Folds Rocks are often bent into a series of wave-like undulations called folds Characteristics of...
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Transcript of Folds Rocks are often bent into a series of wave-like undulations called folds Characteristics of...
Folds
Rocks are often bent into a series of wave-like undulations called folds
Characteristics of folds• Folds result from compressional stresses
which shortens and thickens the crust
Characteristics of folds• Parts of a fold
– Limbs – refers to the two sides of a fold– Axis – a line drawn along the points of
maximum curvature of each layer– Axial plane – an imaginary surface
that divides a fold in half
Axial plane, axis and limbs of fold
Folds
Common types of folds• Anticline – upfolded or arched rock layers• Syncline – downfolds or troughs of rock
layers
A series of anticlines and syncline
Recumbent foldAn extreme example of an overturned fold occurs when the axial plane is horizontal
Plunging anticlines and synclines
Idealized view of plungingfolds in which a horizontalsurface has been added.
View of plunging folds as they might appear after extensive erosion. In a plunging anticline the outcrop in the direction ofThe plunge, while the Opposite is true for pluggingsyncline
Folds
Common types of folds• Monoclines – large, step-like folds in
otherwise horizontal sedimentary strata
• Strata bend dip in one direction between horizontal layers on each side.
Monoclines consisting of bent sedimentary beds that were deformed by faulting in the
bedrock below
Other types of folds• Dome
– Upwarped displacement of rocks– Circular or slightly elongated structure– Oldest rocks in center, younger rocks
on flanks– Strata dip away from center in all
directions, oldest strata in center.
Circular outcrop patterns are typical for both domes and basins
Other types of folds• Basin
– Circular or slightly elongated structure
– Downwarped displacement of rocks
– Youngest rocks are found near the center, oldest rocks on flanks
– Strata dip toward center in all directions, youngest strata in center
Circular outcrop patterns are typical for both domes and basins
Faults
Faults are fractures in rocks that allow displacements to occur
Sudden movements along faults are the cause of most earthquakes
Types of faults• Dip-slip faults
– Movement is parallel to the dip of the fault surface
– May produce long, low cliffs called fault scarps
– Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault)
Concept of hanging wall and footwall along a fault
Faults
• Types of dip-slip faults– Normal fault
– Hanging wall moves down relative to the footwall
– Accommodates extension of the crust
– Larger scale normal faults are associated with structures called fault-block mountains
Morphology of mountain ranges in Nevada bounded by normal faults
A depressed block (down faulted) bounded by extensional faults istermed as graben and an elevated block (unfaulted)
Growth ofa mountainrange along anormal fault.
Faults
• Types of dip-slip faults
– Reverse and thrust faults– Hanging wall moves up relative to
footwall– Accommodate shortening of the crust– Results from compressional forces– Reverse- dip of fault plane is above 45
degrees– Thrust- dip of fault plane is less than 20
degrees.
On a reverse fault, the hanging wall moves up relative to the footwall
Thrust faults formed by crustal shortening
Faults
Strike-slip fault• Displacement is horizontal and parallel to strike of
fault• Types of strike-slip faults
– Right-lateral – as you face the fault, the block on the opposite side moves right
– Left-lateral – as you face the fault, the block on the opposite side moves left
• Because of their large size and linear nature, many strike-slip faults produce a trace that is visible over a great distance
• Crushed and broken rocks produced during faulting are more easily eroded, often producing linear valleys or troughs
A block diagram showing the features along a strike-slip fault
Fault
Strike-slip fault• Transform fault
– Large strike-slip fault that cuts through the lithosphere
– Accommodates motion between two large crustal plates
The San Andreas fault system is a major transform fault