Fig. 2
Strength models for Earth calculated using a constant strain rate of 10−17 s−1 (solid lines) with variation in strain rates between 10−16 and 10−18 s−1 (dashed lines). The strength of shallow parts is controlled mainly by frictional sliding and that of deeper parts by plastic deformation. Arrows indicate the transition from brittle to ductile deformation for each model. An increasing thermal gradient results in a decrease in plastic strength at depth and a shallower brittle–ductile transition. Water reduces the strength of both the brittle and ductile regions; consequently, lithospheric strength is markedly lower under wet conditions compared with dry. The presence of clay results in the markedly low frictional coefficient, and hence further decreases the brittle strength