Thrust/reverse faults are dipslip faults on which the hanging
wall block has moved up
relative to the footwall block.
They accommodate horizontal
shortening of the Earth’s crust.
Reverse faults with dips
shallower than 45 degrees are
called thrust faults.
Basic Terms:
-A thrust sheet, or nappe is a hanging
wall block above a very low angle thrust.
-Allochthon, a thrust sheet that has
moved a large distance, and is thus out of
place geologically.
-Autochthon, a large region of rock that
has not been moved and is close to its
original location.
Shape of Thrust Faults
Most thrust faults exhibit similar
deformation features intrinsic to faults.
Thrust faults usually emplace older rocks
on top of younger rocks, with the
exceptions of when the strata dip more
steeply in the same direction as the fault
or in areas of complex deformation where
the stratigraphy is overturned or folded.
At depth, thrust faults are usually
listric faults that curve toward shallow or
horizonatal dips with increasing depth.
Some other features that one might also
find at a thrust fault are:
-Klippe, an isolated remnant of the
allochthon that has been separated from
the main sheet through erosion
-Window or fenster, a hole through the
thrust sheet that exposes an isolated area
underneath
- Ramp-flat geometry
-Lateral ramps are common features of
low-angle thrust faults
-Tear or transfer faults, sidewall faults
that are steeply dipping
(See Figure 5.4)
Displacement on Thrust Faults
The direction of displacement of the
hanging wall block on a thrust defines its
vergence, which is typically up the dip of
the fault surface.
-Movement of a thrust sheet over a ramp
causes a fault-ramp or fault bend fold to
appear in the thrust sheet.
-Ramp anticlines can appear due to the
steepness of the frontal ramp compared
with the main fault surface.
Structural Environments of Thrust
Faults
At the local level, diapiric structures
(less dense rocks that move upward
through more dense surroundings) can
appear.
Thrust faults can also be associated
with folds in four common ways (Fig.
5.10):
-Folds can develop when limbs of the fold
cannot be rotated any closer together
-Can develop as a result of thrusting to
accommodate
deformation
(faultpropagation folds)
-Can develop a steep or inverted limb that
becomes progressively sheared and
thinned until it becomes a ductile thrust
fault
-When thrust faults have an alternating
flat ramp geometry, movements from the
faults cause fault-bend folds to form in the
hanging wall block
The most common example of large
thrust systems are the thrust faults in
foreland fold and thrust belts which mark
the margins of major orogenic belts. Parts
of this system include:
-Foreland, the area in front of the thrust
towards which the thrust sheet moves,
region behind is the hinterland – systems
are generally curved and extend 100’s
to 1000’s of kilometers.
-In a salient or virgation, faults and folds
form an arcuate belt convex toward the
foreland
-In a re-entrant or syntaxis, the arcuate
belt is concave toward the foreland
-Culminations, or high areas, accompany
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salients while depressions accompany
reentrants
-Decollement, a low angle listric thrust
faults having the same general orientation
that join at depth
-Detachment, a gently dipping basal
decollement
-Imbricate fan or schuppen zone, many
individual listric thrust sheets, all with the
same vergence, overlapping like a series
of roofing tiles
-Thrust duplex, system of imbricate
thrust faults bounded from below and
above
-Thrust faults in a system don’t need to
have the same vergence, back thrusts can
develop with the opposite vergence
through tectonic wedging
Kinematic Models of Thrust Fault
Systems
Model 1. Duplex structures result from
the progressive cutting of the thrust fault
into the footwall block (Fig 5.14)
Model 2. Duplex develops by stepwise
retreat as the thrust fault frontal ramp
steps back into the hanging wall block,
leaving horses in the footwall. Upper
glide horizon remains the same after each
stepwise retreat of the frontal ramp (Fig
5.18 A)
Model 3. Same as 2, but the upper glide
horizon steps up in the structure with each
stepwise retreat of the frontal ramp (Fig
5.18 B)
Model 4. A duplex can develop if an
imbricate fan is truncated by a younger
thrust, which then forms the roof thrust of
the duplex (Fig 5.19)
-Faults that cut progressively into the
hanging wall block toward the hinterland
are called out-of-sequence thrusts and
make up a small proportion of thrust
systems.
Goemetry and Kinematics of Thrust
Systems in the Hinterland
Three
models
to
geometrically
accommodate horizontal shortening :
1. A fold and thrust belt paired with an
extensional terrane
David Tang , 2011
Edited by Ellen Knappe and Sam Birch, 2013
2. Root zone model
3. Subduction model
Analysis of Displacement on Thrust
Faults
Determine displacement direction
shear sense, and amount of displacement
through use of maps and cross sections
(Fig 23). It is best to use more than one
method to determine displacement.
A palinspastic cross section is one that has
restored the geology, geographic features,
and tectonic features as nearly as possible
to their configuration that preceded
deformation
References & Resources
Robert J. Twiss, Eldridge M. Moores,
Structural Geology 2nd edition, (W. H.
Freeman), p. 135-150, 2006
David Tang , 2011
Edited by Ellen Knappe and Sam Birch, 2013
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