A new isostatic model of the lithosphere and gravity field M' K' Kaban, P' Schwintzer, Ch' Reigber J

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A new isostatic model of the lithosphere and
gravity fieldM. K. Kaban, P. Schwintzer, Ch.
ReigberJournal of Geodesy (2004)

• Tijmen van Wettum

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Overview

• Introduction
• Isostatic gravity anomalies
• Influences of crustal structure
• Evaluation of a global isostatic model
• Dynamic vs. Isostatic residual topography
• Discussion and Conclusions
• (Etwas zu trinken)

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Introduction

• Calculations of isostatic anomalies of the
  gravity field
• Topographic masses are closer to point of
  observation than compensating roots thus
  combined effect is non-zero
• Isostatic anomalies reflects what we do not know.

Free-air anomalies (or geoid undulations)
Isostatic compensated crust
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Introduction

• Traditionally anomalies following from the
  idealized Pratt and Airy schemes where often
  assumed effects of elastic support of mass.
• However, isostatic anomalies reflect also all
  heterogeneities within the earth.
• Is it worthwhile to consider the a priory crustal
  structure in the computation of isostatic
  anomalies?

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Introduction

• A new global model of isostatic gravity anomalies
  will be derived based on up-to-date data.
• This comes from
• CHAMP mission for geoid model
• New crustal data
• Re-thought of isostatic concept

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Isostatic gravity anomalies

• Isostasy equation
• Deviation from 0 can reflect the following
  sources
• Density heterogeneities in upper crust
• Density heterogeneities in lower crust/upper
  mantle
• Deep density heterogeneities
• Disturbances if isostatic equilibrium due to (a)
  local disturbances (b) deep dynamics (c)
  visco-elastic response
• Error in initial data set

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Isostatic gravity anomalies

• Example
• Non-compensated ridge
• of 1 km height and 40km width
• Isostatic anomaly about 30 mGal which is of about
  the same order as density inhomogeneities that
  are not included in the isostatic model.

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Influences of crustal structure

• Effect of sedimentary layer
• Construct smooth density-depth relationship.
• Usually data is available

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Influences of crustal structure

• Moho discontinuity
• Significant deviations of, the from airy scheme
  expected, depth exist.
• Seems sometimes to ignore the topography
• Uncertainties in Moho depth can compensated by
  adjusting crust/mantle densities

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Influences of crustal structure

• Density variations in crust
• Only knowledge comes only from seismic
  velocities, and thus very unreliable.
• However effects are big on isostatic anomalies.

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Influences of crustal structure
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Evaluation of a global isostatic model

• From the previous described principles a global
  isostatic model is derived.
• Initial gravity model
• Initial crustal data
• Least Square Method , fitting the initial density
  model of the crust to the observations.
• Solved for 3 layers.

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Evaluation of a global isostatic model
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Dynamic vs. Isostatic residual topography

• Rewrite Equation
• 2 sources explain the (log wavelength) topography
• Density distribution
• Mantle flow

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Dynamic vs. Isostatic residual topography

• Dynamic topography is a present-day problem
• With the assumption dynamic topography is a
  low-wavelength effect they separate the 2 effects.

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Dynamic vs. Isostatic residual topography

• Small-scale isostatic gravity anomalies reflect
  local disturbances if the density distribution of
  the crust was perfectly known.
• Improving initial density model.

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Dynamic vs. Isostatic residual topography

• Results ?

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Discussion and Conclusions

• Important to model the complete isostatic
  compensation scheme to derive their geophysical
  origin.
• Various phenomena can be identified from the
  isostatic gravity anomalies.

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Discussion and Conclusions

• Long wave-length (degreelt6) mantle convection
  and deep density anomalies.
• Medium wavelength (6 to 20 degrees) global
  tectonic features

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Discussion and Conclusions

• Short wave-length anomalies (lt2000 km) useless
  to investigate local disturbances
• However good results in short wavelength
  non-isostatic topography
• Long-wavelength non-isostatic topography a result
  of mantle dynamics
• Density distribution model obtained from
  isostatic adjustment from seismic observations
  as a priori information

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