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