lOMoARcPSD|38629542
N1 - Intro to Structural Geology
Structural Geology (University of Birmingham)
Scan to open on Studocu
Studocu is not sponsored or endorsed by any college or university
Downloaded by Harvey Abala (abalaharvey@gmail.com)
lOMoARcPSD|38629542
Structural Geology
L1 – Deformation, Strain & Stress
Structural geology concerns exploration, mapping and the exploitation of resources (e.g. il
and gas, ores and groundwater). It is the study of the deformation of rock by looking at their
geometrical structures. The most common method is observation and measure, analysis
and restoration thus leading to the mechanism.
Oceanic detachment faults: occur at spreading ridges where magmatic activity is not
enough to account for the entire plate spreading rate. They are characterized by long domes
parallel to the spreading direction (oceanic core complexes of the footwall).
The identification of corrugations on the seafloor led to formulate the existence of oceanic
detachment faults in the Atlantic Ocean.
The breakthrough:
Detachment fault activity provides a new mechanism to
form ocean basins, not by classic magmatic activity but
via tectonic activity (e.g. stretching of the lithosphere)
Structural geology differs from tectonic studies as it is the study of the deformation of rock
(e.g. folding) not global processes such as plate boundaries. However, it does support the
study of tectonics. By analysing small-scale structures, is it possible to reconstruct largescale processes that operate (or operated) at a global scale (e.g. large mountain belt
formation).
Many structures and processes in geology are fractals or scale invariant. This means that an
element observed at the centimetre scale can be extrapolated at the regional (kilometre)
scale.
Deformation: when rocks deform in response to imposed stress they exhibit strain, which is
the differential change in size, shape, or volume of a material.
Downloaded by Harvey Abala (abalaharvey@gmail.com)
lOMoARcPSD|38629542
Deformation can be obtained through:
 Dilation (change in volume)
 Translation (change in position)
 Rotation (change in orientation)
 Distortion (change in shape)
Dilation
Dilation can occur via compaction whereby the space between grains in sedimentary rocks
becomes smaller. Sediments can be compacted by 50% their initial thickness and there are
more things in the same amount of space.
It can also occur by the hydration of mantle rocks into serpentinite. Serpentinization is a
process whereby rock (usually ultramafic) is changed, with the addition of water into the
Downloaded by Harvey Abala (abalaharvey@gmail.com)
lOMoARcPSD|38629542
crystal structure of the minerals found within the rock. A common example is the
serpentinization of peridotite into serpentinite (the metamorphic equivalent).
Translation
Anatolia
Arabia
The Anatolian plate is currently being extruded to the
west due to the northward indentation of the Arabian
plate.
Another example is ophiolites are relics of oceanic crust that have been translated by
several tens of kilometres onto continental margins.
Rotation: normal faults and fault block
Downloaded by Harvey Abala (abalaharvey@gmail.com)
lOMoARcPSD|38629542
Distortion
Distortion is a change in shape and it is also referred to as
strain
Rocks can acquire a permanent distortion because under certain conditions they behave as
a plastic material. At low temperatures (below 350°C), rocks are elastic and brittle. In fact as
their elastic limit is overcome, they break (i.e. generate faults). At high temperatures (above
350°C), rocks are essentially plastic so under a stress they flow as a dense fluid and acquire
a permanent deformation.
Simple shear vs Pure shear
Both result in a shortening in one direction and lengthening in the perpendicular direction
Simple shear: Lines parallel to the strain axes rotate during deformation
Downloaded by Harvey Abala (abalaharvey@gmail.com)
lOMoARcPSD|38629542
Pure shear: Lines parallel to the strain axes do not rotate during deformation
Another example of distortion
Internal shear
Homogenous deformation vs
Heterogeneous deformation
Homogenous deformation: deformation
identical throughout a rock volume so the
originally straight/parallellines remain
straight/parallel
Downloaded by Harvey Abala (abalaharvey@gmail.com)
lOMoARcPSD|38629542
Heterogeneous deformation: deformation not identical throughout a rock volume so the
originally straight/parallellines become irregular
Stress: a force (pressure) that rocks feel (invisible) – you cannot see gravity
Strain: the material (visible) effect of stress - you can see a landslide (effect of gravity)
Typical stress (pressure) affecting rocks is gravity and tectonic stress due to plate motion
Stress (σ) has the dimension of pressure: σ = force/area which is measured in Pascals (Pa)
Rule number one: strain is not stress
Rule number two: strain is not stress
People use skis because the stress produced by their weight is lower if distributed over a
larger area.
Why am I sliding?
The main stress (the skier weight) on an inclined plane can be decomposed into a normal
stress perpendicular to the plane, and a shear stress parallel to the plane. Because the shear
stress induced by my mass is larger then the frictional resistance of snow.
Downloaded by Harvey Abala (abalaharvey@gmail.com)