612
Airy-type loeal isostatie adjustment of the underlying
ernst is redueed by the flexural strength of the bending ernst (ef. Seet. 8.4.1). It deereases away from the
load and even ereates a zone of uplift, the peripheral
forebulge. One ean distinguish two types of endmembers deseribing these proeesses (e.g. Flemings
and Jordan 1989):
- If the overriding teetonie load inereases slowly
with time and the eontinental ernst beneath the thrnst
belt is thiek, subsidenee of the evolving foreland basin proeeeds at a low rate, the basin tends to beeome
overfilled, and the forebulge develops far away from
the load.
- In eontrast, rapidly overriding thiek teetonie loads
on thinned eontinental or transitional ernst generate
fast subsidenee in relatively narrow underfilled basins. The sediment supply from the thrust belt and its
hinterland eommonly lags behind the teetonie event
(e.g. Cant and StoekmalI993).
The deformation front of over-thrust belts rnay advanee at rates up to several tens of mmJa and eause
rapid sediment aeeumulation in the foredeep.
For the foreland of the Swiss Alps, Homewood et al.
(1986) inferred a thrust dip progagation of 7 mm1a and a
convergence rate decreasing from around 8 to 2 mm1a during basin evolution. The foreland basin of central Switzerland migrated at least 210 km northwestward over the European craton. Farther east, the thrust front advanced at
rates of 3 to 30 mm1a toward the northern Alpine foreland
basin in the early (Oligocene) stage (Zweigel 1998); later
the rates declined. The front of the western Alps in France
advanced 5-8 mm1a (Sinclair 1997b). In the sub-Himalayan
foredeep a rate of 20-30 mm1a was determined. In the
northern Apennines, the Neogene deformation front advanced at a rate of 2.5 to 10 mm1a; further south the
depocenters of Miocene foredeeps migrated outward at a
rate of up to 75 mm1a, and sediments accumulated as fast
as almost 1000 m1Ma (Ricci Lucchi 1986; Boccaletti et al.
1990). In the South Pyrenean foreland basin, adeformation
rate of35 mm1a is assumed (Mutti et al. 1985; Labaume et
al. 1985). Overthrusting of the Iberian ranges in the western part of the Ebro basin occurred at a mean rate of 0.7
mm1a (Munoz-Jimenez and Casas-Sainz 1997; see also
Marzo et al. 1998).
Fig. l2.34e demonstrates one eyele of flexuralloading, relaxation and rebound for a c1assieal fore land
basin whieh often ereates an erosional (± angular)
uneonformity within the basin fill. Subsidenee in
front of the advaneing load first inereases. After the
load has eome to rest, subsidenee eontinues and
slows with time (viseo-elastie relaxation), while the
peripheral forebulge migrates toward the load. This
stage rnay be followed by aperiod of more or less
uniform subsidenee in wide areas of the basin. Upon
unloading, as a result of erosion, the underlying ernst
responds by rebound (uplift) whieh in turn rnay generate some subsidenee in the region of the former
peripheral bulge (sag or "anti-peripheral bulge", e.g.
Chapter 12 Basin Evolution and Sediments
Ettensohn 1994). This type of sag basin also migrates
toward the zone of former loading with time. Sediments derived from the thrust belt are deposited in
the basin and thus further eause subsidenee to migrate outward toward the foreland.
Flexural loading and unloading may alternate several times with the result that their effeets beeome
superimposed on eaeh other. In later stages of basin
evolution the advanee of nappes of the overthrust
belt is inereasingly hampered by the thiekening,
flexured and eratonward rising top of the underlying
eontinental ernst (Fig. 12.31d). Some eases have
been deseribed in whieh upper portions of the
underplating ernst and its shelf sediments were
seratehed off and ineorporated into the overthrust
belt (not shown in Fig. 12.31). Early molasse may be
sheared off and folded in front of the overthrust belt,
or overridden by the nappes. In this stage, the
overthrust sheets are foreed to move obliquely upward and reaeh elevations high above sea level, before they finally eome to rest. This in turn promotes
subsidenee and additional flexure of the eontinental
plate and thus expansion of the basin toward the
foreland.
In addition, subsidenee of peripheral fore land basins ean be enhaneed by ongoing subduetion
(subduetion load) and, in the ease of retroare settings, by a subduetion-related "dynarnie slab load"
(Fig. 12.34a,b). Crnstal shortening and overthrnsting
of an orogenie belt mayaiso be assoeiated with the
episodie aeeretion of alloehthonous terranes along
the subduetion zone, as proposed, for example, for
the Paeifie margin of the Roeky mountains of North
Ameriea (e.g. Stoekmal et al. 1992). For all these
reasons, the subsidenee his tory of foreland basins is
eomplex and diffieult to prediet.
Taiwan foreland basin and Timor-Tanimbar trough.
Some deep basins in the modern oceans evidently result
from rapid emplacement of overthrust loads, such as the
1.5 km deep western Taiwan foreland basin (Covey 1986)
and the 2-3 km deep Timor-Tanimbar trough (AudleyCharles 1986; Harris et al. 1998). The latter basin is 10cated to the east ofthe Sunda-Java trench (cf. Fig. 12.23a)
where the Australian continental crust underplates the nonvolcanic island chain of the Banda arc (e.g. Charlton
1997). Collision during the middle Pliocene was followed
by crustal shortening in the foreland region which continues to the Present. It operates at rates of 6 to 12 cmla and
causes both uplift ofthe island arc (1 to 3 mm1a) and subsidence of the foreland basin (l mm1a along the present axis
of the basin). The arc and forearc region seem to be affected by extension and backthrusting. In an upper Quaternary sediment section drilled in the Timor trough, 400 m
consist of fine-grained siliciclastics and biogenic carbonates. The total sediment thicknesses ofthe water-filled foreland basin range from 4 to 7 km.
Western Solomon Sea. This closing ocean basin is another example of incipient arc collision with the Autralian
continent. The modern foredeep shows a relatively long
period of turbidite deposition followed by a short phase of
Airy-type loeal isostatie adjustment of the underlying
ernst is redueed by the flexural strength of the bending ernst (ef. Seet. 8.4.1). It deereases away from the
load and even ereates a zone of uplift, the peripheral
forebulge. One ean distinguish two types of endmembers deseribing these proeesses (e.g. Flemings
and Jordan 1989):
- If the overriding teetonie load inereases slowly
with time and the eontinental ernst beneath the thrnst
belt is thiek, subsidenee of the evolving foreland basin proeeeds at a low rate, the basin tends to beeome
overfilled, and the forebulge develops far away from
the load.
- In eontrast, rapidly overriding thiek teetonie loads
on thinned eontinental or transitional ernst generate
fast subsidenee in relatively narrow underfilled basins. The sediment supply from the thrust belt and its
hinterland eommonly lags behind the teetonie event
(e.g. Cant and StoekmalI993).
The deformation front of over-thrust belts rnay advanee at rates up to several tens of mmJa and eause
rapid sediment aeeumulation in the foredeep.
For the foreland of the Swiss Alps, Homewood et al.
(1986) inferred a thrust dip progagation of 7 mm1a and a
convergence rate decreasing from around 8 to 2 mm1a during basin evolution. The foreland basin of central Switzerland migrated at least 210 km northwestward over the European craton. Farther east, the thrust front advanced at
rates of 3 to 30 mm1a toward the northern Alpine foreland
basin in the early (Oligocene) stage (Zweigel 1998); later
the rates declined. The front of the western Alps in France
advanced 5-8 mm1a (Sinclair 1997b). In the sub-Himalayan
foredeep a rate of 20-30 mm1a was determined. In the
northern Apennines, the Neogene deformation front advanced at a rate of 2.5 to 10 mm1a; further south the
depocenters of Miocene foredeeps migrated outward at a
rate of up to 75 mm1a, and sediments accumulated as fast
as almost 1000 m1Ma (Ricci Lucchi 1986; Boccaletti et al.
1990). In the South Pyrenean foreland basin, adeformation
rate of35 mm1a is assumed (Mutti et al. 1985; Labaume et
al. 1985). Overthrusting of the Iberian ranges in the western part of the Ebro basin occurred at a mean rate of 0.7
mm1a (Munoz-Jimenez and Casas-Sainz 1997; see also
Marzo et al. 1998).
Fig. l2.34e demonstrates one eyele of flexuralloading, relaxation and rebound for a c1assieal fore land
basin whieh often ereates an erosional (± angular)
uneonformity within the basin fill. Subsidenee in
front of the advaneing load first inereases. After the
load has eome to rest, subsidenee eontinues and
slows with time (viseo-elastie relaxation), while the
peripheral forebulge migrates toward the load. This
stage rnay be followed by aperiod of more or less
uniform subsidenee in wide areas of the basin. Upon
unloading, as a result of erosion, the underlying ernst
responds by rebound (uplift) whieh in turn rnay generate some subsidenee in the region of the former
peripheral bulge (sag or "anti-peripheral bulge", e.g.
Chapter 12 Basin Evolution and Sediments
Ettensohn 1994). This type of sag basin also migrates
toward the zone of former loading with time. Sediments derived from the thrust belt are deposited in
the basin and thus further eause subsidenee to migrate outward toward the foreland.
Flexural loading and unloading may alternate several times with the result that their effeets beeome
superimposed on eaeh other. In later stages of basin
evolution the advanee of nappes of the overthrust
belt is inereasingly hampered by the thiekening,
flexured and eratonward rising top of the underlying
eontinental ernst (Fig. 12.31d). Some eases have
been deseribed in whieh upper portions of the
underplating ernst and its shelf sediments were
seratehed off and ineorporated into the overthrust
belt (not shown in Fig. 12.31). Early molasse may be
sheared off and folded in front of the overthrust belt,
or overridden by the nappes. In this stage, the
overthrust sheets are foreed to move obliquely upward and reaeh elevations high above sea level, before they finally eome to rest. This in turn promotes
subsidenee and additional flexure of the eontinental
plate and thus expansion of the basin toward the
foreland.
In addition, subsidenee of peripheral fore land basins ean be enhaneed by ongoing subduetion
(subduetion load) and, in the ease of retroare settings, by a subduetion-related "dynarnie slab load"
(Fig. 12.34a,b). Crnstal shortening and overthrnsting
of an orogenie belt mayaiso be assoeiated with the
episodie aeeretion of alloehthonous terranes along
the subduetion zone, as proposed, for example, for
the Paeifie margin of the Roeky mountains of North
Ameriea (e.g. Stoekmal et al. 1992). For all these
reasons, the subsidenee his tory of foreland basins is
eomplex and diffieult to prediet.
Taiwan foreland basin and Timor-Tanimbar trough.
Some deep basins in the modern oceans evidently result
from rapid emplacement of overthrust loads, such as the
1.5 km deep western Taiwan foreland basin (Covey 1986)
and the 2-3 km deep Timor-Tanimbar trough (AudleyCharles 1986; Harris et al. 1998). The latter basin is 10cated to the east ofthe Sunda-Java trench (cf. Fig. 12.23a)
where the Australian continental crust underplates the nonvolcanic island chain of the Banda arc (e.g. Charlton
1997). Collision during the middle Pliocene was followed
by crustal shortening in the foreland region which continues to the Present. It operates at rates of 6 to 12 cmla and
causes both uplift ofthe island arc (1 to 3 mm1a) and subsidence of the foreland basin (l mm1a along the present axis
of the basin). The arc and forearc region seem to be affected by extension and backthrusting. In an upper Quaternary sediment section drilled in the Timor trough, 400 m
consist of fine-grained siliciclastics and biogenic carbonates. The total sediment thicknesses ofthe water-filled foreland basin range from 4 to 7 km.
Western Solomon Sea. This closing ocean basin is another example of incipient arc collision with the Autralian
continent. The modern foredeep shows a relatively long
period of turbidite deposition followed by a short phase of