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.
 Number-one tool used in oil-and-gas exploration
2. 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:
a. Faulting messes rocks up. The messy looking part of the profile
is likely where the faults are.
b. Start at the right-hand side and pick some prominent reflectors
(heavy lines)
c. Follow these reflectors to the left, looking for truncations and/or
offsets.
d. Once you have found a series of truncations and/or offsets,
connect these together to outline a fault trace.
e. 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.
3. 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:
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?
 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:
a. Horsts and Grabbens
b. Synthetic and antithetic faults
Part-II: Thrust-fault systems
1. Interpret the thrust-fault seismic reflection profile
a. Faulting messes rocks up. The messy looking part of the profile is
likely where the faults are.
b. Start at the right-hand side and left-hand-side and pick some
prominent reflectors (heavy lines)
c. Follow these reflectors towards the middle, looking for truncations
and/or offsets.
d. Try to match up sets of similar reflectors on either side of the
truncations/offsets.
e. Once you have found a series of truncations and/or offsets, connect
these together to outline a fault trace.
f. If the fault is large enough and at a sufficiently low angle, it may form
a reflector or a series of discontinuous reflectors.
g. This fault will sole into a basal detachment surface.
2. 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:
a. Any systematic changes in fault dip
b. The orientations of layers approaching the thrust fault
 Discuss ramps and flats
 Discuss frontal, lateral, and oblique ramps
 Discuss fault-bend folds
a. Snake-head structure in seismic profile
b. Ramp anticlines develop over frontal ramps in thrust faults.
Systematics of fold axis orientations.
c. 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
a. Windows
b. Klippe