Lecture Six - Faults and shear zones Flashcards
What does a cross section of the crust look like when being compressed from one side?
In this diagram, the theta one indicates where maximum compressive stress is occuring.
The theta three indicates where the minimum compressive axis is.
Are faults scale independant?
What is a fault and what is a shear zone?
Faults and shear zones are structures in which a large amount of strain is accommodated within a small volume of rock.
Faults - are brittle features where movement occurs on a discreat surface or an array of surfaces within a Fault Zone.
Shear Zones - are more ductil features in which movememnt occurs across a zone of finite width.
Fault and shear zones occur in conjunction with one another.
Brittle rocks on the surface (fault) and more ductile rocks underneath where it is hotter (shear zones).
Why are faults and shear zones important elements of mountain belts?
The control features of the architecutre of the mountain belt.
They can accomodate large crustal movements.
Juxtapose ‘far travelled’ packages of rock.
Can contain good information on kinematics as well as geometry.
What are the different kinds of faults that can occur?
Normal faults - occur in a extension setting, where the hanging wall moves down in relation to the foot wall. Crust streching and thinning in horrisontal direction.
Reverse faults - occur in compressional settings, where the hanging wall move up in relation to the foot wall. Can also be called a thrust fault when occuring at a low angle.
Strike slip faults - when two pieces of crus tmove parallel to one another. No extension or compression.
What are striations or fibres?
Stiations are the marks made on the faces of rock which are faulting past each other. Essentially the scratch marks made when the rocks move past one another.
These marks are parallel to the fault direction.
Striations are parallel to the net slip.
What are the faults and strain axis orientations?
What is apparent offset?
When looking at a 2D surface of a rock, one can only draw conclusions of the apparent offset of the whole, 3D section of rock.
I.e. The rock may appear to be folded on one surface, but other processes may be affecting the rock in 3D.
What are some of the different fault terminations (decreased displacement, hidden thrusts and splays)?
Decreased displacement - stops being a fault gradually by decreasing the amount of earth it moves.
Hidden thrust - fault hidden by a fold (folds and thrust often occur together).
Splays - when a single fault turns into many (the faults force is displaced and forms a fingering pattern of faulting).
What are strain rates?
Control whether you break or fold rocks (rate of deformation).
If the strain rate is fast, the rock will likely fault (break).
If the strain rate is slow, the rock is more likely to fold.
What are drag folds?
Rocks being dragged on a fold plane.
Hanging wall is climbing over the foot wall.
What is autochton and allochton?
When allochton moves over autochton.
When earth begins in a single plane, with layers aligned.
It is then deformed by one section of the land moving up, over the neighbouring section of land.
This results eventually in the allochton (top potion) climbing over the autochton up a ramp.
What are fold and thrust belts?
Large horisontal motions can be accomodated by a combination of sliding on relatively flat faults (basal flats or decollement) and ramps (thrust faults) with associated folds.
Decollement: a process in which some strata become partly detached from those underneath and slide over them, causing folding and deformation.
System with stacked thrust faults and flats are known as duplexes.
How are collisional belts more complicated regarding their folds and thrusts?
Rarely are collisional belts just a stack of folds and thrusts.
Complex sets of faults develop.
E.g. Those in the Himalayas. India collides with Asia. Thrust faults form along the Himalayan front but behind and around them are strike slip faults and normal faults (high angle and low angle).
What are some of the strucutural symbols used on geological maps?
Antiform.
Synform.
Thrust or reverse faults.
Normal faults.
Strike-slip faults.
What occurs at the x and z axis with regards to faults?
The x axis is the maximum extension axis.
The z axis is the maximum shortening axis.
How are faults and folds related?
The fold axial plane is ~ perpendicular to the strain axis of maximum shortening.
Fault conjugate pair - acute angle to strain axis of maximum shorting (z axis).
