WESTERN PACIFIC STRATIGRAPHY
The ancient crust of the western Pacific plate is the primary input to
the >2000-km-long Izu-Mariana subducting margin. Ideally, this input
would be constrained by drilling several holes through the sedimentary
section and deeply into oceanic crust along the length of the trench.
Because of the great expense and time it takes to drill in ~6000-m water
depth, we are limited in practice to a few drill holes and extending this
information regionally using sedimentation and plate-motion models,
along with seismic stratigraphy. It is thus important to understand the
context of sedimentation and plate history in the western Pacific in order
to maximize the information gained from a small number of reference
sites, such as Leg 185 Sites 801 and 1149.
The crust subducting into the Mariana Trench includes Jurassic seafloor
of the East Mariana and Pigafetta Basins (Fig. 5). Based on magnetic
anomaly lineations, this region was thought to contain the Earth's oldest
in situ oceanic crust formed at ultra fast spreading rates (160 km/m.y. at
Site 801). The basic goal of Leg 129 was to sample Jurassic oceanic crust.
Earlier attempts to recover Jurassic sediments and basement in the
western Pacific had been thwarted by extensive mid-Cretaceous volcanics
and sills and by problems with the drill string sticking in chert horizons.
While drilling two holes (ODP Sites 800 and 801) that also bottomed in
Cretaceous basalt, Leg 129 was the first to succeed in recovering
Jurassic oceanic basement in the Pacific Ocean, and Hole 801C rocks are
still the oldest sampled in the ocean basins, at 167 ± 5 Ma (Pringle, 1992).
During Leg 129 entire sedimentary columns at three sites in the East
Mariana and Pigafetta Basins (ODP Sites 800-802; Fig. 5) were also
successfully sampled. Before this leg, the recovery from nine DSDP and
ODP sites averaged <50 m each. Taking Hole 801C as typical of the region,
the sedimentary stratigraphy consists of Cenozoic brown pelagic clay
overlying Coniacian to Campanian cherts and porcellanite, Albian
seamount volcaniclastics, and Bajocian to Valanginian radiolarites (Fig. 6;
Lancelot, Larson, et al., 1990). This sedimentary history reflects the plate
history, which begins in the Southern Hemisphere in a zone of high
biological productivity, as recorded by the Jurassic radiolarites (Fig. 7;
Lancelot, Larson, et al., 1990). The plate then moved southward until the
Early Cretaceous when it began to move northward again, collecting
volcaniclastics from the nearby Albian Magellan Seamounts and then more
siliceous sediments as it again crossed the high-productivity zone 5°-10°
south of the paleoequator. The Cenozoic was characterized by very slow
accumulation of deep brown pelagic clays, very depleted in biogenic,
terrigenous, or eolian input, as is expected for the open-ocean
environment. This history is typical for the East Mariana and Pigafetta
Basins, and this stratigraphy, particularly the clay/chert and
volcaniclastic intervals, can be traced regionally from seismic records
(Abrams, et al., 1992). Although recovery was generally low (<30%), Leg
129 provided adequate sampling of the different sedimentary components
to characterize the sedimentary geochemical flux into the Mariana Trench.
This contrasts with the existing information to the north, along the entire
1000 km of the Izu margin.
Previous drilling in the Nadezhda Basin, seaward of the Izu-Bonin
Trench, was about as successful as drilling to the south prior to Leg 129.
The chert horizons plagued drilling during Leg 20, which placed five holes
in the region, none of which was to hit basement except DSDP Hole 197,
where only 1 m of undatable tholeiite was recovered. Thus, the M-series
magnetic anomaly ages have never been tested in this region. Along the Izu
Trench, magnetic anomalies predict that the oceanic crust decreases in
age from Jurassic (>M18) in the south to Early Cretaceous (M11) at Site
1149 (Fig. 5). It was unknown if the extensive mid-Cretaceous volcanism
that took place in the south extended north into the Nadezhda Basin.
Average recovery of sediments in the Nadezhda Basin was extremely
low (<15 m) for previous DSDP sites, again because of sticking problems
and spot coring. Leg 20 cores indicate an upper ash- and diatom-rich clay
unit overlying a brown pelagic clay and Cretaceous chert and chalk. The
paleolatitude history for Site 1149 predicts a longer duration beneath
equatorial zones of high biological productivity, and thus, extensive chert
and chalk sequences (Fig. 7). Water-gun seismic profiles collected during a
presite seismic survey show a prominent reflection at ~0.2 s two-way
traveltime (TWT) that corresponds to the chert horizon and another
prominent reflection at 0.42 s TWT (Fig. 8) that corresponds with probable
basement. These two reflections/horizons are prominent features, which
are well correlated across the Nadezhda Basin.
Thus, a primary objective of Leg 185 was to drill the missing inputs
(i.e., subducting sediments and oceanic crust) to the Izu-Mariana recycling
equation. Mariana sediments had already been adequately sampled; the
remaining component was altered oceanic crust. For the Izu margin, both
sediment and oceanic crust were virtual unknowns. Thus, the goal of Leg
185 was to drill a basement site seaward of the Mariana Trench and a
sediment and basement site seaward of the Izu Trench. Hole 801C was
chosen for the Mariana site because it is the only certain window into
Jurassic basement in the western Pacific with a re-entry cone set and
~60 m already drilled into the upper mid-ocean-ridge basalt (MORB)
tholeiites. Other sites could have been chosen closer to the trench, but
this would likely require drilling through a significant section of
Cretaceous volcanics or intrusives above Jurassic basement. Deepening
the MORB section at Hole 801C would help to characterize the chemical
fluxes into the Mariana subduction zone as well as define the aging and
architecture of Layer 2 in fast-spreading crust.
Site 1149 lies within the same spreading compartment as Site 801,
along a flow line in crust ~30 m.y. younger (Fig. 5), formed at a spreading
rate slower than Site 801 (100 mm/yr full rate). Roughly 100 km from the
trench, Site 1149 lies seaward of the main faulting of the plate as it
bends into the subduction zone. The main objective at Site 1149 was to
drill through the inferred 400-m-thick sedimentary sequence and into
basement subducting along the Izu margin, which would enable a
comparison with the fluxes to the south into the Mariana Trench.
To 185 Scientific Objectives
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