592
(Fig. 12.24e). With inereasing distanee from the
modem treneh, inereasingage and overburden pressure, the rocks ofthe thrust sheets deve10p sealy foliation, slaty cleavage, and are finally subjeet to metamorphism.
Summary (Deep-Sea Trenches and Sediment
Accretion)
- Deep-sea trenehes eolleet large volumes of
sediment from the side of the are, the ineoming subdueting oeeanie plate, and sometimes
from distal sourees.
- The proportion of sediment ineorporated into
the aeeretionary wedge is high when a treneh
is entirely filled, the eonvergenee rate is slow,
and the angle of subduetion is gentle.
The major part of these sediments is used for
frontal aeeretion and underplating below a
solid backstop.
- The sediment sueeessions of aeeretionary
wedges refleet, as far as they are still intaet,
the treneh fills (mainly turbidites and mass
flow deposits) and the ineoming pelagie sediments of the subdueting plate.
Dewatering and struetural deformation above
the deeollement zone inereases with the distanee from the wedge front.
Melange zones (ehaotie struetures) may result
from different processes (gravity-indueed
mass movements on the foreare slope,
subduetion-related tectonic deformation, and
liquefaetionlfluidization.
12.5.3 Forearc Basins
Basin Types and General Characteristics
Between a deep-sea treneh and its eorresponding
magmatie are several types of foreare basins ean be
distinguished (Fig. 12.25a-e):
- Aeeretionary basins (mostly slope basins). These
basins evolve at the top of the aeeretionary wedge
and their sediments lie uneonformably on older, defonned strata. A major part of these sediments eonsist of mass flow deposits and turbidites derived from
the uplifted subduetion eomplex or the are massif.
They refleet a eomparatively deep marine environment. The basin fills may be defonned mainly by
eompressional forees exerted during the eontinuing
growth of the underlying aeeretionary wedge.
- Residual basins. These basins aeeumulate sediments on oeeanie or transitional ernst trapped beChapter 12 Basin Evolution and Sediments
tween the are massif and the subduetion zone (Fig.
12.25a).
- Construeted basins (Fig. 12. 25b). This basin type
develops on both the are massif (inner side of the
basin) and defonned strata of the aeeretionary wedge
(outer side of the basin). Its sediments lie uneonformably on the basement of the are and aeereted
sediments of the subduetion eomplex.
- Composite basins (Fig. 12.25e).
- 1ntramassiJ or intra-are basins. The sediments of
these relatively small basins are derived from the are
or baekare region and lie uneonformably on rocks of
the island are or eontinental margin are. The
intramassif sediments of island ares tend to be predominantly marine, whereas those of eontinental
margin ares are eommonly nonmarine. Basins evolving during a later stage of an are-indueed orogenie
belt are ealled intramontane basins (cf. Figs. 1.2b and
1.3a) and deseribed further in Seet. 12.7.2.
The residual and eonstrueted or eomposite foreare
basins are most important and diseussed further in
the following seetion.
The basin classifieation used here was introdueed by
Diekinson and Seely (1979). Foreare basins in the modern
oceans show different stages of evolution (Fig. 12.25d).
Sloped foreare regions (e.g., along New Britain, the Solomon Islands, and the south New Hebrides) may develop
some small slope basins on top of the aeeretionary wedge,
but do not providc larger basins for sediment aecumulation.
Similarly, terraeed foreare regions, as known from the
northeastem Pacifie off Oregon and Washington, cannot
aeeumulate sediments of substantial thickness on top of the
subduction eomplex. Baek-tilted foreares (south Tonga,
Mariana Islands, northem New Hebrides) are eharaeterized
by a partially submerged volcanic are (Fig. 12.25d) and a
kind of intra-are basin direetly adjaeent to the are. Foreare
basins on continental crust. Several basins along the Paeifie eoast of South America evolved on eontinental or
transitional ernst. Off Pern, for example, tectonic erosion
along the subducting oeeanic plate prevented the formation
of a substantial aeeretionary prism. As a result, ernst with
continental affinity extends to the shelf edge or lower slope
of the margin (cf. Fig. 12.20b, Moberly et al. 1982; Shipboard Seientific Party 1988e, Leg 112). After an earlier
period of uplift, aseries of elongated, narrow, eoast-paralleI foreare basins evolved on top of this ernst, i.e., on the
shelf and upper slope. The sediments of these basins refleet
eoastal upweIIing sinee the Mioeene and therefore eontain
blaek shale intervals . and phosphorite horizons; furthermore, they display widespread diagenetie dolomitization.
The thieknesses of these basin fills are limited eompared
with those of the more common foreare basins diseussed
below.
Basin Evolution, Sediment Sources, and
Tectonic Overprint
Residual foreare basins on trapped oeeanie ernst, in
partieular, tend to be initially deep. They ean aeeu-
(Fig. 12.24e). With inereasing distanee from the
modem treneh, inereasingage and overburden pressure, the rocks ofthe thrust sheets deve10p sealy foliation, slaty cleavage, and are finally subjeet to metamorphism.
Summary (Deep-Sea Trenches and Sediment
Accretion)
- Deep-sea trenehes eolleet large volumes of
sediment from the side of the are, the ineoming subdueting oeeanie plate, and sometimes
from distal sourees.
- The proportion of sediment ineorporated into
the aeeretionary wedge is high when a treneh
is entirely filled, the eonvergenee rate is slow,
and the angle of subduetion is gentle.
The major part of these sediments is used for
frontal aeeretion and underplating below a
solid backstop.
- The sediment sueeessions of aeeretionary
wedges refleet, as far as they are still intaet,
the treneh fills (mainly turbidites and mass
flow deposits) and the ineoming pelagie sediments of the subdueting plate.
Dewatering and struetural deformation above
the deeollement zone inereases with the distanee from the wedge front.
Melange zones (ehaotie struetures) may result
from different processes (gravity-indueed
mass movements on the foreare slope,
subduetion-related tectonic deformation, and
liquefaetionlfluidization.
12.5.3 Forearc Basins
Basin Types and General Characteristics
Between a deep-sea treneh and its eorresponding
magmatie are several types of foreare basins ean be
distinguished (Fig. 12.25a-e):
- Aeeretionary basins (mostly slope basins). These
basins evolve at the top of the aeeretionary wedge
and their sediments lie uneonformably on older, defonned strata. A major part of these sediments eonsist of mass flow deposits and turbidites derived from
the uplifted subduetion eomplex or the are massif.
They refleet a eomparatively deep marine environment. The basin fills may be defonned mainly by
eompressional forees exerted during the eontinuing
growth of the underlying aeeretionary wedge.
- Residual basins. These basins aeeumulate sediments on oeeanie or transitional ernst trapped beChapter 12 Basin Evolution and Sediments
tween the are massif and the subduetion zone (Fig.
12.25a).
- Construeted basins (Fig. 12. 25b). This basin type
develops on both the are massif (inner side of the
basin) and defonned strata of the aeeretionary wedge
(outer side of the basin). Its sediments lie uneonformably on the basement of the are and aeereted
sediments of the subduetion eomplex.
- Composite basins (Fig. 12.25e).
- 1ntramassiJ or intra-are basins. The sediments of
these relatively small basins are derived from the are
or baekare region and lie uneonformably on rocks of
the island are or eontinental margin are. The
intramassif sediments of island ares tend to be predominantly marine, whereas those of eontinental
margin ares are eommonly nonmarine. Basins evolving during a later stage of an are-indueed orogenie
belt are ealled intramontane basins (cf. Figs. 1.2b and
1.3a) and deseribed further in Seet. 12.7.2.
The residual and eonstrueted or eomposite foreare
basins are most important and diseussed further in
the following seetion.
The basin classifieation used here was introdueed by
Diekinson and Seely (1979). Foreare basins in the modern
oceans show different stages of evolution (Fig. 12.25d).
Sloped foreare regions (e.g., along New Britain, the Solomon Islands, and the south New Hebrides) may develop
some small slope basins on top of the aeeretionary wedge,
but do not providc larger basins for sediment aecumulation.
Similarly, terraeed foreare regions, as known from the
northeastem Pacifie off Oregon and Washington, cannot
aeeumulate sediments of substantial thickness on top of the
subduction eomplex. Baek-tilted foreares (south Tonga,
Mariana Islands, northem New Hebrides) are eharaeterized
by a partially submerged volcanic are (Fig. 12.25d) and a
kind of intra-are basin direetly adjaeent to the are. Foreare
basins on continental crust. Several basins along the Paeifie eoast of South America evolved on eontinental or
transitional ernst. Off Pern, for example, tectonic erosion
along the subducting oeeanic plate prevented the formation
of a substantial aeeretionary prism. As a result, ernst with
continental affinity extends to the shelf edge or lower slope
of the margin (cf. Fig. 12.20b, Moberly et al. 1982; Shipboard Seientific Party 1988e, Leg 112). After an earlier
period of uplift, aseries of elongated, narrow, eoast-paralleI foreare basins evolved on top of this ernst, i.e., on the
shelf and upper slope. The sediments of these basins refleet
eoastal upweIIing sinee the Mioeene and therefore eontain
blaek shale intervals . and phosphorite horizons; furthermore, they display widespread diagenetie dolomitization.
The thieknesses of these basin fills are limited eompared
with those of the more common foreare basins diseussed
below.
Basin Evolution, Sediment Sources, and
Tectonic Overprint
Residual foreare basins on trapped oeeanie ernst, in
partieular, tend to be initially deep. They ean aeeu-