Geologic Structure

Structural geology

Structural geologists study the architecture and processes responsible for deformation of Earth’s crust

Rock structures matter! Oilfields are contained in structures as are ore deposits

Deformation

Deformation refers to all changes in the original form and/or size of a rock body

Most crustal deformation occurs along plate margins

Occurs from tectonic forces

Deformation

Deformation involves• Stress - force divided by area• Types of stress

– Compressional stress – shortens rocks– Tensional stress – extends rocks– Shear stress – pushing in different

directions.

Deformation

Strain – changes in the shape or size of a rock body from stress

How rocks deform• Rocks subjected to stresses greater than

their own strength deform by fracturing, folding or flowing.

DeformationHow rocks deform

• General characteristics– Elastic deformation – the rock returns to

nearly its original size and shape when the stress is removed

– When a rock breaks, it is called brittle deformation. Any material that breaks into pieces exhibits brittle behavior

– Low temperature and pressure conditions– When rocks bend or flow, like clay, it is called

ductile deformation– High temperature and pressure conditions– Earthquakes release elastic energy

Deformation

How rocks deform• General characteristics of rock

deformation– Factors that influence the strength of a rock

and how it will deform

– Temperature

– Confining pressure

– Rock type

– Time (strain rate)

Folds formed under high grade metamorphic conditions

Mapping geologic structures

When conducting a study of a region, a geologist identifies and describes the dominant rock structures

• Work is aided by advances in aerial photography, satellite imagery, digital topography and Global Positioning Systems (GPS)

Strike and Dip

Strike: Direction of the line of intersection between a tilted plane and a horizontal plane.

• 2-directional line

Strike and Dip

Dip: Angle between a tilted plane and a horizontal plane that is perpendicular to strike.

• Maximum angle ( ≤ 90º )

• Notation: ____ ° N, S, E, W, NE, NW, SE, SW

(E. McBride)

Cross-Section: Dipping Strata

Horizontal

Dip Angle

W E

(E. McBride)

Cross-Section: Dipping Strata

W E

Horizontal

Dip Angle

Map Symbols

strike and dip

strike and dip (vertical)

strike and dip (horizontal)

45º

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