Lecture 5: Structural Geology

1
Lecture 5 Structural Geology

• Questions
• How do you read a geologic map?
• What structural elements of rocks form in
  response to deformation and how to you use the
  structures to reconstruct geologic history?
• How and why do you construct a cross section?
• Reading
• Grotzinger et al., chapter 7

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Conventions of Geologic maps

• A geologic map is a scaled representation of
  geological observations at the surface of the
  Earth
• It shows the intersection between the underlying
  rocks and structures and the topographic surface
  (generally not a plane), projected vertically
  onto a plane
• Topographic contours are usually shown in order
  that an accurate representation of geometry can
  be inferred from the map.

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Conventions of Geologic maps

• Mappable formations (i.e., thicker than a pen
  stroke at the map scale) are assigned colors or
  hatching patterns.
• Other features are marked using the symbols in
  the table. Note especially
• Light black lines for contacts between units,
  solid where known, dashed where inferred, dotted
  where covered (by alluvium, usually)

• Heavy black lines for faults, with sense of slip
  marked if known (also solid where observed,
  dashed where inferred, dotted where covered).
• Short lines with hatch marks on down-dip side
  for orientation (strike and dip) of planar
  features, particularly bedding but also foliation
  or joint sets.
• Line with inward-pointing arrows for syncline,
  outward pointing arrows for anticline, arrow at
  end if fold axis has plunge.

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Properties of topographic contours

• Reading topography from contours is hard for the
  uninitiated, but contours follow some useful
  basic rules
• The difference in elevation between adjacent
  contours is always given on the map as the
  contour interval.
• Contour lines from a V pointing upstream in
  drainages and valleys
• Contours never cross or divide (unless scale is
  so fine that cliffs can overhang). Spacing of
  contours gives the gradient or slope.
• Hills and Knobs make closed contours.
• Closed depressions are marked with
  inward-pointing hatchures on the closed contours.

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Structural Elements of Maps and Rocks Folds

• Fold nomenclature consider a curved surface
• Any point on a surface can be described by a
  maximum and minimum principal curvature (in
  perpendicular directions). If one of these is
  zero, it is called a parabolic or cylindrical
  point.

• If every point on a surface is cylindrical, but
  the orientation of the line of zero curvature
  changes, the surface is conic.
• If every point on the surface is cylindrical and
  the orientation of the line of zero curvature is
  constant then the whole surface is cylindrical.
• FACT most natural folds in rocks are (roughly)
  cylindrical!

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Folds

• Why are natural folds nearly cylindrical?
• Define a property of a surface called Gaussian
  Curvature

• where rmax and rmin are the principal radii of
  curvature.
• For any deformation of a surface that does not
  change its area, CG is constant.
• Since an initially flat surface has CG0, all
  shapes that can be obtained by constant-area
  folding have a principal curvature equal to zero
  all points are cylindrical points (try it with
  a piece of paper).
• A circle around an elliptical point with
  rmaxrmingt0 encloses a larger area of surface
  than the same circle when the surface was flat.
• A circle around a hyperbolic point with
  rmaxrminlt0 encloses a smaller area of surface
  than the same circle when the surface was flat.
• For cylindrical folds, if layer thickness is
  preserved around the fold, measured perpendicular
  to bedding, the fold is parallel. If layer
  thickness is variable as a result of the fold
  deformation, the fold is nonparallel.
• If the shape of each bedding surface in a fold is
  congruent with the shape of adjacent bedding
  surfaces, then the fold is similar.
• Question Can a fold be both parallel and
  similar?
• If curvature varies smoothly along the fold, it
  is curved if there is a sharp maximum in
  curvature or a kink at the hinge, the fold is
  angular.

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Folds

• Curved or angular? Parallel or not? Similar or
  not?

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Fold terminology

• The direction of the line of minimum curvature is
  the fold axis.
• A fold axis is a line (or curve), so instead of
  strike and dip (which describe a plane), its
  orientation is given by trend and plunge.
• A local maximum in curvature perpendicular to the
  fold axis is a hinge.
• The line connecting the hinges along the
  direction of minimum curvature is a hinge line
  (parallel to the fold axis, for cylindrical
  surfaces).
• A point where both principal curvatures is zero
  is an inflection.
• The surface or plane defined by hinge lines in
  successive layers is the axial plane.
• The area between an axial plane and an inflection
  is a limb of a fold.

No plunge
Hinge
Plunge
Inflection
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Fold terminology

• A fold with older rocks in the core is an
  anticline a fold with younger rocks in the core
  is a syncline.
• A fold that is convex upwards is an antiform. A
  fold that is convex downwards is a synform.

• A fold with vertical axial plane is symmetric. A
  fold so asymmetric that the beds on one side are
  overturned (dip gt90 compared to original
  direction of deposition) is an overturned fold.
  A fold with a nearly horizontal axial plane is
  recumbent. A fold with both limbs overturned is
  inverted.
• You have to be careful because an inverted
  anticline is a synform!

10
Fold terminology

• Two special classes of non-cylindrical folds, in
  which the plunge changes along the fold axis, are
  domes and basins

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Mechanisms of folding

• Folds can result from layer-parallel compression
  (buckling), layer-perpendicular variations in
  loading (bending), or from apparent growth of
  small-amplitude folds during homogeneous
  pure-shear strain.
• Parallel folding requires layer-parallel slip
  between layers (c.f. deck of cards) parallel
  folds are dominant in regions of partly brittle
  rheology.
• Non-parallel folding requires layer-parallel flow
  within layers and so is common in fully ductile
  rheology

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Structural elements of maps and rocks Faults

• A crack in a rock indicates brittle failure. If
  there is no relative motion across it is a joint.
  If there is relative motion of the two sides of
  the crack then it is a fault.
• In a ductile regime faults may be replaced by
  shear zones, but that is not quite the same thing
  as a fault.
• Types of Faults Faults are classified first by
  orientation (the simplest observation), then by
  slip (which is harder to measure).
• The orientation (dip) of a fault can be described
  as vertical, high-angle, or low-angle. The
  dividing line between high-angle and low-angle is
  45, more or less.
• When slip is known, faults are classified into
  strike-slip (horizontal displacement), dip-slip
  (vertical displacement), and oblique (some of
  both).
• Strike-slip motion is either right-lateral or
  left-lateral.
• For non-vertical faults, dip-slip motion is
  either normal (hanging wall down with respect to
  footwall) or reverse (hanging wall up). A
  low-angle reverse fault is called a thrust fault.

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Structural elements of maps and rocks Faults
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Normal Faults

• Normal faults often form in conjugate sets (same
  strike, equal but opposite dips) due to the
  symmetry of the Mohr diagram (q.v. lecture 10).
• Pairs of opposing conjugate faults generate
  alternating horsts and grabens.

• In brittle rocks near the surface, extensional
  normal faults are usually high-angle (55-70),
  but they frequently become low-angle with depth
  (listric normal faults) either because they enter
  a weak ductile layer (becoming a detachment fault
  parallel to bedding) or because of the overall
  increase in ductile behavior with depth.
• The spacing between sets of normal faults is
  generally similar to the thickness of the brittle
  layer.

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Normal Faults

• Normal faults form in several characteristic
  settings
• Overall horizontal extension
• Continental rifting (spacing controlled by
  intracrustal brittle-plastic transition depth,
  15 km)
• Oceanic rifting (spacing controlled by very thin
  brittle layer over roof of crustal magma chamber
  at zero age, 1 km fewer, bigger faults at slow
  spreading rate than at high spreading rate).
• Gravity slides On a variety of scales,
  topographic gradients at the surface create a
  driving force for horizontal extension. This
  includes landslides of all scales (the head scarp
  and bed of a landslide is a normal fault) as well
  as whole mountain front scale slides (Heart
  Mountain Fault, Montana-Wyoming) and regional
  slides (Gulf Coast).
• Flexure like the Hot Creek graben in the Long
  Valley resurgent dome.
• Normal faulting without horizontal extension
  Collapse
• Caldera ring faults
• Collapse features due to dissolution (of
  evaporites or carbonates) or differential
  compaction. A sinkhole is a normal fault feature
  of sorts.

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Normal Faults Gravity Slides

• The Heart Mountain Detachment Fault is a famous
  example of gravity driven near-surface tectonics.
  Heart Mountain is an isolated piece of
  Ordovician rocks sitting on Eocene mud 40 km away
  from the nearest similar outcrop.

• It actually slid there in the Eocene
• The system has four parts
• 1. Breakaway normal fault
• 2. Detachment along single bedding plane in
  Ordovician Bighorn Dolomite.
• 3. Thrust ramp or transgressive fault that steps
  up to Eocene land surface.
• 4. Bentonite-lubricated Eocene land surface that
  allowed blocks to slide 40 km into Bighorn Basin.

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Normal Faults

• The Gulf Coast of the U.S. is riddled with normal
  faults, like the island of Hawaii, that
  accommodate sliding of high ground into the sea.

• The Kettleman Hills anticline in California is a
  dome that puts the upper surface in flexural
  tension and caused the development of a system of
  normal faults.

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Normal Faults

• The hanging wall of a normal fault typically
  undergoes secondary folding (rollover), tilting
  (as at Red Rock Canyon) or faulting (antithetic
  faults), especially if the master fault is
  listric.
• The foot wall of a large-offset normal fault
  often undergoes isostatic rebound in response to
  unloading, which can expose deep metamorphic
  rocks of the footwall in a metamorphic core
  complex.

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Strike-slip Faults

• Strike slip faults have dominantly horizontal
  slip and tend to be near-vertical, though they
  may also have some dip (but this increases
  friction and surface area, so it is not favored).
• Strike-slip faults form in two principal tectonic
  settings
• Transform plate boundaries
• Step-overs between normal or thrust faults in
  dominantly compressive or extensional terrains
• Either way, the end of a strike-slip fault is
  generally a region of compression or extension.

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Strike-slip Faults

• Frequently, strike-slip faults are made of
  multiple en échelon (parallel but offset linear
  features) segments
• When the overlap between en échelon segments
  becomes large, there is a transition to multiple
  parallel faults all simultaneously active, like a
  stack of dominoes

Landers earthquake rupture
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Strike-slip Faults

• When strike-slip faults connect the ends of
  normal or thrust faults, they are called tear
  faults
• Like normal faults, strike-slip faults often come
  in conjugate sets. In the strike-slip case, one
  set will be left-lateral and the other
  right-lateral, both at 30 to the direction of
  maximum horizontal compression. Example San
  Andreas and Garlock faults (?).

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Thrust and Reverse faults

• Thrust faults are evidently common at compressive
  plate boundaries, both in foreland
  fold-and-thrust belts and as principal subduction
  zone boundary structures.
• Thrust faults, especially when high-angle, can
  form, like normal faults, conjugate pairs and
  horst-and-graben structures
• More often, thrust faults are lower-angle and the
  structure is asymmetric it has vergence to one
  side (the way the upper plates are going).

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Thrust and Reverse faults

• Thrust faults are frequently imbricated in
  fold-and-thrust belts, and often form duplex
  structures with many small imbricate thrusts
  between major sole and roof thrusts

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Relationships between faults and folds

• Faults and folds are commonly associated, in
  several well-established relationships
• Strike-slip faults generate folding at
  restraining bends or at terminations
• Normal faults generate folding in both hanging
  wall (rollover) and footwall (rebound)
• Thrust faults generate folds if they terminate
  below the surface (fault propagation folds blind
  thrusts) or at a bend in the fault plane (fault
  bend folds)

Fault propagation fold
Fault bend fold
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Structural elements of rocks and maps Fabrics

• Fabric refers to the internal arrangement of the
  constituent particles of rocks (i.e., mineral
  grains, lithic clasts, etc.). These particles
  can have characteristic size, texture, packing,
  preferred shape or crystallographic orientation,
  and inhomogeneity of mineral type or any other
  fabric element.
• Fabrics are important in structural geology and
  metamorphic petrology because they record not
  only the primary formation of a rock but also
  evidence of the deformation. Texture development
  is the microscopic expression of rock plasticity.
• Textural fabric yields quantitative evidence of
  the extent and orientation of finite strain.

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Fabric

• Any fabric that defines surfaces of systematic
  orientation is a planar fabric or foliation.
• Planar fabrics include cleavage (preferred
  cracking, breaking, or splitting direction),
  schistosity (orientation of planar mineral
  grains, particularly mica), bedding (sedimentary
  layers) and gneissosity (secondary compositional
  or mineralogical layering).
• Any fabric that defines lines or curves of
  systematic orientation is a linear fabric or
  lineation.
• Lineations often lie in the plane of associated
  foliations.
• Lineations are defined by oriented mineral grains
  (shape or crystallographic orientation),
  microscopic fold axes, or intersections of
  foliations.
• Foliations and lineations frequently display
  close geometric relationships with associated
  outcrop-to-regional scale structures like folds
  and faults. This makes fabric observations
  essential evidence in working out map-scale
  structure.

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Examples Deduction of Structure from Fabric

• Slickensides
• A lineation in the plane of a fault, defined by
  simple scratches or growth of secondary minerals
  with preferred orientations, gives the slip
  direction (but not always the sense of slip along
  that direction) -- often easier than finding
  piercing points (tells you which way to look for
  a matching piercing point, anyway)
• Axial plane cleavage
• Folds, particularly when not parallel, are
  commonly associated with development of a
  penetrative cleavage parallel to the axial plane.

There is no necessary relationship between
bedding planes and foliation directions!
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Fabric Examples

• The axial plane cleavage may display fanning as
  it passes between layers that are easier or
  harder to deform.
• This property can be used to infer which is the
  direction to the nearest fold hinge, a good
  example of how outcrop-scale fabrics help solve
  map-scale structures.

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Examples deduction of stress orientation from
fabric

• Lattice preferred orientation
• Mechanical anisotropy of minerals may cause them
  to rotate until their crystallographic axes are
  aligned with the applied stress.
• LPO in olivine leads to seismic and electrical
  anisotropy in the sheared mantle. In ophiolites,
  these orientations may be preserved and allow
  mapping of flow patterns under the ridge.

• Growth of new minerals in a stress field also
  results in lattice preferred orientation,
  especially of platy minerals

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Examples deduction of stress orientation from
fabric

• Shape preferred orientation
• In addition to crystallographic orientation, the
  shape of grains or objects can reflect finite
  strain through several mechanisms
• rotation of elongated mineral grains towards
  parallelism with the strain direction
• homogeneous stretching of equant mineral grains
  until they too are elongated along the strain
  direction
• growth of new minerals in pressure shadows around
  hard minerals or grains

Stretched pebbles, initially nearly spherical, in
deformed quartz-pebble conglomerate
Ooids in deformed limestone. Shape is due to
growth of calcite and quartz fibers where matrix
has stretched away from relatively rigid ooids
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Examples deduction of stress orientation from
fabric

• Shape preferred orientation
• growth of new minerals in pressure shadows around
  hard minerals or grains
• Changes in shape of grains by differential
  pressure solution in different directions.

These Stylolites (from the limestone partitions
in the bathrooms here) are planar features
created by accumulation of insoluble material
along planes of concentrated pressure solution in
carbonates. Their orientation is perpendicular
to s1. Similar processes generate shape-preferred
orientation by differential pressure-solution at
grain scale.
Pressure shadows of recrystallized quartz around
rigid, rotating pyrite grain.
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Structural elements of maps and rocks Joints

• A joint is any natural planar crack that is not a
  fault (hence, no slip across it), bedding (a
  sedimentary structure), or cleavage (defined by
  mineral orientation) and is larger than the grain
  size of the rock.

• Joints generally form by tensile fracture. They
  may form open fissures and fill with vein
  material in some cases.
• Systematic joints show regular orientation over
  many joint surfaces. All the joints in a region
  with a roughly common orientation are joint sets.
  Joint sets intersect without offsetting each
  other.

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Joints

• Joints sets are often regionally consistent and
  imply that joints reflect regional-scale
  orientations of stress in the crust

• A joint system is two or more joint sets that are
  genetically related, e.g. the three joint sets
  that result from columnar jointing

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Joints

• The most common way to form joints is by tectonic
  unloading and thermal contraction. During
  uplift, progressive unroofing lowers the mean
  stress, while laterally confined thermal
  contraction generates differential stress. This
  leads to transitional tensile fracture (see
  fracture mechanics in lecture 7).
• In many cases, jointing driven by erosion and
  cooling must take place very close to the
  surface, since joints can be evidently seen to
  follow topography at the 100 m scale and because
  the spacing of joints clearly increases over the
  last few tens of meters of exposure in quarries

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Construction of Cross Sections

• A geologic map can show all data actually
  observable by the geologist, walking around the
  surface looking at outcrops.
• Interpretation of a geological map, however,
  usually requires visualization in 3-D, by
  extrapolation of surface data downwards
  (sometimes with help from drilling or seismic
  data).
• The most common representation of such an
  extrapolated interpretation is a cross section,
  or vertical slice through the map.
• The line (or piecewise linear path) along which
  a cross section is drawn should always be clearly
  marked on the corresponding surface-view map.
  The top of the cross section should match the
  surface data (topography, rock units, contacts,
  dips) exactly.
• Usually, in regions of reasonably simple
  structure, one chooses to draw transverse cross
  sections, i.e. perpendicular to regional strike.
• A cross section is always a model, a non-unique
  (though testable) suggestion of the simplest deep
  structure consistent with observations. It is
  always possible to suggest alternatives.

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Cross Sections and 3-D visualization simple cases

• Case 1 Homoclinal structure, flat topography

• Case 2 Horizontal stratigraphy plus topographic
  relief

• Simpler structure can give more complicated map
  pattern!

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Cross Sections and 3-D visualization simple cases

• Case 3 Simple anticline, no plunge, no topography

• Case 4 Simple syncline, no plunge, no topography

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Cross Sections and 3-D visualization

• Case 5 Plunging anticline, no topography

• Ignoring topography and dip measurements, same
  outcrop pattern in map view as case 2!

• More complicated cases are built up from these
  basic patterns
• Case 6 Plunging folds, angular unconformity,
  topography developed in the flat overlying strata

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Cross Sections and 3-D visualization

• Case 7 Thrust fault and normal faultlow angle
  faults are much more obvious in map view.

• Case 8 Strike-slip faults can be obscure both in
  map-view and cross section, if piercing points
  are absent

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Balanced Cross Sections

• When rocks are deformed from a known intial state
  (e.g. horizontal strata), a cross section is a
  valid interpretation of data only if it can be
  geometrically retrodeformed (unfold the folds,
  backslide the faults) to the original state. This
  is called balancing a cross-section.
• The fundamental assumption in balancing is
  conservation of mass.
• If we are dealing with already-lithified rocks
  (no compaction), then in most cases we can also
  assume conservation of volume.
• If no material moved into or out of the plane of
  our cross section, e.g. for cylindrical folds in
  a plane perpendicular to the generator, this
  reduces to conservation of area.
• Finally, if the structure is parallel (preserved
  layer thickness), this reduces to conservation of
  bed length.

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Balanced Cross Sections

• Conservation of area in balanced cross sections
  can be used, for example, to determine the depth
  of an otherwise unobservable detachment that
  allows shortening above

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Construction of balanced fold cross sections

• For cylindrical, parallel folds, one simple
  assumption is that folds are concentric, i.e.
  made up of circular arcs. In this case dip
  measurements are tangent to the arcs and normals
  to dips intersect at the center of the circle. As
  many centers are introduced as necessary to fit
  the data. This leads to the Busk method.

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Construction of balanced fold cross sections

• Alternatively, parallel folds may be extrapolated
  using the assumption of straight limbs and
  angular hinges. In this case, if layer thickness
  is preserved, an axial surface must bisect the
  angle between the fold limbs. This leads to the
  kink method.

When the fold is not parallel, i.e. layer
thickness differs between the two limbs, the kink
method may be modified using a refraction law