Geology Reference
In-Depth Information
The slip direction is usually perpendicular to the axis of the drag fold at the border
of the fault zone, which can be observed in outcrop or be inferred by dip sequence
(SCAT) analysis (Chap. 9, see also Becker 1995). The axes of folds contained within a
fault zone are approximately normal to the slip direction, but may be arcuate, in which
case the slip direction bisects the arc (Hansen 1971). Slip vector determination con-
tains inherent ambiguities that must be considered, however. Different slip directions
may be overprinted with only the last increment being observed or the slip trajecto-
ries may be curved.
To determine the fault slip, draw the slip vector on the structure contour map of the
fault so that it connects the correlated points on the hangingwall and the footwall as
in Fig. 7.16. If the end points of the slip vector are specified by their
xyz
coordinates,
the length of the slip vector in the plane of the fault is (from Eq. 2.4)
L
=[(
x
2
-
x
1
)
2
+(
y
2
-
y
1
)
2
+(
z
2
-
z
1
)
2
]
1/2
,
(7.1)
where
L
= the slip and the subscripts “1” and “2” = the coordinates of the opposite ends
of the slip vector. If the end points are specified by the horizontal and vertical distance
between them (from Eq. 4.7)
L
=(
v
2
+
h
2
)
1/2
,
(7.2)
where
L
= the slip,
v
= vertical distance between end points, and
h
= the horizontal
distance between end points.
Without the information provided by the correlation of formerly adjacent points
across the fault or knowledge of the slip vector, it is impossible to determine whether
a fault is caused by strike slip, dip slip, or oblique slip. Map interpretation must fre-
quently be done without knowledge of the slip amount or direction, a situation for
which the fault separation, given next, provides the framework for the interpretation.
7.4.2
Separation
Separation is the distance between any two index planes disrupted by a fault (Dennis
1967; Bates and Jackson 1987). Several different separation components are commonly
used to describe the magnitude of the fault offset. Strike and dip separations are, re-
spectively, the separations measured between correlative surfaces along the strike and
directly down the dip of the fault (Billings 1972). The stratigraphic separation (Fig. 7.18)
is the stratigraphic thickness of the beds missing or repeated across a fault (after Bates
and Jackson 1987). Stratigraphic separation is a thickness, and therefore represents a
measurement direction perpendicular to bedding. The stratigraphic separation is equal
to the fault cut, that is, the amount of section missing or repeated across a fault at a
point. It is possible for a fault to have a large displacement and yet show no strati-
graphic separation. For example, strike-slip displacement of horizontal beds produces
no stratigraphic separation. Other combinations of slip direction and dip of marker
beds can also result in zero separation (Redmond 1972).
