A Discussion on natural strain and geological structure - Comparison of the flow properties of rocks at crustal conditions

It is inferred that, although both primary and tertiary creep may be important in certain regions, large-scale ductile deformation in the Earth's crust must be governed by secondary creep (steady state). This flow involves plastic deformation resulting from dislocation motion and diffusion. Geological, geophysical and geochemical observations constrain the temperature (T), strain rate ($\dot{\epsilon}$), and stress difference ($\sigma $) for rocks undergoing secondary creep to: -30-800 degrees C, 10$^{-7}$-10$^{-15}$ s$^{-1}$, and up to 300 MPa (3 kbar). The actual conditions of secondary creep are strongly dependent on rock type and depth of deformation. Useful laboratory data on rocks obtained over wide ranges of T, $\dot{\epsilon}$ and $\sigma $ are limited to ice, halite, marble, dolomite, quartzite and dunite. Steady-state flow results are available for both wet and dry rocks; H$_{2}$O strongly affects the behaviour of both quartzite and dunite, but has a negligible effect on halite and marble. Secondary creep data for each rock are well fitted by $\dot{\epsilon}$ = A exp (-Q/RT) $\sigma ^{n}$, where Q is an activation energy for creep (diffusion) and A, R, n are constants. Comparison between those rocks expected in the deep crust indicates that at the highest T and at $\dot{\epsilon}$ of 10$^{-12}$-10$^{-15}$ s$^{-1}$, $\sigma $ is largest for dry dunite and dolomite, followed by dry quartzite, marble and wet quartzite. Equivalent viscosities ($\eta $) range from 10$^{18}$-10$^{22}$ Pa s (10$^{19}$-10$^{23}$ P). At intermediate depths (at T = 300-500 degrees C), $\sigma $ in dolomite is slightly greater than dry quartzite; both are much stronger than marble. In the shallow crust, secondary creep is expected only in marble (T > 250 degrees C) and in halite (T > 25 degrees C). The $\eta $ of halite at 25-250 degrees C, range from 10$^{21}$-10$^{17}$ Pa s. At the surface and at $\dot{\epsilon}$ of 10$^{-7}$-10$^{-10}$ s$^{-1}$ (glacier flow), $\eta $ of ice would be 10$^{15}$ to 10$^{12}$ Pa s between -30 and 0 degrees C. Values of $\eta $ for all rocks examined appear insensitive to T except wet quartzite and all dunite.