Download STRUCTURAL GEOL PPT(2010).

STRUCTURAL GEOLOGY
PRIMARY & NON-TECTONIC STRUCTURES
1) PRINCIPLE OF ORIGINAL HORIZONTALITY
(NICOLAS STENO 1638-1686) – LAYERS OF
SEDIMENTS ARE ORIGINALLY DEPOSITED
HORIZONTALLY.
• THIS PRINCIPLE IS IMPORTANT TO ANALYSIS OF
FOLDED & TILTED STRATA.
• FIELD OBSERVATIONS REVEAL THAT EARTH IS
DYNAMIC THAT LEADS TO SCIENCE OF PLATE
TECTONICS; THAT PLATE MOVEMENT CAUSES
ROCK DEFORMATION
• SEDIMENTS FORMED IN OCEAN & DEPOSITED
IN HORIZONTAL LAYERS
• NOT ALL SEDIMENTARY LAYERS ARE
DEPOSITED HORIZONTALLY
• COARSE SANDS MAY BE DEPOSITED AT 150
ANGLE CALLED ANGLE OF REPOSE, EXAMPLE IS
SAND DUNES
• SEDIMENTS MAY COVER OVER PRE-EXISTING
INCLINED SURFACE & DEPOSITED
UNCOMFORMABLY TO OLDER
SEDIMENTARIES
• ALTHOUGH NOT UNIVERSALLY APPLICABLE,
POH IS USEFUL IN STUDY OF SEDIMENTOLOGY,
STRUCTURAL GEOLOGY & STRATIGRAPHY
2. PRINCIPLE OF SUPERPOSITION (NICOLAS STENO) SEDIMENTARY LAYERS ARE DEPOSITED IN A TIME
SEQUENCE WITH THE OLDEST AT THE BOTTOM
AND THE YOUNGEST AT THE TOP
• POS APPLIES TO VOLCANIC ROCKS WHICH FLOW
ON OLDER ROCKS BY FLOW BANDING
3. THE PRINCIPLE OF LATERAL CONTINUITY –
SEDIMENTARY LAYERS EXTEND LATERALLY IN ALL
DIRECTIONS
• COULD BE SEPARATED BY A VALLEY OR BY
EROSIONAL FEATURE
3. PRINCIPLE OF CROSS-CUTTING RELATIONSHIP –
THE GEOLOGIC FEATURE WHICH CUTS ANOTHER IS
YOUNGER OF THE TWO FEATURES.
DETERMINATION OF RELATIVE AGES OF STRATA &
OTHER GEOLOGIC STRUCTURES
A – FOLDED ROCK STRATA CUT BY THRUST FAULT
B - LARGE INTRUSION CUTTING THROUGH A
C - EROSIONAL ANGULAR UNCONFORMITY CUTTING OFF
A & B ON WHICH ROCK STRATA WERE DEPOSITED
D – VOLCANIC DIKE CUTTING THROUGH A, B & C
E – EVEN YOUNGER ROCK STRATA OVERLYING C & D
F – NORMAL FAULT CUTTING THROUGH A, B,C &
E
4. PRINCIPLE OF FAUNAL SUCCESSION – SEDIMENTARY
ROCK STRATA CONTAIN FOSSILIZED FLORA & FAUNA
AND THAT THESE FOSSILS SUCCEED EACH OTHER
VERTICALLY IN A SPECIFIC, RELIABLE ORDER THAT CAN
BE IDENTIFIED OVER WIDE HORIZONTAL DISTANCES
ROCK DEFORMATION
DEFORMATION REFERS TO CHANGES IN VOLUME,
SHAPE, POSITION OR IN COMBINATION OR ROCK
UNDER STRESS
STRESS – APPLIED FORCE ACTING ON THE ROCK
STRAIN – RESPONSE OF A ROCK UNDER STRESS
WHICH INVOLVES CHANGE OF VOLUME OR SHAPE
OF THE ROCK – RESULTING DEFORMATION
TYPE OF STRESS:
1. LITHOSTATIC (CONFINING) – STRESS
IN ALL DIRECTION
5. DIFFERENTIAL (DEVIATORIC) – UNEQUAL STRESS
DUE TO TECTONIC FORCES. THERE ARE THREE(3)
BASIC KINDS:
5.1 TENSIONAL(EXTENSIONAL) – FORCES ACT IN
OPPOSITE DIRECTION; ROCK STRETCHES
5.2 COMPRESSIONAL - FORCES ACT TOWARDS
EACH OTHER; SQUEEZES ROCK
5.3 SHEAR – SIDE TO SIDE SHEARING RESULTS TO
SLIPPAGE & TRANSLATION
THREE KINDS OF DIFFERENTIAL STRESS
STRAIN – ROCK DEFORMATION IN RESPONSE TO
STRESS
 CONTROL FACTORS TO ROCK DEFORMATION
DEPENDS ON : 1) PRESSURE &
TEMPERATURE(DEPTH IN EARTH) , 2) ROCK
MATERIAL - MINERAL COMPOSITION OF ROCK
3) VOLATILE CONTENTS, 4) TIME
TYPES OF STRAIN/STAGES OF DEFORMATION
1) ELASTIC DEFORMATION – CHANGES IN
SHAPE OF ROCK IS REVERSIBLE.
2) PLASTIC DEFORMATION – CHANGES IN SHAPE
OF ROCK ARE PERMANENT & NOT REVERSIBLE
3. FRACTURE – ROCK BREAKS
ROCK BEHAVIOR
1) BRITTLE – ROCK BREAKS,
2) DUCTILE – ROCK FLOWS OR BENDS
ONCE ELASTIC LIMIT IS SUPASSED, ROCK WILL
DEFORM PLASTICALLY IF ROCK IS DUCTILE OR
ROCK WILL FRACTURE(RUPTURE) IF ROCK IS
BRITTLE
STRESS - STRAIN
MECHANISM DUCTILE DEFORMATION OCCURS
DEPENDING PRINCIPALLY ON TEMPERATURE
1. PLASTIC FLOW OCCURS AT RELATIVELY LOW
TEMPERATURE IN THE LOWER CRUST &
UPPERMOST MANTLE. ROCK DEFORMS BY
GRADUAL CREEP ALONG CRYSTAL GRAIN
BOUNDARIES & PLANES WITHIN CRYSTAL
LATTICES
2. POWER FLOW CREEP OCCURS WHERE
TEMPERATURE IS GREATER THAN 55% OF THE
MELTING TEMPERATURE FOR THAT
DEPTH(PRESSURE). MOVEMENT OCCURS ALONG
CRYSTALLOGRAPHIC PLANES & GRAIN BOUNDARY
3. DIFFUSION CREEP OCCURS WHERE
TEMPERATURE IS GREATER THAN 85% OF THE
MELTING TEMPERATURE FOR THAT
DEPTH(PRESSURE). DEFORMATION/FLOW IS
LARGELY BY IONIC/ATOMIC DIFFUSION &
CONTINUOUS RECRYSTALLIZATION