Deformation: Unconformities, Folds and Faults Flashcards
Define Stress
Forces applied to a rock
Define Strain
The deformation caused by the stress
Define Elastic Deformation
No permanent deformation, will recover after stress is removed
Define Plastic (Ductile) Deformation
Permanent change in shape
Define Fracture Point
When the rock starts to break
Define Elastic Limit
Marks the start of permanent deformation
Define Brittle Response
Yield strength of the material has been exceeded and the material fails in a brittle way
What are four factor affecting stress and strain?
Temperature, Composition of rock/strength, Confining pressure, Strain rate
Define Strike
Angle/direction of no dip
Define True Dip
The maximum angle of dip a rock can have
Define Dip Direction
The direction in which the rock is tilted
Define Apparent Dip Direction
Any angle between 0 and maximum tilt
Define Unconformity
A boundary that marks a break in deposition, a period of erosion
What are five features providing evidence for the presence of an unconformity?
Erosional surface between two layers, Changes in dip amount and direction wither side of the U/C, Cross cutting relationships, Gap in fossil ages either side of U/C, Conglomerate above U/C
What are the four types of unconformity?
Angular, Nonconformity, Paraconformity, Disconformity
Define Nonconformity
Igneous/metamorphic underlying
Define Paraconformity
Looks like no break in deposition but sequential fossils are missing
Define Disconformity
Horizontal on either side with erosion surface
Define Folding
Occurs when the compressional stresses exceed the yield strength of the rock, causing rock layers to change their attitude or dip
What are the two main shapes of a fold?
Synform, Antiform
Define Anticline
Upwards closing fold with the oldest rocks in the centre
Define Synform
downward closing fold with the youngest rocks in the centre
Define Fold Limb
Section of the fold between crest and trough
Define Hinge
Line at which there is a change in dip direction, the most curved part of the fold
Define Crest
Highest point on a fold
Define Trough
Lowest point on a fold
Define Axial Plane
A plane that joins all the hinges of all the beds
Define Axial Planar Trace (APT)
Outcrop of the axial plane at the Earth’s surface
What are the four attitudes of a fold?
Upright (70-90), Inclined (10-70), Overturned, Recumbent (0-10)
What are the four types of interlimb angles?
Open (135-90), Tight (40-1), Isoclinal (0)
Define Fault
A fracture in a rock along which there has been observable displacement
Define Fault Plane
Plane where movement takes place
Define Hanging Wall
Block above the fault plane
Define Footwall
Block below the fault plane
Define Throw
Vertical displacement
Define Heave
Horizontal displacement
Define Dip-Slip Fault
A fault that moves vertically
Define Strike-Slip Fault
A fault that moves horizontally
What are the three types of dip-slip fault?
Normal (tensional stress), Reverse (compressional stress), Thrust (compressional stress, less than 30 degrees)
What are the two types of strike-slip fault?
Sinistral (move to left), Dextral (move to right)
What stress is associated with strike-slip faults?
Shear stress
What dip do faults usually have with a straight outcrop pattern?
90 degree
The youngest rocks are…
Always on the downthrown side
What does SWAN stand for?
Synclines are Wider on the downthrown side and Anticlines are Narrower
