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GLOBAL TOPOGRAPHY. CONTINENTAL & OCEANIC LITHOSPHERE. CONTINENTAL & OCEANIC LITHOSPHERE. Age. Topography. mid ocean ridge. Heat Flow. mantle. . t. T T. s o. thermal  Thermal boundary layer of mantle convection. t. T T. s o. +. t=0.

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Presentation Transcript
slide3

CONTINENTAL & OCEANIC LITHOSPHERE

Age

Topography

mid ocean

ridge

Heat Flow

mantle

slide4

t

T T

s o

thermal Thermal boundary layer of mantle convection

t

T T

s o

+

t=0

time

Region of T gradient is a

Thermal Boundary Layer

_

t=0

z

z

tectothermal age of plate (ta)

MOR

mantle flow

mantle heat loss (q)

slide5

c

m

t

mechanical : Layer of long term strength

m

chemical/mechanical : Dehydrated Layer (dry=hi viscsoity)

m

tectothermal age of plate (ta)

MOR

mantle flow

mantle heat loss (q)

thermal Thermal boundary layer of mantle convection

t

(cold=hi viscosity)

slide6

Continent

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).

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.

slide7

Cooper et al. 2004

convecting mantle

failed region

compression

failed region

extension

cold

hot

upper crust

“subducting” lithosphere

viscosity = 10 Pa s

lower crust

25

cratonic root

warm mantle

viscosity = 10 Pa s

21

bulk mantle

local

geotherm

slide8

Cooper et al. 2004

Chemical/Mechanical Lithosphere

c

Thermal Lithosphere

t

Dynamic Mantle Sub-Layer

slide9

surface heat flow

mantle heat flow

c

t

0

Upper Crust

Lower Crust

50

100

Depth (km)

150

Chemical Lithosphere

200

Average Thermal Lithosphere

250

300

0 200 400 600 800 1000 1200 1400

Temperature (Celsius)

slide10

700

4.5

4

600

3.5

500

3

Radiogenically

Depleted Root

Temperature Drop Across Sub-Layer (C)

400

Thermal/Chemical BL Thickness Ratio

2.5

Radiogenically

Enriched Root

300

2

200

1.5

100

1

40 60 80 100 120 140 160 180 200

Chemical Boundary Layer Thickness (km)

slide11

Preserving & Destroying Cratonic Lithosphere

The Structure of

4

0

3

100

Chem

Thermal/Chemical Ratio

Therm

depth (km)

200

2

55

50

300

60

45

40

50 100 150 200

65

Latitude

35

400

Chemical Lithosphere (km)

Yuan & Romanowicz 2010

slide12

CRATON INSTABILITY

CRATON STABILITY

Preserving & Destroying Cratonic Lithosphere

UNDERSTAND STABILITY TO UNDERSTAND INSTABILITY

slide13

chemically real light material - crust (has own rheology)

chemically light

material - root

(own rheology)

mantle

failed regions

cold

hot

base of thermal lithosphere

continental lithosphere is cool & more viscous than bulk mantle

21

hot viscosity 10 Pa s

26

cold viscosity 10 Pa s

MODELING CRATON STABILITY

slide14

MODELING CRATON STABILITY

MODELING CRATON STABILITY

Send Continent into Model Subduction Zone

See What it Takes to Save Root & Keep Crust Stable

300+ Simulations Later …

slide15

MODELING CRATON STABILITY - BUOYANCY

Buoyancy

Does Not Lead To

Stability

(even w/ temperature

dependent viscosity)

7 Myr

29 Myr

slide16

MODELING CRATON STABILITY - VISCOSITY

Root 1000X Viscosity of Mantle at = Temp

Viscosity

Does Not Lead To

Stability

50 Myr

Viscosity+

Critical Thickness

Can Lead To

Stability

100 Myr

slide17

1.0

Root/Mantle Viscosity Ratio = 1000

Extreme De-Hydration

0.8

Normalized Root Extent

0.6

0.4

50 Myr

100 Myr

150 Myr

0.2

120 140 160 180 200 250

Root Thickness (km)

Lower Ratio (>100) Can Not Prevent Viscous Root Deformation

MODELING CRATON STABILITY - VISCOSITY

slide18

MODELING CRATON STABILITY - VISCOSITY

Root 1000X Viscosity of Mantle at = Temp

Viscosity

Does Not Lead To

Stability

50 Myr

Viscosity+High

Craton Yield Stress

Can Lead To

Stability

100 Myr

slide19

MODELING CRATON STABILITY - YIELD STRESS

1.0 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

slide20

Auto makers consider it impractical to

make drivers heads stronger so ……...

slide21

MODELING CRATON STABILITY - MOBILE BELTS

Mobile Belts

Can Provide Craton

Stability

(act as crumple zones

to buffer stress)

50 Myr

100 Myr

slide22

REGENERATING MOBILE BELTS (Crumple Zones)

if subduction starts offshore,

forms island arc, then

migrates on shore

- craton will be buffered

if subduction starts at time B

- craton will be stressed

Dietz [1963]

slide23

craton

yield ratio = 0.5

craton

no crumple zone

yield ratio = 1.0

crumple zone model

mobile belt (deep green) yield stress relative to craton (pale green) yield

= 0.5

slide24

IN

STABILITY

Dry Viscosity/Thickness

Rehydrate/Thin from Below

High Yield Stress

Rehydrate

Mobile Belt Stress Buffers

Lack of Buffer

slide25

Loss of > 120 km of

Archaean lithosphere,

Sino-Korean craton

Precambrian

Palaeozoic

Mesozoic

Cenozoic

Silurian

volcanism

Basin development/volcanism

Volcanism and extension

barren

kimberlite

diamond

kimberlite

Archean crust

(3800 Ma)

Asthenosphere

(1300 C)

Asthenosphere

(1300 C)

S-K C

crust

Asthenosphere

(1300 C)

Asthenosphere

(1300 C)

removed

cratonic

root

cratonic root

slide26

Low Angle Subduction Would Allow

For Rehydration Weakening

S-K C

Why Geologically Recent

Instability ? Weakening

Elements in Place in Past

slide27

IN

STABILITY

Increasing Mantle Stress

slide28

Failure Zone

Subducting Slab

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

slide29

Lower Viscosity

Dominates

Stress

Scaling

INCREASE INTERNAL HEATING DECREASE MANTLE VISCOSITY

375

250

Lithospheric Stress (Mpa)

125

0

6 7 7

5x10 1x10 2x10

Internal Heating Rayleigh Number

slide30

MODELING CRATON STABILITY

MODELING CRATON STABILITY

O’Neill et al., Lithos (2010)

Vary Cratonic Properties:

Viscosity,

Yield Stress,

Buoyancy

Vary Mantle Properties:

Clayperon Slope,

Upper/Lower Mantle Viscosity,

Convective Vigor

(increases in past)

slide31

Dehydrated Craton Stress (Mpa)

Mantle Heat Production

Weakened (Hydrated) Craton

Weakened (Hydrated) Craton

Small Disruption, No Recycling

Large Disruption, Recycling

slide32

Reference

(dry)

Weakened

(rehydrated)

Mantle Stress (Mpa)

Craton Yield Stress (Mpa)

Past Present Future

Geologic Time

slide33

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

Mantle Stress Can Increase Over Time Due To Increasing Mantle Viscosity

Greater Potential for INSTABILITY in Geologic Present Vs. Ancient Past