tectonic plates: what are they made of and what …dsw/lect4_geodyn_asthen.pdf · lithospheric...

Tectonic Plates: What are They Made of and What Drives Them ?

Lithospheric Plates

The lithosphere can be defined thermally by an isotherm at the base of the lithosphere which should be around 1350oC.

How are plates created ?

How and Why do plates move ?

Time-Dependent Heat Conduction

dT/dt = d2T/dx2

(Known as the “Heat flow Equation”)

Where = k/Cp

is thermal diffusivity (m2/s).

describes the diffusion of temperature or heat across a body of material


dT/dt = d2T/dx2

Charcteristic diffusion time (t) can be described using where

t = d2/

This gives the time for heat to diffuse across a distance, d.


dT/dt = d2T/dx2

Charcteristic diffusion distance (d) can be described using where

This gives the distance temperature will propogate through the material in a given time period.

d = sqrt(t

Activity

Zhao et al., 1997

P wave tomography image of the Tonga trench subduction zone

High velocity subducting slab is clearly visible (blue) extending down to at least 660 km depth.

Global Seismic Tomography

Subducting Farallon slab is imaged through seismic tomography extending to at least 2000 km depth

Farallon reaches this depth somewhere beyond the east coast of North America

Grand et al., 2001.

Activity

Seismic tomography image of the Pacific plate subducting beneath Japan.

Scientists argue about whether all subducting plates penetrate through the 660 km discontinuity into the lower mantle.

Fukao et al., 2001

Activity

Some authors say some slabs just rest at the 660 and may “thermally assimilate” over time.

Calculate how long it would take a slab to “thermally assimilate”.

Use the thickness of the slab you observe in the images above

Assume thermal conductivity of peridotite, k = 3.0 Wm-1K-1,density = 3250 kg m-3, and heat capacity, Cp = 0.8

kJ/kg K

Fukao et al., 2001

660 km

1000 km

Other Seismic Studies of the Continental Lithosphere

Dayanthie S. Weeraratne, Donald W. Forsyth, Andrew A. Nyblade (Brown University and Penn State)

Meters

Surface wave tomography methodin the continental upper mantle Tanzanian craton

Ethiopian Plateau

Tanzanian Broadband Experiment

Ethiopian Broadband Experiment

50s

2D Phase Velocity Maps

* High phase velocities are observed within craton boundaries.* Low velocities observed beneath the Eastern rift branch.* Disruption of cratonic lithosphere in SE corner.

Shear Wave Velocity Cratonic Lithosphere

High velocity cratonic lithosphere observed to 170 km depth.Disruption of lithosphere in SE corner observed at depths 80 150 km.

Disruption of the lithosphere

Tectonic Plates on Earth and Other Planets

VenusEarth

The Earth has many tectonic plates Other planets only have one plate, Why ?


VenusEarth

Maybe the Earth's lithosphere is weaker and prone to break up ? Any differences in lithospheric thickness, strength ? What allows plates to move ? Do other planets have an asthenosphere ? What is the asthenosphere ?

“One-Plate” Planets

Venus “One-plate” planets such as Venus or Mars are thought to have a shell-like lithosphere which surrounds the planet

This lithospheric shell may rotate as a whole if an asthenosphere is present to allow movement.

How could we measure such tectonic plate movement ?

We know very little about the physical properties of the asthenosphere.What makes the asthenosphere “ductile” ?

?LithosphereAsthenosphere

*Seismic low velocity zone (LVZ)*Competing effects of increasing temperature and pressure at depth*Compositional variations in water content or presence of partial melt

Study of the Oceanic Asthenosphere

Ocean Bottom Seismometer (OBS) DeploymentAuthors: Dayanthie Weeraratne1, Donald W. Forsyth1, Yingjie

Yang1, Spahr Webb2

1. Brown University, 2. Lamont Doherty Observatory

COOK16/Melville November, 2001VANC04/Melville November, 2002

GLIMPSE Experiment

Brown UniversityLamont Doherty ObservatoryOregon State University

(Gravity Lineations and Intraplate Melting Petrology and Seismic Expedition )

Rayleigh Waves in the Earth

S

PRayl

eigh w

ave

RSP

X 10^3

X 1

0^2

Time (s)

Mw = 7.9Aleutian Islands

*Surface waves *Surface and body wave tomography*Rayleigh wave dispersion*Azimuthal anisotropy

Azimuthal Distribution of Earthquakes and Raypaths

* Ideal azimuthal distribution* 155 Earthquakes * 4.5 < Ms < 7.8

Raypath density varies from 1565 to 132 paths with increasing period.18s 33s 59s 100s

1D Shear Wave Velocities of the Low Velocity Asthenosphere

* High velocity lithosphere extends to 60 km +/ 20 km.* Steep positive velocity gradient identifies the base of the LVZ (Low Velocity Zone) at ~110 km associated with the asthenosphere.

Starting Model(NF, 1998)L

VZ

Seismic Tomography Images of the Asthenosphere

* High velocity lithosphere and LVZ are well resolved.

*Low velocities are observed beneath seamount chains.

*Lithospheric thinning beneath Sojourn ridge.

A

A'B

B'

A A'

Distance from EPR (km)

B B'Vs (km/s)

What can Seismic Attenuation tell us?

Seismic attenuation is found to be strongly affected by temperature and volatile content (e.g. Wiens and Smith, 2003).

Small amounts of partial melt, however, have a lesser effect as relaxation occurs at periods outside the seismic frequency band (e.g. Hammond and Humphreys, 2000a).

*Experiments suggest asthenosphere LVZ can be explained by natural increase in temperature and pressure effects alone (Faul and Jackson, 2005).

* But predict very low Q values (Q < 40)

* Can we measure this quantity ?

QsVs (km/s)T *C

Faul and Jackson, 2005

Karato and Jung, 1998

The Oceanic Asthenosphere

Q reaches 220 at 45 km depth and decreases below 60 km but does not require Q lower than 80 at 150 km depth.

Surface Wave Quality Factor (Q)

Period (s)

QR

QR = pi/(T*U*) T = period, U = group velocity

.γ (T) = π/(T*C2) Σ { (Vs*δC/δVs) + ½ (Vp*δC/δVp) }1/Q C = Phase Velocity (km/s), T = Period (s), Q = Intrinsic Shear Wave Quality Factor (Mitchell, 1995)

Q

Dep

th (k

m)

80 100 120 140 160 180 200 220 240

0

50

100

150

GLIMPSE

Yang et al., 2007

Attenuation in the Oceanic Asthenosphere

Qs

Dep

th (k

m)

GLIMPSE

Faul and Jackson, 2005

Vs (km/s)

Dep

th (

km)

* This study indicates that pressure and temp effects alone are not sufficient

* We suggest the presence of partial melt may explain the low velocity and

moderate attenuation in this region of the asthenosphere

What Drives Plate Motion ?

Slab Pull Ridge Push Others ?

Mantle Drag

Ridge Push

The elevation of spreading centers creates gravitational potential energy gradient which “pushes” plates away

Calculating Ridge Push Quantitatively

Forces include stress and strain measurements for tectonic plates (Zoback 1989; Coblentz et al)

“Ridge push” force can be calculated per ridge length

Resultant convergence direction for Indo-Australian plate can be determined

Slab Pull

Slab pull forces are more difficult to estimate or measure

Subducting plates go down during collisional impact – not from pure negative buoyancy...

So downgoing plates may act differently than downwelling forms of convection

Lithospheric Plates and Mantle Flow

Figure 10.1 in your text (Davies) show that mantle flow is more active beneath fractured plates

Do plates drive mantle flow – or the reverse ?

Introduction to Fourier Transforms

Digital Photo: Image Size

Original photo (left) is compressed using a Fast Fourier Transform (FFT)

Smaller image size (left) retains all major features but with reduced clarity and smaller file size


Old photo (left) was cleaned up with a FFT (right)


Fourier Transforms can be used- Recover signal from a noisy record- Electrical Filters- Clean a television picture- reduce image size of digital photos

Fourier Transforms can be done analytically (on paper) or computationally (on a computer)

Fourier Transforms

A single musical note from a trombone is shown above You can see it is made up of a range of frequencies and amplitudes It's Fourier transform is shown to the right

Fourier Transforms

Every signal is made of a series of harmonics or multiples of the fundamental frequency

Each tone is produced by a unique group of phases and amplitudes or harmonics

Fourier Transforms: Amplitudes and Phases

Jerome Karl (left) and Herb Hauptman (right) won Nobel Prizes for work on phase problems in small molecule crystals

Phases from right photo are combined with Amplitudes of left photo giving photo in lower left.

The same was done for lower right

Clearly phase is very important in identifying a signal.

Fourier Analysis

Fourier analysis is used to find the amplitudes and phases which produce a given signal (musical note, seismogram, etc..)

Uses include identification of a valuable violin, detect faulty behavior in an aero-engine, detect heart defect in a cardiogram, detect mantle heterogeneities which produce a seismogram.

Fourier Synthesis

Fourier synthesis is the process used to construct a waveform by adding together a fundamental frequency and other overtones or harmonics (adding various phases and amplitudes)

We combine these harmonics by using cosine and sine terms in a Fourier series.

1


8

Global Seismic Tomography

Subducting Farallon slab is imaged through seismic tomography extending to at least 2000 km depth

Farallon reaches this depth somewhere beyond the east coast of North America

Grand et al., 2001.

11

Other Seismic Studies of the Continental Lithosphere

Dayanthie S. Weeraratne, Donald W. Forsyth, Andrew A. Nyblade (Brown University and Penn State)

Meters

12

Surface wave tomography methodin the continental upper mantle Tanzanian craton

Ethiopian Plateau

Tanzanian Broadband Experiment

Ethiopian Broadband Experiment

13

50s

2D Phase Velocity Maps

* High phase velocities are observed within craton boundaries.* Low velocities observed beneath the Eastern rift branch.* Disruption of cratonic lithosphere in SE corner.

14

Shear Wave Velocity Cratonic Lithosphere

High velocity cratonic lithosphere observed to 170 km depth.Disruption of lithosphere in SE corner observed at depths 80 150 km.

Disruption of the lithosphere

15


VenusEarth

The Earth has many tectonic plates Other planets only have one plate, Why ?

16


VenusEarth Maybe the Earth's lithosphere is weaker and prone to break up ? Any differences in lithospheric thickness, strength ? What allows plates to move ? Do other planets have an asthenosphere ? What is the asthenosphere ?

17

“One-Plate” Planets

Venus “One-plate” planets such as Venus or Mars are thought to have a shell-like lithosphere which surrounds the planet

This lithospheric shell may rotate as a whole if an asthenosphereis present to allow movement.

How could we measure such tectonic plate movement ?

29


30

What Drives Plate Motion ?

Slab Pull Ridge Push Others ?

Mantle Drag

31

Ridge Push

The elevation of spreading centers creates gravitational potential energy gradient which “pushes” plates away

32

Calculating Ridge Push Quantitatively

Forces include stress and strain measurements for tectonic plates (Zoback 1989; Coblentz et al)

“Ridge push” force can be calculated per ridge length

Resultant convergence direction for Indo-Australian plate can be determined

33

Slab Pull

Slab pull forces are more difficult to estimate or measure

Subducting plates go down during collisional impact – not from pure negative buoyancy...

So downgoing plates may act differently than downwelling forms of convection

34

Lithospheric Plates and Mantle Flow

Figure 10.1 in your text (Davies) show that mantle flow is more active beneath fractured plates

Do plates drive mantle flow – or the reverse ?

35


36


Original photo (left) is compressed using a Fast Fourier Transform (FFT)

Smaller image size (left) retains all major features but with reduced clarity and smaller file size

37


Old photo (left) was cleaned up with a FFT (right)

38


Fourier Transforms can be used- Recover signal from a noisy record- Electrical Filters- Clean a television picture- reduce image size of digital photos

Fourier Transforms can be done analytically (on paper) or computationally (on a computer)

39

Fourier Transforms

A single musical note from a trombone is shown above You can see it is made up of a range of frequencies and amplitudes It's Fourier transform is shown to the right

40

Fourier Transforms

Every signal is made of a series of harmonics or multiples of the fundamental frequency

Each tone is produced by a unique group of phases and amplitudes or harmonics

41

Fourier Transforms: Amplitudes and Phases

Jerome Karl (left) and Herb Hauptman (right) won Nobel Prizes for work on phase problems in small molecule crystals

Phases from right photo are combined with Amplitudes of left photo giving photo in lower left.

The same was done for lower right

Clearly phase is very important in identifying a signal.

42

Fourier Analysis

Fourier analysis is used to find the amplitudes and phases which produce a given signal (musical note, seismogram, etc..)

Uses include identification of a valuable violin, detect faulty behavior in an aero-engine, detect heart defect in a cardiogram, detect mantle heterogeneities which produce a seismogram.

43

Fourier Synthesis

Fourier synthesis is the process used to construct a waveform by adding together a fundamental frequency and other overtones or harmonics (adding various phases and amplitudes)

We combine these harmonics by using cosine and sine terms in a Fourier series.

tectonic plates: what are they made of and what …dsw/lect4_geodyn_asthen.pdf · lithospheric...

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