Folds and faults are both structures produced
by the deformation of rocks. Folds1 are
structures where layering is deformed without
breaking so that the layering surfaces are
curved but continuous. Faults represent a
different type of response by rocks to the
stresses imposed on them.
deformation
déformation tectonique (Joly 1997)
souple (ductile)
flexure
2
Avec/sans déplacement
pli
cassante
diaclase
faille2
Monoclines are folds consisting of two
horizontal (or nearly so) limbs connected by a
shorter inclined limb. They can be compared
to anticlines, which consist of two inclined
limbs dipping away from each other, and
synclines, which consist of two inclined limbs
dipping towards each other.
déformation
pli
monocline
monoclinal
faille
anticline
anticlinal
syncline
synclinal
monoclinal
two horizontal limbs [side of a fold]
inclined limb
deux couches
réunies par un pendage
anticlinal
two inclined limbs
dipping away
concave
synclinal
two inclined limbs
dipping together
convexe
With a fault, the surface along which
displacement has taken place is the fault plane.
fault
fault plane
plan de faille
displacement
déplacement
Structure contours can be used to describe the
form of fault planes and these are interpreted
in the same way as structure contours drawn
for any surface.
fault
fault plane
structure contour
structure de contour
displacement
The inclination of a fault plane is described by
its dip. Faults with dips greater than 45° are
sometimes referred to as high-angle faults
those with dips less than 45° are low-angle
faults.
fault
(dip)
45°
45°
high-angle fault
low-angle fault
faille à fort pendage faille à faible pendage
The rocks which lie above a fault plane make
up the hanging wall, those below the foot wall.
fault
above
hanging wall
toit
below
foot wall
mur
The parts of a fault are
(1) the fault plane
(2) the fault trace
(3) the hanging wall
(4) the footwall.
The fault plane is where the action is. It is a
flat surface that may be vertical or sloping.
The line it makes on the Earth's surface is the
fault trace. Where the fault plane is sloping,
the upper side is the hanging wall and the
lower side is the footwall. When the fault
plane is vertical, there is no hanging wall or
footwall.
(vertical) fault
fault plane
plan de faille
fault trace
ligne de faille
(oblique fault)
fault plane fault trace hanging wall foot wall
plan de faille ligne de faille
toit
mur
Any fault plane can be completely described
with two measurements: its strike and its dip.
The strike is the direction of the fault trace on
the Earth's surface.
The dip is the measurement of how steeply the
fault plane slopes—if you dropped a marble on
the fault plane, it would roll exactly down the
direction of dip.
Fault plane
(direction)
strike
direction
(slope)
dip
pendage
fault
dip-slip fault
faille à rejet incliné
normal
strike-slip fault
(faille décrochante); décrochement
reverse
faille normale faille inverse
left lateral right lateral
décrochement sénestre décrochement dextre