1. classic definition = study of deformed rocks in the upper crust

Transcription

1 Structural Geology I. 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 II. 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 CE 437: Structural Geology Notes 1

2 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) CE 437: Structural Geology Notes 2

3 IV. 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 V. 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 CE 437: Structural Geology Notes 3

4 5. Fold Tightness based on interlimb angle A. Flat lying 180 B. Gentle C. Open D. Tight E. Isoclinal 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 VI. 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 CE 437: Structural Geology Notes 4

5 C. Tectonic Stretching e.g. American Southwest any significant horizontal extension will soon cause development of normal faults CE 437: Structural Geology Notes 5