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 Ts o

t=0

t=0

_

+

z

T Ts 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 strengthm

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” lithospheresubducting” lithosphere viscosity = 10 Pa sviscosity = 10 Pa s 2525

warm mantleviscosity = 10 Pa s warm mantleviscosity = 10 Pa s2121

hothothothotcoldcoldcoldcoldconvecting mantle

failed region failed region extensionextensionfailed region failed region extensionextension

failed region failed region compressioncompression failed region failed region compressioncompression

cratonic rootlower crustupper crust

bulk mantle

local local geothermgeotherm

Cooper et al. 2004

Cooper et al. 2004

cChemical/Mechanical Lithosphere

tThermal Lithosphere

Dynamic Mantle Sub-Layer

ct

mantle heat flow

surface heat flowUpper CrustLower Crust

Chemical Lithosphere

Average Thermal Lithosphere

Temperature (Celsius)0 200 400 600 800 1000 1200 1400

Dep

th (

km

) 0

50

100

150

200

250

300

Th

erm

al/

Ch

em

ical B

L T

hic

kn

ess R

ati

o 4.5

4

3

3.5

2

2.5

1

1.5

Chemical Boundary Layer Thickness (km)

40 60 80 100 120 140 160 180 200

Tem

pera

ture

Dro

p A

cro

ss S

ub

-Layer (C

) 700

600

500

400

300

200

100

Radiogenically Depleted Root

Radiogenically Enriched Root

QuickTime™ and a decompressor

are needed to see this picture.

Yuan & Romanowicz 2010

Therm

Chem

Chemical Lithosphere (km)Latitude

dept

h (k

m)

50 100 150 200

4

3

2

400 Therm

al/C

hem

ical R

ati

o

300

200

100

0

65 35

55 4560 50 40

Preserving & Destroying Cratonic Lithosphere

The Structure of

Preserving & Destroying Cratonic Lithosphere

CRATON STABILITYCRATON INSTABILITY

UNDERSTAND STABILITY TO UNDERSTAND INSTABILITY

MODELING CRATON STABILITY

chemically light chemically light material - root material - root (own rheology)(own rheology)

chemically real light material - crust (has own rheology)chemically real light material - crust (has own rheology)

cold cold hothotmantlemantle

base of thermal lithospherebase of thermal lithospherecontinental lithosphere is cool & more viscous than bulk mantle

failed regionsfailed regions

cold cold viscosity 10 Pa s viscosity 10 Pa s

2626hot hot viscosity 10 Pa sviscosity 10 Pa s2121

Send Continent into Model Subduction Zone See What it Takes to Save Root & Keep Crust Stable

MODELING CRATON STABILITYMODELING 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

Nor

mal

ized

Roo

t Ext

ent

Root Thickness (km)

50 Myr100 Myr150 Myr

0.2

0.4

0.6

0.8

1.0

120 140 160 180 200 250

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 StressCan Lead To Stability

50 Myr

100 Myr

Root 1000X Viscosity of Mantle at = Temp

MODELING CRATON STABILITY - YIELD STRESS

0.2

0.4

0.6

0.8

1.0

0.1 0.15 0.2 0.25 0.3 0.35 0.4

Root & Crust; 50myrRoot & Crust; 100myrRoot Only; 50MyrRoot Only; 100Myr

Normalized Root Extent

Continent/Mantle Yield Ratio1.0 1.5 2.0 2.5 3.0 3.5 4.0

Craton Does Not Fail Under Stress Due to High Yield Strength

Buffer Cratons from High Stress and They Will Not Yield

Auto makers consider it impractical to make drivers heads stronger so ……...

MODELING CRATON STABILITY - MOBILE BELTS

Mobile Belts Can Provide Craton Stability(act as crumple zones to buffer stress)

50 Myr

100 Myr

Die

tz [1

963]

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 = 1.0no crumple zone

craton

craton

yield ratio = 0.5

STABILITYSTABILITYININ Dry Viscosity/ThicknessDry Viscosity/Thickness

High Yield StressHigh Yield Stress

Mobile Belt Stress Buffers Mobile Belt Stress Buffers

Rehydrate/Thin from BelowRehydrate/Thin from Below

RehydrateRehydrate

Lack of BufferLack of Buffer

crustcrust

cratonic rootcratonic root

removed cratonic root

removed cratonic root

Precambrian Palaeozoic Mesozoic Cenozoic Silurian volcanism

Basin development/volcanism Volcanism and extension barren kimberlite

diamondkimberlite

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 CLow Angle Subduction Would Allow Low Angle Subduction Would Allow For Rehydration WeakeningFor Rehydration Weakening

Why Geologically RecentWhy Geologically RecentInstability ? WeakeningInstability ? WeakeningElements in Place in PastElements in Place in Past

STABILITYSTABILITYININ

Increasing Mantle StressIncreasing Mantle Stress

Subducting Slab Failure Zone

Horizontal Surface Velocity

Track Temperature, Strain Rate, and Stress ProfilesTo Get Average Lithospheric StressGives a Measure of Convective Mantle Stress

Vary Internal Heating To See How Mantle Stress Varies With Convective Vigor

0

5000

1 104

1.5 104

0 5 10 15 20

Lith

osph

eric

Str

ess

(Mpa

)

Internal Heating Rayleigh Number

5x10 1x10 2x10 6 7 7

250

125

0

375

INCREASE INTERNAL HEATING DECREASE MANTLE VISCOSITY

Lower Viscosity Dominates Stress Scaling

MODELING CRATON STABILITYMODELING 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) CratonSmall Disruption, No Recycling

Weakened (Hydrated) CratonLarge Disruption, Recycling

Deh

ydra

ted

Cra

ton

Str

ess

(Mpa

)

Mantle Heat Production

0

5000

1 104

1.5 104

0 5 10 15 20

Geologic Time

Past Present Future

Craton Y

ield Stress (M

pa)

Man

tle S

tres

s (M

pa)

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