Lect 3 Structural Geol

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Transcript Lect 3 Structural Geol

MECHANICS OF PLASTIC DEFORMATION
> PLASTIC DEFORMATION IS BY FLOW; THE
PROCESSES MAY BE:
1. INTERGRANULAR MOVEMENT
A) DISPLACEMENT TAKES PLACE BETWEEN
INDIVIDUAL GRAINS ALONG GLIDE PLANE
B) INDIVIDUAL CRYSTALS AND GRAINS OF ROCK
MOVE UNDER STRESS
C) INDIVIDUAL GRAINS MAINTAIN THEIR SHAPE
AND SIZE
D) EACH GRAIN MOVES & ROTATES RELATIVE TO
EACH OTHER-
D) GRANITOID ROCKS - LARGER CRYSTALS ARE
BROKEN INTO SMALLER SPHERICAL GRAINS
& ROTATE RELATIVE TO EACH
OTHER(GRANULATION)
2. INTRAGRANULAR MOVEMENT
A) DISPLACEMENT WITHIN INDIVIDUAL
CRYSTALS
B) SLIPPING TAKES PLACE ALONG GLIDE
PLANES
C) ATOMIC STRUCTURE CONTROLS POSITION
& NUMBER OF GLIDE PLANES; GLIDE PLANES
ARE RELATED TO SYMMETRY OF MINERAL.
TYPES OF GLINDING PLANES
1. TRANSLATION-GLIDING
A) LAYERS OF ATOMS SLIDE INTERATOMIC RELATIVE
TO OTHER LAYERS
B) SHAPE IS CHANGED; CRYSTAL LATTICE IS
UNCHANGED
2. TWIN-GLIDING
A) LAYERS OF ATOMS SLIDE A FRACTION IN AN
INTERATOMIC DISTANCE RELATIVE TO ADJACENT
LAYERS
B) DISPLACED PART BEARS TWINNED RELATIVE TO
UNDISPLACED PART
TRANSLATION-GLIDING. A. ARRANGEMENT OF
ATOMS BEFORE GLIDING. B. ARRANGEMENT
OF ATOMS AFTER GLIDING ALONG PLANES G1
G1 & G2 G2
TWIN-GLIDING. A. ARRANGEMENT OF ATOMS
BEFORE GLIDING. B. ARRANGEMENT OF
ATOMS AFTER GLIDING ON GLIDE PLANES
G1G1, G2G2, G3G3 AND G4G4.
C) SHEETS OF ATOMS DON’T SLIP ALONG GLIDE
PLANES IN ANY DIRECTION
GLIDE DIRECTIONS ARE GLIDING LINES OF
DIRECTION OR MOVEMENT
 RECRYSTALLIZATION – PLASTIC DEFORMATION
WITHOUT CHANGE IN SHAPE, E.G. LIMESTONE
TO MARBLE
A) NUMBER OF CRYSTALS PER UNIT VOLUME
DECREASES; SIZE OF INDIVIDUAL CRYSTALS
INCREASES
B) UNDER DIFFERENTIAL PRESSURE CONDITIONS,
SOLUTION & RECRYSTALLIZATION OF ROCK IS
SHORTHENED OR LENGTHENED
LIMESTONE TO MARBLE
RIECKE PRINCIPLE – 1) SOLUTIONS IN PORE
SPACES OF ROCKS DISSOLVE OR OCCUR MOST
READILY UNDER GREATEST EXTERNAL
PRESSURE 2) RECRYSTALLIZATION OCCURS
MOST READILY AT POINTS OF LEAST
EXTERNAL PRESSURE. RP IS APPLICABLE TO
RECRYSTALLIZATION OF METAMORPHIC
ROCKS. RECRYSTALLIZATION FACILITATES
PLASTIC DEFORMATION
> MAJOR TYPES OF DEFORMATION ARE 1)
ELASTIC, 2) PLASTIC AND 3) RUPTURE
DESCRIPTION OF FOLDS
FOLDS – WAVELIKE UNDULATION CAUSED BY
BENDING OF ROCKS USUALLY PRODUCED BY
COMPRESSIVE STRESSES; A BEND IN THE
ROCK LAYERS
 ATTITUDE OF BEDS
• STRIKE – TREND OR DIRECTION OF STRATA
OR BEARING OF THE HORIZONTAL LINE ON
THE PLANE OR DIRECTION OF A LINE
FORMED BY THE INTERSECTION OF THE
BEDDING AND A HORIZONTAL PLANE.
2) Dip- angle between the bedding & a
horizontal plane
PARTS OF FOLDS
• AXIAL PLANE(SURFACE) – PLANE THAT
DEVIDES FOLD SYMMETRICALLY, USUALLY
BISECTS THE LIMBS
• LIMBS(FLANKS) – SIDES OF THE FOLDS
• FOLD AXIS – LINE OF MAXIMUM OF THE
FOLD
• CREST – LINE ALONG THE HIGHEST PART OF
THE FOLD
• CRESTAL PLANE – SURFACE FORMED BY THE
CRESTS
TROUGH – LINE OCCUPYING THE LOWEST
PARTS OF THE FOLD
• TROUGH PLANED – PLANE CONNECTING
TROUGH LINES
• HINGE LINE – CONNECTS POINTS OF
MAXIMUM FOLD CURVATURE
• INFLECTION POINT – LIMB POINT WHERE
SENSE OF CURVATURE CHANGES
•
> PARTS OF FOLD (DRAW DIAGRAMS) Page 36
> NOMENCLATURE OF FOLDS
FOUR GENERAL TYPES OF FOLDS
1. ANTICLINE – LIMBS DIPPING AWAY FROM
EACH OTHER & AWAY FROM THE FOLD AXIS
2. SYNCLINE – LIMBS DIPPING TOWARDS EACH
OTHER & TOWARD THE FOLD AXIS
3. DOME – CIRCULAR OR ELLIPTICAL CONVEX
FOLD WITH LIMBS DIPPING TOWARD AWAY
FROM THE CENTER
4. BASIN – CIRCULAR OR ELLIPTICAL CONCAVE
FOLD WITH LIMBS DIPPING TOWARD THE
CENTER
> ANTICLINE & SYNCLINE CAN BE:
1) SYMMETRIC – LIMBS DIP IN OPPOSITE
DIRECTIONS AT EQUAL ANGLE & AXIAL PLANE
IS VERTICAL
2) ASSYMETRIC – LIMBS DIP IN OPPOSITE
DIRECTIONS AT DIFFERENT ANGLES & AXIAL
PLANE IS INCLINED
3) OVERTURNED - LIMBS DIP IN THE SAME
DIRECTION AT DIFFERENT ANGLES & THE
AXIAL PLANE IS INCLINED
4) RECUMBENT - OVERTURNED FOLD WITH
HORIZONTAL LIMBS AND AXIAL PLANE
5) PLUNGING – FOLD AXIS IS PLUNGING AT AN
ANGLE RATHER THAN BEING HORIZONTAL
OTHER VARIETIES OF FOLDS WHICH MAY
BELONG TO ANY OF THE FOUR TYPES OF
FOLDS
1. ISOCLINAL – TWO LIMBS OF FOLD DIP AT
EQUAL ANGLES IN THE SAME DIRECTION
2. FAN FOLD – BOTH LIMBS ARE OVERTURNED
3. CHEVREON FOLD – AXES ARE SHARP &
ANGULAR
4. MONOCLINE – BEDS INCLINED IN A SINGLE
DIRECTION
5. STRUCTURAL TERRACE – DIPPING STRATA
ASSUME A HORIZONTAL ATTITUDE
6. HOMOCLINE (Gk, “SINGLE INCLINATION) –
STRATA DIPPING IN ONE DIRECTION AT A
UNIFORM ANGLE
7. CLOSE OR TIGHT FOLD – DEFORMATION IS
INTENSE TO CAUSE FLOWAGE OF MORE
MOBILE BEDS
8. OPEN FOLD – FLOWAGE DOES NOT TAKE
PLACE
DRAG FOLD – MINOR FOLDS ARE FORMED WHEN A
COMPETENT BED SLIDES PAST THE INCOMPETENT
BED
 STRCTURAL CONTROLS ON MINERALIZATION
• STRUCTURAL FEATURES (FAULTS, FRACTURES,
FOLDS) IN ROCKS CAN INFLUENCE ORE DEPOSITION
1) HEAT & PRESSURE - FORCES THAT CAN CAUSE
DEFORMATION OF ROCKS :
1.1 FOLDING – BENDING OF ROCKS
1.2 FAULTING – FRACTURING AND DISPLACEMENT
1.3 SHEARING – SLIDING PARALLEL TO THE PLANE
OF CONTACT BETWEEN TWO ROCKS
1.4 COMPRESSION – COLLIDING TOGETHER
OF TWO ROCKS
1.5 EXTENSION- SEPARATING OR
INCREASING THE DISTANCE BETWEEN
TWO ROCKS
HYDROTHERMAL DEPOSITS – HYDROTHERMAL
FLUIDS CIRCULATE & LOCALIZE THE DEPOSITS
ALONG THE STRUCTURES.
• PRE-MINERAL STRUCTURES – REFER TO
STRUCTURES FORMED PRIOR TO
MINERALIZATION
• POST MINERAL STRUCTURES – REFER TO
STRUCTURES FORMED AFTER
MINERALIZATION EVENTS
Figure 10 – 6. Fracture systems in rocks overlying an igneous
intrusion. A & B: radial fractures above a circular intrusion. C &
D: longitudinal fractures above an elliptical intrusion (from Emmons,
1937).
Figure 10 – 7. Cross section of Ft. Knox granite-hosted gold deposit,
Fairbanks District, Alaska, showing late-stage shear zones containing high
grade gold mineralization ( 1.0 ounce per ton) (after Bakke, 1991).