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Dynamics of Lithospheric
Deformation (1)
Topics
Rheology (once more).
Isostasy
Effective elastic thickness.
Instabilities: Folding & RT



Basics
Folding instability on various scales
Rayleigh-Taylor instability examples
Metamorphic reactions and tectonics
Interaction between erosion & tectonics
Rheology used for lithosphericscale deformation
Often a combination of some kind of
“brittle” or “plastic” rheology and a viscous
rheology
Sometimes elasticity is also included.
Rheology of the Lithosphere
Kohlstedt et al. (1995)
Visco-elasto-plastic rheologies
•Describes most of the behavior on lithospheric scale
•From development of faults, to elastic bending of the
lithosphere and viscous creep in the mantle
• Processes not prescribed but develop as function of stress,
strain rate, temperature etc.
Viscous creep-laws typically used
&ij 
eff
1
2eff
 ij
B 
 

2  
2nd
1 n
 E  pV 
exp 

 nRT 
B depends on grainsize (in linear domain)
n=1 : Diffusion creep (Newtonian or linearly
viscous)
n>1 : Dislocation (powerlaw) creep
Christmas trees depend on temperature/plate age
Difference between compression/extension
Burov Treatise on
Geophysics V. 6
(2007)
-> Differences come from the dependence of Byerlee’s law on normal stress.
Compression results in larger normal stress (‘tectonic loading’)
Rheology and some recent (emotional) discussions
Jackson
(2002)
Earthquake focal depths beneath
continents
moho
Most EQs in upper crust, few beneath Moho
Maggi et al
(2000)
Earthquakes beneath Tibet
Jackson
(2002)
Deichman
et al. (2003)
Inferred rheology for the lithosphere
moho
classical
Jackson
(2002)
Preferred
by Jackson
Preferred by
Jackson
Jelly sandwich vs. Crème brulee
Burov & Watts (2006)
Consequences of weak mantle (1).
Burov (2007)
Consequences of weak mantle (2).
Burov (2007)
Christmas tree and rheology
Assumptions of a typical Christmas tree strength
profile:


Either viscous or plastic (brittle) rheology (both
instantaneous)
No vertical variation of strainrate
Possible flaws:


No elasticity (no stress evolution)
Strainrate may be varying with depth
Bottomline:

Do not over-interpret christmas-tree reconstructions!