A cohesive fault rock formed by dynamic recrystallization in a shear zone, representing the lowest grade of mylonitization. In a protomylonite, less than 50 percent of the rock volume has been crushed into a fine-grained matrix, leaving large, identifiable relict crystals from the original parent rock. It marks the transition between a fractured breccia and a fully sheared mylonite.
Heard on the job
The core samples from the shear zone show a transition from undeformed granite into a distinct protomylonite.