Structural Geology Notes

 Introduction 

1. Classic definition = study of deformed rocks in the upper crust deformed includes translation, rotation, and strain (change of shape) All rocks are deformed in some way. 

Sedimentary rocks are the most studied by structural geologists because the initial shape, etc. is very well-known, plus, sedimentary rocks are structural setting for hydrocarbons. 

2. Engineering definition = stress (σij), strain (εij), strain rate, and constitutive laws (Cijkl) of geologic materials. Constitutive law is elastic, plastic, viscous, or frictional. 

3. Driving forces of deformation 

A. Gravitational loading 

B. Tectonics (plate boundaries) 

Convergent or collisional – e.g. Andes, Himalayas, or Cascades 

Divergent – e.g. mid-Ocean ridge (studied by Marine Geologists) 

Basin and Range

Transform - - e.g. San Andreas Fault 

Permanente Limestone Quarry Example

paleomagnetic study indicates ~3000 km translation from near equator (rates – 3,000 km in 30 Ma = 10 cm/yr) tilted layers indicate rotation elliptical ooids (originally spherical) indicate shape change 

 

Field Methods and Tools 

 1. Measuring orientations of lines (vectors): use trend and plunge 

 A. Trend -- measured as degrees clockwise from North 

 B. Plunge – measured as degrees downward from horizontal 

2. Measuring orientations of planes: three options 

A. Strike, dip and dip direction 

 1. strike – horizontal line on plane 

 2. dip -- 

B. Measure trend and plunge of direction of maximum dip 

C. Measure trend and plunge of pole normal to plane

3. Stereograms are used to graphically represent measurements of planar and linear features Imagine downward-pointing normal vectors of planes intersecting a hemisphere, then project that hemisphere onto a plane. Lines shows increments of 10°

 

Example Poles 

1. Horizontal plane, strike = indetermined, dip=0° 

2. Strike =90°, dip = 45°, and dip direction = North 

(90°, 45° N) 

3. Strike=0°, dip=90°, direction=E 

(0°, 90° E) 

4. Strike=45°, dip=45°, direction=SE 

(45°, 45° E)

Faults 

fracture = a discontinuity or break in rock fault = a discontinuity in which one block has slipped past another (Mode II or III) 

joint = a discontinuity in with no slip parallel to fractures some opening (Mode I) 

 1. Descriptive geometry of faults 

A. Hanging wall – HW – block above fault plane 

B. Footwall – FW – block below fault plane 

Think of the English coal miners who coined these terms as they tunneled through a fault. 

 C. Fault surface = planar or listric (concave upward) 

 D. Slip – the direction, sense and magnitude of movement on a fault (a vector, u) 

 2. Classification of faults - using concepts of HW, FW, and Slip, faults are classified into: 

A. Normal Faults – HW slips down relative to FW 

B. Thrust Faults -- FW slips up relative to HW 

C. Strike-slip faults – horizontal slip -- right lateral or left lateral 

3. Anderson’s Theory of Faulting -- There cam be no shear stress at a free surface, therefore principal stresses are oriented perpendicular and parallel to Earth’s surface. 

A. Normal faults dip 60°, vertical max. principal stress 

B. Thrust faults dip 30°, vertical min. principal stress, horizontal max. principal stress 

C. Strike-slip faults dip 90°, vertical intermediate principal stress, horizontal max. and min principal stress 

Folds 

 1. Shapes 

 A. Monoclines – a single bent limb caused by vertical displacement 

 B. Antiform – concave downward, not used for sed. rocks 

 C Synform – concave upward, not used for sed. rocks 

 D. Anticline – concave downward, oldest sedimentary rocks in center 

 E. Syncline – concave upward, youngest sedimentary rocks in center 

 2. Folds in 2D 

 A. Hinge point – point of maximum curvature 

 B. Hinge zone -- curved portion of fold 

 C. Limb – little or no curvature between hinges 

 3. Folds in 3D 

 A. Fold axis – collection of hinge points 

 B. Axial plane – collection of fold axes 

 

 4. Symmetry 

 A. Symmetric – each limb dips the same 

 B. Asymmetric or verging -- one limb has steeper dips than other 

 C. Overturned – one limb has overturned beds

5. Fold Tightness based on interlimb angle 

A. Flat lying 180°

B. Gentle 170°-180°

C. Open 90°-170°

D. Tight 10°-90°

E. Isoclinal 0°-10°

 6. Folds in Map view 

A. Horizontal fold axis – layers will be straight in map view 

B. Plunging fold axis – layers will be curved in map view 

Joints or Fractures 

 1. Characteristics 

A.Very abundant near surface 

B. Occur in Sets (~1 m spacing) 

C. Different sets are often perpendicular to one another 

 2. Importance 

A. Rock stability - Mining, quarrying, and tunneling 

B. Groundwater flow in non-porous rocks 

C. Rock Climbing 

 3. Formation 

A. Thermal Cooling e.g. Columbia River Basalts of Eastern Washington 

σ = (α Ε ∆Τ ) / (1−ν) 

where σ = thermal stress? 

α = thermal coeff of expansion 

Ε = Young’s Modulus

∆Τ = temperatrue change

ν = Poission's Ratio for basalt 

∆T = 80° C, rock will fracture 

∆T = 1000° C (cooling from solidification to ambient temp), 

ε=2.4% or 2.4 mm crack every meter 

B. Gravitational Loading (Poisson Effect) – e.g. granites in Yosemite 

  ∆σ = ρ g ∆z ν / (1−ν)

where ρ = density of removed overburden 

 ∆z = change in overburden thickness 

g = gravitational acceleration

ν = Poission's Ratio

C. Tectonic Stretching e.g. American Southwest  any significant horizontal extension will soon cause development of normal faults.