Structural Geology

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Structural Geology

• 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
• Structural geology provides information about the
  conditions during regional deformation using
  structures

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Main Principles and Concepts

• Original Horizontality (Steno)
• Uniformitarianism (James Hutton)

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Tectonics vs. Structural Geology

• Both are concerned with the reconstruction of the
  motions that shape the outer layers of earth
• Both deal with motion and deformation in the
  Earths crust and upper mantle
• Tectonic events at all scales produce deformation
  structures
• These two disciplines are closely related and
  interdependent

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Definitions

• Tectonics Study of the origin and geologic
  evolution (history of motion and deformation) of
  large areas (regional to global) of the Earths
  lithosphere (e.g., origin of continents building
  of mountain belts formation of ocean floor)
• Structural Geology Study of deformation in
  rocks at scales ranging from submicroscopic to
  regional (micro-, meso-, and macro-scale)

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Structural Geologist

• A geologist who
• Studies deformation of rock and Earths crust
• Identifies and interprets geological structures
  and their tectonic implications

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Field Tectonic Studies

• Many tectonic problems are approached by studying
  structures at outcrop scale, and smaller
  (microscopic) or larger (100s to 1000s of km)
  scales
• Systematically observe/record the patterns of
  rock structures (e.g., fault, fold, foliation,
  fracture). This gives the geometry of the
  structures.

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Next steps

• Explanation of the structures
• Kinematics of formation of the structures
  motions that occurred in producing them
• When integrated over a large area, the motions
  will help to infer the past tectonic motions

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Use of Models

• We use geometric, mechanical, and kinematic
  models to understand deformation on all scales
  (micro, meso, macro)
• Geometric model 3D interpretation of the
  distribution and orientation of features within
  the earth crust
• Kinematic model Specific history of motion that
  could have carried the system from an undeformed
  to its deformed state (or from one configuration
  to another)
• Plate tectonic model is a kinematic model

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Mechanical Model

• Mechanical model Based on laws of continuum
  mechanics
• Study of rock deformation under applied forces
  (laboratory work)
• Model of driving forces of plate tectonic based
  on the mechanics of convection in the mantle is a
  mechanical model

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Analyses

• Descriptive
• Recognize, describe structures by measuring their
  locations, geometries and orientations
• Break a structure into structural elements -
  physical geometric
• Kinematic
• Interprets deformational movements that formed
  the structures
• Translation, Rotation, Distortion, Dilation
• Dynamic
• Interprets forces and stresses from interpreted
  deformational movements of structures

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We Study

• Changes in the original orientation, location,
  shape and volume of a rock body (Deformation
  changes in shape, position, and/or orientation of
  a body)
• Physical and chemical forces that deform rocks
• Geologic structures that form to accommodate
  changes
• Stress - Force applied over an area
• Strain - Observable deformation in the rock
• Brittle deformation e.g., fault the result of
  rapidly applied high stress which "break the
  rock"
• Ductile deformation e.g., fold the result of
  slowly applied, constant, low stress which "bend
  the rock"

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Deformation

• Includes strain, rotation, translation
• Homogeneous strain strain is the same at every
  point in the deformed body
• Undeformed state is used as a reference frame
• Commonly, we do not know the initial state
• Coordinate transformation is used to describe a
  deformation if initial and final states are known

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• Structure
• A geometric feature in a rock whose shape, form,
  and distribution can be described
• MicrostructureThe small-scale arrangement of
  geometric and mineralogical elements within a
  rock
• Texture Preferred orientation of
  crystallographic axes in the sample
• Microfabric Comprises the microstructure and the
  texture of a material

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Fundamental Structures

• Contacts
• Primary Structures
• Secondary structures
• Fractures (Joints, Shear Fractures)
• Vein - Precipitated minerals from fluid flowing
  through fractures
• Fault
• Fold

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Fundamental Structures, cont

• Foliation - Preferred orientation of planar rock
  bodies and/or minerals
• Lineation - Preferred orientation of linear
  minerals and rocks
• Shear Zone
• Zones of deformed rock that have accommodated
  movement