SUMMARY OF WEEK 4 READINGS

T&M pp. 96-127

Chapter 6: Thrust Faults

  1. Some introductory definitions
    1. Reverse faults dip >45 deg. Thrust faults dip <45 deg.
    2. Thrust faulting can happen on all scales from millimeters to kilometers.
    3. Thust Sheet- low angle hanging wall block has large areal extent compared to thickness
    4. Allochthon- sheet shifted out of place from original position. Rocks within sheet are allochthonous.
    5. Autochthon- sheet close to original location (ie. footwall block). Contains autochthonous rocks.
  2. Recognition of Thrust Faults
    1. Vertical section through fault stratigraphy is generally duplicated.
    2. Horizontal separation can vary due to different dips - Figure 6.2
    3. Types of Stratigraphy consistent with thrust faults - are only general indicators and is difficult to diagnose properly in the field.
      1. Plutons/metamorphics brought near the surface from deep in the Earth.
        1. Plutonic/high grade metamorphic on top of unmetamorphic/low-grade igneous.
      2. Similar aged rocks but different sedimentary facies.
      3. Highly deformed allochthonous on top of slightly/undeformed autochthonous.
  3. Shape and Displacement of Thrust Faults
    1. Shape of Thrust Faults- creates irregular map surfaces indicative of path.
      1. Some dip less with depth- form listric fault systems.
      2. Constant dip with depth.
      3. Greater dip with depth- accommodates compression of intrusion or jog in strike-slip.
    2. Klippe- isolated remnant of allochthon eroded away (outcrop).
    3. Window (Fenster)- erosion creates a hole through sheet to reveal layers below.
      1. both Klippes and Windows are indicators of minimum displacement.
    4. Displacement on Thrust Faults- generally up dip.
      1. Ramps can cause oblique motion and folds to develop.
        1. fault-ramp fold/fault-bend fold- usually anticlines since ramps are steeper tat main fault surface.
  4. Structural Environments of Thrust Faults
    1. Local Thrust Faults- local geometry requires convergence or shortening reacting brittly.
      1. Diapiric structures- less dense material move up through denser surroundings.
        1. ie. Salt domes that push rock out of the way (also have normal faults associated with)
      2. Common with bends in strike-slip faults that result in compression.
    2. Thrust Faults Associated with Folds- Figure 6.10
      1. When can't fold anymore thrust faults cut the steep or overturned fold.
      2. Fault Propagation Folds- folds develop to accommodate deformation above tip line.
        1. Eventually shears off steeper fold.
      3. Steep or inverted fold progressively sheared until forms a duplicate shear zone.
      4. Folds in hanging wall block develop going over a ramp.
    3. Thrust Systems.
      1. Foreland fold and thrust belts- margins of major orogenic belts.
        1. belts consist of sets of low-angle listric faults of similar attitudes.
        2. sets of folds and faults more or less parallel.
        3. Foreland- area in-front of thrusting fault or sheet.
        4. Hinterland- behind thrust system.
        5. Salient/Virgation- faults and folds form belt convex toward foreland.
        6. Reentrant/Syntaxis- faults and folds form belt concave toward foreland.
        7. Culminations- relatively high area usually along salients.
        8. Depressions- low regions usually along reentrants.
        9. Cross section view- belts overlie undeformed basement along gently sloping decollement, detachment, or sole thrust.
          1. Deformation usually limited to above decollement.
          2. Wedge shape- thiner cross section towards foreland.
          3. Listric thrust faults asymptote to decollement.
      2. Tear faults take up differential displacements.
      3. Imbricate fan or Schuppen zone- faults stacked on top of each other.
        1. Usually concave up and decrease dip with depth terminating at branch lines in decollement.
      4. Duplex- imbricate thrusts that branch off floor below and upward into a roof thrust(S-shaped)
        1. Results in a stack of horses.
      5. When thrust fault dies out transfers displacement elsewhere forming en echelon thrust structure.
  5. Kinematic Models of Thrust Fault Systems- want to know where new faults form (ie. toward foreland or hinterland)
    1. Usually ramp faults cut into footwall block forming progressive horses.
      1. Hinterland dipping, Antiformal stack, Foreland Dipping duplex.
    2. Ramp faults can cut into hinterland- not as common.
    3. Geometry alone not a good indicator of propagation direction- depends also on stratigraphy.
  6. Geometry and Kinematics of Thrust Systems in the Hinterland
    1. What happens to sole fault beneath Hinterland?
      1. Sole could re-surface somewhere else with extensional features
        1. Paired zone of extension usually with shallow fault systems with small amounts of displacement (on regional scale)
      2. Have deep basement shortening and metamorphosis.
      3. Could end in a subduction zone.
  7. Analysis of Displacement on Thrust Faults.
    1. Direction and Sense of Displacement.
      1. Regional scale- normal to strike
      2. Tendency of thrust system to cut up-section in direction of displacement in stratigraphy.
      3. Imbricate thrust systems branch up from sole in direction of relative displacement.
      4. Assumed "upward" movement of hanging wall block not always valid since fault dips may be altered from folding.
    2. Determining Amount of Displacement from Maps.
      1. Use klippes and/or windows to estimate displacement.
    3. Determining amount of Displacement from Cross-Section.
      1. Useful only if section cut is parallel to displacement direction.
      2. Very difficult for complex structures such as imbricates and duplexes.

Chapter 7: Strike-Slip Faults

  1. Introduction.
    1. Accomodate horizontal shear.
    2. No net addition or subtraction of area to crust.
    3. Tear Faults- small scale common with folds, thrusts, and normal faults.
      1. Occur in hanging wall block of low angle faults.
    4. Transfer Fault- accommodates transfer from one fault to another (usually oblique)
    5. Transform/Transcurrent Faults- major regional strike-slip systems
      1. Transform- plate boundaries.
      2. Transcurrent- no plate boundary.
  2. Characteristics of Strike-Slip Faults
    1. Usually planar and near vertical at the surface.
      1. Straight line fault trace.
    2. Causes Offsets.
      1. Shutter Ridge- ridge displaced in-front of a canyon cutting it off.
    3. Reidel Shears (R)- develop at small angles en echelon synthetic to main fault (Fig. 7.4A).
      1. Antithetic R' shears- high angles opposite to sense of shear.
    4. Thrust and Normal faults can form en echelon along shear surface.
  3. Shape, Displacement, and Related Structures.
    1. Single Faults
      1. May terminate at depth into another fault or fade out into ductile zone.
      2. Bends(Jogs)- curved parts of fault trace connect two non-coplanar segments (trace turns).
      3. Stepovers(Offsets)- region where one fault ends and another en echelon fault begins.
        1. Described as right or left depending on turn as follow fault trace.
            Contractional Extensional
          Dextral(Rt) Left bends Right bends
          Sinistral(Lt) Right bends Left bends
    2. Strike-Slip Duplexes- displacements along bends and step overs.
      1. Forms horizontally stacked horses.
      2. Accomodate extension and contraction of crust of the horizontal free surface- usually oblique motion.
      3. Cross Section shows a flower structure.
        1. Normal, negative structure- concave up (tulip)
        2. reverse, positive structure- convex up (palm tree)
      4. Scissor faults- change from normal fault at one end to reverse fault at the other.
        1. Usually accommodates rotation of horst blocks.
    3. Terminations- get imbricate fans of normal, thrust, or strike-slip splays.
  4. Structural Associations with Strike-Slip Faults
    1. Tear Faults
      1. develop in regions of normal or thrust faulting or fold sheets.
      2. Fold axes tend to terminate along tear faults
    2. Bend and Stepover Duplexes
      1. Complex zones of anastomosing, parallel, or en echelon faults.
    3. Terminations- can turn perpendicular to slip motion and turn into a thrust fault.
  5. Kinematic Models of Strike-Slip Fault Systems
    1. Part of shear is distributed to rock on either side of fault.
      1. square becomes a parallelogram- one diagonal shortened and one lengthened.
        1. Thrust systems form perpendicular to shortened axis.
        2. Normal systems form perpendicular to lengthened axis.
  6. Analysis of Displacements on Strike-Slip Faults
    1. Similar to Dip-slip model (Sect. 5.5) except change orientation. See equation on p. 127.
  7. Balancing Strike-Slip Faults
    1. Appropriate only in map view (horizontal plane).
    2. Difficult to include bends and stepovers- violate 2-D displacement rule since get vertical displacement.
    3. Not common practice method of analysis.