SUMMARY OF WEEK 4 READINGS
T&M pp. 96-127
Chapter 6: Thrust Faults
- Some introductory definitions
- Reverse faults dip >45 deg. Thrust faults dip <45 deg.
- Thrust faulting can happen on all scales from millimeters to kilometers.
- Thust Sheet- low angle hanging wall block has large areal extent compared to thickness
- Allochthon- sheet shifted out of place from original position. Rocks within sheet are allochthonous.
- Autochthon- sheet close to original location (ie. footwall block). Contains autochthonous rocks.
- Recognition of Thrust Faults
- Vertical section through fault stratigraphy is generally duplicated.
- Horizontal separation can vary due to different dips - Figure 6.2
- Types of Stratigraphy consistent with thrust faults - are only general indicators and is difficult to diagnose properly in the field.
- Plutons/metamorphics brought near the surface from deep in the Earth.
- Plutonic/high grade metamorphic on top of unmetamorphic/low-grade igneous.
- Similar aged rocks but different sedimentary facies.
- Highly deformed allochthonous on top of slightly/undeformed autochthonous.
- Shape and Displacement of Thrust Faults
- Shape of Thrust Faults- creates irregular map surfaces indicative of path.
- Some dip less with depth- form listric fault systems.
- Constant dip with depth.
- Greater dip with depth- accommodates compression of intrusion or jog in strike-slip.
- Klippe- isolated remnant of allochthon eroded away (outcrop).
- Window (Fenster)- erosion creates a hole through sheet to reveal layers below.
- both Klippes and Windows are indicators of minimum displacement.
- Displacement on Thrust Faults- generally up dip.
- Ramps can cause oblique motion and folds to develop.
- fault-ramp fold/fault-bend fold- usually anticlines since ramps are steeper tat main fault surface.
- Structural Environments of Thrust Faults
- Local Thrust Faults- local geometry requires convergence or shortening reacting brittly.
- Diapiric structures- less dense material move up through denser surroundings.
- ie. Salt domes that push rock out of the way (also have normal faults associated with)
- Common with bends in strike-slip faults that result in compression.
- Thrust Faults Associated with Folds- Figure 6.10
- When can't fold anymore thrust faults cut the steep or overturned fold.
- Fault Propagation Folds- folds develop to accommodate deformation above tip line.
- Eventually shears off steeper fold.
- Steep or inverted fold progressively sheared until forms a duplicate shear zone.
- Folds in hanging wall block develop going over a ramp.
- Thrust Systems.
- Foreland fold and thrust belts- margins of major orogenic belts.
- belts consist of sets of low-angle listric faults of similar attitudes.
- sets of folds and faults more or less parallel.
- Foreland- area in-front of thrusting fault or sheet.
- Hinterland- behind thrust system.
- Salient/Virgation- faults and folds form belt convex toward foreland.
- Reentrant/Syntaxis- faults and folds form belt concave toward foreland.
- Culminations- relatively high area usually along salients.
- Depressions- low regions usually along reentrants.
- Cross section view- belts overlie undeformed basement along gently sloping decollement, detachment, or sole thrust.
- Deformation usually limited to above decollement.
- Wedge shape- thiner cross section towards foreland.
- Listric thrust faults asymptote to decollement.
- Tear faults take up differential displacements.
- Imbricate fan or Schuppen zone- faults stacked on top of each other.
- Usually concave up and decrease dip with depth terminating at branch lines in decollement.
- Duplex- imbricate thrusts that branch off floor below and upward into a roof thrust(S-shaped)
- Results in a stack of horses.
- When thrust fault dies out transfers displacement elsewhere forming en echelon thrust structure.
- Kinematic Models of Thrust Fault Systems- want to know where new faults form (ie. toward foreland or hinterland)
- Usually ramp faults cut into footwall block forming progressive horses.
- Hinterland dipping, Antiformal stack, Foreland Dipping duplex.
- Ramp faults can cut into hinterland- not as common.
- Geometry alone not a good indicator of propagation direction- depends also on stratigraphy.
- Geometry and Kinematics of Thrust Systems in the Hinterland
- What happens to sole fault beneath Hinterland?
- Sole could re-surface somewhere else with extensional features
- Paired zone of extension usually with shallow fault systems with small amounts of displacement (on regional scale)
- Have deep basement shortening and metamorphosis.
- Could end in a subduction zone.
- Analysis of Displacement on Thrust Faults.
- Direction and Sense of Displacement.
- Regional scale- normal to strike
- Tendency of thrust system to cut up-section in direction of displacement in stratigraphy.
- Imbricate thrust systems branch up from sole in direction of relative displacement.
- Assumed "upward" movement of hanging wall block not always valid since fault dips may be altered from folding.
- Determining Amount of Displacement from Maps.
- Use klippes and/or windows to estimate displacement.
- Determining amount of Displacement from Cross-Section.
- Useful only if section cut is parallel to displacement direction.
- Very difficult for complex structures such as imbricates and duplexes.
Chapter 7: Strike-Slip Faults
- Accomodate horizontal shear.
- No net addition or subtraction of area to crust.
- Tear Faults- small scale common with folds, thrusts, and normal faults.
- Occur in hanging wall block of low angle faults.
- Transfer Fault- accommodates transfer from one fault to another (usually oblique)
- Transform/Transcurrent Faults- major regional strike-slip systems
- Transform- plate boundaries.
- Transcurrent- no plate boundary.
- Characteristics of Strike-Slip Faults
- Usually planar and near vertical at the surface.
- Straight line fault trace.
- Causes Offsets.
- Shutter Ridge- ridge displaced in-front of a canyon cutting it off.
- Reidel Shears (R)- develop at small angles en echelon synthetic to main fault (Fig. 7.4A).
- Antithetic R' shears- high angles opposite to sense of shear.
- Thrust and Normal faults can form en echelon along shear surface.
- Shape, Displacement, and Related Structures.
- Single Faults
- May terminate at depth into another fault or fade out into ductile zone.
- Bends(Jogs)- curved parts of fault trace connect two non-coplanar segments (trace turns).
- Stepovers(Offsets)- region where one fault ends and another en echelon fault begins.
- Described as right or left depending on turn as follow fault trace.
- Strike-Slip Duplexes- displacements along bends and step overs.
- Forms horizontally stacked horses.
- Accomodate extension and contraction of crust of the horizontal free surface- usually oblique motion.
- Cross Section shows a flower structure.
- Normal, negative structure- concave up (tulip)
- reverse, positive structure- convex up (palm tree)
- Scissor faults- change from normal fault at one end to reverse fault at the other.
- Usually accommodates rotation of horst blocks.
- Terminations- get imbricate fans of normal, thrust, or strike-slip splays.
- Structural Associations with Strike-Slip Faults
- Tear Faults
- develop in regions of normal or thrust faulting or fold sheets.
- Fold axes tend to terminate along tear faults
- Bend and Stepover Duplexes
- Complex zones of anastomosing, parallel, or en echelon faults.
- Terminations- can turn perpendicular to slip motion and turn into a thrust fault.
- Kinematic Models of Strike-Slip Fault Systems
- Part of shear is distributed to rock on either side of fault.
- square becomes a parallelogram- one diagonal shortened and one lengthened.
- Thrust systems form perpendicular to shortened axis.
- Normal systems form perpendicular to lengthened axis.
- Analysis of Displacements on Strike-Slip Faults
- Similar to Dip-slip model (Sect. 5.5) except change orientation. See equation on p. 127.
- Balancing Strike-Slip Faults
- Appropriate only in map view (horizontal plane).
- Difficult to include bends and stepovers- violate 2-D displacement rule since get vertical displacement.
- Not common practice method of analysis.