ICTP COLLOQUIUM on Wednesday 25 May 2011

[Info Point]

*ICTP COLLOQUIUM*

*16:30, Wednesday, 25 May 2011*

*Main Lecture Hall, Leonardo Building, ICTP*

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*Professor Peter Molnar*

*University of Colorado at Boulder*

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*Mantle dynamics and the rise and fall of mountain belts***

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*Abstract*

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*Where crust shortens horizontally and thickens, isostatic compensation 
of the thick crust requires that a mountain range or high plateau form. 
Yet, mantle lithosphere must also shorten horizontally, and if it 
thickens, it should provide a weight that maintains the range at a lower 
elevation than it would be if isostatic compensation resulted only from 
thickened crust. Being cold and dense, mantle lithosphere (or at least 
its lower part) should be convectively unstable, and hence should sink 
rapidly into the underying asthenosphere as downwelling sheets or 
plumes.  Indeed, tomographic images of mountain ranges do not, in 
general, reveal a deep root of thickened mantle lithosphere, suggesting 
that some process removed it.  Removal of mantle lithosphere should make 
the remaining lithosphere more buoyant, and when (if) mantle lithosphere 
is removed, the earth's surface should rise.  I exploit seismological 
images of upper mantle structure, numerical experiments of 
Rayleigh-Taylor instability (arising when a dense layer overlies a less 
dense one), and inferences of paleo-altimetry, among other geologic 
observations to argue that mantle lithosphere commonly is removed when 
mountain ranges are built, with unexpected consequences in some cases.  
I will use geological and geophysical data from Tibet, the Andes, the 
Tien Shan, etc. to illustrate apparent consequences of removal of mantle 
lithosphere.  (Anyhow, if all of this interpretation proves to be 
nonsense, I hope to present interesting facts and well executed 
numerical experiments that others will find useful.)***

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