Structural effects of gravity - Introducing Tectonics, Rock Structures and Mountain Belts - page 78

cylindrical in form, commonly 1-2 km
in diameter, and then into a mushroom
shape with a circular cross-section. In
some places the salt may even pierce
the surface, as in one of the salt diapirs
of the Zagros range in Iran, where salt
flows down the flanks of the dome,
rather like a salt 'glacier' (Figure 9.5B).
Where the initial salt body is in the
form of an anticline, it will evolve into
an elongate salt pillow . Any subse-
quent diapir will also be elongate and is
termed a salt wall . Salt walls are often
formed along steeply inclined faults.
The flow of the salt within these
structures creates sets of folds and
associated planar and linear fabrics
that may be used to reconstruct the
geometry of the whole structure. Plug-
type diapirs possess internal structures
whose flow directions are radial at the
base of the structure, become parallel
to the cylindrical stem of the diapir, and
then spread out radially again in the

upper mushroom (Figure 9.6A, B). Folds
formed by the layers within the salt
become tighter and more constricted
as they approach the cylindrical stem
of the diapir (Figure 9.6C, D) such that
the fold axes, which initially would
be straight or gently curved, become
progressively more tightly curved,
as shown in Figure 9.6C, individual
folds assuming a rounded conical,
or sheath-like shape (Figure 9.6F).
Structures also form in the strata sur-
rounding the salt bodies (Figure 9.5A):
the withdrawal of salt from a circular
area around a salt dome will produce
a syncline, which will be accentuated
as the diapir breaks through to form
a plug, as seen in Figure 9.4.4. Anti-
clinal structures formed above a salt
dome are associated with extensional
normal faults and graben . Structures
around the margins of salt diapirs
often form oil traps that are sealed by
the impermeable salt - many oil fields

in the Gulf of Mexico, for example,
have been formed in this way.
Although salt flow can be quite fast
compared to other rocks, salt diapirs
can take many millions of years to
evolve into their final (relatively stable)
form. The numerous salt bodies of
the north German plain arose from a
Zechstein (upper Permian) evaporite
layer which began to move upwards to
form a set of diapirs during the Trias-
sic. However, upward movement was
still occurring in the early Cenozoic.

Granite diapirs and mantled
gneiss domes
The granite diapirs discussed in the pre-
vious chapter were able to rise through
their overburden because they were
(largely) liquid as well as being less
dense than their host rocks. However,
the metamorphic core complexes of
many orogenic belts contain dome-
shaped or anticlinal bodies of largely
granitic composition that represent
older basement and which appear
to have risen through their younger
metamorphic cover. Many of these have
been interpreted as diapirs that have
moved up through the crust in a solid
state and are therefore analogous to the
salt diapirs just described. Such bodies
are particularly common in Archaean
granite-greenstone terrains , so-called

flow
direction

fold
traces

A

D

fold
cross-sections

Figure 9.6 Diagrammatic representation of flow directions
and fold patterns in a salt diapir. A. Cross-section of a
salt diapir showing idealised pattern of flow. B. Plan view
of base of diapir showing radial flow directions; flow is
vertical in the central column. C. The folds are constricted
as they move towards the central column, and the fold
axes become progressively more curved. D. Cross-section
of diapir showing idealised fold traces. E. Plan view
through centre of column showing fold cross-sections.
F. Progressive tightening and elongation of a fold with a
curved axis.

fold axes

E

B

C

F