Attitude of Structural Elements

1
Attitude of Structural Elements

• Structural Geology

2
Some Rocks Have Attitude!

• Attitude A general term for the orientation of a
  line or plane. Has two components
• 1. Bearing (e.g., strike, trend)
• Horizontal angle measured from a line to a
  reference. It is scalar (e.g., N30oE, or 030o)
• The reference is North (000o) or South (180o )
• 2. Inclination (e.g., dip, plunge)
• Vertical angle between a line or plane and the
  horizontal (measured down not up!).
• Inclination ranges between 0o and 90o
• Dip is a vector (e.g., 40oN) plunge is a scalar!

3
Attitude of Linear Structures

• The attitude of linear structures is defined by
  the trend, plunge (together they define a vector)
• Trend is the bearing of the line
• Plunge is the inclination of the line
• Linear structure are also defined by their pitch
  on a given plane
• Pitch The acute angle between the line and the
  strike of the plane on which the line lies
• Example of linear structures Fold axis,
  hingeline, intersection of two planes, stretched
  pebbles, other lineations

4
Attitude of Planar Structures

• The attitude of planar structures is defined by
  the strike, dip
• Strike is the bearing of a horizontal line on the
  plane (a scalar), e.g., N40oE
• Dip is the inclination of the plane, measured
  down
• dip is a vector it gives the direction and
  amount of dip of the plane. Example for dip
  80oN
• 80o is the amount, N is the direction
• Example of planar structure bedding, fault,
  fold axial plane, layering in lava, foliation

5
A Common Mistake by Geologists

• Measuring planar structures as if they are
  linear. e.g.,
• Measuring the trace of an inclined (but not
  vertical) bedding on a non-horizontal ground
  surface
• The trace actually may not be horizontal (i.e.,
  does not represent the strike). Recall that two
  inclined planes generally intersect along a
  non-horizontal line (intersection along the
  horizontal is a special case!).
• In this case what we are really measuring is the
  trend of an inclined line, on a vertical plane,
  which may not even be on the bedding plane!
• The best is to directly measure the (strike, dip)
  or (dip amount, dip direction) of a plane, rather
  than the trend of its trace. This may require
  exposing the surface of the plane with a hammer
  and chisel!
• NOTE It is ok to measure the trace of a
  vertical bedding on a horizontal ground surface.
  In this special case (which does not occur
  frequently), the trend of the trace is the same
  as the strike!

6
Structural Contours

• Lines of equal elevation above or below some
  reference level, on the a specific surface (e.g.,
  contact, fault, coal seam)
• They are not the same as topographic contours,
  which are lines of equal elevation on the surface
  of Earth

7
Construction of Structural Contours

• Follow a contact on the map, and find two points
  at which a contact is intersected by the same
  topographic contour
• A line through these two points is horizontal.
• Since it lies on the plane, it is also the strike
• Draw at least two structural contours for each
  plane
• These are parallel if the surface is planar
• Draw a line perpendicular to two adjacent
  structural contours along the true dip of the
  plane (this is a horizontal line ?x)
• Measure ?x with a ruler find its length in real
  ground scale
• Find the difference in elevation (e.g., in meter)
  of the two adjacent structural contours (?h)
• The dip (?) of the plane is calculated from
• tan ? ?h/?x (rise over run use the same
  scale)
• The arc tangent or tan-1 gives you the dip (i.e.,
  ?)