Chapter 8 Flashcards
is a tabular volume of rock consisting of a central slip surface or core, formed by intense shearing, and a surrounding volume of rock that has been affected by more gentle brittle deformation spatially and genetically related to the fault.
fault
The opposite case, where the hanging wall is upthrown relative to the footwall
reverse fault
Where the hanging wall is lowered or downthrown relative to the footwall, the fault is
normal fault
If the movement is lateral, i.e. in the horizontal plane, then the fault is a
strike slip fault
traditionally means a series of subparallel faults or slip surfaces close enough to each other to define a zone.
fault zone
Two separate normal faults dipping toward each other create a downthrown block known as a
graben
Normal faults dipping away from each other create an upthrown block called
horst
The largest faults in a faulted area
master faults
dips toward the master fault
antithetic fault
dips in the same direction as the master fault.
synthetic fault
Many faults show some deviation from true dip-slip and strike-slip displacement in the sense that the net slip vector is oblique
oblique-slip faults.
A series of displacement vectors over the slip surface gives us
displacement field
The vector connecting two points that were connected prior to faulting indicates
net slip direction
The degree of obliquity is given by the
which is the angle between the strike of the slip surface and the slip vector (striation).
pitch
is the separation of layers observed on a horizontal exposure or map
Horizontal seperation
is that observed in a vertical section
dip separation
In a vertical section the dip separation can be decomposed into the horizontal and vertical separation. These two separations recorded in a vertical section are more commonly referred to as
heave (horizontal component) and throw (vertical component).
The general term for the stratigraphic section missing or repeated in wells drilled through a fault
stratigraphic separation
Brittlely deformed wallrock known as the
fault damage zone
In crystalline rocks, the fault core can consist of practically non-cohesive
fault gouge
In extreme cases, friction causes crystalline rocks to melt locally and temporarily, creating a glassy fault rock known as
pseudotachylyte.
constitute the fault core, particularly for faults formed in the lower part of the brittle upper crust.
catclasite
If the matrix fragment ratio is higher, the rock is called a
fault gouge.
are characterized by their large fragments. They all have less than 10% matrix and are cohesive and hard rocks.
Crush breccias
are distinguished from crush breccias by their lower fragment–matrix ratio.
Cataclasites
which are not really fault rocks although loosely referred to as such by Sibson, are subdivided based on the amount of large, original grains and recrystallized matrix
Mylonite
Layers are commonly deflected (folded) around faults, particularly in faulted sedimentary rocks. The classic term for this behavior is
drag
Faults that fall below seismic resolution are generally referred to as
subseismic faults
Another characteristic feature is the presence of local intervals with rapid but progressive changes in dip and/or dip azimuth
cusps.
Some cored fault rocks may be so non-cohesive that they fall apart to form what is known
rubble zones.
The zone of microfractures (and mesofractures) ahead of the fracture tip zone is called the
process zone.
is the shear zone-like geometry where layers flex toward parallelism with the fault.
Normal drag
is used for the usually larger-scale rollover structures that occur on the hanging-wall side of listric normal faults.
Reverse drag
models the ductile deformation in a triangular fault propagation fold area in front of a propagating fault tip.
trishear method
Fold structures are commonly referred to
Force folds
Folds that form ahead of a propagating fault tip are called
fault propagation folds.
The latter mechanism can take drag to the point where the rotated layer, which typically consists of clay or shale, forms a
smear
where slip accumulates at very sudden seismic slip events, separated by periods of no slip.
stick-slip
Some laboratory experiments show that a gradually increasing force is needed for slip to continue
slip hardening
Below this depth one would expect abundant seismic or stickslip activity until the brittle–plastic transition is reached. This is exactly what earthquake data indicate, and the zone is called the
seismogenic zone.
predicts that both the amount of slip and the rupture length vary from event to event,
variable slip model
considers the slip at a given point to be the same in each slip event.
uniform slip model
Faults tend to nucleate at many different places as a region is critically stressed, for instance during rifting, and we refer to such groups of faults as
fault populations.
Before the tips have reached each other (but after their strain fields have started to interfere) the faults are said to
underlap
Under- and overlapping faults are said to be
soft linked
If the fault interference occurs perpendicular to the slip direction, which for normal and reverse faults means in the horizontal direction, and if the layering is subhorizontal, then the folding is well expressed in the form of a ramp-like fold. The fold itself is called a
relay ramp and the entire structure is known as a relay structure.
Eventually the ramp will break to form
breached relay ramp.
The fracture (or deformation band) has not yet (or just barely) reached the upper and lower boundaries of the layer. Once the fracture touches the layer boundaries it is called
vertically constrained fracture
The ability of faults to affect fluid flow is commonly referred to
transmissivity.
Where sand is completely juxtaposed against shale, the fault is sealing regardless of the properties of the fault itself. This type of seal is called
juxtaposition seal.
in the fault core reduces grain size and therefore reduces porosity and permeability.
Cataclasis
The process where clay or shale is smeared out into a more or less continuous membrane is simply
smearing
If such a membrane represents a physical barrier to fluid flow it is called a
seal, and the fault is a sealing fault.
where clay is tectonically eroded from clay-rich layers along the fault and incorporated into the fault core, is a third mechanism that contributes to clay smearing.
Clay abrasion
(SSF) gives the ratio between fault throw T and the thickness of the shale or clay layer.
shale smear factor
relates to how far a clay or shale layer can be smeared before it breaks and becomes discontinuous.
clay smear potential
