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Merry Christmas 2017

Merry Christmas and a Happy New Year from the Department for Geodynamics and Sedimentology!

After a scientifically very successful year, the executive of the Department for Geodynamics and Sedimentology wishes all our members, students and collaborators a peaceful and joyous holiday season and a prosperous New Year.

 

 
About our Season’s Greeting Movie:

Distinct Element Method model illustrating the formation of a hydraulic breccia.
A bonded particle model comprised of ~11,000 circular particles is initially confined using frictionless, rigid platens so that the ratio of horizontal to vertical normal stress is 0.8; this confinement is kept constant during the entire model run.
The green layers are passive markers.
Hydraulic fracture is modelled by adding small, frictionless particles at 50 randomly positioned injection points. During deformation the positions of these injection points are transferred assuming a homogenous back-ground strain (computed using the positions of the platens).
After the creation of a small particle at each injection point the model is brought to equilibrium in two steps: First, the model is cycled elastically, meaning that fracture is inhibited, so that dynamic effects due to particle placements are eliminated and the ‘fluid’ (frictionless particles) is in equilibrium. Then the model is cycled while fracture is permitted so that the host-rock responds to the increase of ‘fluid pressure’.
This two-step process mimics hydraulic fracture under quasi-static conditions and is repeated 500 times, so that at the final stage 25,000 intrusion particles are added.
The intrusion particles are coloured according to their emplacement time, ranging from yellow (early) to red (late).

Despite its simplicity, the model illustrates the three main stages of hydraulic breccia formation (e.g., Jébrak, M., 1997, Ore Geology Reviews 12, 111-134).

(1) Propagation: First a polygonal fracture network develops, with fractures propagating occasionally from triple junctions at about 120° angles (in other model runs, with fewer injection points and an isotropic remote stress, triple junctions form more frequently). The dominant fracture orientation is, however, initially vertical, i.e. parallel to the maximum principal remote stress direction.

(2) Wear: As the fracture connectivity increases a transition from a solid medium to an assemblage of fragments occurs. The fracture-bounded volumes rotate/translate in various directions and at various rates. Wear abrasion occurs because of the roughness of the fracture walls and the evolving geometry of the conduits.

(3) Dilation: Ultimately the fracture system becomes hydrodynamically continuous and the fractures open with very little wear abrasion or fragmentation. The dominant fracture opening direction is horizontal, i.e. parallel to the minimum principal remote stress direction.

The model was designed by Martin Schöpfer and run using the PFC2D software.


 

Department for Geodynamics and Sedimentology
University of Vienna

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