Geological Structures Flashcards
Define a fault
A fracture in a rock along which there has been an observable amount of displacement
What events occur when stored stres is released?
Earthquakes
How are faults formed and how long can they be?
Tension, compression or shear forces
Few cm to hundreds of km
What types of shattering may occur along the fault plane? What other process occurs?
Brecciation or fault gouge (finely ground). The movement of mineralising fluids/water
How is the downthrow side marked on a geological map?
A tick
What is throw?
The vertical displacement of rocks along the fault plane
Footwall, hanging wall, downthrown wall, upthrown wall
Below fault, above fault, moves down, moves up
Two main types of fault?
Dip slip and strike slip
Types of dip slip faults
Normal
Graben and Horst
Reverse
Thrust
What is a dip slip fault
Movement along the fault plane is parallel to the dip
What type of fault does tension cause?
Normal
What type of fault does compression cause?
Reverse
Describe a normal fault
Principally vertical stress direction due to lithostatic pressure, minimally horizontal
Occur close to surface where crust is stretched and fractured.
Hanging wall = downthrown (gravity).
Tension causes crustal extension
Breaks previously continuous beds of rock
Describe graben
When two normal faults dip towards each other a graben (rift valley) is formed. Graben forms the downthrow between two faults. Occur at the centre of mid ocean ridge and E-African Rift Valley System
Describe Horst
Two normal faults dip away from one another and form Horst - the elevated block as areas either side have dropped down
Describe a reverse fault
Formed by principally horizontal compressional forces - shortening of the Earth’s crust close to the surface. Hanging wall = upthrown. Overlap in the strata causes repetition of formerly continuous bed
Describe a thrust fault
Type of reverse where fault dip is less than 45 degrees (usually between 10-40°). Associated w major fold mountain systems, w displacements measured in km. Can result in inverted strata
Define a strike slip fault
Where the fault plane is (near) vertical and the movement along the fault is horizontal, parallel to the strike
Two main types of strike slip faults
Tear and transform
Describe tear fault
Result of shearing forces applied. Typically large scale with large displacements. e.g. Great Glen Fault
Describe a transform fault
Associated w plate margins (conservative plate margins/transform plate boundaries). common at right angles to MOR. Result from different rates of movement within a plate and allow rigid plates to adjust for difference in rates
Earthquake frequency at transform vs tear fails.
Few to none at tear faults as there is little to no relative movement
What do dextral and sinistral mean
Right handed and left handed. If the block of rock on the opposite side of the fault has moved it the right it is dextral. If it has moved to the left it is sinistral
Define slickensides
The linear striations (grooves and ridges) and polishing found in a fault plane indicating the direction of relative movement (parallel to the orientation of striations) caused by pieces of grinding rock between the walls.
Define fault breccia
The broken fragments from the rocks on either side of the fault plane. They are large and angular and made of hard competent rock, that may be cemented by minerals precipitated in the fault zone at a later stage
Define fault gouge
The very fine particles of rock formed when the grinding of rock along the fault plane occurs at very high pressure and frictional heating.
What is mylonite?
A rock produced by dynamic recrystallisation of minerals on a fault plane (particularly of fault gouge)
Define a fold
A fold is a flexure in rocks where there is a change in the amount of dip in a bed
What causes folds
Horizontal compressive forces, usually at destructive plate margins
Define antiform. What is an anticlinal fold
Upward closing fold with dips pointing inwards. Anticlinal when the age of the rocks is known, antiform describes only the shape
Define synform. What is synclinal?
Downwards closing fold with dips pointing outwards. Synclinal when the age of the rocks is known, synform describes only the shape
In regards to inter-limb angle:
1) Gentle
2) Open
3) Closed
4) Tight
5) Symmetrical/Upright
6) Asymmetrical/Inclined
1) 180-120°
2) 120-70°
3)70-30°
4) <30°
5) AP is vertical
6) AP dips, limbs have different dips
Where are the oldest rocks in an anticline fold?
In the core
Where are the oldest rocks in a syncline fold
The outside
Define hinge
The line along which there is a change in the amount and or direction of dip forming the most sharply curved part of the fold
Define axial plane trace
The outcrop of the axial plane at the earth’s surface
Define plunge
The angle of dip of the axial plane from the horizontal
Symmetrical vs Asymmetrical
S) angle of dip the same
A) angle of dip different
S) limbs same length
A) limbs diff lengths and limb w low angle of dip has wider outcrop than limb with higher angle
S) AP vertical
A) AP inclined
Define dome
An anticline that dips away from the centre in all directions
Define basin
A syncline that dips towards the centre from all directions
Define oversold
Folds which have both fold limbs dipping in the same direction but by different amounts
Define recumbent
Folds w axial planes and fold limbs close to horizontal, less than 30°
Define nappe
Recumbent fold broken along thrust plane
Isoclinal fold
Fold w parallel limbs nearly vertical in very tight folds
Cross symbol on map?
Horizontal
How are dykes drawn ?
Perpendicular to base of cross section
Define stress
The force applied to rocks
Define strain
The change in the shape or volume of a body as a result of applied stress (deformation)
What is the equation linking the original length of the line, the strain and the change of length of the line in regards to how vertical stress affects perfectly elastic material
Strain=change of length/original length
Define competent
Rocks that are strong and brittle that tend to joint and fault, staying the same thickness when they are deformed
Three examples of competent rocks
Sandstones, limestones and most igneous rocks
Define joint
A fracture in competent rocks along which no observable movement has occurred
Define incompetent rocks
Rocks that vary in thickness when they are deformed as they behave in a plastic way. They are weak and plastic and tend to fold and develop cleavage
Two examples of incompetent rocks
Mudstones and shales
Define shear forces
Forces which act along a plane in the rock and promote sliding along that plane
Go look at those sad little graphs sigh (p174)
What three factors affect the stresses applied to a rock and the resulting strains
1) higher temp = more plastic, cold rocks = brittle
2) strength of rocks increases w confining pressure, determined by the weight of overlying rocks
3) time is crucial in determining the type of deformation - pressure applied short period of time rocks may behave brittle manner pressure applied over an extended time can result in plastic deformation
What do tensional forces cause
Fracturing of rocks and crustal extension
What do compressional forces result in
Fracturing or folding of rocks
What do shear forces result in
The deformation of rocks in one plane usually horizontally. May result in faults or folds
How can fossils be used to measure deformation?
Deformation of bilaterally symmetrical fossils or ooliths
What forces may result in joints?
Folding, cooling or unloading of rocks
Where do joints form
Competent rocks - they are brittle and break when put under tension. Joints in sedimentary rocks are usually found perpendicular to the beds
What causes tectonic joints and what are their characteristics
Folding - tension joints parallel to the axial plane and cross joints on the limbs
What causes cooling joints
Perpendicular to cooling surfaces of igneous bodies so oft vertical - Form as a result of contraction on cooling igneous rocks. The rock cools and crystallises, contracting into a series of columnar polygonal structures
What causes Unloading joints
Lower pressure near the surface - often horizontal as well as vertical - load pressure declines as a result of uplift and erosion
Why do rocks fracture under compressive forces
The outward surface, containing the competent rock of the bed is stretched more than the inner forces
What are the two common forms of tectonic joints?
Tension joints (parallel to axial plane trace of the fold)
Cross joints (at angle to APT of the fold)
Describe the cleavage/beddings in a rock in the normal and inverted limb
In both, platy minerals align perpendicular to the principal stress direction. On the normal limb, the bedding dips at a lower angle than the cleavage. On the inverse, the bedding dips at a higher angle than the dip of the cleavage
How can you recognise bedding from photographs and diagrams?
Diff in colour or comp
Diff in grain size
Structures within beds and along planes
How to recognise jointing
Fractures along which there is no displacement
Often perpendicular to bedding
Possible mineralisation along the joints
Only develops in competent rocks such as sandstones and limestones
How to recognise cleavage from diagram or pictur
Rocks splits easily along parallel planes making slates
Often parallel to the axial plane of folds
Planes very close (few mm) together
Only in incompetent rocks like shales
