Rheological Modeling and Deformation of Continental Lithosphere

Rheological models of lithosphere integrate the effects of temperature and fluids to mechanical deformation as rocks undergo different modes of deformation under varying pressure and temperature conditions ( Brace and Kohlstedt, 1980 ; Goetze and Evans, 1979 ; Kirby, 1977 ; Meissner and Strehleau, 1982 ). In a uniform composition model, frictional sliding governs mechanical deformation in the uppermost part where temperature and lithostatic pressure are relatively low. At greater depth, due to increasing pressure and temperature, plastic deformation controls the strength. This type of behavior is seen in the oceanic lithosphere because it is mainly made of olivine-rich rock with a very thin crust. As continental lithosphere consists of Theologically weak quartz-, intermediate feldspar, and strong olivine-rich layers representing the upper and lower crust, and upper mantle, respectively, the strength increases with depth in each layer until the temperature becomes large enough to facilitate plastic deformation in that layer. The transformation from brittle to ductile deformation is called the brittle-ductile (B/D) transition. Experimental studies have shown that quartz and felspar become ductile at 300-450°C for natural strain rates whereas olivine becomes ductile at 700-750°C.