Lecture 1- deformation, stress and strain Flashcards
What is structural geology?
the study of deformation of rock by looking at their geometrical structures
What do you need to get the big picture of an areas geology?
due to the geological record being so discontinuous must look at very small samples
Why is structural geology imporant?
Mapping and exploitation of resources (oil, gas, ores, groundwater) (seismic survey)
Geological reconstruction of areas (alps)
What is the method of structural geology?
Observe and measure which allows for restoration of original environment allowing possibility to know cause, mechanism and timing of deformation
What can be seen from the 3D imaging of the Mid-Atlantic ridge?
Parallel striations on a fault plane
What are corrugations?
the shaping of parallel ridges or grooves
What are oceanic detachment faults?
faults found on the sea floor that allow for spreading of the lithosphere allowing more plate divergence
Why wouldnt you expect to find a fault on the sea floor?
due to the magmatic processes which enact here
What do detachment faults provide a new mechanism for the formation of?
ocean basins
What is tectonics?
Set of processes that operate at a large scale (e.g., mountain building, plate motion) and generate a characteristic set of structures
What is scale invariance/ fractalation?
an element in microscopic scale will be reflected exactly identical at the macroscopic scale
What is an example location where foliation of gabbro linked to mountain building?
Tibet-Himalaya
What are the 4 types of deformation?
Dilation (volume change)
Translation (change in position)
Rotation (orientation change)
Distortion/ strain (shape change)
Is dilation an increase or decrease in volume?
both
What is translation?
change in position of a rigid body
What is rotation?
change in orientation
Does rotation deformation have to occur at a central point in the object?
no it can occur elsewhere which leads to translation also the occurring
What will the displacement field look like under strain?
longer arrows (faster movement) and shorter arrows leading to a skew in the rock
What simple shear do with proportions?
shortening in one direction and extension in the perpendicular direction
With simple shear what happens to the lines not parallel to shear direction?
they rotate
What happens to orthogonal lines after deformation by simple shear?
no longer perpendicular
How is deformation produced with pure shear?
contractional or extensional forces
Do lines rotate in deformation by pure shear?
No they do not
What happens to the orthogonal lines during pure shear?
they remain perpendicular
What is strain like when thought of as a number?
dimensionless
What is stress?
a force (F) applied over a unit area (A) producing deformation
What is the measure of stress?
the same as a pressure (force/area) unit is N/m*2 or pascal (pa)
Is stress visible?
No (i.e. you cant see gravity)
What plate motion is affecting the Anatolian plate?
The northwards movement of the African and Arabian plates which push the Anatolian plate in to the stable (large) Eurasian plate
What faults are created in Anatolian plate due to squeezing?
North Anatolian fault and the Eastern Anatolian fault
What deformation is Anatolian plate experiencing?
Rotation
What is compaction?
A reduction in the pore spaces between sedimentary rocks due to lithostatic pressure
How much can sediments be compacted from their original volume?
50%
What type of deformation does compaction induce?
Strain and dilation
What is the Bolivian orocline?
a section of the Andean cordillera that has been bent into present arcuate shape
What deformation occurred at the Bolivian orocline?
rotation and translation
How did the Himalayan belt form?
When the Indian plate moved northwards ~8000km and collided with Asia
What deformation did India undergo?
Translation and rotation
What is serpentinisation?
the hydration of mantle rocks (peridotites)
How does serpentinisation affect rock volume?
causes up to a 40% volume increase
What rocks are involved in serpentinisation?
olivine breaks down into serpentine
What type of deformation is serpentinisation?
dilation
