Formation of lithospheric shear zones: Effect of temperature on two-phase grain damage

The Generation of Plate Tectonics From Grains to Global Scales: A Brief Review

“…While Venus is mostly in the slowly deforming state, it can make forays into tectonic states, but which are unstable given their proximity to the unstable intermediate branch. (right) Temperature‐dependence of grain size for the Oman ophiolite predicted by Mulyukova and Bercovici (), according to the data from Linckens et al () (black and gray vertical lines, wherein black is for the secondary phase volume fraction >0.3), superposed with the theoretical piezometer obtained with the hysteresis model (solid black curve). The colored line is the field boundary grain size R F colored according to temperature; the dashed black curve marks the mixing transition grain size R m below which the well‐mixed, mylonitic branch becomes stable.…”

“…Analogously, the observed grain size can be translated into the driving stress, thus serving as a paleopiezometer, based on the empirically deduced correlation between grain size and stress (Jung & Karato, ; Karato et al, ; Post, ; Van der Wal et al, ). These piezometric relations were used in Rozel et al () and Mulyukova and Bercovici () to determine the range of possible values for f , by comparing them to the steady state grain size predicted by the grain damage model for different stress and temperature conditions; f is found to be temperature dependent and, starting off roughly at 0.1 for coldest temperatures of 800 K, decreases several orders of magnitude with temperature increasing to 1600 K, depending on the assumed activation energies for grain growth and rheological compliancies, among others.…”

“…The work to produce more interface area is tapped from the phase‐volume average of the total work rate ( ), since any mode of deformation will cause interface distortion by stretching, rending or phase mixing. The range of possible values for f I and f G were determined in Bercovici and Ricard () and Mulyukova and Bercovici (), by comparing the theoretically predicted steady state grain sizes to the ones reported from experimental (Jung & Karato, ; Karato et al, ; Linckens et al, ; Post, ; Van der Wal et al, ) and field data (e.g., Herwegh et al, ; Linckens et al, , ) for different stress and temperature conditions. As before, the factor λ is specific to the lognormal grain size distribution.…”

“…Due to its simplicity, relative to the full set of equations and , the pinned state limit is frequently used in geodynamic applications of grain damage theory (Bellas et al, ; Bercovici et al, , ; Mulyukova & Bercovici, , ).…”

“…However, upon extreme deformation exclusive to plate boundaries, mineral grains recrystallize to smaller sizes, erasing the preexisting viscous anisotropy in the process (Boneh et al, ; Ebert et al, ; Linckens et al, ). Finally, while viscous anisotropy may induce about an order of magnitude weakening (Hansen et al, , ) for rocks deforming by dislocation creep, grain size reduction, and the resulting activation of grain size‐sensitive creep may allow for weakening over multiple orders of magnitude (Bercovici & Ricard, , ; Karato et al, ; Mulyukova & Bercovici, ; Poirier, ; Van der Wal et al, ).…”