Transcript Structural Geology

Structural Geology
Joints and Veins 2
Lecture 15 – Spring 2016
1
Data Collection in the Field
• Data collection can be done as an inventory,
collecting data on all joints within an area
or crossing a line, or by selectively
surveying an area, to visually assess what
the major joints or joint sets are
• Representative joints are measured
2
Inventory Methods
• Figure 7.12 in text
• Inventory methods can be
quite time-consuming, and
may be cluttered by data
on nonsystematic joints
• Inventory methods can
provide data to assess joint
density, joint orientation,
joint spacing, and can be
used with both systematic
and nonsystematic joints
3
Selective Method
• The selective method works well when joints have
large spacings, beyond the scope of a normal area
survey, and does not clutter data collection with
reams of information on nonsystematic joints
which may not be useful
• But it is a selective, and therefore subjective,
process
• People sometimes see what they want to see, data
that match preconceived notions of what happened
in a given region
4
Representation of Joint Data
• Joint data can be represented by plotting the
strike and dip of joints on geologic maps, or
on topographic maps used as base maps
5
Joint
Trajectories
• Figure 7.13a
in text
• Joint trajectories can be plotted to assess joint
attitude in a given region
• Trajectories represent the trend of joints; not
necessarily the trace of individual joints
6
Statistical Maps
• Statistical maps that show the orientation of
many joints within a region can help show
dominant joint orientations within the
region
• If joints are not vertical, they are best
represented on equal area nets
7
Vertical Joints
• If joints are vertical,
so that only twodimensions need be
considered, a
histogram plot of the
joint density versus
strike can be used
(Figure 7.13b in text)
8
Rose Diagram
• The Rose diagram is a
type of polar
histogram
• Bearings are shown
directly on the
diagram
• Figure 7.13c in text
9
Unroofing
• The process is known as unroofing
• Figure 7.14 in text
10
Shrinkage
• As rock cools. it shrinks
• Rock can contract in the vertical direction without causing
secondary effects, since the surface of the earth is a free
surface, and cannot transmit shear stress
• Shrinkage in the horizontal directions causes horizontal
tensile stresses to develop
 Expansion in the vertical direction also leads to contraction in the
horizontal directions, due to the Poisson effect
• Membrane Effect - Each layer acts like a membrane, and
uplift stretches the membrane and increases its radius of
curvature, adding to the horizontal tensile stress
11
Tensile Stress
• If the sum of the tensile stresses overcomes lithostatic
burial pressure and the tensile strength of the rock, it
causes the rock to crack and form vertical joints
 Vertical joints indicate that σ3 is horizontal
 Since the earth’s surface is a free surface, it must be a principal
plane of stress
 The other two principal planes must be vertical
• This type of joint formation is likely to be important in
continental interiors, especially in sedimentary basins.
12
Orogen
• Belt of deformed rocks often accompanied
by metamorphic and plutonic rocks
• An extensive belt of rocks deformed by
orogeny, associated in places with plutonic
and metamorphic rocks
13
Epeirogenic Movements
• Gentle vertical land movements of regional extent
are called epeirogenic movements
• Literally, making of continents
• To be exact, it is an action of uplift or subsidence
of large area of continent or ocean basins
 Uplift creates domes
 Subsidence forms basins
 Sea level can change because of the movement of seabed
14
Sheeting Joint Diagram
• They appear to form where horizontal stresses are
considerably higher than the vertical load pressure
• Joint spacing less near surface
• Figure 7.15a in text
15
Development of Elastic Strain
• Since the wall rock and
pluton behave differently,
elastic strains develop
• The pluton has welded
itself to surrounding
rock, so the differential
strain creates elastic
stress in the pluton
• Figure 7.15b in text
16
Unroofing
• Commonly, the pluton
shrinks more than the
surrounding rock, and tensile
stresses develop
perpendicular to the wall
• The resulting residual stress
may exceed the weak load
pressure, allowing tension
joints to form
• Figure 7.15c in text
17
Exfoliation in Granite
• Tioga Pass Road, Yosemite National Park
• Photography by Dr. Sharon Johnson,
University of California, Berkeley
18
Granite Domes Formed by Exfoliation
• Tanaya Lake, Yosemite National Park
• Photography by Dr. Sharon Johnson,
University of California, Berkeley
19
Natural Hydraulic Fracturing
• We have seen that it is possible to inject fluids in
the ground and cause fracturing, in a process
called hydraulic fracturing
• Sometimes, the process occurs naturally
• The fluid can be water, petroleum, or natural gas
• If the cracks are near the surface, they may
intercept the surface, and allow fluid to leak out
20
Santa Barbara
Channel
• Natural oil seeps in the Santa Barbara
channel off the coast of California lead to the
discovery of oil reserves there
• Hydraulic fracturing due to petroleum
pressure may have played a role in
developing the seepage
21
Opening Stress
• A rock like a cemented
sandstone may have both its
pores and cracks filled with
fluid
• The fluid pressure within the
crack creates an opening
stress (σo)
• Figure 7.16 in text
22
Inward Pressure
• Fluid pressure also pushes inward, creating
two types of closing stress, σcg and σcp
• The “cg” subscript stands for a closing
stress applied through the cracks in contact
with the grain
• The “cp” stress means a closing stress
through the crack’s contact with a pore
23
Joints Related to Tectonic Deformation
• Convergent or collisional orogenic events
produce compressive stresses that affect
rocks over broad regions
• In the foreland region of the orogen, joints
may form for several reasons
• The maximum horizontal stress is
approximately perpendicular to the trend of
the orogen
24
Natural Hydrofracturing
• Natural hydrofracturing produces joints that are
parallel to the σ1 direction of tectonic features,
such as folds
• These joints may contain mineral infilling of a
type consistent with depths and pressures of
several kilometers below the surface, which
indicates they are not formed by near-surface
tension cracking
 They are thought to result from an increase in fluid
pressure due to overburden pressure from thrust sheets,
or from deposition of material eroded from the
continental interior
25
Joints Not Parallel to Fault
• Figure
7.17a in text
• Since faults are
usually inclined to
the remote σ1
direction, the joints
formed in the stress
field that caused the
fault to move will not
be parallel the fault
26
Thrust Fault Joints
• Figure
7.17b in text
• Movement along a
thrust fault,
particularly if the
fault surface is not
planar, may cause
warping, with
associated local
tension and jointing
27
Pinnate Joints
• Wall rock immediately adjacent to the fault may
be affected by tensile stresses, creating short
joints at angles between 30-45º to the fault
• These are called pinnate joints
• Figure 7.17c in text
28
Tension from Folding
• Folding may produce local tensile stresses
as layers bend
• If metamorphic conditions are not
produced, tension cracks may develop
29
Joint Convergence
• In outer-arc environments, this leads to joints
that are parallel to the fold hinge, and which
may converge at depth to the core of the fold
30
• Figure 7.18 in text
Ladder Pattern
• Ladder pattern has
long joints with
much shorter
perpendicular
joints, terminating
at the long joints
• Figure 7.19a in text
31
Grid Pattern
• Alternatively, we
can see joints
which appear to be
mutually crosscutting defining a
grid pattern
• Figure 7.19b in text
32
Orogenic Foreland Region
• Joint sets are usually a strike-parallel set with a set
of orthogonal cross-joints
• Cross-joints are parallel to the regional maximum
horizontal stress trajectory
• They are probably formed as syntectonic natural
hydrofractures
• The strike-parallel joints could be release joints
formed by relaxing of stress, or out-arc extension
of folds
33
Tensile Stress Regions
• Joints may develop perpendicular to the
regional tensile stress
• If this stress relaxes, elastic rebound occurs,
and slight expansion may occur in the
direction perpendicular to the original
stretching
• This causes a new set of joints,
perpendicular to the original joints, to form
34
Uplift
• During uplift, erosion may unload pressure from
rock below
• A joint set perpendicular to the regional σ3
develops
 Continued uplift may open these joints further
• Stress parallel to the existing joints cannot be
relieved this way
 It accumulates and forms a new set of joints orthogonal
to the first
35
Alternating Stress
• Grid patterns may be caused by two joint sets
initiating at the same time, or alternative cracking
episodes on each set
• If both sets form in the principal plane
perpendicular to σ3, it is clear that the stress field
is changing with time
• One possibility is that σ2 and σ3 are switching
back and forth
 This works if both stresses are similar in magnitude.
36
Conjugate Joint Systems
• Some orogenic forelands display conjugate joint
systems with the bisector of the dihedral angle
being perpendicular to the fold axis.
• Such folds often display plumose patterns,
indicating they formed as Mode I fractures
• Traditionally, it was thought that conjugate
fractures were either shear fractures, formed at
about 30̊ to σ1, or “transitional-tensile” fractures,
formed at angles less than 30̊ to σ1
37
Cross-Strike Joints
• Current thinking is that both sets of joints are
cross-strike joints, initially formed perpendicular
to σ3
• The two sets would form at different times, with
different stress fields
38
Devonian Joints in New York
• In south-central New York, there are two joints
sets, separated by an angle less than 60̊
 The rocks are Devonian, and slightly folded
 It is thought the joint sets formed during the late
Paleozoic Alleghanian orogeny
• Two different episodes orogenic activity took
place
 The maximum horizontal stress during the first and
second episodes were not parallel
 Slip lineations on such faults can then be interpreted as
mesoscopic faults reactivated subsequent to their
formation, rather than as shear fractures
39
Joints in Igneous Rocks
• Igneous rocks can form joints, either within the
igneous rock itself, or in the surrounding country
rock
• A plutonic intrusion can stretch the rock around it,
causing tension fractures to form
 The shape of the intrusion determines the joint pattern
 Circular plutons often produce radial fracturing
 The fractures may bend and become parallel to the
regional maximum horizontal stress at a distance from
the pluton.
40
Hypabyssal Intrusions & Lava Flows
• Hypabyssal intrusions or lava flows may also be
subject to rapid cooling and contraction
 Cooling takes the rock below the brittle/plastic
transition, and elastic strain develops
 When tensile stress exceeds the rock strength, it breaks
 Shrinkage is equal in all directions, so several sets of
joints form
 Usually, there are three sets at about 120̊ forming to
create a hexagonal pattern
41
Paleostress Indicators
• Since joints propagate normal to σ3, their
planes define the trajectories of σ3 within
their region
• If the joint is vertical, the strike of the joint
defines the trajectory of maximum
horizontal stress
• However, we don’t know if this is σ1 or σ2
42
Large Joint
• Walls of Entrada Sandstone border Park Avenue in
the Courthouse Towers area, Arches NP
• Joints in the thin bedded shale beneath the sandstone
are much more closely spaced
43
• USGS photo
Stress Shadows
• Two joints may grow toward each other If they
are not coplanar, they may enter each others
stress shadow, and terminate
• Figure 7.21a in text
44
Stress Shadow Overlap
• Map view showing
that joints cannot
pass each other
because they are
too close together –
their stress shadows
overlap
• Figure 7.21c in text
45
Joints Combine
• The joint tips may interact, changing the
local stress field, and the joints may grow
together
• Figure 7.21b in text
46
Other Joint Terminations
• Joint growth can cause local drops in fluid
pressure
 The joint stops growing until fluid pressure builds up
again
 Multiple arrest lines show this type of behavior
• The joint may grow into a different type of rock,
where plastic yielding is easier
 Plastic flow then terminates the joint
 Alternatively, if the joint grows into a stronger rock, it
may not be able to crack it
47
Veins and Vein Arrays
• Veins are fractures that has been filled by
precipitation of material from solution
 By far the most common vein forming materials are
quartz and calcite
 Other substances found in veins include ore minerals,
mostly sulfides, zeolites, and chlorite
• The fractures may be either joints or shear
ruptures
• Veins range in size from the width of a hair to
several meters, and can be many meters long
• Veins can also form in groups, called vein arrays
Planar Systematic
Array
Figure 7.22a in text
• Vein arrays can occur in a variety
of ways
• A planar systematic array has a
group of veins, mutually parallel,
with a nearly constant spacing
• These form by mineralization
during or subsequent to the
formation of a systematic joint
set
49
Stockwork Vein Arrays
• Stockwork vein arrays
are the result of
shattering of rock, either
by extremely high fluid
pressures, or locally
pervasive fracturing
associated with tectonic
faulting and folding
Figure 7.22b in text
50
En Echelon
Veins
Figure 7.23a in text
• En echelon veins may
form by the infilling of
en echelon joints in
the twist hackle fringe
of a larger joint
• They can also form by
shear across a fault
zone
51
En Echelon Vein Formation
Figure 7.23b in text
• The en echelon veins are
initiated parallel to σ1,
often at an angle of about
45º to the shear borders
• As the shear develops,
the fractures open
• This allows them to fill
with vein material
• Once filled, they are
material objects
52
En Echelon
Vein
Formation
Figure 7.23c in text
• Further shearing will
rotate them and the
acute angle increases
• If further vein growth
occurs, it will be at 45º
to the shear borders,
and the veins will
become sigmoidal
53
Blocky Crystals
• Vein material may be blocky or
fibrous
• Blocky crystals form in open
cavities
• Open cavities form near the
surface, where rock strength
allows cavities to stay open, or
fluid pressure if great enough to
hold the fracture open
• Figure 7.24a in text
54
Fibrous Crystals
• The formation of fibrous crystals is somewhat
more controversial
• One mechanism is the crack-seal mechanism
• A rock must contain pore fluid with dissolved
minerals
• If fluid pressure becomes great enough, the rock
cracks a few microns
• Fluid rushes in, locally lowering the pressure
55
Formation of
Fibrous
Crystals
• Lower pressure can lower the solubility of the
mineral, and initiate precipitation, which seals
the crack
• The process then repeats, over and over again
• Figure 7.24b in text
56
Syntaxial Growth
• Cracking occurs at center
of vein
• Composition of fibers and
wall rocks is identical
• Figure 7.26b in text
57
Antitaxial Growth
• Vein and wall rock
composition are
different
• Cracking occurs along
vein margins
• Figure 7.26a in text
58
Fibrous Antitaxial Vein
• Each time the crack
widens a small amount
• The existing crystals
act as nuclei, and the
crystals elongate, with
veins growing outward
from the center
Fibrous antitaxial vein in
limestone – field of view
about 1 mm
59
Antiaxial
Calcite
Veins
• Tapley Hill Formation,
Opaminda Creek, Arkaroola,
South Australia
• Width of view 13 mm,
crossed polars
• Tips of two parallel en échelon antitaxial fibrous calcite veins
• Mean fiber width increases slightly from the median line
outwards, which indicates that growth was outwards (antitaxial)
• Fiber shape is symmetric around the median line, except near the
tips
• Growth and propagation of the veins caused bending of the shale
"bridge" in between
60
Stretch Perpendicular to Vein
• The direction of the fibrous
elongation often marks the
direction of stretching
• Figure shows stretching
perpendicular to the vein, with
the long axis of the fibers
parallel to stretching, and
perpendicular to the wall
61
• Figure 7.27a in text
Stretch Oblique to Vein
• Figure shows the fibers at an
oblique angle to the wall
• Sigmoidal shaped fibers indicate
that the extension direction
rotated relative to vein wall
orientation
• Figure 7.27b in text
62
Change in Stretching Direction
• Figure shows that the order of movement depends
on whether the growth is syntaxial or antitaxial
• Figure 7.27 cd in text
63
Linaments
• Lineaments are linear features recognized
on aerial photos, satellite imagery, or
topographic maps
• They exist at regional scale, but not as
mesoscopic or microscopic features
• They may not be initially recognizable from
the ground
64
Lineament Photo
• Structural lineaments are defined by structurally
controlled alignment of features like ridges,
depressions, or escarpments
• Duncan lake region, Northwest Territories, Canada
• Figure 7.28 in text
65
Brockton-Froid Lineament,
Montana
• The Brockton-Froid
lineament using
analytical hillshading
for visualization
• The lineament is
illuminated by a light
source located at an
azimuth of N35W and
an angle from
horizontal of 45º
66