STRUCTURAL GEOLOGY HATCHER 10 Flashcards
Rupture does not occur on the planes of maximum shear that bisect the angles between he greatest and least principal stresses.
Theory of Shear Fracture
Maximum stress bisecting the acute angle between two conjugate shear planes of stress of the stress elipsoid
Hartmans Rule
Max and Inter principal stresses are oriented horizontally and the minimum oriented vertically and is on a “State of Compression” shear planes are lying 45 deg or less
Thrust Fault
How can thrust faulting be achieved in two ways?
1) Increase the radius of the circles until failure envlope is reached
2) decresae Vertical Min Stress by erosion
Shear planes corresponds to
Fault planes
Max and Min Principal stress are horizontal while intermediate is vertical lying 45 deg or less
Strike Slip
Involves extension on one horizontal axis (X or Y) with the maimum principal stress vertical
Normal Fault
angle between shear planes at a normal fault
45 deg or less than the axes of Max and Min
Effective Normal Stress (S) is equal to
Normal Stress - Fluid Pressure
States that the Tangential force parallel (Shear Force) to a farcture surface necessary to initiate slip is directly proportional to the force normal to the fracture surface
Amonton’s First Law
The frictional resistance of motion is Independent of the contact area
Amonton’s Second Law
whats the effect of Fluid Pressure to Shear Stress
Decreases thus initiating slip
involves Sudden movement on the fault after long term accumulation of stress and is accompanied by elastic rebound which causes the earthquake
Stick-Slip (Unstable Frictional Sliding)
Involves uninterrupted motion along a fault so that stress is relieved continuously and does not accumulate
Stable Sliding (Continuous Creep)
Microscopic and Megascopic irregularities and imperfections on fault surfaces
Asperities
When will shear heating be significant?
When motion >1cm/ yr
shear >50MPa
Evidence of Shear Heating
1) Pseudotachylite - dark vein fillings of glass like material frome by brittlle failure and sudden release of elastic energy and friction generated heat
2) Metamorphic Aureoles in subduction zones
Depth of Brittle Ductile Transition
10-15km
What types of faults have both Birttle and Ductile Zones?
Very large faults that penetrate the entire crust
Zones of weakness and represent localized strain softening which can be caracterized by Zone of closely spaced interleaving anastamosing brittle faults and crushed rocks (Cataclasites) near the surface or zone of ductile faults and associated mylonitic rocks at hreat dept
Shear Zones
Strain softening in Ductile zones are characterized by the formation of
Mylonites
Shear Sense Indicators are used to determine shear sense in
Zones of Ductile Shearing
Large grains that have grown in the rock mass during deformation and metamorphism
Porphyroblast
Relict of earlier larhe grains that have survived shearig
Pophyroclast
Wedge shaped winged/tailed inclusion that DO NOT CROSS THE REFERENCE PLANE tracing tail away from grains
Sigma type (Nangiiwan)
Winged or Tailed inclusion where in tails wrap around tha grain so they CROSS THE REFERENCE PLANE
Delta Type (Yumayakap)
If the rotation is clockwise the shear sense is
Dextral
Inclusions without tails
Theta Type
SAUSAGE SHAPED features formed by EXTENSION of A STRONG LAYER in a more DUCTILE GROUND MASS
Boudins
Mineral lineations or foliation composed of quartz muscovite chlorite magnetite or other mineral that forms in extensional strain field on either side of a large crystals and are commonly oriented parallel to the prominent foliation
Pressure shadows
large, lenticular eye-shaped mineral grains or mineral aggregates visible in some foliated metamorphic rocks
Augen
Parallel alignment of plat minerals or bands in a metamorphic rock of sevaral types develop in shear zones as products of proregssive simple shear
S-C Surface
A progresive shearing-induced foliation which forms at an angle of 18-25 deg to the dominant foliation surface (eto ung horizontal lines)
C-Surface
C means
Cissaillement meaning shear
Exitsing Dominant Foliation parallel to the walls so vertical ito na nadform of the shear zone (Eto ung diagonal)
S-Surface
Mica fish is what type of S-C fabric?
Type II S-C (C-Mica, S-Qtz)
S-C surfaces which originate by aparrent extension and rotation as the shear zone moves and are structures resembling INTRAFOLIAL Folds appera between cveniently oriented active S-Surfaces inclines opposite to the shear direction and C-Surface Orientation
Extensional Crenulation Cleavage
Form early in the movement history of a brittle shear zone and then are either destroyed or ceased movements
En enchelon shear farctures formed at 10-15 deg to the principal shear and have the same displacement sense as the primary shear fractures
Riedel Shear
Forms at high angle with opposite movement
Antiriedel shear
