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 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

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

To interpret the profile: 1.The messy looking part of the profile is likely where the faults are. 2.Start at right-hand side and pick some prominent reflectors (heavy lines) 3.Follow reflectors to the left; look for truncations and/or offsets. 4.Connect truncations and/or offsets together to outline a fault trace. 5.If fault is large enough and at sufficiently low angle, it may form a reflector or a series of discontinuous reflectors.

Basic interpretation

What do you notice about: a.The orientations of sedimentary layers approaching the large normal fault? b.The thickness of beds approaching the large normal fault? c.The down-dip geometry of the large normal fault?

Different normal-fault styles

Symmetric and asymmetric normal- fault systems Rotational Nonrotational

Watch Allmendinger’s movie Growth strata Growth strata: Sed rocks deposited during faulting. Thickest next to fault

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

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

To interpret the profile: 1.Start at the sides and pick prominent reflectors 2.Follow reflectors towards the middle, looking for truncations and/or offsets. 3.Match up similar reflectors on either side of truncations/offsets. 4.Connect these together truncations/offsets to outline a fault trace. 5.Fault may form a reflector or a series of discontinuous reflectors. 6.This fault will sole into a basal detachment surface.

Basic interpretation

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

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

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

Fault-related folds Fold due to faulting

Ramp anticlines

Fault-propagation folds Steeply dipping overturned limb Moderately dipping limb

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