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 fold An extreme example of an overturned fold occurs when the axial plane is horizontal

Plunging anticlines and synclines Idealized view of plunging folds in which a horizontal surface has been added. View of plunging folds as they might appear after extensive erosion. In a plunging anticline the outcrop in the direction of The plunge, while the Opposite is true for plugging syncline

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 is termed as graben and an elevated block (unfaulted)

Growth of a mountain range along a normal 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