Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 167

Hinterland

Imbricate faults
(Schuppen structure)

Thrust and fold belt

Foreland

duplex structure Decollement

Duplex structures

10 km

Hinterland

Foreland

Fig. 4.105 Idealized model of a thrust and fold belt and its representation on a map. The filled triangle along the faults on the map point in the
dip direction of the faults.

Stress trajectories

s 1

s 3

t

2 u

2 a

s n

Fracture trajectories Shear displacement

Fig. 4.106 Stress trajectories can curve at depth when there are stress gradients. Coulomb fractures will bend according to stress trajectories,
which can cause the change from thrust (low-angle) to reverse faults (high-angle).

basement is mostly unaffected. This situation poses impor-
tant mechanical and kinematic problems in the reconstruc-
tion of tectonic processes related to thrusting, due to the
decoupling between the shortening of the basement and
the cover. Common structures in thrust and fold belts are
a low-angle or near horizontal basal shear plane or decolle-
ment , that act as detachment areas and separate a highly
deformed, both folded and fractured upper part or cover
from a relatively undeformed substratum or basement . The
detachment is also called a sole fault , produced where there
is a mechanical contact formed by the presence of a less
frictional weak layer (typically clay, shale, or salts).
Deformed rock wedges over thrust faults are often called
thrust sheets or nappes . The cover is also known as an
allochthonous terrain due to its displaced nature, forming

very extensive and relatively thin triangular rock wedges
that thin in the displacement or tectonic transport direc-
tion. The basement under the main decollement is often
referred to as autochthonous , the rocks there remaining
in situ . Erosion of part of the allochthonous terrain allows
observation of the basement at the Earth's surface in so-
called tectonic windows . Similarly, erosive remnants of an
allochthonous terrain surrounded by autochthonous rocks
are called klippes .
As in normal faults, flat and ramp geometries are com-
mon in thrust faults, lying perpendicular, parallel, or
oblique to block transport direction. Commonly ramps are
formed when a low-angle or horizontal fault rises to a shal-
lower level in the crust cutting competent rocks and form-
ing a high-angle step inclined backward with respect to the