588
tions of landward-dipping thrust faults which tend to
steepen with increasing distance from the deformation front (Fig. 12.22e). Strata may be disrupted even
in porous sediment within 10-20 km of the deformation front. The second generation of thrust faults disrupts the older thrust faults and thus prornotes further
deformationof the accreted sediments. Initial fault
surfaces are abandoned and deformation propagates
into adjacent undeformed sediment. This is caused
by sediment strengthening along the faults due to
dewatering and release of excess fluid pressure. As a
result, the fault and shear zones thicken andcontribute to the intensive deformation of a tectonic
melange. Underplating of sediment is often associated with the formation of duplex structures. Smallscale structures inelude scaly fabric, cataelastic shear
zones, and disrupted veins.
Melange, Wildjlysch
Other portions of the subduction complex consist of
a mixture of various rock types ineluding large solid
blocks in a more uniform, but sheared matrix. Such
chaotic zones are referred to as melange. Principally,
one can distinguish between several types of melange
(Orange and Underwood 1995):
- Gravitationally induced lateral spreading of nearsurface sediment within the slope apron of an
accretionary wedge.
- Large-scale gravity-driven debris flow deposits
(olistostromes, cf. Sect. 5.4.2).
- Diapiric sediment movements (diapiric injection,
see below).
- Tectonic deformation within shear zones at the
front, the base (decollement zone), or within the
accretionary wedge. Melange can also form by viscous flow of poorly lithified sediments in wedgeshaped "channels" of the decollement. The brecciation of carbonate-capped seamounts approaching
the subduction zone (collisional collapse) has been
quoted as a further mechanisms generating a
polymictic melange (Kanmera and Sano 1991; Taira
1993/4). Melange may contain exotic metamorphic
blocks (greenshist and blueshist metamorphism) derived from deeply underplated sediments.
Fig. 12.23. Sunda arc-trench system. a Map view
with Bengal and Nicobar deep-sea fans accumulating
sediments from the Himalayas; such material also
migrates along the Sunda trench axis as far as Sunda
Strait. At present, forearc basins and the outer arc
ridge trap sediments from the island arc (Sumatra and
Java). b-d Cross sections of arc-trench system from
northwest (Andaman Islands) to southeast (Java).
Chapter 12 Basin Evolution and Sediments
The thickness of the disrupted zones in accretionary
wedges grows with progressive deformation. It is
frequently difficult to decide whether a melange results entirely from tectonic processes within the
subduction complex, or whether it represents one of
the other processes mentioned above. Mass flow deposits at the foot of the slope or diapiric structures in
front of the accretionary wedge are later incorporated
into the subduction complex and modified by tectonic deformation.
Furthermore, it occurs that material from the
accretionary wedge is transported again into the
trench and thus recyeled within the arc-trench system. Where a genetic interpretation of a chaotic deposit isdifficult, the descriptive term wildjlysch may
be used.
Mud Diapirs and Mud Volcanoes
In addition to these major structures, deep-sea
trenches, the lower slopes of accretionary prisms, and
some forearc basins may show a variety of medium
to large-scale sedimentary structures. They inelude
seepages on the sea floor (cf. Sect. 13.8.3), mud volcanoes and mud diapirs, wet sediment intrusions
such as vertically and laterally injected sand dikes,
and thick and widely extended "chaotic layers" generated in semi-lithified rocks. The latter can easily be
misinterpreted as debris flows or tectonic melange
zones. The elasts of mud volcanoes, for example
those of the Mediterranean Ridge, can be ~ 1 m in
size and come from great depth below the sea floor.
These phenomena result from liquefaction or
fluidization of water-rich, overpressured sediments
andJor the release of methane from gas hydrates acting as driving force for the uplift of the liquified sediment.
Mud volcanoes in the deep sea and in coastal areas are
known from many locations, e.g. the Caspian Sea, the
Black Sea, the Mediterranean and the Makran coastal region of Pakistan (Makran wedge), the Nankai prism, and
the Indonesian are (Cita et al. 1996; Ivanov et al. 1996).
Mud volcanoes and flattened "mud-pies" can also occur on
the seaward side of accretionary wedges as described, for
example, from the Barbados wedge (Lance et al. 1998).
For further information about aIl these phenomena see,
e.g., Hsü (1974), Raymond (1984), Cowan (1985), Moore
and Byrne (1987), Shipboard Scientific Party (l988b, Leg
Note decreasing thickness of sediment on subducting
oceanic plate, decreasing width and elevation of
outer arc ridge, increasing water depth of forearc
basins, and change in sand provenance from distal
Himalayas to more local sources from the arc. See
text for further explanation. (After Moore GF et al.
1982).
tions of landward-dipping thrust faults which tend to
steepen with increasing distance from the deformation front (Fig. 12.22e). Strata may be disrupted even
in porous sediment within 10-20 km of the deformation front. The second generation of thrust faults disrupts the older thrust faults and thus prornotes further
deformationof the accreted sediments. Initial fault
surfaces are abandoned and deformation propagates
into adjacent undeformed sediment. This is caused
by sediment strengthening along the faults due to
dewatering and release of excess fluid pressure. As a
result, the fault and shear zones thicken andcontribute to the intensive deformation of a tectonic
melange. Underplating of sediment is often associated with the formation of duplex structures. Smallscale structures inelude scaly fabric, cataelastic shear
zones, and disrupted veins.
Melange, Wildjlysch
Other portions of the subduction complex consist of
a mixture of various rock types ineluding large solid
blocks in a more uniform, but sheared matrix. Such
chaotic zones are referred to as melange. Principally,
one can distinguish between several types of melange
(Orange and Underwood 1995):
- Gravitationally induced lateral spreading of nearsurface sediment within the slope apron of an
accretionary wedge.
- Large-scale gravity-driven debris flow deposits
(olistostromes, cf. Sect. 5.4.2).
- Diapiric sediment movements (diapiric injection,
see below).
- Tectonic deformation within shear zones at the
front, the base (decollement zone), or within the
accretionary wedge. Melange can also form by viscous flow of poorly lithified sediments in wedgeshaped "channels" of the decollement. The brecciation of carbonate-capped seamounts approaching
the subduction zone (collisional collapse) has been
quoted as a further mechanisms generating a
polymictic melange (Kanmera and Sano 1991; Taira
1993/4). Melange may contain exotic metamorphic
blocks (greenshist and blueshist metamorphism) derived from deeply underplated sediments.
Fig. 12.23. Sunda arc-trench system. a Map view
with Bengal and Nicobar deep-sea fans accumulating
sediments from the Himalayas; such material also
migrates along the Sunda trench axis as far as Sunda
Strait. At present, forearc basins and the outer arc
ridge trap sediments from the island arc (Sumatra and
Java). b-d Cross sections of arc-trench system from
northwest (Andaman Islands) to southeast (Java).
Chapter 12 Basin Evolution and Sediments
The thickness of the disrupted zones in accretionary
wedges grows with progressive deformation. It is
frequently difficult to decide whether a melange results entirely from tectonic processes within the
subduction complex, or whether it represents one of
the other processes mentioned above. Mass flow deposits at the foot of the slope or diapiric structures in
front of the accretionary wedge are later incorporated
into the subduction complex and modified by tectonic deformation.
Furthermore, it occurs that material from the
accretionary wedge is transported again into the
trench and thus recyeled within the arc-trench system. Where a genetic interpretation of a chaotic deposit isdifficult, the descriptive term wildjlysch may
be used.
Mud Diapirs and Mud Volcanoes
In addition to these major structures, deep-sea
trenches, the lower slopes of accretionary prisms, and
some forearc basins may show a variety of medium
to large-scale sedimentary structures. They inelude
seepages on the sea floor (cf. Sect. 13.8.3), mud volcanoes and mud diapirs, wet sediment intrusions
such as vertically and laterally injected sand dikes,
and thick and widely extended "chaotic layers" generated in semi-lithified rocks. The latter can easily be
misinterpreted as debris flows or tectonic melange
zones. The elasts of mud volcanoes, for example
those of the Mediterranean Ridge, can be ~ 1 m in
size and come from great depth below the sea floor.
These phenomena result from liquefaction or
fluidization of water-rich, overpressured sediments
andJor the release of methane from gas hydrates acting as driving force for the uplift of the liquified sediment.
Mud volcanoes in the deep sea and in coastal areas are
known from many locations, e.g. the Caspian Sea, the
Black Sea, the Mediterranean and the Makran coastal region of Pakistan (Makran wedge), the Nankai prism, and
the Indonesian are (Cita et al. 1996; Ivanov et al. 1996).
Mud volcanoes and flattened "mud-pies" can also occur on
the seaward side of accretionary wedges as described, for
example, from the Barbados wedge (Lance et al. 1998).
For further information about aIl these phenomena see,
e.g., Hsü (1974), Raymond (1984), Cowan (1985), Moore
and Byrne (1987), Shipboard Scientific Party (l988b, Leg
Note decreasing thickness of sediment on subducting
oceanic plate, decreasing width and elevation of
outer arc ridge, increasing water depth of forearc
basins, and change in sand provenance from distal
Himalayas to more local sources from the arc. See
text for further explanation. (After Moore GF et al.
1982).