594
wedge. (2) Isostatic response to the emplacement of dense
oceanic crust beneath the forearc region by subduction. (3)
Rapid cooling of a relatively warm upper plate resulting
from the subducted cooler plate. This mechanism may apply to situations where young oceanic crust is incorporated
as upper plate into a forearc region. Furthermore, the subsidence histories of forearc basins may be influenced by the
angle of subduction and variations in the subduction rates.
In the course of time, a narrow residual basin can
evolve into a broader constructed or composite basin
that onlaps the arc massif on the inner flank and lies
on the accretionary wedge on the outer side (Fig.
12.25b,c). The basin configuration tends to pass from
a ridged to a shelved forearc basin (Fig. 12.25d).
Outer ridges result from the upbuilding of the
accretionary wedge (structural highs) or from tectonic or volcanic structures (observed along intraoceanic arc systems). The ridged forearcs creating a
ponded basin are most effective in trapping and accumulating sediments. Shelved forearc basins may
lose a major part of the arc-derived sediment to the
trench.
During the evolution from a ponded to a shelved
morphology, the depositional environment of the basin passes from open, deep-marine conditions into
the increasingly restricted environments of a wide
shallow-marine and coastal. Rapid deposition of
deep-marine sediments (mainly turbidites) is followed by slower accumulating shallow-marine strata
and finally often by continental deposits (Fig.
12.25e), particularly on the side ofthe arc massif.
A tendency for uplift of both the arc massif and
the outer ridge and subsidence of the basin center
favor the continuation of sedimentation for some
time. Additional deformation of the strata can be
caused by extension, compression, and wrench faulting of the basin fill. In the early stage of such deformation, normal or reverse faults are common on the
side of the arc massif, while folding and overthrusting predominate on the seaward side of the basin fill
(Fig. 12.25e). Strike-slip motions preferentially occur
at the transition from the accretionary prism to the
arc. If the forearc basin fill is exposed to collision, as
in ancient orogenic belts, it is further tectonically
overprinted.
Sediment Successions of Foreare Basins
(General Aspects)
An idealized residual foreare basin may display the
following succession (from top to bottom):
- Fluvial and deltaic sands, partially derived from
the uplifted roots of the arc. On the inner edge of the
basin, these sediments may interfinger with deposits
from pyroclastic flows, relatively coarse grained ash
fall, lava flows, and alluvial fans consisting of both
Chapter 12 Basin Evolution and Sediments
volcanic and nonvolcanic materials (cf. Sect. 2.4).
- Sands and shales deposited in shallow-marine environments (shelf sea); high proportion of volcaniclastic material but a relatively low amount of biogenic components, resulting from the high influx of
terrigenous material and rapid sediment buildup.
- Flysch-like shale-sandstone sequences with upwardly increasing proportions of coarser grained
turbidites, largely composed of volcaniclastic material. The sediments are distributed by seaward and
basin-parallel currents. Mass flow deposits are common, particularly on the landward side of the basin
(basin slope). The shales may contain some carbonate (when deposited above the CCD) and partially
have a relatively high organic carbon content.
-'- Abyssal-plain sediments containing montmorillonitic shales devoid of carbonate, fine-grained ash fall
deposits, and fine-grained turbidites.
A large supply of terrigenous material, including
clastic detritus from the far hinterland of the arc,
commonly results in rapid infilling of the forearc basins. This is often the case in basins associated with
highly elevated non-volcanic continental margins.
Along island arcs, the non-volcanic terrigenous sediment fraction tends to be low and biogenic sediments, such as shallow-marine carbonates can play
some role. In sediment-starved, warm equatorial regions, reefs and carbonate banks can grow along the
shallow basin margins and build up thick carbonate
sections (e.g. in the Mentawai trough).
If a basin is filled up rapidly during its early stage,
which is common in continental margin settings, a
large proportion of its sediments consists of shallowwater and nonmarine deposits. These accumulate
during the late, sediment load-driven phase of subsidence. In contrast, relatively slowly filling basins
during coeval subsidence maintain deep-water conditions for longer time periods and thus lead to the
buildup of a thicker deep-water sequence.
These general trends are superimposed, as in other
basin settings, by relative sea-level changes, climate
change, and certain pulses in the subduction process
and volcanic activity. Increased compression can
steepen the basin slope and the relief of the hinterland and thus promote slides and mass flows (e.g.
Postma et al. 1993). Further significant modifications
can be brought about by jumps or shifts in the position of the subduction zone or a change in the vector
of plate convergence through time. Such processes
complicate the interpretation of the structural evolution and facies successions of ancient forearc basins.
The evolution of forearc basins as depositional systems ends when subduction of oceanic crust is followed by obduction and collision of plates, and the
area is uplifted and exposed to erosion.
The post-depositional deformation of forearc basin
fills is less pervasive than that of the accreted sedi-
wedge. (2) Isostatic response to the emplacement of dense
oceanic crust beneath the forearc region by subduction. (3)
Rapid cooling of a relatively warm upper plate resulting
from the subducted cooler plate. This mechanism may apply to situations where young oceanic crust is incorporated
as upper plate into a forearc region. Furthermore, the subsidence histories of forearc basins may be influenced by the
angle of subduction and variations in the subduction rates.
In the course of time, a narrow residual basin can
evolve into a broader constructed or composite basin
that onlaps the arc massif on the inner flank and lies
on the accretionary wedge on the outer side (Fig.
12.25b,c). The basin configuration tends to pass from
a ridged to a shelved forearc basin (Fig. 12.25d).
Outer ridges result from the upbuilding of the
accretionary wedge (structural highs) or from tectonic or volcanic structures (observed along intraoceanic arc systems). The ridged forearcs creating a
ponded basin are most effective in trapping and accumulating sediments. Shelved forearc basins may
lose a major part of the arc-derived sediment to the
trench.
During the evolution from a ponded to a shelved
morphology, the depositional environment of the basin passes from open, deep-marine conditions into
the increasingly restricted environments of a wide
shallow-marine and coastal. Rapid deposition of
deep-marine sediments (mainly turbidites) is followed by slower accumulating shallow-marine strata
and finally often by continental deposits (Fig.
12.25e), particularly on the side ofthe arc massif.
A tendency for uplift of both the arc massif and
the outer ridge and subsidence of the basin center
favor the continuation of sedimentation for some
time. Additional deformation of the strata can be
caused by extension, compression, and wrench faulting of the basin fill. In the early stage of such deformation, normal or reverse faults are common on the
side of the arc massif, while folding and overthrusting predominate on the seaward side of the basin fill
(Fig. 12.25e). Strike-slip motions preferentially occur
at the transition from the accretionary prism to the
arc. If the forearc basin fill is exposed to collision, as
in ancient orogenic belts, it is further tectonically
overprinted.
Sediment Successions of Foreare Basins
(General Aspects)
An idealized residual foreare basin may display the
following succession (from top to bottom):
- Fluvial and deltaic sands, partially derived from
the uplifted roots of the arc. On the inner edge of the
basin, these sediments may interfinger with deposits
from pyroclastic flows, relatively coarse grained ash
fall, lava flows, and alluvial fans consisting of both
Chapter 12 Basin Evolution and Sediments
volcanic and nonvolcanic materials (cf. Sect. 2.4).
- Sands and shales deposited in shallow-marine environments (shelf sea); high proportion of volcaniclastic material but a relatively low amount of biogenic components, resulting from the high influx of
terrigenous material and rapid sediment buildup.
- Flysch-like shale-sandstone sequences with upwardly increasing proportions of coarser grained
turbidites, largely composed of volcaniclastic material. The sediments are distributed by seaward and
basin-parallel currents. Mass flow deposits are common, particularly on the landward side of the basin
(basin slope). The shales may contain some carbonate (when deposited above the CCD) and partially
have a relatively high organic carbon content.
-'- Abyssal-plain sediments containing montmorillonitic shales devoid of carbonate, fine-grained ash fall
deposits, and fine-grained turbidites.
A large supply of terrigenous material, including
clastic detritus from the far hinterland of the arc,
commonly results in rapid infilling of the forearc basins. This is often the case in basins associated with
highly elevated non-volcanic continental margins.
Along island arcs, the non-volcanic terrigenous sediment fraction tends to be low and biogenic sediments, such as shallow-marine carbonates can play
some role. In sediment-starved, warm equatorial regions, reefs and carbonate banks can grow along the
shallow basin margins and build up thick carbonate
sections (e.g. in the Mentawai trough).
If a basin is filled up rapidly during its early stage,
which is common in continental margin settings, a
large proportion of its sediments consists of shallowwater and nonmarine deposits. These accumulate
during the late, sediment load-driven phase of subsidence. In contrast, relatively slowly filling basins
during coeval subsidence maintain deep-water conditions for longer time periods and thus lead to the
buildup of a thicker deep-water sequence.
These general trends are superimposed, as in other
basin settings, by relative sea-level changes, climate
change, and certain pulses in the subduction process
and volcanic activity. Increased compression can
steepen the basin slope and the relief of the hinterland and thus promote slides and mass flows (e.g.
Postma et al. 1993). Further significant modifications
can be brought about by jumps or shifts in the position of the subduction zone or a change in the vector
of plate convergence through time. Such processes
complicate the interpretation of the structural evolution and facies successions of ancient forearc basins.
The evolution of forearc basins as depositional systems ends when subduction of oceanic crust is followed by obduction and collision of plates, and the
area is uplifted and exposed to erosion.
The post-depositional deformation of forearc basin
fills is less pervasive than that of the accreted sedi-