Transcript Chapter 7 Joints k

Lecture 7 Joints
Form perpendicular to weakest stress, often tensile s3
Mostly Chapter 7
Joints and veins
Joint: a fracture without measurable shear displacement (cracks or tensile
fractures)
Fault: a fracture with measurable displacement
Vein: a fracture filled with minerals precipitated from solution
Calcite veins fill joints
A fault offsets layers of sediment
SS and clay layers
Surface morphology
Plumose structure: wavy structure
on joint
Spreads outward from joint origin
MODES
Divergent
See figure 6.11
Shear (Transform)
Dip-slip & rotation
Surface morphology
Why does the plumose structure form?
Mode 1 loading: should yield smooth fractures perpendicular to s3.
BUT real joints are not perfectly smooth.
Rocks are not homogeneous.
these imperfections distort the local stress field
The stress field at the tip of the propagating crack changes
Joint Spacing in sedimentary rocks
Joints are mostly evenly spaced
Widely or closely spaced, partially
depending on length of time tensile
stress applied
Joint spacing and bed thickness:
Closely spaced in thin bedded rx
Wide spaced in thick bedded rx
Stress shadows
Rigid grid
Greater length of joint has
a wider stress shadow
Cutting one string
causes only a few
remaining strings to
relax.
Cutting many strings in a
row causes a wide band of
strings to relax – larger
area affected
Joint spacing and Lithology:
1) Stiffness = Elastic value E,
Youngs’ modulus
2) Hookes law
s  E e
where e is the elongation
strain
Stiff dolomite fractures a few times
before the sandstone fractured the
first time.
Dolomite stiffness >> Sandstone
1) Stretch a block
2) Stress in each bed controlled by
Hookes law (magnitude of stress
depends on E)
3) Beds with large E (stiffness)
develop a greater stress and fracture
first.
Rocks with low tensile
strength develop more
closely space joints
AND
More tensile strain
(stretching) yields more
joints
Joint arrays
Systematic vs nonsystematic joints
Systematic joints:
Planar joints
Joints are parallel or
subparallel.
Same average spacing
Nonsystematic joints:
Irregular spatial distribution
Not parallel to one another.
Different average spacing
Joints in the field
Why study joints
Questions to answer in the field
1)Tectonics (paleostress)
1) Systematic or nonsystematic joints
2) Geomorphology
(drainage patterns)
2) Orientation of joints strike and dip
3) Cross-cutting relations Steno
Relative Dating
4) Surface morphology planar
plumose
5) Dimensions of joints
6) Joints and lithology which rocks
thicknesses have closer
7) Relations of joints and faults and
folds. DEMO FOAM Pyroxene
DEMO
Joints in the field
Methods
1) Inventory
a) Sample fracture density
b) Sample joint orientation (strike and
dip of joints)
2) Relate to tectonics
Categories of Brittle Deformation
• Frictional Sliding on preexisting fractures
• Cataclastic flow due grain scale fracturing
• Shear rupture at acute angle to max. principle stress
• Tensile cracking perpendicular to dir of min. stess
Cataclastic rocks
A Cataclastic rock is a type of metamorphic rock that has been wholly
or partly formed by progressive fracturing. Rock fragments are
reduced in size by crushing and grinding of existing rock, a process
known as cataclasis. The process and is mainly found associated with
fault zones.
Cataclasite is a type of cataclastic rock that is formed during faulting,
consisting of angular clasts in a finer-grained matrix.
Cataclasite seen in thin section.
Scale is 200 mm
Stress Concentration and Griffith Cracks
• A stress concentration is a location
in an object where stress is
concentrated. An object is strongest
when force is evenly distributed
over its area, so a reduction in area,
e.g. caused by a crack, results in a
localized increase in stress.
• Griffith cracks are preexisting
microfractures and flaws in the rock,
weakening it. Reason rock failure less than theory
Origin and Interpretation of joints
Sheeting joints – uplift and exhumation
Sheeting joints form in a location where s1 is horizontal while s3 is
vertical near the ground surface. Joints become more closely placed
near the ground surface
Origin and Interpretation of joints
1) Sheeting joints – uplift and exhumation
A cooling pluton contracts more
than country rock. Here, st
(tensile stress) is oriented
perpendicular to the intrusive
contact.
After exhumation, joints form
parallel to intrusive contact and
creates an exfoliation dome.
Mechanical Exfoliation in Granite
Yosemite National Park
Source: Phil Degginger/Earth Scenes
Origin and Interpretation of joints
2) Natural hydraulic fracturing
Stresses in the Earth’s crust are
mostly compressive.
How do joints form in such a tectonic
environment?
Effect of pore pressure on fracture.
Increase of pore pressure in a preexisting crack pushes outward
Increase of st (tensile stress) that
allow crack tip to propagate.
Origin and Interpretation of joints
3) Regional divergence
High pore pressures in blocks subject to
divergence, weakens confining pressure
Formation of joints in hangingwall block with normal faults
Tensile stress s3
weakest is horizontal,
joints form
perpendicular to s3
http://www.ged.rwth-aachen.de/
Veins and vein arrays
Terminology (see table 7.2)
Vein: A fracture filled with mineral crystals precipitated from fluids.
Quartz or calcite are common vein fill
Ore minerals occur as vein fill
Vein Array: Groups of veins.
a) Vein array
b) Stockwork array of veins
(rock shattered and filled
by mineral precipitation)
Veins and vein arrays
En echelon vein array
 Fill en echelon joints
 Develop within a fault
zone
Veins and vein arrays
En echelon vein array
 Fill en echelon joints
 Develop within a fault zone.
a) en echelon vein array
b) Sigmodial en echelon veins
due to rotation of older,
central part of veins and
growth of vein material at
~45° to the shear surface
Calcite-filled wing crack with tip splay
Veins and vein arrays
Vein fill: block and fibrous veins
a) Blocky – vein fill equant - open fracture when
mineral precipitated.
b) Fibrous – vein crystals long relative to width.
Lineament:
A linear feature recognized on aerial photos,
topographic maps or remotely sensed images.
Defined only on a regional scale.
Aligned topography, changes in vegetation
Represent faults, joints, folds, dikes, or contacts.
Lineaments are not always confirmed with ground
truth.
Summary
Common questions you should ask when mapping
Should I pay attention to joints and veins?
It depends. What is the purpose of the map?
You should pay attention to them IF the map purpose is, for
example:
1) to locate faults
2) to define variations in permeability
3) to define joint intensity for oil and gas exploration (maybe with
drill core data)
4) to define and explore orientation of veins for ore deposits
5) to predict groundwater transport
But IF the purpose is, for example,
(1) understanding stratigraphy, or
(2) the history of folding in high-grade metamorphic rocks
then joint analysis will not help.