Geology Reference
In-Depth Information
Faulted Surfaces
8.1
Introduction
This chapter covers how to construct maps containing faults, the integration of fault
maps with horizon maps, interpreting faults from isopach maps, mapping overlapping
and intersecting faults, and mapping cross-cutting faults.
8.2
Geometry of a Faulted Surface
Faults cause discontinuities in the map of a marker horizon and result either in gaps,
overlaps, or vertical offsets of the marker surface. Normal faults produce gaps
(Fig. 8.1a,b), reverse faults cause overlaps (Fig. 8.1c,d), and vertical faults produce line
discontinuities on the map of a marker horizon (Fig. 7.17). Structure contours on the
fault plane intersect the offset marker where the contours on the marker and the fault
are at the same elevation. It is best practice to show the fault contours on the map, scale
permitting. Structure contours on a fault surface are easily constructed by connecting
the points of equal elevation where the marker horizon intersects the fault surface on
opposite sides of the fault. This is called the implied fault surface (Tearpock and Bischke
2003). A fault intersects the marker surface along lines known as hangingwall and foot-
wall cutoff lines. If the fault ends within the map, then the contours on the marker
surface are continuous at the ends of the fault.
Heave and throw are the components of fault separation directly visible on the struc-
ture contour map of a faulted marker horizon. Throw is the vertical component of the
dip separation; heave is the horizontal component of the dip separation (Fig. 8.2; also
Sect. 7.4.3), both components being defined in the dip direction of the fault. The throw
is the change in vertical elevation between the hangingwall and footwall cutoff lines in
the direction of the fault dip. The throw and heave are related to the stratigraphic
separation by Eqs. 7.5-7.8.
8.2.1
Heave and Throw on a Structure Contour Map
On a structure contour map the heave is the map distance between the hangingwall
and footwall cutoff lines, measured in the direction of the fault dip (Figs. 8.3, 8.4).
The fault gap (or overlap) is the horizontal distance between the hangingwall and
