The microstructural legacy of mantle deformation during orogenic reactivation

Studies of multiple mantle exposures indicate that a fundamental shift occurs in polymineralic peridotites at ~850° C.  At these temperatures, there is a shift from dislocation creep (plus or minus dislocation accommodated grain boundary sliding) to reaction-facilitated grain-size sensitive creep.  This reaction results in a fine-grained matrix produced by neocrystallization.  The fine-grained shear zones that formed by dislocation creep dynamic recrystallization create increased grain-boundary surface area that localize the reaction-enhanced deformation.  Because the grains are formed by reaction, grain boundary pinning of the different mineral phases occurs.  Moreover, these fine-grain sizes are preserved during exhumation, because of the grain boundary pinning.  Thus, the fine-grain size – once it has been formed by reaction-facilitated deformation – continues to exist even if there is a change in temperature.  

This rheological behavior is not typically shown in deformation strength profiles, because monophase olivine does not show these effects.  Yet, the lithospheric mantle is polyphase, and we have observed evidence for reaction-facilitated deformation that occurred below ~850° C.  Once grain size reduction has occurred in a polyphase material, it is not expected to grow large grain sizes again, due to the role of grain boundary pinning.  Thus, once formed, a reaction-facilitated shear zone with smaller grain size relative to the surrounding mantle rocks would remain a lithospheric “scar”.  The fine-grain shear zones would preferentially reactivate because the zone can deform by grain-size sensitive creep at lower stress conditions that the surrounding mantle material can deform by dislocation creep.  This interpretation could explain the common reactivation of transform faults, and perhaps even extensional faults, in orogenic belts.  Reactivation of transform faults in the mantle may explain:  1) the Neoproterozoic transform faults of the eastern and western United States, which are reactivated by Pennsylvanian and Cretaceous deformation, respectively; and 2) the modern San Andreas System reactivating a Cretaceous – Paleogene proto San Andreas Fault.