Transcript The structure, stability, and potential instability of ancient continental

GLOBAL TOPOGRAPHY
CONTINENTAL & OCEANIC LITHOSPHERE
CONTINENTAL & OCEANIC LITHOSPHERE
Age
Topography
mid ocean
ridge
mantle
Heat Flow
tectothermal age of plate (ta)
MOR
t
mantle flow
mantle heat loss (q )
thermal Thermal
boundary layer of mantle convection
t
Ts
Ts
To
t=0+
time
t=0
z
_
z
To
Region of T gradient is a
Thermal Boundary Layer
tectothermal age of plate (ta)
MOR
c 
m t
mantle flow
mantle heat loss (q )
thermal Thermal
boundary layer of mantle convection
t
mechanical m : Layer of long term strength (cold=hi viscosity)
chemical/mechanical m : Dehydrated Layer (dry=hi viscsoity)
Continent
Oceanic Thermal Lithosphere
defines convection pattern
- it is the cold, overturning
boundary layer.
Oceanic Chemical Lithosphere subducts
- overturning portions of the Earth see
a constant temperature boundary
condition.
Continental Chemical Lithosphere does not
participate in convective mantle overturn
(inherently buoyant).
Provides a more complex thermal coupling
condition for covecting mantle below.
Cooper et al. 2004
convecting mantle
cold hot
failed region
extension
upper crust
lower crust
cratonic root
bulk mantle
failed region
compression
local
geotherm
warm mantle
viscosity = 10 21 Pa s
“subducting” lithosphere
viscosity = 10 25 Pa s
Cooper et al. 2004
Chemical/Mechanical Lithosphere
Dynamic Mantle Sub-Layer
Thermal Lithosphere  t
c
0
surface heat flow
Upper Crust
Lower Crust
Depth (km)
50
mantle heat flow
100
150
200 Chemical Lithosphere
Average Thermal Lithosphere
250
300
0
200
400 600
800 1000 1200 1400
Temperature (Celsius)
c
t
Thermal/Chemical BL Thickness Ratio
700
4
600
3.5
500
3
Radiogenically
Depleted Root
400
2.5
Radiogenically
Enriched Root
2
300
200
1.5
100
1
40
60
80
100
120
140
160
180
200
Chemical Boundary Layer Thickness (km)
Temperature Drop Across Sub-Layer (C)
4.5
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decompressor
are needed to see this picture.
Chem
Therm
65
60
55
50
Latitude
Yuan & Romanowicz 2010
45
40
35
Thermal/Chemical Ratio
Preserving
Destroying
The&Structure
of Cratonic Lithosphere
4
3
2
50
100
150
200
Chemical Lithosphere (km)
CRATON INSTABILITY
STABILITY
Preserving & Destroying Cratonic Lithosphere
UNDERSTAND STABILITY TO UNDERSTAND INSTABILITY
MODELING CRATON STABILITY
chemically real light material - crust (has own rheology)
mantle
cold hot
failed regions
chemically light
material - root
(own rheology)
cold viscosity 10 26 Pa s
21
hot viscosity 10 Pa s
base of thermal lithosphere
continental lithosphere is cool & more viscous than bulk mantle
MODELING CRATON STABILITY
Send Continent into Model Subduction Zone
See What it Takes to Save Root & Keep Crust Stable
300+ Simulations Later …
MODELING CRATON STABILITY - BUOYANCY
Buoyancy
Does Not Lead To
Stability
(even w/ temperature
dependent viscosity)
7 Myr
29 Myr
MODELING CRATON STABILITY - VISCOSITY
Root 1000X Viscosity of Mantle at = Temp
Viscosity
Does Not Lead To
Stability
50 Myr
100 Myr
Viscosity+
Critical Thickness
Can Lead To
Stability
MODELING CRATON STABILITY - VISCOSITY
Normalized Root Extent
Extreme De-Hydration
1.0
Root/Mantle Viscosity Ratio = 1000
0.8
0.6
50 Myr
100 Myr
150 Myr
0.4
0.2
120
140
160
180
200
250
Root Thickness (km)
Lower Ratio (>100) Can Not Prevent Viscous Root Deformation
MODELING CRATON STABILITY - VISCOSITY
Root 1000X Viscosity of Mantle at = Temp
Viscosity
Does Not Lead To
Stability
50 Myr
100 Myr
Viscosity+High
Craton Yield Stress
Can Lead To
Stability
MODELING CRATON STABILITY - YIELD STRESS
Normalized Root Extent
1.0
Root & Crust; 50myr
Root & Crust; 100myr
Root Only; 50Myr
Root Only; 100Myr
0.8
0.6
0.4
0.2
0.1
1.0
0.15
0.2
0.25
0.3
0.35
1.5 2.0
2.5 3.0
3.5
Continent/Mantle Yield Ratio
0.4
4.0
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
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
crumple zone model
mobile belt (deep green) yield stress
relative to craton (pale green) yield
= 0.5
craton
craton
no crumple zone
yield ratio = 0.5
yield ratio = 1.0
INSTABILITY
Dry Viscosity/Thickness
Rehydrate/Thin from Below
High Yield Stress
Rehydrate
Mobile Belt Stress Buffers
Lack of Buffer
Precambrian
Palaeozoic
barren
Silurian
diamond
kimberlite volcanism kimberlite
Mesozoic
Basin development/volcanism
Cenozoic
Volcanism and extension
Archean crust
(3800 Ma)
Loss of > 120 km of
Archaean lithosphere,
Sino-Korean craton
S-K C
Asthenosphere
(1300 C)
Asthenosphere
(1300 C)
Asthenosphere
(1300 C)
Asthenosphere
(1300 C)
crust
removed
cratonic
root
cratonic root
Low Angle Subduction Would Allow
For Rehydration Weakening
S-K C
Why Geologically Recent
Instability ? Weakening
Elements in Place in Past
INSTABILITY
Increasing Mantle Stress
Horizontal Surface Velocity
Failure Zone
Subducting Slab
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
INCREASE INTERNAL HEATING
DECREASE MANTLE VISCOSITY
375
Lithospheric Stress (Mpa)
1.5 10 4
Lower Viscosity
Dominates
Stress
Scaling
4
1 10
250
5000
125
00
0
5
6
5x10
10
7
1x10
15
20
7
2x10
Internal Heating Rayleigh Number
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)
Dehydrated Craton Stress (Mpa)
Mantle Heat Production
Weakened (Hydrated) Craton
Large Disruption, Recycling
Weakened (Hydrated) Craton
Small Disruption, No Recycling
1 10 4
5000
Craton Yield Stress (Mpa)
Mantle Stress (Mpa)
1.5 10 4
Reference
(dry)
Weakened
(rehydrated)
0
0
Past
5
10
15
Present
Geologic Time
20
Future
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