12.5 Backarc and Retroarc Basins
12.5.4 Backare and Retroarc Basins
General Characteristics
Baekare basins develop either behind island ares on
oeeanie erust or, as retroare basins, on normal and
thinned eontinental erust. Retroare basins often
evolve into retro-foreland basins (cf. Fig. 1.2b).
In eontinental margin settings, the early evolution
of a retroare region is eommonly eharaeterized by
subduetion-related extension and rifting, whieh may
be followed by aeeretion of new oeeanie erust (cf.
Fig. 12.18e). Extension behind the magmatie are results from the fact that the rate of subduetion of the
downgoing plate is faster than the subduetion-driven
forward motion of the overriding plate. To eompensate this defieit the backare region of the overriding
plate is stretehed. Later, the arcward side of the
baekare basin may be folded and overthrust (cf. Fig.
1.2b), similar to the tectonic evolution of foreland
basins (Seet. 12.6.2). Therefore, these basins are often addressed as foredeeps or foreland basins, although they have developed behind an aetive magmatie are.
In intra-oeeanie are-treneh systems, the backare
region is an oeeanie basin of limited size whieh aeeumulates pelagie and hemipelagie sediments intermixed with volcanielastie material. Most of the modern oeeanie baekare basins and marginal basins lie in
the northwestern, western, and southwestern Paeifie
(Fig. 12.28) whieh may be addressed as a "supersubduction zone" active since the breakup of Gondwana (~180 Ma). However, the marginal basins present in the modern Paeifie are younger than 80 Ma;
the older basins are gone, probably eaused by
subduetion and elosure.
Marginal oeeanie basins ean be subdivided into
subduction-related marginal basins and basins not
related to subduction. Furthermore, the subductionrelated basins form two groups according to their
origin (Tomaki and Honza 1991; see also Leitch
1984; Maedonald 1993):
- Backarc basins evolve on the backare side of an
aetive are by sea-floor spreading following rifting of
the arc.
- Subduetion-related marginal basins form by entrapment of part of a major ocean basin due to the establishment of a new subduction zone in the oeean basin.
Oceanic backarc basins in a strict sense inc1ude the presently active Mariana trough, the North Fiji and Lau basins
in the western Pacific, and the Sea of Japan (see below).
An example in the Mediterranean Sea is the Tyrrhenian Sea
(cf. Sect. 12.2.3; Fig. 12.l5a,b). The Aegean Sea on continental crost is essentially a retroarc basin which has a relatively complex structural setting (cf. Sect. 12.7.4).
599
Non-subduction-related marginal basins in the Pacific
inc1ude the Caroline Sea and probably the South China,
Sulu, Celebes, Coral Sea, and New Caledonian basins.
Most ofthese basins are not associated with active backarc
spreading, but they may represent remnants of older
backarc basins which have been trapped by the retreat of a
former active arc-trench system.
Marginal basins, ineluding true backarc basins, commonly show the following characteristics:
- The basins preferentially develop on the eastern
side of major continents.
- Oceanic backare basins tend to elose by developing
an own subduction zone.
- The lifetimes of these basins is limited « 25 Ma)
compared with those of major ocean basins.
Sediment Successions of Backare and
Retroarc Basins
The sediments of backare regions vary markedly as a
result of their differing tectonic settings, changes in
sediment supply and depositional environments.
There are no generally applicable, simple facies models, even if retroarc basins on continental crust and
oeeanic backare basins are considered separately.
Ancient backarc basins are normally identified with
the aid of their overall setting in a plate tectonic context. In addition, the following observations can be
useful in recognizing such basins:
- Backarc and retroarc basins are generally asymmetrie, with a steep, often thrust-faulted outer margin
against the volcanic arc and a gentler dipping, possibly block-faulted inner margin.
- The sediments of backarc regions inelude elastics
derived from both margins, i.e., -from the volcanic arc
and drainage areas of the craton, but lateral sediment
influx from distal sources mayaiso playa role, as in
the previously mentioned example of Burma and the
Andaman Sea.
Retroare (Retro-Foreland) Basins on
Continental Crust
The basin fills tend to evolve from an initial phase of
coarse, elastic deposition into shallow-marine environments, and then to thick, molasse-type sediments
derived largely from the volcanic arc and its basement. The sub-Andean zone in South America on the
eastern flanks of the Andes probably displays the
most prominent examples of this type of retroarc
(retro-foreland) basins.
12.5.4 Backare and Retroarc Basins
General Characteristics
Baekare basins develop either behind island ares on
oeeanie erust or, as retroare basins, on normal and
thinned eontinental erust. Retroare basins often
evolve into retro-foreland basins (cf. Fig. 1.2b).
In eontinental margin settings, the early evolution
of a retroare region is eommonly eharaeterized by
subduetion-related extension and rifting, whieh may
be followed by aeeretion of new oeeanie erust (cf.
Fig. 12.18e). Extension behind the magmatie are results from the fact that the rate of subduetion of the
downgoing plate is faster than the subduetion-driven
forward motion of the overriding plate. To eompensate this defieit the backare region of the overriding
plate is stretehed. Later, the arcward side of the
baekare basin may be folded and overthrust (cf. Fig.
1.2b), similar to the tectonic evolution of foreland
basins (Seet. 12.6.2). Therefore, these basins are often addressed as foredeeps or foreland basins, although they have developed behind an aetive magmatie are.
In intra-oeeanie are-treneh systems, the backare
region is an oeeanie basin of limited size whieh aeeumulates pelagie and hemipelagie sediments intermixed with volcanielastie material. Most of the modern oeeanie baekare basins and marginal basins lie in
the northwestern, western, and southwestern Paeifie
(Fig. 12.28) whieh may be addressed as a "supersubduction zone" active since the breakup of Gondwana (~180 Ma). However, the marginal basins present in the modern Paeifie are younger than 80 Ma;
the older basins are gone, probably eaused by
subduetion and elosure.
Marginal oeeanie basins ean be subdivided into
subduction-related marginal basins and basins not
related to subduction. Furthermore, the subductionrelated basins form two groups according to their
origin (Tomaki and Honza 1991; see also Leitch
1984; Maedonald 1993):
- Backarc basins evolve on the backare side of an
aetive are by sea-floor spreading following rifting of
the arc.
- Subduetion-related marginal basins form by entrapment of part of a major ocean basin due to the establishment of a new subduction zone in the oeean basin.
Oceanic backarc basins in a strict sense inc1ude the presently active Mariana trough, the North Fiji and Lau basins
in the western Pacific, and the Sea of Japan (see below).
An example in the Mediterranean Sea is the Tyrrhenian Sea
(cf. Sect. 12.2.3; Fig. 12.l5a,b). The Aegean Sea on continental crost is essentially a retroarc basin which has a relatively complex structural setting (cf. Sect. 12.7.4).
599
Non-subduction-related marginal basins in the Pacific
inc1ude the Caroline Sea and probably the South China,
Sulu, Celebes, Coral Sea, and New Caledonian basins.
Most ofthese basins are not associated with active backarc
spreading, but they may represent remnants of older
backarc basins which have been trapped by the retreat of a
former active arc-trench system.
Marginal basins, ineluding true backarc basins, commonly show the following characteristics:
- The basins preferentially develop on the eastern
side of major continents.
- Oceanic backare basins tend to elose by developing
an own subduction zone.
- The lifetimes of these basins is limited « 25 Ma)
compared with those of major ocean basins.
Sediment Successions of Backare and
Retroarc Basins
The sediments of backare regions vary markedly as a
result of their differing tectonic settings, changes in
sediment supply and depositional environments.
There are no generally applicable, simple facies models, even if retroarc basins on continental crust and
oeeanic backare basins are considered separately.
Ancient backarc basins are normally identified with
the aid of their overall setting in a plate tectonic context. In addition, the following observations can be
useful in recognizing such basins:
- Backarc and retroarc basins are generally asymmetrie, with a steep, often thrust-faulted outer margin
against the volcanic arc and a gentler dipping, possibly block-faulted inner margin.
- The sediments of backarc regions inelude elastics
derived from both margins, i.e., -from the volcanic arc
and drainage areas of the craton, but lateral sediment
influx from distal sources mayaiso playa role, as in
the previously mentioned example of Burma and the
Andaman Sea.
Retroare (Retro-Foreland) Basins on
Continental Crust
The basin fills tend to evolve from an initial phase of
coarse, elastic deposition into shallow-marine environments, and then to thick, molasse-type sediments
derived largely from the volcanic arc and its basement. The sub-Andean zone in South America on the
eastern flanks of the Andes probably displays the
most prominent examples of this type of retroarc
(retro-foreland) basins.