Mariana Trench-Arc-Back Arc System Viewedfrom Submersible Diving and Deep Sea Drilling

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Hiromi Fujimoto

Ocean Research Institute, University of Tokyo, 1-15-1, Minamidai, Nakano-ku, 164 Japan

Abstract The Mariana trench-arc-back arc

system has been an interesting target for diving

surveys and deep sea drilling. The MarianaTrench is an end member of subduction zones

characterized by back-arc spreading and high

angle of subduction. This feature may originatein active mud volcanoes of serpentine peridotite

observed in the forearc region. The Mariana arcis superior for studies of the subduction-relatedhydrogeological processes, because most part

of it is a simple convergent plate margin with

limited continental crust and without

accretionary prism. The oblique subduction at

the southernmost part is associated with the

deepest seafloor in the world and an exposure

of the upper mantle layer on the inner slope.

The Mariana Trough is a back arc spreading

system showing varied activities: rifting in the

northern part, active spreading in the centralpart, and strike-slip faulting in the southern

part.

Introduction

Owing to the increased resolution of thesatellite altimetry, we can recognize tectonic

features in the western Pacific in a map ofsatellite-derived gravity anomalies shown inFigure 1 (Sandwell et al., 1995). The

approximately 2500 km long Izu-Bonin-

Mariana arc system lies along the western

Pacific rim. While the Izu-Bonin arc is rather

straight, the Mariana arc associated with back

arc spreading really looks like an arc.The Mariana arc-trench system is

characterized by a deep trench, high angle of

subduction, limited deep earthquakes, noaccretionary prism, depression at the arc, and

active back arc spreading (Uyeda, 1984). Theoldest part of the Pacific plate is being

subducted at the Mariana Trench beneath the

Philippine Sea plate. The southern part of the

Mariana Trench shows a feature of oblique

subduction.The Mariana trench-arc-back arc system has

been an important target for many researches of

marine geology and geophysics inclusive of

diving surveys and deep-sea drillings. Several

holes were drilled in the Mariana Trough andMariana forearc near 18°N during DSDP Leg 60

(Nussong et al., 1981) and ODP Leg 125(Fryer and Pearce, 1992). Active hydrothermal

activities at the Mariana Trough axis were foundduring diving surveys aboard the submersibleAlvin (e.g., Craig et al., 1987).

The tectonic history around the Mariana Arcis reviewed by Lee et al. (1995). The Izu-Bonin-Mariana arc system is estimated to haveformed around 45 Ma (Meijer et al., 1983),

when a subduction initiated along a formertransform fault (Uyeda and Ben-Avraham,1972). The first episode of rifting began about

32 Ma to split the arc into the remnant Palau-Kyushu ridge and active West Mariana ridge(Mrozowski and Hayes, 1979). Subsequentseafloor spreading in the Parece Vela and

Shikoku basins continued until about 15 Ma. Asecond episode of rifting to form the MarianaTrough began around 7 Ma (Fryer and

Hussong, 1981), separating the remnant WestMariana ridge and the Mariana arc. TheMariana Trough has been spreading since about

3.5 Ma (Yamazaki et al., 1993).Lee et al. (1995) also showed three peaks of

the Mariana arc volcanism based on tephraglasses retrieved from 10 DSDP cores aroundthe Mariana arc system. The latter two peaks

correspond to two major pulses of arcvolcanism recorded in the Circum-Pacific area.Mariana arc explosive volcanism has been

predominantly tholeiitic for the past 40 m.y.

(Lee et al., 1995).Following these researches the Yokosuka and

the Shinkai 6500, JAMSTEC, carried out

surveys in the Mariana region in 1992, 1993,1995, and 1996 (Figure 2). This paper aims to

summarize the results of these surveys as wellas those of deep sea drillings. Researches ofhydrothermal activities in the Mariana Trough is

discussed in another paper in this volume.

Mariana arc system

The Mariana arc system is a simple

convergent plate margin and provides

opportunities for studying the tectonic and

Figure 1. Satellite-derived gravity anomalies in the western Pacific (Sandwell et al.,

1995) originally produced in a color map.

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geochemical processes of intraoceanic plate

subduction without the complexities of

continental crust and accretionary prism.

An E-W profile of crustal structure across the

Mariana arc system was obtained along the

latitude 23°N (Murauchi et al., 1968) and 18°N

(Hussong et al, 1981). The crustal structure of

the Mariana arc is characterized by limited

continental crust. Prominent low density

structure in the upper mantle is estimated

beneath the forearc region near 18°N (Sager,

1980; Yang et al., 1992). However, this may

not be a common feature of the Mariana arc

system because a local negative gravity anomaly

of large amplitude is located in this part of the

arc (Figure 1); we need to observe crustal

structures across the arc system in another part

in order to understand the geophysical

processes at the Mariana arc-trench system.

One of important features of the Mariana

forearc is the presence of active mud volcanoes

of serpentine peridotite. Conical Seamount near

I9 °N is located near the outer edge of the deep-

sea terrace, and carbonate chimneys were found

during an A/vw diving (Fryer et al., 1990). It

was drilled during the ODP Leg 125 (Fryer and

Pearce, 1992; Mottl, 1992). Geochemical

analysis of the pore waters shows extremely

low chlorinity indicating upwelling of pore

waters at a rate around 10 mm/y (Mottl, 1992).

The pore waters require a deep source, and are

enriched in sulfur and carbon. The absence of

an accretionary prism in the Mariana forearc

severely constrains the origin of the fluids

upwelling through the mud volcano. Mottl

(1992) postulates that the fluids probably

originate at the top of the downgoing slab,

about 30 km below the seafloor, by heating of

the sediments and basalt of the subducted

oceanic crust. The water serpentinizes the

overlying mantle wedge. Serpentine seamounts

act as conduits through which water, CO2,

hydrocarbons, and sulfur pass from the slab

into the oceans.

Another serpentine seamount near 14°N was

surveyed by using the Shinkai 6500. The

seamount lying in the forearc east of Guam

Island was named "Chamorro Seamount"

(Fryer, 1996a, 1996b). A structure like a

carbonate chimney was found at the summit,

and samples were collected near the end of a

submersible diving. Small tube worms and

bacteria mat were found in the samples (Fryer,

1996b). The seamount was revisited in 1996

aboard the Shinkai 6500 (Mottl.

Mariana Trough

The Mariana Trough is recognized in Figure I

with the negative anomalies near 140 °E west of

the Mariana arc. The Mariana Trough is a back

arc spreading system showing varied activities:

rifting in the northern part, active spreading in

the central part, and strike-slip faulting in the

southern part. This variability is reflected on

mineral composition of the back arc basin

basalts (Fryer, 1996a).

The Mariana Trough in the north of 20°N is in

the stage of initial rifting with scattered

volcanoes but no real spreading center (Fryer.

1996a). The steep axial valley wall is a good

target for observation of the crustal section

down to the upper mantle. Two diving surveys

aboard the "Shinkai 6500" were carried out in

1996.

Geophysical mapping of the central part

carried out by Geological Survey of Japan

revealed that the trough has been spreading

these 3.5 Ma (Yamazaki et al., 1993). Near-

complete geophysical coverage of a 50 km by

100 km area encompassing the Mariana Trough

near 18°N was obtained during the KH-92-1

cruise of the R/V Hakuho-maru in 1992. The

median valley and rift mountain segmentation,

volcanic morphology, and magnetic signature

compare well with well-studied areas of the

slowly-spreading Mid-Atlantic Ridge,

suggesting that spreading rate, independent of

tectonic setting, is an important variable in

determining crustal accretionary styles at

spreading centers (Kong, et al., 1993). The half

spreading rate is estimated to be 1.5 cm/yr

(Seama and Fujiwara, 1993) to 2 cm/yr

(Yamazaki et al., 1993) based on geomagnetic

anomalies.

Geophysical mapping was carried out over

the Mariana trough axis near 16°N aboard the

Yokosuka in 1996 as the first step of the

geophysical transect of the Mariana arc system

along latitude 16°N. A morphological structure

indicating a fracture zone was observed at

15o40'N (Fujimoto and Tamaki, 1996).

A major strike-slip fault extends in the

southern part from the trench axis across the

forearc, through the volcanic arc, and into the

back arc basin (Fryer, 1996a). Three-

dimensional view of this part obtained by swath

bathymetry by the Yokosuka (Fujimoto et al.,

1996) is shown in Figure 3. Arc magmas

apparently leak along this fault zone into the

forearc and the back arc spreading center

(Fryer, 1996a). The westward extention of the

arc volcanism from this strike-slip was

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to

Figure 2. The areas of swath mapping by the Yokosuka anddiving sites of the submersible Shinkai 6500. Modifiedfrom a map produced by K. Fujikura (personal comm.)

Figure 3. 3-D view of a major N-S fault zone that cuts across

the entire Mariana forearc at 1440l(yE.

confirmed by multi-narrow-beam mapping by

the Yokosuka (Fujimoto et al., 1996).

Southern Mariana Trench

The inner wall of the southern Mariana

Trench shows steep slope, which could be

induced by the oblique subduction of the Pacific

plate underneath the Philippine Sea plate. The

escarpment extends from the water depth of

4,000 m to about 11,000 m, and lies on the

southward extention of the Mariana Trough

spreading system; it provides opportunities for

observation of an oceanic crustal section across

the Moho. Mantle peridotites were observed

and sampled during two Shirtkai 6500 dives on

the landward slope of the eastern Challenger

Deep (Fujimoto et al., 1996). However, large

part of the slope is covered by the sediment,

and the submarine Moho has not been

recognized yet.

A sediment sample was obtained from the

seafloor (10,897 m in water) of the Challenger

Deep by the remotely operated vehicle Kaiko on

4 March 1996, and six benthic foraminiferal

taxa are identified (Akimoto et al., 1996). This

assemblage is the deepest record in the depth

distribution of benthic foraminifers.

Summary remarks

Recent studies have revealed the interesting

features of the Mariana arc system, and

important problems as follows have emerged to

be clarified:

(1) Possible biological activities associated with

the serpentine mud volcanoes in the Mariana

forearc.

(2) Processes to form the continental crust in

the Mariana arc compared with those in the Izu-

Bonin arc.

(3) Tectonicthe Mariana'

and spreading processes in

(4) Lower crustal structures exposed in the

northern Mariana Trough and on the inner slopeof the Challenger Deep.

(5) Upper crustal section of the oldest Pacificplate possibly exposed seaward of the Mariana

Trench.

Acknowledements The author would like to

thank Katsunori Fujikura for arrangements ofthis symposium proceedings. Masao Nakanishi

kindly produced a gravity map shown in Figure

1. The 3-D bathymetric map in Figure 2 was

produced by Mayumi Sekine.

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