Structural Geology: Lecture 2

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Structural Geology Lecture 2

• Review of last lecture.
• What does the subject of tectonics cover and how
  is it related to structural geology?
• Historical development of structural geology and
  tectonics research.
• Basic concept of rock deformation.

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• Summary of lecture 1 - the introduction
• Structural Geology deals with rock deformation,
  which is expressed by the development of geologic
  structures such as faults, folds, and structural
  fabrics.
• To understand structural development, we pose
  three basic questions What is it?, How did it
  form?, and Why did it form?.
• To address these questions, structural geologists
  adopted three sequential approaches (1)
  descriptive, (2) kinematic, and (3) dynamic
  analyses, with an increasing amount of
  interpretative components.

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• Descriptive analysis
• We use maps, cross sections, field photos, drill
  data, seismic profiles, gravity data, etc., to
  describe the geometry of geologic structures.
• We use orthographic projection, stereographic
  projection, and various means of geometric
  methods to analyze the geometric elements of
  individual structures (will be covered by your
  lab exercises).
• We develop geometric models to capture the
  essence of geologic structures and classify them
  according to their natural appearances.

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• Kinematic analysis
• We analyze and reconstruct the sense and
  magnitude of fault slip and sequence of
  deformation for the development of individual
  structures and structural associations (which we
  often refer to as systems).
• We use allied fields such as sedimentology,
  thermochronology/geochronology, geomorphology,
  quantitative metamorphic petrology to deduce the
  kinematics of faulting.
• We establish kinematic models that link rock
  deformation with other related geologic processes
  such as thermal evolution, sedimentation, and
  metamorphism to test the validity of the proposed
  structural evolution model in the broadest sense.
  

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• Dynamic analysis
• Once the kinematic models, which could be
  multiple, are developed, we explore the
  mechanical conditions that are suitable for the
  formation of the structures and proposed
  deformation history.
• Two approaches are taken dynamic analysis (a)
  numerical computation, coupling energy
  (temperature), force, and mass balance equations,
  and (b) analogue models using materials
  appropriate for Earths conditions (i.e., through
  proper scaling using first principles). Both
  approaches require sound understanding of
  continuum mechanics, fracture mechanics, and
  calculus, which will be introduced at an
  elementary level in this class.

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• Like all sciences, research in structural geology
  is also a process of hypothesis testing
• We test geometric models (folds, faults, and
  their specific forms).
• We test kinematic models (how folds or faults
  were developed and their geologic consequences
  not only in rock deformation but also in
  sedimentology, metamorphism, thermochronology,
  etc.)
• We test dynamic models for why the structures
  formed (i.e., we check out what boundary and
  initial conditions are plausible for creating the
  observed geologic structures and deformation
  paths).
• All three types of models are interactive and the
  final geologic solution should have all three
  models internally consistent.

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• Historical Perspectives of Structural Geology
• Structural geology was developed mostly from
  mining and geologic engineering work in the 19th
  century. Geologists use fault geometry and
  throws ( fault offset) to predict where to
  find economic deposits such as coal beds and
  gold-bearing veins. Also, road constructions in
  the Alps inn the 19th century used structural
  geology extensively to predict where the
  competent beds.
• Prior to 1970, geologists treat faults and folds
  as separate entities. They also treat faults in
  the same structural belts such fold-thrust belt
  or extensional provinces as unrelated structures.
• The concept of fault systems and and fault-fold
  relationships were not appreciated until 1970
  when Canadian petroleum geologists started
  applying the concept to make balanced cross
  sections widely used for petroleum exploration
  and tectonic studies.

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Traditional application of structural geology in
coal mining
coal bed
Fault offset
fault
Where do I find the coal be on the other side of
the fault?
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Traditional application of structural geology in
tunneling and road construction
?
Projected tunnel
Where are we going to encounter the competent
layer so we can budget our time and labor?
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Modern application of structural geology seismic
interpretation
Sumatra, Indonesia
Uninterpreted seismic reflection profile
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Modern application of structural geology seismic
interpretation
Interpreted seismic reflection profile
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Uninterpreted seismic reflection profile
Gulf of Mexico
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Interpreted seismic reflection profile
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Brazil offshore
Uninterpreted seismic reflection profile
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Interpreted seismic reflection profile
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Structural Geology is also central to
interpretation of sedimentary data (e.g., growth
strata), metamorphic data (P-T-t paths), and
geomorphologic data (spatial and temporal
evolution of terraces and river systems).
Mastering the field of structural geology
requires a strong 3-D visualization and an
imaginative mind to reconstruct complex
deformation history.
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• What is tectonics and how it is related to
  structural geology?
• Structural geology and tectonics both deal with
  deformation of earths crust and lithosphere.
  However, structural geology usually deals with
  structures at microscopic and regional scales
  (i.e., individual structures or structural
  systems) whereas tectonics deals with regional
  and global scales (i.e., a fold-thrust belt, and
  orogenic system, etc.).
• Tectonics usually deals with the absolute
  geologic time during which a structure was
  developed. However, in the study of structural
  geology, only the relative sequence of
  deformation is examined.
• Tectonics generally deals with the consequence of
  rock deformation, such as the development of
  sedimentary basins, metamorphism, and igneous
  activities related to rock deformation.

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Typical structural geology study deals with a
single fault system at a relatively smaller
scale microscope to regional
A thrust duplex system
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A tectonic study deals with a structural belt or
an orogenic system and its associated products
(melts and metamorphism)
Crustal melt
Metamorphic belt
Major Detachment fault
Major thrust
Foreland basin