Transcript Structures

Rock Structure and Fault
Activity
chapter 9
What is structural geology
The study of the forms of the Earth’s crust
and the processes which have shaped it
• analysis of displacement and changes in
shape of rock bodies (strain)
• reconstruct stress that produced strain
Structural Deformation
Rocks deform when
stresses placed upon
them exceed the rock
strength
• Brittle deformation
(e.g. fractures)
• ductile deformation
(e.g. folds)
Driving Forces
• Plate tectonics – plate convergence and ridge
spreading
• Deep burial of sediments
• Forceful intrusion of magma into the crust
• Meteorite impacts
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Evidence of Crustal
Deformation
Folding of strata
Faulting of strata
Tilting of strata
Joints and fractures
Evidence of Crustal
Deformation
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Folding of strata
Faulting of strata
Tilting of strata
Joints and fractures
Evidence of Crustal
Deformation
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Folding of strata
Faulting of strata
Tilting of strata
Joints and fractures
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Evidence of Crustal
Deformation
Folding of strata
Faulting of strata
Tilting of strata
Joints and fractures
Applications of structural
geology
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subsurface exploration for oil and gas
mining exploration
geotechnical investigations
groundwater and environmental site
assessment
Geological structures
• Geologic bed contacts
• Primary sedimentary structures
• Primary igneous structures
• Secondary structures
Fundamental Structures
Three fundamental types of geologic
structures:
• bed contacts
• primary structures - produced during
deposition
or emplacement of rock body
• secondary structures - produced by
deformation
and other process after rock is emplaced
Bed Contacts
Boundaries which separate one rock
unit from another
• two types:
1. Normal conformable contacts
2. Unconformable contacts
(‘unconformities’)
Conformable Bed Contacts
Horizontal contact between rock units
with no break in deposition or
erosional gaps
• no significant gaps in geologic time
Book Cliffs,
central Utah
Unconformable Contacts
Erosion surfaces representing a
significant break in deposition (and
geologic time)
• angular unconformity
• disconformity
• non-conformity
Angular Unconformity
Bedding contact which discordantly cuts
across older strata
• discordance means strata are at an angle to
each other
• commonly contact is erosion surface
Formation of an angular
unconformity
Disconformity
Erosional gap between rock units
without angular discordance
• example: fluvial channel cutting into
underlying sequence of horizontally
bedded deposits
Nonconformity
Sedimentary strata overlying igneous or
metamorphic rocks across a sharp contact
• example: Precambrian-Paleozoic contact in Ontario
represents a erosional hiatus of about 500 ma
Grand Canyon, USA
Structural Relations
The structural relations between bed
contacts are important in
determining:
1. presence of tectonic deformation/uplift
and;
2. relative ages of rock units
• principle of original horizontality
• principle of cross-cutting
• principle of inclusion
Principle of Original
Horizontality
Sedimentary rocks are deposited as essentially
horizontal layers
• exception is cross-bedding (e.g. delta foresets)
• dipping sedimentary strata implies tectonic uplift and
tilting or folding of strata
Principle of Cross-cutting
Igneous intrusions and faults are
younger than the rocks that they
cross-cut
Mafic dike cutting across older sandstones
Cross-cutting Relations
Often several cross-cutting
relationships are present
• how many events in this outcrop?
Principle of Inclusion
Fragments of a rock included within a
host rock are always older than the
host
Fundamental Structures
Three fundamental types of structures:
• bed contacts
• primary structures
• secondary structures
Primary Sedimentary
Structures
Structures acquired during deposition of
sedimentary rock unit
Stratification - horizontal bedding is most
common structure in sedimentary rocks
Primary Sedimentary
Structures
Cross-bedding - inclined stratification
recording migration of sand ripples or
dunes
Primary Sedimentary
Structures
Ripples - undulating bedforms produced by
unidirectional or oscillating (wave) currents
Ripple
marks
Primary Sedimentary
Structures
Graded bedding - progressive decrease in
grain size upward in bed
• indicator of upwards direction in deposit
• common feature of turbidites
Primary Sedimentary
Structures
Mud cracks - cracks produced by
dessication of clays/silts during
subaerial exposure
Primary Sedimentary
Structures
Sole marks - erosional grooves and marks
formed by scouring of bed by
unidirectional flows
• good indicators of current flow direction
Primary Sedimentary Structures
Fossils – preserved remains of organisms, casts or
moulds
• good strain indicators
• determine strain from change in shape of fossil
• relative change in length of lines/angle between
lines
Primary Igneous Structures
Flow stratification
• layering in volcanic rocks produced by
emplacement of successive lava
sheets
• stratification of ash (tephra) layers
Primary Igneous Structures
Flow stratification
• layering in volcanic rocks produced by
emplacement of successive lava
sheets
• stratification of ash (tephra) layers
Primary Igneous Structures
Pillow lavas - record extrusion and
quenching of lava on sea floor
Importance of Primary
Structures
1. Paleocurrents - determine paleoflow directions
2. Origin – mode of deposition, environments
3. Way-up - useful indicators of the direction of
younger beds in stratigraphic sequence
4. Dating - allow relative ages of rocks to be
determined based on position, cross-cutting
relations and inclusions
5. Strain indicators - deformation of primary
structures allows estimates of rock strain
Secondary Structures
Secondary structures - deformation
structures
produced by tectonic forces and other
stresses in crust
Principle types:
• fractures/joints
• faults/shear zones
• folds
• cleavage/foliation/lineation
Fractures and Joints
Fractures – surfaces along which rocks
have broken and lost cohesion
Joints - fractures with little or no
displacement parallel to failure
surface
• indicate brittle deformation of rock
Fractures and Joints
Faults
Faults - fracture surfaces with appreciable
displacement of strata
• single fault plane
• fault zone - set of associated shear fractures
• shear zone - zone of ductile shearing
Shear Zones
Shear zone - zone of deformed rocks that are more
highly strained than surrounding rocks
• common in mid- to lower levels of crust
• shear deformation can be brittle or ductile
Fault Terminology
Hanging wall block- fault block toward which
the fault dips
Footwall block - fault block on underside of
fault
Fault plane – fault surface
Fault Slip
Slip is the fault displacement described
by:
• direction of slip
• sense of slip
• magnitude of slip
Fault Types
Dip-slip faults - slip is parallel to the
fault dip direction
normal
reverse
thrust
Fault Types
Normal fault - footwall block dispaced
up
Fault Types
Reverse (thrust) fault - footwall block
displaced down
Fault Types
Strike-slip – fault slip is horizontal,
parallel with strike of the fault plane
• right-handed (dextral)
• left-handed (sinistral)
Fault Types
Oblique slip – Combination of dip- and
strike-slip motion
• dextral-normal
• dextral-reverse
• sinistral-normal
• sinistral-reverse
Faults
What type of faults are shown here?
Faults
What type of faults are shown here?
Faults
What type of faults are shown here?
Faults
What type of faults are shown here?
Folds
Folds – warping of strata produced by
compressive deformation
• range in scale from microscopic features
to regional-scale domes and basins
• indicators of compression and shortening
Fold Terminology
Hinge (Axial) plane - imaginary plane bisecting fold
limbs
Hinge line - trace of axial plane on fold crest
Plunge - angle of dip of hinge line
horizontal fold axis
plunging
fold axis
Fold Terminology
Anticline - convex in direction of
youngest beds
Syncline - convex in direction of oldest
beds
Antiform - convex upward fold
(stratigraphy unknown)
Synform - concave upward fold
Anticline / Antiform?
Syncline
Synform?
Fold Terminology
Synformal Anticline - overturned anticline
Antiformal Syncline - overturned syncline
Fold Terminology
Monocline - step-like bend in strata
Foliation and Cleavage
Foliation - parallel alignment of planar fabric elements within a
rock
Cleavage - tendency of rock to break along planar surface
cleavage is a type of foliation
• resemble fractures but are not physical discontinuities
Foliation and Cleavage
Foliation - parallel alignment of planar fabric elements within a
rock
Cleavage - tendency of rock to break along planar surface
cleavage is a type of foliation
• resemble fractures but are not physical discontinuities
Lineations
Lineation - sub-parallel to parallel alignment of
elongate linear fabric elements in a rock body
• e.g. slickenlines and grooves on fault plane surface
Structural analysis
Involves three steps
1. Descriptive or geometric analysis
2. Kinematic analysis
3. Dynamic analysis
Geometrical analysis
Measurement of the 3-dimentional
orientation and geometry of
geological structures
simplified into:
• lines
• planes
lines or linear geological
structures
• liniation
– any linear feature observed in a rock or
on a rock surface
– any imaginary line – such as a fold axis
Orientation of linear
structures
LINES
Trend – azimuth direction measured clockwise
from north 360°
Plunge – angle of inclination of line measured from
the horizontal (0 - 90°)
Examples of linear
structures
• Primary – flute casts, grooves, glacial
striae
• Secondary – slickenlines, mineral lineations
Glacial striations on bedrock
sole marks
Examples of linear
structures
• Primary – flute casts, grooves, glacial
striae
• Secondary – slickenlines, mineral lineations
Grooves on fault plane
Slickenlines on fault surface
Orientation of Linear
Structures
linear structures on an other planar
surface:
• pitch angle
– angle from horizontal measured within
the plane
Striations
on a fault
plane
Pitch
angle
Planar Geological Structures
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bedding planes and contacts
foliation
joint surfaces
fault planes
fold limbs
fold axial planes (imaginary surface)
Examples of Planar
Structures
Bedding planes – most common
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primary depositional surface
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erosional surface
inclined bedding plane
Examples of Planar
Structures
Foliation – cleavage planes produced by
metamorphism
• common in slates and phyllites
foliated phyllite
Examples of Planar
Structures
Joint planes – planar fracture
surfaces caused by brittle failure
Examples of Planar
Structures
Fold axial plane - imaginary plane
bisecting limbs of fold
Orientation of Planar
Structures
The attitude of a plane can be
established from any two lines
contained in the plane, provided they
are not parallel
Orientation of Planar
Structures
Strike – azimuth direction of a
horizontal line in a plane
Dip – angle of inclination of line measured
from the horizontal (0 - 90°)
Orientation of Planar
Structures
Appearent dip
– dip measured along
a line other than
90 to strike
– apparent dip will
always be less than
the true dip angle
Measurement of orientation
Strike (plane)
Trend (line)
azimuth orientation measured with a compass
Measurement of orientation
Strike (plane)
Trend (line)
azimuth orientation
measured with a
compass
Dip (plane)
Plunge (line)
inclination measured using
an inclinometer
Measurement of Strike
Direction
Right hand rule???
When your thumb (on your right hand)
is pointing in the direction of strike
your fingers are pointing in the
direction of dip!!
Measure of Dip Angle
The angle between the horizontal and
the line or plane
Structural Data
Symbols represent different structural
data
Symbols are placed on the map:
– in the exact field orientation
– where the data is measured
Standard Structural
Symboles
Exercises
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geological maps
structure contour and structure maps
three-point problems
cross sections
sterionets
Geological Maps
• distribution of rock types and
contacts
– symbols on map represent structures
(strike and dip, fold axes, faults etc.)
– map and structure symbols allow you to
infer subsurface structures
Outcrop patterns
Outcrop patterns controlled by
attitude (strike and dip) of beds and
topographic relief
“V” Rule
• Beds dipping downstream “V” –
downstream
• Beds dipping upstream “V” – upstream
Vertical beds cut straight
Vertical oriented beds cut in a straight
line regardless of topography!!
Horizontal beds
• layers always at the same altitude –
do NOT dip in any direction
– layered cake
Outcrop Patterns
Which direction are the beds dipping?
Outcrop Patterns
Which direction are the beds dipping?
Outcrop Patterns
Which direction are the beds dipping?
Outcrop Patterns
Which direction are the beds dipping?
Block models
Relations between outcrop patterns and
subsurface structures
map view on bottom – cross sections in blocks on top
Bryce 3-D modeling blocks
Structural Contour Maps
Map showing the relief of a subsurface
geological surface
– top or bottom of bedding planes, faults
or folded surface
– constructed from borehole data
Structure Contour Maps
Structure contour lines are lines of equal elevation
• show elevation relative to horizontal datum
• values are often negative since subsurface
elevations are commonly below sea level
Datum Surface
Datum is a horizontal reference
surface
• regional stratigraphic surface
Constructing Structural
Contours
Points of equal elevation along a bed contact
• intersection of contact with topo contour
• draw structure contours through points of equal
elevation
Planar surfaces
Uniformly dipping plane – contours are
parallel
folded planar surfaces
Contours have variable spacing
Rules of Contouring
1) contours cannot cross or bi-furcate
2) contours cannot end in the middle of the
map, except at a fault or other
discontinuity
3) same contour interval must be used across
the map and elevations must be labelled
4) elevation is specified relative to datum
(e.g. m above sea level)
Determining Dip
Dip direction and angle can be determined from structure
conour maps
• measure horizontal separation X and find difference in Z
• tan  = Z/X,  = tan –1 (Z/X)
• e.g.  = tan –1 (10m/100m),  = 6°
Three-point problem
A minimum of three points are required
to uniquely define the orientation of a
plane
Three-point problem
• Find min and max
values
• Draw line between
these and divide
distance into
intervals
• Connect points of
equal elevation
• Two points in a plane
at the same elevation
lie in the line of
strike
Three-point problem
• Find min and max
values
• Draw line between
these and divide
distance into
intervals
• Connect points of
equal elevation
• Two points in a plane
at the same elevation
lie in the line of
strike
Isochore Map
Drill hole logs giving the thicknesses in the drilled (often
vertical) direction
Apparent thickness – true thickness = perpendicular to
bedding
Isopach Map
Map showing “true” thickness measured
perpendicular to bedding
Cross-sections
Cross-section is a 2-D slice
through stratigraphy
• construct perpendicular to dip
= true dip
• constructed at any other
direction = apparent dip
Engineering properties of
faulted or folded rock
• shear strength
– loose materials
– compressive materials
– permeable materials
hydrology of fault zones
• water in fault zones common due to
fractured rock
– fault zone may be either an aquifer or an
aquiclude
• crushed to gravel
• crushed to clay
hydrology of fault zones
• water in fault zones common due to
fractured rock
– fault zone may be either an aquifer or an
aquiclude
• crushed to gravel
• crushed to clay
Problems due to water in
fault zones
• leakage of waste water under a landfill
• leakage of water under a dam
• sudden collapse and inflow of water into a
tunnel
• hydrothermal alteration of rocks to clay
minerals along faults – variable physical,
mechanical and hydrological properties
• soluble rocks - cavities
Activity of faults?
• Risk for further movement
– active fault – has moved in the last 100 000 to
35 000 years
– dormant fault – no recorded movement in
recent history
Indicators of fault
movement
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fault scarps
stream displacement
sag ponds lineaments
vegetation displacement
Risk potential depends upon:
1. duration of the quake
2. intensity of the quake
3. recurrence of the quake
Potential trigger’s
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stess > stength
water in a reservoir – added weight and
lubrication
storage of fluids in old mines
blasting
surface excavation
ground water mined from aquifers
extraction of oil and gas from aquifers
Case studies
• Auburn Dam – wide slender arch dam on
the American River, upstream of
Sacramento, California
• Fig. 9.31
• pre investigations
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detailed mapping
8 km trenches
2 km exploratory tunnels
30 km borings
Auburn Dam
• geology
– metamorphic competent amphybolite
– metasediments
– included vertical weak zones and lenses of chlorite
schist, talc schist and talcose serpentinites up to 30 m
wide, aligned with foliation
– series of sub parallel mineralized reverse faults with
strike transverse to the dam axis dipping 40 to 55
degrees into the abutment
– two of the longest faults are tangential to the dam, close
or under the dm on the left abutment
– no active faults in the area
– the area was supposed to be low seismicity
Auburn Dam
• foundation construction
– earthquake occurred 5.7
– regional fault study
– reassessment
• 32 km trenches
• more borings
• surface excavations
aim to establish the time relationship of
the faults
Auburn Dam
Concluded that the faults wee formed in
another tectonic setting than the present
(compressional rather that extensional
stress field)
A review of the dam – will it withstand
vibrations from a 6.5 magnitude quake on a
fault < 8 km from the dam??
“Off set” design recommended to withstand
25 mm to 900 mm
NO DAM built due to discussions on safety!
Baldwin Hill reservoir – failed
1963
• 1 principle embankment, 47 m
high, and 5 smaller
embankments
• excavated hollow in between at
the top of a mountain range
Baldwin Hill reservoir – failed
1963
• geology
– friable deposits of the Pliocene Pico Formation, massive beds
of clayey, sandy siltstone
– Pleistocene Ingewood Formation. interbedded layers of sand,
silt, and clay, with some thin linestone beds; some of the
sand and silt beds are unconsolidated and erodable
– Both formations contain calcareous and limonitic concretions
– bedding dips slightly 5 to 7 degrees, striking roughly parallel
to the Inglewood fault
– major active fault, Inglewood, passes just 150 m west of the
reservoir
– the fault is a right lateral strike slip with a vertical
component
– fault acts as a subsurface dam for a major oil field in the
hills
Baldwin
• Excavation phase
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7 minor faults wee mapped
mostly normal faults
3 to 100 mm silty gouge
largest fault had a total displacement of
more than 8 m
Baldwin
• Design
– rock foundation lined with
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asphalt and
gravel drain layer
covered with compacted clay
covered with asphalt
Baldwin
• Construction phase 1947-51
– fault 1 caused problems
– slide initiated revealing that the fault
passed beneath the inlet/outlet tower
– the tower was relocated 48 ft
Baldwin
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liner cracked along the trace of the fault
emptied in 1957
cracks repaired
cracks were also observed in the surrounding
area of the reservoir
the cracks dipped steeply
trend NS parallel to the faults
some exhibited small sinkholes – indicative of
extensional strain
offset dip slip
Baldwin
• nearby oil fields – oil was being extracted
– resulted in subsidence due to collapse of the aquifer
– subsidence of 2.7 m between 1917 and 1962
Baldwin
• Failure 1971
– emptied completely in 4 hours
– seepage along the fault had enlarged to
a pipe
– then to a tunnel and
– then the collapse of the roof
– a canyon eroded completely through the
all of the reservoir
Baldwin
• Failure 1971
– Why??
• cracks in the floor extended across the entire
reservoir along the trace of the fault
• 50 mm displacement
• open voids along the fault
• movement along the fault had fractured the lining
• rupture of the asphalt membrane
• water eroded cavities into the foundation rock