Hello All,

 

    I just returned for a field trip to find this discussion about brittle ductile transition. In 1986 Tectonophysics I published a short article about the brittle-ductile transition in rocks to try to clarify nomenclature. In summary, there are two types of transition; between deformation mechanisms, such as deformation by fracturing and frictional sliding (which can be distributed and is always pressure dependent and relatively temperature independent) and by (pressure independent but more temperature dependent) plastic or diffusive transfer processes.  Both of these can lead to either localized deformation (faults, shear zones) or to distributed flow (whether cataclastic flow or flow by crystal plasticity). The second type of transition is therefore a mode of failure transition, from localized to distributed (ductile deformation). Thus the term brittle-ductile transition is a misnomer – I never use it, preferring faulting (or localized deformation) to distributed flow transition (mechanism not specified).

     In rock mechanics, ductility is the capacity for flow without localization (as defined by Hugh Heard in 1960 GSA special publication vol 73, in which there is no prejudice about mechanism), and after a few percent strain the mode of failure can change from distributed to localized deformation. Look how much more carefully experimentalists talk about these concepts than do others.  In my 1986 paper there is a diagram that illustrates the difference between mode of failure and deformation mechanism transitions. I hate it when geolophysicists talk about THE brittle ductile transition in the Earth, as if it is at a specific place or particular depth, or independent of mineralogy. The concept gains credence through careless and unwarranted repetition – ugh!

 

Ernie Rutter

 

 

Hi Olivier and other,