Fractured granitic layers in gneisses sheared during Devonian extensional collapse in the SW Norwegian Caledonides (Bergen area).
Detailed view of fracture. Note how it extends downwards into a ductile deformation structures, like a fault propagation fold or little shear zone before dying out.
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.
Haakon Fossen
Very interesting feature of deformation. I have been seen a lot of outcrops with contrasting gneiss layers and different deformation behaviour. Do you think that always implies in a example of brittle-ductile transition?
(Semi)brittle deformation of highly competent layers can occur below the brittle-ductile transition as well, an example being boudinage. However, the type of fractures that can be seen in these picture, with staining of K-spar and epidote and other minerals filling the fractures is, from my experience at least, typical for ca. 300 °C. However, thermochronologic data will always be useful to constrain crustal depth/temperatures.
In a ductile-brittle shear zone, the structures brittles and ductiles can to form together?
Yes, at the same time, depending on the local rheologic properties of the layers/components of the rock.