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Structural Geology

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What is a geologic structure ?

A geometric feature in rock whose shape, form, and distribution can be described.

It is the branch of geology that deals with:

  • Form, arrangement and internal architecture of rocks
  • Description, representation, and analysis of structures from the small to moderate scale
  • Reconstruction of the motions of rocks

What is structural geology ?

It is the branch of geology that studies the 3D geometry from micro to macro scale of rocks to explain the deformation processes the rocks experienced since their origination.

It introduces the physical side of Geological Sciences and emphasizes:

  • Geometry (shape, orientation, position, size, etc.)
  • Motion (beginning and ending positions and paths of particles and bodies—deformation or change in geometry)
  • Mechanics (explanations of why the geometry and motion are asthey are)

Includes lots of observations from the field (but also some from the laboratory and the computermodelling)
Teaches you not only facts, but also skills and techniques that are necessary in advanced classes and central to geologic practice.
Structural geology provides information about the conditions during regional deformation using structures.

Structural Geology
Structural Geology
Structural Geology Wall Stone
Structural Geology Wall
Structural Geology Rocks
Structural Geology

What are the job opportunities for a structural geologist ?

  • Oil and gas industry (Field exploration)
  • Mining (Field exploration)
  • Underground Water resources (Field exploration)
  • Engineering (Construction, Industry, Hazard)
  • Academics & Research in various subjects (Earthquakes, Classical
    Geology, Planetary Sciences, NASA).

What do we study in structural geology?

Structural geology studies the strain, which is the end product of deformation in
extremely heterogeneous materials.
We infer the stress that causes strain; we never observe stress while it is
happening.

Strain ——> Shortening or lengthening (extension)
Stress —–> Compression or Tension

 We measure attitude of planes and lines.

 Attitude: the orientation of a plane or line
in space.

Scale Terms

Global:A scale covering almost the entire world
Regional or Provincial:Roughly definable; generally corresponds to a physiographic province. Taurus Mountains, Himalayan Plato.
Macroscopic or Map Scale:Larger than an area one can see from a particular point on the ground.
Mesoscopic:An area visible from a particular point on the ground (outcrop to hand sample)
Microscopic: Visible with the help of an optical microscope.
Submicroscopic:Visible the with help of an advanced microscopic device like TEM (Transmission electron microscopy) or SEM (scanning electron microscope)
Penetrative:Characterizes the entire body of the rock
Non-penetrative:Does not characterize the entire body of the rock (ex. a part of the body)

Structures

Primary structures:

Are structures that develop during the formation of the rock. (Ex. Bedding, ripple mark or cross bedding in sedimentary rocks). Primary structures represent the local conditions of the environment within which the rock forms (Davis & Reynolds, 1996).

Secondary structures:

Are structures that develop in sedimentary or igneous rocks after lithification, and in metamorphic rocks during or after their formation. Fundamental secondary structures are joints and shear fractures; faults, folds, cleavage, foliations, lineations, shear zones (Davis & Reynolds, 1996).

Important Terms in Measurement

  • Trend: The direction of a horizontal line specified by its bearing or azimuth.
  • Bearing: The horizontal angle measured east or west from the true north or south.
  • Azimuth: The horizontal angle measured clockwise from the true north.
  • Strike:the trend of a horizontal line on an inclined plane. It is marked by the line of the intersection with a horizontal plane. (Davis & Reynolds, 1996).
  • Inclination: Theverticalangle, measureddownwardfromthehorizontaltoa slopingplaneorline.

Fracture

Is a local separation or discontinuity plane in a geologic formation, such as joints or faults.

Galway Lake Road scarp formed along the Emerson fault during the 28 June 1992 Landers, California, earthquake (Photo: Ramon Arrowsmith).
Galway Lake Road scarp formed along the
Emerson fault during the 28 June 1992 Landers,
California, earthquake (Photo: Ramon
Arrowsmith).
Rectangular joints in siltstone and black shale within the Utica Shale (Ordovician) near Fort Plain, NewYork.
Rectangular joints in siltstone
and black shale within
the Utica Shale (Ordovician)
near Fort Plain, NewYork.
 8 km long surface rupture that opened during the 19 May 2009 earthquake at Lunayyir, Saudi Arabia. (Photo: John Pallister).
 8 km long surface rupture that opened
during the 19 May 2009 earthquake at
Lunayyir, Saudi Arabia. (Photo: John
Pallister).

Joints

A joint is a separation in rock where the amount of separation is not greater than the displacement associated with the opening of the fracture.

Rectangular joints in siltstone and black shale within the Utica Shale (Ordovician) near Fort Plain, NewYork.
Rectangular joints in siltstone
and black shale within
the Utica Shale (Ordovician)
near Fort Plain, NewYork.

Faults

Distinct fracture surfaces along which rocks have been offset by movement parallel to the fracture surface.

Macroscopic scale

The San Andreas Fault system is a right lateral strike-slip fault.
The San Andreas Fault
system is a right lateral
strike-slip fault.

Mesocopic scale

Thrust faulting of Jurassic sedimentary rock at Ketobe knob along the San Rafael swell in central Utah (Shortening occures; Davis & Reynolds 1996).
Thrust faulting of Jurassic
sedimentary rock at Ketobe knob
along the San Rafael swell in
central Utah (Shortening occures;
Davis & Reynolds 1996).

Microscopic scale

Thin section of feldspar in the Salihli granodiorite cut by microfaults; note bookshelf order of feldspar fragments,
Thin section of feldspar in the
Salihli granodiorite cut by
microfaults; note bookshelf order
of feldspar fragments, (Hetzel
etal., 1995, JGSL)

Folds

Folds are planar surfaces that are curved or bent due to external forces.

Macroscopic scale

Landsatimage of large anticlines in the ZagrosMountains (NASA).
Landsatimage of large anticlines in the ZagrosMountains (NASA).

Mesocopic scale

Anticlines at SandymouthBay at Cornwall UK .(http://www.stacey.peak-media.co.uk/bude-walk/)
Anticlines at SandymouthBay at Cornwall UK .(http://www.stacey.peak-media.co.uk/bude-walk/)

Microscopic scale

Microfolds in a metamorphic rock; phyllite from the eastern Alps of Italy
Microfolds in a metamorphic rock; phyllite from the eastern Alps of Italy

Foliation

Foliation is any penetrative planar fabric or layering in a rock.
(Marshak & Mitra, 1988)

 Examples of planar fabric.
Examples of planar fabric.
Gneiss with foliation
Gneiss with foliation
Foliationin folded,metamorphic rocks . Devonian Gile Mountain Formation, Quechee Gorge Vermoont, US. (Vermont Geological Survey)
Foliationin folded,metamorphic rocks . Devonian Gile Mountain Formation, Quechee Gorge Vermoont, US. (Vermont Geological Survey)

Lineation

Is a prefered linear alignment of elements in rocks. (Davis & Reynolds,
1996)

This photograph illustrates a pronounced and prominent stretching lineation plunging steeply to the north, as a rake upon the main shear foliation ...
This photograph illustrates a pronounced and prominent stretching lineation plunging steeply to the north, as a rake upon the main shear foliation …
Block diagram of minearl lineation
Block diagram of minearl lineation

Shear zones

Is a general term for a relatively narrow zone with subparallel boundaries in which rocks are more highly deformed than rocks adjacent to the zone. (Marshak &
Mitra, 1988)

Simplified modelof the connection betweenfaults, which normally form inthe upper crust, and classicductile shear zones(Fossen, 2010, Cambridge Press).
Simplified modelof the connection betweenfaults, which normally form inthe upper crust, and classicductile shear zones(Fossen, 2010, Cambridge Press).

Symbology in structural geology

Some common symbols whose usage is well established:

Cite this article as: Geology Science. (2019). Structural Geology. [online] Available at: http://geologyscience.com/methods-of-geology/structural-geology/ [14th December 2019 ]
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