Crustal loading, isostatic models and lithospheric flexure

Examples  from the Eastern Alps

Airy isostasy

 
(Left) The Airy isostatic gravity field, which was calculated with a density contrast of 400 kg/m³ and and normal crustal thickness of 30 km. (Right) The Airy isostatic residual field. This field shows the differences between the measured Bouguer anomaly of the Eastern Alps and the Airy isostatic gravity field.

Flexural rigidity of the Eastern Alps

1. Modelling of the 3D-density structure of the study area under consideration of all available constraining information.


The figure above presents a cross-section through a 3D density model of the Eastern Alps, which is constrained by a seismic
velocity model.


2. Estimation of the surface in equilibrium of loading and buoyancy and of the internal loading of the plate.


This figure shows the absolut load distribution calculated along the same cross-section as the density profile above. The
loads are directly calculated by IGMAS.
 
 
The internal, inhomogeneous part of the loads, in respect to a reference "normal"  density of 2800 kg/m³. The topographic loading of the Eastern Alps.

3. Calculation of the pseudo topography by summing of internal and topographic loading.
 
The pseudo topography of the Eastern Alps. The pseudo topography was calculated by summing the loads of point 3 and converting the loads with a pseudo density of 2670 kg/m³.


4. Calculation of an "elastic" Moho in respect to a variable flexural rigidity.
Parameters: window size (90 km), step size (15 km), normal crustal thickness (30 km) and density contrast at Moho (400 kg/m³).
 
 
The elastic Moho of the Eastern Alps, which resulted from the
pseudo topography, presented in the point 4.

 

5.    Comparison of modelled and elastic Moho.
 
 
The Moho of the Eastern Alps as resulted by the density model presented in point 1. Difference between the Moho to the left and the elastic Moho, presented in point 5.

6.    The flexural rigidity, the fit between the elastic and modelled Moho is best for, is taken.
 
 
Flexural rigidity of the Eastern Alps. The effective, elastic thickness of the Eastern Alps. For the calculation a Poisson-ration of 0.3 and a Young modulus of 1E11 Pa.

References: The presented examples are taken of publications by Braitenberg et al., 2002, Ebbing, 2002, and Ebbing et al., 2001.