Please enable JavaScript.
Coggle requires JavaScript to display documents.
A2 Rock Deformation (Key definitions (Ductile deformation (Permanent and…
A2 Rock Deformation
Key definitions
Ductile deformation
Permanent and significant strain without fracture
Fold
Structures formed by ductile deformation
Plastic deformation
Ductile deformation achieved by slow internal flow or creep
Flow
Permanent and on going deformation by means of both brittle and plastic deformation
Yield point/ elastic limit
The point on a stress-strain diagram that marks the transition from elastic to permanent deformation
Fault
Rock fracture formed by brittle deformation showing evidence of displacement
Brittle failure
Sudden and permanent change in a rock by fracturing or faulting
Plasticity
The ability of a material to permanently change without fracturing
Elastic deformation
Recoverable strain
Fold terminology
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
Plunging Antiform- Eroded V-shaped outcrop points in same direction as plunge
Plunging Synform- Eroded V-shaped outcrop points in opposite direction as plunge
Faults and principal stress directions
Normal Faults
Max= vertical, Intermediate and Min= horizontal
Reverse Faults
Max= horizontal, Intermediate= horizontal, Min= vertical
Strike-slip Faults
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.
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 reactivation
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
Calculating bed dimensions (Trig)
SoH
CaH
ToA
Nature of rock deformation
Competency of rock
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
Geological conditions
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
Relationship between fold style, temp and depth
Flexural 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
Flow folds
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
Identifying faults
Features associated with faults
Fault Breccia
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
Fault Gouge
Fine grained and clay rich, soft rock located along centre of fault zone, gouge created by combination of crushing and chemical alternation
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