Chapter 7 Flashcards
Grain boundary sliding is influenced by friction, and the mechanism is therefore called
frictional sliding
The grains translate and rotate to accommodate frictional grain boundary slip, and the whole process is called
granular flow
are fractures restricted to single grains.
Intragranular fractures
are fractures that extend across a few grains.
Intergranular fractures
The fracture and crushing of grains, coupled with frictional sliding along grain contacts and grain rotation
cataclasis
More moderate cataclastic deformation can occur in somewhat wider brittle or cataclastic shear zones. In this case the fragments resulting from grain crushing flow during shearing
cataclastic flow.
Strong grain crushing without evidence of shear offset has also been observed and is called
pulverization
any planar or subplanar discontinuity that is very narrow in one dimension compared to the other two and forms as a result of external (e.g. tectonic) or internal (thermal or residual) stress.
fracture
Fractures can be separated into:
- shear fractures (slip surfaces)
- opening or extension fractures (joints, fissures and veins)
are very narrow zones, often thought of as surfaces, associated with discontinuities in displacement and mechanical properties (strength or stiffness).
Fractures
is a fracture along which the relative movement is parallel to the fracture
is used for fractures with small (mm- to dm-scale) displacements
shear fracture
is more commonly restricted to discontinuities with larger offset.
fault
used for fractures with fracture-parallel movements regardless of the amount of displacement and is consistent with the traditional use of the term fault.
slip surface
are fractures that show extension perpendicular to the walls.
Extension Fractures
have little or no macroscopically detectable displacement, but close examination reveals that most joints have a minute extensional displacement across the joint surfaces, and therefore they are classified as true extension fractures.
Joints
When filled with air or fluid we use the term
fissure
Mineral-filled extension fractures are called
veins
while magma-filled fractures are classified
dikes
have contractional displacements across them and are filled with immobile residue from the host rock.
Contractional planar features
are compactional structures characterized by very irregular, rather than planar, surfaces.
Stylolites
as they nicely define one of three endmembers in a complete kinematic fracture framework together with shear and extension fractures.
Contraction fractures
can be used to test the uniaxial compressive or tensile strength of rocks.
Uniaxial rigs
is used to explore the effect of large shear strain under vertical compression of up to about 25 MPa.
ringshear apparatus
where rock cylinders are loaded in the axial direction while the sample is confined in fluid that can be pumped up to a certain confining pressure.
Triaxial tests
where the resistance against shear is explored. The higher the normal stress, the higher the shear stress necessary to activate the fracture.
Shearbox experiment
ideally develop perpendicular to σ3 and thus contain the intermediate and maximum principal stresses. In terms of strain, they develop perpendicular to the stretching direction under tensile conditions, and parallel to the compression axis during compression tests.
Extension Fractures
If extension fractures form under conditions where at least one of the stress axes is tensile, then such fractures are
tensile fracture
The line that tangent the Mohr circles represents the Coulomb fracture criterion
Coulomb failure envelope
is the envelope or curve in the Mohr diagram that describes the critical states of stress over a range of differential stress, regardless of whether it obeys the Coulomb criterion or not.
Mohr failure envelope
In this zone microdefects expand and connect so that the macrofracture can grow
Process zone
far-field stress, we mean the stress that exists away from the local anomaly, or the state of stress if the anomaly was not there.
Remote stress
Long, narrow planes slightly oblique to the main fracture surface named
Hackles
This would typically be a bedding interface or some other boundary between two rock types of different mechanical properties, in which case a series of twisted joints or twist hackles form
fringe zone
are thus locations of minimum propagation velocity and form parabolic (elliptic in massive rocks) irregularities sometimes referred to as arrest lines.
Ribs
which are tensile fractures at the end of shear fracture
Wing Cracks
In other cases, a whole population of minor, typically tensile fractures occur in the tip zone. These are asymmetrically arranged with respect to the main fracture and referred to
horsetail fractures.
If the secondary fractures in the tip zone represent a fan shaped splaying of the main fracture, then the commonly used term is
splay faults.
While splay faults are synthetic with respect to the main fault___________ may also occur in the tip zone of fractures
, antithetic fractures
describes the vertical increase in critical shear stress (stress required for faulting) through the frictional upper crust
Byerlee’s law
is the difference between the applied or remote stress and the fluid pressure.
Effective stress
In highly porous rocks and sediments, brittle deformation is expressed by related, although different, deformation structures referred to as
are restricted to highly porous granular media, notably porous sandstones.
deformation bands.
develop by shear-related disaggregation of grains by means of grain rolling, grain boundary sliding and breaking of grain bonding cements; the process that we called particulate or granular flow.
Disaggregation bands
(also called framework phyllosilicate bands) form in sand(stone) where the content of platy minerals exceeds about 10–15%.
Phyllosilicate bands
typically show striations and classify as slip surfaces rather than deformation bands.
Clay smears
form where mechanical grain breaking is significant (Figure 7.39). These are the classic deformation bands first described by Atilla Aydin from the Colorado Plateau in the western USA
Clastic bands
