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SGES 1302INTRODUCTION TO EARTH SYSTEM

LECTURE 7: Geological Structures: Joints & Faults

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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?

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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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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

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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

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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

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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.

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Exfoliation joints: a set of a set of joints subparallel to the joints subparallel to the ground surface topographyground surface topography

Columnar joints: occurs in occurs in hypabassal intrusive and lava hypabassal intrusive and lava flows, where the rock is broken flows, where the rock is broken into joint-bounded columns, into joint-bounded columns, roughly hexagonal in cross sectionroughly hexagonal in cross section

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Faults

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Fault: Fractures with measurable slip Fault: Fractures with measurable slip parallel to the fracture planeparallel to the fracture plane

Fault zone – a brittle structure where loss of Fault zone – a brittle structure where loss of cohesion and slip occur on arrays of fault cohesion and slip occur on arrays of fault with definable width.with definable width.

Shear zone – a ductile structure across in Shear zone – a ductile structure across in which a rock body is mesoscopically cohesive, which a rock body is mesoscopically cohesive, displacement/strain is distributed across a band displacement/strain is distributed across a band of definable width.of definable width.

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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

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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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