7/28/2019 Geo L13

http://slidepdf.com/reader/full/geo-l13 1/5

Lecture 13: Structural Geology

Chap. 8 p. 251-269;

DEFORMATION

•Deformation: alteration of size and/or shape•Structural geology: Earth science discipline studying

- The processes responsible for the deformation of the Earth’s crust

- The geological structures produced by deformation: faults, joints, foldsSTRESS AND STRAIN

•Stress (σ): Force applied per unit area [N/m2] σ = force/area

• Normal stress: component of stress perpendicular to a given plane- Compressional: to shorten a body

- Tensional: to pull apart a body

- Shear: component of stress applied parallel to a given plane

•Strain (ε): Change in the shape and/or size of a body as a result of stress [dimensionless]- Elastic and Plastic deformation ε = ∆L/L 

- Elastic deformation: returns to original shape

•Rocks typically behave as combination of ideal materials

•Some rocks have high modulus (strong) while others have a low modulus (weak)•Some rocks will exhibit elastic deformation if the stress is small or over a short time period

•Some rocks deform plastically AFTER observing other types of deformation

STRENGTH OF THE DIFFERENT ROCK 

TYPES 

•Igneous rocks generally strong

- Especially plutonic rocks due tolarge, interlocking crystals

•Sedimentary rocks vary

- Salt, mudstones weak 

- Quartz-rich sandstones strong

•Metamorphic rocks vary- Quartizites strong

- Schists weak due thin layering

•At shallow depth (low pressure)

- Rocks behave elastically to elastic limit before brittle failure

- Forces primarily vertical, weight of overlying materials•Middle to lower crust (higher pressure)

- Rocks first behave elastically

- Forces/Temperature from different directions

- Ductile failure

•Above the elastic limit, two scenarios:- Brittle rocks fail abruptly producing fractures

-Ductile rocks undergo plastic deformation producing undulations called folds

*Remember that layers are always deposited horizontallyMAPPING PLANAR FEATURES

•Requires coordinate system

- With respect to North- Planar features are expressed by Strike and Dip

•Strike: intersection of planar structure with a horizontal plane

- Expressed as compass angle from North (clockwise)- 0° ≤ strike ≤ 360°

7/28/2019 Geo L13

http://slidepdf.com/reader/full/geo-l13 2/5

MAPPING PLANAR FEATURES

•Dip: inclination of planar structure, measured 90° from strike line- Specify angle and direction

- Water will flow in direction of dip

- Dip always measured perpendicular to strike

•When measuring Strike and Dip, we apply the right hand rule- Thumb in direction of Strike

- Fingers in direction of Dip

FRACTURES

•Brittle rocks produce fractures•Fractures are the most common geological structure

- Fracturing occurs in all rock types

- Fracturing occurs at several scales→ Meters to hundreds of kilometers

•Factors controlling the “brittleness” of a rock:

- Rock composition and texture

- Temperature and pressure- Presence of fluids

•Two types of fractures, scale dependent

- Faults: major fractures, showing appreciable movement between rock blocks- Joints: minor fractures, showing little or no movement between rock blocks

- Both faults and joints have significant engineering implications

•Stresses building up in the Earth's crust are relieved by relative motion between rock blocks•Fault: fracture in the Earth's crust resulting from the displacement of one rock block with respect to the other 

•Hangingwall: rock block above the fault

•Footwall: rock block below the fault

•Sudden movement along active faults are the cause of mostearthquakes

•Many faults are inactive

- Evidence of past deformation

•Faults are classified according to the relative movement between blocks

- Dip-slip fault: movement in the direction of dip

→ Normal fault & Reverse fault

- Strike-slip fault: lateral movement along strike

- Several faults display a combination of dip-slip and strike-slip movement

THRUST FAULTS

•Thrust fault: low-angle reverse fault

•Moves older rocks (hanging wall) over younger rocks (foot wall)

•Associated with plate collision and mountain building

•Large displacements (up to 100s km)•Typical dip < 20°

7/28/2019 Geo L13

http://slidepdf.com/reader/full/geo-l13 3/5

ENGINEERING CONSIDERATIONS: FAULTS

•Faults can introduce a number of conditions that can have a negative impact on engineering projects- Differing rock types on either side of the fault - Presence of weaker rock material

- Faults provide access to water - Movement between rock blocks

•Narrow zone of intense deformation

•Rocks within the zone might be weaker - Fault breccia: pieces of broken rocks

- Fault gouge: clay material resulting from rock pulverized during movement

•Surrounding rock is intact and strong•The obvious: Earthquakes!!!

JOINTS

•Joints are a concern for road cuts, slope stability, tunneling, mining operations•Joint: a fracture with little or no movement between rock blocks

•Joint set: a group of parallel joints

•Joints are fractures

•Frequently form parallel to pre-existing zones of weakness:Bedding planes → Bedding joints

Foliations → Foliation joints

Slaty cleavage → Cleavage joints

•Joint frequency is not necessarily constant throughout a rock mass- In sedimentary rock, regular joints - In granite, irregular joints

JOINTS: THE CAUSE

•Joints result from internal stresses- Stresses transmitted into continents by plate tectonics

- Expansive joints: loading (burial) and unloading (removal of overlying rocks by erosion)

- Cooling joints: thermal contraction/expansion in relation to igneous processes•Systematic joints

- parallel, regularly-spaced fractures - Created by a regional uniform stress

•Non-systematic joints

- randomly orientated fractures with irregular or curved joint faces

- Created by local non-uniform stressesJOINTS: MEASUREMENT

•Stereonet plots strike and dip- Stereographic projection

- Points closer to the circumference represent vertical faces

- Points closer to the center represent horizontal faces

SIGNIFICANCE OF JOINTS•Impact on the strength (quality) of the rock 

•Water flow: increased permeability and fluid movement along joints

- In soluble rocks, dissolution occurs preferentially along joints

- Concentration of chemical/mechanical weathering along joints

- Favors circulation of mineral-rich hydrothermal fluidsENGINEERING CONSIDERATIONS: JOINTS

•Orientation- Orientation of joints are a major concern for slope stability

- Take advantage of planes of weakness during quarrying

•Anisotropy: characteristic of a property having a different value when measured in different directions- Rock masses with non-systematic joints have less anisotropy than masses with systematic joints

- Rock masses with systematic joints might have significantly weaker properties in a specific direction

•Spacing- Closely-spaced joints tend to cause numerous rock falls

- More widely-spaced joints tend to cause massive rock failures

7/28/2019 Geo L13

http://slidepdf.com/reader/full/geo-l13 4/5

FOLDS

•Ductile rocks produce folds•Fold: geological structure formed when rocks are bent or curved as a result of 

 plastic deformation

- Folds are produced by lateral compression of the crust

- There might be multiple phases of deformation- Folds can be re-folded by a later event

- Folding occurs at several scales

•Composition of a fold- Hinge: point of maximum curvature

- Limbs: parts of fold that are not curved; interlimb angle

- Axial plane: imaginary plane equidistant from each limb, bisects angle betweenlimbs

- Axis: intersection of hinge and axial plane

- Plunge: angle between horizontal and hinge

ANTICLINE AND SYNCLINE

•Anticline: arched fold in which the central part contains theoldest rock layer 

- Convex upwards

•Syncline: arched fold in which the central part contains the

youngest rock layer - Convex downwards

•Neutral: axial plane horizontal

•Folds can be complex when considering all the parameters

•Note double anticline forms a domeATTITUDE OF AXIAL PLANE

•Four types of folds based on dip of axial plane

SYMMETRY ABOUT AXIAL PLANE•Symmetric: lengths of limbs L1 and L2 equal

•Asymmetric or Overturned: limb lengths not equal, L1 > L2

7/28/2019 Geo L13

http://slidepdf.com/reader/full/geo-l13 5/5

PROGRESSION OF FOLDING

Map Example

•Note dip arrows still point away from plunging antiforms, in to synformsENGINEERING CONSIDERATIONS: FOLDS

•Unequal stresses can be present in a folded rock mass

- Event within the same rock unit

•Stresses are a function of:- Position in the fold

- Style of the fold

- Variations in bedding, foliation, etc.•Civil engineering operations may meet with unexpected results when the stresses are released

CASE STUDY: SUDBURY STRUCTURE

•The Sudbury structure formed by meteoritic impact (1.85 Ga)- Over the time, structure has been deformed by compressional forces from a circular to an oval shape

•Major mineral deposits (Ni, Cu)

- Renew interest in the economic potential of other impact craters

SUDBURY: INCO R&D PROJECT

•Joints are mapped to estimate the “quality” (structural integrity) of the rock •It is difficult to map joints underground

- Harsh environment- Poor lighting conditions

- Manual, requires compass measurements

 Business drivers

•Quantitative structural analysis of joint orientation and block size for planning support•Data archiving

•Money