heat flow in young oceanic crust: is earth’s heat flux 44 tw or 31 tw
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Will Gosnold. Heat Flow in Young Oceanic Crust: Is Earth’s Heat Flux 44 TW or 31 TW. 2008 Joint Assembly, Ft. Lauderdale T21A-01, May 27, 2008 T-21A Thermotectonic Models of the Oceanic Lithosphere and the Problem of Hydrothermal Circulation: A New Look. Outline. Statement of problem - PowerPoint PPT PresentationTRANSCRIPT
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Heat Flow in Young Oceanic Crust:
Is Earth’s Heat Flux 44 TW or 31 TW
2008 Joint Assembly, Ft. Lauderdale
T21A-01, May 27, 2008
T-21A Thermotectonic Models of the Oceanic Lithosphere and the Problem of Hydrothermal Circulation: A New Look
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Will Gosnold
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Outline
• Statement of problem• Heat flow data• Continental and marine heat flow• Heat flow vs. age models• 2-D numerical models• Sumary and conclusions
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20,201 heat flow sites recognized by the International Heat Flow Commission
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q = 510 t -.5
q = 480 t -.5
q = 473 t -.5
At issue is the accuracy of models of heat flow vs. age.
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Conductive heat flow at the surface is described by Fourier’s law of Heat conduction
Assuming we know heat flow, temperature at depth “z” may be calculated by
q
n
i i
iz
qzT1
Surface Heat Flow
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Conductive heat flow is predictable
Continental heat flow decreases with depth
Sources are heat contained with the crust and mantle and radioactive heat production
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In a conductive environment with constant heat flow, the temperature gradient varies with thermal conductivity.
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A fundamental assumption is that the temperature gradient is vertical and heat flow calculated from the gradient is vertical heat flow.
q
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Topography and complex structure with thermal conductivity contrasts or transient sources and sinks such as water flow invalidate this assumption.
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Heat flow in conductive environments is predictable and the heat flow map of North America demonstrates this predictability on the continents and in the ocean basins.
High heat flow:
young crust and recent tectonics
Low heat flow:
old thermally stable crust
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Variation in conductive heat flow within heat flow provinces on the continents is due to variation in radioactive heat production.
q = q0+ AD
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Heat flow within ocean basins correlates with age.
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Variability of q vs distance east of the Rocky Mts.
Continental heat flow exhibits low variability in non-tectonic areas.
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Marine heat flow exhibits high variability everywhere.
Variability of q vs distance from ridge
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Side-by-side comparison of marine and continental heat flow suggests the presence of non-conductive and transient signals in marine environments and in young tectonic environments.
Bullard’s Law
"Never take a second heat flow measurement within 20 km of the original for fear that it differ from the first by
two orders of magnitude."
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2-D finite-difference heat flow model
• Temperature profile for the ridge crest and intraplate from D.H. Green
• Temperature at base of intraplate lithosphere 1370 C
• Thermal conductivity profile from Hofmeister (1999) and van den Berg, Yuen, and Steinbach (2001)
• Half-spreading velocities of 1, 2.5, 5,&10 cm y-1
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Surface Temperature 0 C
Base of Lithosphere = 1370 C
T = 1370T = 1370
T = 1410T = 1410
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i
ii
a TbKPK
TT
kPTk
3
00
00 )1)](298()3/14exp[)298(),(
Thermal Conductivity: Hofmeister (1999); van den Berg, Yuen, Steinbach (2001)
k0 = 4.7 WK-1m-1
T in deg K, P in PaGruenheissen Paramteter, γ = 1.2Thermal expansion coefficient, α = 2.0 x 10-5 K-1
Bulk modulus, K0 = 261 GPaPressure derivative of the bulk modulus, K0' = 5The fitting parameter, a = 0.3
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Each node in the model exchanges heat with its eight nearest neighbors in two processes: conduction and advection. Iteration time for each calculation is controlled to maintain stability in the model.
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Surface Temperature 0 C
Base of Lithosphere = 1370 C
T = 1370T = 1370
T = 1410T = 1410
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Temperature and heat flow gradient from ridge crest to 19 Ma (474 km @ 2.5 cm y-1)
0 40000 80000 120000 160000Age (y/100)
-120,000
-100,000
-80,000
-60,000
-40,000
-20,000
0
Dept
h (m
)
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HF at 0 Ma = 1577 mW m-2
HF at 0 Ma = ∞HF at 0 Ma = ∞
HF at 0 Ma = ∞
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Summary and conclusions
• Conductive heat flow is predictable.• Side-by-side comparison of marine and
continental heat flow suggests extreme non-conductive and transient signals in marine environments and in young tectonic environments.
• To test analytical models of heat flow at ocean ridges, we created a 2-D finite-difference model of lithosphere spreading.
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Summary and conclusions
• The output of the model is a 2-D temperature-depth grid that provides a comparison with various analytical models of oceanic heat flow.
• We tested the reliability of the computations using different half-spreading rates and different node spacings and verified that the models yield equivalent results at equivalent times and depths.
• Our results show that the GDH1, HSC, and PSM models overestimate heat flow close to the ridge, but the differences are small.
• Our model does not provide evidence that heat flow is less than 44 TW.
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0 40000 80000 120000 160000Age (y/100)
-120,000
-100,000
-80,000
-60,000
-40,000
-20,000
0
Dep
th (m
)
010020030040050060070080090010001100120013001400
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• Heat is transported laterally by advection. Plates move at different rates at different times and heat flow is higher farther out in fast moving plates.
• After 10 MA of not moving, the 12 km thick lithosphere lost all of its heat for advection.
• Do separate segments of the plates move at different rates?
• It is the difference in velocity at the ridge that matters because there is no other source of heat.