Kink strengthening and rank-1 connection of crustal rocks.

Kink bands are localized folds or bends formed in layered materials, such as millefeuille. They are common in rocks within the earth's crust that have undergone compression deformation. While the formation of kinks has been studied extensively, their effects on material strength remain unclear in geologic materials. This study investigates the relationship between kink formation and compressive strength through experiments on crustal-material biotite. In our results, kink bands are observed pervasively in samples and show hardening behavior. The observed kinks have symmetric tilt boundaries, satisfying the rank-1 connection condition, a key condition for material strengthening. Therefore, the hardening behavior observed in this study is due to "kink strengthening". Furthermore, we reviewed whether kink bands of various scales in geologic materials satisfy the rank-1 connection and discussed its effect on the strength. Notably, it is indicated that the formation of mega kinks (km-scale kinks) strengthens the crust and influences earthquake rupture.