| Rheological profile of the continental lithosphere | ||||||
| The figure below shows the rheological profiles for a compressional and an extensional tectonic regime. The rheological parameters for the crust and the mantle are that of a granite (quartz >40%) and a dunite (peridotite with >80% olivine) respectively. The power law and Dorn creep law are dependent on both the temperature T and the strain rate. Therefore, these parameters must be defined first. The geotherm here is such that the temperature at the Moho is 460ºC and the choosen strain rate is 10-15 s-1. The straight parts of the profiles (in the upper crust and the upper mantle) represent brittle failures. The red lines are the Dorn law creep curves, whereas the black curved lines are the power law flow curves. One can see that the differential stress for both the power law and Dorn law creep strongly decrease with T. |
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| A few things to note: • With the Dorn law, brittle failure does not occur in compression. Dorn law creep significantly reduces the strength of the upper mantle. • The brittle part of the crust is thicker in extension that it is in compression. • For a normal geotherm (TMoho <650ºC), the upper crust and the upper mantle are the strongest layers of the lithosphere, the lower crust and the lower lithospheric mantle are extremely weak. • Because quartz deforms by ductile flow at lower temperature (~300ºC) than olivine (ductile at ~600ºC) the upper mantle is much stronger than the lower crust for a normal geotherm (TMoho <650ºC). |
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