602
- Stage 1. Rifting of an island arc along a zone of
crustal wealmess and subsequent backarc spreading
create a narrow trough between the active arc and
inactive remnant arc. The basal fill in the trough is
dominated by gravity mass flows, i.e., slides, slumps,
debris flows, and turbidity currents, which transport
basaltic pyroclastic and epiclastic material (eroded
older pyroclastic deposits) into the basin. Around the
backarc spreading center, special hydrothermal deposits may accumulate. On the side of the remnant
arc, epiclastic and some pelagic sediments are common. The basin attains only moderate depth, and sedimentation rates are generally high during this stage
(at least 100 to 200 m1Ma).
- Stage 2. Continued backarc spreading causes basin
widening and subsidence of previously formed oceanic crust as a result of cooling. Episodic volcanic
activity in the arc region further provides large quantities of basaltic volcaniclastic material, which is deposited in aprons at the base of the arc or dispersed
as ash falls over wider areas. Deposition of epiclastic
material on the side of the remnant arc is replaced by
deposition of pelagic sediments, which also play
some role on the active arc side. The basin floor is
commonly still above the CCD (Sect. 5.3.2) and thus
allows preservation of calcareous rnicrofossils. Dissolution of siliceous fossils may be hampered by relatively high Si0 2 contents in sea water due to the
presence of volcanic ash and glass. In the more distal
parts of the basin, the sedimentation rate drops to
around 50 mJMa, but may maintain high values
arcward in the volcaniclastic apron. Increased arc
volcanism leads to propagation of the areas previously dorninated by pelagic sedimentation. If volcanism wanes, pelagic, ash-poor sedimentation will
prevail.
- Stage 3. Ocean spreading in the existing basin
ceases, and splitting of the volcanic arc initiates the
formation of a new backarc basin. The old basin still
receives some epiclastic material and subaerially dispersed ash from the active arc, but is dorninated by
pelagic sedimentation. Continued subsidence may
lower the basin floor below the CCD and thus cause
carbonate dissolution, resulting in the deposition of
brown clays. The sedimentation rate drops to values
characteristic of normal pelagic deposits in the
oceans (5-15 m1Ma for nannofossil ooze and around
5 m1Ma for brown abyssal clays). The subsequent
sediment succession will be controlled mainly by
factors unrelated to arc volcanism and backarc
spreading.
Sediment successions of small ocean basins. Basins resulting from the rifting of continental crust or
predating the formation of a volcanic island arc frequently begin with normal clastic sediments containing quartz, feldspar and mica. They are followed by
pelagic and hemipelagic sediments and display a
Chapter 12 Basin Evolution and Sediments
shallowing-upward trend, with increasing influence
from volcaniclastic material if a volcanic source is
nearby. In addition, they show marked differences
between their outer and inner margins.
Examples of Modern Backare Basins
Southwestern Pacific. Tertiary sediment sueeessions
drilled in backare basins (Fig. 12.30a) largely follow the
rules outlined above. Young, narrow basins behind an aetive are or older basins adjaeent to an island are display
sueeessions rieh in various voleanielasties and ealcareous
nannofossil ooze. The sedimentation rates of the
volcanielastie sequenees mostly range between 50 and 100
m/Ma. However, the sediments of oldermargin basins are
dominated by slowly deposited nannofossil oozes and silty
elays, with minor proportions ofvoleanielastie material.
These results were gained during the Deep Sea Drilling
Projeet (Leiteh 1984; Klein 1985b) west ofthe Tonga are
(Site 203) in the New Hebrides basin (Site 286) west ofthe
Vanuatu islands, and in the South Fiji basin (Site 205). The
older margin basins are generated by rifting of eontinental
erust (Sites 210 and 287 in the Coral Sea basin, and Site
206 in the New Caledonia basin).
The upper portion of Site 210 in the New Caledonia basin eontains about 2000 muddy turbidite beds altemating
with ealcareous ooze, deposited during a time span of 10
Ma (sedimentation rate around 50 m/Ma). This inerease in
sediment supply ean be explained by a rejuvenated sediment souree in the remnant are region. The narrow New
Caledonia basin is filled with a 3000 m thiek sueeession of
submarine slumps derived largely off the Lord Howe Rise.
The Tasman Sea basin adjaeent to the Australian eontinent
is eharaeterized by a sequenee rieh in silieeous ooze and
ehert (not shown in Fig. 12.30a). Early to mid-Oligoeene
uneonformities observed in several basins are assoeiated
with aceelerated bottom eurrents.
Information about speeifie sedimentary basins in the
South Paeifie, ineluding New Zealand, is found in Ballanee
(1993). Foreare and baekare basins older than Mioeene are
mostly diffieult to reeognize. The basins of New Zealand,
in partieular, are strongly affeeted by lateral displacement.
Sea of Japan. This basin is weIl studied and therefore
an example of a relatively eomplex basin history. The basin
is a relatively old (-25-15 Ma) and presently inaetive
backare basin whieh is beginning to elose by the development of a subduetion zone on the western side ofthe Japanese islands (cf. Figs. 12.24a,b and 12.28). The basin is
more than 3500 m deep and separated'from the open sea by
a shallow sill. The basin floor is only partially made up of
young oeeanie erust whieh formed along an E-W direeted
spreading axis (perpendieular to the strike of the presentday subduetion zone to the east of the Japanese islands).
The remainder of the basin floor is normal, rifted, or
thinned eontinental erust (e.g. Ingle 1997, Tamaki et al.
1997).
During the early rifting and extensional phase of basin
evolution, the rate of subsidenee was limited and non-marine sediment and voleanie material aeeumulated. With the
onset of backare spreading, aecompanied by basaltie
magmatism, subsidenee aeeelerated (100-500 m/Ma) and
the basin beeame marine with inereasing water depth. A
- Stage 1. Rifting of an island arc along a zone of
crustal wealmess and subsequent backarc spreading
create a narrow trough between the active arc and
inactive remnant arc. The basal fill in the trough is
dominated by gravity mass flows, i.e., slides, slumps,
debris flows, and turbidity currents, which transport
basaltic pyroclastic and epiclastic material (eroded
older pyroclastic deposits) into the basin. Around the
backarc spreading center, special hydrothermal deposits may accumulate. On the side of the remnant
arc, epiclastic and some pelagic sediments are common. The basin attains only moderate depth, and sedimentation rates are generally high during this stage
(at least 100 to 200 m1Ma).
- Stage 2. Continued backarc spreading causes basin
widening and subsidence of previously formed oceanic crust as a result of cooling. Episodic volcanic
activity in the arc region further provides large quantities of basaltic volcaniclastic material, which is deposited in aprons at the base of the arc or dispersed
as ash falls over wider areas. Deposition of epiclastic
material on the side of the remnant arc is replaced by
deposition of pelagic sediments, which also play
some role on the active arc side. The basin floor is
commonly still above the CCD (Sect. 5.3.2) and thus
allows preservation of calcareous rnicrofossils. Dissolution of siliceous fossils may be hampered by relatively high Si0 2 contents in sea water due to the
presence of volcanic ash and glass. In the more distal
parts of the basin, the sedimentation rate drops to
around 50 mJMa, but may maintain high values
arcward in the volcaniclastic apron. Increased arc
volcanism leads to propagation of the areas previously dorninated by pelagic sedimentation. If volcanism wanes, pelagic, ash-poor sedimentation will
prevail.
- Stage 3. Ocean spreading in the existing basin
ceases, and splitting of the volcanic arc initiates the
formation of a new backarc basin. The old basin still
receives some epiclastic material and subaerially dispersed ash from the active arc, but is dorninated by
pelagic sedimentation. Continued subsidence may
lower the basin floor below the CCD and thus cause
carbonate dissolution, resulting in the deposition of
brown clays. The sedimentation rate drops to values
characteristic of normal pelagic deposits in the
oceans (5-15 m1Ma for nannofossil ooze and around
5 m1Ma for brown abyssal clays). The subsequent
sediment succession will be controlled mainly by
factors unrelated to arc volcanism and backarc
spreading.
Sediment successions of small ocean basins. Basins resulting from the rifting of continental crust or
predating the formation of a volcanic island arc frequently begin with normal clastic sediments containing quartz, feldspar and mica. They are followed by
pelagic and hemipelagic sediments and display a
Chapter 12 Basin Evolution and Sediments
shallowing-upward trend, with increasing influence
from volcaniclastic material if a volcanic source is
nearby. In addition, they show marked differences
between their outer and inner margins.
Examples of Modern Backare Basins
Southwestern Pacific. Tertiary sediment sueeessions
drilled in backare basins (Fig. 12.30a) largely follow the
rules outlined above. Young, narrow basins behind an aetive are or older basins adjaeent to an island are display
sueeessions rieh in various voleanielasties and ealcareous
nannofossil ooze. The sedimentation rates of the
volcanielastie sequenees mostly range between 50 and 100
m/Ma. However, the sediments of oldermargin basins are
dominated by slowly deposited nannofossil oozes and silty
elays, with minor proportions ofvoleanielastie material.
These results were gained during the Deep Sea Drilling
Projeet (Leiteh 1984; Klein 1985b) west ofthe Tonga are
(Site 203) in the New Hebrides basin (Site 286) west ofthe
Vanuatu islands, and in the South Fiji basin (Site 205). The
older margin basins are generated by rifting of eontinental
erust (Sites 210 and 287 in the Coral Sea basin, and Site
206 in the New Caledonia basin).
The upper portion of Site 210 in the New Caledonia basin eontains about 2000 muddy turbidite beds altemating
with ealcareous ooze, deposited during a time span of 10
Ma (sedimentation rate around 50 m/Ma). This inerease in
sediment supply ean be explained by a rejuvenated sediment souree in the remnant are region. The narrow New
Caledonia basin is filled with a 3000 m thiek sueeession of
submarine slumps derived largely off the Lord Howe Rise.
The Tasman Sea basin adjaeent to the Australian eontinent
is eharaeterized by a sequenee rieh in silieeous ooze and
ehert (not shown in Fig. 12.30a). Early to mid-Oligoeene
uneonformities observed in several basins are assoeiated
with aceelerated bottom eurrents.
Information about speeifie sedimentary basins in the
South Paeifie, ineluding New Zealand, is found in Ballanee
(1993). Foreare and baekare basins older than Mioeene are
mostly diffieult to reeognize. The basins of New Zealand,
in partieular, are strongly affeeted by lateral displacement.
Sea of Japan. This basin is weIl studied and therefore
an example of a relatively eomplex basin history. The basin
is a relatively old (-25-15 Ma) and presently inaetive
backare basin whieh is beginning to elose by the development of a subduetion zone on the western side ofthe Japanese islands (cf. Figs. 12.24a,b and 12.28). The basin is
more than 3500 m deep and separated'from the open sea by
a shallow sill. The basin floor is only partially made up of
young oeeanie erust whieh formed along an E-W direeted
spreading axis (perpendieular to the strike of the presentday subduetion zone to the east of the Japanese islands).
The remainder of the basin floor is normal, rifted, or
thinned eontinental erust (e.g. Ingle 1997, Tamaki et al.
1997).
During the early rifting and extensional phase of basin
evolution, the rate of subsidenee was limited and non-marine sediment and voleanie material aeeumulated. With the
onset of backare spreading, aecompanied by basaltie
magmatism, subsidenee aeeelerated (100-500 m/Ma) and
the basin beeame marine with inereasing water depth. A