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ES204 Plate Tectonics Lecture Eli Silver X 2266 Email: [email protected] Room: A142 EMS
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Outline
1. What is a plate? How Thick? 2. Why does the Earth have Plate Tectonics?
3. Are all tectonics a result of plate edge effects?
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Topography of the Earth’s Surface
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What is a Plate?
Plate thickness can be measured:
• Seismically
• Thermally
• Elastically
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Heat Flow through the lithosphere is governed by Fouriers’s Law:
q = -k ∂T/ ∂z Where q is the rate of heat flow/area k is thermal conductivity z is positive down and q positive upward T is Temperature
This describes conductive heat flow
Thermally
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For the upper mantle, thermal conductivity, k, Is about 3 Watts/meter/°C
And q is generally expressed as mW/m2
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For 140 my old lithosphere, the temperature is 1000 °C at 100 km. If k = 3 W/m/ °C, what is q?
Ans: 0.03 W/m2 or 30 mW/m2
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Temperature at base of TBL ~ 1200 oC
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Elastic Thickness Of the Lithosphere
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from Watts, 2001
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wo
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w(x) = w0e(-x/α) [cos(x/α) + sin(x/α)]
Where w0 is the maximum deflection at the load
α = [4D/g(ρm – ρw)]1/4
Known as the flexural parameter Where D = Eh3 / 12(1-υ2) D is the flexural rigidity of the plate h is the elastic thickness of the plate υ is Poisson’s ratio E is Young’s Modulus g is gravity ρm and ρw are mantle and water densities
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To solve for the elastic thickness, h:
Choose a value for h, solve for D, then α. Next solve for w at a number of x locations. Plot, and compare your curve with the observed data. Change h to a higher or lower value of h, depending on this comparison.
Constants: E = 1011 Pa υ = 0.282 (ρm – ρw) = 2400 kgm/m3
g = 9.8 m/s2
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wo
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-2500
-2000
-1500
-1000
-500
0
500
0 50000 100000 150000 200000 250000 300000 350000
30 Km 30 Km
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-2500
-2000
-1500
-1000
-500
0
500
0 20000 40000 60000 80000 100000 120000 140000 160000
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wo
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Estimating Lithospheric Thickness, z, as a function Of Plate Age, t:
With z in km, and t in Ma, z ~ 11√t
t = 1 My, z = 11 km
t = 10 My, z = 35 km
t = 100 My, z = 110 km
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What thicknesses do you get from Crustal Flexure studies?
Less than half these numbers.
Often 10 km or less, and 25 to 30 km is Definitely on the high end for Oceanic Crust.
Why the large difference???
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Rock strength varies with depth in the Crust and upper mantle depending on:
• Temperature • Rock composition • Crustal Thickness • Strain rate • Water content
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Strength of the Lithosphere
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The Jelly Sandwich Crust – Upper Mantle
Top: Variable Crustal Thickness
Middle: Variable Temperature
Lower: Variable Strain rate
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Wet vs. Dry
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East African Rifts: Earthquake depths and Crustal Rheology
Albaric et al., 1009; Tectonophysics
Effects of crustal composition and heat flow
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Earthquake Depth-Frequency● Yield Strength Envelopes
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Frequency-Depth distribution of Earthquakes In California – A function of:
• Thermal structure • Crustal Thickness • Petrology and Geochemistry • Strain rate • Water content