Sediments deposited in basins of this type tend to
undergo folding, which will result in good structural
traps. Geothermal gradients will be relatively high,
and allow thorough maturation.
8.12 Converging Plate Boundaries
When plates collide (converge), there can be
boundaries between:
• Oceanic crust and continental crust, such as along
the west side of South America (Andes type).
• Continental crust against continental crust (Alpine
or Himalayan type mountain chain).
• Oceanic crust against oceanic crust, with formation
of volcanic islands.
The oceanic plate has a higher density than the
continental crust, so in a subduction zone the oceanic
plate will be carried downwards under the continental
one. The oceanic plate is relatively cold and its geothermal gradient is low because since it is descending,
the upward heat flux is reduced as a function of the rate
of subduction. Where there is active subduction, ocean
trenches are formed, such as the 10–11 km deep ones
outside the Philippines and Japan. This is because the
underlying rocks are cold and therefore dense.
In subduction zones the downward-moving part of
the oceanic crust is characterised by extremely low
geothermal gradients. This is because the heat flow
must move counter to the direction of movement of the
plate undergoing subduction which is therefore colder
and denser than the average old ocean floor crust. The
subduction zones form the deepest parts of the ocean
(up to 10–11 km deep) where there is not a great deal
of sedimentation. Ordinary, cold, 100–150 million
year old ocean floor is in isostatic equilibrium at a
depth of 5–7 km without sediment loading.
Where we have subduction of oceanic crust beneath
continents we find the following types of basin:
• Trench basin (oceanic sedimentation within the
trench).
• Accretionary prism.
• Fore-arc basin (relatively shallow basin in front of
island arcs).
• Inter-arc basin (sedimentary basin between island
arcs of continental crust).
• Back-arc basin (sedimentary basin on the oceanic
crust).
• Retro-arc basin (formed if continental blocks are
upthrust and faulted, creating a secondary basin in
the continental crust).
Trench basins seldom contain large amounts of
sediment if separated from the continent by an island
arc preventing sediment supply from the continents.
One of the reasons why they are the deepest oceanic
areas is because they do not fill up with sediment. This
is because the ocean floor beneath trench basins is part
of a dynamic system and is being consumed through
subduction as sedimentation occurs. The downward
movement of the oceanic crust helps to maintain a
low geothermal gradient in the trench since heat
must flow up through rocks that are descending.
Because of the continual lateral displacement of the
rock floor under the trenches, the sediment thickness
will be limited. The island arcs are the main source of
sediment supply to the trench basins.
Most of the sediment coming from the continents will
be trapped in back-arc basins. The sediments in trench
basins consist of deep-water conglomerates, turbidites
and pelagic sediments. Trench basins lie mostly below
the carbonate compensation depth and therefore contain
little carbonate and are also usually starved with respect
to clastic sediment supply. Sedimentary transport of very
fine-grained pelagic sediments tends to occur along the
axis of the trench. The inner slope up to the island arcs is
steeper than the outer one, and the subduction zone
(Benioff zone) is at the foot of this slope (Fig. 2.48).
When the oceanic plate descends into the Benioff
zone, sediments deposited on the sea bed may be
scraped off, forming wedges of sediment. This is
called an accretionary prism and consists of a complicated pattern of folded and brecciated sedimentary
rocks exhibiting overthrusts and underthrusts (Fig. 2.
49). The sedimentary sequence is the right way up
within each thrusted unit, but the sequence of thrusted
units is inverted. The youngest sediment wedges are at
the bottom because they are the last sediments to have
been scraped up from the seafloor.
On the inside of the trench, slope basins are formed
with sediment supplied by the island arcs. These
deposits are folded, deformed and reworked by the
subduction processes.
270
K. Bjørlykke
undergo folding, which will result in good structural
traps. Geothermal gradients will be relatively high,
and allow thorough maturation.
8.12 Converging Plate Boundaries
When plates collide (converge), there can be
boundaries between:
• Oceanic crust and continental crust, such as along
the west side of South America (Andes type).
• Continental crust against continental crust (Alpine
or Himalayan type mountain chain).
• Oceanic crust against oceanic crust, with formation
of volcanic islands.
The oceanic plate has a higher density than the
continental crust, so in a subduction zone the oceanic
plate will be carried downwards under the continental
one. The oceanic plate is relatively cold and its geothermal gradient is low because since it is descending,
the upward heat flux is reduced as a function of the rate
of subduction. Where there is active subduction, ocean
trenches are formed, such as the 10–11 km deep ones
outside the Philippines and Japan. This is because the
underlying rocks are cold and therefore dense.
In subduction zones the downward-moving part of
the oceanic crust is characterised by extremely low
geothermal gradients. This is because the heat flow
must move counter to the direction of movement of the
plate undergoing subduction which is therefore colder
and denser than the average old ocean floor crust. The
subduction zones form the deepest parts of the ocean
(up to 10–11 km deep) where there is not a great deal
of sedimentation. Ordinary, cold, 100–150 million
year old ocean floor is in isostatic equilibrium at a
depth of 5–7 km without sediment loading.
Where we have subduction of oceanic crust beneath
continents we find the following types of basin:
• Trench basin (oceanic sedimentation within the
trench).
• Accretionary prism.
• Fore-arc basin (relatively shallow basin in front of
island arcs).
• Inter-arc basin (sedimentary basin between island
arcs of continental crust).
• Back-arc basin (sedimentary basin on the oceanic
crust).
• Retro-arc basin (formed if continental blocks are
upthrust and faulted, creating a secondary basin in
the continental crust).
Trench basins seldom contain large amounts of
sediment if separated from the continent by an island
arc preventing sediment supply from the continents.
One of the reasons why they are the deepest oceanic
areas is because they do not fill up with sediment. This
is because the ocean floor beneath trench basins is part
of a dynamic system and is being consumed through
subduction as sedimentation occurs. The downward
movement of the oceanic crust helps to maintain a
low geothermal gradient in the trench since heat
must flow up through rocks that are descending.
Because of the continual lateral displacement of the
rock floor under the trenches, the sediment thickness
will be limited. The island arcs are the main source of
sediment supply to the trench basins.
Most of the sediment coming from the continents will
be trapped in back-arc basins. The sediments in trench
basins consist of deep-water conglomerates, turbidites
and pelagic sediments. Trench basins lie mostly below
the carbonate compensation depth and therefore contain
little carbonate and are also usually starved with respect
to clastic sediment supply. Sedimentary transport of very
fine-grained pelagic sediments tends to occur along the
axis of the trench. The inner slope up to the island arcs is
steeper than the outer one, and the subduction zone
(Benioff zone) is at the foot of this slope (Fig. 2.48).
When the oceanic plate descends into the Benioff
zone, sediments deposited on the sea bed may be
scraped off, forming wedges of sediment. This is
called an accretionary prism and consists of a complicated pattern of folded and brecciated sedimentary
rocks exhibiting overthrusts and underthrusts (Fig. 2.
49). The sedimentary sequence is the right way up
within each thrusted unit, but the sequence of thrusted
units is inverted. The youngest sediment wedges are at
the bottom because they are the last sediments to have
been scraped up from the seafloor.
On the inside of the trench, slope basins are formed
with sediment supplied by the island arcs. These
deposits are folded, deformed and reworked by the
subduction processes.
270
K. Bjørlykke