12.5 Ancient Subduction-Related Basins
clude slumps and mass flows, which contain material
from shallower or terrestrial environments. Both the
forearc and backarc or retroarc basins mostly contain
abundant volcaniclastics deposited from mass flows,
turbidity currents, or air-fall. The typical shallowingupward trend of the forearc basins mayaiso be present in oceanic backarc basins. The presence of oceanic basalts associated with deep-marine sediments
in a subduction complex or melange is taken as evidence for accreted sea floor, but mayaiso indicate
the former existence of a residual forearc basin or an
oceanic backarc basin. Furthermore, paleo-current
directions and textural trends of subaerially and
subaquatically deposited beds (e.g., fluvial transport,
sediment gravity flows, turbidity currents) can be
used to distinguish between forearc and backarc
depositional systems. The occurrence of specific detrital minerals in sandstones (e.g., clinopyroxene and
amphibole; Morris 1987) indicate nearby active arc
volcanism.
605
The composition of volcaniclastic material derived either
from continental margin arcs (dominated by intermediate
and silicic, largely calc-alkaline magma types) or oceanic
arcs (mainly basaltic and andesitic material) is frequently
quoted as a means of discriminating these tectonic settings
(e.g., Crook 1974; Dickinson 1982; Maynard et al. 1982;
Valloni and Mezzardi 1984; Dürr 1996; cf. Sect. 2.4.3).
Large proportions of feldspar and other minerals characteristic of granitic and granodiorjtic rocks may testify to the
fact that the arc massifwas already deeply eroded. Interpretations of ~ncient sedimentary successions based solelyon
composition should, however, be regarded with caution
(e.g., Moore GF et al. 1982; Lash 1985; Velbe11985; Zuffa
1987). Several modern examples of trench fi11s, forearc
basins and backarc basins have shown that distant sediment
sources and sediment recycling can play an important role.
clude slumps and mass flows, which contain material
from shallower or terrestrial environments. Both the
forearc and backarc or retroarc basins mostly contain
abundant volcaniclastics deposited from mass flows,
turbidity currents, or air-fall. The typical shallowingupward trend of the forearc basins mayaiso be present in oceanic backarc basins. The presence of oceanic basalts associated with deep-marine sediments
in a subduction complex or melange is taken as evidence for accreted sea floor, but mayaiso indicate
the former existence of a residual forearc basin or an
oceanic backarc basin. Furthermore, paleo-current
directions and textural trends of subaerially and
subaquatically deposited beds (e.g., fluvial transport,
sediment gravity flows, turbidity currents) can be
used to distinguish between forearc and backarc
depositional systems. The occurrence of specific detrital minerals in sandstones (e.g., clinopyroxene and
amphibole; Morris 1987) indicate nearby active arc
volcanism.
605
The composition of volcaniclastic material derived either
from continental margin arcs (dominated by intermediate
and silicic, largely calc-alkaline magma types) or oceanic
arcs (mainly basaltic and andesitic material) is frequently
quoted as a means of discriminating these tectonic settings
(e.g., Crook 1974; Dickinson 1982; Maynard et al. 1982;
Valloni and Mezzardi 1984; Dürr 1996; cf. Sect. 2.4.3).
Large proportions of feldspar and other minerals characteristic of granitic and granodiorjtic rocks may testify to the
fact that the arc massifwas already deeply eroded. Interpretations of ~ncient sedimentary successions based solelyon
composition should, however, be regarded with caution
(e.g., Moore GF et al. 1982; Lash 1985; Velbe11985; Zuffa
1987). Several modern examples of trench fi11s, forearc
basins and backarc basins have shown that distant sediment
sources and sediment recycling can play an important role.