tectonics_lecture-4
DESCRIPTION
Gravity, Isostasy and Creep Physics of the Solid Earth Supriyo Mitra Figure 2: Deflection of plumbline due to Himalayas Local Isostatic compensation Local Isostatic compensation Gravitational Potential and Acceleration Gravity of the Earth The Shape of the Earth Gravity Measurements Absolute Gravity measurement Relative Gravity measurement Free Air Correction δgF = (2h/R)g Free Air Anomaly g F = g obs – g(λ) + δg F Bouguer CorrectionTRANSCRIPT
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Gravity, Isostasy and Creep
Physics of the Solid EarthSupriyo Mitra
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Figure 2: Deflection of plumbline due to Himalayas
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Local Isostatic compensation
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Local Isostatic compensation
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Gravitational Potential and Acceleration
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Gravity of the Earth
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The Shape of the Earth
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Gravity Measurements
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Absolute Gravity measurement Relative Gravity measurement
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Gravity Corrections and Anomalies
Free Air Correction δgF = (2h/R)g
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Free Air Anomaly
gF = gobs – g(λ) + δgF
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Bouguer Correction
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SLAB APPROXIMATIOM
∆g = 2πGρt
Therefore
∆g = 42ρt milligals
Where ρ is the density in km/m3 and t is thickness or bathymetry in km.
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Terrain Correction
(r,θ) = γρθ { ( r0 – ri ) + ( ri2 + ∆z2 )1/2 – ( r0
2 + ∆z2 )1/2 }
Correction is small if r > 20z,
where r is the average distance from the compartment to the station.
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Bouguer Anomaly
gB = gobs – g(λ) + δgF – δgF + δgT
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Isostatic Anomaly
Actual Bouguer anomaly – computed Bouguer anomaly for a proposed density model
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Synthetic Examples
100% Compensation
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70% Compensation
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0% Compensation
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Observed Gravity Anomalies
Rockall, t = 2 km
∆g should be= 140 mgals
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Long wavelength topography (large scale surface features) are normally in Isostatic equlilibrium
Therefore the mantle is, on a long time scale, not particularly strong.
Observed anomaly = 20 mgals, So the topography must be compensated
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Models of compensation and density-depth tradeoff
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Geoid Height Anomalies g∆h = -∆V
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Mantle convection and geiod height anomalies
From McKenzie et al 1980
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Not all topography is isostatically compensated ….
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Not all topography is compensated. The Hawaiian Ridge, with t = 4 km, and both the observed and calculated anomalies are about 300 milligals. So the ridge is notcompensated, and must be supported by elastic forces in the plate.
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Wavelength of deflection will provide a measure of the elastic thickness
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CreepHow does compensation occur?
We need to understand long term behaviour of stressed solids:
Homologous Temperature τ = T / Ts
Where T is the temperature of the solid and Ts is the melting temperature both in K and
Homologous stress σ / μ
Where σ is the stress and μ the shear modulus.
Creep or long term behaviour of solids is determined by τ
Only at temperatures larger than a certain homologous temperature certain types of creep can occur. Creep also depends on the stress applied.
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Diffusion creepPower-law creep / Dislocation creep
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Stre
ss
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~ 60 km in oceans
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