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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 2eff 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!