTINENTAL & OCEANIC LITHOSPHERE

Age Topography Heat Flowmid ocean ridge mantle CONTINENTAL & OCEANIC LITHOSPHERE

tectothermal age of plate (ta) mantle heat loss (q )mantle flow  t MOR thermal  Thermal boundary layer of mantle convection t T s o t=0 _ + z T s o time z Region of T gradient is a Thermal Boundary Layer

 m mantle heat loss (q )mantle flow MOR thermal  Thermal boundary layer of mantle convection t mechanical : Layer of long term strength m   c chemical/mechanical : Dehydrated Layer (dry=hi viscsoity) m  (cold=hi viscosity)  t tectothermal age of plate (ta)

Oceanic Thermal Lithosphere defines convection pattern - it is the cold, overturning boundary layer. Continental Chemical Lithosphere does not participate in convective mantle overturn (inherently buoyant). Continent Oceanic Chemical Lithosphere subducts - overturning portions of the Earth see a constant temperature boundary condition. Provides a more complex thermal coupling condition for covecting mantle below.

“subducting” lithosphere viscosity = 10 Pa s viscosity = 10 Pa s warm mantle viscosity = 10 Pa s warm mantle viscosity = 10 Pa s 21 hothotcoldcold convecting mantle failed region extension extension failed region extension extension failed region failed regioncompression compression cratonic root lower crust upper crust bulk mantle local localgeotherm Cooper et al. 2004

 c Chemical/Mechanical Lithosphere  t Thermal Lithosphere Dynamic Mantle Sub-Layer

 c  t mantle heat flow surface heat flow Upper Crust Lower Crust Chemical Lithosphere Average Thermal Lithosphere Temperature (Celsius) Depth (km)

Thermal/Chemical BL Thickness Ratio Chemical Boundary Layer Thickness (km) Temperature Drop Across Sub-Layer (C) Radiogenically Depleted Root Radiogenically Enriched Root

Yuan & Romanowicz 2010 Therm Chem Chemical Lithosphere (km) Latitude depth (km) Thermal/Chemical Ratio Preserving & Destroying Cratonic Lithosphere The Structure of

Preserving & Destroying Cratonic Lithosphere CRATON STABILITYCRATON INSTABILITY UNDERSTAND STABILITY TO UNDERSTAND INSTABILITY

MODELING CRATON STABILITY chemically light material - root (own rheology) chemically real light material - crust (has own rheology) coldhotmantle base of thermal lithosphere continental lithosphere is cool & more viscous than bulk mantle failed regions cold viscosity 10 Pa s cold viscosity 10 Pa s26 hot viscosity 10 Pa s 21

Send Continent into Model Subduction Zone See What it Takes to Save Root & Keep Crust Stable MODELING CRATON STABILITY 300+ Simulations Later …

7 Myr MODELING CRATON STABILITY - BUOYANCY 29 Myr Buoyancy Does Not Lead To Stability (even w/ temperature dependent viscosity)

MODELING CRATON STABILITY - VISCOSITY Viscosity Does Not Lead To Stability Viscosity+ Critical Thickness Can Lead To Stability 50 Myr 100 Myr Root 1000X Viscosity of Mantle at = Temp

MODELING CRATON STABILITY - VISCOSITY Normalized Root Extent Root Thickness (km) 50 Myr 100 Myr 150 Myr Root/Mantle Viscosity Ratio = 1000 Extreme De-Hydration Lower Ratio (>100) Can Not Prevent Viscous Root Deformation

MODELING CRATON STABILITY - VISCOSITY Viscosity Does Not Lead To Stability Viscosity+High Craton Yield Stress Can Lead To Stability 50 Myr 100 Myr Root 1000X Viscosity of Mantle at = Temp

MODELING CRATON STABILITY - YIELD STRESS Craton Does Not Fail Under Stress Due to High Yield Strength Buffer Cratons from High Stress and They Will Not Yield

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MODELING CRATON STABILITY - MOBILE BELTS Mobile Belts Can Provide Craton Stability (act as crumple zones to buffer stress) 50 Myr 100 Myr

Dietz [1963] if subduction starts offshore, forms island arc, then migrates on shore - craton will be buffered REGENERATING MOBILE BELTS (Crumple Zones) if subduction starts at time B - craton will be stressed

mobile belt (deep green) yield stress relative to craton (pale green) yield = 0.5 crumple zone model yield ratio = 1.0 no crumple zone craton yield ratio = 0.5

STABILITY STABILITYIN Dry Viscosity/Thickness Dry Viscosity/Thickness High Yield Stress High Yield Stress Mobile Belt Stress Buffers Mobile Belt Stress Buffers Rehydrate/Thin from Below Rehydrate Lack of Buffer

crust cratonic root removed cratonic root removed cratonic root PrecambrianPalaeozoicMesozoicCenozoic Silurian volcanism Basin development/volcanism Volcanism and extension barren kimberlite diamond kimberlite Asthenosphere (1300 C) Asthenosphere (1300 C) Asthenosphere (1300 C) Asthenosphere (1300 C) Archean crust (3800 Ma) S-K C Loss of > 120 km of Archaean lithosphere, Sino-Korean craton

S-K C Low Angle Subduction Would Allow For Rehydration Weakening Why Geologically Recent Instability ? Weakening Elements in Place in Past

STABILITY STABILITYIN Increasing Mantle Stress

Subducting Slab Failure Zone Horizontal Surface Velocity Track Temperature, Strain Rate, and Stress Profiles To Get Average Lithospheric Stress Gives a Measure of Convective Mantle Stress Vary Internal Heating To See How Mantle Stress Varies With Convective Vigor

Lithospheric Stress (Mpa) Internal Heating Rayleigh Number 5x10 1x10 2x INCREASE INTERNAL HEATING DECREASE MANTLE VISCOSITY Lower Viscosity Dominates Stress Scaling

MODELING CRATON STABILITY Vary Cratonic Properties: Viscosity, Yield Stress, Buoyancy O’Neill et al., Lithos (2010) Vary Mantle Properties: Clayperon Slope, Upper/Lower Mantle Viscosity, Convective Vigor (increases in past)

Weakened (Hydrated) Craton Small Disruption, No Recycling Weakened (Hydrated) Craton Large Disruption, Recycling Dehydrated Craton Stress (Mpa) Mantle Heat Production

Geologic Time Past Present Future Craton Yield Stress (Mpa) Mantle Stress (Mpa) Reference (dry) Weakened (rehydrated)

Mantle Stress Can Increase Over Time Due To Increasing Mantle Viscosity Greater Potential for INSTABILITY in Geologic Present Vs. Ancient Past High Craton Viscosity Leads to Stability in Thick Root Limit. INSTABILITY: Rehydrate to Lower Viscosity High Yield Stress Relative to Ocean & Peripheral Continental Lithosphere Leads to Stability INSTABILITY: Lower Yield Stress (water) or No Peripheral Buffer