Geology Reference
In-Depth Information
8.6.1
Relay Overlap
Faults that transfer displacement from one to the other without intersecting constitute a
relay pattern (Fig. 8.27). The displacement is said to be relayed from one fault to the other.
The displaced horizon in the zone of fault overlap forms a ramp that joins the hangingwall
to the footwall (Figs. 8.30, 8.31a). The ramp may be unfaulted or may itself be broken by
faults. An unbroken ramp provides an opportunity for pore fluids to migrate across the
main fault zone. Second-order faults within a ramp typically trend at a low angle to the
strike of the ramp and are therefore at a high angle to the relay faults. Faults exhibiting a
relay pattern may appear to be unrelated on a map because a single fault will have the dis-
placement pattern of an isolated fault that dies out along strike (Figs. 7.22, 7.25). The covari-
ation of displacement on the two faults reveals their relationship to be that of displace-
ment transfer (for example, Fig. 7.25c). Detailed examination of large fault zones may reveal
that the main fault consists of multiple segments with relay overlaps between the segments.
On the structure contour map of a horizon displaced in a relay zone, the ramp is the
region between the two faults where the contours on the marker horizon are at a high
Fig. 8.30.
Displacement transfer at a re-
lay overlap.
Arrows
indicate
amount of dip separation on
the relay faults
Fig. 8.31.
Structure contour maps based
on the relay overlap geometry
in Fig. 8.30. Both maps are at
the same scale.
a
Displaced
stratigraphic horizon.
Solid
contours
are on the offset
marker,
dashed contours
are
on faults
A
and
B
.
b
Portions
of the two fault planes; con-
tours on fault
A
are
dashed
where they lie below fault
B
