1 SGES 1302 INTRODUCTION TO EARTH SYSTEM LECTURE 7: Geological Structures: Joints & Faults

2 Lecture 7: Geological Structures What will happen, if you drop  a piece of glass,  a rubber ball,  a piece of plasticine  a spoon water Why they behave the way they do?

3 Geological Structures When rocks are subjected to forces/stresses greater than their strength, they begin to deform. Deformation: all changes in the original shape and/or size of a rock body. Most deformation of rocks occurs at plate boundaries. The way a rock body behave during deformation depends on the rheology of the rock, which in turn controlled by the condition of deformation (P, T, time). Elastic – recoverable (below yield strength) Brittle – breaks / fractures (low P, T; fast) Ductile – bend / flow (high P, T; slow)

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6 BRITTLE & DUCTILE DEFORMATION Brittle Deformation (break)  Permanent deformation of a solid due to development of fractures. Brittle Structures include:  Fracture- discontinuity surface.  Vein- fracture in which minerals have precipitated from solution.  Dike- fracture filled by a rock intrusion  Joint- fracture with no measurable shear displacement  Fault- fracture surface with measurable displacement Ductile Deformation (bend)  Permanent deformation of a solid by bending, folding or flowing (without breakage)  Achieved through grain boundary sliding, kinking, dissolution, crystal plasticity Ductile Structures include:  Fold, foliation, lineation Shear Zones- mixed brittle/ductile deformation

7 Faults and Joints Joints and faults are the most common geological structure, found in all rock types Plane/crack across where there is no cohesion Results of brittle deformation FAULT: presence of displacement across fracture plane, mainly parallel to the fracture plane JOINT: no or negligible displacement parallel to the fracture

8 Displacement Modes of fracture surface displacement: 3 basic fracture modes representing “end members” can describe any combination of joint and shear fracture  Mode I: Tensile cracks - Fractures open slightly in direction perpendicular to crack surface  Mode II: Shear fractures (sliding) - Shear by horizontal sliding, rocks on one side of crack surface move slightly parallel to fracture surface, perpendicular to the fracture front  Mode III: Shear fractures (tearing) – move parallel to the crack in a direction parallel to the fracture front

9 Joints Most common geological structure, present in almost all rock mass. Important in controlling landscape morphology – preferential erosion along joints or zones with intense jointing. Also influence strength and permeability of a rock mass. Often occur in sets, giving blocky appearance to rock mass. Systematic Joints: Group of joints that are parallel or subparallel to one another, and maintain roughly the same average spacing over the region of observation. Nonsystematic Joints: Have an irregular spatial distribution, not parallel to neighbouring joints and often non planar.

10 a set of joints subparallel to the ground surface topography Exfoliation joints: a set of joints subparallel to the ground surface topography occurs in hypabassal intrusive and lava flows, where the rock is broken into joint-bounded columns, roughly hexagonal in cross section Columnar joints: occurs in hypabassal intrusive and lava flows, where the rock is broken into joint-bounded columns, roughly hexagonal in cross section

11 Faults

12 Fault: Fractures with measurable slip parallel to the fracture plane Fault zone – a brittle structure where loss of cohesion and slip occur on arrays of fault with definable width. Shear zone – a ductile structure across in which a rock body is mesoscopically cohesive, displacement/strain is distributed across a band of definable width.

13 Classification of Faults Faults are classified based on displacement or slip Dip-Slip Faults  Normal faults  Reverse faults  Thrust faults Strike-Slip Faults  Left-lateral strike-slip faults  Right-lateral strike-slip faults Oblique-Slip Faults Fault motion provide information on the nature of the forces at work within the Earth

14 Hangingwall: block above a (non vertical) fault Footwall: block below a (non vertical) fault Strike slip / transcurrent fault: displacement parallel to strike  sinistral / left lateral – opposite block moves to the left  dextral / right lateral – opposite block moves to the right Dip slip fault: displacement parallel to dip  normal fault – hangingwall moves down  reverse fault – footwall moves down  thrust fault – reverse fault with dip of fault <45°  low angle normal fault – normal fault dipping < 45° Oblique slip fault: has both strike and dip displacements

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