L5 cracks and crumples deformation processes

Deformation

a process by which rocks bend, break or flow. In response to compression, tension or shearing.

• Types of deformation

  • displacement: change in location, one block of rock moves from one place to another.

  • distortion: change in shape

  • rotation: change in orientation

Stress

• Not the same as force → important to distinguish between them.

• Stress is the force applied per unit area (S=F/A)

Strain

• When material is loaded with a force, it produces a stress which causes a deformation.

• Strain is the response of a system to an applied stress

  • e.g. it’s the deformation

  • It is represented by the change in shape arising from the deformation

Types of stress

• Compression

• Tension

• Shear

• Pressure

Compression

Horizontal compression drives plates collision and orogeny (mountain building)

Tension

Horizontal tension drives crustal rifting

Shear

Shear develops when surfaces slide past one another

Brittle deformation

• Fracture in response to stress

• A continuous, force is applied to a rock. As the force is gradually increased. A little change occurs in the rock until it suddenly fractures/faults.

Ductile deformation

• Flow leading to permanent deformation in response to stress

• Gradually increasing force will cause the rock to undergo smooth and continuous plastic deformation. The rick will contort and change shape without fracturing (see folding and foliation later)

Ductile flow vs Brittle flow

• Temperature

• Confining pressure

  • Temp and pressure can increase with depth in Earth

  • Change from Brittle to ductile with depth

  • Brittle-ductile transition

  • In continental crust about 15km depth

Rock properties

• Rock strength can control nature of deformation

  • Mudstone are often weak and are more prone to ductile flow

  • granites are often strong are more prone to fracturing

Strain rate

• The rate at which a stress and associate deformation or strain is applied

• Apply stress more rapidly then rock is more likely to deform by brittle failure. Like hitting it with a hammer.

Fluids

The presence of fluids e.g. water within a rock has the effect of weakening the rock and making it more prone to ductile deformation

Depth

As both temperature and pressure increase with depth

• Upper levels favour brittle behaviour → faults. Earthquakes therefore are often concentrated in upper 10-15km of the earth’s crust

• Higher T and P → ductile behaviour - folds

Ductile-Brittle transition

How are earthquakes recorded?

Seismographs record the seismic waves generated by earthquakes

Seismic waves associated with earthquakes

• Generated by earthquakes naturally, energy travels as waves.

  • Body waves

  • Surface waves

Body waves

Waves that move through the Earth

• P waves

• S waves

Surface waves

Waves that travel along the earth’s surface which is detected by a seismograph

• L waves

• R waves

Hypocentre

It is the location where the fault slip occurs. Usually on a fault surface

Epicentre

It is the land surface directly above the hypocentre. Maps often portray the location of epicentres.

Rock friction

It resists movement, so faults move via stick-slip behaviour.

• Stick is the period between earthquakes

• Slip is the earthquake

Earthquakes

• They are a form of brittle deformation and in California they are restricted to the upper ca 13km of the crust.

• However, earthquakes can be much deeper than 10-15km depth where cold ocean crust is subducted under continental crust.

Seismic tomography

• Spatial variation in seismic travel times (P waves velocity) provides details of Earth’s structure

• Based on assumption that faster waves = cooler and/or more dense rock

Plate tectonics

The source of stress for deformation of rocks is due to plate movements/interactions

Different types of faults

• Normal fault

• Reverse fault

• Thrust fault

• Strike-slip fault

Convergent plate

Divergent plate

Transform plate

Folding

• Ductile response to compression

• Anticlines and Synclines, axial plane

  • Imaginary plane that divides the fold as symmetrically as possible, with one limb either side of the line.

  • Horizontal and plunging folds

  • Vertical and inclined axial planes

Symmetrical folds

Have limbs that dip symmetrically from the axial plane

Asymmetrical folds

Have on limb that digs more sleeply than the other

Overturned folds

Have limbs that dip in the same direction, but one limb has been tilted beyond the vertical

Fold and thrust belt

Often occur at margins of mountain belts

• Fault slices stacked one on top of the other

• Process acts to shorten and thicken crust

• Due to horizontal compression

• Piggy-back thrusting

Divergent boundaries

• East African rift zone

• Red sea

• Atlantic ocean

Tensional stress, normal faulting

Ancient normal faults are from rifts in offshore NW Europe

Convergent boundaries

• Ocean-Ocean convergence

  • Accretionary wedge, sediment scraped off the down going slab → developed (folds & thrust)

• Ocean-Continent convergence

  • Fold and thrust belt on over-riding plate

• Continent-Continent convergence

  • Deformation: High mountain belt forms by folding, foliations, thrust-faulting and doubling of the crustal layers

  • Fold and thrust belts either side of the mountain belt

Accretionary wedge

• Contains thrusts.

• Thrust faults form above the down going oceanic crust.

• Folds form between the thrust faults.

Continent-Continent convergence: an old orogeny

• Caledonian Orogeny → 400 mya, formed continuous mountain belt

• Fold and thrust belt → Intense deformation (metamorphism and igneous intrusions