598
Tbe Great Valley Foreare Basin, California. All three
major components of the arc-trench system, i.e., the (Franciscan) subduction complex, the (Great Valley) forearc
basin, and the (Sierra Nevada) magmatic arc are exposed
on land in their original relationship (Fig. l2.26a; Ingersoll
1982). This relationship is preserved, because further compression ofthe arc-trench system was terminated by strikeslip movement along the San Andreas Fault, converting the
convergent margin into a trans form margin.
Tbe forearc basin developed on trapped oceanic crnst in
the Great Valley area, whereas to the north and south such
basins are missing. Tbe forearc region evolved from a terraced or ponded, narrow residual basin with a deeply submerged ridge into a wider, shelved composite forearc basin
behind a high, broad outer ridge.
Sedimentation began in the upper Jurassic and lower
Cretaceous, with muddy slope deposits and conglomeratic
channel fills. Sand and coarser grained components are
derived from the underlying ophiolites, metamorphic and
plutonic rocks of tbe former collision (suture) zone, and
volcanic material from the magrnatic arc. A great part of
the suture-derived material came from tbe Klamath Mountains in the north (Fig. l2.26a). The proportion of
volcanics decreases upsection, while the quartz content of
the sandstones increases (Fig. l2.26c). Tbis reflects the
erosion of the magrnatic arc and the former suture zone.
Later, arkosic sands and sandstones point to the fact that
the batholithic roots of tbe magmatic arc were exposed to
erosion.
Concurrent with the westward growth and upbuilding of
the Franciscan subduction complex, the forearc basin was
enlarged and filled. Sands were dispersed by submarine
slope channels and deposited in large fan systems. However, the Sierran magmatic arc migrated eastward (Fig.
l2.26d) and lost its dominant role as a sediment source.
Tbe oceanward parts of the basin first underwent strong
subsidence and then uplift as a result of the growing
subduction complex (Moxon and Graham 1987). At the
same time, tbe landward parts of the basin subsided due to
the thermal contraction and eastward migration of the
Sierran magmatic arc.
Summary (Foreare Basins)
Forearc basins develop between deep-sea trenches and magmatic arcs, either on trapped oceanic crust and/or on the inner part of an accretionary wedge. Their fills may onlap or, in some
cases, rest on the subsided basement of the arc.
Many basins evolve from a ridged, deep phase
to a shallow-marine shelf-type and finally to a
continental stage.
The early stage of basin filling is characterized
by thick deposits of relatively small, sand- and
gravel-rich deep-sea fans on top of pelagic or
hemipelagic sediments. The channel-fan systems
are fed by several point sources from the side of
the arc.
In intraoceanic settings, the clastic sediments are
derived mainly from the volcanic arc; along conCbapter 12 Basin Evolution and Sediments
Tbe total sediment tbickness in the western part of the
basin amounts to about 13 km; in the eastern part it is
much less. The sedimentation rate during tbe upper Cretaceous was about 280 m/Ma in the basin center. By the end
of the Cretaceous, the forearc basin had attained the morphology of a wide shelf, allowing arc-derived clastic material to migrate westward and to accumulate in the trench.
Paleogene sediments were deposited predominantly under
shallow-marine and nonmarine conditions; the Neogene is
represented by primarily nonmarine strata, apart from some
smalI, deep basins caused by Neogene wrench tectonics
associated with tbe development of the San Andreas fault
system. These movements finally terrninated the evolution
of tbe Great Valley basin and converted the subducting
margin into a transform margin (Fig. l2.26b).
Tbe Xigaze foreare basin, Tibet. This residual basin is
associated with the closure of the Tethys ocean between the
Indian plate and the Lhasa block, a microplate of Eurasia.
The folded flysch-type forearc deposits are well exposed
over a distance of 600 km along the Indus-Yarling suture
zone (Einseie et al. 1994; Dürr 1996). Parts of tbe trapped
oceanic plate are exhumed in an ophiolite belt along the
southeni. boundary of the basin. Figure 12.27 shows the
basin evolution in three phases, characterized by finingupward megasequences with some dark and marly shales at
their tops which may result from 3rd order sea-level
changes. Several deep-sea channel systems could be identified supplying the basin with both volcaniclastic material
from the arc (-65% of the basin fill) and detritus of older
rocks from the hinterland (-35%). The arc provided material for a mean sedimentation rate of -1 00 m/Ma in the basin. Relics of a carbonate buildup and carbonate-bearing
coarse-grained channel fills indicate the former existence
of a fringing reef along the arc side. Shallow-marine
forearc sediments are preserved only locally, but alluvial
fan deposits derived from the roots of the arc are still present on the inner side ofthe basin.
tinental margins, the hinterland of the arc may
act as an additional significant sediment source.
Later stage sediments tend to be shallow-marine
and largely controlled by oceanic processes
(e.g. coastal upwelling leading to black shales
and phosphorites). In sediment~starved settings,
carbonate buildups and pelagic sediments play
some role.
Sediment buildup can be affected by syn-depositional subsidence of the basin center, upward
growth of the outer ridge, and uplift of the arc.
The depocenters first migrate seaward, but may
later return to more landward sites.
Tbe Great Valley Foreare Basin, California. All three
major components of the arc-trench system, i.e., the (Franciscan) subduction complex, the (Great Valley) forearc
basin, and the (Sierra Nevada) magmatic arc are exposed
on land in their original relationship (Fig. l2.26a; Ingersoll
1982). This relationship is preserved, because further compression ofthe arc-trench system was terminated by strikeslip movement along the San Andreas Fault, converting the
convergent margin into a trans form margin.
Tbe forearc basin developed on trapped oceanic crnst in
the Great Valley area, whereas to the north and south such
basins are missing. Tbe forearc region evolved from a terraced or ponded, narrow residual basin with a deeply submerged ridge into a wider, shelved composite forearc basin
behind a high, broad outer ridge.
Sedimentation began in the upper Jurassic and lower
Cretaceous, with muddy slope deposits and conglomeratic
channel fills. Sand and coarser grained components are
derived from the underlying ophiolites, metamorphic and
plutonic rocks of tbe former collision (suture) zone, and
volcanic material from the magrnatic arc. A great part of
the suture-derived material came from tbe Klamath Mountains in the north (Fig. l2.26a). The proportion of
volcanics decreases upsection, while the quartz content of
the sandstones increases (Fig. l2.26c). Tbis reflects the
erosion of the magrnatic arc and the former suture zone.
Later, arkosic sands and sandstones point to the fact that
the batholithic roots of tbe magmatic arc were exposed to
erosion.
Concurrent with the westward growth and upbuilding of
the Franciscan subduction complex, the forearc basin was
enlarged and filled. Sands were dispersed by submarine
slope channels and deposited in large fan systems. However, the Sierran magmatic arc migrated eastward (Fig.
l2.26d) and lost its dominant role as a sediment source.
Tbe oceanward parts of the basin first underwent strong
subsidence and then uplift as a result of the growing
subduction complex (Moxon and Graham 1987). At the
same time, tbe landward parts of the basin subsided due to
the thermal contraction and eastward migration of the
Sierran magmatic arc.
Summary (Foreare Basins)
Forearc basins develop between deep-sea trenches and magmatic arcs, either on trapped oceanic crust and/or on the inner part of an accretionary wedge. Their fills may onlap or, in some
cases, rest on the subsided basement of the arc.
Many basins evolve from a ridged, deep phase
to a shallow-marine shelf-type and finally to a
continental stage.
The early stage of basin filling is characterized
by thick deposits of relatively small, sand- and
gravel-rich deep-sea fans on top of pelagic or
hemipelagic sediments. The channel-fan systems
are fed by several point sources from the side of
the arc.
In intraoceanic settings, the clastic sediments are
derived mainly from the volcanic arc; along conCbapter 12 Basin Evolution and Sediments
Tbe total sediment tbickness in the western part of the
basin amounts to about 13 km; in the eastern part it is
much less. The sedimentation rate during tbe upper Cretaceous was about 280 m/Ma in the basin center. By the end
of the Cretaceous, the forearc basin had attained the morphology of a wide shelf, allowing arc-derived clastic material to migrate westward and to accumulate in the trench.
Paleogene sediments were deposited predominantly under
shallow-marine and nonmarine conditions; the Neogene is
represented by primarily nonmarine strata, apart from some
smalI, deep basins caused by Neogene wrench tectonics
associated with tbe development of the San Andreas fault
system. These movements finally terrninated the evolution
of tbe Great Valley basin and converted the subducting
margin into a transform margin (Fig. l2.26b).
Tbe Xigaze foreare basin, Tibet. This residual basin is
associated with the closure of the Tethys ocean between the
Indian plate and the Lhasa block, a microplate of Eurasia.
The folded flysch-type forearc deposits are well exposed
over a distance of 600 km along the Indus-Yarling suture
zone (Einseie et al. 1994; Dürr 1996). Parts of tbe trapped
oceanic plate are exhumed in an ophiolite belt along the
southeni. boundary of the basin. Figure 12.27 shows the
basin evolution in three phases, characterized by finingupward megasequences with some dark and marly shales at
their tops which may result from 3rd order sea-level
changes. Several deep-sea channel systems could be identified supplying the basin with both volcaniclastic material
from the arc (-65% of the basin fill) and detritus of older
rocks from the hinterland (-35%). The arc provided material for a mean sedimentation rate of -1 00 m/Ma in the basin. Relics of a carbonate buildup and carbonate-bearing
coarse-grained channel fills indicate the former existence
of a fringing reef along the arc side. Shallow-marine
forearc sediments are preserved only locally, but alluvial
fan deposits derived from the roots of the arc are still present on the inner side ofthe basin.
tinental margins, the hinterland of the arc may
act as an additional significant sediment source.
Later stage sediments tend to be shallow-marine
and largely controlled by oceanic processes
(e.g. coastal upwelling leading to black shales
and phosphorites). In sediment~starved settings,
carbonate buildups and pelagic sediments play
some role.
Sediment buildup can be affected by syn-depositional subsidence of the basin center, upward
growth of the outer ridge, and uplift of the arc.
The depocenters first migrate seaward, but may
later return to more landward sites.