EARTH STRUCTURE: An Introduction to Structural Geology and Tectonics 2 nd edition Authors: Ben A. Van der Pluijm & Stephen Marshak Publisher: Norton, 2004.

EARTH STRUCTURE:An Introduction to Structural

Geology and Tectonics2nd edition

Authors: Ben A. Van der Pluijm & Stephen Marshak

Publisher: Norton, 2004

Lecturer: Dr. Masdouq Al-Taj

• 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.

1. Introduction 1.1 Historical Survey

• During the late 18During the late 18thth century and century and through the 19through the 19thth century the century the geological discovery have been geological discovery have been quickened.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 down-slope 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 subsidening 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 three-dimensional 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).• 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 = F / AStress(σ) =force/area =mass*acceleration/area =kg.m.s-2/m²=Newton/m²=N/ m²=Pascal (Pa)

Sign of stress:+ve: in case of compression. -ve: in case of tensions.

• •Deformation refers to any change in shape, 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 size (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 analysisWhat 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.