A rheological weak zone intensified by post-rift thermal relaxation as a possible origin of simple shear deformation associated with reactivation of rifting

Rheological heterogeneity brought about by the grain-size refinement associated with the spinel/plagioclase–lherzolite reaction is discussed in the context of post-rift thermal relaxation in continental rifts and passive margins. Above a critical temperature, the effect of grain-size refinement is not sufficient to obtain significant rheological heterogeneity. A critical time-scale for thermal relaxation, defined as a period when the temperature at the boundary of a reduced grain-size segment falls below the critical temperature, is evaluated in this study. It is found that a segmental rheological weak zone with a significant strength contrast can develop if the following conditions are satisfied: (1) a substantial grain-size reduction associated with spinel/plagioclase–lherzolite reaction occurs during an earlier extensional event, and (2) a successive extensional event occurs after the critical time-scale for thermal relaxation (a few tens of million years). The development of such rheologically weak zones may contribute to simple shear deformation in rift-related sedimentary basins developed by multi-rifting events.