Metamorphic rocks found at the surface have travelled through the plastic-brittle transition (ductile-brittle transition), where they journey from the regime dominated by (quasi)plastic deformation mechanisms to that characterized by brittle mechanisms. Rocks that deform continuously or repeatedly through that journey contain sets of structures that record this transition, with (semi-)brittle structures overprinting slightly older ductile structures.
As I am preparing a presentation for the bi-annual Norwegian winter meeting in Oslo in January I thought I would share an example of this from near Bergen, Norway. The setting is exhumation of the subducted margin of Baltica, and the pictures show ductile gneiss structures affected by semi-brittle fractures that selectively form in the stiffest (mica-poor) layers. They can be seen to extend into little ductile shear zones or small fracture propagation folds in mica-bearing lithologies.
It is typical for this transition that some layers behave brittley while others (less stiff ones) still deform ductilely. Ar/Ar mica ages (ca. 400 Ma) and U-Pb dating (ca. 396 Ma) of sphene in the early fractures show that the transition into the brittle regime happened in the earliest-Middle Devonian, while other evidence (AFT and stratigraphic evidence) shows that the rocks reached near-surface conditions in the Late Jurassic.