Geology Reference
In-Depth Information
Fig. 8.47.
Evolution of crossing reverse faults having opposed dips, similar to those in Fig. 8.46c.
a
Displacement on the first fault, with location of the second fault
dashed
.
b
Displacement on the sec-
ond fault.
Dotted lines
bound the zone of combined stratigraphic separation for the
shaded
horizon.
T
indicates regions of local structural thinning of the shaded unit
ment on each fault has been assumed to be constant, a reasonable assumption over
a small portion of a fault, but not likely to hold over a long distance along the fault
surface.
8.7.2
Contemporaneous Faults
Intersecting faults may be of the same age, that is, contemporaneous. (The term
contemporaneous faulting has also been used for faults in which the displacement
is contemporaneous with deposition (Hardin and Hardin 1961) but the term
growth fault is now widely used for that concept.) According to the Andersonian
theory of faulting (Sect. 1.6.4) a biaxial state of stress is expected to produce two
conjugate fault trends of the same age that intersect with a dihedral angle of 40-65°.
A triaxial stress state may cause three or four fault trends (Oertel faults) to form si-
multaneously.
The fault pattern in the zone of intersection of contemporaneous faults tends to
be more complex than the patterns for sequential faults discussed in the previous
section. Multiple small faults may occur in the zone of intersection (Fig. 8.48). In
the experimental studies of normal faults by Horsfield (1980), two crossing conju-
gate faults formed initially and with continuing extension the initial horst and
graben were segmented by smaller conjugate faults. Many of the crosscutting re-
lationships described above are probably the result of contemporaneous conjugate
faults that cut one another as their displacement increases and they grow along
strike and down dip.
As yet there is little published on the geometry of crossing contemporaneous faults.
This style of structure may have been overlooked because of the incorrect assump-
tion that crossing faults cannot be contemporaneous or because the geometric rela-
tionships are complex. Where timing information, such as growth strata, is available,
the time relationships of the fault sets can be determined and used to substantiate the
interpretation of fault contemporaneity.
