Lecture 11 Structural Geology

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Lecture 11 Structural Geology

• Rock deformation and structural geology
• Folds in Rock
• Rock fractures Joints and Faults
• Geologic maps

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• Rock deformation and structural geology
• Rocks deform under stress. Deformation refers to
  all changes in volume and/or shape of a rock
  body.
• Structural geology examines the present state of
  crustal deformation and determines the original
  geologic setting and the nature and direction of
  the earth forces (tectonic forces) that produced
  these rock structures.
• Folding in a rock occurs in the plastic range of
  the rock.
• Faulting occurs when the rocks break.

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• Laboratory experiments of deformation of marble
  by compressive forces under confining pressures
  similar to shallow crust (middle) and deeper
  crust (right). (M.S. Patterson).

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• Deformation of A) flat-lying strata. B)
  compressional stress tend to shorten a rock body,
  often by folding. C) Tensional stress acts to
  elongate or pull apart a rock unit. D) Shear
  stress acts to bend and break them. (Tarbuck and
  Lutgents)

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• Anticlines and synclines in the Calico Hills
  near Barstow, California. (Hamblin and
  Christiansen)

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• Descriptions of an inclined plane strike and
  dip
• Strike is the direction of the line produced by
  the intersection of the inclined plane with the
  horizontal plane. It is expressed as the angle of
  the line from the north.
• Dip is the angle between the inclined plane and
  the horizontal plane.
• Strike and dip directions are always mutually
  perpendicular.

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• Strike and dip of a rock layer. (Tarbuck and
  Lutgents)

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• Folds in Rock
• A fold is a bent structure that originally was
  planar, such as a sedimentary bed. Folds may be
  produced by either horizontal compression or
  vertical forces in the crust, just as pushing in
  on opposite sides of a paper or up from below.

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• Terms used to describe the parts of a fold
• limb The two sides of a fold are called limbs.
• axis A line drawn along the points of maximum
  curvature of a layer of a fold. More strictly, it
  is called hinge line.
• axial plane an imaginary plane surface that
  divides a fold as symmetrically as possible.
• plunge If the fold axis is not horizontal, the
  angle of the axis with the horizontal plane is
  called plunge.

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• Terms describing a fold. (Tarbuck and Lutgents)

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• Types of folds
• anticline upfolds or arches of rock layers
• syncline downfolds or troughs of rock layers.
• monocline only one direction of dip prevails in
  a fold system.
• symmetrical fold the axial plane is vertical
  with the limbs dipping symmetrically from the
  axis
• asymmetrical fold the axial plane is tilted from
  the vertical with one limb dipping more steeply
  than the other.
• overturned fold one limb is tilted beyond the
  vertical
• recumbent fold this is an overturned fold
  "lying on its side" so that the axial plane is
  nearly horizontal.

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• An illustration of principle types of folds.

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• Anticlines and synclines in the Calico Hills
  near Barstow, California. (Hamblin and
  Christiansen)

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• Folds of deformed sedimentary strata near
  Palmdale, California. The dashed-line indicates a
  fault also present. (E.J. Tarbuck)

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• (Top) The San Rafael monocline, Utah. (S.
  Trimble) (Bottom) Illustration of monocline
  consisting of bent sedimentary beds caused by
  faulting in the bedrock below.

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• Overturned fold. (G.S. Of Israel). One limb has
  been turned completely upside-down with older
  beds on the top.

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• A recumbent fold in Precambrian rocks of the
  Umanak area, Greenland. (T.C.R. Pulvertaft)

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• Outcrop (map) view of folds
• Anticline the oldest beds are in the center and
  the beds become progressively younger in each
  direction.
• Synclines the youngest bed is in the
  center and the beds get progressively older in
  each direction.
• Symmetrical folds have equal bed widths on
  opposite sides of the axial plane, but
  asymmetrical folds will have different bed widths
  on the opposite sides.
• For a plunging anticline, the nose (formed by the
  intersection of the fold system with a horizontal
  plane) points in the same direction as the
  plunge. For a plunging syncline, the nose points
  in the direction opposite to that of the plunge.

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• The diagram shows the surface of eroded remnants
  of a syncline and the characteristic core of
  younger rocks flanked on both sides by older
  rocks dipping toward the core. (Press and Siever)

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• Anticlines and synclines. The numbers 1 through 6
  indicate strata of progressively younger ages.
  (West, p.203)

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• Symmetrical and asymmetrical anticlines and
  synclines. Symmetrical folds have equal bed
  widths on opposite sides of the axial plane, but
  asymmetrical folds will have different bed widths
  on the opposite sides. (West, p.203)

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• Plunging folds. Note the nose of a plunging
  anticline in outcrop points in the direction of
  the plunge, while the opposite is true of
  plunging synclines. (Tarbuck and Lutgents)

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• A plunging anticline forms a a V-shape pattern
  pointing in the direction of plunge. This example
  is from near St. George, Utah. (Hamblin and
  Christiansen)

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Domes (dome-shaped folds) and basins (bowl-shaped
folds). (W.W. Norton)
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• Rock fractures Joints and Faults
• Joints
• A joint is a crack along which no appreciable
  movement has occurred.
• Most joints are produced when rocks are deformed
  by tectonic forces, with some exceptions.

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• Eroded joints, Arches National Park, Utah. (W.
  Clay)

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• Devils Tower, Wyoming. The columnar joints form
  when igneous rocks cool and develop shrinkage
  fractures producing elongated columns.
  (Thomason/Stone Images)

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• Joints in granitic rocks near the top of Lembert
  Dome, Yosemite National Park. The joints were
  enhanced by weathering. (E.J. Tarbuck)

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• Faults
• A fault is a fracture with relative movement of
  the rocks on both sides of it, parallel to the
  fracture.
• Fault terminology
• strike, dip of fault plane, hanging wall,
  footwall.

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A fresh fault scarp after an earthquake in
Nevada. (S. Marshak)
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(a)
(b)
(a) Slip lineations on a fault surface. (b)
Breccia, broken-up rocks along this fault. (S.
Marshak)
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• The names of hanging wall and footwall came from
  miners mining along fault zones, who hung their
  lanterns on the hanging wall and walked on the
  footwall. (Tarbuck and Lutgents)

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• Types of faults
• dip-slip fault normal fault, reverse fault,
  thrust fault
• strike-slip fault left lateral, right lateral
• oblique-slip fault has both strike-slip and
  dip-slip component. Note The textbook calls it
  "translation fault", which is rarely used.

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• Types of faults. a) Normal faults, caused by
  tensional forces, result in extension. b) Reverse
  faults, caused by compressional forces, result in
  shortening. c) Strike-slip faults associated with
  shearing forces. d) Oblique slip suggests a
  combination of shearing and compression/tension.
  (Press and Siever)

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• A normal fault. (Tarbuck and Lutgents)

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• A small normal fault along the road to Kolob
  Terraces just north of Toquerville, Utah.

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• Normal faulting in the Basin and Range Province.
  Tensional stresses elongated and fractured the
  crust into numerous blocks. Movement along the
  fractures tilted the blocks producing parallel
  mountain ranges. (Tarbuck and Lutgents)

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• The relative movement of a reverse fault

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• The Keystone thrust fault of southern Nevada.
  Dark-colored limestone (Cambrian) has been thrust
  over light-colored Jurassic sandstone, younger by
  some 350 million years. (J.S. Shelton)

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• Diagram for a strike-slip fault (right-lateral).
  Note how the stream channels have been offset by
  fault movement. (after R.L. Wesson et al.)

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• Strike-slip faults are commonly expressed by a
  series of straight linear ridges and troughs that
  can be traced for long distances. Here the San
  Andreas fault in southern California offsets a
  drainage system.

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• The San Andreas Fault system runs from the Gulf
  of California and enters the Pacific from
  northern California. The accumulated
  displacement, from earthquakes and creep, exceeds
  560 km over its 29-million-year history. (Tarbuck
  and Lutgens)

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• Geologic maps
• Geologic maps are among the best sources of
  information for preliminary site location and
  design. Thus, engineers need to become familiar
  with the construction and use of these maps.
• Elements of a geologic map include rock types,
  relative ages of the rocks, geologic contacts
  between different rock units, geologic structures
  (e.g., faults), and maybe topographic contours.