A2 Rock Deformation

Key definitions

Nature of rock deformation

Relationship between fold style, temp and depth

Faults and principal stress directions

Identifying faults

Fold terminology

Fault reactivation

Calculating bed dimensions (Trig)

Features associated with faults

Ductile deformation

Fold

Plastic deformation

Flow

Yield point/ elastic limit

Fault

Brittle failure

Plasticity

Elastic deformation

Recoverable strain

Sudden and permanent change in a rock by fracturing or faulting

The point on a stress-strain diagram that marks the transition from elastic to permanent deformation

Ductile deformation achieved by slow internal flow or creep

Permanent and significant strain without fracture

Structures formed by ductile deformation

Permanent and on going deformation by means of both brittle and plastic deformation

Rock fracture formed by brittle deformation showing evidence of displacement

The ability of a material to permanently change without fracturing

Competency of rock

Geological conditions

Resistance to deformation, Competent rock respond in brittle way, stay same thickness, Crystallised rocks, sandstone and limestone. Incompetent rock responds in ductile way, thickness changes, clay, mudstone, shale

Temperature- Cold temps= brittle failure with fractures and faults, increasing temps decreases yield strength so rocks behave more plastically. 25 degrees= brittle fracture and faults, 200 degrees= flexural folds, 700 degrees= flow folds and distorted rock

Confining pressure- Strength increases with confining pressure, rocks are stronger at depth so more pressure is needed for deformation

Strain rate- Type and amount of strain deformation depends on time, over short periods rocks will strain only if subjected tom large stresses and usually behave in brittle manner, over longer periods rocks can slowly deform plastically and at lower stresses, Geologists can measure strain rate over time, rock is stronger over shorter periods of time

Flexural Folds

Flow folds

Formed by ductile deformation of warm or wet rock, rock is relatively competent means orthogonal thicknesses do not alter, slickensides created by slippage along bedding planes, low temps, ductile deformation of competent rock, 200 degrees up to 5km deep

Caused by ductile deformation of hot, incompetent rock, alters orthogonal thicknesses, maintains thickness parallel to axis surface, formed by shearing and associated with high temps, ductile flow deformation, 500 degrees at 50km deep

Normal Faults

Reverse Faults

Strike-slip Faults

Max= horizontal, Intermediate= horizontal, Min= vertical

Max= horizontal, Intermediate= vertical, Min= horizontal

Faulting results when applied tectonic stresses exceed fracture strength of rock, when rock is subjected to differential stress, the stress regime can be resolved into 3 principal stress directions that act at right angles to each other, Stress max, Stress min, Stress intermediate.

Max= vertical, Intermediate and Min= horizontal

When a rock is compressed it shortens and gradually swells until it suddenly fractures, it occurs due to development of 2 shear planes, which are failure surfaces created by the stress, the shear fractures are known as conjugate shear surfaces due to their symmetrical relationship with the principal stress directions

Fault Breccia

Fault Gouge

Slikensides

Finely polished and scratched fault plane formed by high pressure abrasion and synkinematic (growth during deformation) mineral growth during movement, usually straight and reveal trend of fault movement

Rock composed of coarse angular fragments in matrix of fine grained debris, created by crushing or catalysis (rock crushed along fault plane) of brittle rock along centre of fault zone

Fine grained and clay rich, soft rock located along centre of fault zone, gouge created by combination of crushing and chemical alternation

Occurs when later tectonism or crustal movement reactivates an earlier formed fault, in Britain reactivation of faults has occurred since last ice age in response to isostatic adjustment of crust due to glacial unloading

The reactivation of faults can lead to structural inversion if the direction of initial fault movement is reversed, 2 obvious examples-

  • Reverse faulting along normal fault due to compressional forces
  • Normal faulting along thrust fault due to crustal extension

Recognising Reactivation

Quantocks Head Fault, West Kilve, Somerset

Fault has 40-50cm normal throw, the deformation of the HW caused by reverse drag demonstrates it has been reactivated

Wessex Basin Structural Inversion

One of most studied examples of fault reactivation, Mesozoic extensional sedimentary basin that extends across a large part of Dorset, normal faults of basin were affected by structural inversion during Cenozoic in response to compression resulting from the combined effects of the Atlantic opening and Alpine orogeny, faults had been buried under Cretaceous Chalk and Tertiary Clays, reverse faulting deformed the overlying rocks, most well known structure created by this structural inversion is the Lulworth Crumple, a parasitic fold created by reactivation of Purbeck 'normal' fault

  • Gentle= 180-120 degrees
  • Open= 120-70 degrees
  • Closed= 70-30 degrees
  • Tight= 30-0 degrees
  • Isoclinal= 0 degrees
  • Upright
  • Inclined
  • Highly inclined
  • Recumbent
  • Overturned
  • Hinge lines
  • Axial plane
  • Axis
  • Angle of plunge
  • Axial plane cleavage
  • Crest
  • Trough
  • Fold height
  • Wave length
  • Amplitude
  • Interlimb angle

SoH

CaH

ToA

Plunging Antiform- Eroded V-shaped outcrop points in same direction as plunge

Plunging Synform- Eroded V-shaped outcrop points in opposite direction as plunge