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

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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 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 = k d2T/dx2 (Known as the “Heat flow Equation”) 2 Where k = k/rCp is thermal diffusivity (m /s). k describes the diffusion of temperature or heat across a body of material Time-Dependent Heat Conduction dT/dt = k d2T/dx2 Charcteristic diffusion time (t) can be described using k where t = d2/k This gives the time for heat to diffuse across a distance, d. Time-Dependent Heat Conduction dT/dt = k d2T/dx2 Charcteristic diffusion distance (d) can be described using k where d = sqrt(k t) This gives the distance temperature will propogate through the material in a given time period. Time-Dependent Heat Conduction dT/dt = k d2T/dx2 Charcteristic diffusion distance (d) can be described using k where d = sqrt(k 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, -3, density = 3250 kg m 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 2-D 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 ªductileº ? 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 ave w igh ayle P R S *Surface waves *Surface and body wave tomography *Rayleigh wave dispersion *Azimuthal anisotropy Aleutian Islands 10^2 Mw = 7.9 X 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. 1-D Shear Wave Velocities of the Low Velocity Asthenosphere Z Starting Model V (NF, 1998) L * 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 (Qm) 0 QR = pi/(T*U*g) T = period, U = group velocity m) 50 GLIMPSE (k h h t R Q Dep 100 Yang et al., 2007 150 80 100 120 140 160 180 200 220 240 Period (s) Qm .γ (T) = π/(T*C2) Σ { (Vs*δC/δVs) + ½ (Vp*δC/δVp) }1/Qm C = Phase Velocity (km/s), T = Period (s), Qm = Intrinsic Shear Wave Quality Factor (Mitchell, 1995) Qm 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 (km) pth De ) m k ( h t p e Qs D * 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 2-D Phase Velocity Maps 50s * High phase velocities are observed within craton boundaries.
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