STRUCTURAL GEOLOGY
Lectures 33-34
Mechanisms for Ductile Flow
(dislocations, recrystallization, pressure solution)

In addition to brittle fracture several other mechanisms are responsible for the structures of the earth.  Each mechanism alone or combined with other mechanisms acts to facilitate the changes in the shape of rocks at various levels in the crust of the earth.  The higher the temperature (i.e. the deeper within the crust), the more likely that ductile mechanisms dominate over brittle mechanisms in controlling the style of deformation.  Ductile mechanisms can be divided into intergranular and intragranular processes.  Higher temperatures favor intragranular processes.
Near the surface of the crust pressure solution is the most common intergranular deformation mechanism.  It is manifested by cleavage in low-grade metamorphic rocks, the subject of lecture 17.  The basic process involves a dissolution at contact points between grains of rock.  Characteristically one grain will dissolve faster than the other.  The product of this differential dissolution is a stylolite in fine-grained rocks or pits and beards in pebbles of a conglomerate.  The dissolution process is believed to be activated by the high normal stress between grain contacts.  The rock, dissolved from the high stress contacts, then diffuses into the pore spaces of the rock where pore fluid exerts the only normal stress at pressures much less than found along grain-to-grain contacts (Fig. 33-1).

(Fig. 33-1)

In two or more component systems one material will dissolve faster than another.  For example, in a pelagic limestone the calcite dissolves and clay forms as an insoluble residue.  In a one component system, the single component dissolves uniformly.  In a shale (clay) the composition of the shale after 50% shortening is the same as the initial shale.  In quartzites the grain boundaries are irregular as the grains penetrate each other.
There are several possible sinks for the dissolved material.  In conglomerates beards appear in the pore space.  Many rocks contain veins opening normal to the maximum principal stress.  These veins are filled with the material dissolved from grain-to-grain contacts.  In other geologic settings it is clear that the dissolved material is carried completely out of the system.
Figure 33-2 shows the orientation of s1 relative to the formation of stylolites and pressure solution cleavage.  The maximum rate of dissolution is in the direction of s1.  This favors the formation of a cleavage with its "planar" direction normal to s1.

(Fig. 33-2)

The rate limiting process in cleavage formation by the pressure solution is the diffusion of the dissolved rock.  Two steps in the diffusion process include:  1.) Circulation of pore water within the open pore space, and 2.)  grain-boundary diffusion.  Because grain-boundary diffusion occurs on the scale of mm to cm, circulation of pore water can account for larger amounts of strain.
A second intergranular deformation mechanism is Coble Creep.  This process occurs by grain-boundary sliding.  The sliding may be enhanced by films on the grain boundaries.  Cataclasis may be a low temperature example of Coble creep involving brittle fracture (Fig 33-3).

(Fig. 33-3)

Intragranular deformation involves processes internal to grains and crystals of rocks.  No crystalline material is perfect.   In a crystalline lattice there are always single atoms (i.e. point defects) or rows of atoms (i.e. line defects or dislocations) missing.  These defects can migrate or diffuse within the crystalline lattice.  In fact it is the motion of these defects that allows a crystalline lattice to strain or change shape without brittle fracture.  A crystalline lattice may contain as many as 108 defects per cm3.  The motion of these defects is controlled by the stress and the temperature to which the crystal is subjected.  As both the stress and temperature increase these defects move more often (i.e. the strain rate increases as a function of differential stress and T (Fig. 33-4).

(Fig. 33-4)

Figure 33-4 shows the cross section of an edge dislocation.  The edge dislocation is the plane of missing atoms extending downward in the figure.  The motion of line defects or dislocation is mapped using the Burger's Vector.  For Figure 12-4 the Berger's vector is horizontal.  In two dimensions a line defect moves by changing places with a row or column of existing atoms (Fig.  33-5).  The column of atoms marked on black in Figure 33-5 is about to have its bond on the bottom broken to then occupy the position above the plane of missing atoms.  By this process the atoms in the upper half of the diagram move to their right relative to the crystalline lattice in the bottom half of the diagram.  We would say that the edge dislocation is moving to the left.

(Fig. 33-5)

 This process of dislocation motion in a single plane of atoms is called slip.  If a second line defect intersects the migrating dislocation, the dislocation can climb to another plane of atoms (Fig. 33-6).  In Figure 33-6 the lattice in the top of the figure is moving to the right relative to the bottom portion of the lattice.  A vacancy is shown.  When the lattice hits the vacancy then the edge dislocation grows upward into the lattice in the upper portion of the figure.  The edge dislocation moves to the right to intersect the line vacancy.

(Fig. 33-6)

  Many dislocations can become piled up in a single plane.  Because lattice strain occurs over each dislocation, the crystal has a different birefringence in the vicinity of the dislocation.  A line of these dislocations can cause the crystal to have a different birefringence just above the line.  This is known as a deformation lamellae and marks line piled-up dislocations (Fig. 33-7).

(Fig. 33-7)

Another type of dislocation plane can spiral through the crystal.  This is a screw dislocation.  Twin boundaries in calcite are screw dislocations (Fig. 12-8).

(Fig. 33-8)

Recovery and recrystallization are two more intragranular processes.   Recovery is the process of returning the crystal to its original undeformed state.  If dislocations pile-up in a line perpendicular to their direction of motion or Burger's vector, then a subgrain boundary is formed (Fig. 33-9). 

(Fig. 33-9)

 Polygonization is the process where edge dislocations climb by atom or vacancy diffusion and glide to low-energy polygonal networks.   Here the subgrain boundaries are at very small angles to neighbors.  Recrystallization takes place when high angle grain boundaries sweep through the crystal.   the high angle grain boundaries sweep out many of the defects in the crystal.  The process of normal grain growth tries to reduce grain boundary tension by putting grains next to each other with the smallest energy.  Here grains meet at dihedral angles (fig.  33-10).

(Fig. 33-10)