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Tectonophysics 389 (

Incipient subduction of the Ontong Java Plateau

along the North Solomon trench

A. Tairaa,*, P. Mannb, R. Rahardiawana,1

aOcean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, JapanbInstitute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, 4412 Spicewood Springs Road, Bldg. 600,

Austin TX 78759, United States

Received 11 July 2002; accepted 14 July 2004

Available online 17 September 2004

Abstract

The Ontong Java Plateau (OJP) in the western central Pacific is the largest and thickest oceanic plateau and one of a

few oceanic plateaus converging on an island arc (Solomon island arc—SIA). To better understand the evolution of the

North Solomon trench (NST), active oblique convergence between the OJP and SIA, and late Neogene development of

Malaita accretionary prism (MAP), we present 850 km of multichannel seismic reflection data integrated with 7832 km2 of

IZANAGI side-scan sonar coverage. We have focussed the study at the transition area between the well-defined

northwestern end of the North Solomon trench and a diffusely deformed area where the trench is actively propagating in a

northwestward direction. The deeper structure beneath the survey area is discussed by Phinney et al. [Oceanic plateau

accretion in the Malaita accretionary prism inferred from multi-channel seismic reflection data, this issue] using deeper

penetration, multichannel seismic reflection lines. The serial cross sections provided by multichannel seismic profiling

combined with the IZANAGI backscattering imagery provides a time series evolution for the development of the North

Solomon trench. The main evolutionary stages include (1) the incipient trench in the northern area marked by a diffuse

zone of deformation above a broad arch in the crust. Deeper penetration profiles by Phinney et al. show the bulge is

related to a deeper decollement fault that is propagating upward and seaward through the crust. (2) The formation of a

continuous thrust front in the central area. Deeper penetration profiles by Phinney et al. show this thrust front is surface

expression of the same decollement present at depth to the north. The boundary between the surface trace of the thrust and

the diffuse area of deformation in the northern area is inferred as a vertical, high-angle tear fault with left-lateral offset. (3)

The formation of a deep, elongate trench which controls gravitationally related slumping and sedimentation around the

steep edges of the trench fill basin. The areas to the southeast are those that have undergone convergence for the longest

period of time and therefore show better developed trench structures and a reduced width of the MAP. Areas to the

0040-1951/$ - s

doi:10.1016/j.tec

* Correspon

cho, Yokosuka 2

E-mail addr1 Now at: M

2004) 247–266

ee front matter D 2004 Elsevier B.V. All rights reserved.

to.2004.07.052

ding author. Center for Deep Earth Exploration, Japan Marine Science and Technology Center (JAMSTEC), 2-15 Natsushima-

37-0061, Japan. Tel.: +81 468 67 9252; fax: +81 468 67 9255.

ess: ataira@jamstec.go.jp (A. Taira).

arine Geological Institute of Indonesia, Bandung, Indonesia.

A. Taira et al. / Tectonophysics 389 (2004) 247–266248

northwest have undergone convergence for a shorter period of time and show less developed trench structures and a wide

area of the MAP.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Subduction; Ontong Java Plateau; North Solomon trench; Solomon Islands; Collision

1. Introduction

1.1. Tectonic setting of the convergent zone

The Solomon Islands form part of the Cenozoic

Northern Melanesian arc system and are located along

an obliquely convergent plate boundary separating the

Pacific and Indo-Australian plates (Mann and Taira,

this issue) (Fig. 1A). The Solomon island arc is

bounded to the north by Kilinailau–North Solomon

trench system and the south by the New Britain–San

Cristobal trench (Fig. 1B). The obliquely convergent

zone between the Ontong Java Plateau (OJP) and the

Solomon Islands Arc (SIA) produced the 140-km-

wide, late Neogene Malaita accretionary prism (MAP)

(Phinney et al., 1999, 2004).

The North Solomon trench defines the deformation

front of the MAP in a 900-km-long belt extending

from north of Choiseul Island to the southeast of

Ulawa Island (Fig. 2). The deformation front of the

MAP forms a major D-shaped salient in the trend of

the contact zone between the Solomon Islands and the

OJP (Fig. 2). The eastern Solomon Islands including

Santa Isabel and Malaita Islands expose well-studied

outcrops of oceanic plateau rocks (Petterson et al.,

1997) that can be correlated to subsurface rocks of the

Ontong Java plateau known from seismic profiling

and ODP drilling (Phinney et al., 1999). High

topography on Bougainville and the western Solomon

Islands (New Georgia Group) is related to the

construction of large stratovolcanoes in late Neogene

time. Mann (1997) and Mann and Taira (this issue)

postulate that the elevated, nonvolcanic topography

and greater seismicity the eastern Solomon Islands

(islands of Guadacanal, Makira, and Malaita) may

reflect their position in the vice-like convergent zone

between thickened Ontong Java oceanic plateau on

the Pacific plate and thickened oceanic plateau or

continental crust of the Lousiade plateau on the

Australian plate (Fig. 2).

The Ontong Java Plateau as defined by the 4000-

m isobath has an average water depth of about 2 km

and stands ~3 km higher than the Jurassic–Creta-

ceous ocean crust (Fig. 1B). The Ontong Java Plateau

has a crustal thickness of 33 km at the North

Solomon trench and beneath the MAP—or about

3–4 times that of normal oceanic crust (Miura et al.,

2004). The deeper basins adjacent to the plateau

appear to be normal Mesozoic oceanic crust charac-

terized by widespread magnetic anomalies and

fracture zones (Nakanishi et al., 1992) (Fig. 1B).

Earthquake hypocenters indicate that the Ontong Java

Plateau (or the oceanic crust it was adjacent to) is

presently subducting beneath the Malaita accretionary

prism and Solomon Islands (Mann and Taira, this

issue).

1.2. Rationale for the study

The Ontong Java Plateau (OJP) is one of the few

oceanic plateaus which actively converge on an

island arc and therefore can provide important

insights into the question of whether ancient oceanic

plateaus were subducted beneath (Mann and Taira,

this issue) or were accreted to arcs and continents

(Cloos, 1993). If oceanic plateaus as large as the OJP

are preserved at subduction zones, plateaus would

become a major contributor to the accretion of

continental crust over geologic time, and would rival

the continental contribution of arcs (Kroenke, 1972;

Nur and Ben-Avraham, 1982; Abbott and Mooney,

1995).

The study area described in this paper was chosen

for a short but intensive 2-day survey for several

reasons: (1) previous work by Phinney et al. (1999,

2004) using widely spaced MCS lines had identified a

low-angle decollement in the area where the North

Solomon trench died out to the northwest (Fig. 2);

this survey was conceived to provide a higher

resolution image of the seafloor deformation across

Fig. 1. (A) General location map of the Solomon Islands in the southwest Pacific Ocean. (B) Extent of the Ontong Java oceanic plateau (OJP)

shaded in gray as defined by Coffin and Eldholm (1994) using the 4000-m isobath. The Kilinailau–North Solomon–Cape Johnson trench is a

continuous feature separating the Ontong Java plateau from the Malaita accretionary prism (MAP—light grey area) and the Northern

Melanesian arc system exposed in New Ireland, Bougainville, and the Solomon Islands. The dashed line indicates the crest of the outer bulge

produced by flexure of the OJP as inferred from satellite-derived, free-air gravity (Sandwell and Smith, 1997). Thin black lines east of the

Ontong Java Plateau are Mesozoic oceanic spreading anomalies and fracture zones identified by Nakanishi et al. (1992). Plate vector shows

direction and rate of the Pacific plate relative to the Australia plate. Key to abbreviations: New Britain trench (NBT), San Cristobal trench

(SCT), Woodlark spreading ridge (WSR), North Solomon trench (NST), Stewart arch (SA, outer bulge adjacent to Solomon Islands), Cape

Johnson trench (CJT), Vitiaz trench (VT). Numbered black dots are DSDP or ODP wells into the Ontong Java Plateau with numbers in

parentheses indicating the age of basaltic basement in millions of years.

A. Taira et al. / Tectonophysics 389 (2004) 247–266 249

Fig. 2. Tectonic map of the Solomon Islands arc modified from Phinney et al. (this issue) showing major faults at the convergent zone between

the Ontong Java Plateau and the Solomon island arc. Phinney et al. (2004) divided the Malaita accretionary prism into four, distinctive, fault-

bounded structural domains: Choiseul, Santa Isabel, Malaita, and Ulawa. The boxed study area described in this paper covers the northern parts

of the Choiseul and Santa Isabel structural domains at the Northern limit of the North Solomon trench. Studies by Phinney et al. (2004) and

Cowley et al. (2004) constrain a post–early Pliocene southeast-to-northwest progression in obliquely convergent deformation between the

Ontong Java Plateau, the Malaita accretionary prism, and the Solomon island arc. Left-lateral oblique shortening is partly accommodated by left-

lateral trench-parallel slip on the Kia–Korigole–Kaipito fault zone (KKKFZ). Bold broken lines are locations for Fig. 13A and B cross-sections.

Key to abbreviations: Bougainville (BG), Shortland Islands (SH), Santa Isabel (SI), New Georgia Islands (NG), Guadalcanal (G), Malaita (M),

Makira (MK), and Ulawa (U).

A. Taira et al. / Tectonophysics 389 (2004) 247–266250

this zone; (2) previous workers had questioned

whether the North Solomon trench presently accom-

modates oblique convergence between the Pacific and

Australia plate (Kroenke, 1972; Kroenke et al., 1986);

this survey addresses the issue of active deformation

of the seafloor in the vicinity of the North Solomon

trench; and (3) Mann (1997), Phinney et al. (1999,

2004) postulate that deformation of the MAP and

North Solomon trench is migrating from early

Pliocene in the southeast to active deformation in

the northwest north of Choiseul Island; by systemati-

cally surveying a 150-km-long segment of the trench

in detail, we can test the idea of southeast to

northwest deformation.

2. Data acquisition and methods

Structures in the zone of obliquely convergent

deformation in the North Solomon Trench were mapped

using the IZANAGI side-scan sonar imagery and multi-

channel seismic profiles. The data was collected using

the RV Hakuho Maru operated by the Ocean Research

Institute of the University of Tokyo. The marine

geophysical data shown in this paper was collected over

a period of 2 days (January 27–28, 1998) during the first

leg of the KH98-1 cruise that was dedicated to multi-

disciplinary investigations of various aspects of the

structure and stratigraphy of the Ontong Java Plateau

(Larson et al., 1998). The study area is located in the

A. Taira et al. / Tectonophysics 389 (2004) 247–266 251

northernmost part of the Malaita accretionary prism

(northern North Solomon Trench), within coordinates

5840VS to 7800VS and 157820VE to 159815VE (Fig. 3).

2.1. IZANAGI side-scan sonar system

The IZANAGI is designed to be highly portable and

quickly mounted onto research vessels of almost any

size. Three components make up the basic system as

commonly used for seafloor mapping: a control/display

unit (recorder/processor); a towed transducer assembly

(towfish); and an electromechanical cable which

connects the towfish to the ship. For routine seafloor

mapping, the IZANAGI is towed at a depth of 100 m

below the surface at a ship speed of up to 8 knots. The

system is capable of acquiring high-resolution seafloor

images and bathymetric maps across a 10-km-wide

swath. In water as deep as 10 km, the system can

produce backscattering images up to 40 km in swath

width. The instrument emits a signal of 11 kHz to port

and 12 kHz to starboard. The IZANAGI backscattering

images are composed of 1024 pixels on each side of the

swath (Fig. 4). The phase angle data were processed for

Fig. 3. Track lines of IZANAGI side-scan sonar and multichannel seismic re

trench during cruise KH98-1. The lines are spaced at a distance of 7 km

multichannel seismic data collected along lines 1–16 is 850 km; the area

multichannel seismic lines described by Phinney et al. (2004).

production of a bathymetric map with contour interval

of 100 m (Fig. 5).

The IZANAGI backscattering images record varia-

tions in the acoustic intensity of surficial structures on

the seafloor of the North Solomon trench (Fig. 4). Ship

tracks were oriented at an oblique angle (roughly east–

west direction) to trench-related structures of the North

Solomon trench to maximize their seafloor reflectivity

on the backscatter image. Contrasts in seafloor

reflectivity allow high-resolution geological and struc-

tural interpretations of the seafloor especially when

combined with results from coincident seismic reflec-

tion profiles (Fig. 3). Linear or sinuous structures on the

seafloor including fault scarps and the hinge regions of

folds are apparent on the image and can be verified on

the bathymetric map (Fig. 5) and on intersecting

seismic profiles. High-intensity zones (black zones)

on the image produced by high-intensity backscatter

are inferred to represent areas of rocky or irregular

seafloor or steep slopes, especially if these slopes are

inclined toward the towfish. Medium intensity, grey

zones on the image generally correspond to a relatively

flat or gently sloping seabed with sediment cover.

flection data collected at the northwestern end of the North Solomon

to insure overlap of adjacent side-scan swaths. The line length of

l extent of the side-scan survey is 7832 km2. Heavy lines indicate

Fig. 4. Mosaic of processed IZANAGI side-scan data from the northwestern end of the North Solomon trench. Inset shows the location of lines relative to the nearest land areas of the

Solomon Islands (Choiseul Island). Higher intensity backscatter is shown in black and lower intensity backscatter is shown in white. White lines on mosaic represent ship tracks

numbered lines 1–16. The study area is divided into three zones: (1) a northern area imaged by lines 1–5 that is marked by diffuse and discontinuous seafloor deformation; (2) a

central area imaged by lines 6–13 of developing, semicontinuous trench deformation; and (3) a southern area imaged by lines 14–16 of established trench deformation. Two zoom

figures of the boundary regions are also shown. Trends of seafloor scarp and en echelon lineament are marked by bold dots.

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Fig. 5. Bathymetric map with a contour interval of 100 m of the northwestern end of the North Solomon trench (cf. Fig. 4 for side-scan map of same area). Dotted lines labeled lines

1–16 are ship tracks. Northern, central, and southern areas of deformation inferred from side-scan data in Fig. 4 are indicated. Note progressive, northwestward decrease in the amount

of bathymetric relief along the trench from southeast to northwest. The seafloor in the northern area is essentially flat.

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2.2. Multichannel seismic reflection

Serial, 24-channel seismic profiles were collected

along coincident lines with the IZANAGI side-scan

survey (Fig. 3). These multichannel seismic profiles

are used to image structures and stratigraphic sequen-

ces of the upper crustal section. Wider spaced by

deeper penetrating 112- to 118-channel images of the

same area are discussed by Phinney et al. (2004) (Figs.

2 and 3). The sound source for the KH98-1 acoustic

survey was a 1900-cc airgun array fired at 10-s time

intervals, plus or minus a randomly generated time

interval to attenuate any previous shot noise during

stacking. The signal reflection was received by a 24-

channel streamer with group spacing of 25 m, yielding

common midpoint (CMP) data at an interval of 12.5 m.

Ship positioning was determined by the shipTs GlobalPositioning System.

Seismic processing was done using the Focus

Interactive software package at the Ocean Research

Institute. CMP geometry was configured based on the

receiver spacing, offset, and ship speed. This geom-

etry produced fold in CMP gathers with a maximum

of 100 valid traces. Velocity analyses were performed

at all lines. Prestack processing included t2 gain,

spiking deconvolution, normal moveout correction,

stretch mute, and neartrace mute for multiple attenu-

ation. The resulting stacking velocity function was

used to provide an external mute and an internal mute

for the near 465-m offsets just above the water column

Table 1

Summary of interval velocities from multichannel seismic reflection

lines, cruise KH98-1

Sequence CDP range Velocity (m/s) Ave. velocity (m/s)

OJP 3b

and 3c

28–1265 1505.5–1966.6 1736.1F230.5

1304–1425 1504.5–1966.6 1730.2F225.7

1461–1740 1506.6–1971.5 1738.6F232.9

OJP 3a 28–1265 2008.3–2283.3 2145.8F137.5

1304–1425 2001.0–2282.3 2141.6F140.6

1461–1740 2030.6–2270.1 2150.4F119.8

OJP 2 28–1265 2401.2–3160.8 2781.0F379.8

1304–1425 2398.7–3171.6 2785.1F386.5

1461–1740 2406.8–3169.8 2788.3F381.5

OJP 1 28–1265 N3362.8–3397.8 N(3380.3F17.5)

1304–1425 N3360.8–3392.2 N(3376.5F15.7)

1461–1740 N3364.8–3398.2 N(3381.5F16.7)

OJP 3: Eocene–Recent pelagic section.

OJP 2: Cretaceous–Eocoene chert–limestone interbed section.

OJP 1: Cretaceous OJP basaltic basement.

multiple. CMPs were stacked and a poststack finite-

difference algorithm-migration was applied. A band-

pass filter and automatic gain control were applied to

the final processed seismic sections and relative

amplitudes were preserved. A linear time variant gain

was used as an approximation for spherical diver-

gence correction. The velocity model is a smoothed

function of the interval velocities computed from

reliable picks in the stacking velocity functions. A

summary for the velocities of each of the major

sequences provided in Table 1.

3. Results of IZANAGI backscatter image and

bathymetric map

The IZANAGI backscatter mosaic of the study

area reveals a grey seabed signature of moderate

intensity with numerous dark spots related to bathy-

metric highs (Fig. 4). Structural interpretation of the

backscatter image and bathymetric map is shown in

Fig. 6. Based on the structural map, we subdivide the

survey area into three areas (northern, central, and

southern) based on their common structural and

bathymetric characteristics.

3.1. Northern area

The northern area is north of 6810VS and was

imaged on lines 1–5 (Fig. 3). The northern area of this

survey lies outside and to the northwest of the

Choiseul structural domain of Phinney et al. (2004)

because of its apparent lack of large-scale deforma-

tional features (Fig. 2). The backscatter image of the

northern area shows the low intensities over wide

areas characteristic of a little deformed seafloor

covered by soft, pelagic sediment (Fig. 4). The

bathymetric map in Fig. 5 reveals a relatively flat

seafloor ranging in depth from 2500 to 3000 m with

isolated highs corresponding to areas of high reflec-

tivity on the backscatter map. Bathymetric contours

trend northeast and indicate a gradual slope to the

southeast in the direction of the southeastward-deep-

ening North Solomon trench (Fig. 5). Several small,

elgonate bathymetric depressions and ridges with

northwest and north trends were mapped between

lines 2 and 3 and fall on the northwestward projection

of the North Solomon trench (Fig. 5).

Fig. 6. Structural interpretation of seafloor deformation based on IZANAGI side-scan and bathymetric data shown in Figs. 4 and 5. Thicker line

with triangles on upthrown side represents a thrust fault marking the deformation front of the North Solomon trench. Folds parallel this zone of

thrust deformation. The northern part of the central area of deformation is characterized by backthrusting with thrust faults dipping to the

northeast. The boundary between young trench deformation of the central area and diffuse deformation of the northern area is marked by a left-

lateral tear fault. Note the northwestward termination of the trench fill deposit (dark grey area) as the seafloor becomes flatter and less deformed.

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3.2. Central area

The central area lies between latitudes 6810VS–6835VS and was imaged on lines 6–11 (Fig. 3). The

central area contrasts sharply with the northern area

because of the appearance of linear scarps and folds

indicative of active seafloor deformation along the

northwest-trending North Solomon trench (Fig. 4).

The central area of this study corresponds the western

edge of the Choiseul structural domain of Phinney et

al. (2004) (Figs. 2 and 3).

Theboundarybetween thenorthernandcentral zones

is a linear scarp. A bend of structural ridge together with

en echelon arrangement of lineaments [Fig. 4(a)] along

the south side of the scarp suggests that the scarp is a left-

lateral tear fault generated during northeast–southwest

shortening between the OJP and SIA with left-lateral

sense of slip (Fig. 6). In the central part of central area,

the floor of the developing North Solomon trench is

bounded to thewest by a seriesN708–808Wfault zones.

The prominent ridge flanking the upthrown block of

the major thrust fault forming the North Solomon

trench is about 2–10 km wide and 100 km long. The

edges of the ridge are characterized by steep, fault-

controlled slopes with high backscatter (Fig. 4).

3.3. Southern area

The southern area imaged on lines 12–16 shows

continued deepening of the North Solomon trench and

extensive areas of slumping related to this trenchward

tilting and bending of the OJP in response to

overthrusting along the trench (Fig. 6). The structural

ridge forming the trench slope of the North Solomon

Fig. 7. Multichannel seismic line 2 from northern area of diffuse deformation of the North Solomon trench (cf. Figs. 4, 5, and 6 for location).

The line shows the three megasequences of the Ontong Java Plateau and Malaita accretionary prism that are discussed in detail by Phinney et al.

(1999, 2004). The depth and location of the decollement fault and intrabasement reflectors are also constrained by higher resolution

multichannel seismic data described by Phinney et al. (2004). Note the anomalous, elongate seafloor block that is aligned with the North

Solomon trench to the southeast (cf. Fig. 6), faulting to the south of the seafloor block that overlies the decollement fault at depth, and the broad

arch developed in the area of the anomalous seafloor block. Vertical exaggeration is approximately 15:1.

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trench becomes wider and more complex (Fig. 6). The

southern part of the southern area (south of 6845V) iscomposed of two parallel, thrust-bounded ridges

separated by deep basins. The two ridges exhibit

Table 2

Summary of seismic stratigraphy

Unit Seismic characteristics

Trench-fill sequence Low-amplitude, parallel and continuous reflec

onlap relation with OJP 3

OJP 3

c Low-amplitude, parallel and continuous reflec

bContinuous and conformable reflectors

a

Erosional unco

OJP 2 High-amplitude and semicontinuous reflectors

unconformity surface

OJP 1 Low-amplitude, subparallel and semicontinuo

nested anticlines with fold axes trending N808E,N908E, and N1058E. The western, landward ridges

is more prominent and reaches an average water depth

of 2000 m (Fig. 5).

Lithological correlation

tors with Late Neogene to Recent terrigenous turbidites (?)

tors Late Neogene to Recent pelagic sediment

Middle to late Miocene chalk

Eocene to early Miocene chalk

nformity

with multiple Cretaceous to Eocene chert and

limestone interbeds

us reflectors Cretaceous basaltic basement

Fig. 8. Multichannel seismic line 4 from northern area of diffuse deformation of the North Solomon trench (cf. Figs. 4, 5, and 6, for location). The line shows the three megasequences

of the Ontong Java Plateau and Malaita accretionary prism that are discussed in detail by Phinney et al. (1999, 2004). The depth and location of the decollement fault and

intrabasement reflectors are also constrained by higher resolution multichannel seismic data described by Phinney et al. (2004). Note the absence of the anomalous, elongate seafloor

block seen on Line 2 to the north (Fig. 7). A large arch is seen along between from the elongate block to the northwest and the North Solomon trench of the central area to the

southeast. The arch is a manifestation of active motion along the decollement indicated at depth. Vertical exaggeration is approximately 15:1.

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Fig. 9. Multichannel seismic line 5 at the boundary between the northern area of diffuse deformation and the central area of developing trench deformation of the North Solomon

trench (cf. Figs. 4, 5, and 6, for location). The line shows the three megasequences of the Ontong Java Plateau and Malaita accretionary prism that are discussed in detail by Phinney et

al. (1999, 2004). The depth and location of the decollement fault and intrabasement reflectors are also constrained by higher resolution multichannel seismic data described by

Phinney et al. (2004). Note the tear fault with prominent seafloor relief and left-lateral motion that separates the northern and central areas (cf. side-scan data in Fig. 6 for its strike

direction, parallel to the direction of Pacific–Australia plate motion). The North Solomon trench of the central area southeast of the tear fault is marked a large surface scarp that

overlies a shallowing thrust decollement. Vertical exaggeration is approximately 15:1.

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Fig. 10. (A) Multichannel seismic line 7 crossing the northern part of the central area of developing trench deformation of the North Solomon trench (cf. Figs. 4, 5, and 6, for

location). The main structures seen on line 7 include the elongate main ridge, a pop-up block (Main Ridge) defined by the thrust of the North Solomon trench to the east–northeast and

two backthrusts to the west–southwest. The zone of backthrusting extends 25 km in the landward direction. Deformation in the seaward direction includes an active zone of fault

propagation folds with seafloor relief produced by thrust faults. Vertical exaggeration is approximately 4:1. (B) Zoom of trench deformation. Vertical exaggeration is approximately

15:1. The zone of thrusting and folding is propagating seaward or to the east–northeast 13 km from the main thrust scarp forming the North Solomon trench.

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Fig. 11. Multichannel seismic line 12 crossing the southern part of the central area of the developing North Solomon trench (cf. Figs. 4–6 for location). The prominent features

observed on this line include trench-fill sequence which onlaps unit OJP3, steep frontal fault scarp of the North Solomon trench and uplifted and folded sequences of the Ontong Java

plateau (Main Ridge) and overlying slope sequence. At the seaward margin of the trench, high-angle reverse fault is developed. Vertical exaggeration is approximately 15:1.

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The boundary between the central and southern

areas is marked by a topographic discontinuity similar

to the boundary between the northern and southern

areas. There is a clear bend in the trend of the Main

Ridge and en echelon lineaments associated ENE–

WSW trending scarp [Fig. 4(b)]. In addition, this

scarp extends trenchward forming a small ridge chain

(Fig. 5) which borders the northward limit of trench-

fill sediments. We interpreted this scarp offsetting the

frontal structural ridge as a tear-fault with left-lateral

strike-slip component.

A part of trench-fill sediments may be derived

from the areas above and near sea level along the

Stewart arch (Fig. 4). A large slump with a width of

6 km is observed at the base of the slope and gullies

on the slope of the Stewart arch indicate debris flows

with considerable erosive power. Some slumps may

be generated along normal fault scarps parallel to the

trend of the Stewart arch (Phinney et al., 1999,

2004).

4. Results of multichannel seismic reflection

Multichannel seismic reflection profiles were

acquired along all ENE- to WSW-oriented survey

tracks (Fig. 3). Their alignment is roughly perpendic-

ular to the North Solomon trench and thus provides

structural profiles across the main tectonic elements of

the trench system. There are no drill sites located

within or near the study area to correlate sequence

stratigraphic interpretations. Stratigraphic interpreta-

tions particularly for deeper horizons not well imaged

on our lines rely heavily on the interpretations of

deeper penetration and wider spaced MCS lines from

Phinney et al. (2004) that overlap our lines (Figs. 2

and 3). We use the Shipboard Scientific Party (1975,

1991 and 2001) reports and stratigraphic correlations

presented in Phinney et al. (1999, 2004) for inter-

pretations of seismic stratigraphy.

4.1. Seismic stratigraphy

4.1.1. Sequence OJP1

Based on seismic stratigraphy and interval veloc-

ities, the OJP imaged on all of the seismic lines is

broken into three sections: basaltic basement of early

Cretaceous age (OJP1) and two overlying sedimentary

units of Cretaceous through Recent age: Cretaceous to

Eocene chert–limestone section (OJP2), Eocene–

Recent pelagic section (OJP3) (Phinney et al., 1999;

2004) (Fig. 7) (Table 2). The base of Eocene chert–

limestone section is observed on all lines in this area.

The total sediment thickness is 650 m in the northern

area, thinning to less than 600 m in the southern area.

Based on spectral analysis of stacking velocities

(Table 1), the OJP1 seismic sequence is correlated

with early Cretaceous igneous basement of the

Ontong Java Plateau whose top horizon is charac-

terized by a high-amplitude continuous reflection

(Fig. 7). Velocities at depth exceed 3379 m/s (Table

1) and are characteristic of basaltic lithologies. The

basement reflectors dip to the southwest at 28 to 38and are traceable at all lines crossing the northern area

(Figs. 7 and 8). In contrast, the dip of basement

reflectors steepen to about 48 to 58 in the southern

area. In all seismic sections, the OJP1 sequence

consists of low-amplitude, subparallel, semi-continu-

ous reflectors with infrequent intrabasement reflectors

that are better imaged on deeper penetration seismic

data of Phinney et al. (2004) (Figs. 7, 8 and 9). The

interval thickness between the top of basement and

intrabasement reflectors ranges from 1400 to 1900 m

below the seafloor and thickens in a landward

direction.

4.1.2. Sequence OJP2

The base of sequence OJP2 is a late Cretaceous

unconformity expressed as a high amplitude and

semicontinuous reflector that is horizontal and

maintains a uniform sub-bottom depth of 140 m

(Figs. 7, 8 and 9). The high interval velocities of

OJP2 (averaging 2785 m/s) indicate a predominately

sedimentary section of interbedded chert and lime-

stone. Petterson et al. (1997) postulated that the

OJP2 passed through the CCD during deposition

based on an observed increase in carbonate content

toward the top of the sequence. The semi-

continuous reflector and multiple unconformities of

OJP2 suggest deposition of sequence OJP2 in a

relatively moderate energy marine environment near

a fluctuating CCD manifested by several dissolution

events. This erosional boundary separating OJP2

from overlying Eocene–Miocene pelagic and turbi-

ditic mudstone section of OJP3 was correlated by

Phinney et al. (1999) to the Kwaraae Mudstone

A. Taira et al. / Tectonophysics 389 (2004) 247–266262

Formation mapped by Petterson et al. (1997) on

Malaita Island. The top of sequence OJP2 exhibits a

southeastward dip with thinning of the unit as it

approaches the North Solomon trench.

4.1.3. Sequence OJP3

The OJP3 section can be subdivided into OJP3a,

OJP3b and OJP3c (Figs. 7, 8 and 9) (Table 2). OJP3a

and OJP3b vary laterally in thickness as well as in

velocity and reflection character. The average interval

velocity for OJP3a is 2146 m/s and OJP3b is 1735 m/

s (Table 1). Both OJP3a and b show time thicknesses

of over 300 ms. Within the MAP, velocities of OJP3a

increase up to 2200 m/s. Reflectors within OJP3a are

continuous and conformable relative to the under-

lying OJP2 sequence. The reflectors separating

OJP3b and OJP3c show an increase in velocity and

may represent an ooze/chalk transition. The OJP3c

sequence is the youngest sequence in the OJP

sedimentary cap and is known from drilling to

correlate with a Miocene–Recent pelagic shallow

marine section (Berger et al., 1992). In the shallow

sequence, average interval velocities of OJP3 are

1641 m/s. The conformable nature of reflectors

within sequence OJP3c suggests its deposition as a

pelagic drape in a low-energy marine environment.

Uneven and truncated reflectors indicate syndeposi-

tional ocean current scouring and multiple stages of

faulting. The thickness of sequence OJP3c decreases

from 135 to 90 m in a southwestward direction

probably as a result of the recent erosion by trench

fill sedimentation.

4.1.4. Trench-fill sequence

Late Neogene clastic sediment filling the North

Solomon trench is characterized by low-amplitude,

parallel reflectors which show an onlap relationship

with underlying OJP3c sequence (Fig. 11). The wedge

shape trench-fill is clearly recognized from seismic

lines 11–16 averaging 400 m in thickness.

Fig. 12. Comparison of main structures formed at the northern, central, a

presented in this paper. The horizontal distance is approximately 50 km in

(OJP2 and OJP3) is about 600 m. (A) In the northern area (cf. Fig. 6 fo

expressed by diffuse faulting and broad arching of the seafloor. The dec

section (Phinney et al., 2004). (B) In the central area, the decollement has p

propagation folding overlies blind thrusts that have propagated seaward (w

the decollement has penetrated the seafloor and formed an even larger scar

sided and asymmetrical, trench-fill basin. High-angle thrusts have propag

4.2. Surficial structures

The MCS lines were used to map the extent and

relationship between the accreted section of the MAP

landward of the North Solomon trench and the

subducting section of the OJP. In the northern area,

the MCS profiles reveal that the seafloor sedimentary

units in this area are cut by small faults with seafloor

offsets up to 50 m. Based on interpretation of

IZANAGI images, the fault trends of the northern

area between lines 1–2 trend NW–SE and are

colinear with the North Solomon trench to the south

(Fig. 6).

Most of the blocks labelled on Fig. 6 in the

central area contain numerous vertical offsets but

these faults are discontinuous and cannot be

confidently mapped between adjacent lines. The

central area south of approximately 6810VS is

characterized by a more continuous fault. The

Main Ridge of the North Solomon trench and its

adjacent thrust front is terminated to the northwest

by a tear fault striking ENE (Fig. 6). This fault

separates the diffusely faulted OJP section in

northern area from the area of more pronounced

convergent deformation to the south (Fig. 9). This

fault forms a very steep scarp with a 90-m-high

vertical offset of the seafloor and associated

erosional truncation. In this same area, some

seismic profiles show an intensely faulted basement

ridge rising up to 300 m (Fig. 9).

Fault propagation folds overlying blind thrusts are

well imaged on seismic section profile Line 7 (Fig.

10). Blind thrust zones are well developed within the

OJP3b and OJP3c sections. The blind thrusts extend

3–37 km seaward of the toe of the trench slope.

Elongate highs on the seafloor are interpreted be

small-ramp anticlines above the blind thrusts, which

are propagating seaward into the fill of the North

Solomon trench. A small-ramp anticline with land-

ward vergence is inferred as associated with back

nd southern areas of the North Solomon trench based on the results

each figure and thickness of Ontong Java Plateau sedimentary cover

r location), the decollement has not penetrated the seafloor and is

ollement is mainly present in the upper part of the oceanic plateau

enetrated the seafloor and formed a prominent scarp. An area of fault

est–northwest) in units OJP3b and OJP3c. (C) In the southern area,

p that has tilted the downgoing Ontong Java Plateau to form a steep-

ated the surface in the trench fill.

A. Taira et al. / Tectonophysics 389 (2004) 247–266 263

A. Taira et al. / Tectonophysics 389 (2004) 247–266264

thrusts judging from topography and the nature of

reflector offset (Fig. 10).

In the southeastern area, the trench and associated

ridge of the North Solomon trench become more

prominent than observed in the central area. From the

axis of the North Solomon Trench (more than 3500 m

in depth) to the foot of the southeastern Main Ridge,

the bottom topography is rough and is characterized

by a discontinuous succession of ridges elevating the

seafloor in three levels from 3500 m up to 2250 m

(Fig. 6). These ridges are underlain by deformed

sediments more than 800 m thick (Fig. 11). The

thickness of the sedimentary section here obscures the

depth to the top of the basement horizon of OJP1. The

deformation of the sedimentary section, as in all other

areas of the trench, extends upward to the seafloor.

Farther to the east, the southwest dipping trench outer

slope is underlain by about 1100 m of sediment

overlying an undisturbed basement (Fig. 11). The

trench slope break dominates the deformation front in

central area to seaward throughout study area, and

continues along the length of the trench, where image

a distinctive set of three, 8–10-km-wide, trench-slope

faults are easily seen in the IZANAGI backscattering

image.

5. Discussion and conclusions

The serial cross sections provided by multi-

channel seismic profiling (Figs. 7, 8, 9, 10 and

11) combined with the IZANAGI backscattering

imagery and bathymetry (Fig. 6) provides a time

series evolution for the development of the North

Solomon trench. The main evolutionary stages

Fig. 13. (A) Schematic regional cross section modified from Cowley et a

where a bathymetric trench separates the Malaita accretionary prism (San

Java Plateau (line of section shown on Fig. 2). The top of the downgoing O

lines shown by Phinney et al. (2004) and is instead inferred from earth

convergence is highly oblique to the plane of these sections and is partial

KKK fault zone. The synclinal structure of the Central Solomon intra-ar

regional cross section across the northern part of the North Solomon trenc

Malaita accretionary prism (Choiseul structural domain) from the obliquel

Dark gray area represents oceanic plateau crust of the Ontong Java Plateau

arc. The top of the downgoing Ontong Java Plateau is based on deeper

synclinal structure of the Central Solomon intra-arc basin (Bougainville sub

two subduction zones.

include (1) the incipient trench in the northern area

marked by a diffuse zone of deformation above a

broad arch in the crust (Fig. 12A). Deeper pene-

tration profiles by Phinney et al. (2004) show the

bulge is related to a deeper decollement fault that is

propagating upward and seaward through the crust.

(2) The formation of a continuous thrust front in the

central area (Fig. 12B). Deeper penetration profiles

by Phinney et al. (2004) show this thrust front is

surface expression of the same decollement present

at depth to the north. The boundary between the

surface trace of the thrust and the diffuse area of

deformation in the northern area is a vertical, high-

angle tear fault with left-lateral offset. (3) The

formation of a deep, elongate trench which controls

slumping around the steep edges of the trench fill

basin (Fig. 12C).

The level of detachment in the trench fill basin of

the North Solomon trench is variable. In the central

area, the decollement penetrates into the upper

sedimentary part of the trench fill basin (Fig. 12B)

while in the southern area thrust faults appear

steeper and rooted in the underlying basement

(Fig. 12C).

The areas to the southeast are those that have

undergone convergence for the longest period of time

and therefore show better developed trench structures

and a reduced width of the MAP (Fig. 13A). Areas to

the northwest have undergone convergence for a

shorter period of time and show less developed trench

structures and a wide area of the incipient MAP (Fig.

13B). Deeper penetration imaging by Phinney et al.

(2004) shows that a subhorizontal decollement exists

beneath this area of a relatively flat and undeformed

seafloor.

l. (2004) across the north-central part of the North Solomon trench

ta Isabel structural domain) from the obliquely converging Ontong

ntong Java Plateau was not observed on deeper penetration seismic

quake hypocenters from the ISC database (open dots). Note that

ly accommodated by active left-lateral slip along the trench-parallel

c basin (Mborokua and Russell basins) is apparent. (B) Schematic

h that is currently evolving into a bathymetric trench separating the

y converging Ontong Java Plateau (line of section shown on Fig. 2).

; light gray pattern represents island arc crust of the Solomon island

penetration seismic lines shown by Phinney et al. (2004). Note the

-basin of Cowley et al., 2004) that is actively shortened between the

A. Taira et al. / Tectonophysics 389 (2004) 247–266 265

A. Taira et al. / Tectonophysics 389 (2004) 247–266266

Acknowledgements

The authors thank the captain and crew of the RV

Hakuho Maru for their expert assistance during the

KH-98 cruise to the Ontong Java Plateau. We thank H.

Tokuyama, S. Saito, K. Mochizuki, F. Yamamoto, T.

Kanehara and K. Shimizu for discussions and assis-

tance during the acquisition and processing of the

seismic and IZANAGI sidescan sonar data. Special

thanks are extended to M.G. Petterson and other

anonymous reviewers of this paper for significant

improvement of the manuscript. This paper formed

part of a master’s degree by R. Rahardiawan com-

pleted at the Ocean Research Institute and supported

by a fellowship from the INPEX Foundation. UTIG

contribution no. 1670.

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