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2002

GEOL 3810

Structural GeologyT & Th, 8:00 - 11:50 AMDr. Luther M. Strayer

NS 353Office: (510) 885-3083

[email protected]

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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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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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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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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 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 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 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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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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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


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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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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.


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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.


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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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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

[email protected]

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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3810 - Deformation

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