Dip-Slip Faults

Goal: To interpret dip-slip faults on seismic sections and to build on your interpretations to understand normal-fault and thrust-fault systems.

Part-I: Normal-fault systems

  1. Seismic-reflection profiles
  • The squiggly lines on these profiles are reflectors. Reflectors are sound waves reflected off of density contrasts (geologic contacts). They therefore represent different rock layers.
  • You can think of a properly processed seismic reflection profile as a sound-based impressionist picture of the structure of the earth’s crust.
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  1. Interpreting the profile
  • Remember, we are focusing on faults. Look for offsets and truncations of rock layers.
  • There is one large normal fault and lots of small normal faults. Concentrate on finding the large fault first.
  • To interpret the profile:
  • Faulting messes rocks up. The messy looking part of the profile is likely where the faults are.
  • Start at the right-hand side and pick some prominent reflectors (heavy lines)
  • Follow these reflectors to the left, looking for truncations and/or offsets.
  • Once you have found a series of truncations and/or offsets, connect these together to outline a fault trace.
  • If the fault is large enough and at a sufficiently low angle, it may form a reflector or a series of discontinuous reflectors. These will often cut across bedding.
  1. Learning from the interpretation
  • Look at your interpretation and the key (Don’t worry, key is a lot more detailed than my interpretation too). What do you notice about:
  • The orientations of sedimentary layers approaching the large normal fault?
  • The thickness of beds approaching the large normal fault?
  • The down-dip geometry of the large normal fault?
  • Discuss block rotation and roll-over folds
  • Discuss growth strata — View Allmendinger growth faulting movie
  • Discuss listric faults, low-angle detachment faults, and normal-fault systems
  • Additional Terms:
  • Horsts and Grabbens
  • Synthetic and antithetic faults

Part-II: Thrust-fault systems

  1. Interpret the thrust-fault seismic reflection profile
  1. Faulting messes rocks up. The messy looking part of the profile is likely where the faults are.
  2. Start at the right-hand side and left-hand-side and pick some prominent reflectors (heavy lines)
  3. Follow these reflectors towards the middle, looking for truncations and/or offsets.
  4. Try to match up sets of similar reflectors on either side of the truncations/offsets.
  5. Once you have found a series of truncations and/or offsets, connect these together to outline a fault trace.
  6. If the fault is large enough and at a sufficiently low angle, it may form a reflector or a series of discontinuous reflectors.
  7. This fault will sole into a basal detachment surface.
  8. Learning from the interpretation
  • Look at your interpretation and the key (Don’t worry, key is a lot more detailed than my interpretation too). What do you notice about:
  • Any systematic changes in fault dip
  • The orientations of layers approaching the thrust fault
  • Discuss ramps and flats
  • Discuss frontal, lateral, and oblique ramps
  • Discuss fault-bend folds
  • Snake-head structure in seismic profile
  • Ramp anticlines develop over frontal ramps in thrust faults. Systematics of fold axis orientations.
  • Fault-propagation folds Grow at tip of ramp. Typically have over-turned limb.
  • Thick-skinned vs. thin-skinned reverse-fault systems. High-angle, basement-involved reverse faults
  • Features of dissected thrust belts
  • Windows
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