notes utk exam esok PART 2
Definitions:
Fault: A fracture surface along which rocks have moved relative to each other.
Fault zone: A zone of sheared, crushed or foliated rock, in which numerous small
dislocations have occurred, adding up to an appreciable total offset of the undeformed walls.
All gradations may occur between multiple fault planes and single shear zones.
Fault rock: Rock formed as a result of deformation in a fault zone.
Mylonite: A fault rock which is cohesive and characterised by a well developed schistosity
resulting from tectonic reduction of grain size, and commonly containing rounded
porphyroclasts and lithic fragments of similar composition to minerals in the matrix.
Fine scale layering and an associated mineral or stretching lineation are commonly present.
Brittle deformation of some minerals may be present, but deformation is commonly by
crystal plasticity. Mylonites may be subdivided according to the relative proportion of finergrained
matrix into protomylonite, mesomylonite and ultramylonite.
When the protolith is known terms such as mylonitised granite, granite mylonite or granitederived
mylonite can be used.
Note: in the field it is often not possible to distinguish a foliated fault rock formed by brittle
deformation (foliated cataclasite) from one formed by crystal plastic or grain boundary
sliding processes, or a combination of different deformation mechanisms. Hence these are all
defined in the field by the term mylonite.
Protomylonite: A mylonite in which less than 50% of the rock volume has undergone grain
size reduction.
Mesomylonite: A mylonite in which more than 50% and less than 90% of the rock volume
has undergone grain size reduction.
Note: As mylonite is the general term, to be consistent it is necessary to have a prefix for the
more specific term intermediate between proto- and ultramylonite.
Ultramylonite: A mylonite in which more than 90% of the rock volume has undergone grain
size reduction.
An ultramylonite need not be ‘ultra’ fine grained.
Augen Mylonite: A mylonite containing distinctive large crystals or lithic fragments around
which the foliated fine-grained matrix is wrapped, often forming symmetric or asymmetric
trails.
Blastomylonite: A mylonite that displays a significant degree of grain growth related to or
following deformation.
Phyllonite: A phyllosilicate-rich mylonite that has the lustrous sheen of a phyllite.
Cataclasite: A fault rock which is cohesive with a poorly developed or absent schistosity, or
which is incohesive, characterised by generally angular porphyroclasts and lithic fragments
in a finer-grained matrix of similar composition.
Generally no preferred orientation of grains of individual fragments is present as a result of
the deformation, but fractures may have a preferred orientation. A foliation is not generated
unless the fragments are drawn out or new minerals grow during the deformation. Plastic
deformation may be present but is always subordinate to some combination of fracturing,
rotation and frictional sliding of particles.
Cataclasite may be subdivided according to the relative proportion of finer-grained matrix
into protocataclasite, mesocataclasite and ultracataclasite.
Protocataclasite: A cataclasite in which the matrix forms less than 50% of the rock volume.
Mesocataclasite: A cataclasite in which the matrix forms more than 50% and less than 90%
of the rock volume.
Note: As cataclasite is the general term, to be consistent it is necessary to have a prefix for
the more specific term intermediate between proto- and ultracataclasite.
Ultracataclasite: A cataclasite in which the matrix forms more than 90% of the rock volume.
Fault Breccia: A medium- to coarse-grained cataclasite containing >30% visible fragments
Fault Gouge: An incohesive, clay-rich fine- to ultrafine-grained cataclasite, which may
possess a schistosity and containing <30% visible fragments. Lithic clasts may be present.
Pseudotachylite: Ultrafine-grained vitreous-looking material, usually black and flinty in
appearance, occuring as thin planar veins, injection veins or as a matrix to pseudoconglomerates
or breccias, which infills dilation fractures in the host rock.
