Transcript Structural Geology - Illinois Wesleyan University

Structural Geology
• Sedimentary and volcanic rocks are originally deposited in sub-horizontal layers. Any deviation shown by
rocks is considered a structural deformation and is found/shown on geologic maps.
• Most deformation is due to stresses and strains at plate margins; some is due to cooling-driven contraction
and heating-driven expansion.
• Rocks deform permanently by breaking (joints and faults) and flowing or oozing (folds).
• Basins and domes are largely due to vertical movements of the crust.
v 0030 of 'Structural Geology' by Greg Pouch
at 2011-10-24 09:57:29
LastSavedBeforeThis 2011-09-07 16Structural.ppt
Structural Geology
Processes (Rheology)
3 Rheology > Stress
4 Rheology > Strain
5 Rheology > Controls on Strain Style
6 Rheology on one slide
7 Rheology Images
8 Geologic maps (how we know about deformation)
Products (Geologic Structures)
9 Geologic Structures>Fractures
10 Geologic Structures>Fractures > Slides
11 Geologic Structures>Flexures
12 Geologic Structures>Flexures>Slides
13 Basins and Domes
Rheology > Stress
•Stress Stress is the force applied divided by the area (the continuum
replacement for force).
–Can be compressive, tensile or shear. Compressive and tensile
stresses are called normal, because they act normal (perpendicular) to
the elemental area.
–Any stress field is equivalent to a set of three perpendicular, normal
stresses called the principal stresses. Pressure is stress uniform in all
directions (homogenous, pressure). Stresses can be separated into
pressure (average stress) and stresses deviating from uniform. Pressure
controls material properties.
–Effective stress = (stress on grains)-(fluid pressure) [the effect of
increasing fluid pressure is equivalent to decreasing confining
pressure]
–If the principal stresses are equal (rare), only pressure is felt by the
material.
–If the principal stresses are not equal (almost always), there will be
shear stresses at directions between principal stresses.
•Strain Deformation of material.
•What controls the style of strain?
Rheology > Strain
•Stress (the continuum replacement for force).
•Strain Deformation of material.
Strain is the change in length divided by the initial length. (continuum replacement of
displacement)
–Causes of strain
•Strain can result from an applied stress or force. (For example, stepping on mud applies a
force or strain, and the soil strains [either elastically or plastically or viscously]).
•Strain can occur and cause a stress (When something cools, it contracts, and causes a force or
stress)
–Styles of strain
•Elastic strain is reversible. When stress is removed, item goes back to original dimensions.
Many materials are linear-elastic (strain is proportional to stress)
Engineers consider elastic good in final products like buildings and roads.
•Brittle general term for irreversible strains that includes jointing and faulting. In brittle
deformation, there are discontinuities. Breaking glass is brittle.
•Ductile (continuous, dispersed deformation, no discontinuities develop)
–Viscous the material responds to any shear stress by flowing to eliminate the stress and
deformation rate is proportional to applied stress. Same as fluid (liquid or gas).
–Plastic the material has a level of shear stress required to initiate deformation, called the
plastic limit, after which it strains/squishes. When you see them, 'plastics' are not usually
plastic: the showed plasticity at the handling stage. Properly made mashed potatoes and
puddings are plastic.
•"Rigid" means no deformation occurs. This is a useful idealization to simplify calculations
and design when deformation is "negligibly small" and usually elastic.
•"Flexible" is the opposite of rigid: it means the thing can bend or deform somehow. It is
hugely vague.
•What controls the style of strain?
Rheology > Controls on Strain Style
•Stress (the continuum replacement for force).
•Strain Deformation of material.
•What controls the style of strain?
–Material Under a set of conditions, one material may deform
plastically while others may be elastic or brittle. (For example, at room
temperature and for low stresses, steel is elastic, fresh potato chips are
brittle, and mashed potatoes are elastic or plastic.)
–Magnitude of stresses (At low stress, most deformation is elastic. At
higher stresses, failure—brittle or ductile— occurs)
–Confining pressure (Brittle failure common at low pressure, plastic
common at high pressure)
–Temperature (Elastic/brittle->Plastic->Viscous)
–Fluid pressure (pressure increases have the same effect as decreasing
confining pressure but not shear)
–Fluid composition (reactive fluids, like water and saltwater, can
influence the mode of deformation. Fluid viscosity can influence the
style of deformation.) (Stale potato chips are more plastic than brittle.)
–History of the material (pre-existing cracks and crystal defects)
Rheology
on
one
slide
•Stress Stress is the force applied divided by the area (the continuum replacement for force).
–Can be compressive, tensile or shear. Compressive and tensile stresses are called normal, because they act normal
(perpendicular) to the elemental area.
–Any stress field is equivalent to a set of three perpendicular, normal stresses called the principal stresses. Pressure is
stress uniform in all directions (homogenous, pressure). Stresses can be separated into pressure (average stress) and
stresses deviating from uniform. Pressure controls material properties.
–Effective stress = (stress on grains)-(fluid pressure) [the effect of increasing fluid pressure is equivalent to decreasing
confining pressure]
–If the principal stresses are equal (rare), only pressure is felt by the material.
–If the principal stresses are not equal (almost always), there will be shear stresses at directions between principal stresses.
•Strain Deformation of material. Specifically, it is the change in length divided by the initial length.
–Causes of strain
•Strain can result from an applied stress or force. (For example, stepping on mud applies a force or strain, and the soil
strains (either elastically or plastically or viscously).
•Strain can occur and cause a stress (When something cools, it contracts, and causes a force or stress)
–Styles of strain
•Elastic strain is proportional to stress and reversible
•Brittle general term for irreversible strains that includes jointing and faulting. In brittle deformation, there are
discontinuities. Breaking glass is brittle.
•Ductile (continuous, dispersed deformation)
–Viscous the material responds to any shear stress by flowing to eliminate the stress and deformation rate is
proportional to applied stress.. Fluid.
–Plastic the material has a level of stress required to initiate movement (plastic limit), after which it strains at a
uniform rate
•What controls the style of strain?
–Material Under a set of conditions, one material may deform plastically while others may be elastic or brittle. (e.g., at
room temperature and for low stresses, mashed potatoes are plastic, steel is elastic, and cookies are brittle)
–Magnitude of stresses (At low stress, most deformation is elastic. At higher stresses, failure—brittle or ductile—
occurs)
–Confining pressure (Brittle failure common at low pressure, plastic common at high pressure)
–Temperature (Elastic/brittle->Plastic->Viscous)
–Fluid pressure (pressure increases have the same effect as decreasing confining pressure)
–Fluid composition (reactive fluids, like water and saltwater, can influence the mode of deformation. Fluid viscosity can
influence the style of deformation.)
–History of the material (pre-existing cracks and crystal defects)
Rheology Images
Geologic
maps
(how
we
know
about
deformation)
• There are many types of maps produced by geologists, but the type meant when used without other
modifiers shows
–the formation (lithology+continuity) at bedrock;
–orientation of beds (strike and dip) or omitted if horizontal or not measured;
–geologic structures (faults, fold axes…) (discussed later)
–basemap for orientation. Usually a topo map, but might be counties, states, roads….
• A formation is a mappable unit, ideally of uniform lithology but more often a set of rock types deposited in
continuity and distinctive from those above and below. Usually, this map is largely inferred based on
scattered outcrops: Canadians clearly separate observations at outcrops from inferences between,
Americans show the whole area as conclusion or observation.
• Strike and dip are a way of specifying the orientation of an inclined plane. Strike is the direction of the
line of intersection between the plane and a horizontal plane. Dip is perpendicular to strike and is the
vertical angle of the plane.
–Water would flow down the dip direction. If the outcrop were immersed, the intersection of the plane
with the surface of the water (horizontal surface) would be a strike line. Water level changes would shift
the position but not direction of the strike lines.
–To specify the direction of a line, you typically use the vertical angle downward, and the plunge direction
(azimuth of the downward side of the line). The dip direction is the plunge direction of the dip-line
(steepest sloping line) More on this in lab.
• When interpreting a geologic map to get three-dimensional structure, remember that the surface is nearly
horizontal: a 30° slope is very steep, and probably unstable, and slopes usually only go on for a short
distance. Only in very rugged terrain will the topography deviate significantly from a horizontal plane.
Geologic
Structures>Fractures
• Fractures (discontinuities/cracks in rock) indicate brittle deformation and are common near the surface.
They can be due to ductile deformation at depth, due to more-or-less elastic bending of the crust, high fluid
pressures, applied strains due to cooling, or other even less interesting causes.
• Once rock has been fractured, strain tends to occur along pre-existing fractures.
• Fractures tend to concentrate fluid flow and are significant for groundwater, ore formation, and control
emplacement of magmas. They are also preferred planes of failure and are significant for mass wasting and
their effects on weathering.
• Joints do not have demonstrable displacement in the plane of the fracture, but may have displacement
perpendicular to the fracture plane. Joints are the result of tension or effective tension. Joints usually occur
in sets with the same direction. Joints are usually not mapped, but they should be.
• Faults have demonstrable displacement in the plane of the fracture and indicate stress was differential.
Faults often occur alone, but frequently occur in sets with the same direction (fault zones). Faults are rarely
seen, but are usually inferred based on differences in rock outcrop; when they are seen, they are not usually
nice clean planes, but some ground up mess.
–Faults stop moving when the stress is removed, but are frequently re-activated by later stress fields.
–Fault nomenclature The type of fault is based on the dip of the fault plane and which fault moved up
relative to the other one. The upper block is called the hanging wall, the lower block the footwall.
Type
Dip
Meaning
Common or
rare
Normal
>45°, usually >60° often 75°,
hanging wall down
Horizontal tension, crustal extension. Divergent plate boundaries
Very common
Reverse
>45°, 60°,
hanging wall up
Horizontal compression with a weird stress field
Rare
Thrust
<45°, usually <15°, often
around 0°
hanging wall up
Horizontal compression with a typical stress field. Often associated
with folding.
Convergent plate boundaries
Common
Strike-slip
90
Transform plate boundaries
Rare
Oblique slip
any
Anything other than pure dip-slip or strike-slip motion.
Surprisingly
rare
Geologic Structures>Fractures > Slides
Geologic
Structures>Flexures
At high pressure and temperature, rock can behave plastically or viscously (ductilely) and accommodate
Syncline
Axis
Limb
Axis
b
Lim
Lim
b
Axis
compressive stresses by folding. Flexures (bends) can also occur in brittle rocks by movement along
faults, with "hinges" at faults.. Ductile flexure results in smoothly curving shapes. Brittle flexure results in
sharp, angular shapes.
• Folds Elongated folds are usually due to horizontal compression perpendicular to the axis (line of
maximum curvature). Folds typically occur in sets of parallel folds in collisional mountain ranges. The
amplitude dies away from the center of the mountain range. Think of a blanket being shoved into a pile.
• Anticlines usually have an A shape (beds tilt out) and always have the oldest rock in the middle
• Synclines usually have a Y shape (beds tilt in) and always have the youngest rock in the middle
• The axis is the line of maximum curvature and is more-or-less the middle of the fold. The "plane"
containing the axes of successively deeper layers is the axial plane. The crest is the highest point on an
anticline, the trough is the lowest point on a syncline. The limb is the more or less planar side of a fold.
The left limb of a syncline is usually the right limb of an adjoining anticline.
• Folding can result in oil-traps, jointing, and faulting.
• Folds can be horizontal (fold axis is horizontal) or plunging; symmetrical (both limbs have the same dip
amount) or asymmetrical, vertical (the axial plane is vertical) or inclined, open (internal angle at axis >90)
or closed, isoclinal (both limbs dip the same amount in the same direction), overturned (at least one limb
has dip>90), recumbent (axial plane is more-or-less horizontal).
• Typical folds of interest in oil are plunging, have dip<20, axial plane is nearly vertical, and are almost
always asymmetrical.
• Within an area, a certain types of folds and faults will be common. Generally, they'll have consistent
orientations. This is known as the "structural style". Areas with different structural styles usually have had
very different histories and physical properties.
b
m
i
L
Syncline
Anticline
Basins and Domes
• Equant Basins and Domes are usually due to vertical buoyant forces and usually occur alone.
• Domes and basins are often unrelated to plate margins, and do not fit into plate tectonics well.
• Basins are often the sites of very thick accumulations of sediment.
• (Basin is also used to refer to any thick accumulation of sediment, regardless of shape or cause.
Sometimes, it refers to a topographic basin.)
Summary
• Rocks can deform (strain) in
response to stress, or stress in
response to strain (like coolinginduced stresses). The stress-strain
relationship can be elastic, brittle,
plastic, or fluid, depending on a
number of factors.
• Rocks can deform by jointing,
faulting, or folding.
• Deformations can be seen in
outcrop, on geologic maps, and
even in thin section.
• In an extensional environment,
typical deformation includes
jointing, normal faults, and
regional tilting.
• In a compressional environment,
typical deformation includes thrust
faults, folding, and secondary
tensional features
• Go back and look at first slide.