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Dip-slip faults

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. Seismic-reflection profile of a large normal fault. Seismic-reflection profiles.

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Dip-slip faults

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  1. 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.

  2. Part-I: Normal-fault systems Seismic-reflection profile of a large normal fault

  3. Seismic-reflection profiles • The squiggly lines on these profiles are reflectors • Recorded by sound waves reflected off of density contrasts (geologic contacts) • Represent different rock layers. • Seismic reflection profile = sound-based impressionist picture of earth. • Number-one tool in oil-and-gas exploration

  4. Interpreting the profile • Look for offsets and truncations of layers • Concentrate on finding the large fault first

  5. To interpret the profile: • The messy looking part of the profile is likely where the faults are. • Start at right-hand side and pick some prominent reflectors (heavy lines) • Follow reflectors to the left; look for truncations and/or offsets. • Connect truncations and/or offsets together to outline a fault trace. • If fault is large enough and at sufficiently low angle, it may form a reflector or a series of discontinuous reflectors.

  6. Basic interpretation

  7. 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?

  8. Different normal-fault styles

  9. Symmetric and asymmetric normal-fault systems Nonrotational Rotational

  10. Watch Allmendinger’s movie www.geo.cornell.edu/geology/faculty/RWA/movies Growth strata Growth strata: Sed rocks deposited during faulting. Thickest next to fault

  11. What do normal-fault systems really look like? The Tetons are a rotated fault block

  12. Part-II: Thrust-fault systems Seismic-reflection profile of a thrust fault

  13. To interpret the profile: • Start at the sides and pick prominent reflectors • Follow reflectors towards the middle, looking for truncations and/or offsets. • Match up similar reflectors on either side of truncations/offsets. • Connect these together truncations/offsets to outline a fault trace. • Fault may form a reflector or a series of discontinuous reflectors. • This fault will sole into a basal detachment surface.

  14. Basic interpretation

  15. What do you notice about: • Any systematic changes in fault dip • The orientations of layers approaching the thrust fault

  16. Ramp Flat Ramp: Dipping segment of the fault. Fault cuts up section Flat: Subhorizontal segment of the fault. Fault follows beds.

  17. Frontal ramp: 90° to transport direction Lateral ramp: parallel with transport direction Oblique ramp: oblique to transport direction

  18. Fault-related folds Fold due to faulting

  19. Ramp anticlines

  20. Fault-propagation folds Moderately dipping limb Steeply dipping overturned limb

  21. Dissected thrust belts Window: Hole eroded through hanging wall of a thrust fault that exposes footwall rocks Klippe: Isolated remnant of thrust sheet. Typically topographic highs

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