Geology Reference
In-Depth Information
Fig. 3.4.
Dip-domain map of a portion
of the Triassic Gettysburg half
graben, Pennsylvania.
Num-
bers
are domain dips and
ar-
rows
are dip directions.
Heavy
line
is a normal fault, down-
thrown to the southeast.
(Modified from Faill 1973b)
sharp hinges and have sharp corners on the map. The characteristic dip-domain ge-
ometry is regions of planar dip separated by narrow hinges. A map of dip-domain
compressional folds in the central Appalachian Mountains (Fig. 3.3a) shows long, rela-
tively planar limbs and very narrow, tight hinges. The cross sections (Fig. 3.3b) show
a chevron geometry. Extensional folds may also have a dip-domain geometry. The
extensional dip domains in a portion of the Newark-Gettysburg half graben (Fig. 3.4)
have been synthesized from numerous outcrop measurements. Structure contour maps
with straight lines and sharp bends are appropriate for dip-domain structures.
Computer programs may allow smoothing to be performed after contouring, if
smoother surfaces are desired. For structural interpretation it is recommended that
the unsmoothed contours should always be examined first because they provide more
insight into the interpretation of the data points themselves.
3.4
Contouring Techniques
A given set of points can be contoured into a nearly infinite number of shapes, de-
pending on the methodology followed. There is no absolute best technique for con-
touring and the overall appearance of the map is not necessarily an indication of its
quality (Davis 1986). Geological interpretation must ultimately be part of the process.
It is usually a good idea to start the interpretation process with a map that is con-
structed using standardized and reproducible procedures. Data points may either be
contoured directly, using the triangulated irregular network (TIN) method, or can be
interpolated into the elevations at the nodes of a grid (gridding) and then contoured
(Jones and Hamilton 1992). In order to illustrate the process of constructing a struc-
ture contour map, the points in Fig. 3.5 will be treated as if they come from locations
where there is no knowledge of the shape of the surface between the points.
