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2002
GEOL 3810
Structural GeologyT & Th, 8:00 - 11:50 AMDr. Luther M. Strayer
NS 353Office: (510) 885-3083
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Chapter 1
The Nature of Structural Geology
Structural geology addresses the architecture
of the Earth - the physical components orstructures that make up the Earths crust, and
that form in response to applied forces and
stresses.
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Deformation of the Earths CrustDeformation results from stressesthat exceed
a rocks strength.
When peak strength is reached, failure is
either brittle (fracture) or ductile (flow),depending on how the physical environment has
affected rock strength (i.e. temperature, strain
rate, etc.).Stresses are applied to rocks in countless
ways: burial, cooling/heating, intrusion, plate
motion, im acts from s ace...
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Architecture & StructuresThe structural geologist is faced with a finished
product and has the inverse task of learning how itcame to be - effectively the opposite task that faces an
architect.
What is the structure? What were the startingmaterials? Whats the geometry? How did it change
shape? Source of stresses? Sequence of deformation?
These lead to more questions:
When? How long? P&T? Strength of meterials?
Why does this happen?
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Plate Tectonics & Structural Geology
Structural geology is a field where the result of plate
tectonics is directly observable.
Plate motions are directly responsible for many ofthe stresses that cause deformation of rock.
Distortions of the Earths crust are most prominent
at convergent plate margins - called orogens - wheremountain belts are the physiographic expression of
orogenic belts.
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QuickTime and aAnimation decompresso r
are needed to see this picture.
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Fundamental
Structures
Contacts:are the most
basic structures, they
separate one rock unitfrom another -
depositional,
unconformities, faults,
intrusive, shear zones.
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Fundamental Structures
Primary Structures:These are sedimentarystructures that may be in stratapriorto
deformation. They may be quite useful as strain
markers (giving us an initial state) and as way-up indicators, etc.
They must notbe mistaken for secondary
structures, which are the resultof deformation.
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Bedding Laminations
Graded Bedding
up
Primary Structures
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Cross-Beds (asymmetric)
Oscillation Ripples (symmetric)
up
up
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M d C k
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up
up
Mud Cracks
Rain Drops / Footprints
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Load Casts
Tool Marks
up
up
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Root Casts / Worm Burrows
Stromatolites
up
up
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Fundamental Structures
Secondary Structures:These are what we arehere for!
Form in Seds and Igneous rocks after
lithification and cooling, and in Metamorphic
rocks during or after formation.
Include: joints and shear fractures; faults;folds; cleavage, foliations and lineations; and
shear zones.
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Secondary Structures
Joints:
Smooth, planar cracks that cause loss of cohesion
of the rock, and upon which there has been almost
imperceptable movement.
They typically ocurr in families and swarms.
Basic joints are tensional (mode I) fractures. Their
surfaces often haveplumose structuresthat canindicate direction of crack propagation
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Pl S
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Plumose Structures
S d S
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Secondary Structures
Shear fractures:
Form in response to a very slight shearing
movement parallel to the plane of the fracture.
Commonly found in conjugate sets, in rocks
that have been folded or faulted.
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S d S
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Secondary Structures
Slickensides, slickenlines:
Effectively, they are small scratches that form
in response to motion on a fault.
May be the result of very large or very small
displacements.
Lines indicate directionof motion. Steps inrock or mineral coatings may indicate sense of
slip.
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S d S
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Secondary Structures
Transitional tensile fractures (tension gashes):
Combines tensile opening behavior (joints)
with shearing (shear fratures).
En echelon tensile fractures (veins) link up to
form throughgoing faults.
Tips of the fractures point to major stressdirection.
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S d St t
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Secondary StructuresFaults:
Faults are discrete fractures or discontinuities alongwhich some amount of offset occurred, in the plane
parallel to the discontinuity.
Magnitude of offset can be from mm to km.
Fault motion may create fault gouge (clayey),
polished surfaces (slickensides) or breccia (angularfragments).
Reverse / Thrust - Normal - Strike Slip
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Secondary Structures
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Secondary StructuresFolds:
Folds form when layers are bent into curved
or kinked shapes.
Their forms can tell a lot about how, why andunder what conditions they formed.
They can form in many ways - buckling, due
to motion on a fault or shear zone, flow at high
temperatures, gravity sliding, etc.
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Secondary Structures
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Secondary StructuresCleavage, Foliations and Lineations:
These arefabricswith the rock that form underconditions of elevated P & T, when mineral grains can
change shape, dissolve or precipitate, and
recrystallize.These arepenetrativestructures - they pervade the
rockmass - and are internal, not surficial.
Cleavage and foliations are planar features (2D),
while lineations are linear (1D).
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Secondary Structures
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Secondary StructuresShear Zones:
Shear zones are like faults in that they
accommodate displacement parallel to the shear
zone plane except there is no clear discontinuity
or fracture across the zone - the rockmass
appears to stay continuous.
They are typically the higher P & Trepresentation of faults, but there are brittle
shear zones.
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Concept of Detailed Structural
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Concept of Detailed Structural
Analysis
Detailed structural analysis - with particular
emphasis on strain analysis is the basis of
structural geology. It is predicated on the
notion that most structures contain in, or
adjascent to them, is the information necessary
to decipher them.
There are 3 fundamental parts to DSA
Concept of detailed structural analysis
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p f y
Descriptive Analysis: is the most important
and most fundamental aspect of DSA. It
consists of identifying and accurately
describing the location, attitude/orientation,
and geometries of structures.
This is generally done in the field, although
remote sensing is also used.
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Concept of detailed structural analysis
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Kinematics Analysis: this consists of
determining the direction and magnitude (if
possible) of the motions that were
responsible for the deformation.
It is concerned with basic motions -
translations, rotations, distortions, and
dilations
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Concept of detailed structural analysis
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Dynamic Analysis: is concerned with theforces
andstresses involved with deformation.
Generally the most interpretive aspect of DSA, but
is based on rock mechanics and materials theory,
and should ultimately be based on natural
observations.
Often done by applying theory and/or analogand/or numerical models.
Descriptive Analysis:
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p y
Its the heart of structural geology. It focuses on the
exact details of geometry: 3D spatial and angularrelationships.
Angles between lines and planes - orientation of a
lines, and of the intesection between 2 planes -changes in lengths of lines
This leads to the use of orthographicand
stereographicprojection - so called sick fun
Descriptive Analysis:
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Descriptive Analysis:
Should ideally be done free of interpretation -
w/out preconceived notions of what (we
think) shouldbe there.
On the other hand, our experience, and ourknowledge of tectonics and structural style
will often guide or investigation...
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Structural Elements:
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Every structure we encounter is composed of structural
elements, which must be identified and described in order
to carry out descriptive analyisis.
There are 2 types:
1) Physical Elements: these are real and tangible - like fold
limbs or fault surfaces - which have measurable geometries
and orientations.
2) Geometric Elements: are imaginary lines and planes (that
can also be measured) which help us describe a structuresgeometry.
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Structural Elements:
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By careful observation of specific features and by
their systematic plotting graphically, we canidentifysetsof features that might have a common
orientation or appearance. A number of sets of
structures can define asystemof structures.
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Concept of detailed structural analysis
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Kinematic Analysis: takes off where descriptive analysis
leaves off.
Kinematic analysis deals with the recognition of changes
in shape, angle, area/volume, and locationof materials
during deformation - specifically:Translation: rigid-body motion from A to B;
Rotation: about a pole or series of poles;
Distortion: changes in angular relationships, and;
Dilation: area/volume loss or gain during defm.
Kinematic Analysis:
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The goal of kinematic analysis is to determine the
deformation path- the seriesof translations,
rotations, distortions and dilations that take the
structure from its originalto its defortmedstate.
This is done at all scales- from plate-tectonicmotions, down to the grain-scale in a thin section.
Strain Analysisis one aspect
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St a a ys s s o e aspect
of kinematic analysis that
focuses on changes of shapeand size of deformed objects.
Penetrative Deformation:
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How we treat a specific area kinematically often
depends on whether or not the deformation is
penetrative,at the scale of observation.
For structures to be penetrative, they must be closely
spaced enough to appear to be everywhere -clearly this is a notion that is closely tied to that of
scale of observation
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The notion of penetrative deformation is strongly scale dependent.
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Slip and Flow: scale dependent descriptions
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The phenomena of flow, that is, the continuous (no
discontinuities) shearing of material, may at a
closer scale of observation, in fact be accomodated
upon a series of (relatively) small slip surfaces or
faults.
This can be seen at the grain and sub-grain scale in
mylonitic rocks, and at the outcrop scale in large
mountian belts.
Modest systematic movements on relatively close-
spaced slip surfaces can produce significant
distortions of a rockmass.
Concept of detailed structural analysis
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Dynamic Analysis: interprets the forces, stresses and
mechanics that produce structures.
A major goal is to determine the magnitude and
orientation of stresses that produce structures, and
the mechanical response of the rockmass to those
stresses.
Concept of detailed structural analysis
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This is done using:
Physical or analog models - where physical models
of natural processes are made using either actual
rock materials, or rock analogs - clay, gelatin,
silly-putty, sand, butter, wax, etc.
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Concept of detailed structural analysis
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This is also done using:
Analytical theoretical modeling - a mathematical
exercise where a solution is derived from
mechanical theory.
Numerical models - these are computer models,
systems of differential equations that are
numerically solved to simulate natural phenomena.
Distinct-Element Models: Thrust Faults,
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Normal Faults, and the Big One
Luther M. StrayerCal State University, Hayward
Distinct-Element Particle Model
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Sand Model
0% Seds
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0% Seds
25% Seds
50% Seds
75% Seds
100% Seds
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Detailed structural analysis
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The Famous Pizza Model
San Manuel ore body
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