Faults Flashcards
Fault
-a fracture or discontinuity in a
volume of rock across which there has
been significant displacement.
-brittle
-can be planar or curved
- length can vary from a few centimetres to thousands of kilometres.
fault zone vs shear zone
Fault zone: a tabular region containing many parallel or anastomosing faults
Shear zone:
- blocks of rock have been displaced in a fault-like manner, but without development of visible faults
-no loss of continuity/cohesion
on top of fault zone -> brittle
on top of shear zone -> ductile
types of faults based on dip angle
Horizontal fault (dip 0-10°)
Shallowly dipping (dip 10-30°)
Moderately dipping (dip 30-60°)
Steeply dipping (dip 60-80°)
Vertical fault (dip ~90°)
Listric fault dip decreases progressively with depth
Components of slip
Net slip = Strike-slip + Dip-slip
Types of faults – based on slip direction
Strike slip fault:
net slip vector approximately parallels the strike of the fault
dip slip fault:
net slip vector approximately parallels the dip line of the fault
oblique slip fault: has both strike and dip slip components
Transcurrent fault or wrench fault: strike slip fault with very steep to vertical dips
Types of faults – based on shear sense
dip slip faults
-normal if footwall goes up and hanging wall goes down
-reverse if hanging wall goes up and footwall goes down
strike slip faults
-right lateral/dextral
-left lateral/sinistral
oblique slip ->if movement is parallel to neither strike or dip
-combinations
left lateral/normal
left lateral/reverse
right lateral/normal
right lateral/reverse
scissor (rotates about a fixed point)
(amount of slip varies from one part of the fault to another)
Thrust fault
Thrust fault: a low-angle reverse fault
Thrust sheet: regional package of rocks above a thrust that has moved along the thrust.
Nappe: A thrust sheet that has moved more than ~10 km relative to the footwall.
Allochthon: regional thrust sheet that moved a great distance (allochthonous)
Autochthon: rocks (below a regional thrust sheet) that retain their original location (autochthonous)
Window (fenster): area of autochthonous rocks surrounded by allochthonous rocks.
Klippe: isolated allochthonous rocks
Separation along faults
-offset of markers observed along a
fault
->in outcrops, map patterns, structural profiles
refers to apparent sense and
magnitude of offset along a fault.
-not the same as net slip
Dip separation
(Heave and throw)
Strike separation
Net Slip
determine the net slip vector:
-determine its magnitude, direction of slip, sense of slip.
-requires knowledge of the positions on either side of the fault of two originally contiguous points
(Piercing points; e.g. intersection of two
planar markers).
If slip direction is known, only one
planar marker needed.
If the slip direction is unknown, only minimum slip may be constrained.
Net slip vector
Fault Bends
Flats: parallel to bedding (hanging wall flat, footwall flat)
Ramp: cuts across bedding (hanging wall ramp, footwall ramp; frontal ramp, lateral ramp, tear fault)
Extensional vs. contractional faulting
Extensional fault:
- results in lengthening of a layer,
e.g. a normal fault
Contractional fault: a
-results in shortening of a layer,
e.g. a reverse fault
describe the 4 main types of brittle fault rocks
how are they classified?
Breccia: >30% large angular fragments in fine-grained matrix; noncohesive, but can be cemented by later deposits (e.g. calcite or quartz)
Cataclasite: <30% large angular fragments in fine-grained matrix; cohesive
breccia to cataclasite
undeformed -> crackle breccia
-> mosaic breccia -> chaotic breccia
-> cataclasite
Fault gouge:
pulverized rock, very fine-grained (generally <0.1 mm in diameter);
non cohesive; clayey fault rock.
Pseudotachylyte (pseudotachylite):
cohesive, glassy, very fine-grained
fault rock; formed by local melting
due to frictional heating during slip
on a fault.
classification based on
- size of fragments
- if they are cohesive or noncohesive
Slickenslides and striations (or striae)
slickenfiber
Slickensides are smooth or shiny shear surfaces in rocks.
commonly display striations (or striae).
The striae are believed to be parallel to the movement direction during their
formation.
were widely believed to result from brittle deformation, but recent studies show that they can also form
by brittle-ductile transitional
processes.
Slickenfiber
Mineral fibers that grew during the fault movement. They show the direction of displacement.
Normal fault systems
- Normal fault systems
Related to extension
Divergent plate boundaries
rifts, passive margins, mid ocean ridges
Synthetic fault – subsidiary faults that parallel the major faults
Antithetic faults – subsidiary faults whose dip is opposite to that of the
major
Detachment fault: low-angle, separating unfaulted rocks below from
faulted rocks above
Horst: an up-faulted block between high-angle faults; the foot wall block
The faults bordering horsts and grabens are usually normal faults.
Graben: a relatively down-faulted topographic trough between high angle
normal faults that are dipping towards each other. Graben is German word
for trough
Half graben: bounded by fault only on one side, usually involves rotation of
fault blocks
Thick skinned vs thin skinned tectonics
Thin skinned tectonics: Deformation not involving the basement, only the overlying sedimentary cover.
Thick skinned tectonics: Deformation involving the basement, not only the overlying sedimentary cover
- Reverse (thrust) fault systems
Regional shortening, collision orogenies
Convergent plate boundaries
Fold-thrust belts
Imbricate fan – a series of reverse faults dipping in the same direction and soling out on a floor thrust.
Duplex system - thrusts that span the interval of rocks between a higher level detachment and a lower detachment (roof thrust, floor thrust)
Stair step flat and ramps, thrust duplex
- Strike-slip fault systems
Transpression (positive flower structure)
Transtension (negative flower structure)
