Download Faults and Faultings - National Taiwan University

Faults and Faultings
Aerial Photo of Wallace Creek and San Andreas Fault
North American Plate
The San Andreas Fault in
the Carrizo Plain with
offset in Wallace Creek
Provided by: David Lynch,
http://epod.usra.edu/
(91 m)
Jyr-Ching Hu, Department of geosciences, National
Taiwan University
Fault
• In a general sense, a fault is any surface or zone
in the Earth across which measurable slip (shear
displacement) develops.
• In a more strict sense, faults are fractures on
which slip develops primarily by brittle
deformation.
This second definition serves to distinguish a
“fault” (sensu stricto) from a fault zone and shear
zone.
Fault Zone
• Brittle structures in which loss of cohesion and
slip occur on several faults within a band of
definable width. Displacement in fault zone can
involve formation and slip on many small,
subparallel brittle faults (fault splays), or slip on a
principal fault from which many small faults
divergent, or slip on a anastomosing array of
faults.
Shear Zones (剪切帶)
• Shear zones are ductile structures across which
a rock body does not lose mesoscopic cohesion,
so that strain is distributed across a band of
definable width.
• In ductile shear zone, rocks deformed by
cataclasis, a process involving fracturing(破裂),
crushing(壓碎), and frictional sliding(克服摩擦力
的滑動) of grains or rock fragments, or, more
commonly, by crystal plastic deformation
mechanisms.
Fault, Fault Zone and Shear Zone
分支斷層
標誌層
Fault Geometry and Displacement
Listric Fault (鏟形斷層)
FaultsPlanar
whosenormal
dip decreases
progressively
faults and
listric normalwith
faultdepth.
Fig. 16.616
Chapter
Formation of a rollover anticline above a
listric normal fault
捲筒式背斜
Different Types of Faults
Extensional and Contractional Faulting
Representing of Faults on Maps and Cross
Sections
Different Symbols for Dip-slip Fault and
Strike-slip Fault
切開線
Klippe, window, allochthon, and autochthon
in a thrust-faulted region
Klippe (飛來峰)
Allochthon
(外來岩體)
Autochthon
(在地岩體)
Thrust-fault (逆斷層)
Window (構造窗)
Fault Separation (斷距) and Determination
of Net Slip
Heave: Horizontal component of dip separation.
Throw: Vertical component of dip separation.
S: Strike separation
D: Dip separation
H: Horizontal separation
V: Vertical separation
Fault Bends
斷坡
斷坪
Releasing bend and restraining bend
Restraining bend
Releasing bend
Transpression and Transtension
• Transpression: movement across a segment of
strike-slip fault results in some compression.
• Transtension: movement across a segment of
strike-slip segment faults results in some
extension.
Fault Terminations and Fault Length
A: Ground
surface
B: Pluton
C and D: one
fault cut another
D: Unconformity
A: Merging with another
fault;
B: Horsetailing
C: Dying out into a zone
of ductile deformation
(c) A series of ramps merging at depth with a basal detachment.
Tip lines (尖端線) for an emergent fault and
a blind fault
Tip line: The boundary between the slipped and
unslipped region at the end of a fault
斷層跡
尖端點
尖端線
Fault Terminations and Fault Length
• Horsetail: A fault splits into numerous splays near
its end, thereby creating a fan of small fractures.
• Tip line: The boundary between the slipped and
unslipped region at the end of the fault.
• Emergent fault: If the fault intersected the ground
surface while it was still active.
• Exhumed fault: If it intersected the ground surface
because the present surface of erosion has
exposed an ancient, inactive fault.
• Blind fault: A fault that dies out in the subsurface,
thus does not intersect the ground surface.
斷層的成長是由中心向兩
端擴展。斷層中心的位移
最大，向兩端逐漸變小，
端點的位移為零。
斷層線的長度與斷層位移
的關係：
斷層線的長度(L)越長，所
累積的位移(D)越大。
D = cLn
n為碎形維度。
D = 0.03 L1.06
Characteristics of Fault and Fault Zone
• Brittle fault rock: Process of brittle faulting tends to break
up rock into fragments.
• Fault breccia: Creation of random array of nonsystematic
mesoscopic fractures that surrounding angular blocks of
rock. Random fabrics, no distinctive foliation.
• Fault gouge: fine-grained rock flour
• Cataclasite: a cohesive brittle rock that differs from
gouge or breccia in that the fragments interlock, allowing
the fragmented rock to remain coherent even without
cementation. Random fabrics, no strong foliation or
lineation
Characteristics of Fault and Fault Zone
• Protocataclasite: 10-50% of the rock is matrix
• Cataclasite: 50-90% is matrix.
• Ultracataclasite: 90-100% is matrix.
Characteristics of Fault and Fault Zone
• Psuedotachylyte (psuedotachylite): A glassy or
microcrystalline material that forms when friction
heating melts rock during slip on a fault.
Pseudotachylyte flows into cracks between
breccia fragment or into cracks penetrating the
wall of the fault.
• Argille scagliose: A fault rock that forms in very
fine-grained clay or mica-rich rock (shale or slate)
and characterized by the presence of a very
strong wavy anastamosing foliation, the rock
breaks into little scales or platy flakes.
Characteristics of Fault and Fault Zone
• Brittle fault rock: Process of brittle faulting tends
to break up rock into fragments.
• Fault breccia: Creation of random array of
nonsystematic mesoscopic fractures that
surrounding angular blocks of rock. Random
fabrics, no distinctive foliation.
• Fault gouge: Continued displacement across the
fault zone crush and further fragment breccia
and/or break off microscopic asperities
protruding from slip surfaces in the fault zone.
Fine-grained rock flour.
Slickensides and Slip Lineations
Growth of slip fibers
Steps along a fiber-coated fault surface
Subsidiary Faults and Fracture Geometry
The orientation of Riedel (R),
Conjugate Riedel (R’) and pshears. Note that the acute
bisector of the R and R’-shear
is parallel to the remote 1
direction.
Fault-Related Folding
(a)A small flexure develops during
shortening of the layer, and
anticline-syncline pair develops;
(b) En echelon (or stepped) gashes
form in the fold;
(c) A fault breaks through the fold;
cutting through a gentle flexure.
(d) Geometry of a fault-propagation
fold.
Fault-Related Folding
(a) 斷層彎曲褶皺
(Fault-bend fold)
(c)底滑褶皺
(Detachment or
decollement folds)
(b)層內褶皺
(Intraformational folds),
(d)垂簾褶皺
(Drape fold).
Recognizing and Interpreting Faults
• Surface rupture: Seismic faulting along an
emergent strike-slip fault, manifesting by broken
ground and fissures.
• Fault scarp (斷層崖): Displacement on an
emergent dip-slip fault creates a step in the
ground surface.
• Fault-line scarp (斷層線崖): Caused by the
occurrence of a resistant stratigraphic layer that
has been uplifted on one side.
Fault-line scarp(斷層線崖)
Fault-line scarp (斷層線崖)
Recognition of Faults from subsurface Data
1. Abrupt steps on structure-contour maps;
2. Excess section (repeating of stratigraphy) or
loss of section in a drill core;
3. Zone of brecciated rock in a drill core;
4. Seismic-reflection profiles
5. Linear anomalies or abrupt change in the
wavelength of gravity and/or mangetic
anomalies.
Change in fault character with depth
糜稜岩
綠色片岩相
角閃岩相
Anderson’s theory of faulting
1. Faulting represents a response of rock to shear stress,
so it only occurs when differential stress (d=1-2=2s)
does not equal zero.
2. Faults that initiate as Coulomb shear fractures will form
at an angle of about 30o to the 1 direction and contain the
2 direction.
Why isn’t 1 at 45o to the fault plane, where
the shear stress is maximum?
• Recall the role of normal stress, where the ratio
of shear stress to normal stress on planes
orientated at about 30o to 1 is at a maximum.
• The earth surface is a “free surface” (contact
between ground & air/fluid) that cannot transmit
a shear stress. Thus, regional principal stresses
are parallel or perpendicular to the surface of the
Earth in the upper crust.
Limitation of Anderson’s theory
1. Faults do not necessarily initiate in intact rock.
The frictional sliding strength of a preexisting
surface is less than the shear failure of intact
rock, thus preexisting joint surfaces or faults
may be reactivated before new faults initiate.
2. The orientation of a fault may change as the
rock body undergoes progressive deformation.
3. Systematic changes in stress trajectories are
likely to occur with depth in mountain belts (e.g.,
along a regional detachment).
Formation of Listric Fault: Changes in stress
field with depth
Curved principal stress trajectories and listric fault in a
sheet of rock pushed from left side.
1. Top: Free surface
2. Bottom: Frictional sliding surface
Formation of Listric Fault
Predicted pattern of reverse faulting
Fault forms at ~30o to the 1 trajectory
Fluids and Faulting
• Fault rocks are altered by reaction with a fluid phase
(clay: feldspar + water).
• Fault zone: Abundant veins precipitated from fluids (Qz,
calcite, Chlorite, economic minerals)
• Fault valving (活辮) or seismic pumping (地震灌水作用):
Faulting-triggered fluid motion.
Increasing in open space, fluid pressure in the fault zone
temporarily drops relative to the surrounding rock. Fluidpressure gradient drive groundwater into the fault zone
until new equilibrium.
How presence of water in fault zone affects
the stress at which faulting occurs?
1. Alteration minerals in the fault zone have lower shear
strength than minerals in the unaltered rock. It permits
the fault to slip at a lower frictional stress.
2. Presence of water in a rock cause hydrolytic weakness
of silicate minerals; allow deformation to occur at lower
stresses.
3. Pfluid in the fault zone decreases the effective normal
stress in a rock body; thus decreases the shear stress
necessarily to initiate a shear rupture in intact rock or
frictional sliding on a preexisting surface.
Thrust Sheet Paradox: Hubbert-Rubey
hypothesis
(a) Thrust sheet on a
frictional surface.
(b) Because of frictional
resistance, the shear
stress to initiate sliding
exceeds the yield strength
of frontal end of the thrust,
causing fracturing and
folding.
(c) Thrust sheet moves
coherently if it rides on a
“cushion” of fluid.
Stress and Faulting: A Continuing Debate
How large the shear stress must be to initiate faults or to
reactivate preexisting faults?
The magnitude of s necessary to trigger faulting depends
on fluid pressure, lithology, strain rate, temperature, and the
orientation of preexisting fault.
Variation in differential stress necessary to initiate
sliding on reverse, strike-slip, and normal faults as
a function of depth
d  gz
ddifferential stress
(d = 2s)
: 3 for reverse fault
1.2 for strike-slip fault
0.75 for normal fault
: ratio of pore-fluid &
lithostatic pressure (
ranges from 0.4, for
hydrostatic fluid pressure, to
1 for lithostatic pressure
Shear heating
• If movements on a fault surface in the brittle
regime of the crust involves frictional sliding,
some of the works done during fault movement is
transferred into heat.
 s *  = Ee + Es + Q
 : Amount of slip on the fault
 Ee: Energy radiated by earthquakes
 Es: Energy used to create new surfaces
(breaking chemical bonds)
 Q: Heat generated.
Shear heating
s *  > Q
The San Andreas fault is a huge active fault, it is
not bordered by a zone of high heat flow as
predicted by the shear-heating model.
Weak-fault hypothesis
• Direct measurement of stress in the crust around
the San Andreas Fault suggests that the 1
trajectory bends to that it is nearly perpendicular
in the immediate vicinity of the fault. The San
Andreas Fault is behaving like a surface of very
low friction, and therefore could not support a
large s.
Stress drop (應力降)
• Stress drop: When an earthquake occurs,
energy is released and the value of s across
the fault decrease.
• Stress drops for earthquake estimated range
between ~ 0.1 MPa and 150 MPa, but are
typically in the range of 1-10 MPa.
Fault Systems
(a) Parallel array
(b) Anastomosing array
(c) En-echelon array
(d) Relay array
(e) Conjugate array
(f) Random array
Normal Fault Systems
Synthetic Fault
(同向斷層)
Half graben
Graben
Antithetic Fault
(反向斷層)
Reverse Fault Systems
Thrust systems are common along the margin of
convergent plate boundaries and collisional orogen.
Thrusting occurs in conjunction with the formation of
folds, resulting tectonic provinces called fold-thrust
belts (褶皺逆衝斷層帶).
Imbricate fan (疊瓦狀排列) in thrust system
Reverse Fault Systems
Duplex : Consist of thrust that span the interval of rock
between a high-level detachment called a roof thrust
and a lower detachment called a floor thrust.
Strike-Slip Fault System
Flower structures: They splay into many separate
faults in the near surface, resembles the head of
flowers.
Faulting and Society
1989 Loma Prieta earthquake damage
Museum of the City of San Francisco
Faulting and Resources
1. Oil: oil traps, oil migration and reservoirs.
2. Ore minerals:
(a) hydrothermal fluids and veins
(b) fault breccia
3. Groundwater: A fault zone may act as a
permeable zone through which fluids migrate.
Faulting and Earthquakes
• Inactive fault: they haven’t slipped in a long time,
and are probably permanently stuck.
• Active faults: they have slipped recently or have
the potential to slip in the near future.
• Seismic fault: If an increment of faulting causes
an earthquake.
• Aseismic fault: Offset occurs without generating
an earthquake. Aseismic fault = fault creep
Active faults in Taiwan
Fault creep
造山作用之見證二:花東縱谷中池上
斷層的潛移
東
西
如何觀察破裂面的變形?
釘網成果：人造建物破壞面之變形速率
s
te
r
ho
n
ni
/ yr
m
m
27
:
te
a
gr
in
n
rte
o
sh
yr
/
m
m
23
:
ate
r
g
地殼變形精密測量儀
地殼變形精密測量儀之精密度
大坡國小地殼變形精密測量儀之成果
Stick-slip (粘滑) behavior
Laboratory frictional
sliding experiments on
granite. The stress
drop (dashed lines)
correspond to slip
event.
Recurrence interval
• Recurrence interval: Average time between
successive faulting events.
霧峰 鳳梨園
台大地質科學系 陳文山教授
南投 竹山
N-1 event
N-2 event
Chichi Earthquake
台大地質科學系 陳文山教授
古地震的垂直位移量
•
槽溝位置 921
•
•
•
•
•
•
‧
•
•
萬豐 1.3m
名間 2.5m
鳳梨園 1.6-2.1m
釋迦園 1.8m
仙公廟 1.1m
東陽里
中正公園 5-6m
文山農場 0.5m
竹山
1.8m
n-1(<300)
0.4m
1.0m
1.0m
n-2(~700-800)
n-3(1900) n-4(3100)
1.2m
2.0m
1.1m
1.8m
1.3m
0.2m
0.3m
2.2m
1.0m?
1.0m?
?
0.5m
7.3m(<3340BP)
台大地質科學系 陳文山教授