Download Fault Elements and Structural Reactivation

Fault Reactivation
E2: Rock Deformation KI1: (d)
What you need to know .
A fault is a weakness in crust that happens as a result of tectonic activity. As a line of
weakness if the crust is subjected to stress at a later time (a later orogeny or fold mountain
event) is sometimes moves again (reactivates), but this time even in a different direction
(thus a normal fault could become a reverse fault or visa versa). Of course, the fault
evidence is in the offset of beds on either side so the type of fault you actually see is the
result of all the movements along the fault (i.e. the net movement).
This is shown in the diagram below.
The quartzite bed has apparently been moved by tension (i.e. a normal fault) whilst the
SAME fault shows the unconformity to be the result of (later) crustal shortening
(compression).
Check this out by adding relative movement arrows for the quartzite and unconformity.
Which came last? Why? Can you add σmax and σmin arrows to the diagram for BOTH areas of
the fault.
Most faults are in fact reactivated and the type of fault we see is the combination of all the
movements. The LAST movement may be shown by the orientation and/or direction of the
slickensides.
Slickensides
E2: Rock Deformation KI1: (b)
What you need to know .
Slickensides occur when the rocks along a fault rub against each other, making their surfaces
smoothed, lineated, and grooved. Slickenside formation may involve simple friction, or if the
fault surface was once deeply buried, actual growth of oriented mineral grains may respond
to the forces on the fault. Slickensides can be confused with glacial striations so be careful.
The later can occur on any rock surface but the former only along fault planes.
Slickensides record the LAST orientation of movement on the fault plane as previous
slickensides are eroded. Slickensides may show direction of movement in some cases where
small ledges formed during deformation result in a smooth and rough direction along the
fault plane when the hand is rubbed in the orientation of the slickensides.
Rough direction
Smooth direction
Orientation of slickensides
Can you decide which way the block on the left moved?
This block moved to the left
Fault Gouge and Fault Breccia
E2: Rock Deformation KI1: (b)
What you need to know .
Fault gouge is the name given to the rock “flour” which results from the grinding up of rock
along the fault plane. It is unconsolidated with a very small grain size. Fault gouge has no
cohesion, it is normally an unconsolidated rock type, unless cementation took place at a later
stage. Fault gouge forms in the same way as fault breccia, the latter also having larger clasts.
Fault breccia is a breccia (a rock type consisting of poorly sorted, angular clasts) that was
formed by tectonic forces as the fault moved. The included clasts are the rocks found along
the fault plane. Fault breccia has no cohesion, it is normally an unconsolidated rock type,
unless cementation or mineralisation took place at a later stage.