Structural Geology USTP Team
By Ali Maher Geophysicist
Contents Structure Geology Types of Structures Primary Structures Sedimentary Rocks Igneous Rocks Deformation Secondary Structures Cleavage Joints Fault Fold Conclusion
Structure Geology
Structural Geology Structural Geology • Structure comes from latin; struere (to build). • It is the study of deformed rocks. • Structures are classified according to the affecting forces, into: a- Tectonic; due to stresses from plate tectonics. b- Non-Tectonic; due to forces that aren’t directly linked to plate tectonics (Gravity).
• It deals with the origin, geometry & kinematics of structures of formation.
Structural Analysis Structural Analysis (Levels of Structural study) 1. Geometric Analysis; the description of rocks in 3D (present). 2. Dynamic/Mechanical Analysis; (forces, stresses & mechanics). 3. Kinematic Analysis; (how rock particles re-arrange themselves from the initial position).
Types of Structures
Types of Structures Types of Structures
A) Primary Structures - Formed during or shortly after deposition (Sedimentary) or formation (Igneous) of rock & absent in Metamorphic R.
B) Secondary Structures - Formed after its host rock is formed (Found in all types of rocks).
in Sediments before it become Sedimentary R. in Lava or Magma before it become Igneous R. absent in Metamorphic R., because it originated from either Sedimentary or Igneous.
ď ą Sometimes distinguishing Primary & Secondary Structures are difficult.
Cleavage. (Met. R.) Joint. Fault. Fold.
Primary Structures
Primary Structures of Sedimentary R. A) Primary Structures of Sedimentary Rocks 1.Stratification/Bedding:[the structural unit of stratified rocks] • Strata; [Strata < Laminae (<1cm)] • Bedding; is a surface in a sedimentary rock (Upper & Lower) separates the overlying and underlying beds with different (Composition – Texture – Color – Grain Size – Cement – etc.). • Different beds represent different (Source – Sedimentary Processes – Environment of Deposition). 2.Graded-Bedding:• Progressive Fining upward. • Ex: Turbidities.
upward
or Coarsening
UB
LB
Primary Structures of Sedimentary R. A) Primary Structures of Sedimentary Rocks 3.Cross-Bedding:- (False Bedding/ Current Bedding) â&#x20AC;˘ Oblique surfaces within the master bed, which caused by wind or water current. 1) Ripple Marks; Formed by the current over the unconsolidated surface layers. a. Wind or Water (Costal); Asymmetric ripple marks (Planar). b. Deep Water (Wave Base); Symmetric ripple marks (Trough). 2) Aeolian Cross-Bedding; (Sand Dunes) Occurs in places which affected by changing in wind direction. 3) Coastal Cross-Bedding; Occurs in places which affected by changing in current of water direction.
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a
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Primary Structures of Sedimentary R. A) Primary Structures of Sedimentary Rocks 3.Cross-Bedding:- (False Bedding/ Current Bedding) 4) Torrential Cross-Bedding; (Tropical/Alluvial fans) Occurs in places which affected by heavy rains in winter (Coarse grains) & weak in summer (Fine grains). 5) Delta Structure; Formed in the transitional zone between land & sea, by river sediments deposited in the sea. Consists of; a. Top Set. 5 b. Foreset. c. Bottom set.
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fine coarse
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Primary Structures of Sedimentary R. A) Primary Structures of Sedimentary Rocks
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4.Lensing of Strata:• A) Flaser Bedding; mostly sand ripples. • B) Wavy Bedding; mixture of sand ripples & mud. • C) Lenticular Bedding; mostly mud.
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5.Mud Cracks:• Occurs in the mud after dehydration by sunset. • Clays has the ability to associate with 10 molecules of H2O & also lose it after dehydration by sunset, which caused the formation of Mud Cracks. 6.Consolidation:- (Compaction or Cementation) Unconsolidated Consolidated [Mud – Sand – Gravels] [Mudstone – Sand Stone – Conglomerate] • Consolidation decreases the porosity of the original material. • Compaction, is affected by gravity & produced a decrease in the volume (Shales are formed by the compaction process). • Cementation, doesn't change the volume but increases the specific gravity & strengthens the rock.
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Primary Structures of Sedimentary R. A) Primary Structures of Sedimentary Rocks
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7.Debris Flow:• Collection of sediments with varied grain sizes without sorting, because of an initial dip for beds. • The angle of initial dip (Sand < Clay).
8.Contact:Boundary between two geologic units of any kind, it may be: 1. Conformable; a boundary between adjacent beds or units which doesn’t represent a gap in time. 2. Unconformable; a boundary between beds which represents stopping in deposition (erosion or non-deposition) of strata. [gap in time] Identifying Unconformity: a) Basal Conglomerates. b) Topographic relief. c) Paleosols, (recognized by color change & soil structures).
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Primary Structures of Sedimentary R. A) Primary Structures of Sedimentary Rocks
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Types of Unconformity:
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1. Disconformity/Parallel Unconformity [it causes the presence of conglomerate deposition] 2. Angular Unconformity; there is an angle between the rocks. 3. Non-Conformity; between two types of rocks (Igneous & Sedimentary). 4. Para-Conformity; has a simple separation bedding plane with no erosional surface. 5. Blended Unconformity; has no distinct separation plane or contact & may be consists of soils or pebbles derived from the underlying rock. Para-Conformity & Blended Unconformity are distinguished only by Fossils. [Hiatus/gap in time]
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Evidences of Unconformity: a) Structural; such as Angular Unconformity. b) Sedimentologic; Disconformity & Non-Conformity. c) Paleontologic; Para-Conformity & Blended Unconformity.
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Primary Structures of Igneous R. B) Primary Structures of Igneous Rocks 1.Structure due to Flow:Igneous rocks represent the cooled products of magmatic melt, which rise from within the earth’s crust & flow to form structures. 1) Linear Flow Structure. 2) Platy Flow Structures (Schlieren/Flow Layer). Leucocratic, colored minerals (Muscovite). Melanocratic, dark minerals (Biotite). 3) Flow Wrinkles. 4) Pillow Lava.
2.Structure due to Fracture:1) Columnar Joints; fractures formed in basic rocks due to cooling and shrinkage, such as Basalt.
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Deformation
Deformation Deformation Deformation common at plate margins, which may be a change in the original location, orientation, shape or volume which result from stresses that exceed rock strength.
Rocks & Deformation Types Rocks will deform in different ways, depending upon
the properties of the rock & the nature of the applied stress. 1.Elastic Rock; [Non-Permanent Deformation] 2.Plastic Rock; [Permanent Deformation] 3.Ductile Rock; [Continuous Deformation] 4.Brittle/Rupture Rock; [Discontinuous Deformation]
Deformation (Force) Deformation Concepts 1. Force.
2. Stress.
3. Strain.
Force A property or action that changes or tends to change the state of rest or velocity or direction of an object in a straight line. Velocity ; change in distance per time. V = L/T = LT-1 (ms-1) Acceleration; change in velocity per time. (due to gravity) g = LT-1/T = LT-2 (ms-2) Force; mass X acceleration. F = Mg = MLT-2 (N=Kg.ms-2)
Force Types 1. Body Forces; act on every point within the body. Always present. Due to gravity or inertia. Its magnitude are proportional to the mass of the body.
2. Surface Forces; act on a specific surface area in a body. Reflect pull or push of the atoms on one side of a surface against the atoms on the other side. Its magnitude are proportional to the magnitude of the area.
Deformation (Force) Factors controlling Rock Strength & Deformation style 1. Temperature and Confining Pressure; - Low T. & P. = Brittle Deformation. - High T. & P. = Ductile Deformation. 2. Rock Type; mineral composition controls strength. 3. Time; Stress applied for a long time generates change. 4. Depth of rocks.
- Fluid or Gas In the pores of a rock counteract the effects (decrease the magnitude) of burial and tectonic stresses. * Effective Stress (σe) = burial & tectonic stress – (pore pressure + vertical stress).
Porepressure depth
Pore Pressure & Effective Stresses
σ1 Porepressure
σ2 Porepressure
σ3
Porepressure
Deformation (Stress) Deformation Concepts 1. Force.
2. Stress.
3. Strain.
Stress A force applied over an area (determines the concentration of force). [σ = F/A] Uniform Stress (Confining Stress) [Stress equal from all directions, referred to as; Pressure]
Differential Stress [Stress isn’t equal from all directions]
Normal Stress (σn) [Components of Stress _|_ Plane Surface]
Shear Stress (σs) [Components of Stress // Plane Surface] - Slippage of one rock mass past another. - Leads to change in shape.
Tensional Stress (Extension) * Pull-apart stress (away from a plane) * Stretches rock (lengthening) * May increase volume * Thins Crust.
Compressional Stress * Push-together stress (toward a plane) * Squeezes rock (shortening) * May decrease volume * Thickens Crust.
Deformation (Strain) Deformation Concepts 1. Force.
2. Stress.
3. Strain.
Strain Change in the shape or size of a rock body caused by stress (Strain occurs when stresses exceed rock strength). When a rock is subjected to increasing stress, it passes through three successive stages of deformation. * Stages of Deformation: 1. Elastic Deformation; where the strain is reversible. 2. Ductile/Plastic Deformation; where the strain is irreversible. 3. Fracture/Brittle Deformation; irreversible strain where the material breaks.
Components of Strain (Types of Deformation Mechanism) Kinematic analysis includes four components of deformation. 1. Rigid Body Deformation; a) Translation (Change in Position). b) Rotation (Change in Orientation). 2. Non-Rigid Deformation; a) Distortion (Change in Shape). b) Dilation (Change in Size/Volume).
Deformation (Strain) Deformation Concepts 1. Force.
2. Stress.
3. Strain.
Strain Types of Strain
Homogeneous - Produces the same distortion everywhere. - Straight lines stay straight. - Parallel lines stay parallel.
Continuous (No Break) * Such as; Ductile Deformation.
Heterogeneous - Produces a varied distortion from place to place. - Straight lines donâ&#x20AC;&#x2122;t stay straight. - Parallel lines donâ&#x20AC;&#x2122;t stay parallel.
Discontinuous (Break) * Such as; Brittle Deformation.
Stress-Strain Relationship Typical Stress-Strain Curves for Experimentally Deformed Rocks “elastic” part of curve; if load is removed, all deformation is recovered
abrupt failure at yield point or elastic limit by macroscopic faulting “brittle” behavior. strongest (most competent) rock because it supports the greatest load before permanent deformation
Elastic limit or yield point is the Stress at rupture
“plastic” part of curve; even if load is removed, rock remains permanently deformed (“ductile” behavior)
differential stress (σ1 – σ3)
weakest (incompetent) rock because it supports the smallest load before permanent deformation
strain
Strike & Dip Sedimentary strata and lava flows generally flows the law of horizontality, so for inclined strata we have to define two terms: 1. Strike (trend); The compass direction of any horizontal line on the plane. 2. Dip (inclination); The line of angle between a horizontal plane and the inclined plane.
Strike _|_ Dip A – Strike. B – True Dip Direction. C – True Dip Angle. D – Apparent Dip Direction. E – Apparent Dip Angle.
Secondary Structures
Cleavage of Rocks Cleavage of Rocks Is the ability of rocks (Metamorphic) to break along parallel surfaces of secondary origin & it may be inclined to bedding or parallel to bedding. [Rock Cleavage differs from Mineral Cleavage] Cleavage may be considered as evidence that major folding of the rock has occurred.
Schist ď&#x192;&#x2DC; Cleavage could be classified into: 1. Flow Cleavage; (Zone of Full Flow) It is due to parallel orientation of platy minerals, such as Mica or of elongate minerals, such as Hornblende. 2. Fracture Cleavage; (Zone of Intermediate Flow) It is a true rupture & essentially a closely spaced jointing, with no visible displacement takes place along the planes of rupture.
Joints of Rocks Joints fracture without displacement.
Joints may be: 1. Systematic Joints; straight & form parallel sets or systems. 2. Non-Systematic Joints; irregular & curved, usually terminate on systematic joints.
Three fundamental Fracture (Joints)
modes: 1. Mode I (Tensile/Opening); Crack opening _|_ Fracture Surface/Plane. 2. Mode II (Sliding); Sliding in direction // Fracture Surface/Plane. 3. Mode III (Tearing); Scissoring // Fracture Surface/Plane.
Joints form under: 1) Shallow depth with low confining Pressure (PC). 2) High Pore fluid Pressure (PF). 3) Low Temperature (T). 4) Elastic regime.
Joints of Rocks Joints Definitions:
Sheet Joints; parallel to Topography, found in plutonic rocks that are already exposed. Columnar Joints; found in tabular extrusive igneous rocks. Joint Plane; a fracture surface of a block. Joint Arrays; group of joints. Joint Set; group of systematic joints (planes of one group). Joint System; two or more of related sets of joints. Joint Spacing; average distance between neighbor joints. Joint Ribs; represent positions of joints in past. Conjugate Joints; two perpendicular sets of joints. Strike Joints; strike // strike of the bedding. Dip Joints; strike // structure dip. Bedding Joints; parallel to the bedding.
Non-Systematic
Faults Faults Fracture with displacement in Rocks, as a result of Tensional Stress.
Fault Components I – Foot Wall (Up Thrown). II – Hanging Wall (Down Thrown). cb – Dip Slip. db – Strike Slip. ab – Net Slip (Rake). ae – Vertical Throw. ed – Heave.
1. Foot Wall (Up Thrown). 2. Hanging Wall (Down Thrown). 3. Foot Wall (Down Thrown). 4. Hanging Wall (Up Thrown).
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Faults Types of Faults
Strike-Slip (Horizontal displacement) Lateral = Tear = Shear
Dip-Slip (Vertical displacement) Types (Simple)
Left-Lateral (Sinistral)
Normal = Gravitational = Tensional
Right-Lateral (Dextral)
Normal > 450 Dip
Reverse = Compressional Thrust = Reverse < 450 Dip
Oblique-Slip (Horizontal + Vertical dis.)
Forms (Complex) TransTension
Step Grabens Half-Grabens Horst Listric Growth Relay Ramp Ring Radial Rotational = Bivotal = Scissors = Hybrid
TransPression
Left-Lateral Normal
Left-Lateral Reverse
Right-Lateral Normal
Right-Lateral Reverse
Faults Strike-Slip Faults (Horizontal displacement)
Left-Lateral (Sinistral)
Right-Lateral (Dextral)
Faults Dip-Slip Faults (Vertical displacement)
Types (Simple) Normal = Gravitational = Tensional Normal > 450 Dip Reverse = Compressional Thrust = Reverse < 450 Dip
Forms (Complex) Step Grabens Half-Grabens Horst Listric Growth Relay Ramp Ring Radial Rotational = Bivotal = Scissors = Hybrid
Faults Oblique-Slip Faults (Horizontal + Vertical dis.)
TransTension
Left-Lateral Normal
Right-Lateral Normal
TransPression
Left-Lateral Reverse
Right-Lateral Reverse
Folds Folds Rocks are bent or curved as a result of Compressional Stress.
Fold Components
Folds Types of Folds
Structure Styles
Conclusion
Conclusion o o o o o
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Structural Geology; Study of deformed rocks. Deformation Types; Elastic, Plastic, Ductile & Brittle. Deformation Concepts; Force – Stress – Strain. Types of Structure; 1.Primary Structures (Sedimentary – Igneous). 2.Secondary Structures (all types of rocks). Primary Structures of Sedimentary Rocks: 1) Stratification/Bedding 2) Graded Bedding. 3) Cross Bedding. 4) Lensing of Strata. 5) Mud Cracks. 6) Consolidation. 7) Debris flow. 8) Contacts. Primary Structures of Igneous Rocks: 1) Due to Flow (Linear – Platy – Wrinkles – Pillow Lava). 2) Due to Fracture (Columnar Joints). Secondary Structures: 1) Cleavage. 2) Joints. 3) Faults. 4) Folds. Types of Faults; 1. Strike-Slip Fault. 2. Dip-Slip Fault. 3. Oblique-Slip Fault.