Deformation

• Deformation - any change in the shape or volume of a rock, such as when a rock is folded or fractured.

  • stress is a force of deformation
  • strain is the change in shape that results when stress is applied
• Types of Stress:

  • Compression - rocks are squeezed along the same line; compression shortens the rock layers by folding or faulting (shortening strain common at convergent plate boundaries)
  • Tension - forces along the same line act in opposite directions; tension lengthens the rock or pulls them apart causing fractures and faults (common at divergent plate boundaries; rift valleys)
  • Shear - forces act parallel to one another but in opposite directions; deformation occurs along closely spaced planes (common at conservative plate boundaries)

  
• Hooke's Law - Law of elasticity discovered by the English scientist Robert Hooke in 1660; 'for relatively small deformations of an object, the size or displacement of the deformation is directly proportional to the deforming force or load'.
• • Elastic Behaviour - if a material returns to its original size and shape when you remove the forces stretching/deforming it (reversible deformation).
  • Plastic/Inelastic Behaviour - if a material stays deformed after the force is removed (irreversible deformation).
• If you apply too big a force a material will lose it's elasticity. The amount a spring streches is proportional to the amount of force applied. Rocks will deform elastically until they reach the elastic limit unless the force is applied quickly.
• Ductile materials show a great amount of plastic strain/bending before they fracture. Ductile material shows elastic then plastic deformation before rupture. Rocks fold easily (incompetent rock) and show cleavage + evidence of plastic flow. With very ductile material, a small load produces plastic deformation.

  
• Brittle materials fracture after only a small amount of plastic strain. The material shows little or no plastic deformation before rupture; common in competent rock + joints/faults formed.

  
• Factors affecting stress/strain:

  • Temperature - increased temperature decreases the strength of the rock + the higher the temperature, the more plastic the rocks will be; at high temperatures rocks will display ductile behaviour and will fold (even if competent) + cold rocks behave brittle so will fracture/fault.
  • Rate of deformation - very slow rates of deformation (plate tectonics) produce ductile behaviour + fast rates of deformation (earthquakes) produce brittle behaviour.
• Factors affecting stree/strain:

  • Confining Pressure - increased confining pressure raises the strength of the rock; therefore rocks at depth are more ductile and rocks nearer the surface are more brittle (mass of overlying rocks causes confining pressure)
  • Type of Rock - some rocks are naturally more brittle or ductile than others; high strength=brittle deformation (granite/quartzite) and low strength=ductile deformation (shale/halite)
• • Elastic Strain - reversible, rock can return to original shape.
  • Plastic Strain - rock flows, strain rate increases faster as stress increases (irreversible).
  • Brittle Strain - rock breaks suddenly as stress increases (irreversible).
• Deformation at divergent plate margins:

  • plates move apart and this divergence causes tensional stress - some ductile stretching of crustal rocks occur, but mainly faulting caused by brittle failures that occurs here
  • tension results in a series of normal faults causing the crust to move downwards in blocks (step-faulting); if on either side of a plate margin, the central block subsides in a rift valley
• Deformation at convergent plate margins:

  • compression produces a different set of tectonic features, often forming a mountain belt (orogeny); large fold mountain belts are formed by continent-continent collision where sedimentary rocks are faulted, folded, and uplifted (eg. Himalayas)
  • during orogenies, cooler rocks near surface behave brittle producing reverse faults/large-scale thrust faulting + in deeper, warmer more ductile zones intense folding produces tight, isoclinal folding and metamorphism
• Deformation at convergent plate margins:

  • where two continents collide, the compression can be so intense that large scale folds are forced upwards within rising mountain chains and slide down the outer flanks as folds called nappes (eg. Dalradian Tay Nappe, Scotland)
• Deformation at conservative plate margins:

  • shear stresses - lateral movement is produced at the transform faults characteristic of these margins, resulting in strike-slip faults (eg. San Andreas Fault, California)
• Dipping Beds

  • Bed - a layer of sedimentary rock that has been laid down under a specific set of environmental conditions, and can vary in thickness.
  • Bedding Planes - a surface that seperates each bed and is parallel to the surface of despoition. They represent a break in sedimentation, or a change in composition/grain size/colour.
  • Dip - the maximum inclination of a bed measured from the horizontal ising a clinometer; measured as the maximum angle between a horizontal line and the dipping bed plane.
• Dipping Beds

  • True Dip - the actual angle of the dip measured at right angles to the strike.
  • Apparent Dip - a dip that is measured to be less than the maximum inclination; measured when you can only see intersections between beds/strata.
  • Strike - the horizontal line on a bedding plane, measured as a bearing from north using a compass.

  
• Distinguishing betwen beds:

  • colour
  • grain size/shape
  • sorting
  • composition
  • bed thickness
• Folding

  • caused by compressional forces - folds result from the slow deformation of rocks; happens deep underground where rock are under higher temperatures/pressures
  • folds form where layered rocks have been shortened; folds common in orogenic belts (regions where lithosphere has undergone shortening as result of plate movements)
• • Fold Limb - the section of a fold between one hinge and the next, forming the side of a fold.
  • Hinge - the line along which there is a change in the amount and/or direction of dip, forming most sharply curved part of fold.
  • Axial Plane - a plane that joins the hinges of all beds (bisects the fold).
• • Axial Plane Trace - the outcrop of the axial plane at the surface.
  • Plunge - the angle of dip of the axial plane from the horizontal (where the fold hinge line is not horizontal)
  • Wavelength - the distance between the fold crests or troughs.
  • Amplitude - half the vertical distance between the crest and trough.
• Fold Keywords

  A) crest

  B) limb

  C) axial plane

  D) axis

  E) trough

  
• • Antiform - an upward closing fold (no indication of the relative age of the beds).
  • Synform - a downward closing/trough-shaped fold (no indication of the relative age of the beds).
  • A dome is an antiform (beds dip outwards in all directions), and a basin is a synform
• • Anticline - an antiform where the oldest bed is in the core and the beds get progressively younger away from the fold core.
  • Syncline - a synform where the youngest bed is in the centre and the beds get progressively older away from the fold core.

  
• As the intensity of folding increases and the angle between the fold limbs decreases the folds are decribed as:

  • Gentle - interlimb angle 180-120$^o$
  • Open - interlimb angle 120-70$^o$
  • Closed - interlimb angle 70-30$^o$
  • Tight - interlimb angle <30$^o$

  
• • Asymmetrical - the axial plane dips and limbs have different dips.
  • Symmetrical - the axial plane is vertical and limbs have same dips.
  • Isoclinal - limbs are parallel to each other.
  • Monocline - a fold with only one limb.
  • Overturned Fold - axial plane tilted beyond the vertical (with inverted limbs).
  • Recumbent Fold - folds are lying down (near horizontal axial planes).
• Faulting

  A fault is a fracture in the earth's crust along which movement has occured. These movements can be horizontal, vertical or both. Faults are planar surfaces within rocks along which movement has taken place. Faults are created by tensional, compressional or shear forces.
• Dip-Slip Faults (movement occurs vertically/parallel to strike):

  • Normal Fault - caused by tensional (pulling apart) forces resulting in crustal extension.
  • Reverse Fault - caused by compressional (pushing together) forces resulting in crustal shortening.
  • Thurst Fault - a reverse fault that dips <45$^o$.
• Strike-Slip/Tear Faults (movement occurs horizontally/parallel to dip):

  • Dextral Tear Fault - relative movement is to the right due to shear forces.
  • Sinistral Tear Fault - relative movement is to the left due to shear forces.
• • Fault Plane - a plane of fracture along which the rocks have been displaced (narrow zone of shattering along the fault).
  • Upthrow - the side of the fault where the movement is upwards in relation to the other side.
  • Downthrow - the side of the fault where the movement is downward in relation to the other side.
• • Footwall - the side of the fault below the fault plane.
  • Hanging Wall - the side of the fault that lies above the maximum inclination of the fault plane as measured from the horizontal.
  • Throw - the vertical displacement of rocks along the fault plane.
  • Heave - the horizontal displacement of rocks along the fault plane.
• Features of a Fault:

  A) normal

  B) reverse

  C) strike-slip

  
• After faulting, the landscape of the upthrown block will be higher than on the downthrown block; the upthrown block will be eroded to match the level of the downthrown block, so the rocks exposed at the surface on the downthrown block will be younger than those on upthrown block.

  • erosion of the upthrown block exposes strata at the surface previosuly at depth
  • marker beds used to establish relative displacement of rocks across a fault
• Normal Faults

  • normal faults are dip-slip faults in which the hanging wall has been downthrown (moved down relative to the footwall)
  • normal faults are caused by tension forces so relfect extension of crustal rocks + commonly dip at angles between 45-70$^o$

  
• • Step Faults - consist of a set of parallel normal faults which all downthrow in the same direction; the displaced bed appears at successively different levels.
  • Block Faults - divide the crust into blocks which may form mountains (eg. Basin-and-Range Province, Western USA).
• Horsts and Grabens

  • A horst is an upthrown block bounded by twp normal faults, eg. Vosges and Black Forest Mountains, France and Germany.
  • A graben/rift valley is a downthrown block or a long fault trough bounded by normal or step faults, eg. East African Rift Valley + Midland Valley, Scotland.

  
• Reverse Faults

  • a reverse fault is a dip-slip fault in which the hanging wall has moved up relative to the footwall, it has been upthrown
  • reverse faults are caused by compressional forces, so indicative of crustal shortening

  
• Thurst Fault

  • these are reverse faults where the angle of dip is less than 45$^o$ + thrust faults are associated with crustal shortening and continetal collision zones (common in mountaineous areas)
  • eg. Moine Thrust, NW Scotland; brings precambrian rocks over 560Ma to rest above Cambrian rocks <560Ma during Caledonian Orogeny
• Tear Faults (Strike-Slip)

  • involve lateral movements only; dextral tear faults are relative movements to the right, and sinistral tear faults are relative movements to the left
  • formed by shear forces which cause rocks to slide horizontally past each other; crust is neither shortened or extended, eg. Great Glen Fault, Scotland (sinistral fault)
• Oblique-Slip Faults involve vertical and lateral movement.
• Transform Faults

  • strike-slip faults in the ocean; end by running into and appearing to transform into other structures (oceanic ridges/trenches)
  • in oceanic ridge, the crust is pulling apart while in the trenches oceanic crust is being pushed down
  • transform faults connect structures where movement is different to that on the fault + result of different rates of movement