Transcript Geologic Structure Part I – Folds

Types of Metamorphism
Regional metamorphism
– Large scale – large volume of rock is affected
– Associated with convergent plate margins and mountain
building
– Folding and faulting increase thickness of the crust
– Occurs over a range of temperatures and pressures
– Fluids are also present
– Low grade to high grade metamorphism
– Will have zones of contact metamorphism
Regional Metamorphism
Metamorphic Grade
The following diagram illustrates the rock cycle. Match the
lettered responses to the blank ovals on the diagram. (Note:
some letters are used more than once.)
Magma
1
Metamorphic rock
•
•
•
•
•
a) Cementation & Compaction
(lithification)
b) Heat and Pressure
4
c) weathering, transportation,
deposition
d) Cooling and solidification
e) Melting
Sedimentary Rock
2
3
Igneous rock
6
5
7
8
Sediment
Rock Cycle
Geologic Structure
Rock Deformation
• Stress
– Force applied to an area
– Pressure
– Differential stress – magnitude of stress is
greater in one direction
• Rock Strength
– Rock’s resistance to deformation (stress)
Rock Deformation Cont.
• Strain
– A rock’s response to stress
– Deformation resulting in change of shape or position
• Elastic deformation
– Deformation is reversible, after stress is removed, materials
return to original shape, i.e. A rubber band
• Plastic deformation
– Deformation is permanent
– Rock flow in response to stress
– Need high pressures and temperatures, generally deep in
earth’s interior, i.e. Taffy
Rock Deformation Cont.
• Brittle deformation
– Deformation is permanent
– Rock break (fracture) or lose cohesion
– Generally low temperature and low pressure,
common in shallow crust, i.e. Peanut brittle
Types of Stress
• Tensional (Extension) Stress
• Compressional Stress
• Shear Stress
Tension or Extensional Stress
• Stress move away from each
other (pull apart)
• Rock lengthens
• Common in divergent
boundaries, but not sole
locale
• Ductile deformation –
stretching and thinning of
rock
• Brittle deformation –
fracturing and faulting
Compressional Stress
• Stress presses (squeezes)
rock together
• Rock shortens
• Common at convergent
boundaries
• Ductile deformation –
shortening and
thickening of rock – rock
folds
• Brittle deformation –
fracturing and faulting
Shear Stress
• Stress creates a
lateral shift in the
rock
• Rock slides past other
rock
• Common at
transform boundaries
• Results in faults
Folds
• Ductile deformation
• Compressional stress
• Horizontal rocks are deformed
Fold Terminology
• Limb – the sides of the fold
• Axial plane (hinge plane) –
imaginary plane that divides
the fold into two equal parts,
marks location of maximum
curvature
• Fold axis (hinge line) – the
line formed by the
intersection of the hinge
plane and a bedding plane
Syncline
• A fold with the
youngest beds in
the middle
• Generally trough
shaped, with
limbs dipping
toward the center
Anticline
• A fold with the
oldest beds in the
middle
• Generally arched
shaped, with
limbs dipping
away the center
Monocline
• Folds with only one limb
• Often Associated with faults
Fold Symmetry
• Symmetrical fold –
limbs of the fold dip at
the same angle
• Asymmetrical fold –
limbs of the fold dip at
different angles
Overturned
Fold
• Fold where one
limb has been
tilted beyond
vertical
Applied
to a
1
2
causes
Daily Exercise-Complete the
concept map
3
5
4
The material
The material
7
8
Defined by
youngest
rock in middle
9
no displacement
10
a)
b)
c)
d)
e)
f)
g)
Joints
Folds
Anticline
Tension
Faults
Break
Plastic
deformation
h) Syncline
i) Brittle
deformation
j) Force/stress
k)
l)
m)
n)
o)
Deformation
Normal
Reverse
Rock
Compression
6
displacement
11
Each letter can be used
only once, and it is possible
that it will not be used.
Faults
• A results of brittle deformation
• Fractures in the Earth's crust where
displacement has occurred
Fault Terminology
• Hanging wall – block
that lies below the
fault plane
• Footwall – block that
lies above the fault
plane
Classes of Faults
• Dip-Slip
– Movement is primarily
vertical
• Strike Slip
– Movement is along
strike
• Oblique-Slip
– Movement is both
vertical and along
strike
Dip-Slip Fault – Normal Fault
• Extensional Stress
• Hanging wall moves
down with respect to
footwall
FOOTWALL
HANGINGWALL
Normal Fault
• Horst – raised
block of material
bounded by two
normal faults
• Graben – trough
(valley) bounded
by two normal
faults
Dip-Slip Fault – Reverse Fault
• Compressional Stress
• Hanging wall moves
up with respect to
footwall
Dip-Slip Fault – Thrust Fault
• Compressional Stress
• Low-angle reverse
fault (fault plane <
45o)
• Displacement can be
100’s km (~60 miles)
• Typically associated
with folds
Strike-Slip
Fault
Left-lateral strike-slip faults:
movement of opposite plate
is to the left
• Shear Stress
• No vertical movement
• Movement parallel to
fault plane
Right-lateral strike-slip faults:
movement of opposite plate is
to the right
Joints
• Brittle deformation at low
pressure
• Tension fractures with NO
DISPLACEMENT