Attitude of Structural Elements: Attitude of Structural Elements Structural Geology


Some Rocks Have Attitude!: 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!


Attitude of Linear Structures: 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


Attitude of Planar Structures: 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


A Common Mistake by Geologists : 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!


Structural Contours: 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


Construction of Structural Contours: 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., )