Download Earth Structure: An Introduction to Structural Geology and Tectonics

The Hashemite University
Faculty of Nat. Res. and Env.
Department of Earth and Environmental Sciences
STRUCTURAL GEOLOGY (111201350) 3 CH (2+3)
Lecturer: Dr. Masdouq Al-Taj
nd
OBJECTIVES:
The aims of this course are:
• Introduces the basic mechanical principles in structural geology, like
stress, strain, elastic and plastic deformation in materials and rocks.
• Enables the student to recognize and describe the different geological
structures (joints, faults, folds, foliation…etc.).
• Study the mechanism of the formation of different structures.
• Helps the student to determine time-structural event relationships.
• Review the skills of using the geological compass and the
stereographic methods in structural geology.
• Study geological structures in the lab and in the field through field
trips to the surrounding areas.
In this course we will cover three
parts of the text book:
• PART A: Fundamentals
– Chapter 1: Introduction
– Chapter 2: Primary and Nontectonic
Structures
– Chapter 3: Force and Stress
– Chapter 4: Deformation and Strain
– Chapter 5: Rheology
• PART B: Brittle Structures
– Chapter 6: Brittle Deformation Processes
– Chapter 7: Joints and Veins
– Chapter 8: Faults and Faulting
• PART C: Ductile Structures
– Chapter 9: Ductile Deformation Processes
and Microstructures
– Chapter 10: Folds and Folding
Week
Subject
Chapter
1
Introduction and basic terms in structural geology
1
2
Primary and nontectonic structures
2
3
Force and stress: Normal and shear stress
3
4
Mohr diagram for stress
3
5
Strain measurement
4
First exam
6
Rheology
5
7
Brittle deformation processes
6
8
Initiation of brittle deformation
6
9
Joints
7
10
Faults and faulting, Fault systems
8
11
Recognizing and interpreting faults
8
Second exam
12
Ductile deformation processes and microstructures
9
13
Folds
10
14-16
Fold classification
10
The mechanics of folding
EARTH STRUCTURE:
An Introduction to Structural
Geology and Tectonics
2nd edition
Authors: Ben A. Van der Pluijm & Stephen
Marshak
Publisher: Norton, 2004
Lecturer: Dr. Masdouq Al-Taj
Other References:
• Park, R., (1997): Foundations of structural
Geology. Chapman and Hall, London.
• Hobbs, B., Means, W. and Williams, P., (1989):
An outline of structural geology, 3rd ed., John
Wiley, New York.
• Ramsay, J. and Huber, M., (1987): The
techniques of modern structural geology.
Academic Press, London.
INTERNET REFERENCES:
http://www.uwrf.edu/~iw00/Structural_Geology.html
http://www.geo.cornell.edu/geology/classes/RWA/GS_326/GEOL
326.html
http://earth.usc.edu/geol321/
http://www.geo.cornell.edu/geology/faculty/RWA/maintext.html
http://www.geologyshop.co.uk/struct~1.htm
EXAMS:
First exam: 20 %
Second exam: 20 %
Final exam: 35 %
Lab. + quizzes + attendance: 25 %
1. Introduction
1.1 Historical Survey
• Leonardo de Vinci (1452-1519) drew carefully
shape of rock bodies in sketches to understand
the natural shape of the Earth.
• Perhaps the first description of rock deformation
came in the 17th century by Nicholas Steno
through the principle of original horizontality. He
examined outcrops and observed that the bedding
of the rocks wasn’t horizontal. So, he recognized
these rocks were deformed.
• During the late 18th century and
th
through the 19 century the
geological discovery have been
quickened.
In 1785, James Hutton introduces the doctorine
of uniformitarianisim (the present is the key to the
past).
+ A group of scientists started to recognize
themselves as geologists. Their main aims were:
* To make geological maps.
* Reported the formation of rocks.
* The origins of specific structures and
mountain ranges.
Later, ideas about the origin of mountains have
evolved gradually.
• Firstly, they believed that movement of
magma upward generated mountains and the
associated folds were generated by downslope movement along the flanks of these
mountains. (G. P. Scrope (1825)).
• Subsequently, horizontal forces were
emphasized, and the scientists were believed
that mountain ranges evolved due to
contraction of the earth that resulted from the
progressive cooling.
• Later, James Hall recognized that the
Paleozoic strata in the Appalachian in North
America were much thicker than correlative
strata in the interior of the continent. This led
to the development of geosyncline theory
where deep subsiding sedimentary basin
evolved into mountain range.
• In the 20th centaury, the foundations of
structural geology solidified, but by the
1960s it became a real science by the
formulation of PLATE TECTONICS
THEORY and considered as a revolution
in earth sciences.
• This figure after Isacks et al., 1968.
Structural geology: is the study of the threedimensional distribution of rock units with respect to their
deformational histories.
The primary goal of structural geology is to use
measurements of rock to uncover information about the
history of deformation (strain) in the rocks, and ultimately,
to understand the Stress field that resulted in the observed
strain and geometries.
This understanding of the stress field can be linked
to important events in the regional geologic past; a
common goal is to understand the structural
evolution of a particular area with respect to
regionally widespread patterns of rock deformation
(e.g., mountain building, rifting) due to plate
tectonics.
WHAT IS THE JOB OF STRUCTURAL
GEOLOGISTS :
(1) measure rock geometries.
(2) reconstruct their deformational histories.
(3) calculate the stress field that resulted in that
deformation.
1.2 GEOLOGIC STRUCTURES:
Firstly, let us define what we mean by geologic structure:
It is a geometric feature in a rock whose shape, form and
distribution can be described.
Examples of geologic structures are: folds, faults , joints,
veins, cleavage, foliation and lineations.
Consequently, there are many schemes for classification
of these structures.
1.2.1 Classification of Geological
Structures
I. Classification based on geometry (shape and form
of a particular structure):
a. planer surface
b. linear surface
c. curviplaner surface
This classification may be the most important because
it includes: folds, faults , joints, veins, cleavage,
foliation and lineations.
II. Classification based on
geological significance:
a. primary: ripple mark, cross bedding, mud cracks.
b. local gravity driven: slumping.
c. local density –inversion driven: salt dome (form due
to variation in rock density).
d. fluid-pressure driven: injection of unconsolidation
material due to sudden release of pressure.
e. tectonic: due to interaction between lithospheric
plates.
First four usually primary and nontectonic structures while the
fifth is the main aspect of structural geology.
III. Classification based on
timing of formation :
a. synformational: structure forms with initial
deposition of rock.
b. penecontemporaneous: structure forms
before full lithification, but after initial
deposition.
c. postformational: structure forms after the
rock has fully lithifide.
IV. Classification based on Process
of formation
(the deformation mechanism)
• Fracturing: related to cracks in rocks.
• Frictional sliding: related to slip of one body of
rock past another.
• Plasticity: deformation by internal flow of
crystals without loss of cohesion.
• Diffusion: material transport in either solid-state
or assisted by a fluid (dissolution). Stylolotes
• Combination: combinations of deformation
mechanisms contributing to the overall strain.
V. Classification based on
Mesoscopic cohesiveness during
deformation
• Brittle: structure forms by loss of cohesion.
• Ductile: structure forms without loss of cohesion.
• Brittle/ Ductile: deformation with both brittle and
ductile aspects.
VI. Classification based on Strain
significance, in which a reference
frame must defined (usually earth
surface or the deformed layer):
• Contractional: shortening of a region (convergence).
• Extensional: stretching of a region (divergence).
• Strike-slip: movement without either shorting or
stretching (lateral slip).
VII. Classification based on
Distribution of deformation in a
volume of rock
• Continuous: occurs at the rock body at all
scales.
• Penetrative: occurs throughout the rock body at
observation scale.
• Localized: structure in continuous or penetrative
only within a definable region.
• Discrete: structure occurs as an isolated feature.
Finally, most crustal structures are a
consequence of plate tectonics
activities that include; convergence,
divergence and transform (lateral
slip) movements.
1.3 Stress, strain and deformation
• Stress is the main cause of deformation in the crustal rocks.
• The stress (σ) is the force (F) per unit area (A) of the acting plane σ
Stress(σ) =force/area
=mass*acceleration/area
=kg.m.s-2/m²=Newton/m²=
N/ m²=Pascal (Pa)????
=F/A
Sign of stress:
+ve: in case of compression.
-ve: in case of tensions.
•
•Deformation refers to any change in shape, volume, position, or orientation of
a body resulting from the application of a differential stress.
Deformation in general has three components: -
•
Translation: movement of rock from place to
another ( i.e fault)
•
Rotation: pivoting of a body around a fixed
axis (i.e fold)
•
Strain: change in volume (dilation) and/or
change in shape (distortion) of a rock.
Strain is of two types:
1. Homogeneous strain: the deformation
is the same throughout the rock.
2. Heterogeneous strain: the deformation
is different throughout the rock.
1.4 Structure analysis
What do structural geologists do?
Structural geologists do structural analysis, which
involves many activities such as:
1. Descriptive analysis:
The characterization of the shape and
appearance of geologic structures.
Attitude, strike, dip angle, dip direction ,plunge, trend,
rake (pitch), apparent dip, trace, cross section,
profile plane……
2. Kinematic analysis:
Involve the determination of the movement paths that
rocks or parts of rocks have taken during transformation
from the undeformed to deformed state. (use of features in
rocks to define the direction of movement on a fault).
3. Dynamic analysis:
Involve development of an understanding of how stress
related to deformation (stress and its direction).
4. Strain analysis:
The development of mathematical tools to quantifying the
strain in a rock.
5. Deformation – Mechanism analysis:
The study of processes on the grain scale to atomic scale
that allow structures to develop , ex: sliding, fracturing,
plasticity.
6. Tectonic analysis:
The study of the relation between structure and global
tectonic process: divergent, convergent, transform.
Structural Analysis and Scales of
Observation
1. Descriptive analysis (shape and appearance,
vocabulary, 3D orientation).
2. Kinematic analysis (define the direction of
movement)
3. Strain analysis (quantifying the strain (maths)).
4. Dynamic analysis (How stress is related to
deformation, used microstructure).
5. Deformation – mechanism analysis (structural
development in grain to atomic scale, fracture
and flow of the rock).
6. Tectonic analysis (relation between structure
and global tectonic).
We used four relative scales of
observations
Scale of observation
1. Micro scale (thin section): microscope
2. Meso scale (isolated outcrop): hummer
3. Macro scale (regional): helicopter
4. Mega scale (plate): Satellite, Global
Positioning System (GPS)
• Good observation, recognition
and description of rocks and
their structure are very
important for field analysis.
Some guideline for the
interpretation of deformed area
• Law of original horizontality (bed deposited
horizontally).
• Law of superposition (strata follow one another
in chronological).
• Stratigraphical continuity for the same
lithological sequence.
• Sharp discontinuities in lithological pattern are
faults, unconformities or intrusive contacts.
• Deformed area can be subdivided into a
number of region contain consistent structural
attitude (structural domain).
• Principle of least astonishment (simplest
interpretation is most correct).
• Additional subsurface data (drilling, seismic
and other geophysical techniques) are important
for structural geologist interpretation.
• It is important to imagine all geological
structure in a MODEL 3D and even more than
that.