Fractures, faults and earthquakes - Introducing Tectonics, Rock Structures and Mountain Belts - page 36

The stratigraphy in Figure 5.5A can
be calibrated from borehole data, and
shows a set of normal faults of Upper
Jurassic age that have continued to be
active, though with smaller throws, into
the Cretaceous, displacing the basal
Upper Cretaceous unconformity.

As indicated in Figure 5.4, the gravi-
tational effect of the extension is to
elevate the mantle together with the
earlier formed faults, which rotate into
a sub-horizontal attitude. Ultimately,
as shown in Figure 5.6A, the oldest and
lowest fault, now a basal crustal detach-
ment, becomes back-tilted, and mantle
material is exposed at the surface.

(Figure 5.7B) in the overlying sheet.
These fault-generated folds are dis-
cussed further in the following chapter.
Eventually, the gravitational load on
a rising thrust sheet causes the thrust
plane to stick - or lock up - in which
case, continuing compression will cause
a new thrust to develop. This second
thrust usually forms in advance of the
original ramp by extending the basal
thrust along the detachment horizon,
then cutting upwards to form a new
ramp. A succession of such structures
can result in a very complex assemblage,
as shown, for example, in Figure 5.7C.
Thrust faults play an important
role in mountain belts by transferring
rocks from deep crustal levels up to the
surface, moving them over younger,
originally higher-level, rocks. In this
way they respond to crustal shorten-
ing by increasing crustal thickness.
Present-day mountain belts such as
the Alps, the Himalayas and the North
American Rockies ( see Chapter 11) all
exhibit sets of thrusts that have under-
gone displacements of many tens or
even hundreds of kilometres. A famous
example of a much older thrust system
is the Moine thrust zone of north-west
Scotland (Figure 5.7C), which has been
known and studied since the 1880s - see
Chapter 12. It is the marginal regions

Formation of a passive continental
margin
The role of extensional faulting is criti-
cal in the initiation and evolution of a
constructive plate boundary, and the
subsequent formation of an ocean
basin. Since much of the evidence
for this is necessarily below sea level,
studies of passive continental margins
have largely relied upon seismic sur-
veying, aided by deep-ocean drilling
and the use of submersible dives. Such
methods, employed in the investiga-
tion of the (non-volcanic) west Iberian
rifted continental margin has revealed
a complex array of extensional faults
belonging to several generations,
each of which has become inactive
(locked) and subsequently replaced
by a younger, steeper set (Figure 5.6).

Compressional fault systems
A special category of reverse fault,
termed a thrust fault, is typically
gently inclined or even horizontal.
Major thrusts occur in zones in which
numerous individual thrusts and
reverse faults are linked together in
a complex manner, all contributing
to the overall movement. In bedded
rocks, thrusts may describe a staircase
path by alternately following a weak
layer, often a bedding plane, known
as a detachment horizon , then rising
up a ramp to achieve a higher level
(Figure 5.7A). Progressive movement
of the upper thrust sheet causes these
ramps to be transported across flat-
lying strata to form fold structures

stage 3
fault

stage 2
faults

stage 1
faults

CMB

mantle

crust

Figure 5.6 Structure of the west Iberian rifted continental margin. A. This
seismic section, constrained by several deep-sea drill cores, reveals a
complex array of extensional faults belonging to several generations,
each of which has become inactive (locked) and subsequently replaced
by a younger, steeper set; the gravitational effect of the extension is to
elevate the mantle together with the earlier formed faults, which rotate
into a sub-horizontal attitude, ultimately becoming back-tilted, and
exposing mantle material at the surface. B. A partial restoration shows
the crustal section before fault movements on the (red) second-stage
faults. CMB, crust-mantle boundary; MCR, mid-crustal reflector. A, B,
based on Reston (2007).

A

5km

MCR
CMB

B