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

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

Lithospheric Plates


 The lithosphere can be defined thermally by an isotherm
                                                    o
                                                         at the
base of the lithosphere which should be around 1350 C.

 How are plates created ?

 How and Why do plates move ?

Time-Dependent Heat Conduction

              dT/dt =  d T/dx
                          2     2

      (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

Time-Dependent Heat Conduction

                      dT/dt =  d T/dx
                                   2     2

    Charcteristic diffusion time (t) can be described using  where
                             t = d2/

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

Time-Dependent Heat Conduction

               dT/dt =  d T/dx
                            2      2

 Charcteristic diffusion distance (d) can be described
using  where
                   d = sqrt(t

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

Time-Dependent Heat Conduction

               dT/dt =  d T/dx
                            2      2

 Charcteristic diffusion distance (d) can be described
using  where
                   d = sqrt(t

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

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

    Fukao et al., 2001


 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.

Activity

         660 km
         1000 km

          Fukao et al., 2001

  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

Other Seismic Studies of the Continental Lithosphere
      Dayanthie S. Weeraratne, Donald W. Forsyth, Andrew A. Nyblade
                    (Brown University and Penn State)

                                                    Meters

Ethiopian Broadband Experiment

Surface wave tomography method
in the continental upper mantle
   ­ Tanzanian craton
   ­ Ethiopian Plateau

                                  Tanzanian Broadband Experiment

2­D Phase Velocity Maps

        50s

* 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

                                                        Disruption of
                                                        the lithosphere

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

Tectonic Plates on Earth and Other Planets

          Earth                          Venus

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

Tectonic Plates on Earth and Other Planets

              Earth                             Venus

  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 ?

Study of the Oceanic Asthenosphere

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

     Lithosphere
      Asthenosphere

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

Ocean Bottom Seismometer (OBS) Deployment
  Authors: Dayanthie Weeraratne1, Donald W. Forsyth1, Yingjie
                     Yang1, Spahr Webb2
           1. Brown University, 2. Lamont Doherty Observatory

GLIMPSE Experiment
(Gravity Lineations and Intraplate Melting
   Petrology and Seismic Expedition )

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

Brown University
Lamont Doherty Observatory
Oregon State University

Rayleigh Waves in the Earth

                  e
                av
               w
          gh
    l  ei
 a y                       P
R

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

                                                Aleutian Islands
                 X 10^2

                                                     Mw = 7.9
                           P   S   R
                          X 10^3          Time (s)

Azimuthal Distribution of Earthquakes and Raypaths

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

   18s                 33s                59s                100s

Raypath density varies from 1565 to 132 paths with increasing period.

1­D Shear Wave Velocities of the Low Velocity Asthenosphere

                           Starting Model
LVZ

                           (NF, 1998)

      * 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.

Seismic Tomography Images of the Asthenosphere
A                                  A
                                   '
                                              A

                                                                  A'
                                          B

                                                             B'

                             Vs (km/s)
B                                   B'
                                         * High velocity lithosphere
                                           and LVZ are well resolved.

                                         *Low velocities are observed
                                          beneath seamount chains.

                                         *Lithospheric thinning
                                          beneath Sojourn ridge.
        Distance from EPR (km)

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).

The Oceanic Asthenosphere
                                                          Faul and Jackson, 2005

Karato and
Jung, 1998

                         T *C            Vs (km/s)              Qs

      *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 ?

Surface Wave Quality Factor (Q)
                                                                0

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

                                                   Depth (km)
                                                           50
                                                                                             GLIMPSE
QR

                                                         100

                                                                                              Yang et al., 2007
                                                         150
                                                           80       100   120   140   160   180   200   220   240
                    Period (s)                                                        Q

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

Attenuation in the Oceanic Asthenosphere
                                                                 Vs (km/s)

             Faul and Jackson, 2005

                                      GLIMPSE
Depth (km)

                                                    Depth (km)
                               Qs

   * 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

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

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

Digital Photo: Image Size

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

Introduction to Fourier Transforms


    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.

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

                                          1

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.
                                         8

Other Seismic Studies of the Continental Lithosphere
      Dayanthie S. Weeraratne, Donald W. Forsyth, Andrew A. Nyblade
                    (Brown University and Penn State)

                                                    Meters            11

Ethiopian Broadband Experiment

Surface wave tomography method
in the continental upper mantle
   ­ Tanzanian craton
   ­ Ethiopian Plateau

                                                                     12
                                  Tanzanian Broadband Experiment

2­D Phase Velocity Maps

        50s

* High phase velocities are observed within craton boundaries.
* Low velocities observed beneath the Eastern rift branch.       13
* Disruption of cratonic lithosphere in SE corner.

Shear Wave Velocity Cratonic Lithosphere

                                                        Disruption of
                                                        the lithosphere

High velocity cratonic lithosphere observed to 170 km depth.        14

Disruption of lithosphere in SE corner observed at depths 80 ­ 150 km.

Tectonic Plates on Earth and Other Planets

          Earth                            Venus

      
          The Earth has many tectonic plates
      
          Other planets only have one plate, Why ?
                                                     15

Tectonic Plates on Earth and Other Planets

              Earth                             Venus

  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 ?                      16

  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.
                                                                17

    How could we measure such tectonic plate movement ?

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

                                         29

What Drives Plate Motion ?

      
        Slab Pull
      
        Ridge Push
      
        Others ?
      
          Mantle Drag        30

Ridge Push

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

                                                            31

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

                                               32

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        33
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 ?
                                                              34

Introduction to Fourier Transforms

                                     35

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                             36

Digital Photo: Image Size


    Old photo (left) was cleaned up with a FFT (right)
                                                         37

Introduction to Fourier Transforms


    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)

                                                   38

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                    39

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

                                      40

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.
                                                41

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.

                                                                   42

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.

                                                          43