Geology and Offshore Resources of the Solomon Islands, Joint Cruise Report

GEOLOGY AND OFFSHORE RESOURCES OF

THE SOLOMON ISLANDS

JOINT CRUISE REPORT •

R/V S. P. LEE 1982 ,

Editors

J. G. Vedder

and

K. S. Pound

1984

U.S. Geological Survey

345 Middlefield Road

~nlo Park

california 94025 U.S.A.

'!'ABLB OF COIfl'BHTS

Introduction to Geology and Offshore Resources of the Solomon Islands J. G. Vedder, F. I. Coulson 1

PART 1

Navigation for CCOP!SOPAC Cruise, Leg 3, Solomon Islands W. C. Steele, K. L. Kinoshita 17

Submarine Topography of the Solomon Islands Region T. E. Chase, B. A. Seekins, K. E. Lund 18

Single-Channel Seismic, Uniboom, and 3.5-kHz Systems Used in Solomon Islands D. L. Tiffin .20

Multichannel Seismic Operations for CCOP/SOPAC Cruise, Leg 3, Solomon Islands D. M. Mann 22

Wide-Angle Seismic Reflection and Refraction Data from the Solomon Islands A. K. cooper, R. A. Wood 24

Sampling Methods, Solomon Islands J. B. Colwell, J. G. Vedder 30

PART 2

Geology of the central and western Solomon Islands F. I. Coulson, J. G. Vedder 36

Correlation of Rock Units in the Solomon Islands K. S. Pound 67

Regional Offshore Geology of the Solomon Islands J. G. Vedder, F. I. Coulson 76

Tectonics of the Southeastern Solomon Islands: Formation of the Malaita Anticlinorium L. W. Kroenke, J. M. Resig, P. A. Cooper 88

Tectonic Implications of Seismicity Northeast of the Solomon Islands P. A. cooper, L. W. Kroenke, J. M. Resig 95

Gravity Anomalies of the Solomon Islands Between 1560 and 161°E Longitude L. A. Beyer 101

ii Crustal Structure of the Solomon Islands Intra-Arc Basins from Sonobuoy seismic studies A. K. Cooper; T. R. Bruns, R. A. Wood 112

Deep Structure of the central and Southern Solomon Islands Region: Implications for Tectonic Origin A. K. Cooper, M. S. Marlow, T. R. Bruns 127

seismic Stratigraphy and Structure of sedimentary Basins in the Solomon Islands Region T. R. Bruns, A. K. Cooper, D. M. Mann, J. G. Vedder 142

Description and Interpretation of Dredged Rocks, Solomon Islands J. B. Colwell, J. G. Vedder 168

PrelLminary Descriptions of Gravity Cores from New Georgia Sound J. B. Colwell 177

Recent Depositional Patterns in the central Solomons Trough of the Solomon Islands J. B. Colwell, D. L. Tiffin 178

Foraminiferal Stratigraphy and paleobathymetry of Dredged Rock, R/V s. P. Lee Cruise, Solomon Islands J. M. Resig 184

Elevation of the Pacific Province, Solomon Islands, at the Pacific and Indo- Australia Plate Boundary J. M. Resig, L. W. Kroenke, P. A. Cooper 188

Source-Rock Evaluation of OUtcrop Samples from Guadalcanal, Malaita, and the Florida Islands, Solomon Islands B. Buchbinder, R. B. Halley 195

Offshore Petroleum Potential, Solomon Islands J. G. Vedder, T. R. Bruns 202

Summary of the Geology and Offshore Resources of the Solomon Islands J. G. Vedder 214

iii IN'l'RODDCTION TO GEOLOGY AND OFFSHORE RB$CXlRCKS OF THE SO:t.C:K)N ISLANDS

J. G. vedder U. S. Geological Survey, Menlo Park, California, 94025

F. r, Coulson Institute of Geological Sciences, Nicker Hill, Keyworth, Notts, England

PURPOSEANDSCOPE

This report is intended to serve a dual purpose: to review the geology of the Solomon Islands and to present new findings from a multinational marine surv-ey in 1982•.. ,A brief summary of geography •. hiStory, and culture proVides additional background on the physical features of the islands as well as their socia-political aspects. A two-part format is used for the entire report. The first part consists of general descriptions of shipboard operations, data acquisition and processing, analytical methods and tabular material. Large bathymetric maps that accompany the first part are included in a separate packet. The second part contains topical and interpretive papers. Because each contribution to the second part is intended to stand alone, some background information reappears throughout, particularly that concerning regional tectonics. Collectively, these reports are a direct outgrowth of a program of marine geoscience and mineral resource investigations arranged under the auspices of the ANZUs (Australia, New Zealand, United States) Tripartite Agreement with CCQP/SOPAC(United Nations Committee for the Coordination of Joint Prospecting for Mineral Resources in South Pacific Offshore Area). The U.S. Department of State was instrumental in negotiating the agreement and establishing the support for cooperative research. Funding for implementation and operation was ?rovided by the Office of Energy of the United States Agency for International Development (AID). Additional funds were furnished by the Australian government. The various facets of research were coordinated by the office of CCOP/SOPACin FiJi. Scientific personnel were supplied by the U.S. Geological Survey, Australia, New Zealand, and other CCOP/SOPACmember nations. The investigations were made aboard the U.S. Geological Survey R/V ~ l:!§. during 23 days at sea. The cruise began at Honiara, Guadalcanal, Solomon Islands on May 19, 1982, and concluded at Rabaul, Papua NewGuinea on June 11, 1982. The principal study area was the Central Solomons Trough; although additional tracklines were run across adjoining basins, intrarc ridges and flanking trenches in order to help resolve regional structure.

Veddex , Coulson: Introduction 1 GEOGRAPHY, HISTORY ANDCULTURE

Geographically, the northwest-trending Solomon Islands archipelago (Fig. 1) stretches across more than 900 km of the southwestern Pacific OCean between 5"S and 11"s latitude, and 154"E and 163"E longitude. As a political eneiey, however, the Solomon Islands are spread over a considerably different area. The Eastern Outer Islands, which are geologically related to the Fiji Basin and the Vanuatu (formerly New Hebrides) archipelago, are governmentally managed by the Solomon Islands. Bougainville, which is geologically part of the Solomon Islands, is administered by Papua NewGuinea. Tectonically, the Soloman Islands form a segment of the Melanesian island-arc complex, which extends as a sinuous belt southeastward from the islands of the Admiralty Group in Papua New Guinea through Vanuatu and Fiji to Tonga and the Kerrnadec Islands. Names that have been applied to this region include Outer Melanesian Zone (Glaessner, 1950), Melanesian Re-entrant (Coleman, 1970), and Melanesian Borderlands (Coleman and Packham, 1976). The six large islands of Choiseul, New Georgia, Santa Isabel, r.uadalcanal, Malaita, and San Cristobal form two chains that are separated by cw Georgia Sound at the northwest and Indispensable Strait at; the southeast. Smaller islands within the double chain include Nggela (Florida Is-lands-), Russell Islands, Ulawa, and Uki. Each of the large islands is elo~- gate and oriented northwest except Guadalcanal, which is broadly sigmoidal and oriented nearly east-west; and Malaita, which has a north-northwest trend. The islands of the northeastern chain are arranged in a left-stepping echelon pattern. OUtlining the northwest end of NewGeorgia Sound are the Shortland Islands, the largest of which are Alu, Fauro, Mono, oveu , and Cerna. Nearly 400 Jan of open ocean separates the main islands from the Easeern OUter Islands, which include the santa Cruz group (Nendo, Vanikolo, Utupua, Reef Islands, Duff Islands). Still farther east are Tikopia, Anuta, and Fatutaka, which lie within the North Fiji Basin. None of these eastern islands are described in this report. The large atoll of Ontong Java is about 275 km north of santa Isabel, and remote Sikaiana is about 200 kJn east of northern Malaita. The low-lying islands of Bellona and Rennell are about 175 km south of GuadalcanaL Guadalcanal is the largest (-6,000 kJn2in area - 150 km long, 45 km wide) and highest (Mt. Makarakombuo, 2,447 m altitude) of the Solomon Islands. The total land area of the Solomons is estimated to be about 27,750 smz , The ••ndigenous population is approximately 235,000: a number that is expected to double by the year 2000. Most of the large islands have steep relief and are covered by tropical rain forest, except for the grasslands along the northern coastal belt on GuadalcanaL The archipelago is outside the usual hurricane belt and lies within a zone of oceanic equatorial climate, where temperatures usually range between 22" and 32"C and the humidity averages about 80 percent. Southeast tradewinds generally prevail between March and December. As much as 635 cm of rain per year falls on the southern coast of Guadalcanal (Hackman, 1980). Although sea and air services link the main islands, limited roads make travel on the islands difficult. Beyond the influence of the larger towns, Melanesian tribal culture dominates. The earliest inhabitants probably moved into the archipelago more than 3,000 years ago. First contact with Europeans was in February 1568 when a two-ship Spanish expedition under the commandof Alvara de Mendana de Neyra made landfall at Santa Isabel and spent the next six months exploring some of the larger islands. An attempt at colonization on a second voyage in 1595

Vedder, Coulson; Introduction 2 ended in failure when Mendana died, and the encampment at Graciosa Bay on Nendo was abandoned. Other early explorers were Quieros (1595-1606), Carteret ( 1767) , Bougainville ( 1768) , Surville ( 1769) , Short land ( 1788) , O'Entrecasteaux (1791), and La Perouse (1799). Visits by Europeans virtually ended until the period between 1840 and 1860, when the trochus shell and sandalwood trade, together with whaling, brought ships to the islands. The arrival of planters (after 1870), blackhirders (slavers) between 1850-1900, and missionaries (after 1850) had severe cultural impact on the native populace. Up to 1920, the population was sparse and actually declined in certain areas as a result of blackbirding, introduced diseases, and tribal head- hunting. Since World War II, the number of Melanesians liVing in the islands has increased more than twofold. English and pisin (pidgin) are the languages commonly used although numerous dialects are spoken on individual islands. The Solomon Islands were a virtual political nonentity within the sphere of influence of both Germany and Great Britain until 1893. At that time, the British established the Solomon Islands protectorate, which included only the southeastern part of the group. In 1900, santa Isabel, Choiseul, and Ontong Java were transferred by the Germans to the British Protectorate. In May 1942, some of the islands were occupied by Japanese troops, but most were driven out after heavy fighting against American forces between August 1942, and February 1943. Remaining Japanese soldiers starved in the jungle or WP~'3 killed or captured by New Zealand troops in 1944 and 1945. After the war, a self-governing movement called "Marching Rule" developed in the islands, but was suppressed by the British in 1947. On July 7, 1978, the Solomon Islands became an independent member of the British Commonwealthand elected its oen parliament.

REVIEWOF GEOLOGICINVESTIGATIONS

Before the establishment of the British Solomon Islands Geological Survey in 1950, very little was known about the geology of the islands. In fact, the only substantive published account was that by Dr. H. B. GUppy, Royal Navy Surgeon (GuPPY, 1887), who visited the Short land Islands, Choiseul, the Florida Islands, San Cristobal, and seve in 1881 while serving on the hydro- graphiC survey vessel H.M.S. ~(Grover, 1965). Some of the kinds of diffi- culties experienced by early workers are graphically recounted by Grover (1965) in the following:

In 1896 the "Albatros" Expedition, sponsored by the Geographical Society of Vienna, landed on Guadalcanal. Eighteen armed sailors and scientists, ••••ith four beach natives as guides, crossed the Guadalcanal Plains, and five days later reached the steeper slopes of the sacred mountain 'raeuve , The local natives informed the newcomers of their belief that if anyone climbed the mountain of their Great Spirit all their people would die. To which the Austrians replied that they had come a long way to climb it, and could not go back without doing so. On the morning of the attempt the party was quietly surrounded while breakfasting, but as "great pity was felt for the white men who were about to die", the natives decided that the least they could do would be to let them fight on a full stomach. 1 They waited awhile for the party to split into two, before attacking simultaneously on a signal in

Vedder, Coulson: Introduction 3 overwhelming numbers: naked painted warriors ar~d with battle axes. One mate lot bravely hurled his attackers one by one over the nearby precipice until he was chopped down. Others went down beneath weight of numbers. The attackers were driven off. Five of the Europeans lost their lives: their leader, the brilliant Austrian geologist, Henrich Foullon von Norbeeck, badly wounded, refused medical attention so that his sailors might have it, and died before midday. And so ended the second attempt to study the geology of the Solomons.

1 Told to Grover in 1950 by the last survivor of the attackers, a very old blind man.

Onshore Geology Programs

The Geological Survey, which began with one geologist and later (1954) increased to three, initiated a program of reconnaissance geological mapping t a scale of 1: 200,000 in a collaborative effort with the University of Sydney. The first University of Sydney expedition started field work on Guadalcanal, Malaita, and Santa Isabel in 1950 and 1951 (University of Sydney, 1956; Rickwood, 1957; and Stanton 1961) and in 1954 was followed by a joint survey of Guadalcanal (Pudsey-oawson and Thompson, 1958; COleman, 1960a). In 1956, reconnaissance surveys of both San Cristobal (Thompson and pudse y-' Dawson, 1958) and the Florida Islands (Thompson, 1958) were completed. work on Choiseul began in 1957 (Coleman, 1960b) and in New Georgia in 1959 (Stanton and Bell, 1965). In 1962, after 12 years of collaborative effort, the early reconnaissance work was compiled into the first geologic map of the Solomon Islands (Coleman, 1965a, Coleman et al., 1965) at a scale of 1:1,000,000. A second edition showing additional geology, and corrections to the original geology was published in 1969. In 1963, after acquiring 1: 50, OOO-scale topographic maps of the protectorate, a regional geological mapping program commenced, initially on the island of Guadalcanal (Dennis and Hackman, 19721 Hughes, 1977a, 1977b; Hackman, 1979, 1980; Turner and HacJanan, in press). This regional program continued at a reduced level from 1963-1975. By the end of 1975, south Malaita (Hughes and Turner, 1976), the Eastern Juter Islands (Hughes et; ai , 1981), northwest San Cristobal (Jeffery, 1976), seve (Proctor and Turner, 1977), the Russell Islands and Mborokua (Danitofea and Turner, 19B1), western Florida (Taylor, 1977), and Ulawa (Danitofea, 1978) were mapped at 1:50,000. Commencing in 1976, the regional mapping program regained momentumas a British Technical cooperation Project, and senior staff were supplied by the Institute of Geological Sciences, United Kingdom. By 1979, geologic maps of the Shortland Islands and Choiseul were finished (Ridgway and Coulson, in press). Mapping of the New Georgia group was completed in 1983 (Dunkley et aI, unpublished mapping). Upon completion of the New Georgia project in 1983, approximately 66 percent of the Solomon Islands was mapped geologically at a scale of 1:50,000. Santa Isabel, Malaita, san Cristobal and the eastern Florida Islands remain to be surveyed and constitute a significant gap in the knowledge of island geology. Of particular importance is the need for detailed mapping of santa Isabel, where two tectonostratigraphic terranes apparently are juxtapose d.

Vedder, Coulson: Introduction 4 Complementing the mapping program, topical research has been done 'at a number of .•••or-ke z ej for example, paleontology (Coleman, 1965bi Coleman and McTavish, 1964; Hughes 1982), ultramafic petrology (Stanton and Ramsay, 1975; Neef and Plimer, 1979), alnoites (Nixon and Coleman, 1978; Nixon, 1980) and isotopic dating (Richards et aI, 1966; Snelling et aI, 1970; Neef and McDougall 1976).

Onshore Geophysics Programs

The first gravity survey, made in 1960 across the plains of north-central Guadalcanal, discovered a broad gravity high that strikes north .•••ard and has gradients of as much as 9.5 m gal/km on its eastern flank (Coleman and Day, 1965). In 1961, T. S. Laudon of the university of Wisconsin linked the Solomons airfields into the .•••orld gravity survey net .•••ork and subsequently undertook a regional land graVity survey (Laudon, 1968). From 1965 to 1968, a major aerogeophysical survey .•••as made as a joint venture of the United Nations Developnent Program and the Government of citish Solomon Islands. In addition to the aerogeophysics, projects in photogeology, reconnaissance stream-sed1ment geochemistry, and follOW'up geological and geophysical investigations were contracted 'at the United Nations to ABEM Company of Stockholm, Sweden, .•••hich combined airborne magnetometer, electromagnetometer, and scintillometer surveys (ABEM,1967). A total of about 40,000 line kilometers .•••as flown, and about 2,800 line kilometers of inter-island magnetometer survey .•••ere added in order to establish the regional magnetic field. The line spacing was mostly 400 m, in some areas 800 m, and mean terrain clearance was 125 m, A regional interpretation of the results was published by Winkler (1968).

Offshore Geophysics Programs

Geophysical surveys and drilling in the seas adjacent to the Solomon Islands have been carried out since 1964 by a variety of naval, institutional, and oil company vessels. After the discovery of oil seeps in Tonga in 1968, the petroleum industry began marine geophysical investigations in the Solomons region. The Tongan occurrence stimulated hydrocarbon exploration on shallo .•••- .narine shelves throughout the southwest Pacific, .•••here buried carbonate reefs were the prospective targets. Surveys were made either 'at "ships of oppor- tunity" transiting bet .•••ee n southeast Asia and the Tonga-Fiji region, or by world-wide reconnaissance programs such as those of Mobil, Gulf, and Shell. Commercial and CCOP/SOPACsurveys in the Solomons after October 1969, are listed in Table 1. Selected marine seismic data have been integrated into topical syntheses of parts of the Solomon Islands (navenne et a L, 1977; Katz, 1980; Maung and Coulson, 1983), but no work of a comparable nature has been published for offshore magnetic and graVity data. Following a short test run of a gravity meter aboard the USS WANDANKin 1964 (Rose et a L, 1968), a cnree-ecnen marine gravity survey was run by the HMSDAMPIERin 1965. In 1966, the Hawaii Institute of Geophysics launched a three-ship seismic-refraction survey in the SolomonS Sea (Woollard et a L, 1967). The R/V CONRADand the R/V VEMAalso did geophysical work in the region during 1966 and 1967 (Ewing et aI, 1969; Houtz et aI, 1968). The findings of these early cruises gave impetus to investigations of the Ontong Java

Vedder, Coulson: Introduction 5 Plateau in 1967, 1968, 1970, and 1971 (Kroenke, 1972, Fig. 13). As part of the Deep sea Drilling Program, the D/S GLOMARCHALLENGERdrilled Site 64 on the Ontong Java Plateau in 1969 (Winterer et aL, 1971) followed by two holes at Sites 2S7 and 288 during Leg 30 in 1972. Site 286 in the Coral Sea also was dri}.led on Lag 30 (Andrews et aI, 1975). In 1971, the R/V CHAINof Woods Hole Oceanographic Institution cruised the WOodlark Basin region (Leg S of Cruise No. 100). In 1972, HMSHYDRA conducted a detailed bathymetric-gravity-magnetic survey of New Georgia Sound~ as well as Bougainville and Manning Straits. ORSTOMfrom Noumea carried out a seismic survey using AUSTRADECIII IN 1975. As part of the Solomon Islands work prOgram of the Committee for Coordination of Joint Prospecting for Mineral Resources in South Pacific Offshore Areas (CCOP/SOPAC),two marine geophysical cruises were undertaken in 1979 and 1981 using the vessel MACHIAS. In 1982, two cruises were sponsored as part of a Tripartite Agreement between CCOP/SOPAC,Australia, New zealand, and the United States. The R/V S. P. LEE Leg 3 cruise was concentrated in New Georgia Sound (the "Slot") (Fig. 2), using seismic-reflection, refraction, -rr av.Ltiy , and magnetics systems. R/V KANAKEOKI investigations were done ..!inly in the Woodlark Basin and New Georgia regions, where heat-flow, seismic-reflection, bottom-sample, and magnetics data were collected.

REGIONALRELATIONS

Geomorphic Setting

The Solomon Islands archipelago generally is described as a fragmented island arc that is situated along the boundary between the Ontong Java Plateau/Central Pacific Basin and the Solomon Sea/Woodlark-Torres Basins (Fig. 3). An intra-arc basin separates the two chains of large islands that form the main part of the archipelago. This basin, called the central SOlomons Trough by Katz (19S0), is about 350 Ian long, as much as 90 Jan wide, and l,SOO m deep in its deepest part. To the south of the double island chain is a well defined but discontinuous linear trench that is more than 6,000 m deep Sa Jan south of san Cristobal and more than 8,500 m deep 110 Jan southwest of Bougainville. The western part of the trench is known as the New Britain ~rench, the eastern as the San Cristobal (South Solomon) Trench. The trench is Jbscure south of the New Georgia group where northeast-trending features in the Woodlark Basin abut the Solomon chain. The limits of the linear segment of the trench system are clearly defined by sharp bends in the Solomon Sea Basin west of Bougainville and in the Torres Basin southeast of San Cristobal where the San Cristobal (South Solomon) Trench meets the North New Hebrides Trench. South of the trench system, a complex area of troughs, basins, and ridges underlies the Coral Sea. To the north and east of the Solomon Islands archipelago. a less well defined trench system includes the Kilinailau, North Solomons, cape Johnson, and Vitiaz Trenches (Fig. 3). This system is as much as 6,000 m deep near the eastern end of the Solomons where it forms an acute-angle bend 50 Jan east of the island of Ulawa. Flanking this trench system on the north is the broad, anomalously shallow Ontong Java plateau.

Vedder, Coulson: Introduction 6 Tec~onic Set~ing

Complex interactions between the Australia-India plate and the Pacific plate created the sinuous chain of island arcs that extend from the Bismarck Sea of Papua ~w Guinea to Tonga and the Kermadec Islands (Coleman and Packham, 1976). At present, the Australia-India plate is moving northward at an absolute rate of 7 cm per year (Minster and Jordan, 1978) and is being consumed beneath the Pacific plate and its subsidiary microplates along a series of trenches that flank the southern and western sides of the island arcs. The Pacific plate has a northwestward absolute rate of motion of 10.7 cm per year (Minster and Jordan, 1978). The Solomon Islands are on a northwest-trending segment of the leading edge of the Pacific plate where the rate of oblique convergence may exceed 10 em per year (Johnson and Molnar, 1972). presumably a combination of subduction and sinistral shear accomodates this rapid convergence. Intense seismicity characterizes the northwestern and southeastern parts of the Solornon Islands arc where the trench system is well developed (Fig. 4). Conversely, seismic activity is much reduced along the central part of the arc Jpposite the Woodlark spreading center where the trench system is weakly expressed bathymetrically. Regional patterns of seismicity are reviewed and interpreted by Cooper et al (this volume). Another relatively -quiet zone between the main island group and the santa Cruz group is interpreted as a transform (Ravenne et aI, 1977, Fig. 6-2a). Clearly defined Benioff zones dip steeply northeastward in the vicinity of Bougainville and the Santa Cruz Islands (Figs. 4A, 4B). Near San Cristobal, the Benioff zone is not well defined and may be steeply inclined. Deep-seated earthquakes in the range of 500-700 kIn occur beneath Bougainville and the Santa Cruz group. It has been suggested that these deep shocks (Fig. 3) may originate along a detached lithospheric slab derived from an old, southwest-directed episode of subduction (Halunen and von Herzen, 1973; Kroenke, 1972). Early studies (Denham, 1969, 1971) indicated that seismicity could be ascribed largely to the northward movement of the Australia-India plate coupled wi~h an east-to-west shear along its northern edge. Interpretation of focal mechanisms of large earthquakes for the period 1963 to 1967 showed that slip vectors weze.vr-ouqh Ly orthogonal to the NewBritain Trench, a pattern that was not in accordance with simple underthrusting of the Australia-India plate oenea t.h the Pacific plate in the Solomons sea area (Ripper, 1970). In order to explain the pattern of hypccenee r-a and the direction of slip vectors in terms of plate tectonics, minor plates are believed to have been sandwiched along the boundary zone of the two major plates,the minor plates deriVing their relative motion from the collision. Different authors have identified various microplates, the names and locations of which vary. The nomenclature of these microplates is summarized below and in Figure 5.

1. Solomon sea plate (Johnson and Molnar, 1972; Johnson, 1979; Curtis, 1973; Ramsey, 1982), or simply the Solomon plate (Luyendyk et aL, 1973; Weissel et aI, 1982) lies between the Woodlark spreading ridge in the Solomon Sea and the NewBritain Trench. 2. South Bismarck plate (Johnson and Molnar, 1972; Connelly, 1976; Johnson, 1979; Taylor, 1979) is in the South Bismarck sea between the New Britain Trench and the Bismarck Sea seismic lineament (Denham, 1969; Ripper, 1975a, 1975b, 1977; Connelly, 1976; curtis 1973; Krause, 1973) and thus includes the Island of New Britain.

Vedder, Coulson: Introduction 7 3. North Bismarck plate (Johnson and Molnar, 1973) lies north of the Bismarck sea seismic lineament and south of the Manus Trench. 4. NewBritain plate (Curtis, 1973) is almost synonymous with the South Bismarck plate. 5. Manus plate (Curtis, 1973) lncludes the North Bismarck plate as well as the Solomon Island archipelago south of the Pacific Province of Coleman (1965a).

Crustal structure in the Solomons region was first investigated by Rose et a L, (1968). In the central Solomons, crustal thickness was estimated to vary from about 27 kIn under Indispensable Strait to 17 Ian southwest of Guadalcanal. From five other profiles across the entire Solomons/northern New Hebrides region, crustal thickness was estimated to vary from 9 to 29 Ian, the higher figures being associated with New Georgia Sound and Indispensable Strait. Thus, the Solomon Islands appear to have little or no "root", and crustal thickness seems to be similar to that of the oceanic region to the north. seismic refraction studies by Furumoto et al (1970) suggest a linear, "lock-like character for the Solomons and a crustal thickness that varies from . 5 to 20 Jan. The crust .beneath the Ontong Java Plateau is as much as 40 kIn thick (Furumoto et aI, 1976) but only 10 to 12 kIn in the Solomon Sea south of the arc. A complex set of relations including current underthrusting of the Pacific plate beneath San cristobal, the occurrence of deep remnant lithospheric slabs beneath the central part of the island chain, and underthrusting of the Pacific plate beneath Bougainville and San Cristobal was proposed by Denham (1975). Interpretations of crustal structure from sonobuoy, seismic, and gravity data (A. Cooper et; a L, this volume; Bruns et a L, this volume) indicate that the Central Solomons Trough contains three subsidiary basins in which as much as 6 Jon of Cenozoic sediment has accumulated. The sedimentary sequence consists of lew velocity (1.6-2.6 km/sec) strata in the upper part and higher velocity (2.8-4.4 kIn/sec) strata in the lower part. Volcanic rocks (5.0-5.5 kIn/sec) and lewer crustal rocks (6.2-7.0 km/sec) underlie the basins. On the basis of preliminary interpretations of seismic and gravity data, two different arc reconstructions are plausible. One implies a rootless arc in which high-velocity upper mantle rocks 12 to 15 Ian beneath the intra-arc basin are juxtaposed against deformed oceanic crust along the faulted northeast side of ';he arc. The other, which is preferred, suggests (1) low-density lithosphere under the Woodlark Basin, (2) a tongue of lithosphere subducted to a depth of 150 km beneath the modern arc, and (3) shallow, possibly relict mantle beneath the intra-arc basin, and (4) a remnant of subducted Pacific lithosphere faulted against old arc rocks. High heat flow is reported in the Woodlark Basin and Solomon Sea (Halunen and von Herzen, 1973; Macdonald et al, 1973; Taylor and Exon, 1983). However, heat flew values in the central Solomons Trough are low (Taylor and Exon, 1983). According to Coleman and Kroenke (1981) and Kroenke et al (this volwne) the northeastern flank of the Solomon Island arc is an abducted piece of the oceanic Ontong Java Plateau, and the central large islands are the remnants of an early Tertiary northeast-facing arc that collided with the plateau about 8 m.y. ago. The modern southwest-facing arc is marked by the late cenozoic volcanic centers that extend from Bougainville to cuada LcenaL, This postulated reversal in arc polarity (Karig and Ma.mmerickx, 1972; Halunen and von Herzen, 1973) presumably was a direct result of the older arc-plateau colli-

Vedder, Coulson: Introduction 8 s Lon, Coleman and Kroenke (1981) explained the absence of subduction-related volcanism east of Guadalcanal as an effect of cool, thick, depleted oceanic lithosphere of the Ontong Java Plateau being juxtaposed against the downgoing Australia-India slab and the resultant nongeneration of arc magmas. Dunkley (1983), however, stated that volcanism is not to be expected in the area of inactive subduction directly east of San Cristobal; the descending slab probably is not in contact with the lithosphere of the Ontong Java Plateau at depth; and the absence of volcanism in eastern Guadalcanal and San Cristoba"l does not represent an unusual gap in spacing of volcanoes along the entire arc.

ACKNOWLEDGEMENTS

To those energetic and prescient individuals who drafted and implemented the Tripartite Agreement that led to the R/V S. P, LEE cruise in the Solomon Islands, we owe our gratitude. Without the skillful supervision and enter- prising initiative of H. Gary Greene, program director, the cruise might not "

PARTICIPANTS

Listed below are the names of the scientists and technicians (and their position and shipboard responsibilities) who participated in the third leg or Solomon Islands portion of the three-leg Tripartite expedition:

Larry A. !\eyer, USGS, Menlo Park, california, U.S.A.; Geophysicist; navigation watch. Donna K. Blackman, USGS,Menlo Park, California, U.S.A.1 Physical Science Technician; multichannel watch. Terry R. Bruns, USGS, Menlo Park, California, U.S.A.; Marine Geophysicist; multichannel watch. GUy R. Cochrane, USGS, Menlo Park, California, U.S.A.; Physical Science Technician; multichannel watch. James B. Colwell, Bureau of Mineral Resources, Canberra, Australia; Sedimentologist; general geophysics watch. • Alan H. Cooper, USGS, Menlo Park, California, U.S.A.; Marine Geophysicist; Chief, sonobuoy watch. Frank I. Coulson, Chief Geologist, Solomon Islands Geological Survey; Geologist; general geophysics watch.

vedder, Coulson: Introduction 9 David J. 8ogg. USGS Marine Facility, Menlo Park, California, U.S.A.; Electronics Technician. Kay L. Kinoshita, USGS. Menlo Park, California, U.S.A.; Chief; navigation watch. Lawrence D. KOoker. USGSMarine Facility. Menlo Park. California. U.S.A.; Electronics Technician Loren W. Kroenke, Research Associate, Hawaii Institute of Geophysics; Geophysicist; general geophysics watch. Gregory Lewis. USGS. Menlo Park, California. U.S.A.; Physical SCience Technician; naVigation watch. Michael S. Marlow, USGS, Menlo Park. California, U.S.A.; Marine Geophysicist; Chief. general geophysics watch. J. Kevin O'Toole, USGS, Marine Facility, Menlo Park, California. U.S.A.; Marine Technician Donald L. Tiffin, CCOP/SOPAC. Suva; Marine Geophysicist; Co-chief Scientist. John G. Vedder. USGS. Menlo Park. California, U.S.A.; Marine Geologist; Co-chief Scientist. Paul A. Wenberg. USGSMarine Facility, Menlo Park, California, U.S.A.; Marine Technician. Raymond A. Wood, New Zealand Geological Survey. rower Butt, New zealand; Geophysicist; general geophysics watch. .,-

Other scientists participated by taking lead roles in post-cruise studies of collected data, or by contributing maps, charts or other information necessary for interpretation of the data collected. These lead contributors are:

Tau Rho Alpha - USGS, Menlo Park, California, U.S.A.; physiographic diagram. Binyamin Buchbinder - University of Israel. Jerusalem, Israel; onshore hydrocarbon study David Bukry - USGS. La Jolla, California, U.S.A.; calcareous nannofossils Thomas E. Chase - USGS. Menlo Park, California. U.S.A.; maps and charts Patricia Cooper. Hawaii Institute of Geophysics; tectonics Robert B. Halley - USGS. Denver, Colorado, U.S.A.; onshore hydrocarbon study. Katherine S. Pound - USGS, Menlo Park, California, U.S.A.; correlation of rock units.

Vedder, Coulson: Introduction 10 REFERENCES

A.B.E.M., 1967, Report on an airborne geophysical survey in the British Solomon Islands: Aktiebdag Elektriks Malmletning (Stockholm) 1965-1966, v. 1 and 2, 316 p. Andrews, J.E., G.H. Packham, and others, 1975, Initial Report~ of the Deep Sea Drilling Project: U.S. Government prining Office, Washington, D.C., v.30, 753 p. Coleman, P.J., 1960a, North-central Guadalcanal, an interim geological repcr-es British Solornon Islands Geological Record (1957-1958), v, " p. 4-13. ------1960b, An introduction to the geology of Choiseul in the western Solomons, 1957; British Solomon Islands Geological Record (1957-1958), v, 1, p, 16-26. ------1965a, Stratigraphical and structural notes on the British Solomon Islands with reference to the first geological map: British Solornon Islands Geological Record (1959-1962), v. 2, Report no. 29, p. 17-31. ------1965b, Tertiary assemblages of larger Foraminifera in the Solomon Islands and New Hebrides Archipelago: Contributions to the Annual Report, NewHebrides Geological Survey, p. 48-51. ------1970, Geology of the Solomon and NewHebrides Islands, as part of the Melanesian re-entrant, Southwest Pacific: Pacific Science, v, 24, p. 289-314. ------and A.A. Day, 1965, Petroleum possibilities and marked gravity anomalies in north-central Guadacanal: British Solomon Islands Geological Record (1959-1962), v, 2, p, 112-119. ------and L.W. Kroenke, 1981, Subduction without volcanism in the Solomon Islands arc: Geo-Marine Letters, v, 1, p, 129-134. ------and R.A. McTavish, 1964, An association of larger and planktonic Foraminifera in single sampLes from middle Miocene sediments, Guadalcanal, Solomon Islands, southwest Pacific: Journal of the Royal Society of Western Australia, v. 47, p. 13-24. ------and G.H. Packham, 1976, The Melanesian Borderlands and India-Pacific Plates' boundary: Earth-Science Reviews, v. 12, p. 197-233. ------et a L, 1965, A first geological map of the British Solomon Islands, 1962, .i£. Reports on the geology, mineral resources, petroleum possibilities, volcanoes, and seismiscity in the Solomon Islands: Record of the Geological Survey of the British Solomon Islands (1959-1962), -r, 2, Report no. 28, p. 16-17. Connelly, J. B., 1976, Tectonic development of the Bismarck Sea based on gravity and magnetic rnodeling: Geophysical Journal of the Royal Astronomical Society, v. 46, p. 23-40. Curtis, J.w., 1973, Plate tectonics and the Papua New Guinea-Solomon Islands region: Journal of the Geological Society of Australia,v. 20, pt. 1, p. 21-36. Danitofea, S., 1978, The Geology of Ulawa Island: Solomon Islands Geological Survey Bulletin No.4 (unpublished). ------and C.C. Turner, 1981, The geology of the Russell Islands and Mborokua: Solomon Islands Geological Survey Bulletin No. 12 (unpublished) • Denham, D., 1969, Distribution of earthquakes in the New Guinea-Solomon Islands region: Journal of Geophysical Research, v. 74, p. 4290-4299.

Vedder, Coulson: Introduction 11 ------1971, seismicity and tectonics of New Guinea and the Solomon Islands: Recent crustal movements: Royal Society N. 2. Bulletin No.9, p. 31-38. ------1975, Distribution of underthrust lithosphere slabs and focal mechanisms--Papua Ne••••Guinea and Solomon Islands region (aba.): Bulletin of the Australian Society of Exploration Geophysicists, v. 6, p. 78-79. Dennis, R.A., and B.D. Hackman, 1972, Geological map of Cape Esperance-Ndoma, Guadalcanal: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. DUnkley, P.N., 1983, volcanism and the evolution of the ensimatic Solomon Islands Are, in D. Shinozura and I. Yokoyama, ede , , Arc volcanism: physics and tectonics: Terrapub, Tokyo, p. 225-241. Ewing, M., R. Houtz, and J. Ewing, 1969, South Pacific sediment distribution: Journal of Geophysical Research, v. 74, p. 2477-2493. Furumoto, A.S., D.M. Hussong, J.F. Campbell, G.H. SUtton, A. Malahoff, J.e. Rose, and G.P. Wollard, 1970, Crustal and upper mantle structure of the Solomon Islands as revealed by seismic refraction survey of Nov-Dec 1966: Pacific Science, v, 24, p. 315-332. ------J.P. Webb, M.E. Odegard, and D.M. Hussong, 1976, Seismic studies on the Ontong Java Plateau, 1970: Tectonophysics, v. 34, p. 41-90 • .•.laessner, M.F., 1950, Geotectonic position of sew Guinea: American Association of Petroleum Geologists Bulletin, v. 34, p. 856-881. Grover, J.C., 1965, A brief history of geological and geophysical investigations in the British Solomon Islands 1881-1961: British Solomon Islands Geological Record (1959-1962), v. 2, Report no. 27, p. 9-15. GuPPY, H.B., 1887, The Solomon Is lands, their geology, general features, and suitability for colonization: S••••ann, Sonnerscheim, Lowrey and Co., London, 152 p. Hackman, B.D., 1979, The geology of the Honiara area, Guadalcanal: Solomon Islands Geological Survey Bulletin, no. 3, 40 p. ------1980, The geology of Guadalcanal, Solomon Islands: Overseas Memoirs of the Institute of Geological Science, Her Majesty'a Stationery Office, London, no. 6, 115 p, Halunen, A.J., and von Herzen, R.P., 1973, Heat flow in the western equatorial Pacific Ocean: Journal of Geophysical Research, v. 78, p. 5195-5208. Houtz, R., J. Ewing, and X. re Pichon, 1968, Velocity of deep sea sediments from Sonobuoy data: Journal of Geophysical Research, v, 73, p. 2615- 2641. ~ughes, G.w., 1977a, The geology of the IA.ir1'ggaBasin area, Guadalcanal: Solomon Islands Geological Survey Bulletin No.6 (unpublished). ------1977b, The geology and foraminiferal micropaleontology of the Lungga and Itina Areas, western Guadalcanal, Solomon Islands: Unpublished Ph.D. Thesis, University College of Wales, Aberystwyth. ------1982, Stratigraphic correlation between sedimentary basins of the ESCAP region, Volume VIII, Solomon Islands, ESCAPAtlas of Stratigraphy III: Mineral Resources Development Series, United Nations, New York, no. 48, p.115-130. ------P.M. Craig, and R.A. Dennis, 1981, The geology of the Eastern OUter Islands: Solomon Islands Geological Survey Bulletin No.4, 108 p. ------G.W., and C.C. Turner, 1976, Geology of southern Malaita: Solomon Islands Geological Survey Bulletin No.2, 80 p. Jeffery, D.H., 1976, The geology of north ••••estern San Cristobal, Uki Ni Masi and Pia, and the Three Sisters: Solomon Islands Geological Survey unpublished Bulletin No. 10.

Vedder, Coulson: Introduction 12 Johnson, R.W., 1979, Geotectonics and volcanism in Papua New Guinea; a review of the late Cainozoic: Bureau of Mineral Resources Journal of Australian Geology and Geophysics, v, 4, p. 181-207. Johnson, T., and P. Molnar, 1972, Focal mechanisms and plate tectonics of the southwet Pacific: Journal of Geophysical Research, v. 77, p. 5000-5032. Karig, D.E., and J. Mammerickx, 1972, Tectonic framework of the New Hebrides island arc: Marine Geology, v. 12, p. 187-205. Katz, H. R., 1980, Basin development in the Solomon Islands and their petroleum potential; CCOP/SOPACTechnical Bulletin No.3, p. 59-75. Krause, D.C., 1973, Crustal plates of the Bismarck and Solomon seas, in R. Fraser, compiler, Oceanography of the South Pacific: New zealand National Commission for UNESCO,wellington, p. 271-280. Kroenke, L.W., 1972, Geology of the Ontong Java Plateau: Hawaii Institute of GeophySics, Report no. HIG-72-5, University of Hawaii, 119 p. Laudon, T.S., 1968, Land gravity survey of the Solomon and Bismarck Islands, in The crust and upper mantle of the Pacific area: American Geophysical Union Mongram No. 12. Luyendyk, B.P., K.C. MacDonald, and W.B. Bryan, 1973, Rifting history of the Woodlark Basin in the Southwest Pacific: Bulletin of the Geological Society of America, v. 84, p. 1125-1134. Macdonald, K.C., B.P. Luyendyk, and R. Von Herzen, 1973, Heat flow and plate boundaries in Melanesia: Journal of Geophysical Research, v. 78(14), p. 2537-2546. Maung, T. v., and F.I. Coulson, 1983, Assessment of petroleum potential of the central Solomons Basin: CCOP/SOPACTechnical Report No. 26, 68 p. Minster, J.B., and T.H. Jordan, 1978, Present day plate motions: Journal of Geophysical Research, v. 83, p. 5331-5345. Neef, G., 1978, A convervent subduction model for the Solomon Islands; Bulletin of the Australian Society of Exploration Geophysicists, v. 9, p. 99-103. ------and I. McDougall, 1976, potassium-argon ages on rocks from Small Nggela Island, British Solomon Islands, s.w. Pacific: Pacific Geology, v, 11, p. 81-96. ------and I.R. Plimer, 1979, Ophiolite complexes on Small Nggela Island, Solomon Islands; summary: Geological Society of America Bulletin, part 1, v. 90, p , 136-138. Nixon, P.H., 1980, Kimberlites in the southwest Pacific: Nature, v; 287, p. 718-720. ----- and P.J. Coleman, 1978, Garnet-bearing Iherzolites and discrete nodule suites from the Malaita alnoite, Solomon Islands, and their bearing on the nature and origin of the Ontong-Java plateau: Bulletin of the Australian Society of Exploration Geophysicists, v. 9, no. 3, p. 103-107. Proctor, w.O., and C.C. Turner, 1977, The geology of save Island: Solomon Islands Geological Survey Bulletin No. 11, 44 p. Pudsey-oawson, P.A., and R.B. 'nlompson, 1958, The detailed geological survey of western Guadalcanal, 1954: Geological Survey of the British Solomon Islands Memoir No.2, p. 43-56. Ravenne, C., C.E. de Brain, and F. Aubertin, 1977, Structure and history of the Solomon-New Ireland region, l:.!:.. International symposium on geodynamics in the SW Pacific, lew caledonia, August-September 1976: Editions Technip, Paris, p , 37-50.

Vedder, Coulson: Introduction 13 Richards, J.R., A.W. Webb, J.A. Cooper, and P.J. Coleman, 1966, Potassium- argon measurements of the ages of basal schists in the British Solomon Islands: Nature, v, 211, p. 1251-1252. Rickwood, F.K., 1957, Geology of the Island of Malaita .i!!.. Geological Reconnaissance of parts of the central islands of the British Solomon Islands Protectorate: Colonial Geology and Mineral Resources, v. 6, no. 3, p. 300-306. Ridgway, J., and F.I.E. Coulson, in press, The Geology of Choiseul and the Shortland Islands, Solomon Islands: Solomon Islands Geological Survey Division Bulletin No. 16. Ripper, 1.0., 1975a, Some earthquake focal eechenfsme in the New Guinea- Solomon Islands region, 1963-1968: Bureau of Mineral Resources Australia, Report 178.

------1975b, Seismicity and earthquake focal eechani.ees in the New Guinea- Solomon Islands region (extended abstract), Bulletin of the Australian Society of Exploration Geophysicists, v. 6, p. 80-81. ------1977, Some earthquake focal mechanisms in the New GUinea/Solomon Islands region, 1969-1971: Report of the Bureau of Mineral Resources, Geology and Geophysics, Australia, no. 192. Rose, J.C., Wollard, G.P., and Malahoff, A., 196B, Marine gravity an~ magnetic studies in the Solomon Islands, in 'Ihe crust and upper mantle of the Pacific area: Monogram of the A;;rican Geophysical Union, no. 12, p. 379-410. Snelling, N.J., I.H. Ingram, and K.P. Chan, 1970, KlAr age determinations on samples from the British Solomon Islands Protectorate: Institute of Geological SCiences, Geochemistry Division, Isotope Geology Unit Report, no. 17-14. Stanton, R.L., 1961, Explanatory notes to accompany a first geological map of Santa Isabel, British Solornon Islands Protectorate: Overseas Geology and Mineral Resources, London, v. B, no. 2, p. 127-149. ------and J.D. Bell, 1965, New Georgia Group, a preliminary geological statement: British Solomon Islands Geological Record (1959-1962), v, 2, Report no. 31, p. 35-36. ------and W.R.H. Ramsay, 1975, Ophiolite basement ccep Lex in a fractured island chain, Santa Isabel, British Solomon Islands: Bulletin of the Australian Society of Exploration Geophysicists, v. 6, no. 2/3, p. 61-65. Taylor, B., 1979, Bism.q.rck Sea: evolution of a backarc basin: Geology, v. 7, p. 171-174. ------and N.F. Exon, 19B3, 1982 R,!V~ Keoki cruise in the Woodlark- Solomons region (abs.): Basic geo-scientific marine research requr re o for assessment of minerals and hydrocarbonsa in the South Pacific, A workshop, Suva, Fiji, October, 1983. ------1984, An investigation of ridge subduction in the WOodlark-- Solomons region: introduction and background, .i!!.. N.F. Exon and B.R. Taylor, compilers, Seafloor spreading, ridge subduction, volcanism and sedimentation in the offshore WOodlark-Solomons region and Tripartite cruise report for ~ Keoki cruise 82-03-16, Leg 4, CCOP/SOPACTechnical Report no. 34, p , 1-42. Taylor, G.R., 1977, Florida Islands Geological Map Sheet FL 1: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000.

Vedder, Coulson: Introduction 14 Thompson, R.B.M., 1958, The geology of the Florida Group, 1956, in The Solomon Islands-geological exploration and research, 1953-1956: Memoir of the Geological Survey of the British Solomon Islands, no. 2, p. 97-101. ------and P.A. Pudsey-Dawson, 1958, The geology of eastern San Cristobal: Memoir of the Geological Survey of the British Solomon Islands 1955-1956, v» 2, p. 90-95. Turner, C.C., and B.D. Hackman, in press, The geology of the Beaufort Bay area, Guadalcanal: Solomon Islands Geological Survey Bulletin No.9. University of Sydney, 1956, Geological reconnaissance of parts of the central islands of the British Solomon Islands Protectorate: Colonial Geology and Mineral Resources, v.6, p. 267-306. weisse I, J.K., B. Taylor, and G.D. Karner, 1982, the opening of the Woodlark Basin, subduction of the Woodlark spreading system, and evolution of the northern Melanesia since the mid-pliocene time: Tectonophysics, v, 87, p. 253-277. Winkler, H.A., 1968, Regional geophysical structure of the British Solomon Islands Protectorate: UN Special Development Programme Aerial Geophysical Survey Project Report (1965-1968), Solomon Islands Geological survey Report A12• •nterer, E.L., W.R. Reidel~ and others, 1971, Initial reports of the Deep sea Drilling Project, v.7., U.S. Government Printing Office, Washington, D.C. Wollard, G.P., and others, 1967, Crusie report on 1966 seismic refraction expedition to the Solomon Sea: Hawaii Institute of Geophysics, Report HIG-67-3, 31 p.

Vedder, Coulson: Introduction 15 TABLE 1 SUMMARYOF OIL EXf'LORATlQol IN rHE SOLOfl:lNISLANDS (!"ROM MA~G ANDCOIJt,.StW, 198J)

DATE SURVEYEDSY SURVEY'LOCATlQol L!NGTH O£SCRIPTIOl

OCt.ober- M"qell"n ~tl:'olewn Shol:'t.land Shelf 769 kill seismic ellrvey Novembel:' {AulSt.l:'alial Lt.d. Bouqainville Sl:.rait. (ail:'qun "nd ap"l:'ke>;) 1969 Manning St>;ait. di.cont.inllolls Indillpen •• ble strait. t.raver.ell Reqion bet.veen ~lai~a and Guacialcanal

19651·1970 Bataafse Int.el:'nat.ional. Short.lands Field examination of Pet.roleum ",aat.schappij Choiselll elCpolled.edilllentary l:'ocks N.v , sant.a babel Malait.a Guadalcanal

HIIy Shell Int.e>;nat.ional Short.lands, Nev Geol:'gia 1971 Pet.roleum Sound, Malait.a 2,764 kill Mult.ichannel .eismic reflection Indispensable strait., Maillet.OlIlet.er, Gravimeter San Cristobal, and RiIInnell I.land

Oct<:. W=stern Geophysical Manning St.rait. 141 kill 12-fold diqital ,lIiSlll.ic 1971 and Teledyne Explorat.ion l:'eflection International, Inc. for SOl:lthern Pacific - Petroleum

OCtober- Teledyne Exploration Shortlands 2,458 Ian seismic survey (sparker) November Company aouqainville St.l:'ait 1971 Oloiselll, Malai t.a santa Isabal, Ulawa Florida Islands N. eoalSt Guadalcanal

June Mobil Oil N. eoast $an Crietobal, 4,447 Ian Multichannel SlIismic retlettion 1972 Corporation Malaita, NE eoast Guadalc:anal SOnobl:loyseillmic refraction (7) NWand SWof Russell Islands Manninq Stl:'ai t N. eoast of OIoillllul Bougainv111e strait South of New Georgia

197· ""stral.ian Gulf Shortlands Shelf 2,480 kill S1nqle and multichannel seismic Oil COqpany Bouqainville Strait reflect.ion N. coast Oloiseul Sonobuoy refraction profile (1l New Georgia Sound Mailletometer, Gravimeter S. coast Santa Isabel RiIIqionbet ••.•en Malait.a, Guacialcanal, and san Cristobal

December Western Geophysical ;\;rea between Guadalcanal, 100 kill Multichannel seismic reflee-tion 1978 for seve , and the P.cifi~et'gy and Florida Islands Minerals, Ltd.

1979 CCOP/SOPAC New Geor••ia Sound 4,064 lcm Sinqle-channel seismic refleetion

t.rackline spacing 15-25 Ian ------",-y- CCOP/SOPAC Shortland blanda 5,500 lcm COPMultichannel (24 fold) June FVVs. P. LEE Leg' J New Georgi. Sound H1gh-resolution.uniboom seism.i.c 1982 Indispensable Strait. track line reflection Iron BottOlll Sound lIpacinq 3.5 kHz hiqh resolution seismic Mala1t.a fold belt 5·10 lcm reflection Ne" Georgia-Guadale-anal 12 kHz bat.hymet.ric profiles tor •• ee area Wlde-anqle 8ei~c reflection/ l:'efract1on SOnobouy profiles (J6) Maqnetometer, Gravimet.er

PART 1

This is the first of two parts of the Joint

Cruise Report. The purpose of the study, previous geologic investigations, regional geology, and other topics are summarized in the Introduction.

The remainder of Part 1 consists of general descriptions of shipboard operations, data acquisition and processing, analytical methods and tabular data.

Large bathymetric maps and profiles, physiographic diagrams and trackline plots are included in an accompanying map packet.

16 NAVIGATION FOR CCOP/SOPAC CRUl:SE, LEG 3, SOLOMAN'ISLANDS

w. C. Steele. L L. nnoshita U.S. Geological Survey, Menlo Park, California 94025

Navigational control of the survey was by satellite fixes. These fixes averaged about one every two hours and had an estimated accuracy of 8400 m, All fixes were integrated with dead-reckoning and radionavigation (bridge radar) • The computer processing was done both aboard ship and on shore. )igi tal navigation data were recorded on magnetic tape and were processed on shore by a multi-purpose Honeywell 68/80 MULTICSsystem. After completion of the cruise, the digital navigational data were demultiplexed to extract satellite data, naVigation shot points, operator requests and comments, and position updates. The position updates consisted of satellite-induced position corrections and position corrections based on radionavigational information. Allowable updates were restricted to satellites whose azimuths were between 15° and 75° and that had fewer than 4 itera- tions and a latitudinal standard deviation of less than .005 and a longitUd- inal standard deviation of less than .010. Navigation shot points were adjusted assuming linear deviations with time between adjacent position updates and not on satellites directly. Deviant navigational shot points based on noncruise-specific criteria were then edited using an interactive graphics editor. These data were plotted for review to determine deletions, omissions, and modifications based on cruise-specific criteria. Appropriate corrections were made, and naVigation and satellite fixes were statistically compared. The mean discrepancy between navigational and satellite fixes is 360 m. with a standard deviation of 569. A track chart was generated, a reduced and simpli- fied version of which is shown in Fig. 1. The long-term storage media for the naVigational data consist of: (1) microfilm of the Shipboard navigational computer output; (2) paper copies of naVigational logs prepared during the cruise; OJ magnetic tapes (digital data) of naVigation and operator commands during cruise; (4) magnetic tapes (digital data) of processed data. These data are kept in permanent U.S.Geological Survey library archives.

Steele, Kinoshita: Navigation 17

SUBMARINE TOPOGRAPHY OF THE SOI..CKJH ISLAHDS REGION

T.I. Cha.se, B.A. seekins. K:.B. ~d U.S. Geological Survey, Menlo Park, california 94025

INTRODUCTION

Included in the map packet that accompanies this report is a series of maps, physio.graphic diagrams, and bathymetric profiles of the Solomon Islands and adJacent regions. Sources of information, including that collected during 'the 1982 cruise of the R/V S.P. Lee (Cruise L7-82-SP), are noted on the maps and diagrams. Acquisition and pr~ssing of navigation and trackline information for this cruise is described by Kinoshita and Steele (this volume). Additional information regarding procedures used in preparing the maps, diagrams, and profiles listed below can be obtained by contacting the authors.

SUMMARYOF MAPS,DIAGRAMS,ANDPROFILES (see map packet)

Figure 1. Index map of submarine topographic maps and companion physiographic diagrams in the Solomon Islands and adJacent regions of the southwestern Pacific.

Figure 2. Submarine topography of the Vanuatu and southeastern Solomon Is- lands region. Contour interval is 200m (based on depth-corrected acoustic soundings). Detailed topography of the southeastern Solomon Islands region is shown in Figure 7.

Figure 3. Submarine physiography of the Vanuatu and southeastern Solomons region. This diagram is scale-matched to the topographic map of the same area (Figure 2) to assist the user in rapidly perceiving the general configuration of the sea floor.

Figure 4. Submarine topography of the Solomon Islands-papua New Guinea region. Contour interval 200m (based on depth corrected accoustic soundings). Detailed topography of the region is shown in Figures 10andl1.

Figure 5. Submarine physiography of the Solomon Islands-Papua NewGuinea region. This diagram is scale-matched to the topographic map of the same area (Figure 4) to assist the user in rapidly perceiving the general configuration of the sea floor.

Chase, Seekins, Lund: Topography 18 Figure 6. Detailed submarine topography of the southeastern Solomon Islands region. Contour interval is 100m (based on depth-corrected acoustic soundings).

Figure 7. Detailed submarine physiographic diagram of the southeastern Solomon Islands region. Companion sheet for Figure 6.

Figure 8. Detailed submarine topography of the northwestern Solomon Islands region. Contour interval is 100m (based on depth-correctea acoustic soundings).

Figure 9. Detailed submarine physiographic diagram of the northwestern Solomon Islands region. companion sheet for Figure 8.

Figure 10. Detailed submarine topography of the southeastern New Guinea region. Contour interval is 100m (based on depth-corrected acoustic soundings).

igure 11. Detailed submarine physiographic diagram of the southeastern Papua New Guinea region. Companion sheet for Figure 10.

Figure 12. Trackline map of R/V .g. ~ cruises L6-82-SP and L7-82-SP in the Vanuatu and southeastern Solomon Islands region.

Figure 13. Trackline map of the R/V ~ ~ cruise L7-82-SP in the northwestern Solomon Islands-Papua New Guinea region.

Figure 14. Detailed trackline map of the R/V ~ ~ cruise L7-82-SP in the southeastern Solomon Islands region.

Figure 15. Detailed trackline map of the R/V S.P. Lee cruise L7-82-SP in the northwestern Solomon Islands region.

Figures 16-20. Bathymetric profiles of the Solomon Islands-papua New Guinea region made from enlargements of microfilms of original bathymetric recordings. See Figures 12-15 for locations of profiles.

Chase, Seekini, Lund: Topography 19 SIlGLB-cHAHHEL SBISMIC, 'ORIEKX)M. AIm 3.S-kHz SYSTEMS USED IN SOLOfI)N ISLANDS

D. L. Tiffin U.N. ESCAP, CCOP/SOPAC,c/o Mineral Resources Dept., Suva, Fiji

INTRODUCTION

Single-channel airgun (SCAG)equipment, Uniboom, and 3.S-kHz systems were used on the USGS R/V S. P. Lee during the Solomon Island phase of the Tripartite program. These are only three of the wide selection of geophysical , systems carried on the ship. All geophysical data were acquired simultaneously.

SINGLE-eHANNELAIRGUNSYSTEM

The SCAGseismic-reflection system utilized the acoustic impulse generated by the rrultichannel seismic airgun array as the sound source. The tuned array consisted of five Bolt air guns totaling 1,326 in.3, towed from the after deck of the vessel. Reflections from bottom and sub-bottom were received on channel 24 of the multichannel hydrophone streamer (the channel closest to the ship), which was used as the SCAGreceiving array. Bottom and sub-bottom reflection signals from the streamer were recorded on a Globe Universal Sciences (GUS) multichannel seismic recorder, and, after a slight

delay I read off the tape by a read head before being amplified by a Geospace III seismic arrplifier. After passing through a Krohnhite band-pass filter, the signals were displayed on two Raytheon Line Scan Graphic 19-in. dry-paper recorders. Direct water arrivals were usually muted in the GUSrecorder. The SCAGsystem pz-ov.Lde d the necessary monitoring of the multichannel operation and of the GUS recording equipment. Because the multichannel seismic (MCS) array was fired at a rate of 16 - 19, the SCAGgraphic recorders were operated in a START-STOPmode in which the first few seconds of data follOWing the beginning of the shot were recorded, after which the recorders stopped to await the next shot. The sweep rate ordinarily was 4 sec, but along 13 lines in the western part of the area, a 5- sec sweep was recorded. Sweep delays were adjusted to de-emphasize the water column when necessary, and to record the maximum sub-bottom penetration, usually to and beyond the first bottom multiple. Band-pass filters were normally in the range of 15 to 101) Hz, rot sweep rates and filter settings were different for each recorder. Vertical exaggeration on the principal recorder was about 5.5 for 4-sec sweep times, and about 4.5 for 5-sec sweeps. The large sound source resulted in excellent penetration and good resolution, until the time of arrival of the bottom multiple which effectively

Tiffin~ Single Channel 20 obscured most SCAGarrivals beyond that time. Often, almost 2 sec of travel time in the sub-bottom was clearly recorded. The weather during the survey usually was fair and the seas calm, aiding in the recording of low-noise data. The single-channel airgun records were photographed aboard ship using a Polaroid camera mounT.edover one of the recorders, and overlapping photographs were trimmed and taped together to make a continuous record of each seismic line. These spliced photographs proved extremely useful for reference, discussion, and planning, particularly in selecting bottom sampling sites.

UNIBOOMHIGH-RESOLUTIONSYSTEM

The Uniboom system consisted of four hull-mounted EG&Gtransducers mounted back-to-back, with a total energy of 1,200 joules. The return signals were received by two side-by-side short hydrophone streamers (about 15 m long) towed just be Lew the water surface beside the Ship. The echoes were processed through a Raytheon Correlation Echo Sounding Processor (CESP II) amplifier, filtered outside the pass-band of 250 - 1,500 Hz, and recorded on a Raytheon. t a-c n, graphic recorder. Sweep rate was 0.5 second, firing was programmed according to depth, and a delay was adjusted to maintain the bottom and sub- bottom echoes w;lthin the 0.5 - second gat.e~ Exagger"ation on the records is apprOximately 10X. The Uniboom system consistently gave extremely clear and highly detailed records on the sub-bottom sediments to depths in excess of 100 m below the sea floor, even in water depths of 1.7 Jan in New Georgia Sound. The records reveal substantial and significant infomation on the Holocene near-surface sedimentary structures, discussed by Colwell and Tiffin (this volume), and Resig, Kroenke, and Cooper (this volume).

3.S-kHz SYSTEM

The 3.S-kHz system consisted of four hull-mounted transducers, all of which transmit the 3.S-kHz acoustic wave, and receive the returning echoes. The returns were processed and amplified with a Raytheon CESP II correlation processor using 25-ms or 100-ms delays on the cor-r-e Let or-, The output was displayed on a Raytheon 19-in. recorder using a 1-sec sweep.

The 3.S-kHz records show details of depositional features in the shallow sub-bottom, but penetration is limited, usually about 10 m or less, and on many lines does not provide mrcb more information than the 12-kHz echo sounder. After completion of the survey, the SCAGand high-resolution data records were photographed from the t s-dn, graphic record onto continuous 35-mm film strips. Copies were made on vellum and paper. Fifty-two lines were recorded on the SCAGand high-resolution seismic systems, as well as througl1-turns between nultichannel lines which were not recorded on the GUS system. A total of epproximaee Iy 3,500 krn of data was obtained in the Solomon Islands region.

Tiffin: Single Channel 21 JlDLTICBANHEL SBISMI.C OPERATIONS FOR CCOP/SOPAC CRUISE. LEG 3. SQLOtI)N ISLANDS

D. M. Mann U.S. Geological Survey, Menlo Park, California 94025

EQUIPMENTANDDATAFORMATS

Multichannel seismic-reflection data were collected on the U.S. ]eological Survey R/V S.P. lee using a Globe Universal Sciences Inc. (GUS) model 4200 seismic recording system, a Seismic Engineering (SEI) towed acoustic array, and a seismic source of five; Bolt airguns.

The acoustic array by SEI consists of 48 active sections, each 50 m long and each containing 30 Teleeyne MD-Shydrophones; two 66-m stretch sections at the head end, and one at the tail end; ten bird transducer sections spaced at regular intervals along the streamer; and a 476-ft. heavy lead!n cable to isolate the streamer from ship noise. The depth of the streamer sections is determined by transducers in each of the variable wing birds that are placed on the bird sections. A control panel in the multichannel lab allows each bird to be indiVidually controlled to a desired depth.

The data enters the multichannel lab through a deck leader, and into a SEI-DSSV signal conditioner/line equalizer. In this unit the signal strength is balanced to a set level to correct for the offset difference in each section of the streamer. The 48 sections are combined into 24 channels by a patch panel before the data enters the GUSrecording system.

Data enters the GUS recording system through a bank of gain ranging amplifiers, high (110Hz) and. low (5 Hz) cut filters, and an A-D (analog to digital) converter. The output is sampled every 2 ms by the MCU(Main Control Unit). Data is then recorded on AMPEX-DMAhigh speed tape transports in BDDR format, 14-track, 4000-ft reels at 8000 bpi. Each track is written separately with the odd tracks in the forward direction, and the even tracks in the opposite direction. A complete data record consists of a 256-bit preamble, 50 bit sync check, data words of 24 bits each plus a sign bit, and a 256-bit postamble. Quality of the data •.•.as monitored using a Raytheon LSR recorder for a near-field single-channel playout, and an SEI wiggle trace camera for all 24 traces. Seismic energy •.•.as provided by five Belt airguns of the following sizes (in cubic inches): 309, 466, 194, 194 and 148. Total volume was 1311 in3 at an air pressure of 1700 to 2000 psi. The airguns were timed to within 0.5 milliseconds of each other using a Litton Industries model LRS-100 gun timing computer and an oscilloscope to monitor the signals from sensors in the firing solenoids.

Mann: Multichannel 22 DATAPROCESSING

Data tapes were brought frorn the srip ta the U.S. Geological Survey Marine Multichannel Processing Facility in Menlo Park, California. The first stage of processing consists of reading the tapes on an AMPEXHDDRtape drive similar to those on the ship. Data is read into a Data General Model 2QO Eclipse computer which checks quality and prints out the header information. At this time the tapes were found to have many bad tracks due to tape drive problems on board the Ship. Using high-density 14-track tape drives allows for fewer data tapes to be written, but also causes a greater loss of data when problems occur. Many of the lines collected on the Solomon Islands Cruise have record gaps caused by bad tracks on the tapes which cannot be read with the present processing system. Arrangements are being made to have these tracks read, and most of the gaps will be filled in the future. Using the information provided by the header listings, an editor makes up a deck of processing parameters for the demultiplexing program. Some of the -ar emeeexs include the static delays, streamer geometry, water blanking, and various in-house infarrna,tion. The demultiplexing is also done on the Data General 200 Eclipse computer.'· Output tapes are 9-track, 1600 bpi, and in_~." Phoenix I format. Further processing is done on a Data General model 230 Eclipse computer. First the demultiplexed tapes are sorted to obtain COP (common- depth-point) data to be used in Velocity Spectrum Analysis. Data are also stacked using a set of brute velocities to give the analyst a seismic section to work with when picking spectrum Analysis. After the velocity file is made from this analysis the data are stacked into 24-fold CDP integer data and output on a 9-track Phoenix format reel. The stacked data can then be plotted an a Versatec model 2160 plotter. Post-stack plotting information is derived using the stack tapes and a Power Spectrum Analysis program which uses a Fourier transform routine on the data. 'n1e data in this paper have had a predictive deconvolution and a band-pass filter applied. The band-pass filter was 5-10, 5--60 (hz), and the deconvolution used a gap of 32 ms, an operator of 100 ee , and the derive gate length was 2000 ms, Final plotting was also an the Versatec model 2160 electrostatic plotter.

Mann: Multichannel 23 WIDE-A!CLE SEISMIC RBFLECTION AND REFRACTION DATA FROM THE SOLOMON ISLANDS

A. It. Cooper U.S.Geological Survey, 345 Middlefield Road, Menlo Park, Ca.

R. A. Wood New zealand Geological Survey, P.O. Box 30368, Lower Hutt, New zealand

INTRODUCTION

In May 1982, the U.S. Geological Survey (U.S.G.S.) recorded 35 seismic sonobuoy stations in the Solomon Island region and 1 sonobuoy in Rabaul harbor, New Britain of Papua New Guinea (Fig. 1, Table 1). The survey was part of a larger offshore hydrocarbon research project undertaken in cooperation with the united Nations Committee for Coordination of Joint Prospecting for Mineral Resources in South Pacific Offshore Areas (CCOP/SOPAC) • During the survey (U.S.G.S. cruise L7-82-SP), sonobuoy data were collected along geophysical tracklines, in conjunction with multichannel seismic-reflection, high-resolution seismic-reflection, gravity, and magnetic data. The purpose of this paper is to describe the data-collection, reduction, and interpretive procedures that were used for the sonobuoy data and to note some of the analytical methods and geologic factors that may introduce errors into the results. Based on these data, further studies have been done on the crustal structure of the Solomon Islands intra-arc basins (Cooper, Bruns, and Wood, this volume) and on the regional tectonics of the Solomon Islands region (Cooper, Marlow, and Bruns, this volume) Other sonobuoys, recorded in 1972 by Mobil Oil Corporation and by Gulf Oil Corporation (Mobil, 1972; Maung, 1983), were obtained from CCOP/SOPAC and interpreted for this study (Fig. 1, Table 1).

SONOBUOY DATA COLLECTION

Sonobuoy station sites were selected during the cruise on the basis of several criteria. Where possible, the sites were (1) in areas where geologic or bathymetric features were thought to be either flat or uniformly dipping over the length of the sonobuoy station, (2) in areas where the sedimentary section was believed to be thickest, and (3) in areas adjacent to islands where geologic units mapped onshore could be correlated with the acoustic units in the offshore section. Two types of seismic sonobuoys were used for the study. U.S. Navy sonobuoys (176 mhz, type 41-8) were used at all but one station; at station 31, a commercial REFTEK sonobuoy (76 mhz), with a lengthened antenna for

A. Cooper, Wood: Wide-Angle Seismic 24 extended range, was deployed. The offset range in which useful seismic data were recorded by the Navy sonobuoys averaged between 25 and 40 km; the offset of the REFTEl< buoy was about 35 km , Horizons at depths of 12 to 14 km and apparent refraction velccities of 7.42 to 8.2 kIn/sec were recorded at the greatest offset ranges. The seismic source was a 5-airgun tuned array with a total volume of 1,326 cu in. that was fired every 50 m (approximately every 17 sec) at an air pressure of 1800 psi. During normal station operations, the Navy sonobuoys were launched from the ship to a distance of 50 m abeam to avoid entanglement of the sonobuoy hydrophone in the multichannel seismic streamer. The wide-angle seismic data transmitted from the sonobuoy were recorded unfiltered on analog magnetic tape and were filtered and displayed in real time on a Raytheon Line Scan Recorder. The sonobuoy station ended when seismic returns were no longer visible in real time on the graphic recorder. Data on magnetic tape were later replayed at different filter and gain settings to enhance the lower frequencies, and to better display the deep refraction horizons. Real-time cords were normally recorded in the 5-64 hz frequency band and replays were filtered 5-30 hz.

SONOBUOY DATA REDUCTION

The data reduction procedures and computer algorithms used by the U.S.G.S. for the analysis of the wide-angle reflection and refraction data (Childs and Cooper, 1978) have been applied to the data both at sea for preliminary interpretation (Cooper and others, 1982; Tiffin and others, 1983), and onshore for final adjustments to the data (Table 2). A typical sonobuoy recorded over the sedimentary section in the New Georgia Sound is shown in Figure 3 with the interpretation added.

Re fraction Da ta

The refraction velocities for each sonobuoy record were determined by selecting and digitizing segments of the seismic first arrivals, fitting these egments by linear least square lines, and using the slope and intercept of the fitted lines to determine the thickness and velocity for each layer. Because a refractor is not generally observed for the sea floor, a hypothetical refractor with an assumed velocity (1.6 kIn/sec) was used. On most records, refractors with velocities less than 6.0 km/sec are seen as distinct linear segments, separated by sharp breaks in slope, that indicate distinct sub surface velocity boundaries. At higher velocities, the refractors are weak and often wavey, which indicates either lateral or vertical velocity gradients, or geologic structure at great depth. Where geologic structure is not evident on the seismic reflection data, curved refractions have been approximated by a series of linear segments. In the presence of geologic structure, only those refraction events occurring away from the structure are used. In the initial selection of refractors, only those seismic events that were easily identifiable were used; we did not attempt to establish regional continUity of refraction horizons. Later interpretation added picking weaker and second arrivals, correlating refractors between sonobuoy stations, verifying picks for the deeper high-velocity refractors, adding a refractor

A. Cooper, Wo~d: Wide-Angle Seismic 25 with an assumed velocity for the sea floor and applying slope corrections based on the dips of reflection horizons determined from the single-channel seismic-reflection profiles.

Wide-angle Reflection Data

Wide-angle reflection arrivals on sonobuoy records are hyperbolas with zero-offset times equal to the corresponding reflection horizons in the vertical-incidence seismic data. Interval velocities between selected reflection horizons have been determined by digitizing each of the hyperbolic wide-angle reflection arrivals, calculating the root-mean-square (rms) velocity for each hyperbola, and iteratively solving for the interval velocity between successively deeper pairs of hyperbola by the methods of LePichon et aI, 1968. Normally, the deepest reflection horizon used in the solution is the acoustic basement (velocity of 4.5-5.5 km/sec) and thus the interval velocities are all within the sedimentary section. Several criteria are used to evaluate and select the reflection hyperbola used for the interval velocity solutions: 1. The reflection horizon on the vertical-incidence seismic profile should be continuous and dip uniformly over the first 4-6 km of the sonobuoy station. This is the distance over which the wide-angle reflection is digitized and the rms velocity is determined. 2. Reflection horizons should be separated, in depth, by at least 200 to 300 m (0.2 to 0.3 sec two-way time); this is the general limit of resolution for stable velocity solutions in intermediate water depths (1,000-2,000 m (LePichon et al,196B). 3. When refractors are present, the wide-angle reflection that is associated with the refractor has been selected; if a refractor is not clearly associated with a wide-angle reflection then the closest identifiable reflector has been used. 4. Prominent reflection horizons, such as unconformities, tops of acoustic units, and basement reflectors, that are present in the vertical- incidence seismic profiles are also selected in the sonobuoy data. Slope corrections, based on correlations with reflection horizons identified in the single-channel seismic reflection profiles, have been applied to the data in areas of steep dip near local geologic structures and along the flanks of sedimentary basins. These corrections are generally small, normally less than 10%.

SOURCES OF ERROR

The sonobuoy velocity determinations for both wide-angle reflection and refraction data are affected by errors from three sources: field operations, data-reduction procedures, and geologic structures:

Field Operations

The largest field errors arise from uncertainties in the distance from the ship to the sonobuoy during the sonobuoy station. Variations in ship heading and speed, as wel+ as sonobuoy drift, preclude the use of navigational information for finding the exact range from the ship to the sonobuoy. This

A. Cooper, Woo,d: Wide-Angle Seismic 26 distance has been estimated by multiplying the direct-arrival (D-wave) travel time by the velocity of sound in water, a value derived from surface-water temperature measurements. Surface temperatures ranged from 29 to 3J degrees Celsius (acoustic velocities of 1,539 to 1,541 mise c) throughout the study area. If lower water temperatures (hence lower acoustic velocities) are present over the D-wave travel path, then the calculated velocities in Tables 2 will be low, in direct proportion to error in water temperature (about 0.13\ low per degree celsius). In most cases, this error has negligible effect on data accuracy.

Data Reduction Procedures

Errors in picking and digitizing the D-wave and the reflection and refraction arrivals yield the largest source of computational errors in the velocity solutions. Least-square calculations for refraction slopes and \ntercepts as well as reflection rms velocities are sensitive to these .igitizing errors and cause subsequent errors in the solutions for layer thicknesses and interval velocities. The magnitude of the error varies-- usually larger at higher ve·locities and greater depths--and is difficult to estimate1 comparison of velocities from a few buoys that have been digitized more than once indicate that the error is a 1-2 ,.

Geologic Structures

Sonobuoy solutions are based on the assumption that layers have uniform velocity, thickness, and constant dip over the extent of the sonobuoy station. These assumptions are violated when anomalous geologic features are encountered during the sonobuoy station. The largest uncertainties in the layer velocity and thickness determinations arise from the presence of geologic structures (such as faults, folds, basement uplifts, and intrusives), irregular basement relief, and lateral depositional variations (such as wedging, onlap, overlap, termination, and development of localized bodies such as reefs and channels). These geologic features manifest themselves on the sonobuoy records as disrupted, wavy, and offset reflectors and refractors, that often have associated sidelobes, crossovers, and diffractions. we minimized the errors from geologic structures by using the vertical- incidence seismic-reflection data to select reflection and refraction events, that are away from-obvious structures. The amplitude of Wide-angle reflections and refractions at large horizontal offset distances varies systematically with geographic area and sometimes causes uncertainty in the picks. In areas with volcanic-rich and limestone sediment, the sonobuoy seismic amplitudes are usually weak. The strongest amplitudes from the largest offsets are achieved along sonobuoy lines parallel with regional structures and in areas away from chaotic acoustic units (such as the southern Russell Basin). Amplitudes are diminished in part by structural relief, but mainly by lateral and vertical compositional variations. For the Solomon Island region, the type of sediment at the sonobuoy station appears to have a larger effect on the seismic amplitudes than the total thickness of sediment; strong basement refractors are often recorded beneath thick (5-6 km) sediment south of Santa Isabel Island away from active

A. Cooper, Wood: Wide-Angle Seismic 27 volcanic centers whereas weak refractors are found beneath thin sediment (2-3 krn) near the active New Georgia Island volcanos.

Other Corrections

Slope corrections have been made for dipping events. However, in the presence of large lateral variations, especially te~nation of layers, the slope corrections are not fully effective in correcting the velocities. The errors in refraction velocities caused by geologic structures generally increase with greater sub-sea floor depths because of a greater uncertainty in the velocity structure and unknown geometry of deep structures. For high- velocity sub-basement horizons (greater than 5.5 km/sec) the velocities are highly sensitive to small changes in the slope of the horizons. Consequently, the high-velocity refraction horizons have the greatest potential errors. A few sonobuoys and selected velocity solutions have been rejected because of equipment malfunctions, uncorrectable operational problems (such as major changes in course, speed, and airgun fire rate), poor data quality, and unmanageable geologic conditions. Field data have been collected on many of these buoys and in sorne cases are shown in Chase et aI, (this volume) and in reports by Cooper et aI, (1982) and Tiffin et aI, (1983). However, the final results are included here (Table 2) only for those sonobuoys that are deemed useable and reliable after all corrections have been applied.

SUMMARY

Wide-angle reflection and refraction data from seismic sonobuoys have been used to compute slope-corrected crustal velocities at 43 sites in the Solomon Island region shown on Figure 2 and 1 site near New Britain. velocities representative of sedimentary (v=1.6-4.2 krn/sec) and igneous (v-4.5-8.2 km/sec) rocks, at maximum sub-sea-floor depths of 14 km, were measured throughout the region. In general, the most reliable velocity and thickness solutions are derived from sonobuoy stations where crustal layers are flat lying and where concentrations of coarse debris in the overlying sedirrentary section are minimal.

A. Cooper, Wood: Wide-Angle Seismic 28 REFERENCES

Childs, J.R. and A.K. Cooper, 1978, Collection, reduction, and interpretation of marine seismic sonobuoy data: U.S.G.S. Open-file report 7f-442, p. 1- 219. Cooper, A.K., R.A. Wood, T.R. Bruns, and M.S. Marlow, 1982, Crustal structure of the Solomon Islands arc from sonobuoy refraction data: EOS, v, 63, p , 1120. Furumoto, A.S., D.M. Hussong, J.F. Campbell, G.H. Suttan, A. Malahoff, J.C. Rose, and G.P. Woolard, 1970, crustal and upper mantle structure of the Solomon Islands as revealed by seismic refraction of November - December, 1966 :Pacific Science, v. 24, p. 315-332. LePichon,X., J. Ewing, and R.E. Houtz, 1968, Deep sea sediment velocity determination made while reflection profiling: Journal of Geophysical Research, -r, 73, no. 8, p , 2597-2614. Maung, Tun U, 1983, Assessment of petroleum potential of the central Solomon basin: CCOP/SOPAC, Surna-Fiji, Technical report no. 26, p. 1-57. Mobil oil Corporation, 1972, Sonobuoy data, Open file, Geology DiVision, Ministry Natural Resources. Solomon Islands. Tiffin,D.L., J.G. Vedder, and A.Cooper, 1983, Multichannel seismic and geophysical survey of "The Slot" and adjacent areas in the Solomon Islands, CCOP/SOPAC, cruise report no. 71, p. 1-16.

A. Cooper, Wood: Wide-Angle Seismic 29

SAMPLING lCBTHODS, SOLOMON ISI.AJmS

J. B. Colwell Bureau of Mineral Resources, Geology and Geophysics, canberra, Australia

J. G. Vedder u. S. Geological Survey, Menlo Park, california 94025

INTRODUCTION

In order to correlate seismic stratigraphy with composition and age of ea-if Loor rocks, bottom. samples must be acquired. Time limitations, bcwever , precluded a thorough sampling program on Leg 3 of the cruise. Nevertheless, rocks and unconsolidated sediments representing some of the shallC7W seismic sequences were obtained at nine of the eleven stations occupied {Fig. 1, Table 1}. Sampling stations were selected by choosing previously noted areas of interest on the multichannel monitor records and then checking against the high-resolution seismic records for the best possible targets. Final site selection also took into account the time available on site and the distance between stations. Because of the widespread occurrence of Virtually flat- lying sediments in the deeper parts of New Georgia Sound, the dredge sites were restricted to the margins of the basin. However, a horst-like feature in the central part of New Georgia Sound between Kolombangara and Choiseul was sampled during ~ Keoki tripartite cruise KK82-03-16-4 (Exon and Taylor, 1984, Figure 1).

SHIPBOARDANDPOST-CRUISELABORATORYTECHNIQUES

Cored and dredged material was photographed, briefly described, and split .nto subsamples onboard. Representative sample splits were distributed to the U.S. Geolog.l.cal Survey, the Bureau of Mineral Resources (BMR), Australia, and the sc i.oecn Islands Geological survey for analysis and az-ch Lv.i.nq , After lengthwise splitting and subsampling, the unsampled half splits of the cores were frozen for storage in the U.S. Geological Survey's refrigerated core archives. Post-cruise laboratory work was done mainly at BMR. Chemical analyses for calcium carbonate and organic carbon content were made at the Australian Mineral Developnent Laboratories in Adelaide. Standard laboratory techniques used at BMRincluded preparation of thin sections, wet-sieving, smear-slide analysis, staining of carbonate components (Alizarin Red-S and Titian Yellow), X-ray diffraction, and point-counting (carver, 1971). Subsamples were taken at the laboratories of all three countries for micropaleontological examination by various investigators inclUding David Bukry, U.S. Geological Survey, George Chaproniere, BMR,and Johanna Resig, University of r.:awaii.

Colwell, Vedder: Sampling 30 Grain-size Analyses

Grain-size data were obtained on all samples by standard wet-sieving techniques. Samples were sieved into <45 , 45-63 , and >63 fractions. No attempt was made to subdivide the <63 (silt and clay) fractions.

component Analyses

Analyses of the constituents in the rocks and sediments were made using smear slides, thin sections, x-ray diffraction, and a binocular microscope. Abundances of components in the cored sediments were estimated, whereas in the dredged rocks, point-counts were made (500-600 points per thin section). Smear-slide estimates of component abundances. which were made using the <45 fraction, were based upon the area of the slide covered by each component. The only major difficulty encountered was estimating the ratio of calcareous nannofossils to terrigenous clay. Nevertheless, there generally was good agreement between the estimated abundance of calcareous nannofossils and the expected abundance based upon the total carbonate content of the sediment (determined chemically), the relative proportions of the three size fractions, and the composition of the two coarser size fractions detennined by using a binocular microscope. X-ray diffraction analyses were made by J. Fitzsimmons of BMRusing standard techniques.

Organic Carbon and Carbonate Analyses

Carbonate-carbon (C02) was determined by acid evolution. Samples were digested for 3S min in phosphoric acid. The evolved CO2 was then passed through a series of chemical filters and driers and finally absorbed into a pre-weighed Micnale tower containing soda asbestos and magnesium perchlorate. The increase in weight percent of the tower was then expressed as a percentage of initial sample weight, that is. percent CO2' Percent caco3 was then calculated by using molecular weights. Organic carbon was determined by using a LECOinduction furnace. The samples were first boiled in HCI to remove any carbonates. The residues were then filtered onto Whatman glass-fiber pads and the pads transferred to LECO crucibles. The crucibles were then burned in the induction furnace. A direct readout of percent C was thus obtained. The instrument was calibrated using standard reference samples, LECOstandard carbon rings, and pure dry CaC03'

SAMPLESITE DESCRIPTIONS

Locations. coordinates, and approximate water depths for all sample sites are shown in Figure , and Table 1. Brief annotations on the retrieved rocks are given in Table 2. The following descriptions include seismic line number. bathymetric configuration, expected rock type. and uncorrected water depth or range of depth.

colwell, Vedder: Sampling 31 ;)redge Stations

Station 1.--(Gravity core).

Station 2.--Station 2 was selected to sample a short ridge that crosses Line 26 on tht. southern margin of Russell basin (Fig. 1). Acoustic basement is shown on the seismic profile at this ridge. The ridge forms part of a series of isolated elongate knolls that extend from the Russell Islands to the New Georgia Group. The site is on the shallOW'est segment of Line 26, ranging from approximately 1,200 to 750 m of water. Its shape {Fig. 2Al, seisrni;c character, and pOSition between the islands of Mborokua and Nggatokae suggested a volcanic origin. Rapid southeastward drift prevented the ship from maintaining course directly on Line 26 with the result that the steepest and highest parts of the target were missed.

Station 3.--Station 3 was intended to be on Line 27 approximately 30 Jon southeast of Station 2, where a prominent south-facing scarp truncates north- dipping strata, features which shOW'up as well-defined reflectors on the seismic profile (Fig. 2B). Because of drift and problems in locating the target, the selected site was overrun, and the feature was dredged about 3 Jan south- 'ast of the line. Water depth at the dredged site is approximately 980 m, considerably less than that at the proposed site.

Stations 4 and 4A.--Station 4 was an unsuccessful dredge site that was repeated as 411.. Both were located on Line 13 on the northern edge of the relatively shallow-water platform. that joins Guadalcanal and the Russell Islands. The aim was to sample the rock sequence directly below the upper surface of the platform (Fig. 2C).

Station 5.--Station 5 was selected because it was on an apparent fault scarp on Line 13 about 3 km northeast of Station 4A. The aim of the haul was to retrieve rocks from a sequence along the deeper part of the Guadalcanal- Russell Islands platform. where well-defined reflectors are shown on the seismic profile (Fig. 2C). Although no rocks were recovered after several "bites" at 8,000 lb of tension, small amounts of gritty mud adhered to the dredge jaws.

Station 6.--Station 6 was sited on Line 30 just outside the line of low islets that forms the northern margin of the Russell Islands. Water depths a the station range from approximately 1,000 to 400 m. As at Stations 4A and 5, the aim was to sample the sequence o~ rocks that forms the southern margin of, and that at some places may extend beneath, the Russell basin (Fig. 20).

Station 7.--Station 7 was on Line 31 approximately 15 km southeast of San Jorge Island. The intent was to sample the base of the prominent scarp that forms the southwestern edge of the shallow-water platform joining Santa Isabel and the Florida Islands. A raised rim is present along part of the southern side of the platform (Colwell and Tiffin, Fig. 1, this volume). At this station, the steep scarp ranges from approximately 400 up to 60 m of water. Large slump deposits shown on the seismic profile extend from the base of the scarp do.in slope to the basin floor in approximately 1,500 m of water (FJ.g. 2E) •

Colwell, Vedder: Sampling 32 Station 8.--(Gravity core).

Station 9.--Station 9 was sited on Line 22 northeast of the island of Vangunu at the southeast end of the NewGeorgia group. It was chosen to sample a low, double-crested ridge that appears to be upfaulted acoustic basement on the seismic profile (Fig. 2F). On this line, the ridge crest is in about 1,100 m of ••••eee r, A probable southeast ••••ard continuation of the same ridge crosses Line 24 in about 1,300 m of water.

Station 10.--Station 10 was on Line 51 approximately 35 Jan west of Vella Lavella near the western end of the survey area. It was selected in order to sample a ridge beneath ••••hich seismic reflectors are deformed. These deformed reflectors possibly extend under the Shortland basin (Fig. 2G). The southern flank of the ridge was sampled in approximately 700 m of water.

Station 11.--Station 11 was chosen on Line 51 about 4 km north of Station 10 to sample beds that appear as a series of well-defined reflectors on the seismic profile (Fig. 2G) and apparently extend beneath the Shortland basin. The reflectors indicate a northward-dipping sequence that suggests uplift of he southern side of the basin. Water depths of the sampled interval range irom approximately 550-600 m,

Gravity-Core Stations

Station 1. --Station 1 ••••as used to check the condition of the wire before beginning dredge operations, as well as to sample the top of the Holocene section in the deep part of the Central Solomons Trough of Katz (1980). It was on Line 26 on the axis of the Russell basin in about 1,800 m of ••••ater (Fig. 2A) •

Station 8.--Station 8 was on Line 31 in a zone of faults and slumps at the base of the slope leading up to the shallo ••••-water platform that joins Santa Isabel and the Florida Islands. It ••••as selected in order to sample the top of the Holocene section northwest of Savo. The water depth was approximately ',280 m (Fig. 2E).

MATERIAL SAMPLED

Brief descriptions of the amount and type of material recovered at the sampling stations are given in Table 2. Four main types of rock and sediment were reexaeveds 1) reef limestone and coral; 2) unconsolidated hemipelagic clay, silty clay, and sandy mud; 3) sedimentary rocks of mixed biological and Volcanic origin; and 4) hornblende-bearing crystal tuff. Detailed descriptions of the materials recovered are given in Colwell (this volume), and Colwe11 and Vedder (this volume). Results of chemical and paleontologic analyses on selected samples are summarized in Tables 2 and 3 and are described in more detail in Resig (this volume), Colwell and Vedder (this volume), and Colwell (this volume).

Colwell, Vedder: Sampling 33 REFERENCES carver, R. E., 1971, Procedures in sedimentary petrology: New York, Wiley, Interscience, 653 p. Exon, N.F., and B.R. Taylor, 1984, Seafloor spreading, ridge subduction, volcanism and sedimentition in the offshore Woodlark=Solomons region and Tripartite cruise report for .!:!!!.!. Keoki cruise 82-03-16, Leg 4: CCOP!SOPACTechnical Report no. 34, 386 p. Katz, H. R., 1980, Basin Development in the Solomon Islands and their Petroleum Potential: ~ UNESCAP, CCOP!SOPACTech. Bull. 3, p. 59-75.

Colwell, Vedder: Sampling 34

TABLE2.--BRIEF DESCRIPTIONOF MATERIALRECOVERED

AMOUNTOF MATERIAL STATION RECOVERED ROCICAND(OR) SEDIM£NT AGE , 1 192 cm Light yellowish-qrey to

2 1/4 dredge bag 90\: ~- and Mn-.oxide-st",ined, heavily bored Ql.iaurnary2 calcarenite (reef limestone) lind coral. Pliocene or OUlltern",ryJ

10\, Dark olive-g-rey, slightly cllicareous, Qu.lIternllry (N:23, CN.14l3,2 volcaniclastic Sllndy siltstone.

J 1/4 dred9'" boa" 98\, Ol.i.ve-"rey, IIlOderately indurated, partly Quaternary (N.23, CN.14alJ•2 bored siltstone. weakly calc;u'eou.... Trace of ForAminiferA.

1\ Olive-91'ey. _ll_indurated, carbonate-free - siltstone. Clllyey. Finely bedded.

4 a Lost dredge.

4A 1/2 dred9" bolo" 100\, Fe- and Mn-oxide-stained reef limestone and Quaternary2 J coral. D~a98netically altered. Well cemented. »hocene or Quaternary

S Trace Smears of Silndy IlUd on dredge jaws. Po5sibly volcanic or volcaniclastic material.

6 1 pc. One piece of highly bored, llllInganese-oxide-stained reef. limestone (calcarenite l.

7 Full dredge b",,, 100\, Coralline algae-bound and encrusted reef QUllurnary2 detrit\ls. !'lainly rhodoliths.

8 262 = Dus'"y-yellow to olive-grey •.•• lightly sandy hemi- Q1,l4ternary (late N.22-N.23l' pelag~c mud. Sandy in part.

9 1/8 dred"e bag 95\: Buff. fos5iliferou5. fine-91'4ined IlUddy QUllter",ary (t<.23, (;N14a)3.2 sandstone an

S\, Pieces of l\Wlnganese-oxide-stained branching corili.

10 1/5 dredge bag 70\' Olive-grey, fossilifero1,ls, fine- to mediwn- Q1,1aternary grained volcaniclastic sandstone. M1,ldd'j. Bored. (late N.22, (;!'J.14lJ,2 Coated with lNlnganese Oxide.

29\, Light yellowish-grey, I\'Clderately indurated, Quatern ••.ry sandy calcareous mudstone. lear Ly N.22. CN.14"'lJ,2

1\, ane rounded cobble, light-grey. crystal tuff.

" Full dredgll bag 90\' Fe- lind Mn-.oxide-eo",t=d, light yellowish- QI1ater"ary 3,2 orange auc:ritic ealcarenite (biol!licrite) eeee ••.in- ing minor volcanogenic materi",l. !:lttensively bored. Evidence of s".b",el:'ial expos".re.

10\, c;rellnuh-gl:'ey. sl~ghtly sandy hellUplIlagJ.c: Quaternary (CN.14a)2 mudstone. bte phocene (14te",t N.21)3

N. - pl&nktlC: for&m.inifllral 'l'one 1 Ch4proniere (pers. comm., ,ge2) _ pla"ktic Foraminifer", CN. - CalC4l:'eOUIln",nno!ossil acne 2 Bukry (writ ••.•n c:omm., 1982) _ eall;areoull ",annofossils 3 RlIsi" (thill volume) - pl",nktic: ",nd benthic Foraminifera

PART 2

This section of the Joint Cruise Report contains topical and interpretive papers that resulted directly from the R/V S. P. LEE 1982 Cruise as well as articles that are based upon a combination of the new cruise data and earlier published work.

- -"0'

35 GEOLOGY OF TIlE CENTRAL AND WESTERN SOLOMON ISLANDS

F. I. Coulson Institute of Geological Sciences, Nicker Hill, Keyworth, England

J. G. Vedder U.S. Geological Survey, Menlo Park, california 94025

ABSTRACT

The central and western Solomon Islands are composed of an exceptionally thick succession of Lower Cretaceous to Holocene rocks. Although the stratigraphic sequences and rock compositions vary from island to island and show local complexities, regional correlations indicate an orderly pattern of island-arc genesis from northeast to southwest across the archipelago. On Malaita and the northeast flank of santa Isabel, the oldest rock sequences consist of Cretaceous and Paleocene pillowed tholeiitic to alkalic basalt that is intercalated and overlain by pelagic mrdecone and limestone. These pelagic deposits are succeeded by Tertiary deep-water strata, in part turbidites, that contain increasing amounts of terrigenous material upward in the Miocene and Pliocene parts of the section. Along the chain of islands that includes Choiseul, southwest flank of Santa Isabel, Florida Islands, Guadalcanal, and San Cristobal, so-called basement rocks consist chiefly of basaltic flows that range from unaltered to amphibolite-facies schist. Embodied in this pre-late Eocene (?) basement complex are intrusions of gabbro and diorite as well as diapirs, thrust sheets, and rrelanges of ultramafic rocks that possibly represent dismembered ophiolite or arc-root rocks. Southwest-directed subduction that may have begun as early as late Eocene time created a northeast-facing island arc in which pillowed tholeiitic basalt and basaltic andesite of Oligocene and early Miocene age were extruded together with their pyroclastic counterparts. These arc-related volcanic rocks are succeeded by marine volcaniclastic and carbonate strata of middle Miocene to Holocene age. Along the southwest edge of the archipelago, magmatism resulting from northeast-directed subduction followed a reversal in arc polarity near the end of Miocene tirre. Calc-alkaline effusive rocks and cogenetic volaniclastic strata of late Miocene to Holocene age predominate. These volcanogenic rocks extend from the Shortland Islands through the New Georgia island group into Guadalcanal where they overlap eastward onto older rocks. Pyroclastic deposits are less extensive than epiclastic strata. These partly subaerial volcanogenic rocks are largely andesite and basaltic andesite except in New Georgia, where porphyritic olivine basalt and picrite commonly are present. Aphyric magnesium-rich basalt also occurs in NewGeorgia. Intrusive complexes that range in coceos ae i.cn from gabbro to quartz monzonite are exposed in the Shortland Islands, NewGeorgia, and Guadalcanal.

Coulson, Vedder: Island Geology 36 Thicknesses of stratigraphic sequences are variable. The exposed Cretaceous to Holocene sequence on southern Malaita, including the basaltic lavas, is as much as 3,750 In thick; and the correlative sedimentary section on northern Malaita may be as much as 2,200 m thick. Pre-Qligocene basaltic flows on northeastern Santa Isabel are reported to be as Ill.l.chas 3,500 m thick. So-called basement rocks are overlain by at least 2,500 m of Oligocene and early Miocene volcanogenic rocks on Guadalcanal and by nearly 1,000 m of equivalent sedimentary section on Choiseul. Middle Miocene strata are nowhere more than a few hundred meters thick, but late Miocene strata in the northern Shortland Islands are about 1,000 m thick. The thickest section of post-Qligocene clastic strata 1s in east-central Guadalcanal, where more than 5,000 m of late Miocene, Pliocene, and Pleistocene (7) conglomeratic sandstone and mudstone form a lenticular body of intertongued lithofacies. Miocene and Pliocene volcaniclastic strata on northwestern Choiseul have a composite thickness of about 1,050 m, Even though the volcano that forms Kolombangara in New Georgia is deeply eroded, pyroclastic material and flows still rise nearly 1,700 m above sea level. Faults are the dominant post-Miocene structures; most are high-angle normal and reverse faults that trend northwest. A subordinate set trends north- -as c , Thrust faults of uncereaan age displace basement rocks on oieaeec i , .:ianta Isabel, and Guadalcanal. The thrust planes are cut by younger high- angle faults. Folds, where present, are chiefly of Pliocene and Pleistocene_ age and generally are broad, parallel, and northwest trending, except on Malaita where they are en echelon and relatively steep flanked. A major fault system that may involve large amounts of left slip transects Santa Isabel from northwest to southeast. This fault system may be the surface expression of an inferred tectonic suture between unmetamorphosed Cretaceous and Paleogene ocean-floor rocks of the Ontong Java Plateau and highly tectonized basement rocks that underlie late Eocene (7) to early Miocene island-arc rocks and that were metamorphosed approximately 35 to 50 Ma.

INTRODUCTION

This paper is a summary of the findings and conclusions of previous geologic investigations in the Solomon Islands. It is an attempt to synthesize and integrate the available geologic literature, which in many cases is disseminated in obscure or out-of-print publications that are not easily -bna Lneb Le , The arrangement of the text is unconventional in that regional patterns of tectonic evolution are reviewed before the rock sequences and local structural features are described. This departure seems justified as a simple means of introducing the causes of the contrasting stratigraphy of the various island groups. The Eastern Outer Islands are not discussed, as they are outside the purview of the marine geologic study for which this report was ••••ritten. An introductory statement on the inferred origins of the islands is followed by a brief description of the traditionally used geologic prov1.nces and shortcomings of this usage. The synopsis of rock units is organized into three chronostratigraphic divisions that best reflect the major phases of island-arc development. Primary sources for stratigraphic names and rock compositions for each unit are parenthetically referenced where the name is introduced in the text.

Coulson, Vedder: Island Geology 37 Origin of the Islands

Interaction bet .••.een the Australia-India and Pacific plates since Eocene time presumably resulted in the creation of the complex island arc that now is manifested by the Solomon Islands (Fig. 1). The absence of allochthonous continental material indicates that the Solomons formed entirely in an ocean environment. Cretaceous and early Tertiary oceanic basement rocks, generally lacking terrigenous material and extensively metamorphosed, contrast with overlying, predominant.ly volcaniclastic sequences that are characterized by abrupt facies changes and relatively simple deformat.ion. Subduct.ion-related volcanism occurred between late Eocene (1) and early Miocene time and again between the late Miocene time and the present. Late Oligocene and younger strata are dominated by thick volcaniclastic wedges that are interspersed with reef and shelf limestone bodies, which were constructed during volcanic quiescence. Although it is na.l generally agreed that south-west directed subduction, impingement of an oceanic plateau, and reversal of arc polarity were controlling factors in the tectonic development of the Solomon Island region, ·ecapitulation of the evolVing ideas about the geologic history points out aoce unsolved problems. Early concepts stressed the taphrogenic nature of the region (Colemanr 1965, 1975, 1976; Coleman and Packham, 1976; Hackman, 1973) and proposed that. volcanism may have originated along major transcurrent fractures. Substantial strike-slip was invoked in order to accommodate oblique convergence between the Australia-India and Pacific plates. The foregoing models interpreted the origin of the islands as late Mesozoic linear geanticlinal welts that progressively broke up by tension and shear into an echelon arrangement. Terms such as "fractured arc" (Coleman, 1970; Hackman, 1973) and "non-arc" (Coleman, 1975) were used to describe the island chain. Taylor (1976) emphasized the importance of carey's (1958) Tethyan Shear System, which includes the Solomons Megashear, and interpreted the primary tectonic element to be a westward-migrating rhombochasmin a zone of sinistral shear wit.hin the system. Although it has been suggested (Curtis 1973, Neef 1978) that the Solomon Islands might be the site of double subduction, most models involve a polarity reversal of the are, probably within the last 10 m.y. (Kroenke 1972, 1984; Karig and Mammerickx, 1972; Packham, 1973; Falvey, 1975; Ravenne et aI, 1977; Coleman and Kroenke, 1981; Dunkley, 1983). The consensus of the polarity- -ever-ee r advocates is that a large part of the Solomon Islands arc was built on late Mesozoic oceanic basement by Paleogene southward-dipping subduction along what is now the northeastern flank and that this episode was followed by nort.heast-directed subduction beginning in late Miocene time. Coleman (1975), however, contended that several conditions apparently contradict the hypothesized northeast - facing Paleogene arc. Amongthese conditions are 1) t.he comparative dearth of calc-alkaline rocks and absence of a tholeiite- calcalk.aline-high K progression; 2) the aseismic gap and absence of a trench between Bougainville and Guadalcanal; 3) the anomalous near-trench location, abnormal lava composition, and high heat flow in the New Georgia island group. On the other hand, most of these apparent anomalies are reconcilable, particulary if subduction and local occlusion of the active Woodlark spreading axis (Fig. 1) are taking place, as seems likely (Weissel et a L, 1982). Further work is required to fully explain a variety of geologic features such as 1) the absence of late Miocene and younger volcanism adjacent to the San cristobal Trench in the region between Guadalcanal and the Eastern OUter

Coulson, Vedder~ Island Geology 38 Islands, 2) the origin and evolution of the central Solomons Trough, 3) the position and configuration and apparent disassembly of the Solomons segment within the Melanesian arc system, and 4) the nature and origin of the pre- Oligocene basement rocks and their suspected juxtaposition. Without detailed mapping of Santa Isabel and san Cristobal and until the rocks throughout the region are more completely dated and genetically correlated, many questions on the origin of the islands will remain unanswered.

Geological Provinces of Coleman

Coleman (1965, 1970) subdivided the central and western modern Solomon Islands arc (Fig. 1) to geological provinces (Fig. 2) whose value as a means of identifying contrasting stratigraphic and structural domains has been accepted by many subsequent workers. Because of this commonusage, brief descriptions of these provinces are given below. However, increased understanding of the geology of the Solomon Islands limits usage of the province concept as originally presented by Coleman. Some of these newer interpretations are included in the province descriptions. Stratigraphic -o tumne shown in Figure 3 and the correlation chart of Pound (this volume) _llustrate some of the complexities among the rock sequences of the three main provinces defined by Coleman (1965, 1970).

Pacific Province.--Malaita, Ulawa, and the northeastern flank of Santa Isabel are included in this province. It is composed of ocean-floor rocks that may have formed a segment of the leading edge of an anomalously thickened portion of the Pacific plate (Ontong Java Plateau), part of which may have been abducted onto the northeastern front of the Solomons arc during Miocene time (Kroenke, 1972; Kroenke et aL, this volume). The basement rocks consist of unmetamorphosed tholeiitic basalt that occurs on Malaita as pillowed and massive flows of Early (1) and Late Cretaceous age. The overlying sedimentary rocks include thick sequences of pelagic carbonate and range in age from Late Cretaceous to Holocene (Fig. 3). The entire pre-Quaternary sequence is folded along northwest-trending axes into a broad anticlinoriwn. Because they contain small amounts of volcanic material, post-Eocene strata in parts of the Pacific Province possibly represent deformed and uplifted remnants of distal forearc deposits. Increasing amounts of volcanic detritus and shoaling upward 'n the stratigraphic succession tend to support the concepts of an evolving, .ortheast-facing arc and incipient obduction of part of the Ontong Java Plateau.

Central Province.--Within this province (Choiseul, southwestern side of Santa Isabel, the Florida Islands, Guadalcanal and san Cristobal), the islands have intensely faulted cores of pre-late Eocene (?) mafic lava and associated intrusions of gabbro and diabase, in part metamorphosed to greenschist and amphibolite facies. Bodies of serpentinized ultramafic rocks that may represent pieces of arc roots or ophiolite are Widespread within this basement complex. The overlying sedimentary succession ranges in thickness from more than 5,000 m in east-central Guadalcanal to less than 700 m on San Cristobal. The diverse sedimentary rocks include biogenic limestone, calcarenite, and volcaniclastic sandstone that range in age from early Miocene to Holocene. In general, the sedimentary sequences display shallow dips,

Coulson, Vedder: Island Geology 39 extensive block faulting, and 10000-amplitudedrape folds that usually reflect basement structures. On several islands within the central Province (Florida Islands, Guadalcanal), tholeiitic basalt of late Oligocene to early Miocene age together with dioritic intrusive complexes are interpreted as representing an initial phase of arc volcanism above a southwest-directed subduction zone.

Volcanic Province.--This province includes the reversed, southwest-facing modern arc and incorporates the New Georgia island group, the Shortland Islands, parts of Choiseul, the Russell Islands, northwest Guadalcanal and seve, The volcanic rocks presumably were generated by northeastward subduction of the active WOodlark spreading-axis segment of the Australia- India plate beneath the overriding Pacific plate. This ongoing subduction constitutes the phase of arc volcanism that began in late Miocene time. The province, which is typified by rocks in the NewGeorgia group island, forms a series of emergent volcanic centers and lava piles that consist largely of subalkaline basalt and smaller amounts of andesite. These islands are surrounded by fringing and offshore reefs that provide a framework for the accumulation of varied volcaniclastic and biogenic sediments. Diorite stocks ~re present on Guadalcanal and NewGeorgia. Kavachi submarine volcano in the ;outheastern NewGeorgia island group is currently active. Sikaiana, Roncador Reef, and Ontong Java to the north of the main island chain were assigned to the Atoll Province by Coleman (1965). To the south, the uplifted atolls of Rennell and Bellona were placed in the same province even though they form a distinct and separate geographic entity. None of these atolls is discussed in detail in this report. The northern group consists of barely emergent Quaternary reef limestone and calcareous sand atop the Ontong Java Plateau. Rennell and Bellona are composed of uplifted Pleistocene reefs that are built on remnants of a possible Eocene island arc.

CRETACEOUSTO LOWEROLIGOCENE(1): OCEANICBASEMENTROCKS

Regional Relations

In the Solomon Islands, the oldest known exposed rocks consist of unmetamorphosed ocean-floor tholeiites of Early (1) and Late Cretaceous age on Malaita (Fig. 3). Elsewhere, on Choiseul, southwestern santa Isabel, Florida tslands, san Cristobal and Guadalcanal, possibly correlative, largely ~etamorphosed igneous and sedimen~ary rocks form a basement complex upon which later arc volcanism and sedimentation were superimposed. The distribution of these rocks is shown on Figure 4. Similar metamorphosed basement rocks may be deeply buried beneath late Miocene to Holocene volcanic rocks in the New Georgia island group, the Shortland Islands, and Bougainville. The unmetamorphosed, basement rocks on Malaita typically consist of deep-water nonvesicular pillowed and massive tholeiitic basalt and associated pyroclastic and volcaniclastic strata that are intercalated with and overlain by pelagic limestone and mudstone beds of Late Cretaceous and Paleocene age. At places, the flows are cut by intrusions of gabbro and diabase. A younger set of basalt flows in southern Malaita is alkalic and apparently ranges in age from Late Cretaceous to Eocene (Hughes and Turner, 1977). Pillowed to massive basalt flO'N's on northeastern santa Isabel are intercalated in the upper part with Paleocene pelagites. On Guadalcanal and Choiseul, unmetamorphosed basalt

Coulson, Vedder: Island Geology 40 of probable Cretaceous and (or) early Tertiary age grades into metabasalt of greenschist to amphibolite facies (Hackman, 1980; Stanton and Ramsay, 1975; Ramsay, 1978; Arthurs, 1981). The schist and unmetamorphosed basalt commonly are in fault contact. On southwestern Santa Isabel, the basement rocks consist mainly of greenschist and amphibolite that probably were derived from andesitic flows and pyroclastic strata (Stanton, 1961; Coleman, 1965). Radiometric (K!Ar) dates on the basement metamorphism on Choiseul give a range of 32.4 :!; 6.8 to 51.5 :!; 6.8 Ma, with a mean of 44 :!; 18 Ma and a preferred date of 50 Ma (Richards et aL, 1966). On the Florida Islands, samples of two metamorphic rocks give dates of 35.2 ± 1.4 and 44.7 =- 2.1 Ma (Neef and McDougall, 1976). Presumably this Widespread Eocene and (or) early Oligocene metamorphism began during the initial phase of southwest-directed subduction of the Pacific plate beneath the Australia-India plate (Dunkley, 1983) • Tectonically emplaced Alpine-type ultramafic complexes, consisting predominantly of serpentinized harzburgite disrupt the basement rocks on Choiseul, Santa Isabel, the Florida Islands, Guadalcanal, and San Cristobal. The large ultramafic bodies of the Solomons generally are arranged in a linear fashion, parallel to the elongate trends of the major islands. Exceptions are the slab-like thrust sheet on eastern Choiseul and one northeast-trending mass in central Guadalcanal. Although one body in the Florida Islands has gabbroic constituents that are as young as latest Eocene or earliest Ollgoce'ne, both the age range (K!Ar, 38.4 ± .07 and 36.7 ±. 04 xe , Neef and McDougall, 1976) and mechanism of emplacement of the ultramafic complexes are imprecisely known. On Guadalcanal, Eocene to late Miocene emplacement ages have been assigned to them (Thompson, 1960; Coleman, 1965; Hackman, 1980). On Choiseul, the nearly horizontal thrust sheet overlies deformed and metamorphosed basement rocks and is unconformably overlain by strata of early Pliocene age (Hughes, 1981). Its age of emplacement therefore, is Miocene or older. According to Coleman (19661, the earliest erosion products from the ultramafic complexes are early Miocene. Possibly, the ultramafic complexes represent multistage protrusions of long duration. Some geologists assert that the tectonically emplaced ultramafic rocks are ophiolite suites on Choiseul (Arthurs, 1981; Hughes, 1981 ) , Florida (Neef and Plimer, 1979), and Santa Isabel (stanton and Ramsay, 1975). Alternatively, they could be remobilized fragments of arc roots.

Cretaceous to Lower Oligocene Stratigraphy

Malaita.--on the northern half of Malaita, mafic lava flows and pyroclastic rocks at the base of the section (Alite Volcanics of Rickwood, 1957) are overlain by a sequence of Cretaceous mudstone and limestone beds (lower part of Malaita Group of Rickwood, 1957) (Fig. 3). The volcanic rocks are composed of unrnetamorphosed pillow lava, diabase and subordinate andesite (Fiu Lavas of Rickwood, 1957). The pyroclastic strata include thin-bedded tuff and probable ignimbrite (Fo'ondo Clastics of Rickwood, 1957) that are as much as 600 m thick; they may be correlative with the lava flows. Siliceous and locally limy mudstone beds (Kwara'ae Mudstones of Rickwood, 1957) as much as 270 m thick overlie the basaltic flows and pyroclastic beds. The oldest of these beds are Albian{?) and cenomanian (van Deventer and Postuma, 1973). The siliceous mudstone beds represent a lithified deep-sea ooze that was deposited at depths greater than 4,000 m (Hughes and Turner, 1976).

Coulson, Vedder: Island Geology 41 In southern Malaita, Early (1) Cretaceous oceanic tholeiitic basalt flows (Malaita Volcanics of Hughes and Turner, 1976) are equivalent to the Alite Volcanics of northern Malaita. The overlying rrudstone correlates .••.ith the siliceous mudstone unit of northern Malaita but is increasingly calcareouS up section. The nudstone is eucceede d by pelagic limestone (Are'are Limestones of Hughes and Turner, 1976) that include zones of chert and intercalated peperite. The limestone sequence ranges in age from Late Cretaceous to Eocene. On Small Malaita (Maramasike), the siliceous mudstone section is absent; and as IlD.Jchas 550 m of pelagic limestone and minor chert beds rest conformably upon tholeiitic basalt ("older basalts" of Hughes and Turner; 1977). At places, this limestone sequence (Apuloto Limestone of Hughes and Turner, 1976) contains thickly interbedded flows of alkalic basalt ("younger basalts" of Hughes and Turner, 1977). As in southern Malaita, the limestone sequence ranges in age from Late Cretaceous to Eocene and 1s large ly a lithified calcareous ooze that was deposited in an open-ocean environment.

Ulawa.--A stratigraphic succession s~lar to that on Malaita is exposed on Ulawa. Basement rocks consisting of unmetamorphosed massive and pillowed ceeru,c tholeiitic basalt (Oroa Basalts of Danitofea, 1978) are pre-Late retaceous and probably correlate .••.ith the mafic lava and diabase of northern Malaita and the "older basalts" of southern Malaita. These basalt flows are conformably overlain by a pelagic limestone and chert sequence (Arau Limestone of Danitofea, 197B) that is as much as 400 m thick and that ranges in age from Late Cretaceous to late Eocene.

San Cristobal.--Basement rocks exposed on San Cristobal (San Cristobal Basement Complex of Jeffery, 1977) .••.ere briefly described by Thompson and Pudsey-Dawson (1 958) and Coleman (1 965) • The complex includes pre-Miocene pillow basalt, massive lava and pods of gabbro (Warahito Lavas of Coleman et a L, 1965) that are extensively fractured, sheared and altered by low-grade metamorphism. At places, lenses of lo .••.er Eocene pelagic limestone are incorporated in the flow sequences. Discrete limestone masses as much as 200 m thick (Ravo Limestones of Thompson and PUdsey-oawson, 1958) rest directly on the flow rocks and are probably Paleocene or Eocene (Coleman in Hackman, 1980). Assemblages of ultramafic rocks occur in fault-boundedtracts in eastern San Cristobal and apparently were tectonically emplaced. Thrust . locks of serpentinized ultramafic rocks structurally overlie basalt in the estern part of the island (Hughes, 1982). The widespread alteration of the basement lavas and the emplacement of the ultramafic rocks probably occurred during Eocene and Oligocene time.

Guadalcanal. --The pre-Miocene basement rocks on Guadalcanal have been diVided into two main groups (Mbirao Group and Guadalcanal Ultrabasics of Hackman, 1980). The Mbirao Group consists of a thick sequence of mafic volcanic rocks, subsidiary limestone, diabase sills and local intrusions of gabbro. The volcanic rocks (Mbirao Volcanics of Hackman, 1980) are composed predominantly of relatively unaltered pillowed and massive basalt that include small bodies of recrystallized limestone, and chert. As nuch as 1,200 m of pillow lava occurs along the south side of the island. Minor, but discrete belts of recrystallized pelagic( 1) limestone (Tetekanji Limestone of Hackman,

Coulson, Vedder: Island Geology 42 1980} as auch as 200 m thick are present in the Mbirao Group in the eastern part of the island where they commonly coincide with faults. A belt of greenschist-facies metamorphic rocks (Mbirao Metabasics of Hackman, 1980) is included in the Mbirao Group and forms a major east-west zone on eastern Guadalcanal where it consists primarily of brecciated and schistose mafic rocks. Bodies of gabbro dated at 92 ;l- 20 Ma (Guadalcanal Gabbro of Hackman, 1980) that intrude the mafic volcanic sequence are considered part of the Hbirao Group. The gabbro occurs in sheared and faulted contact with the metamorphic rocks. . The other group of basement rocks is composed of three roughly linear belts of ultramafic rocks that were designated the Marau, suea , and Ghausava- Itina Ultrabasics by Hackman (1980) • All are predominantly serpentinized harzburgite and probably were emplaced during Eocene or possibly Oligocene time.

Choiseul.-~oleman (1960a; 1962; 1965) described the basement complex on Choiseul as faulted schistose and granulitic rocks (Choiseul Schists), that are overlain by' a sequence of pre-Miocene andesitic and basaltic volcanic -ccxe (Voza Lavas). Ramsay (1978) Arthurs (1981) ,and Ridgway and Coulson (in ""ress) showed that some of the schistose rocks are intensely deformed equivalents of the ~~trusive and extrusive rocks and that the volcanic and sohistose rocks are juxtaposed in thrust sheets and fault-bounded blocks. The flows are pillowed to massive, brecciated and sheared ocean-floor tholeiites that display alteration that ranges from zeolite facies through greenschist to amphibolite facies. The relatively unaltered flows are unconformably overlain by lower Miocene strata and generally are assigned Oligocene and older ages. Ramsay (1978) infers a Cretaceous age for part of the flow sequence, the thickness of which is unknown. A minimumthickness of 800 m was estimated by Smith (1980) for incomplete exposures in the east-central part of the island. The Choiseul Schists are mainly amphibole schist that has dominant northwest-to southeast-trending foliation. One body of schist in the southeastern part of the island yielded a K/Ar mean age of metamorphism of 44 * 18 Ma (Richards, et aI, 1966). Igneous intrusives into the basement complex include diabase dikes and a body of altered microgabbro (the oaka Metamicro- gabbro of Hughes, 1981) that invades the flows in east-central Choiseul. In southeastern Choiseul, a slab-like mass of serpentinized harzburgite as much as 560 m thick (Siruka Ultrabasics of Thompson, 1960) structurally overlies ...•oth the unaltered basalt, flows and the schist as a nearly horizontal thrust aheet; (Hughes, 1981).

Santa Isabel.--Little detailed geologic information is available on santa Isabel. Basement rocks composed largely of schistose amphibolite altered from protoliths of andesitic lava and pyroclastic rocks form a belt along the southwest side of the Kaipito-Korigole fault zone (Stanton, 1961; Coleman, 1965). The schistose rocks are intruded by gabbro and diorite (Vitora Microgabbro of Stanton and Ramsay, 1975) that range from massive, slightly retextured bodies to completely recrystallized amphibolite-facies grade rocks. Stanton and Ramsay (1975) described the basement on southeasternmost Santa Isabel as an ophiolite and noted that the basalt-gabbro part of the sequence is at least 6.5 Jan thick. Alternatively, these rocks may represent tectonically thickened fragments of arc roots rather than an ophiolite complex•.

Coulson, Vedder: Island Geology 43 On the southeastern part of the island, ultramafic rocks (Kolomola Ultramafites of Stanton and Ramsay 1975) form a series of elongate pods of serpentinized harzburgite that appear to be genetically related to the Kaipito-Kongole fault zone. On the island of san Jorge, a plug-like body of ultramafic rocks (San Jorge Ultramafites of sc aneon and Ramsay, 1975) may be a diapir. Overlying the belt of schistose basement along the southwest coast are about 200 m of pillowed basalt flows of unknown age. To the northeast across the Xaipito-Korigole fault zone, as nuch as 3.5 }an of pillow lavas and thin flows are sparsely intercalated with volcaniclastic strata (Sigana Volcanics of Stanton, 1961). Coleman et al (1978) described this thick sequence of volcanic rocks as deep-water pillowed and massive lava and tuff that chemically are oceanic tholeiitic basalt. These tholeiitic rocks are assigned a Late Cretaceous to Paleocene age on the basis of a 66 z. 3.0 Ma K/Ar date (Hackman, 1980). Stanton {1961} recognized that the tuffaceous sandstone beds overlying the so-called Sigana Volcanics southwest of the Kaipito-Korigole fault zone are fundamentally different than the limestone-muds tone-tuff sequence overlying the Sigana Volcanics along the northeast side of Santa Isabel. 'ec.reover , the oldest tuffaCeo\ls sandstone beds southwest of the fault are early Miocene (Coleman, 1965)" Furthermore, the _sedimentary succession along the northeast coast (Tanakau Group of Stanton, 1961) probably is thicker than 1,800 m (Fig. 3), and is highly deformed in the lower part (Coleman, 1965). Lenticular and massive pelagic carbonate beds containing chert are intercalated at places in the volcanic sequence northeast of the fault zone. Planktic foraminiferal assemblages indicate that these intercalated pelagites range in age from late Paleocene to early Oligocene (Coleman et at., 1978). This pelagic sequence resembles the Paleogene sedimentary section in northern Malaita and probably is genetically related to it. Stanton (1961) suggested that the Sigana Volcanics and overlying beds possibly were thrust southwestward over the basement complex along the Kaipito-Korigole fault zone. It is not certain, hccevez-, that this fault zone is entirely a compressional feature (see Summary).

Florida Islands.--Basement rocks of the northwestern Florida island group were described by Taylor (1976, 1977) as an in-situ island-arc ophiolite suite, although they equally as well could represent dismembered, deep-seated ar-c roots. '!he basement complex consists of repetitive flows of oceanic pillow basalt more than 2,800 m thick (Kasika Metabasics and Naghotano Volcanics of Taylor 1977). Much of this mafic sequence is metamorphosed to zeolite and greenschist grade. All the mafic basement rocks are intruded by ultramafiC rocks (Nggela Ultrabasics of Coleman, 1965) that include harzburgite, serpentinized peridotite, and a swarm of diabase dikes, as well as gabbroic rocks (vatilau Gabbro and Vatilau Microgabbro of Taylor, 1977). Basement rocks on small Nggela are cceeceed of serpentinized, deformed harzburgite, dunite and wehrlite together with minor gabbroic rocks and mafic pillow lava (Hanuvaivine ultramafite belt of Neef and McDougall, 1976). At places the ultramafic rocks are tectonically admixed with a younger sedimentary sequence. A gabbroic body within the ultramafic belt pz-ovddes K/Ar apparent ages of 38.4 :II: 7 Ma and 36.7 :I; 0.4 Ma (Neef and McDougall, 1976). Small patches of ultramafic rocks also occur as melanges in northern Small Nggela (Siota Ultrarnafite Belt of Neef and McDougall, 1976). Both

Coulson, Vedder: Island Geology 44 groups of basement rocks were called incomplete parts of an ophiolite sequence (Neef and Plimer, 1979) and both probablY are diapiric.

OLIGOCENETO UPPERMIOCENEROCKS

Regional Relations

Presumably, the widespread Eocene and (or) early Oligocene tretamorphic event that is recorded in basement rocks from Choiseul to san Cristobal occurred in response to the initiation of southwestward subduction of the Pacific plate beneath the Australia-India plate (Dunkley, 1983). The onset of subduction-related arc volcanism led to the emergence of ancestral islands during the Oligocene. Some of the magmatism may have overlapped metamorphism and vice versa. Widespread extrusion of tholeiitic lavas on Guadalcanal the Florida islands, and the Shortland Islands marks the first episode of island- arc volcanism related to subduction. Oligocene and lower Miocene volcanic rocks on Bougainville (Blake and Miezitis, 1967) probably represent the sarne episode of volcanism. Subduction also seems to be indicated by intrusive igneous complexes on Guadalcanal, the Florida islands, and the Shortland Islands. On Guadalcanal, a body of diorite is dated at 24.4 -: 0.3 Ma (Chivas and McDougall, 1979). The active arc environment provided abundant volcaniclastic material to localized Miocene depocenters now exposed on Guadalcanal, san Cristobal, the Florida Islands, Santa Isabel and chot eeul , Subsequent widespread deposition of early Miocene limestone, calcarenite and foraminiferal marl across the shelves of Guadalcanal, the Florida Islands, and San Cristobal attest to waning volcanism in these areas. Pelagic sedimentation continued undisturbed in the Malaita region. Although increasing amounts of fine-grained volcanogenic detritus occur upward in the Miocene sequence on Malaita, the source is unknown.

Igneous Rocks

Subduction-related mid-Tertiary volcanic rocks throughout the Solomons consist mainly of flows that contain intercalated volcaniclastic and carbonate strata. The distribution of these rocks is show-non Figure S. Tholeiitic pillow lavas are common; and most of the associated volcanogenic rocks 'Jrobably are seafloor extrusives, although some volcanism may have been .subaerial on Choiseul (Hughes, 1982) and Bougainville (Blake and Miezitis, 1967>.. Basalt and basaltic andesite predominate, whereas andesite and more sa lie rocks account for only a small proportion of the total volume (Dunkley, 1983). Intrusive rocks are mostly calc-alkaline and dioritic.

Guadalcanal.--Extensive volcanic-arc activity is indicated by exposures on GUadalcanal where at least 2,500 m of basaltic andesite flows are intercalated with pyroclastic, volcaniclastic, and carbonate strata (Suta Volcanics of Hackman, 1968, and Marasa Volcanics of Thompson, 1960). These pillowed and massive flows are assigned a late Oligocene and early Miocene age on the basis of benthic foraminiferal assemblages in interbedded limestone. Intrusive diorite (Poha Diorite of Hackman, 1980) has been dated at 24.4 : 0.3 Ma (Chivas and McDougall, 1978).

COUlson, Vedder: Island Geology 4S Florida Isiands.--

Santa Isabel.--A narrow belt of volcanic rocks overlying schistose basement southwest of the Kaipito-Korigole fault zone is overlain by early Miocene strata (Coleman, 1965). Although these partly fragmented basaltic flows were correlated "'ith a thick flow sequence northeast of the fault acne (Sigana Volcanics of Stanton, 1961), they may be unrelated (see preceding section). Possibly this southwestern belt of flows represents Oligocene arc volcanism.

Choiseul.--Pillowed and massive to brecciated tholeiitic basalt flows and intrusive diabase (Voza [Vosa} Lavas of Coleman, 1960a) are widely distributed on Choiseul. Although Coleman (1960a) suggested that these rocks are middle Oligocene to early Miocene in age, they are more likely to be older (see discussion in preceding section: Cretaceous to lower Oligocene (1) ';tratigraphy) •

Short land Islands.--Volcanic rocks on Fauro and Alu are of uncertain age but some of them probably are related to nearby Oligocene and lower Miocene igneous rocks on Bougainville (Kieta Volcanics of Blake and Miezitis, 1967). More than 500 m of altered volcanic flows form the so-called basement lavas on Fauro, where the sequence consists of massive, pillowed, and brecciated basalt, icelanditei and tholeiitic dacite (MasamasaVolcanics of Turner, 1978; Turner and Ridgway, 1982). On Alu, more than 400 m of altered massive and brecciated lava flows are composed mainly of basalt and basaltic andesite (Alu Basalts of Turner, 1978).

Unusual Intrusive Rocks

In north-central, Malaita a pipe-like body of brecciated alnoite intrudes early Tertiary mudstone and apparently is overlain by mid-Tertlary strata. Pb/U dates obtained from zircons enclosed in the alnoite are 33.9 and 34.1 Ma 'Oavis l!L Nixon, 1980). The alneite contains xenoliths that are characteristic of kim.berlites in continental cratonic settings (Nixon and Coleman, 1978). The occurrence of garnet lherzolite xenoliths interpreted as unmodified mantle material is especially unusual in a supposed island arc setting. Mineralogic data from the alnoite and calculations based upon the pyroxene geothermometer imply a lithospheric thickness of 110 kIn (Boyd in Nixon, 1980). Isotopic stratigraphy studies by Bielski-Zyskind et al {1984) concluded that these rocks and associated basalt were derived by partial melting of relatively undepleted mantle under older depleted upper mantle and that the undepleted mantle occurs deeper than 100 km, They further state that the results of their study are incompatible with the ontong Java Plateau being a piece of old continent.

Coulson, Vedder: Island Geology 46 sedimentary Rocks

Upper Oligocene and Miocene sedimentary sequences in the Solomon Islands are dominated by volcanogenic clastic strata that were deposited chiefly in deep-water basins and extensive carbonate deposits that were built in shallow- water shelf areas. Rapid erosion of the newly emerged and tectonically unstable island blocks led to the denudation of both the basement and early arc volcanic rocks. The resulting voluminous volcanogenic sediments accumulated in fault-bounded basins and troughs that have since been uplifted and tilted. These strata probably began to be deposited locally as early as late Oligocene time; their distribution is shown on Figure 5.

Choiseul.--As much as 2,500 m of volcaniclastic breccia, sandstone, and mudstone (Mole Formation of Coleman, 1960a) unconformably overlie the basement rocks on chc.rseut , At several places on the western part of the island, a rudaceous facies (Komanga Grit of Coleman, 1960a, and Koloe Brecc.ias of Hughes, 1982) forms a basal zone of breccia and conglomerate as much as 250 m thick. Its sporadic distribution, restricted occurrence, and abrupt thickness changes suggest that the coarse detritus fills hollows or small fault- controlled basins within· the basement. .The br$ccia consists of angular basalt.ic clasts set in a basaltfc grit matrix and both the clasts and the matrix we-re derived from nearby exposures' of volcanic rocks and ~eefs, possibly as debris flaws. The breccia grades upward into finer-grained clastic strata that constitute the bulk of the formation. In ••••est-central Choiseul, lower to middle Miocene limestone (Mount Vuasa (Vasu, Vavasa] Limestone of Coleman, 1960a) forms a series of discontinuous lenses as much as 50 m thick within the lo ••••er , coarse-grained part of the volcaniclastic sequence. These carbonate lenses include calcisiltite, calcarenite, and biocalcirudite that represent both reef and fore-reef deposits. Most of the Miocene sedimentary section consists of repeated sequences of interbedded microbreccia, and well-bedded, color-banded marine sandstone, siltstone and mudstone. The succession generally is increasingly finer grained and calcareous upsection. Clasts within the breccia beds were derived mainly from the basaltic and schistose basement, although clasts of limestone, mudstone and siltstone occur higher in the section. Color banding is well developed in the finer grained and thinner bedded strat-a-. Dark bands contain abundant basalt and black pumice fragments; light bands, abundant plagioclase grains. Sedimentary structures including graded and cross-bedding, load- casts, ripple marks and scour are evident (Coleman, 1962)• Much of the sequence probably reflects mass-flow depositional processes onto the slope, although some parts may represent shallo ••••-marine and estuarine environments (Hughes, 1981). Deposition of these strata commencedin late Oligocene time in north-eentral Choiseul, but did not begin until the middle or late Miocene in the northwestern part of the island where the beds are only a few hundred meters thick. According to Hughes (1982), beds as young as early Pliocene occur at places in the uppermost part of the sequence.

Guadalcanal.--A variety of sedimentary rocks were deposted on an irregular surface of heterogeneous pre-Qligocene basement rocks on Guadalcanal. The variable nature of the stratigraphic sequences is illustrated in Figure 3 and

Coulson, Vedder: Island Geology 47 in Pound (this volume,). In the southwestern part of the island, graywacke and sparsely calcareous clastic sediments accumulated in late Oligocene to Miocene deep basins as marine turbidites and debris f Lcws, In the south- central part of the island, a series of thick, lenticular poorly sorted graywacke and conglomerate beds (Kavo Graywacke Beds of Hill, 1960 and Coleman, 1965) are more than 2,500 m thick. Within this sequence, dark massive shale beds possibly indicate a euxinic environment (Hill, 1960). The graywacke sequences both overlie and interdigitate with Oligocene to Miocene volcanic rocks (Suta Volcanics) from which they were principally derived. Biogenic limestone and back-reef calcarenite (Mbetilonga Group of Hackman, 1980) are the predominant sedimentary rock types directly above the Oligocene and early Miocene Volcanogenic sequences. These carbonates are sporadically distributed over llJJ.chof the island and are mainly of early and middle Miocene age, although some are reported by Hughes (1981) to be as young as Pliocene in eastern Guadalcanal. From west to east, five stratigraphic units have been differentiated (Mbonehe [Bonegi] Limestone of Coleman, 1957, Mbetilonga [Betilonga] Limestone of Coleman, 1960b, Tina calcarenite of Hackman, 1980, Lake Lee calcarenite of Coleman, 1960b, and Valasi Limestone of Coleman, 1960b). These units, which rest unconformably upon older rocks, range in minimum thickness from 100 to 400 m and have a maximumestimated .hickness of 1,000 m (Valas.i Limestone). The group is composed largely of impure shelf-carbonate rocks of variable texture, grain size and organic content and- characteristically contain a admixtures of terrigenous' ma:terial.~ The biostromal limestone, which consists of pure carbonate, is massive and partly recrystallized and constitutes as mrch as 70 percent of some units (Mbonehe Limestone). Sinlilar recrystallization is well developed in other units (upper half of the Mbetilonga Limestone, basal beds of the Lake lee Calcarenite). A coralgal reef facies forms the western part of the youngest unit in the group (Valasi Limestone). At places, a foraminiferal biocalcarenite facies, usually thick bedded, is intercalated with or grades laterally into massive recrystallized limestone. These biocalcarenite facies are commonin some units (Lake Lee Calcarenite, and the eastern exposures of the Valasi Limestone). One calcarenite unit (Tina calcarenite) is composed mainly of well-bedded flaggy strata that contain large amounts of terrigenous material inclUding sporadic carbonized wood fragments. Where present, the terrigenous content of the carbonate deposits generally is greatest in the lowermost parts of the sequence (Mbetilonga and Valasi Limestones). Basal conglomerate beds contain locally derived clasts of volcanic and ultramafic rocks. Benthic foraminiferal assemblages suggest water ~pths in the range of 40 to 80 m for most of the impure shelf-carbonate units in the group. However, the purer biostromal limestone probably accumulated in quieter back-reef conditions or as fringing reefs sheltered from terrigenous contamination (Hackman, 1980). In western Guadalcanal, a succession of late Tertiary volcaniclastic arenite and wacke beds (Lungga Beds of Wright, 1968) overlies the pre-middle Miocene volcanic-graywacke-limestone sequence. These volcaniclastic strata contain subordinate conglomerate, mudstone and andesitic lava flows. The diversity of lithic fragments in these coarse clastic deposits is greater than in the underlying graywacke beds. Near the west coast, these strata rest upon oceanic basalt (Turner and Hackman, 1977); and directly south of Honiara, they overlie biostromal limestone and are as much as 1,200 to 1,500 m thick (Wright, 1968, Hughes, 1977). According to Hackman (1979), this volcaniclastic sequence ranges in age from middle Miocene to late Pliocene ( ? ), but Hughes

Coulson, Vedder: Island Geology 48 (1982) assigns an age range of late late Miocene to late Pliocene or early Pleistocene. Abrupt facies changes and internally deformed beds are characteristic of this sequence indicating active tectonism and slumping during accumulation.

Santa Isabel.--Most of the island of Santa Isabel has been mapped only in reconnaissance fashiom consequently, little is known about facies relations and ages of the strata that overlie the volcanic basement complex. The distribution of sedimentary facies on the island is complicated by the apparent juxtaposition of two tectonostratigraphic terranes along a major northwest-trending fault system. These contrasting rock sequences are illustrated in Figure 3 and in Pound (this volume, Fig. 1). Stanton (1961) described the sedimentary rocks and incorporated them in a single informal group (Tanakau Group) in which stratigraphic and facies conno- tations were implied but not named. Nevertheless, the contrasting sedimentary successions on opposite sides of the axial volcanic spine were recognized. Along the northeast coast and southeast end of the island, a stratal succession containing an abundance of pelagic limestone in the lower part was distinguished from a sequence containing terrigenous material and no limestone along the southwest coast. The early Miocene and younger part of the sedimentary section along the northeast coast includes zones of pink chert, minor sedimentary breccia, bedded ~alcisiltite and calcilutite, and rare tuffaceous sandstone. These strata probably are conformable with the underlying volcanic and pelagite sequence, although large-scale penecontemporaneous or post-depositional slumping created apparent unconformities in some areas. Volcanic wacke, consisting of an admixture of noncalcareous volcaniclastic sandstone and siltstone and tuffaceous calcarenite form the remainder of the succession, which may be nonmarine in part. carbonized plant remains representing the first incursion of terrigenous material occur in strata as old as late Oligocene in the northeastern Santa Isabe I successions. Abundant terr igenous detritus, however, first appears in late Miocene strata (Coleman et aI, 1978). The post-oligocene section has been locally subdivided into named units in southern santa Isabel, where the oldest strata (ser-e Beds on the southwest coast, Loguhutu Beds on san Jorge) consist of early Miocene coarse-grained volcaniclastic and tuffaceous sandstone that grades up section into clay-rich, tuffaceous fine-grained sandstone (Rob Roy Beds) of early and middle Miocene age (Coleman, 1965). Middle and late Miocene graywacke beds overlie the fine- Jrained tuffaceous sandstone (Hughes, 1982). The post-oligocene succession in southwest santa Isabel is at least 2.2 km thick, and locally includes interbedded shale, mudstone, and minor amounts of conglomerate. Ultramafic detritus commonlyoccurs in the early Miocene and younger strata along the southwest coast. The section along the southwest coast is slightly deformed in contrast to that in the northeastern part of the island where dips are steep and folds verge northeastward (Stanton, 1961).

Florida Islands.--Because detailed mapping has been completed only on parts of the Florida Islands; stratigraphic relations are not completely resolved. Coleman (1965) described the sedimentary succession above the pre- Miocene volcanic rocks as commencing with calcareous coarse-grained lithic sandstone and subordinate siltstone of early Miocene age. These basal

Coulson, Vedder: Island Geology 49 sandstone beds apparently were derived entirely from volcanic rocks as they contain no ultramafic detritus. calcarenite, fine-grained volcaniclastic sandstone, and siltstone succeed the basal strata and in turn, are overlain by a prominent zone of thick-bedded foraminiferal calcarenite (Anuha calcarenite of Coleman, 1965), also of early M.iocene age. This calcarenite unit forms a discontinuous, north-trending belt of outc:;rops in eastern Big Nggela and is the first in the succession to contain ultramafic detritus. The lower Miocene strata are overlain by a sequence of middle and upper Miocene calcareous sandy siltstone and fine-grained tuffaceous sandstone that probably was deposite;d rapidly in shelf environments. These sandy strata are increasingly calcareous upsection and, on small Ng'gela, grade upward into a Pliocene fringing-reef deposit (Florida Limestone of Coleman, 1965). The Miocene sequence may have a composite thickness of as much as 2,200 m thick. The northwestern part of the Florida island group contains a late Oligocene to Pliocene sedimentary succession (Mboli Beds of Taylor, 1977) that rests unconformably on altered basaltic flows. Within this succession, which has a minimum thickness of 600 m, a turbidite sequence forms the dominant member (the Kombuana Sandstone of Taylor, 1977). These turbidites are composed largely of volcaniclastic arenite, lutite and epiclastic rudite. Although direct correlations have not been made, the Mholi Beds are probably in part equivalent to the Anuha calcarenite and Florida Limestone farther east. On small Nggela, IC7llllerMiocene sedimentary rocks on the north end of the island (Siota Beds of Neef, 1979) consist of massive sandstone and sporadically inter layered mudstone that represent deep-water mass-flow deposits. The sequence is as much as 850 m thick. These sandstone-mudstone beds are geographically separated by an east-trending ophiolitic(?) wedge from a lower Miocene sequence of arenite, rudite and pillow lava (Ghumba Beds of Neef, 1979) on the southern part of the island. Neef (1979) attributes the deposition of the lenticular, coarse strata to deep-water debris flow and slumping; the sequence is progressively finer grained upsection and as much as 1,500 m thick. In western Small Nggela, a thin-bedded to laminated fine- grained sandstone (Ndandala sandstone of Neef, 1979) forms the youngest part of the section. These beds are of middle and late Miocene age and are as much as 280 m thick. The late Oligocene to Pliocene strata throughout the Florida group are affected by numerous high-angle faults and, minor folds.

San Cristobal.--Because san Cristobal probably is geologically the least known of all the major islands, the stratigraphic relations are uncertain. Lenses and irregular masses of upper Oligocene (?) pelagic limestone (Ravo Limestones of Coleman, 1965) and partings of calcareous siltstone are intercalated with and directly overlie the flows to form bodies as much as 200 m thick. Fault-bounded blocks of coarse-grained strata less than 700 m thick (Cristobal Group of Coleman, 1965) are believed to be underlain by basaltic flows. These strata constitute one of the thinnest post-oligocene depositional sequences in the Solomon Islands. On the eastern part of the island, conglomeratic strata (Hariga Conglomerates of the Cristoval Group of Coleman, 1965) both interdigitate with and overlie pillowed basaltic lavas. Included in the conglomeratic sequence are coarse agglomerate and sandstone lenses that have a calcareous tuffaceous matrix. The sandstone contains early Miocene foraminiferal assemblages, al-

Coulson, Vedder: Island Geology 50 though the entire sequence probably ranges in age from early to late Miocene. Stratigraphically, the conglomeratic beds are partly equivalent to foraminiferal calcarenite beds that occur in northeastern san Cristobal. On northwestern san Cristobal, a middle Miocene succession consists of a basal pelagic limestone (Hautarau Limestone of Jeffery, 1975a) and calcareous mudstone, volcaniclastic wacke and slump breccia (Ruawai Beds of Jeffery, 1975a). A minimum thickness of 410 m is estimated for the Miocene section in this area. A volcanogenic sequence (the Waihada Volcanics of Jeffery, 1975a), which is in part equivalent to and in part younger than the Miocene sedimentary rocks, consists of tUff, agglomerate and pillow breccia that show evi:" denee of submarine reworking and slumping. Faults cut all of the Miocene sequences; folds have not been mapped, except on Uki N1 Masi. The serpentinized ultramafic bodies in the western part of the island have been emplaced along thrust faults (Hughes, 1982).

Malaita. --In the northern part of Malaita, late Eocene(?) to middle Miocene strata (Alite Limestone of Rickwood, 1957) consist of as much as 900 m of bedded limestone that contains zones of chert. These limestone beds are 'ucceeded by as much as 760 m of thick-bedded chalk (Sauba Chalk of Rickwood, ,957), .•••hich has an age range of middle to late Miocene and possibly early Pliocene. In the southern part of the island, the Cretaceous to Eocene pelagic limestone beds are overlain by hard calclsiltite beds (Haruta calcisiltites of Hughes and Turner, 1976) that contain an increasing number of brown llUdstone beds upsection. The rrudstone beds may represent distal turbidite deposits or primary products of nearby volcanism. These beds are believed to range in age from late Eocene to middle Miocene. On Small Malaita, a sequence of calcilutite, calcisiltite, and minor calcarenite beds (Hada Calcisiltites of Hughes and Turner, 1976) gradationally overlies the Eocene to middle Miocene strata and is as much as 200 m thick. Sporadic, thin noncalcareous mudstone beds are present at places in this upper Miocene to Pliocene sequence. Most faults and folds on Malaita post-date the late Miocene to Pliocene depositional sequences (see section on upper Miocene to Holocene rocks) •

Ula.•••a.--A 380 m-thick succession of calcisiltite and calsilutite beds containing thin layers of mudstone and intercalations of pillowed and massive lkalic basalts (Waipaina Calcisiltites and Haumela. Basalts of Danitofea, .978) overlies the Cretaceous to Eocene beds on Ula.•••a. These strata range in age from early Oligocene to late Miocene and probably are lateral equivalents of the Oligocene and Miocene calcisiltite beds of South Malaita. A gradual change from deep-vaee r to shallower environments and an increasing influx of volcanic detritus are evident. According the Coleman (1965) the island is a faulted anticine.

UPPERMIOCENETO KOLOCENEROCKS

Regional Relations

Upper Miocene to Holocene largely calc-alkaline volcanism built a series of eruptive centers along a broad, irregular belt that stretches from the

Coulson, Vedder: Island Geology 51 Short land Islands to northwest Guadalcanal. This volcanic activity, which seems to have peaked in Pliocene and Pleistocene time, was eccompanLed by empLecement; of several igneous intrusive ccmpLexes , This episode of volcanism was preceded by middle to late Miocene tectonism and presumably was generated by a r-ever-aaI in arc polarity and incipient northeastward subduction about eight million years ago. At the same time, renewed uplift heralded a period of tectonism and intense alluviation, particularly on Guadalcanal. Ultramafic bodies, probably initially emplaced in late Eocene or Oligocene time, began to be protruded, overthrust, and extensively exposed to subaerial erosion. Early Pliocene and older strata generally are warped by northwest-trending open folds; exceptions are on northeastern Santa Isabel and Malaita where folds are relatively closely spaced and steep limbed. According to Coleman (1965), there are no recognizable folds on San CristobaL Intersecting high-angle normal and reverse faults form northeast-and northwest-trending sets in pre- Pliocene strata on most of the islands. Renewed subduction created a new pattern of sedimentation at the end of Miocene time. In the region of the volcanic axis, rapid uplift and instability curtailed reef growth, and high erosion rates led to the shedding of huge quantities of volcanic detritus into adjacent newly developed intra-arc basins. Farther behind the new are, however, sheltered marine conditions allowed the deposition of extensive platform carbonates.

Igneous Rocks

General features.--Volcanic rocks that are related to northeast-directed subduction occur chiefly along the southwest side of the arc from Bougalnville to Guadalcanal. The distribution of these rocks is shown on Figure 6. Lava and epiclastic breccia derived mainly from flows generally predominate over pyroclastic deposits. Ash-flow tuff and pumice flows of intermediate composition are limited in extent and are associated with andesite domes on some quae scene centers. Shallow-intrusive complexes are present at places in these suites of effusive rocks. Coleman and Kroenke (1981) described a 75 ~ km spacing of volcanoes along the southwest side of the Solomons arc. Even though they fall outside the general trend, two late Cenozoic volcanoes on Choiseul (Maetambe and Kornboro) are included within the belt of southwest-facing arc magmatism. Other off- trend "displaced" volcanism is indicated by the sites of some New Georgia volcanoes, which seem to be anomalously near the trench. For example, the southernmost volcanic center on Simbo lies almost directly above the inferred axis of the clogged trench south of Vella Lavella. Compositionally, the second-episode volcanic rocks are variable, although they are chiefly calc-alkaline throughout the arc. In the NewGeorgia Group, large volumes of olivine-rich basalt and picrite have been erupted and may reflect unusual or abnonnal subduction processes. Excluding the apparently anomalous NewGeorgia volcanism, the upper Miocene to Holocene volcanic rocks generally are more salic than the Oligocene arc eruptives and range in composition from basalt to rhyodacite. Basaltic andesite and andesite are the most abundant extrusive rock types (Dunkley, 1983). In addition to the extrusive centers, a series of irregularly shaped stocks occur along the main arc in the Shortland Islands, New Georgia and Guadalcanal. The calc-alkaline rocks that form these stocks range in composition from gabbro to granodiorite and quartz monzonite; quartz dio-rite coulson, Vedder: Island Geology 52 and tonalite predominate, and all exhibit late-stage sodium enrichment (Dunkley, 1983). Pliocene to Pleistocene radiometric ages have been assigned to several of these stocks.

Gua daLcarra Ls e-e-Orr Guadalcanal, late cenozoic igneous activity was concentrated in the northwestern part of the island where it reached a climax near the end of Pliocene time. Plugs and dissected volcanic cones are composed predominantly of hornblende andesite flows (Gallego Lavas of Thompson and Pudsey-uawson, 1958). These flow sequences are as much as 900 m thick at·the type section. Aprons of pyroclastic breccia and coarse volcaniclastic strata flank the extrusive centers. A single K/Ar age of 6.39 ± 1.95 Ma (Snelling in Hackman 1980) indicates that much of this flow sequence may be older than Pleistocene, although geothermal areas occur nearby and intermittent vulcanicity continues on nearby Savo Island. The hornblende andesite flows are blanketed by as much as 300 m of volcanic agglomerate (Tiaro Tuff Breccia of Hackman, 1979) which probably represents volcanic mudflows. The largely unstratified tuff breccia interfingers with poorly stratified lithic tuff near the volcanic centers and grades laterally into stratified volcanic wacke and rudite (Lungga Beds of Wright, 1968). These bedded strata, which are predominantly sandstone and siltstone, were deposited in narrow channels and basins between volcanoes and contain air-fall ash as well as material derived from the underlying andesite flows. Farther east, in the Gold Ridge area of central Guadalcanal, a thick sequence of extrusive andesite, now eroded and buried by late Pliocene strata, probably represents an isolated calc-alkaline volcanic center. Dioritic rocks (Koloula Diorite of Hackman, 1980), which outcrop in south-central Guadalcana1, yield K/Ar apparent ages that range from 4.47 :t- 0.19 to 1.55 :t- 0.05 Ma and indicate polyphase emplacement (Chivas and McDougall, 1978).

Savo. --Lying about halfway between the northwestern tip of Guadalcanal and the Florida Islands, the island of Savo is a quiescent Pelean volcano that is the easternmost in the chain of volcanoes along the southwest-facing arc. Lavas are composed mainly of hornblende andesite, and the volcanic edifice consists largely of agglomeratic and tuffaceous deposits (Proctor and Turner, 1977). According to Coleman (1965) the most recent sequence of eruptions was in the period 1830 to 1840. Active steam vents and solfataras occur on the southern and eastern slopes of the volcano (Taylor, 1965).

Russell Islands.--The Russell Islands, which lie about midway between Guadalcanal and eastern New Georgia, are the remnants of an emergent island volcano composed of basaltic andesite breccia overlain by basalt lavas (Pavuvu Breccias and Banika Lavas of Danitofea and Turner, 1981). The volcanic core of the islands is encircled by uplifted siltstone and reef limestone of Pleistocene age.

Mborokua.--Mborokua, a small conical island situated between the Russell Islands and New Georgia, is an extinct volcano composed of massive basaltic lava and volcanic breccia (Turner, 1975). The south side of the crater has been breached by the sea.

Coulson, Vedder: Island Geology S3 cnc.Leeuf, --Near the center of the island of Choiseul, Mount Maetarnbe is composed predominantly of andesi tic pyroclastic deposits. Chemically, these are calc-alkaline rccke that presumably are related to northeast-directed subduction along the southwest side of the arc. The pyroclastic strata (Maetambe volcanics of Coleman, 1960a), consist predominantly of water-laid andesitic tuff, ash and breccia and have an estimated minimumthickness of 500 m, Flows have not been recognized. These volcanogenic rocks generally unconformably overlie basement rocks and upper Oligocene to upper Miocene turbi';' dites (Mole Formation). At places, the pyroclastic rocks are interbedded in the upper part of the turbidite sequence indicating contemporaneous volcanism and rapid sedimentation on steep submarine slopes during the early stages of this eruptive episode. Volcanism possibly began as early as middle Miocene time and may have continued spasmodically into the Pleistocene (Hughes, 1981). Although no crater is preserved, geothermal springs on Mount Maetamhe suggest the waning of relatively recent volcanic activity. In the area of Xomboro Peak and Laena Island in southeastern Choiseul, the volcanic rocks consist of a sequence of andesitic breccia and tuff (XomboroVolcanics of Coleman, 1960a). Schist and ultramafic detritus is pre- .ent; at places in t;he breccia beds. to direct evidence for the age of these volcanic rocks is available, but ash-t'all deposits derived from Komboro Peak occur in nearby strata of early Pliocene age. The well-preserved cone and large amount of andesitic material in adjacent Pleistocene deposits suggest that volcanic activity continued into late Quaternary time (Strange, 1981a).

NewGeorqia.--The islands of NewGeorgia represent the most extensive and voluminous development of the late Miocene to Holocene episode of arc volcanism. Virtually the entire island group is formed by a complex of emergent and coalescing volcanoes that are encircled by fringing reefs and lagoons. The volcanoes, however, seem to be anomalously near the NewBritain- San Cristobal Trench, and many lavas are chemically atypical of the calc- alkaline suites that occur elsewhere in the arc. These apparent anomalies are attributed to the subduction of the active Woodlark spreading axis beneath New Georgia (Taylor and Exon, 1984; Dunkley, 1983). The volcanic rocks consist of large volumes of highly porphyritic olivine basalt and pic rite basalt lava and breccia. Hornblende basaltic andesite and -naes Lee flows are subordinate. Less commonbut widespread are fine-grained .phyric magnesium-rich basalt flows. The basalt and picrite are hypersthene normative and show a trend to magnesium enrichment, whereas the minor andesite with which they are associated shows a broad calc-alkaline trend akin to the so-called normal volcanic rocks on other islands in the arc (Dunkley, 1983). The petrogenetic nature of the NewGeorgia suite is difficult to define. The rocks are subalkaline and have a low Ti02 content characteristic of arc volcanics, but the suite is high in potash and the basalt contains as much as 2.6 percent ~O. Two-pyroxene andesite that is relatively rich in MgOoccurs at Simbo. Intrusive rocks include gabbro to tonalite stocks and andesite plugs in the central part of the island group. Typically, the volcano flanks display lateral facies that range from massive flows to volcaniclastic strata, the latter showing an increasing degree of reworking away from the eruptive centers. Slopewash detritus is a significant facies within the volcanic edifices (Dunkley, oral commun., 1982).

Coulson, Vedder: Island Geology 54 Shortland Islands.--Calc-alkaline suites of igneous rocks include both intrusive and extrusive rocks in the Shortland Islands. In the Fauro Island group, two intrusive bodies (TaunaMicrodiorite, Fauro Dacite of Turner, 1978) invade the basement lavas, and farther north a shallow intrusion of hornblende andesite forms CemaIsland. The horseshoe-shaped bay of north Fauro is believed to have been created by caldera collapse of a large volcano which produced reworked crystal tuff and lava flows at least 1,500 m thick as well as the overlying 600 m of volcanic breccia, tuff and minor flows (Karia Sandstones and Toqha Pyroclastics of Turner, 1978). The age of the pyroclastic rocks is uncertain but may be Pliocene and Pleistocene based on comparison with similar rocks elsewhere in the SolomonIslands (Turner, 1978). In the Alu Island area, the pre-Miocene (?) basaltic lavas are intruded by a microdiorite co~lex, whose possible extrusive equivalent is pyroxene andesite (H1$iai Complex and Kamaleai Pyroxene Andesite of Turner, 1978). Because the pyroxene andesite is overlain by middle and late Pliocene sedimentary rocks (Hughes, 1981" the dioritic and andesitic rocks possibly range in age from Mioceneto early Pliocene.

Sedimentary Rocks

General features.--ouring the episode of volcanism that produced "ebe islands along the southwest flank of the are, sedimentation continued in a variety of environments throughout the Solomonsregion. Uplift of the frontal part of the are, coupled with active volcanism, resulted in high rates of deposition in the newly developed intra-arc basins. Abrupt facies changes were caused by deposition around a complex of emergent volcanoes, local basins, and in inter-island channels. Reefs did not flourish except on narrow shelves that were protected from intense alluviation. Elsewhere, low-energy conditions prevailed, and fine-grained calcareous deposits aceurnulated with few incursions of primary volcanic material. However, increasing amounts of terrestrial detritus in depositional sequences throughout the eastern and northeastern parts of the region indicate uplift that ultimately resulted in shoaling and island errergence. The distribution of late cenozoic strata is shownon Figure 6.

Guadalcanal.--oetailed mapping by Hackman.(19~0) has delineated a number If sedimentary rock units that span late Miocene to Pleistocene time. In east-central Guadalcanal, a sequence composedlargely of poorly sorted arenite forms a lithosome as muchas 4,000 m thick (MbokokimboFormation of Hackman, 1980). Complexlithofacies relations and large .eecurre.eof volcanic detritus typify the unit. Although muddyfine-grained sandstone and siltstone beds predominate, claystone and coarse-grained volcaniclastic sandstone form parts of the section; and sporadic channel-fill conglomerate lenses occur throughout. Carbonized wood and other plant remains are common,and dark siltstone zones conta.in pyritic concretions indicative of local anaerobic conditions. Sedimentary structures including chaotic slump features suggest steep gradients and rapid accumulation in envirorunents that ranged from flUVial to outer shelf and that were periodically disturbed by seismic events. Hughes (1982) assigned a late Miocene to late Pliocene or early Pleistocene age to the litho some.

Coulson, Vedder: Island Geology 55 Farther west, in the north-central part of the island, a sequence of volcaniclastic rudite and arenite (TOni Formation of Hackman,1980) ranges in age from middle (?) Miocene to late Pliocene. Subsidiary strata consist of pyroclastic and extrusive rocks and limestone. Facies changes characterize the sequence, which may be partly nOMaarine a.nd which is composed predominantly of conglomeratic material derived largely from volcanic rocks of the Gold Ridge area. Beds of paraconglomerate and orthoconglomerate, as well as volcanic and lithic arenite are included; these are locally calcareous and grade into micrite and marl. Hackman(1980) divided the formation into 10 mappable subformational units. Lenses of massive coralgal limestone and bedded calcarenite (KombusoeLimestone of Hackman,1980) occur at places but nowhere are they more than 100 m thick. MJst of the coarse clastic strata were deposited by slumping and turbidity flows in tectonically unstable shelf and slope environments, although someof the beds maybe fluvial. The intense alluviation may have derived its energy from pliocene volcanism in the vicinity of Gold Ridge. In western Guadalcanal, strata that are laterally equivalent and partly correlative with those in the north-central part of the island form a repetitive succession of arenite and wacke together with subsidiary conglomerate and mudstone (Lungga Beds of Wright, 1968). The clastic fragments were derived principally from volcanic rocks. ~ An age range of late (1) Miocene to late Pliocene or early Pleistocene is assigned to these strata (Hughes, 1982). In contrast to the high-energy terrigenous environments reflected by the coarse clastic strata in the central and western parts of the island, more protected conditions in northeastern Guadalcanal permitted the growth of fringing reefs (Valasi Limestone of Hackman,1968c). The reef limestone is succeeded by a sequence of conglomerate and subordinate mudstone and siltstone beds that has a maximumthickness of 700 m (Vatumbulu Beds of Hackman, 1968). These upper Pliocene to Pleistocene alluvial and shallow-shelf deposits were derived mainly from pre-Miocene volcanic rocks. Pleistocene deposits along the northern fringe of the island consist largely of reef complexes and shallow-marine and estuarine sediments (Honiara Beds of Coleman, 1957). Included in this sequence are coralgal limestone, calcarenite, and epiclastic conglomerate. The section is as muchas 800 m thick but generally is muchthinner. Gentle folds of probable Pliocene age affect upper Miocene and lower Pliocene strata; only minor drag folds adjacent to faults have been recognized in Quaternary strata. North-northwest and north-northeast trending high-angle faults cut Pliocene rocks in central Guadalcanal. Oblique-slip step faults affect Quaternary rocks along the southern part of the island (Hackman,1980).

Choiseul.--Pliocene strata on western Choiseul are conformable on Miocene beds and consist largely of calcareous deposits approximately 400 m thick (Pemba Formation [Pemba Siltstone] of Coleman, 1960a). In general, the section is cceeosed of calcarenite overlain by marl. The lower 200-250 m of section, consists of lower to middle Pliocene calcarenite beds (Sui Calcarenite Memberof Strange, 1981b) and includes zones of non-calcareous mudstone, calcisiltite and paraconglomerate. Commonslump and graded bedding in the calcarenite possibly represent seismically generated turbidites from the western slopes of MaetambeVolcano. The bedded calcarenite section grades upward into upper Pliocene calcisiltite beds (Mbani calcisiltite Memberof Strange, 1981) that are as much as 120 m thick. There are no sedimentary

Coulson, Vedder: Island Geology 56 structures in the uniformly bedded calcisiltite section, which grades upward into massive calcilutite. Increasing fineness upsection mayindicate a transition to deeper water conditions or reduction of source-area relief in middle to late Pliocene time. Quiet, shallow-water conditions prevailed in southeastern Cnoiseul, where lower Pliocene clastic strata (Vaghena Formation of Hughes,1981) were deposited unconformably upon the basement. These strata are at least 40 m thick in the type area and have an estimated minimumthickness of 110-150 m farther west. They consist of thin-bedded calcareous arenite, siltstone and mudstone that contain varying amounts of tuffaceous debris possibly derived from the nearby volcano at KomboroPeak. Deposition probably was in a low- energy shelf environment less than 200 m deep (Hughes, 1981). Following uplift, tilting and erosion, Pleistocene coralgal reefs (Nukiki Limestone of Coleman, 1960a) formed above the Mioceneand Pliocene formations and overlapped them onto basement rocks. These largely recrystallized limestone beds are preserved along the southwestern side of the island as isolated uplifted and tilted blocks as nuch as 150 m thick. They include fore-reef talus and reef-wall deposits as well as locally developed back-reef and estuarine deposits (Hughes, 1981; Strange, 1981 a, b). On northwestern Choiseul, correlative limestone occurs as high as 440 m above sea level and is ~ilted, warped, and .faulted (Str~nge, 1~81a). Andesite.Breccia (Laena Breccia of Strange, 1981a) containing a calcarenite matrix is present on Laena Island east of KornboroPeak. "

NewGeorgia Island Group.--Sedimentary rocks are limited in distribution along the frontal part of the active are, particularly in NewGeorgia. On Giza, limestone and arenite beds contain foraminiferal assemblages of early Pliocene age {Hughes, 1981}. Late Pliocene shelf sed1Illents occur on Vella Lavella, but most of the section consists of Pleistocene reef platform deposits (Hughes, 1982). pleistocene reef limestone occurs on Ranonggaand on western NewGeorgia Island. On the uplifted islands of Tetepare and Rendova, sequences of Pleistocene deep-water siltstone and sandstone contain chaotic masses of rudaceous material consisting of volcanic, plutonic, and coral-reef debris that presumably slumped from high-standing parts of the arc to the north. Someof the reefal deposits on Tetepare are uplifted as muchas 800 m above sea level, and benthiC foraminiferal assemblages indicate deposition at depths of 1.5 to 2.0 kIn, implying uplift of 2-3 kIn during the Quaternary (Dunkley, 1983).

Shortland Islands.--In the Shortland Islands, Pliocene and (or) Pleistocene sediments are represented on the islands of Alu and Monoand record a general transition from hemipelagic deposition of fine volcanic detritus to shallow-water reef construction. On Alu, about 300 m of Pliocene siltstone, claystone and fine-grained sandstone (Kulitanai Siltstones of Turner, 1978) represent open-marine shelf environments that received detritus from Pliocene volcanoes on Bougainville and possibly Fauro. Late Pliocene and Pleistocene calcareous sandstone, chalky limestone and recrystallized reef limestone (Lafang Limestone of Turner, 1978) form an intertonguing sequence of shallow marine deposits as muchas 150 m thick. Late Pleistocene deposits (Toanapina Conglomerates of Turner, 1978) are composedof locally derived alluvial and littoral gravel.

Coulson, Vedder: Island Geology 57 On MonO, about 250 m of pliocene siltstone (Mono Siltstones of 'rurner , 1978) is largely an outer shelf deposit probably representing ash falls from Bougainville volcanism (Hughes, 1981). Reef limestone (Kolehe Limestones of Turner, 1977) constitutes a late Pliocene carbonate-platform facies within t.he ash-fall siltstone unit. Early Pleistocene reef limestone (Bare Reef Limestone and Toloko Reef Limestone of Turner, 1978) were deposited in association with lagoonal siltstone (Soanatalu Siltstone of Tllrner, 1978). Pliocene (?) tuffaceous sandstone beds on Fauro and neighboring islands (Koria Sandstones of Turner, 1978) probably represent rapid deposition of locally derived volcaniclastic sediment on a shallow shelf (Hughes, 1981). The sequence 1s as much as 1,400 m thick. Minor limestone occurs near the base, and primary tuff beds are present higher in the section. According to Turner (1978), folds are essentially absent and faults are small on the Shortland Islands. The northern part of Fauro probably is a large caldera-collapse structure.

Florida Islands.--Pliocene strata on the eastern Florida Islands are largely residual patches of limestone (Florida Limestone of Thompson, 1958) and generally consist of massive to thick-bedded calcarenite (Thompson, 19581 Neef, 1979). A sandy siltstone marker bed is present at places. The limestone is at least 250 m thick on Small .Nggela where 11: rests unconformably upon folded lower Miocene strata. Outcrops along the east coast represent patch-reef accumulations and subordinate lagoonal deposits and contain foraminiferal assemblages of early and middle Pliocene age. Pliocene strata are gently folded and cut by high-angle faults. South- ward tilt of the island platform and submergence occurred during Quaternary time (Coleman, 1965).

San Cristobal. -w<:alcisiltite, calcilutite, siltstone and muddy sandstone beds (Uki Beds of Jeffery, 1975b) of late Miocene to middle Pliocene age occur on Uki Ni Masi and are unconformably overlain by Pleistocene reef limestone (Hughes, 1982). On the main part of san Cristobal, Pliocene(?) and Pleistocene deposits are confined chiefly to the northern and western coastal fringes. Pleistocene reef facies (Arosi Beds of Jeffery, 1975b) include fore- reef coralgal limestone and back-reef calcarenite. The island probably was emergent during the Pliocene, and Pleistocene sea-level fluctuations led to the establishment of fringing reefs on wave-cut benches along the northern coast (Jeffery, 1975b).

Malaita.--In northern Malaita, a pelagic chalk sequence (aeube Chalk of Rickwood, 1957), the upper part of which may be Pliocene, is conformably overlain by about 250 m of siltstone and fine-grained sandstone (Tomba Silts of Rickwood, 1957). These clastic deposits consist mainly of foraminiferal and tuffaceous debris; they are overlain by Pleistocene reef limestone. Along the west coast in the southern part of the island, upper Miocene to Pliocene limestone beds ('Are'are Limestones, upper part of Hughes and Turner, 1976) are unconformably overlain by 180 m of interbedded conglomerate, sandstone and subordinate siltstone (Hauhui Conglomerates of Hughes and Turner, 1976). These poorly sorted, locally cross-bedded coarse clastic strata probably accumulated in a shallow-marine, possibly fan-delta environment

Coulson, Vedder: Island Geology 58 during early Pleistocene time (Hughes and Turner, 1976). Clasts in the conglomerate include subrounded pebbles and cobbles of limestone, basalt, and chert. On small Malaita, an upper Miocene to Pliocene sequence (Hada Galcisiltites of Hughes and Turner, 1976) consists of well-bedded calcisiltite and minor laminated calcarenite about 350 m thick. This sequence is unconformably overlain by uplifted coralgal reef limestone of Pleistocene age (Rokera Limestone of Hughes and Turner, 1976). The reef limestone generally is 20 to 30 m thick and massive, vugular and recrystallized. Large, relatively tight north •••.est-trending folds form an echelon pattern in northern Malaita and deform Pliocene and older rocks (Rickwood, 1957). Folding, faulting and island emergence took place during Pliocene time' in southern Malaita followed by minor Quaternary faulting and regional tilting (Hughes and Turner, 1976). The dominant structures trend north •••.est.

Ulawa.--As much as 330 m of Pliocene calcareous mudstone and slumped conglomerate beds (the Holohau Mudstones of Danitofea, 1978) occur on Ula•••.a. They are overlain by Pleistocene reef limestone as much as 80 m thick (Ngorangora Limestone of Danitofea, 1978).

SUMMARY

Massive to pillOW'ed tholeiitic basalt and' its metamorphosed cOUl1.terparts form most of the basement rocks of the Solomon Islands. Volcaniclastic strata are a minor constituent in the basement complexes. These cretaceous and Paleocene ocean-floor rocks are exposed on all of the large islands except those in the Ne•••.Georgia groupi they also occur on the smaller Florida Islands and U1a•••.a. Isotopic studies of basalt and alnoite from Malaita suggest that these rocks were derived from an undepleted mantle source and not from old, depleted oceanic mantle or subducted old continental crust (Bielski-Zyskind et aI, 1984). other bodies included in the basement are intrusions of gabbro and diabase. An altered gabbro that intrudes metamorphosed basalt on Guadalcanal yielded an age of 92 .• 20 Ma (cencmanaant ) , On Malaita, the upper part of the flow sequence is inter layered with pelagic limestone that contains foraminiferal assemblages as old as early late Cretaceous (early Cenomanian) and possibly as old as late Early Cretaceous (Albian). Else •••.here, these igneous rocks ShCM varying degrees of metamorphism, largely zeolite to greenschist and amphibolite facies. A 66 Ma radiometric age has been assigned to a flCM in the basement sequence on Santa Isabel, where intercalated pelagites near the top are as old as late Paleocene. K/Ar apparent ages of metamorphism on Choiseul range from 32 to 51 Ma and on the Florida Is lands from 35 to 44 Ma. Elongate belts, pods and slab-like bodies of ultramafic rocks, chiefly serpentinized harzburgite, are present on Choiseul, Santa Isabel, Florida Islands, Guadalcanal, and San cristobal. It seems likely that they are protruded masses and thrust sheets that represent either dismembered and remobilized ophiolite or fragmented roots of an island arc. A gabbroic rock from the largest belt on Small Nggela in the Florida island group gave K/Ar apparent ages of 36.7 and 38.4 Mai elsewhere these rocks have not been dated. Detritus shed from these bodies first appears in strata of early Miocene age.

Coulson, Vedder: Island Geology 59 Island-arc tholeiites and related intrusive complexes dominate the late Eocene(?) to early Miocene subduction-generated volcanic sequences on the Florida Islands and Qladalcanal. The volcanic rocks are chiefly basalt and basaltic andesite and subordinate andesite. Intrusions of dioritic rocks are ccemcnpf.ace, and mantle-derived alnoite breccia was ert;)laced in diatreme-like structures on Malaita. Interlayered with and overlying the flow sequences are volcaniclastic strata and limestone. Volcanic flows diminish in volume in the early Miocene sequences, and redeposited volcaniclastic strata increase in thickness and abundance. Miocene carbonate beds, inclUding hemipelagic4 shallow-shelf and reef deposits, are interbedded in the volcaniclastic sequences and generally are increasingly cOIl'lmonupsection. These Oligocene to middle Miocene igneous and sedimentary rocks may extend beneath the younger volcanogenic rocks of the NewGeorgia island group. Late Miocene to Holocene calc-alkaline volcanic rocks and cogenetic epiclastic deposits form most of the NewGeorgia island group as well as Savo, the Russell Islands, Mborokua,and parts of the Shortland fs Lande, Choiseul, and Guadalcanal. Rock types include pyroxene andesite, olivine basalt, picrite (New Georgia), dacite (Fauro), and intrusive diorite and gabbro (Guadalcanal). Pyroclastic deposits are common,particUlarly in NewGeorgia rnd Guadalcanal. Nearshore clastic strata ranging from clayey mudstone to conglomerate are present on many of the islands but are especially well developed on Guadalcanal, where they are more than 5,000 m thick. Alluvial sand and gravel underlie most of the coastal plain of northern Guadalcanal but are very limited on the other islands. Cort;)ositional, distributional and deformational patterns of exposed rock sequences reflect three well-defined episodes of Eocene and younger tectonism. After ocean-floor generation and accumulation of Cretaceous and lower Tertiary basalt and pelagic limestone, an episode of Eocene and (or) early Oligocene metamorphismaltered the oceanic basement rocks to greenschist and amphibolite at manyplaces and resulted in a regional unconformity. The metamorphismmayhave been a response to incipient southward subduction of the Pacific plate beneath the Australia-India plate. The second episode of tectonism both accompanied and followed the late Eocene to early Miocene subduction-related Oligocene magmatic event and created an influx of large amounts of volcaniclastic detritus onto upwarpedshelves and slopes. During the latest Oligocene and early Miocene, relatively extensive deposition of limestone, calcarenite, and marl signified cessation of volcanism. Uplift, tilting, and thrusting probably occurred throughout the Miocene and are anifested in rocks on Guadalcanal, Choiseul, and Santa IsabeL A contributing factor to regional tectonism late in the second episode mayhave been oblique impingementof the Ontong Java Plateau on the old forearc area of the Solomons, a- circumstance that possibly led to transform structures and initial deformation of the Malaita anticlinorium. The third episode was concurrent with the postulated reversal of arc polarity and a new phase of subduction-related magmatism that began near the end of Miocene time. Emergenceof the southwestern row of islands, extension, and developnent of the Central Solomons Trough were direct results of this third episode of tectonism, which seems to have been most active in Pliocene and Pleistocene time. Thick aprons of volcaniclastic sediment accumulated on the back (northeast) side of the ne.•••arc and were dammedby remnants of the older northward-facing arc to form the upper part of the depositional sequence in the intra-arc basin. Post-Miocene faulting and minor folding probably continued on Guadalcanal, Choiseul, Santa Isabel, and San Cristobal, and

Coulson, Vedder: Island Geology 60 relatively intense folding developed on Malaita during the Pliocene. The building of volcanic edifices on the Shortland Islands, Choiseul, and Guadalcanal and the complete burial by the volcanoes of the Ne••••Georgia group obscured the middle Miocene and older arc rocks throughout the southwestern flank of the intra-arc basin. Late Miocene and younger fringing reefs were built around inactive volcanic islands, and associated shelf and slope carbonate deposits ••••ere laid downat places in environments a••••ay from active volcanic centers. Detailed mapping of santa Isabel and san Cristobal and additional paleontologic and radiometric dates are required on all of the islands before many of the geologic enigmas of the SOlomonIslands can be resolved. Among the most perpleXing problems are the poorly understood relations along the K.1a-Korigole-Kaipito fault system of santa Isabel. This fault system almost certainly reflects a major suture between two tectonostratigraphic terranes represented by rocks on Ulawa, Malaita and northeastern santa Isabel on the northeast and san Cristobal, Guadalcanal, southwestern Santa Isabel, and Choiseul on the southwest. Whether it is a compressional or transform feature, or both, is uncertain, and the timing of movementis unresolved. "nother problem is the imprecisely knownage of emplacementof the ultramafic bodies, ••••hose parent terranes remain unidentified. The axis of the northeast- facing Oligocene arc, and the location of its forearc and backarc basins are not yet established. Nor are the amountsof rotation and translation, if any, of various parts of the arc. The offset or superposition of the southwest- facing late Miocene-Holocene arc ••••ith respect to the older arc are conjectural, and the reasons for gaps in seismicity and volcanism are in question. Effects of the subduction of the active Woodlarkspreading axis are not entirely resolved. In short, nuch more needs to be learned about the geology of SolomonIslands before a plethora of questions can be answered.

Coulson, Vedder: Island Geology 61 REFERENCES

Arthurs, J.W., 1981, The geology of the Mbarnbatana area, chct seut , An explanation of 1:50,000 scale geological map sheet CH 5: British Technical Cooperation western Solomons Mapping Project, Report no. 5, 99 p. Bielski-Zyskind, M., G.J. Wasserburg, and P.R. Nixon, 1984, sm-Nd and Rb-Sr systematics in volcanics and ultramafic xenoliths from Malaita, Solomon Islands, and the nature of the Ontong Java Plateu: Journal of Geophysical Research, v. 89, no. B4, p. 2415-2424. Blake, D,H., and Y. Miezitis, 1967, Geology of Bougainville and Buka Islands, New Guinea: Bureau of Mineral Resources, Geology, and Geophysics, Australia, Bulletin no. 93, PNG1, 56 p. Carey, S.W., 1958, The tectonic approach to continental drift, in Continental drift--a symposium: Geology Department, University of Tasmania, Hobart, p , 177-355. Chivas, A.R., and I. McDougall, 1978, Geochronology of the Koloula porphyry copper deposit, Guadalcanal, Solomon Islands: Economic Geology, v, 73, P 678-689. "oLeman, P.J., 1957, Geology of Western Guadalcanal, .ia Marshall, C.E., et aI, ede ; , Geological reconnaissance of parts of the central islands of the British Solomon Is-Iands Protectorate: Colonial Geology and Mineral Resources, v. 6, no. 3, p. 267-307. 1960a, An introduction to the geology of Choiseul in the western Solomons, 1957; British Solomon Islands Geological Record (1957-1958), v , " p. 16-26. 196Gb, North-central Guadalcanal an interim geological report: British Solomon Islands Geological Record {1957-19581, v. 1, p. 4-13. 1962, An outline of the geology of Choiseul, British Solomon Islands: Journal of the Geological Society of Australia, v. B p. 135-158. 1965, Stratigraphical and structural notes on the British Solomon Islands with reference to the first geological map: British Solomon Islands Geological Record (1959-1962), v, 2, p. 17-31. 1966, The Solomon Islands as an island arc: Nature, v. 211, p. 1249- 1251. 1970, Geology of the Solomon and New Hebrides Islands, as part of the Melanesian Re-entrant, Southwest Pacific: Pacific Science, v, 24, p , 289-314. 1975, The Solomons as a non-arc: Bulletin of the Australian Society of Exploration Geophysicists, v. 6, no. 213, p. 60-61. 1976, A re-evaluation of the Solomon Islands as an Arc System, l.£. G.P. Glasby and H.R. Katz, eds ; , Marine geological investigations in the Southwest Pacific and adjacent area: CCOP!SOPACTechnical Bulletin, no. 2, p. 134-140. and L.W. Kroenke, 1981, Subduction without volcanism in the Solomon Islands arc: Geo-Marine letters, v, 1, p , 129-134. and G.H. Packham, 1976, The Melanesian Borderlands and India-Pacific Plates' boundary: Earth-Science Reviews, v. 12, p. 197-233. ---:;-;:0 et aL, 1965, A first geological map of the British Solomon Islands, 1962, 2:!!.. Reports on the geology, mineral resources, petroleum possibilities, volcanoes, and seismiscity in the Solomon Islands: Record of the Geological Survey of the British Solomon :;:slands (1959-1962), v, 2, Report no. 28, p , 16-17.

Coulson, Vedder: Island Geology 62 ___ .,.._ B. McGowran, and W.R.H. Ramsay, 1978, New, early Tertiary, ages for basal pelagites, northeastern Santa Isabel, Solornon Islands (central southwest flank, Ontong Java Plateau): Bulletin of the Australian Society of Exploration Geophysicists, v. 9, no. 3, p. 110-114. :urtis, J. W., 1973, Plate tectonics and the Papua Ne••••Guinea-Solomon Islands region: Journal of the Geological Society of Australia, v. 20, pt. 1, p. 21-36. Danitofea, 5., 1978, The Geology of Ulawa Island: solomon Islands Geological Survey Bulletin No.4 (unpublished). and C.C. Turner, 1981, The geology of the Russell Islands and Mborokua: Solomon Islands Geological survey Bulletin No. 1"2 (unpublished) • Dunkley, P.M., 1983, Volcanism and the evolution of the ensimatic Solomon Islands Are, in o. Shi.mozuru and I. Yokoyama, eds ,, Arc volcanism: physics and tectonics: Terrapub, TOkyo, p. 225-241. Falvey, O.A., 1975, Arc reversals, and a tectonic model for the North Fiji Basin: Bulletin of the Australian Society of Exploration Geophysicists, -e, 6, no. 213, p. 47-49. Hackman, B.D., 1968, Observation on folding in the Oligocene-Miocene limestones of central Kwara'ae, Malaita: British Solomon Islands Geological Record (1963-1967), v. 3, Report no. 76, p. 47-50. 1973, The Solomon Islands fractured are, .!!L P.J. Coleman, ed,, The Weste.rn Pacific: Island arcs, marginal seas, geochemistry: University of Western Australia Press, p. 179-191. 1979, The geology of the Honiara area, Guadalcanal: Solomon Islands Geological Survey Bulletin, no. 3, 40 p. 1980, The geology of Guadalcanal, Solomon Islands: Overseas Memoir of the Institute of Geological Sciences, no. 6, Her Majesty's Stationery Office, London, 115 p. Hill, J.H., 1960, Further exploration in the Betilonga area of Guadalcanal: British Solomon Islands Geological Record (1957-1958), v. 1, p. 81-94. Hughes, G.W., 1977, The geology of the Lungga Basin area, Guadalcanal: Solomon Islands Geological Survey Bulletin No.6 (unpublished). 1981, The geology of the Ririo area, ChoiseUl, an explantation of the 1:50,000 Geological Map Sheet CH 4: British Technical Cooperation Western Solomons Mapping Project, Report No.5, 55 p. 1982, Stratigraphic correlation between sedimentary basins of the ESCAP region, Solomon Islands, in UN ESCAP Atlas of Stratigraphy III, Mineral Resources Development Series: United Nations, New York, p. 115-130. and C.C. Turner, 1976, Geology of South Malaita: Solomon Islands Geological Survey Bulletin No.2, 80 p., 3 maps. 1977, Upraised Pacific Ocean floor, southern Malaita, Solomon Islands: Geological Society of America Bulletin, v. 88, p. 412-424. Jeffery, D.H., 1975a, Arosi, San Cristobal Sheet SC 1: Geological survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. 1975b, Arosi--West Bauro, San Cristobal sheet SC 2: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. Karig, D.E., and J. Mammerickx, 1972, Tectonic framework of the New Hebrides island arc: Marine Geology, v. 12, p. 187-205. Kroenke, L.W., 1972, Geology of the Ontong Java Plateau: Hawaii Institute of Geophysics, Report no. HIG-72-5, University of Hawaii, 119 p.

Coulson, Vedder: Island Geology 63 1984, Solomon Islands: San Cristobal to Bougainville and Buka, in LeW. Kroenke, ede, Cenozoic tectonic developnent of the southwest Pacific: CCOP/SOPACTechnical Bulletin, nOe 6, che 4, 22 p. Neef, G., 1978, A convergent subduction model for the Solomon Islands: Bulletin of the Australian Society of Exploration Geophysicists, v, 9, no. 3, p. 99-103. 1979, cenozoic stratigraphy of Slnall Nggela Island, Solomon Islands-- early Miocene deposition in a forearc basin followed by Pliocene patch reef deposition: New zealand Journal of Geology and Geophysics, -r, 22, no. 1, p , 53-70 and r , McDougall, I., 1976, Potassium-argon ages on rocks from small Nggela Island, British Solomon Islands, s. W. Pacific: Pacific Geology, v» 11, p. 81-86 and I.R. Plimer, 1979, Ophiolite complexes on small Nggela Island, Solomon Islands; summary: Geological Society of America Bulletin, pt. 1, v. 90, p.136-138. Nixon, P.H., 1980, Kimberlites in the southwest Pacific: Nature, v- 287, p; 718-720. 1978, Garnet-bearing lherzolites and discrete nodule suites from the Malaita alnoite, sotceon Islands, and their bearing on the nature and origin of the Ontong Java Plateau: Bulletin of the Australian Society of Exploration Geophysicists, v. 9, nO. 3, pe 103-107. Packham, G.B., 1973, A speculative Phanerozoic history of the Southwest Pacific, in P.J. Coleman, ed., The western Pacific: island arcs, marginal se;s, geochemistry: University of Western Australia Press, p , 369-388. Plimer, I.R., and G. Neef, 1980, Early Miocene extrusives and shallow intrusives from Small Nggela, Solomon Islands: Geological Magazine, v, 117, no. 6, p , 565-578. Proctor, W.D., and C.C. Turner, 1977, The geology of save Island: Solomon Islands Geological Survey Bulletin No. 11, 44 p. Ramsay, W.R.H., 1978, Field, mineralogical and structural observations on some basement rocks, southeast Choiseul, Solomon Islands: Bulletin of the Australian Society of Exploration Geophysicists, v. 9, p. 107-110. Ravenne, C., C.!. de aroin, and F. Aubertin, 1977, Structure and history of the Solomon-New Ireland region, ~ International symposium on geodynamics in the SW Pacific, New Caledonia, August-september 1976: Editions Technip, Paris, p. 37-50. Rickwood, F.K., 1957, Geology of the island of Malaita, .!E.. Geological reconnaissance of pair of the central islands of the British Solomon Islands Protectorate: Colonial Geology and Mineral gescurces , v, 10, no. 2, p. 112-145. Richards, J.R., A.W. webb, J.A. Cooper, and P.J. Coleman, 1966, Potassium- argon measurements of the age of basal schists in the British Solomon Islands: Nature, v, 211, p, 1251-1252. Smith, A., 1980, The geology of the Nuatambu area, Choiseul: an explanation of 1:50, 000 Geological Map Sheet CH 8: British Technical Cooperation western Solomons Mapping Project, Report no. 8, 83 p. Solomon Islands Department of Geological Surveys, 1969, Geological map of the British Solomon Islands: Department of Geological Surveys, Honiara, Guadalcanal, Solomon Islands, 1: 1,000,000.

COUlson, Vedder: Island Geology 64 Stanton, R.L., 1961, Explanatory notes to accompany a first geological map of Santa Isabel, British Solomon Islands Protectorate: Overseas Geology and Mineral Resources, v. 8, no. 2, p. 127-149. and W.R.H. Ramsay, 1975, Ophiolite basement complex in a fractured island chain, santa Isabel, British Solomon Islands: Bulletin of the Australian Society of Exploration Geophysicists, v, 6, no. 2/3, p. 61- 64. strange, P.J., 1981a, The geology of the Xomboro and Rob Roy Island area, Choiseul: an explanation of the 1:50,000 Map Sheet CH 11: British Tech- nical Cooperation Western Solomons Mapping Project, Report no. 11, 72 p. ______~ 1981b, The geology of the Katurasele area, Choiseul; an explanation of the 1:50,000 Map Sheet CH 6: British Technical Cooperation Western Solomons Mapping project, Report no. 6, 57 p. Taylor, B.R., and N.F. Exon, 1984, An investigation of ridge subduction in the Woodlark--Solomons region: introduction and background, .!!!.. N.F. Exon and B.R. Taylor, compilers, Seafloor spreading, ridge subduction, volcanism and sedimentation in the offshore Woodlark-Solomons region and Tripartite cruise report for Kana Keoki cruise 82-03-16, Leg 4, CCOP/SOPACTechnical Report no. 34, p , 1-42. Taylor, G.R., 1965, The Paraso thermal area, Vella teve Lfa , preliminary report: Solomon Islands GeOlogical Survey Bulletin, no. 1, 12 p. 1976, Styles of mineralization- in the Solomon Islands--a review, in G.P. Glasby and H.R. Katz, eds •., Marine geological investigations in the Southwest Pacific and adjacent areas: UNESCAPTechnical Bulletin 2, p. 83-91. 1977, Florida Islands Geological Map Sheet FL 1: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. Thompson, R.B.M., 1958, The geology of the Florida Group, .!!:!. The Solomon Islands-geological exploration and research, 1953-1956: Memoir of the Geological Survey of the British Solomon Islands, no. 2, p. 97-101. 1960, The geology of the ultrabasic rocks of the British Solomon Islands: Unpublished Ph.D. thesis, University of Sydney, Australia. 1968, Southwest Guadalcanal--the Itina River Basin: British Solomon Islands Geological Record (1963-1967), v. 3, p. 9-14. and P.A. PUdsey-Dawson, 1958, The geology of eastern san Cristobal: Memoir of the Geological Survey of the British Solomon Islands 1955-1956, v, 2, p. 90-95. Turner, C.C., 1975, The geology of Mborokua: Solomon Islands Geological Survey Bulletin No.7, 15 p. 1978, Shortland Islands, Shortland Islands Geological Map Sheet SH 1A: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. and B.D. Hackman, 1977, The geology of the Beaufort Bay area, Guadalcanal: Solomon Islands Geological Survey Bulletin No. 9 (unpublished) • and J. Ridgway, 1982, Tholeiitic, calc-alkaline, and (1) alkaline igneous rocks of the Shortland Islands, Solomon Islands: Tectonophysics, v. 87, p. 335-354. van Oeventer, J., and J.A. Postuma, 1973, Early Cenomanian to Pliocene deep marine sediments from North Malaita, Solomon Islands: Geological society of Australia Journal, v. 20, part 2, p. 145-152.

Coulson, Vedder: Island Geology 65 weissel, J. K., B. Taylor, and G. D. Karner, 1982, The opening of the Wbodlark Basin, subduction of the Wbodlark spreading system, and the evolution of northern Melanesia since mid-Pliocene time: Tectonophysics v, B7, p. 253. Wright, P.C., 1968, Western Guad,~lcanal--thegeology of the Lungga and Tenaru River systems: British Solomon Islands Geological Record (1963-1967), v. 3, p. 25-40.

Coulson, Vedder: Island Geology 66

CORRELATIONOF ROClt UNITS IN THS SOLOfI)N ISLANDS

L S. Pound U.S. Geological Survey, Menlo Park, california 94025

INTRODUCTION

The stratigraphic columns shown in this paper were compiled from ene available literature on the Solomon Islands (See map packet). Each colwnn represents the rock succession on parts of islands or on entire island groups, as designated on Figure 1. Rock type, thickness, contact relations, and chronology are indicated on the columns. The stratigraphic names used are from the most recent literature, except where otherwise indicated in the text. Where previous work has resulted in conflicting stratigraphic nomenclature and ages (Ls e , san Cristobal), the available data are tabulated (Table 1). Interpretations of regional tectonic activity are given in the right-hand column and are discussed at the end of this paper. The columns presented in this paper differ from those of Coulson and Vedder (this volume) both in their format, and in the correlation of some of the rock units. Some of the most critical problems of stratigraphic correlation are described belo,., on an island-by-island cae re , particularly at points where they differ from other correlations (Hughes, 1982; Coulson and Vedder, this voLumeL, For a detailed description of the on-land geology, see Coulson and Vedder (this volume). In some regions stratigraphic data are sparse because of the limited amount of geologic exploration, particularly in such key areas as Santa Isabel. Recent work (Turner and Hughes, 1982; Turner and Ridgway, 1982; Dunkley, 1983) has focused on regional syntheses of the sedimentary and igneous rocks, but earlier studies focused on local formational units rather then regional cor re.Lat.Lcns, Where rocks of similar lithologies are now in fault juxtaposition, such as the Miocene strata (the Tanakau Group and the Longuhutu and Bero Beds) or the Eocene~ligocene volcanic rocks (the Sigana Volcanics) on santa I$abel, correlations have, in the past, been made without reservation across major faults. These correlations probably should be reevaluated. Age control is limited to Cenozoic and late Mesozoic dates based upon foraminiferal assemblages in the sed1mentary rocks and radiometric dates on a few volcanic and intrusive rocks. Hughes (1981a) summarized age determinations for Solomon Islands rocks. The tectonic structure of the Solomon Islands region is complex and not fully understood (Vedder and Coulson, this volume). Development of a definite model for the tectonic evolution of the region is hampered by the shortage of detailed petrogenic data, the structural compleXity of the region, and a tendency to keep the model eImpLe,

Pound: Correlation 67 SHORTLANDISLANDS

The geochemical data obtained from the volcanic and intrusive bodies of the Shortla.nd Islands remain problematic; for details refer to Turner (1978), Turner and Ridgway (1982) and Dunkley (1983). other workers have coxre Je'ee d the basal volcanics on Mono and Alu with basement volcanics (Voza Lavas) on Choiseul (Coulson and Vedder, this volume), althoUgh no radiometric ages have been obtained for the basement rocks on Mono and Alu.

NEWGEORGIAGROUP

Recently published geologic maps of Vella Lavella, Kolombangara, Kohinggo, Parara, Ranongga, Simbo, and Ghizo Islands in the New Georgia Group show many new formation names for the highly variable volcanogenic and sedimentary sequences comprising the variable island successions (Smith et aI, 1982: Abraham et aI, 1984( Abr.aham, Smith, and Hughes, 1982). These new formation names are not shown on the correlation chart, which- is based on the reconnaissance work of Stanton and Bell (1969). The recent mapping confirms the highly variable and complex nature of the volcanogenic rocks, which include large volumes of olivine-rich basalts and picrites as well as minor amounts of hornblende andesite and two-pyroxene andesite (Dunkley, 1983).

CHOISEUL

The Choiseul Schists and the Voza Lavas were treated as two separate units by Coleman (1965). Ramsay (1978), however, showed that in Ologholata Harbour in southeast Choiseul the Voza Lavas may be the unmetamorphosed equivalent of the Choiseul Schists - an interpretation which recent mapping elsewhere on Choiseul seems to confirm (Arthurs, 1981; Hughes, 1981b, 1981c, 1981d. 1981el Philip, 1981; Smith, 1981: Strange, 1981a, 1981b, 1981c). In southeast Choiseul, a 44 ms y , +/- 18 m.y. radiometric age is reported for the Choiseul Schists (Richards et; aI, 1966). Schists in central and northwest Choiseul are interpreted in this report as t¥ne' equivalent to those in southeast CnoLaeuL, some:w-orkers"(Arthurs; 1981; Smith, 1981) have included amphibolites and greenschists within the Voza Lavas. However, in this report metamorphic rocks are considered as Choiseul Schist. The contact between the Choiseul Schist and the Voza Lavas has been described as both gradational (Strange, 1981a) and faulted (Hughes, 1981b). Arthurs (1981 l used the term Upper Kolombangara Tectonic Unit for an amphibolitized zone of faulted Choiseul Schists and Voza Lavas that marks the contact. In southeast and central Choiseul, the Voza Lavas are intruded by gabbroic rock (the Oaka Metamicrogabbrol that has been aecemorpnceed to amphibollte facies. Gabbroic and diabasic dikes intruding the Voza Lavas in northwest Choiseul are assumed to be coeval with the Qaka intrusive body (Strange, 1981a) of southeast Choiseul. In southeast Choiseul the Siruka Ultramafics overlie the Choiseul Schists and Voza Lavas and have been interpreted as a nearly horizontal thrust-emplaced slab (Hughes, 1981d). The timing for the emplacement of the Siruka Ultramafics is unclear. In Southeast Choiseul, the Voza Lavas, Choiseul Schists, and Siruka

Pound: Correlation 68 Ultramafics are either unconformably overlain by, or in fault contact with, the early Pliocene and younger KomboroVolcanics or the Vaghena Formation. The xomboro Volcanics and Vaghena Formation are discrete time-equivalent sequences. Hughes (1981e) suggested that the Vaghena Formation is in part volcanic. Northward, in central Choiseul, the Vaghena thins and contains mid- Miocene to Pliocene foraminifers. The relationship between the Vaghena Formation with the underlying Mole Formation in central Choiseul is unclear (Smith, 1981; Philip, 1981). In central and northwest Choiseul, the Choiseul Schist and the Voza Lavas are unconformably overlain by the Mole Formation. There are conflicting interpretations regarding a pyroclastic unit that Coleman (1965) and Hughes (1981f) described as the basal Koloe Breccia member of the Mole Formation; the unit described by Coleman (1965) and Hughes (1981f) contains late Oligocene foraminifers (Hughes, 1981f). Philip (1981) described the upper Voza Lavas as containing pyroclastic material. Smith (19B1) postulated that the Maetarnbe Volcanics lie on the voaa Lava basement and are interbedded throughout the Mole Formation. The xoLoe Breccia is here shown as the basal eember of the Mole Formation, and the Maetambe Volcanics are shown to overlie the Mole Formation gradationally. In northern northwest Choiseul, the Mole Formation grades upward into the Pemba Formation. The relationship of the Pemba to the time-equivalent Maetambe Volcanics of central Choiseul has not yet been established. The Pleistocene Nukiki LiJnestone is at the top of the sedimentary sequence throughout the island.

SANTAISABEL

Relatively little work has been done on Santa Isabel. Most workers (Stanton, 1961; Coleman, 1965) have assumed that the volcanic piles to the north and south of the Kaipito-Korigole-Kia faults are related, as is implied by their commonname, Sigana Volcanics. Furthermore, it has been assumed that the sedimentary sequences overlying the Sigana Volcanics to the north and south of the Kaipito-Korigole-Kia faults have a common origin. However, Coulson and Vedder (this volume) suggest that because these rocks are separated by a maJor tectonic structure (the Kaipito-Korigole-Kia fault system) they are unrelated. The sedimentary rocks northeast of the Kaipito-Korigole-Kia faults have collectively been called ~he Tanakau Group (stanton, 1961; Coleman, 1965). The coeval sedimentary rocks to the south .••.est of the faults have been termed the Longuhutu beds on San Jorge or the Sero and Rob Roy beds on Santa Isabel. The age of the Kia-Kaipito-Korigole faults is not known, and their extent and nature beyond the northwest end of the island is uncertain. Likewise, the age and time of eIlFlacement of the ultramafiC bodies is unknown. The original spatial relationship of the Sigana Volcanics to the basement complex and the Vitoria Microgabbros is not evident from the rock descriptions or from the geologic map. The location of the volcanic axis for the Sigana VolcaniCS is not eva denu,

FLORIDAISLANDS

Mapping in northwestern Nggela Sule shows that the geology differs significantly from that of Nggela Pile. No attempt is made here to correlate

Pound: Correlation 69 the geology of Nggela Sule and Nggela Pile. On Nggela Pile the east-trending Hanuvaivine ultramafic belt separates the northern and southern sedimentary sequences. The Siota beds to the north of the Hanuvaivine ultramafic belt are cut by the north-trending, possibly diapiric, Siota Ultramafics. The timing and nature of the ultramafite eltq)lacement is not clear. To the south of the Hanuvauvine, the Ghumba beds in the eastern part of the island were folded during middle to late Miocene time.

GUADALCANAL

Hackman (1980) provided the most up-to-date synthesis of the geology of Guadalcanal, as well as a review of the previous geologic investigations. Hackman (1980) divided the pre-Miocene basement into (1) The Mbirao Group and (2) The Guadalcanal Ultrabasics. Nomenclature within the pre-Miocene basement rocks is confusing. The Guadalcanal Ultrabasics are termed the Ghausava, Itina, Suta, or Marau Ultrabasics, depending on their geographic location and structural position. Likewise, the Mbirao Group has been subdivided (Hackman, 1980) into (1) The Mbirao Metabas'ics; (2) The Guadalcanal Gabbro, (3) The Mbirao Dolerites, (4) The Tetekanji Limestones, an~ (-S). The Mbirao Volcanics. Some of these units are further subdivided locally (Hackman, 1980). Limi ted field data hinder both correlations and structural interpretations within the pre-Miocene basement. Geochemical analyses of the basement rocks are not adequate to determine their affinities or deformational history. The 92 +/- 20 ms y , K/Ar age (Snelling, 1970, as cited in Hackman, 1980) for the Guadalcanal Gabbro is the only age control within the pre- Miocene basement. The place the basement units initially formed in relation to their present position, or the position of the Ontong-Java Plateau cannot be determined from the on-land geology of Guadalcanal. The Miocene and Pliocene sedimentary sequences show complex intertonguing relationships, and their correlation across the island is uncertain. Pliocene and Pleistocene volcanic and intrusive rocks on the western part of the island have complex and varied stratigraphic and intrusive relationships with the associated sedimentary units.

SANCRISTOBAL,UKI NI MASIANDPIO

Stratigraphic nomenclature for San Cristobal is here based on that of Jeffery (1975 a,b). For regions not covered by Jeffery (1975 a,b), the stratigraphic nomenclature of Turner and Hughes (1982), Hughes (1982), and Coleman (1965) is integrated with Jeffery's (1975 a,b) system. The changes in stratigraphic nomenclature and age assignments (Table 1) on San Cristobal reflect the lack of extensive rgional geologic data for San Cristobal. Rapid facies changes probably also account for the variability in ages and lithologies reported for the late Oligocene through the Pliocene; otherwise, the stratigraphic succession varies only slightly across the island. The basement is described as essentially a flat-lying sequence of volcanics haVing undergone low-grade rretamorphism (Coleman, 1965; Jeffery, 1975a,b). The amount of internal stacking or deformation is unclear. The contact between the basement and the overlying sedimentary sequence has been variously interpreted (Jeffery, 1975a,bl Turner and Hughes, 1982). Data available do not document in detail changes or hiatuses in the sedimentary

Pound: Correlation 70 column, or the nature and extent of the lateral facies changes in the Miocene- Pliocene sedimentary sequence. Extensive block faulting of the basement forned grabens in which portions of the Mi.ocene-pliocene sediments are now preserved. T~ng of the faulting is not well constrained.

MALAITA

Formation names and descriptions of the rocks in northwest Malaita are from Rickwood (1957). In Small Malaita (Maramasike) the sedimentary succession is generally similar to that 1n northwest Malaita, but different formation names are used (Hughes and Turner, 1976, 1977). In SInall Malaita lateral variation in the lithologies and poorly defined gradational boundaries are characteristic. The succession in south-central Malaita is intermediate between that of northwest Malaita and Small Malaita, and Rickwood's (1957) nonenclature is used. Thus the Are1are Limestone of Hughes (1982) is equLvaLent; to Rickwoods' (1957) Alite Limestone. The term Alite Limestone is here retained for south-central Malaita. The alnoitic breccia within the Alite Limestone of northwest -.Malaita is interpreted as having a diapiric origin, but the nature of "its contact is unclear (Hackman, 1968; Allen and Deans, 1965). Zircons from the alnoitic breccia provide a pb/ur date of 33.9 - 34.1 ms y , The peperite zones of south-central Malaita and small Malaita have been grouped with the Malaita Volcanics by Hughes and Turner (1976). Recently cotrq:liled maps of Malaita (Turner, 1979; Hughes, Proctor and Hackman, 1975; Turner, 1977, Turner, 1976) provide more detailed information on the geology than is shown on the correlation chart.

DISCUSSION

Tectonic setting

Three broad phases of development can be recognized in the rocks of the Solomon Islands: 1. Construction of the Cretaceous Ontong Java Plateau, and deposition of the overlying pelagic sedimentary cover, probably far from its present site. Formation of the oceanic basement rocks which now form the basement for most of the islands; mOst of these rocks are now metamorphosed. 2. Development of a northward-propagating Oligocene arc on the oceanic basement, an event associated with faulting, diapiric ultramafic intrusions, and followed ~ deposition of both forearc and backarc wedges of sediment. 3. Generation of Pliocene and Holocene arc volcanism, an event associated with rapid regional uplift. The timing and recognition of these three broad phases prOVides a background into which the complex stratigraphic and structural relationships of the Solomon Islands can be placed, and the problems identified.

Basement Rocks

The Malaita Volcanics and their sedimentary cover have been correlated with the Ontong Java Plateau sequence, on the basis of age and rock type (Hughes and Turner, 1977). Likewise the Sigana Volcanics and the overlying basal pelagites on northeast Santa Isabel have been correlated with the

Pound: Correlation 71 sequences on Malaita and the Ontong Java Plateau. It is unclear whether the Warahito Lavas and the associated sediments on san Cristobal represent a younger, nearer-ridge (?) portion of the Ontong Java Plateau, or a remnant. of oceanic crust derived from the west. The basement rocks of Choiseul, southwest santa Isabel and Guadalcanal are mostly metamorphosed to greenschist and amphibolite facies (Coleman, 1962; Stanton, 1961; Hackman, 1980); the age of netamorphism appears to vary. Some basement rocks, however, are unmetamorphosed (e.g. the Voza Lavas of Choiseul, the Sigana Volcanics of southwest santa Isabel, and the Mbirao Group of Guadalcanal). The relationship of the unmetamorphosed basement rocks to the metamorphic rocks has been variously interpreted (e.g. Stanton and Ramsay, 1975). It is not known whether the basement rocks and their unmetamorphosed counterparts shared a commonorigin or were juxtaposed during the formation of the Oligocene arc. Baserrent rretamorphism could have resulted from (1) near-ridge alteration of oceanic crust associated with a high thermal gradient and low pressures, or (2) from tectonic burial and deformation associated with high confining pressures and frictional heating on thrust planes. The metamorphic mineral assemblages developed in each of the two cases outlined above are destinctive (Apted and Liou, 1983)1 the low-pressure mineral assemblage (calcic- plagioclase-actinolite-chlorite) is the mineral assemblage reported from Guadalcanal (Hackman, 1980). The high-pressure assemblage (albite-epidote- hornblende), however, is the characteristic assemblage reported from Choiseul and Santa Isabel (Hackman, 19801 coleman, 1965). Clearly, the relationship between the R'etamorphic basement rocks and the development of the Oligocene arc is complex.

Oligocene to Middle Miocene Events

The transition from an oceanic pelagic environment to an arc environment is indicated in the deep-water sediments by the increasing amountS of volcanogenic material, as recorded on the rocks of Malaita and San Cristobal. On land, Oligocene arc development is recorded by the Suta Volcanics and the Poha and Lungga diorites of Guadalcanal, the older volcanics of the Shortland Islands, and the vcee Lavas of Chci.eeuL, Arc volcanism was followed by extensive erosion of regions of high relief, followed by basin development and rapid sedimentation, as is indicated by thick accumulations of clastic strata on Guadalcanal and Choiseul. Diapiric intrusion of ultramafic bodies such as those on the Florida Islands, Santa Isabel, and .Malaita probably began during this time. This phase of Arc magmatism and southwest-directed subduction was shut down by the collision of the Ontong Java Plateau, which subsequently created the reversal of arc polarity.

Pliocene to Holocene Events

The geochemically anomalous volcanic rocks on NewGeorgia, together with the volcanic centers on Choiseul and Guadalcanal attest to recent arc volcanism (Dunkley, 1983; Coleman and Kroenke, 1981). These volcanics were generated by northeast-directed subduction of the active Woodlark spreading ridge, which commenced during latest Miocene. Rapid uplift is indicated by uplifted limestone reefs throughout the Solomons region.

Pound: Correlation 72 REFERENCES

Abraham, 0., A. Smith, and G.W. Hughes, 1982, Ranongga, Simbo, and Ghizo Islands, New Georgia Geological Map Sheet NG 2: Geological Survey, Honiara, Guadalcanul, Solo~n Islands, 1:100,000. P.N. Dunkley, G.W. Hughes, and A. Smith, 1984, Kolombangara, Kohinggo, and Parara Islands, New Georgia Geological Map Sheet NG 3: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:100,000. Allen, J.B., and T. Deans, 1965, An alnoite breccia associated with the ilrnenite-pyrope gravels of Malaita, 1962: The British Solomon Islands Geological Record (1959-1962), v. 2, Report no. 49A, p. 136-138. Apted, M.J., and J.G. Liou, 1983, Phase relations among greenschist, epidote amphibolite, and amphibolite in a basaltic system, in H.J. Greenwood, e d.; , Orville volume, Studies in metamorphism and metasomatism: American Journal of Science, 283-A, p. 238-354. Arthurs, J.W., 1981, The geology of the Mbambatana area, Choiseul, an explanation of the 1:50,000 Geological Map Sheet CH 5: British Technical Cooperation Western Solomons Mapping ProJect, Report no. 5, 99 p. Coleman, P.J., 1962, An outline of the geology of Choiseul: Journal of the Geological Society of Australia, v. 8, pp. 135-58. 1965, Stratigraphical and structural notes on the British Solomon Islands with reference to the first geological map: British Solomon Islands Geological Record (1959-1962), v. 2, Report no. 29, p. 17-31. B. McGowran, and W.R.H. Ramsay, 1978, New, early Tertiary, ages for basal pelagites, northeastern Santa Isabel, Solomon Islands (central southwest flank, Ontong Java Plateau): Bulletin of the Australian Society of Exploration Geophysisists, v. 9, no. 3, p. 110-114. and L.W. Kroenke, 1981, Subduction .••.ithout volcanism in the Solomon Islands arc: Geo-Marine Letters, v. 1, p. 129-134. Dunkley, P.N., 1983, Volcanism and the evolution of the ensimatic Solomon Islands Are, in D. Shinozura and I. Yokoyama, eds., Arc volcanism: physics and tect;nics: Terrapub, Tokyo, p. 225-241. Hackman, B.D., 1968, Observations on folding in the Oligocene-Miocene limestones of central KWara'ae, Malaita: British Solomon Islands Geological Record (1963-1967), v. 3, Report no. 76, p. 47-50. 1980, The geology of Guadalcanal, Solomon Islands: OVerseas Memoirs of the Institute of Geological Science, Her Majesty's Stationery Office, London, no. 6, 115 p. Hughes, G.w., 1981a, catalogue of age determinations of Solomon Islands rocks: Occasional Paper, Geological Survey, Honiara, Guadalcanal, Solomon Islands, unpaginated. 1981b, The geology of the Ririo area, Choiseul: an explanation of the 1:50,000 Geological Map Sheet CH 4: British Technical Cooperation Western Solomons Mapping Project, Report no. 5, 55 p. 1981c, The geology of the Panggoe area, Choiseul; an explanation of the 1:50,000 Geological Map Sheet CH 7: British Technical Cooperation western Solomons Mapping Project, Report no. 7, 50 p. 1981d, The geology of the Claka and Siruka Bay area, Choiseul; an explanation of the 1:50,000 Geological Map Sheet CH 10: British Technical Cooperation Western Solomons Mapping project, Report no. 10, 92 p. 1981e, The geology of Vaghena Island, Choiseul; an explanation of the 1:50,000 Geological Map Sheet CH 12: British Technical Cooperation Western Solomons Mapping Project, Report No. 12, 59 p.

Pound: Correlation 73 1981f, The micropaleontology of the Shortland Islands and Choiseul: British Technical Cooperation Western Solomons Mapping project, Report no. 17, 42 p. 1982, Stratigraphic correlation bet~een sedimentary basins of the ESCAP region, Volume VIII, Solomon Islands, ESCAPAtlas of Stratigraphy III: Mineral Resources Development series, United Nations, New York, no. 48, p. 115-130. ____~~W.D. Procter, and B.D. Hackman, 1975, North 'Are'are, Malaita Map Sheet ML 12: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. and C.C. Turner, 1976, Geology of southern Malaita: Solomon Islands Geological Survey Bulletin, no. 2, 80 p. 1977, Upraised Pacific Ocean floor, southern Malaita, Solomon Islands: Geological Society of America Bulletin, v. 88, p. 412-424. Jeffery, D.H., 1975a, Arosi, san Cristobal Sheet SC 1: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. 1975b, Arosi-West Bauro, san Cristobal Sheet SC 2: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. Neef, G., 1979, Cenozoic stratigraphy of Small Nggela Island, Solomon ~s~~~ds- early Miocene deposition in a forearc baaIn followed by Pliocene patch reef deposition: New zealand Journal of Geology and Geophysics, v, 22, no. 1, p. 53-70. ____~cand I. McDougall, 1976, Potassium-argon ages on rocks from Small Nggela Island, British Solomon Islands, S.W. Pacific: Pacific Geology, v, 11, p. 81-96. and loR. Plimer, 1979, Ophiolite complexes on small Nggela Island, Solomon Islands; summary: Geological Society of America Bulletin, part 1, v, 90, p. 136-138. Nixon, P.H., 1980, Kimberlites in the southwest Pacific: Nature, v, 287, p , 718-720. ______and P.J. Coleman, 1978, Garnet-bearing lherzolites and discrete nodule suites from the Malaita alnoite, Solomon Islands, and their bearing on 'the nature and origin of the Ontong-Java Plateau: Bulletin of the Australian Society of Exploration Geophysicists, v. 9, no. 3, p. 103-107. Philip, P.R., 1981, The geology of the Mt. Sambe area, Choiseul; an explanation of the 1:50,000 Map Sheet CH 9: British 'recnn.rcaI Cooperation Western Solomons Mapping Project, Report no. 9, 38 p. Ramsay, W.R.H., 1978, Field, mineralogical, and structural observations on sorre basement rocks, S.E. Choiseul, Solomon Islands: Bulletin of the Australian Society of Exploration Geophysicists, v. 9, no. 3, p. 107-110. Richards, J.R., A.W. Webb, J.A. Cooper, and P.J. Coleman, 1966, Potassium- argon measurements of the ages of basal schists in the British Solomon Islands: Nature, v, 211, p. 1251-1252. Rickwood, F.K., 1957, Geology of the Island of Malaita in Geological Reconnaissance of parts of the central islands of the British Solomon Islands Protectorate: Colonial Geology and Mineral Resources, v. 6, no. 3, p. 300-306. Smith, A., 1981, The geology of the Nuatambu area, Choiseul, an explanation of the 1: 50,000 Map Sheet CH 8: British Technical Cooperation western Solomons Mapping Project, Report no. 8, 83 p. ___ .,,-_P.R. Philip, P.J. Strange, and G.W. Hughes, 1982, Vella Lavella Island, New Georgia Geological Map Sheet NG 1: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:100,000.

Pound: Correlation 74 Stanton, R.L•• 1961, Explanatory notes to accompany a first geological map of santa Isabel, British Solomon Islands Protectorate: Overseas Geology and Mineral Resources, London, v. 8, no. 2, p. 127-149. _ __ -=::-:-;~, and J.D. Eell, 1965, New Georgia Group, a preliminary geological statement: British Solomon Islands Geological ~cord (1959-1962), v, 2, Report no. 3', p. 35-36. , 969, Volcanic and associated rocks of the NewGeorgia group, British Solomon Islands Protectorate: Overseas Geology and Mineral Resources, v, 10, no. 2, po 113-145. and W.R.H. Ramsay, 1975, Ophiolite basement complex in a fractured island chain, santa Isabel, British Solomon Islands: Bulletin of the Australian Society of Exploration Geophysicists, v. 6, no. 2/3, p. 61-65. Strange, P.J., 1981a, The geology of the O1.oiseul Bay and O1.orovanga area, Choiseul, an explanation of the 1:50,000 Map Sheets CH 1 and CH 2: British Technical Cooperation western Solomons Mapping Project, Feport no. 1-2, 95 p , 1981b, The geology of the Katurasele area, Choiseul; an explanation of the 1:50,000 map sheet CH 6: British Technical Cooperation Western Solomons Mapping Project, Report no. 6, 57 p. 1981C, The geology of the Komboro and Rob Roy Island area, Choise~l; an explanation of the 1: 50, 000 Map Sheet CM 11: British Technical Cooperation Western Solomons Mapping Project, Report no. 11, 72 p. Taylor, G.R., 1977, Florida Islands geological map sheet FL 1: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. Turner, C.C., 1976, South Small Malaita, Malaita Geological Map Sheet ML 17: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. 1977, Central Small Malaita, Malaita Geological Map Sheet ML 16: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. 1978, Shortland Islands Geological Map Sheet, 5H 1A: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:100,000. 1979, South 'Are' are, Malaita Geological Map Sheet ML 15: Geological Survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. and G.W. Hughes, 1982, Distribution and tectonic illlplications of cre eeceoua-cue te rne ry sedimentary facies in Solomon Islands: Tectonophysics, v, 87, p. 127-146. ______and J. Ridgway, 1982, Tholeiitic, calc-alkaline and (?}alkaline igneous rocks of the Short land Islands, Solomon Islands: Tectonophysics, v. 87, p. 335-354. van Deventer, J., and Postuma, J.A., 1973, Early Cenomanian to Pliocene deep marine sediments from North Malaita, Solomon Islands: Geological Society of Australia Journal, v. 20, part 2, p. 145-152.

Pound: Correlation 75

REGIONAL OFFSHORE GEOLOGY OF THX SOLOMON ISLANDS

J. G. Vedder U.S. Geological Survey, Menlo Park, california 94025

P. I. CoulSOD Institute of Geological Sciences, Nicker Hill, Keyworth, England

INTRODUCTION

Marine- surveys- to investigate geologic problems and resource potent±al in the Solomon Islands began about 20 years ago (Vedder and Coulson, this volume). Reconnaissance geophysics cruises and DSDP drilling have provided a great deal of information on regional tectonics and stratigraphy, yet detailed knowledge of offshore geology remains relatively meager. This paper is intended not only as a synopsis of findings from previous work, but also as a summary of new information from the 1982 CCOP!SOPACR/V s.P. Lee Leg 3 cruise. In order to facilitate discription, the Solomon Islands segment of the Melanesian Borderland is divided into three physiographic-structural elements (Figures 1 and 2):

1. Central Solomons Trough and adjacent intra-arc basins. 2. Northeastern Solomons sea-Coral sea trench systems and related troughs and rises. 3. Ontong Java Plateau and adjoining structures.

CENTRALSOLOMONSTROUGH

Regional setting and Nomenclature

In the central and western Solomon Islands, the composite sedimentary and structural basin that separates the two main island chains was called the Central Solomons Trough by KatZ (1980). This structural trough supposedly is part of a larger feature that was designated the Solomon Basin by Ravenne et al (1977), a dismembered regional structure that purportedly extends northwestward along the southwest side of Bougainville into the New Ireland area. Individual parts of the composite intra-arc basin that forms the central Solomons Trough and an adjoining, separate basin were named later as a means of easy reference (Tiffin et aL, 1983). From northwest to southeast, these structural basins are: Short land basin, Russell basin, Iron Bottom basin, and Indispensable basin, which lies east of the Florida Islands platform (Fig. "1). The same names were used by Maung and Coulson (1983) with slight mod- ification. An additional name, New Georgia wedge, is applied by A. Cooper et

Vedder, Coulson: Offshore Geology 76 a L, (this volume) to a large lenticular body of high-velocity material that underlies the southwest side of the central Solomons Trough (Fig. 1). As discribed by de Brion et; al (1977) and Ravenne et al (1977), the Solomon Basin extends from the New Ireland region 1,600 >em southeastward to the Florida Islands; whereas the Central Solomons Trough of Katz (1980) is restricted to the basin that underlies NewGeorgia Sound. In his study of offshore basins in the Solomon Islands, Katz (1980) divided the region Lnt.c. two geologic provinces separated by a major zone of tectonism. He applied the informal names oceanic-pelagic Malaita province and main Solomon pecvance , which contains the Gentral Solomons Trough. Katz recognized internal structural complexities in the trough, including possible fragmentation by a fault zone beneath Manning Strait which he believed caused echelon segmentation of the basin. He stated that sediment thickness increases correspondingly with water depth and that the axial part of the trough is underlain by at least 2.5 to 4.5 kIn of relatively undeformed strata. He described Pliocene and younger folds and faults that locally are present along the basin margins. In a more recent report, Maung and Coulson (19B3) proposed the term central Solomons Basin for the entire semienclosed sea that stretches from Bougainville 850 km southeastward to its termination in the area between Ma-laiee- and San· Cristobal. Kroenke (198'4),' in his regional tect.bliiic synthesis, attributed the development of the central Solomons Trough to the reversal of arc polarity which resulted in the transformation of the former back-arc basin into the present intra-arc basin.

Stratigraphic and Structural Intra-arc Features

Shortland basin. --This basin, which is approximately 140 Jan long and 60 km wide, is bounded by the Shortland Islands on the northwest, Choiseul on the northeast and Vella Lavella on the southeast (Fig. 2). seismic-reflection and refraction data show that the basin contains as much as 5 kIn of strata above acoustic basement (A. Cooper, et aI, this volume; Bruns et aI, this volume). Three large, lenticular sedimentary bodies, are delimited by conspicuous unconformities. The upper two bodies, each as much as 2 km thick, probably were derived mainly from the late Miocene to Holocene volcanic arc to the south and west and presumably consist largely of Pliocene and younger volcaniclastic strata. The lowermost body, which at places is as much as 3 km thick, was derived from the late Eocene (?) to early Miocene arc to the north and northeast and is likely to be composed mainly of detritus eroded from basaltic flows and schistose basement. This oldest sedimentary body is interpreted to be of late Oligocene and Miocene age. Faults and folds generally trend northwest in the Short land basin. In the central part of the basin, folds deform only the lower part of the section, whereas along the northeast flank, folding affects all but the youngest Quaternary strata. On the southern margin of the basin the upper two sedimentary bodies are arched over the ridge, possibly in response to Q.laternary tectonism along the inner wall of the NewBritain Trench.

New Georgia wedge. --A lenticular mass of relatively high-velocity material occurs at shallow depth beneath the northeast margin of the New Georgia island group (A. Cooper et aI, this volume). This high-velocity mass may be more than 4 kIn thick directly east of the main island. The wedge probably is

Vedder, Coulson: Offshore Geology 77 compc aed largely of Pliocene and Pleistocene (1) effusive rocks and volcaniclastic strata that were generated by volcanoes to the south and west. A horst-like feature (Fatu 0 Moana of Taylor and Exon, 1983) overlies the nor-ehse see rn end of the wedge midway between Choiseul and Kolombangara. Until 1982, the highest part of this uplifted feature was interpreted as a submarine volcanic center based upon its morpholoqy (Katz, 1980; Coleman and Kroenke, 1981). However, the surveys by R/V Kana Keoki (Exon and Taylor, 1984) and R/V S.P. Lee (Bruns et aI, this volume) showed conclusively that the specious volcanic center consists of warped and upfaulted strata of late Pliocene (1) and younger age.

Russell basin. -~his basin is centered between santa Isabel and the Russell Islands and coincides with the southeastern half of the central Solomons Trough (Fig. 2). It is the largest single structure beneath New Georgia Sound where it forms a flat-floored bathymetric depression apprOXimately 300 km long and as much as 80 km wide. \di~ seismic reflection profiles show that three well-defined unconformities disrupt the 5-km thick sedimentary section in the Russell basin (Bruns et al, ~t,l:J,is- YQl,.WlIet~ This stratigraphic sequence presumably consists _of .~"- volcaniclastic rudstone and sandstone, largely turbidites, and subordinate carbonate deposits derived from fringing reefs. A segment of the NewGeorgia wedge is included on the southwestern flank of the basin. At times, sediment transport may have been down the trough axis from sources to the north, south and east. The Russell basin is a downwarped structure that is bordered by northwest-trending step faults on its northern and southern edges. Several small{1) northeast-trending faults transect the northwestern part of the basin and have orientations similar to faults on the islands of the New Georgia group. Many of the faults affect sea floor topography and apparently have been recently active (Colwell and Tiffin, this volume). Katz (1980) and Maung and Coulson (1983) suggested that folds are late Pliocene in age and that the unconformable cover is Pleistocene. However, low-amplitude, northwest-trending folds and faults south of san Jorge deform strata as young as Holocene. Larger folds of unknown orientation deform all but the youngest beds in the ~aternary section in the southeastern part of nd Russell basin (Bruns et a L, this volume).

Iron Bottom basin. -r-r ne expanse of water known as Iron Bottom Sound between Guadalcanal, Savo Island and the Florida islands group is underlain by the smallest of the bathymetric basins within the central Solomons Trough (Fig. 2). This small feature is approximately 4S km long and 30 km wide. Strata beneath this bathymetric basin are continuous with those in Russell basin. seismic-reflection profiles show well-defined and continuous sedimentary reflectors throughout most of the section (Vedder et aL, 1983) I which has a maximumthickness of about 4.5 km directly north of Honiara (Bruns et al, this volume). Most of the strata probably were derived from igneous and metamorphiC rocks on Guadalcanal, mainly as turbidites with secondary contributions from similar rocks on the Florida islands group. Quaternary volcaniclastic material from Savo, detrital carbonate, and reef limestone probably are subordinate constituents in the sequence. Minor, low-amplit1lde folds and faults of unknown orientation affect the strata in Iron Bottom basin.

Vedder, CcuLecm Offshore Geology 78 less than 1.a Jan thick probably consist of ~aternary reef detritus and volcaniclastic beds. The platform under Bougainville Strait is floored by Quaternary reefs, beneat.h which pliocene and olde!~ rocks similar to Miocene and Pliocene clastic strata on Choiseul and Fauro are believed to be present. Dredge samples (Colwell and Vedder, this volume) from the subsea ridge west of Vella Lavella are predominantly Q.laternary volcaniclastic sandstone that represents the upper part of a 2.0-to 2.S-Jan thick sequence of intercalated flOW', pyroclastic, and volcaniclastic rocks of probable late Miocene and younger age. These strata appear to be gently arched over the ridge and probably are faulted along the southwestern flank where the slope begins to steepen into the NewBritain Trench. The same beds dip northward and thicken into the Short land basin.

Int.ra-arc Depositional Environments

From the foregoing, it is concluded that the Central Solomons Trough is a largely extensional intra-arc basin that began to trap sediment as early as the late Miocene and that continued to enlarge since that time. The islands and submergent ridges that enclose the trough, together with the unconformities in it attest to rapid uplift and deep erosion accompanied by volcanism. The sediments that formed coalescing aprons behind (southwest of) the old, north-facing arc presumably consist of detritus that was derived directly from Oligocene and older rocks on Choiseul, Santa Isabel, the Florida islands group, and Guadalcanall and substantial lithofacies variation within them is expect.abLe, By analogy with island successions (Vedder and coulson, this volumef Pound, this volume) Miocene reef limestone and shelf carbonate deposits are likely to be interspersed with coarse clastic strata in offshore areas close to the islands. Bathyal turbidites and slump deposits are likely to typify sedimentary sequences in the deeper parts of the trough (Katz, 1980). Deposits similar to those of the late Miocene to Pleistocene lithosornes on Guadalcanal (Hackman, 1980) probably occur at places in the trough, particularly along the southwest flank where active volcanism prevailed at the time. Katz (1980) suggested that the floor of the trough may have subsided as mioh as 2,000 m during the last 1.0 m.y., a rate that corresponds to some of those implied by analysis of dredge samples (Colwell and Vedder, this volume; Fasig, this volume). The compleXity of the central Solomons Trough and its flanking ridges defies classification in standard island-arc terminology and reflects the changes in pattern of tectonism in the region. The thickness of sedimentary fill and lack of a throughgoing central magnetic anomaly seem to disqualify the Central Solomons Trough as a typical back-arc basin. If a northeast- facing ancestral arc is represented by the remnants of a volcanic axis through San Cristobal, Guadalcanal, southwestern Santa Isabel, Choiseul and Bougainville, then it seems that the northeastern flank of this belt must have constituted the fringe of a fore-arc basin in Oligocene and early Miocene time as suggested by Neef (1979). Furthermore, if reversal in arc polarity and upl.i.ft and extension related to late Miocene to Holocene northeast-directed subduction are accepted, then the thick sedimentary fill is attributable to rapid erosion of the new as well as the old island chains.

Vedder, Coulson: Offshore Geology 80 SOLOMON5EA-cORALSEAREGION

General setting

An irregular tract of deep-sea floor characterized b¥ a rami form pattern of ridges, basins, troughs and plateaus lies south of the New Britain-San Cristobal Trench system (Fig. 1). This segment of the Oligocene to HoLeeene Outer Melanesian Arc (carey, 1958) may include remnants of the older (Eocene?) Inner Melanesian Arc (Glaessner, 1950; Hackman, 1980). Although the crustal structure and tectonic history of this complicated plate-boundary region have not been completely resolved, Kroenke's (1984) summary provided a reasonable interpretion of the early evolution of the various arc components. In his reconstruction, he postulated early Tertiary convergence and formation of the Rennell are, early Eocene termination of seafloor spreading in the Coral sea Basin and initiation of subduction along the Rennell Trench segment of the Papuan-New caledonia subduction Zone. Be further proposed that subduction ended in late Eocene time when the Louisiade Plateau obstructed the trench.

Tectonic Elements

Woodlark Basin. --This basin lies southwest of the Solomons arc and bor- ders the southern side of the New Britain-san Cristobal Trench system. The bathymetric basin is 3 to 4 Jan deep and is bounded on the northwest by the Woodlark Rise and on the southeast by the Pocklington Rise. Sediment thickness varies from less than 50 m in the center of the basin to more than 500 m in local ponds near the margins (Taylor and Exon, 1983). The area is characterized by high heat flow, at least 3 to 4 H.F.U., reflecting the young age of the oceanic crust (Halunen and von Herzen, 1973; Taylor and Exon, 1983) and is floored by basaltic pillow lavas that appear to be typical mid-ocean ridge type (Taylor and Exon, 1983). Following Carey's (1958) suggestion that the Woodlark Basin originated as an extensional feature, Luyendyk et al (1973) and Curtis (1973) delineated plate boundaries on the basis of a few magnetic seafloor-spreading anomalies and seismicity. Weissel et al. (1982) further defined the spreading axis, which extends southwestward from the New Georgia island group to the D'Entrecasteaux Islands. Recent investigations of the Woodlark Basin during the R/V ~ Keoki cruise 82-03-16, Leg 4 have shown that the spreading center is marked by an axial rift valley about 10 km wide, with 500 to 1,OOO.m of relief. The pattern of seafloor spreading is characterized by east-west trending magnetic lineations sinistrally offset by north-south fracture zones. Simbo Ridge, which marks a major fracture zone, offsets the anomalies by about 65 km. (Taylor and Exon, 1983). Exon and Taylor (1984) concluded that initial separation of the woodlark and Pocklington Rises occurred approximately 4 Ma and that since then, seafloor spreading has averaged a total opening rate of 6 to 7 em/yr. They found, however, that this spreading has been asymmetric, as the rate is approximately twice as fast on the northern limb. During the 4-m.y. opening history of the Woodlark Basin, the spreading ridge and newly formed crust have been subducted at the New Britain-San Cristobal Trench during convergence of the Pacific and Australia-India Plates. There is no apparent flexure of Woodlark Basin crust into the subduction zone, as the basin crust simply abuts

Vedder, Coulson: Offshore Geology 81 the fore-arc lower slope; furthermore, there is no recognizable bathymetric trench opposite New Georgia, and seismicity is low (Exon and Taylor, 1984). This active spreading system is entering an oceanic subduction zone at a high angle, a relation that may be unique at present. The influence of this nearly orthogonal convergence on the petrochemistry of the volcanic rocks as well as on regional tectonism is not entirely clear but may be the cause of some of the enigmatic features of the arc.

Pocklington Trough.--Lying to the north of the Louisiade Plateau the Pocklington Trough stretches in a sigmoid pattern from the Louisiade Islands almost to Q1adalcanal (Fig. 1). The trough contains a thick sedimentary sec- ion and is marked by a large negative gravity anomaly (Coleman and Packham, 1976). seismically inactive, it has been interpreted as a relict subduction zone by Karig (1972). Recy et al (1977) suggested that the gross morphology of the trough supports the contention that it is a fossil north-directed subduction zone. If correct, this feature may constitute another fragment of the Eocene (1) Inner Melanesian Arc.

Rennell Arc.--South of Guadalcanal, the islands of Rennell and Bellona, Indispensable Reef and adjoining Rennell Trench form a northwest-trending trench-ridge pair about 300 km long (Fig. 1). The Rennell Trench generally is less than 4,500-m deep and is flat floored and partly filled with undeformed sediments of Oligocene (?) and younger age. On the northeast, the trench abuts a 1,300-m deep submarine plateau that is crowned by the uplifted atolls of Rennell and Bellona and barely emergent Indispensable Reef. Landmesser (1974) proposed that the Rennell Trench and adjoining ridge are vestiges of a former northeast-directed subduction zone along an intermediate plate boundary that once was contiguous with New caledonia. Asymmetric trench morphology suggestive of a former subduction zone is evident in reflection profiles across the trench (Recy et a L, 1977), but correlation of the undisturbed ponded sediments with DSDP data is uncertain. Kroenke (1984) favored an Eocene subduction event in the Rennell Trench, coeval with that in NewGuinea and eew caledonia; if correct, then the Rennell Arc represents the earliest subduction period in the Solomon Islands region.

South Rennell Trough.--This northeast-trending trough (Fig. 1) lies south of the Rennell Arc and is about 700 km long, as much as 30 km wide and as deep as 5,000 m, On the basis at magnetic spreading-- anomalies, graVity data, and bottom samples, Larue et al (1977) postulated that this trough is a remnant of a spreading ridge, possibly of Oligocene Age.

Santa Cruz Basin (Torres Basin) .--Little is known about the Santa cruz Basin, which lies southwest of the Juncture of the san Cristobal and North New Hebrides Trenches between San cristobal and the Santa Cruz island group (Fig. 1). Luyendyk et al (1974) and Ravenne et al (1977) described a subhorizontal, well-bedded sedimentary section less than 1,000 m thick. Ravenne et al (1977) alluded to steep slopes in the eastern part of the basin and indicated that it might represent a swell along the outer edge of the subduction zone. Klein et a L, (1975), reporting on the Glomar Challenger Leg 30 DSDPresults, described

Vedder, Coulson: Offshore Geology 82 a 650-m thick middle sccene to Pleistocene sedimentary succession in the southern santa Cruz Basin. Probable turbidites in the lower two-thirds of the succession are overlain by pelagic sediments.

ONTONGJAVAPLATEAUANDADJOININGSTRUCTURES

General setting

The North Solomon Trench, a relatively shallow, poorly defined and partly filled trench, separates the Solomon Islands segment of the OUter Melanesian Arc from the Ontong Java Plateau and Pacific OCean floor (Fig. 1). To the northwest, the North Solomon Trench connects with the Kilinailau Trench; and to the southeast apparently intersects the cape Johnson Trench. According to Kroenke (1984), subduction probably began along the North Solomon Trench in the late Eocene and was accompanied by uplift, metamorphism, and magmatism. Volcanism along the northeast-facing arc probably culminated in the Oligocene, and subduction apparently ended in the early Miocene when the Ontong Java Plateau entered the trench.

Ontong Java Plateau.-~he oceanic Onton-g Ja"va Plateau lies north of the main group of the Solornon Islands (Fig. 1). The plateau is more than 1,600 kIn long and 800 kIn wide and its long dimension is nearly parallel to the island chain. Water depths average less than 2,000 m over the central portion, which culminates in several atolls. The crustal structure was described by Furumoto et al (1976), Murauchi et al (1973), and Hussong et al (1979). The crust is anomalously thick (43 kIn); layer velocities are similar to normal oceanic crust, but each layer is abnormally thick. Kroenke (1972) demonstrated that a uniformly thick (1,000+ m), stratified sedimentary sequence blankets the plateau. Where it adjoins the Solomons along its southwestern edge, the entire plateau sequence plus the underlying basement rocks are highly deformed. Drilling at DSDPSites 64, 288 and 289 disclosed the plateau stratigraphy (Winterer, Reider et aI, 1971; Andrews et aI, 1975). Strata at Site 64 are correlative with laterally continuous seismic reflectors that are traceable to Malaita (Kroenke, 1972; this volume) where they match the Lower Cretaceous to Holocene stratigraphic succession on north Malaita. In all three holes, cherty limestone occurs near the base of the section; and at Site 289, these overlie Lower Cretaceous basalt. The younger strata are primarily nannofossil-foraminiferal limestone, chalk and ooze. Although basaltic flows were not drilled in the older limestone sequence in the three holes, Kroenke (1972) interpreted seismic reflectors south of the plateau as lava flows or sills that could be lateral equivalents of the younger basalts and peperites on southern Malaita (Hughes and Turner, 1977). Nixon (1980) suggested that some of the seismic discontinuities may represent alnoite pipes similar to those exposed in north Malaita. The southwestern flank of the plateau, adjacent to the Solomon Islands, is dominated by the Roncador homocline-Stewart arch, a lithospheric flexure believed to have resulted from severe bending during unsuccessful subduction of the plateau as part of the Pacific Plate at the North Solomons Trench prior to late Miocene arc reversal (Kroenke, 1984; this volume). The DSDPdrilling results indicate the presence of a fracture zone that offsets the plateau

Vedder, Coulson: Offshore GeolOgy 83 between DSDP Sites 288 and 289. This dislocation was projected southward between Santa Isabel and Malaita by Coleman and Kroenke (1981) but is not shown by Kroenke (1984; this volume). Kroenke (1972) suggested that the plateau arose about an exceptionally slow spreading axis and that the maeuave outpourings of basalt (6 million kIn3) may mark an early stage of continent generation. The highly deformed nature of the southwestern margin of the Ontong Java Plateau and the apparent correlation of the stratigraphic sequence on the plateau with that of Malaita led Kroenke (1972; 1984; this volume). to postulate that Malaita and the northwestern margin of Santa Isabel together constitute an obducted slice of the plateau, tectonically welded to the Solomon Islands during collision in late Miocene time. Bielski-Zyskind et al (1984) suggest that the Ontong Java Plateau is composed of undepleted mantle overlain by older depleted upper mantle.

North Solomon Trench.--Kroenke (1984, Fig 4.3) depicts the North Solomon Trench as a structural hinge line or regional syncline. Presumably it represents the site of a late Eocene to early Miocene subduction zone that was oppilated by the impingement of the Ontong Java Plateau during late Miocene time (Coleman and Kroenke, 1981). sediments in the narrow part of the trench directly north of ;the northwest end of Malaita are 1. Q to 1.5 kIn thic:k'·and, essentially undeformed. Uniform acoustic reflectors suggest that the sequence consists largely of turbidites.

Malaita Anticlinoriurn.--The North Solomon Trench is bordered on the south by the Malaita anticlinorium of Kroenke (1972), a northwest-trending set of structures that is the offshore counterpart of the post-Miocene echelon fold belts on the island of Malaita. On both the northern and southern ends of the island, some of the folds are overturned and verge southwestward (Rickwood, 1957; Hughes, 1975; Turner, 1976, 1977). Offshore, the folds appear to more symmetrical northwest of the island (Kroenke, this volume). Steep submarine scarps reflect the presence of large faults. Farther northwest, the folds decrease in number and amplitude and tend to die out north of santa IsabeL Only a single northwest-trending trough occurs north of Choiseul.

Vedder, Coulson: Offshore Geology 84 REFERENCES

Andrews, J.E., G.H. Packham, et aI, 1975, Initial Reports of the Deep sea drilling Project, 30: U.S. Government Prining Office, Washington, D.C., 753 p. Bielski-Zyskind, M., G.J. Wasserburg, and P.H. Nixon, 1984, sm-Nd and Rb-Sr systematics in volcanics and ultramafic xenoliths from. Malaita, Solomon Islands, and the nature of the Ontong Java Plateu: Journal, of Geophysical Research, v. 89, no. 54, p. 2415-2424. Carey, S.w., 1958, The tectonic approach to continental drift, in Continental drift--a symposium: Geology Department, University of Tasmania, Hobart, p. 177-355. Coleman, P. J., and A.A. Day, 1965, Petroleum possibilities and marked gravity anomalies 1n north-central Guadacanal: British Solomon Islands Geological Record (1959-1962), v. 2, p. 112-119. ------and L.W. Kroenke, 1981, Subduction without volcanism in the Solomon Islands arc: Geo-Marine Letters, v. 1, p. 129-134. ------and G.H. Packham, 1976, The Melanesian Borderlands and India-Pacific plates' boundary: Earth-Science Reviews, v. 12, p. 197-233. Curtis, J.w., 1973, Plate tectonics and the Papua New Guinea-Solomon Islands req-:i.on:; Journal of the Geological Society- of Australia, -e, 20, pt. 1, p. 21-36. de Brion, C.E., F. Aubertin, and C. Ravenne, 1977, Structure and history of the Solomons-New Ireland region: International Symposium on Geodynamics 1n Southwest Pacific, Editions Technip, Paris, p. 37-50. Exon, N.F., and B.R. Taylor, 1984, seafloor spreading, ridge subduction, volcanism, and sedimentation in the offshore Woodlark-Solomons region and Tripartite cruise report for Kana Keoki cruise 82-03-16, leg 4: CCOP!sOPACTechnical Report no. 34, p. 1-42. Furumoto, A.s., J.P. webb, M.E. Odegard, and D.M. Hussong, 1976, seismic studies on the Ontong Java Plateau, 1970: Tectonophysics, v. 34, p. 41- 90. Glaessner, M.F., 1950, Geotectonic position of NewGuinea: A.A.P.G. Bulletin, v, 34, p. 856-881. Hackman, B.D., 1980, The geology of Guadalcanal, Solomon Islands: Overseas Memoirs of the Institute of Geological Science, Her Majesty's Stationery office, London, no. 6, 115 p. Ha Lunen, A.J., and von Herzen, R.P., 1973, Heat; flow in the western equatorial Pacific Ocean: Journal of Geophysical Research, v. 78, p. 5195-5208. Hughes, G.W., 1975, Dorio, Malaita ~ological Map Sheet ML 11: Geological survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. ------and C.C. Turner, 1977, Upraised Pacific Ocean floor, southern Malaita, Solomon Islands: Geological Society of America Bulletin, v; 88, p. 412- 424. Hussong, D.J., L.K. Wipperman, and L.W. Kroenke, 1979, The crustal structure of the Ontong Java and Manihiki oceanic plateaus: Journal of Geophysical Research, v. 84, p. 6003-6010. Karig, D.E., 1972, Remnant arcs: Geological Society of America Bulletin, v, 83, p. 1057-1068. Katz, H. R., 1980, Basin development in the Solomon Islands and their petroleum potential; CCOP/SOPACTechnical Bulletin No.3, p. 59-75. Klein, G.D., 1975, Sedimentary tectonics in southwest Pacific Marginal basins based on Leg 30 Deep sea Drilling Project cores from the South Fiji,

Vedder, Coulson: Offshore Geology 85 Hebrides, and Coral Sea Basins: Geological Society of America Bulletin, v, 86, p. 1012-1018. Kroenke, L.W., 1972, Geology of the Ontong Java Plateau: Hawaii Institute of Geophysics, Report no. HIG-72-5, University of Hawaii, 119 p. ------1984, Solomon Islands; san Cristobal to Bougainville and Buka, ~ L.W. Kroenke, e d; , cenozoic tectonic developnent of the southwest Pacific: CCOP/SOPACTechnical Bulletin, no. 6, ch. 4, 22 p. Landmesser, C.W., 1974, Submarine geology of the eastern Coral Sea Basin, Southwest Pacific: M.S. thesis, University of Hawaii, Honolulu, Hawaii, 64 p. ------1977, Evaluation of potential hydrocarbon occurrence in the so Lomon Islands: CCOP/SOPAC,Technical Report, V. 1, no. 5, p. 47-53. Larue, B.M., J. Daniel, C. Jouannic, and J. se cy , 1977, The South Rennell trough; evidence for a fossil spreading zone: International symposium on Geodynamics in Southwest Pacific, Editions Technip, Paris, p. 51-62. Luyendyk, B.P., K.C. MacDonald, and W.B. Bryan, 1973, Rifting history of the Woodlark Basin in the Southwest Pacific: Bulletin of the Geological Society of America, v. B4, p. 1125-1134. ------W.B. Bryan, and P.A. Jezek, 1974, Shallow structure of the New Hebrides arc: Geological Society of America Bulletin, v. 85, p. 1287-1300. Maung, T~ -v., and F.I. Coulson, 1983, Assessment of petroleum potential of the central Solomons Basin; CCOP/SOPACTechnical Report No. 26, 68 p. Murauchi, S., W.J. Ludwig, N. Den, H. Hotta, T. Asanuma, T. Yoshii, A. Kubotera, and K. Hagiwara, 1973, seismic refraction measurements Ontong Java Plateau northeast of New Ireland: Journal of Geophysical Research, v. 78, p. 8653-8663. Neef, G., 1979, cenozoic stratigraphy of Small Nggela Island, SOlomon Islands- early Miocene deposition in a forearc basin followed by Pliocene patch reef deposition: New Zealand Journal of Geology and Geophysics, v, 22, no. 1, p , 53-70. Nixon, P.R., 1980, Kimberlites in the southwest Pacific: Nature, v, 287, p. 718-720. Ravenne, C., C.E. de Broin, and F. Aubertin, 1977, Structure and history of the Solomon-New Ireland region, in International symposium on geodynamics in the SW Pacific, lew ca Led"'Onia, August-September 1976: Editions Technip, Paris, p. 37-50. Recy , J., J. Dubois, J. Can.LeL, J. Dupont, and J. reunay , 1977, Fossil subduction zones: examples in the south-west Pacific: International Symposium on Geodynamics in Southwest Pacific, Technip, Paris, p. 345- 356. Rickwood, F.K., 1957, Geology of the Island of Malaita in Geological Reconnaissance of parts of the central islands of the British Solomon Islands Protectorate; Colonial Geology and Mineral Resources, v. 6, no. 3, p. 300-306. Taylor, B.R•• and N.F. Exon, 1983, 1982 R/V Kana Keoki cruise in the Woodlark- Solomons region (abs.): Basic geo-scientific marine research required for assessment of minerals and hydrocarbonsa in the South Pacific, A workshop, Suva, Fiji, October, 1983. ------1984, An investigation of ridge subduction in the Woodlark-- Solomons region: introduction and background, in N.F. Exon and B.R. Taylor, compilers, Seafloor spreading, ridge subduction, volcanism and sedimentation in the offshQre Woodlark-Solomons region and Tripartite cruise report for ~ Keoki cruise 82-03-16, Leg 4, CCOP/SOPACTechnical Report no. 34, p. 1-42.

Vedder, Coulson: Offshore Geology 86 Tiffin, D.L., J.G. Vedder, A. Cooper, and Shipboard SCientific Party, 1983, Multichannel seismi.c and geophysical survey of "The Slot" and adjacent areas in the Solomon Islands, 19 May - 11 June 1982: CCOP/SOPACWork Program, Cruise Report No. 71, 16 p. Turner, C.C., 1976, South small Malaita, Malaita Geological Map Sheet ML 17: Geological survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. ------1977, central small Malaita, Malaita Geological Map Sheet ML 16: Geological survey, Honiara, Guadalcanal, Solomon Islands, 1:50,000. Vedder, J.G. et aI, 1983, leg 3, central Solomons Trough, .l:.!! H.G. Greene and F.L. WOng, eda ; , Hydrocarbon resource studies in the southwest Pacific: u.s. Geological Survey Open-File Report 32-293, p. 11-24. Weissel, J.L, B. Taylor, and G.D. Karner, 1982, The opening of the Woodlark Basin, subduction of the WOodlark spreading system, and evolution of the northern Melanesia since the mid-Pliocene time: Tectonophysics, v. 87, p. 253-277. Winterer, E.L., W.R. Reidel, and others, 1971, Initial reports of the Deep Sea Drilling Project, v.7., U.S. Government Printing Office, Washington, D.C.

Vedder, Coulson: Offshore Geology 87

TBCTORICS OF THE SOOTBEASTERN SOLOI«)N ISLAHDS: FORMATION OF TIlE MALAITA ANTICLI!I)RIUM*

L •. W•. lCroenke, J •. M~ Resiq, Po. A•. Cooper Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822

INTRODUCTION

The Solomon Islands, first deac r-Lbe d by cc Leman (1966) as an isl,an(L.a:r;c, form a northwest-trending double chain of islands (Fig. 1) along the convergent boundary between the Pacific and Indo-Australian plates. To the northeast, on the Pacific Plate, lies the Ontong Java plateau (OJP), an extensive area of shallO'W sea floor underlain by anomalously thick oceanic crust (Hussong, Wipperman and Kroenke, 1979). To the southwest, deep trenches are the surface manifestation of the currently active subduction zone which dips northeast beneath the volcanic arc (Cooper, Kroenke and Resig, this volume). Island arc volcanic activity, however, is absent at the southeastern end of the group, northeast of the San cristobal trench (Coleman and Kroenke, 1981). The tectonic setting is also complicated by the current subduction of an active spreading ridge and the newly formed lithosphere contained in the adjoining Woodlark Basin (Taylor, 1984). The Solomon Islands were divided by Coleman (1965) into three geologic prcvi ncee on the basis of their lithology and structure (Fig. 2), i.e.: the Pacific Province, embracing tn.awe, Malaita, Small Malaita, and the northeast flank of santa Isabel; the central Province, including san Cristobal, Guadalcanal, the Florida Islands, the southwest flank. of Santa Isabel, and Choiseul; and the Volcanic Province, containing westernmost Guadalcanal, Savo, the Russell Islands, the New Georgia Group, and the Shortland Islands. Bougainville~ although politically part of Papua New Guinea, was later included in both the central and Volcanic prOVinces (Coleman, 1970). An en echelon series of submarine ridges and troughs northeast of the group (Fig. 3) constitute a large fold belt, the Malaita Anticlinorium (Kroenke, 1972). These folds crest subaerially on the Island of Malaita and the northeastern flank of Santa Isabel. The basement rocks and sedimentary section of the anticlinorium are correlated with those of the OJP (Kroenke, 1972; Hughes and Turner, 1977; Kroenke, this volume). Two autochthonous island arcs of opposing polarities, each bounded by trenches, and an allochthonous accreted terrane of open ocean origin form the regional framework of the Solomon Islands (Kroenke, 1984, cenozoic Tectonic

*Hawaii Institute of Geophysics Contribution No. 1548

Kroenke, ReSig, P. Cooper: Malaita Anticlinorium 88 Development). The island arcs (Fig. 3) include: the northeast-facing North Solomon Arc (Melanesian Arc) active during Oligocene and early Miocene time (-38-22 Mal and the southwest-facing South Solomon Arc, active from late Miocene to the present (.....10-0 Ma). Except for a brief resurgence of arc volcanism in the mid~le Miocene (-15 Ma) along the adjoining Manus and Vitiaz arc segments, the intervening middle to late Miocene time was relatively quiescent. The allochthon, comprising both the Pacific Province of the Solomon Islands and the Malaita Anticlinorium, is believed to be a segment of the southwestern margin of the OJP which collided with and overthrust ~e former North Solomon Arc. This Chapter summarizes arguments leading to these conclusions and presents a revised chronology for the sequence of tectonic events.

THEONTONGJAVAPLATEAU

The geology of Santa Isabel includes two principal units (Stanton 1957, 1961): an undifferentiated basement complex, overlain by m1ldy folded mudstones, shales, and tuffs of the Tanakau Group, and an intensely folded succession of basaltic pillOlof lavas (Sigana Volcanics), conformably overlain by pelagic limestones of 1;he Tanakau G:(oup. At least .one major thrust fault is. prest;lnt"7_-t;.he.Ka"ipito furigole Fault (Stanton,~j)-!~lJ.::-,..alongwh~ch \ll:t:r.~fip,:. rocks occur as alpine-type lenses. These ultramafic rocks have also been described as ophiolites (Neef, 1978) • Stanton (1961l noted simi larities between the Sigana pillow lavas and overlying Tanakau limestones of Santa Isabel and the lithostratigraphic succession found on Malaita (Rickwood, 1957). Coleman et al (1965) noted that the Kaipito Koregole thrust divides the island chain into two broad structural zones: a northeastern one, embracing Malaita and southern Santa Isabel and dominated by folding, and a southwestern one, inclUding Guadalcanal and San Cristobal and characterized by block faulting. Geological reconnaissance mapping of the island of Malaita (Rickwood, 1957) provided an initial insight into the stratigraphic succession and the structural relationships present there. Seismic reflection profiling across the OJP northeast of the Solomon Islands (Kroenke, 1972) revealed the presence of the large anticlinorium adjoining Malaita and santa Isabel and established the continuity of the acoustic stratigraphy from DSDPSite 64 on the plateau into the Anticlinorium (Kroenke, 1972). Similarities between the geology mapped onshore and that investigated offshore led to the correlation of the Malaita and the OJp sedimentary sequences and to the conclusion that all of the Malaita Anticlinorium, inclUding the island of Malaita and the northeastern flank of Santa Isabel, are a part of the OJP. Refinement of the stratigraphy of Malaita by Hughes and Turner (1977) reinforced this conclusion, as did their recognition of two phases of igneous activity on Malaita: a widespread "older" tholeiitic phase, assumed to be part of the lower Cretaceous OJP volcanism that formed the basement, and a restricted "younger" alkalic phase that intruded the overlying Cretaceous and lower Tertiary pelagic sediments. Of 14 "older" basalts analyzed, 11 strongly resemble mid-ocean ridge (MORl tholeiites, particularly in TiOZ and KZO contents, whereas 3 are primitive alkalic basalts, moderately high in ~O, similar to basalts from mid-ocean islands, and extremely low in Ti02, similar to basalts from mid-ocean plateaus (Tokuyama and Bitaza, 1981). Kroenke (197Z) suggested that the OJP originated by emplacement of submarine flood basalts and compar-ed it to other oceanic plateaus including

Kroenke, Resig, P. Cooper: Malaita Anticlinorium 89 Iceland (Kroenke, 1974). Major element constituents of the basalts from Deep Sea Drilling Project (DSDP) Site 289 on the northern OJP are similar to those of MORtholeiites as are the minor and trace element concentrations (Stoeser, 1975). Like Icelandic basalts, however, the Site 289 basalts have more K20 and Sr than normal MORbas aI t s, Moreover, the older Malaitan basalts are massive and pillowed, without major dike formation (Hughes and Turner, 1977), which also suggests a flood basalt origin. Drilling at DSDP Sites 288 and 289 produced evidence for an age progression across the OJP, (Summary and Conclusions, Site 288, Andrews, Packham et a L, 1975). Coleman, acccvren and Ramsay (1978), and Hussong, Wipperman and Kroenke (1979), using the Aptian age (110 Ma) obtained for basement at Site 289, extended the progression to Santa Isabel where ages for basement were derived from fossils in the sediments overlying basement (-58 Ma) and directly from a radiometrically dated basement dolerite (66 Ma). They concurred on a spreading ridge origin for the Plateau as did Coleman and Kroenke (1981). In addition, Hussong, Wipperman and Kroenke (1979) concluded that a simple thickening of all oceanic crustal layers by a factor of 5 was compatible with known seismic crustal structure of the plateau. Bielski-Zyskind, Wasserburg and Nixon (1984), in presenting results of a Sm-Nd and as-s- study on samples of alnoite, ultramafic xenoliths, and the basalt country rock from Malaita, report that isotopic data from one sample of

F'_ the Malaita- basalt. falls' to the right of the mantle array for ~-$F'-;-:1'"" NdJ~ ,.'>0. close to the ocean island field. More important, however, they emphasize that there is no hint in the available data from Malaita of the isotopic signature of old continental crust. Indeed, Butler (in prep.) concludes that Walker's (1977) observations of high-frequency Po phases across the OJP indicate that the underlying lithosphere is oceanic in character and not of continental origin. Thus, the available evidence suggests that the OJP originated along a mid-ocean ridge, perhaps in similar manner to the Icelandic Plateau.

THEMALAITAANTICLINORIUM

Multichannel seismic data, acquired during a 1982 R/V S. P. LEE cruise, permit a direct correlation of strata between the southern margin of the OJP and the Malaita Anticlinorium just west of Malaita (Kroenke, this volume). These data substantiate the correlation proposed earlier by Kroenke (1972). The deep troughs northeast of both the anticlinorium and the Solomon Islands are part of the fossil North Solomon Trench (Kroenke, 1972). The presence of deformed Miocene strata overlain by ponded sediment in the troughs northeast of the anticlinorium and the occurrence of terrigeneous debris in the deep water sediments of Miocene-Pliocene age on Malaita suggest that the southwestern margin of the plateau collided with and overthrust the Solomon Arc from the northeast in late Miocene time (Kroenke, 1972). Upper Oligocene-lower Miocene Suta and Marasa Volcanics on Guadalcanal (Hackman, 1980), the latter intruded by the Poha Diorite dated at 24.4 :: 0.3 Ma (Chivas and McDougall, 1978), and the Oligocene Kieta vc Icaru.ce , intruded by the upper Oligocene Umumand Kunai Hills plutons (Blake and Miezitis, 1967), attest to the presence of Oligocene-early Miocene arc volcanism. Furthermore, aspects of late Oligocene-early Miocene sedimentation and geochemistry of contemporaneous lavas on Small Nggela also indicate that a northeast-facing arc was present during late Oligocene-early Miocene time (Neef and Plimer, 1979).

Kroenke, Resig, P. Cooper: Malaita Anticlinorium 90 Hughes and Turner (1977) supported the concept of a pre-late Miocene trench and subduction zone of the northeast side of the Solomon Islands and agreed that a shallowing of the Malaita area occurred during late Miocene to early Pliocene time when a greater amount of terrigenous detritus was introduced into the section. They aLe. noted that folding and faulting occured on Malaita during Pliocene time when the island first emerged. Additionally, extensive slumping within deposits of Oligocene to early and middle Miocene age indicate unstable depositional conditions on an inclined substratus, and Oligocene distal turbidites, later described as tuffaceous beds (Turner and Hughes, 1982), suggested a fixed source location relative to the area of deposition. Piercement structures with lateral reflectors interpreted as sills were recognized in seismic reflection profiles on the outer trench slope north of Malaita and along the crest of the Roncador Homocline and Stewart Arch (Kroenke, 1972). These structures might be equivalent to the intrusive alkalic basalts on Malaita (Kroenke, 1972), or some might also be kimberlite pipes, emplaced in similar fashion to the alnoitic intrusions of Malaita (Nixon, 1980). The Malaita alnoites apparently were derived in Oligocene time (.....34 Ma) as a partial melt from the base of the lithosphere (Nixon and Coleman, 1978). Coleman and Kroenke (1981) proposed that intrusions of the same type occurred prior to the late Miocene arc reversal along tensional ruptures formed, during severe bending of the OJF lithosphere over the ere-noh outer-rise. This deformation and igneous activity would create slope instability and a provenance for the distal turbidites (tuffaceous beds) described by Hughes and Turner (1977). Paleomagnetic analysis of sediment and basalt samples from DSDPSite 289 on the OJF indicate a latitude of formation for the site of about 33°S in Aptian time (..•110 Ma) (Hammondet a L, 1975). Therefore, since formation, the OJP and its marginal areas, inclUding the Pacific Province Islands, must have undergone a substantial northward displacement. However, because of past convergence azimuths between the Pacific and Indo-Australian plates, convergence vectors were almost congruent with latitude (Fig. 8.4, Kroenke, Cenozoic Tectonic Developnent, 1984) and thus any change in paleolatitude between Malaita-QJP and the remainder of the Solomon Islands should be negligible. Apparently the North Solomon Arc overlay a southwest-dipping subduction zone that was active during Oligocene and early Miocene time (-38-22 Ma) {Kroenke, 1984bl, an appropriate time for the emplacement of alnoitic and alkalic intrusive rock suites seaward of the subduction zone. A late Miocene ("'10 Mal arc reversal followed a quiescent period spanning early to late Miocene time (Kroenke, this volume). A brief resurgence of igneous activity punctuated that quiet along the adjoining west Melanesian (Manus) and Vitiaz arc segments in middle Miocene time ("'15 Ma), implying reactivation of the southwest-dipping subduction zone. Reactivation of a segment of the southwest-dipping subduction zone may be occurring tic day, Cooper, Kroenke and Resig, this volume, report that a southwest-dipping zone of low-level earthquake activity, associated with the North Solomon Trench, extends to 200 km in depth. They believe that the feature is part of a relict W-Bzone and conclude that the area over which the zone is observed represents a preViously subducted segment of Pacific crust.

Kroenke, Resig, P. Cooper: Malaita Anticlinorium 91 CONCLUSIONS

That part of the southwestern margin of the OJP now forming the Malaita Anticlinorium encountered the Nbrth Solomon subduction zone in late Oligocene to early Miocene time. Bending of the thick crust and lithosphere over the trench outer rise facilitated intrusion of alkalic basalts such as are found on Malaita as well as the sills and dikes (piercement structures) observed in reflection records across the Roncador homocline and Stewart arch. Subduction apparently ended when the thickest part of the plateau crust and lithosph""re encountered the trench in early Miocene time (-22 Ma), causing the convergent boundary to shift elsewhere in the region. Follol.Jing the initial collision (22-20 Ma) a period of relative quiescence ensued along the North Solomon Arc. Except for a brief resurgence of activity along the West Melanesian (Manus trench) and Vitiaz segments of the subduction zone in middle Miocene time, the quiescent period lasted about 10 Ma, after which convergence recurred on the south side of the old North Solomon Arc. When convergence shifted back to the Solomon Islands in late Miocene time (10-8 Ma), it formed the northeast-dipping South Solomon subduction zone and created the New Britain - san Cristobal trenches. During the formation of the South Solomon subduction zone, the Solomon Island platform was pushed northeastward into the southwestern margin of the OJP, which began to overthrust the fore-arc of the old North Solomon Arc. The ensuing overthrusting resulted in uplift and folding of the oceanic crust of the plateau to form the Malaita Anticlinorium and Pacific Province of the Solomon Islands and caused the emplacement of ophiolites along the Kaipito- Korigole fault system. Contact between northeast-dipping Indo-Australian lithosphere and southwest-dipping Pacific lithosphere and the absence of a wedge of asthenosphere above the W-B zone may have inhibited island-arc volcanism at the southeastern end of the Solomon Islands. In fact, the process of overthrusting, folding, and ophiolite emplacement may still be occurring today.

ACKNOWLEDGMENTS

We are grateful for the constructive criticism of W. T. Coulbourn of BIG and D. L. Tiffin of CCOP/SOPACTechnical secretariat. We appreciate the secretarial and graphics support provided by RIG. In particular •..••e thank E. Norris, R. Rhodes, and M. Prins.

Kroenke, Resig, P. Cooper: Malaita Anticlinorium 92 REFERENCES

Andre.•••s, J. E., Packham, G., et e L, 1975, Initial Reports of DSDP: Washington, U.S. Government Printing Office, v. 30, p. 175-230. Bielski-Zyskind. M•• wasserburg, G. J., and Nixon, P. H., 1984, Sm-Nd and Rb- Sr systematics in volcanics and ultramafic xenoliths from Malaita, Solomon Islands, and the nature of the Ontong Java Plateau: Journal of Geophysical Research, v. 89, p. 2414-2424. Blake, D. H. , and Miezitis, Y. , 1967, Geology of Bougainville and Buke Islands, tew Guinea, Commonwealth of Australia: Department of National Develofll\ent, Bureau of Mineral gesourcee , Geology and Geophysics, Bulletin No. 93, Bulletin No. PNG1. Butler, R., in preparation, Regional seismic observations of the Ontong Java Plateau and East Mariana Basin. Chivas, A. R., and McDougall, I., 1978, Geochronology of the Koloula porphyry copper prospect, Guadalcanal, Solomon Islands: Economic Geology, v. 73, p. 678-689. Coleman, P. J., 1965, Stratigraphical and structural notes on the British Solomon Islands with reference to the first geological map, 1962, in The British Solomon Islands Geological Record, Volume II, 1959-62: Department of Geological surveys, HOniara, p. 17-31. "~ Coleman, P• .1., 1966, The Solomon Islands as an island arc: Nature, v, 211, p. 1249-1251. Coleman, P• .1., 1970, Geology of the Solomon and te .•••Hebrides Islands, as part of the Melanesian re-entrant, Southwest Pacific: Pacific Science, v. 24, p.289-314.

Coleman, P• .1., Grover, J. C't Stanton, R. L't and Thompson, R. B., 1965, A first geological map of the British Solomon Islands, 1962 in Grover, J. C. et aI, The British Solomon Islands Geological Record, Volume II, 1959- 62: Department of Geological Surveys, Honiara, p. 16-17. Coleman, P• .1., McGowran, B., and Ramsay, W. R. H., 1978, New, early Tertiary, ages for basal pelagites, northwest Santa Isabel, Solomon Islands (central southwest flank, Ontong Java Plateau): Australian Society of Exploration and Geophysics Bulletin no. 9, p. 110-114. Coleman, P• .1., and Kroenke, L. W., 1981, Subduction without volcanism in the Solomon Islands arc: Geo-Marine Letters, v. 1, p. 129-134. Hackman, B. D., 1980, The geology of Guadalcanal, Solomon Islands: Institute of Geological-Sciences, Natural Environment Research, Overseas Memoir, v. 6, 115 p , Hammond, S. R., Kroenke, L. w., Theyer, F., and keling, D. L., 1975, Late Cretaceous and Palaeogene palaeolatitudes of the Ontong Java Plateau: Nature, v. 255, p. 46-47. Hughes, G. W'i and Turner, C. C., 1977, Upraised Pacific OCean floor, southern Malaita, Solomon Islands: Geological Society of America Bulletin, v. 88, p. 412-424. Hussong, D. M., Wipperman, L. K., and Kroenke, L. W., '979, The crustal structure of the Ontong Java and Manihiki Oceanic Plateaus: Journal of Geophysical Research, v. 84, p. 6003-6010. Kroenke, L. W., 1972, Geology of the Ontong Java Plateau: Ph. D. thesis, 119 p., University of Hawaii, HIG-72-5. Kroenke, L. w., 1974, Origin of continents through development and coalescence of oceanic flood basalt plateau: EQS Transactions, American Geophysical Union, v. 55, p. 443. Kroenke, L. W., 1984a, Interpretation of a multichannel seismic profile

Kroenke, Resig, P. Cooper: Malaita Anticlinorium 93 northeast of the Solomon Islands, from the southern flank of the Ontong Java Plateau across the Malaita Anticlinorium, this volume. Kroenke, L. w., 1984b, cenozoic tectonic development of the Southwest Pacific: U.N. ESCAP, CCOPjSOPACTechnical Bulletin 6, in press. Neef, G., 1978, A convergent subduction model for the Solomon Islands: Australian Society of Exploration and Geophysics, v. 9, p. 99-103. Neet, G. and Plimer, I. R., 1979, Ophiolite complexes on small Nggela Island, Solomon Islands: Geological Society of America Bulletin, part II, .p. 313-348. Nixon, P. H., and Coleman, P. J., 1978, Garnet-bearing lherzolites and discrete jodule suites from the Malaita Alnoite, Solomon Islands, and their bearing on the nature and origin of the Ontong Java Plateau: Australian Society of Exploration and Geophysics Bulletin, v, 9, p. 103- 107. Nixon, P. H., 1980, Kimberlites in the southwest Pacific: Nature, v. 287, p. 718-720. Rickwood, F. K., 1957, Geology of the island of Malaita, in Marshall, C. E., et; a 1, Geological Reconnaissance of parts of the central Islands of the British Solomon Islands protectorate: Colonial Geology and Mineral Resources, v. 6, p. 300-305. Stanton, R~ L.,- 1957, Geology of southeastern Santa Ysabel and5an. .Jorge Island in Marshall, C. E. et e L, Geological Reconnaissance of Parts of the central Islands of the British Solomon Islands Protectorate: Colonial Geology and Mineral Resources, v. 6, p. 269-286. Stanton, R. L., 1961, Explanatory notes to accompany a first geological map of Santa Ysabel, British Solomon Islands protectorate: Oversea Geology and Mineral Resources, v. 8, p. 127-149. Stoeser, D. B., 1975, Igneous rocks from Leg 30 of the Deep sea Drilling Project, in Andrews, J. E., Packham, G., et aL, Inltial Reports of DSDP: Washington, U.S. Govt. Printing Office, v. 30, p. 401-414. Taylor, B., 1984, A geophysical survey of the Woodlark Solomons region in

Taylor, B., Exon, N. F., eda s r American Association of Petroleum Geologists Earth Science Series, in preparation. Tokuyama, H. and Batiza, R., 1981, Chemical composition of igneous rocks and origin of the sill and pillO\ri-basalt cOIt\'lex of Nauru Basin, southwest Pacific in Larson, R. L., Schlanger, S. 0., et aI, Initial Reports of DSDP: Washington, u.S. Goverment Printing Office, v. 61, p. 673-687. Turner, C. C. and Hughes, G. w., 1982, Distribution and tectonic implication of Cretaceous - Quaternary sedimentary facies in Solomon Islands: Tectonophysics, v, 82, p , 127-146. Walker, D. A., 1977, High-frequency Pn and Sn phases recorded in the Western Pacific: Journal of Geophysical Research, v. 82, p. 3350-3360.

Kroenke, Resig, P. Cooper: Malaita Anticlinorium 94

TBC'l'OHIC IMPLICATIONS OF SEISMICITY II.lRTREAST OF TBB SQLC.K)N ISLANDS*

P. A. Cooper, L. W. Kroenke, J. M. Resig Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822

INTRODUCTION

The Solomon Islands lie along a NW-SEconvergent boundary between the Pacific and Indo-Australian plates (Fig. 1). The structural complexity of the area results from at least two tectonic events: The introduction of ma:s'sive quasi-continental lithosperic units into the subduction zone, and an arc reversal. About 22-20 Ma the Ontong-Java Plateau (OJP) converged with the southern North Solomon Trench in an older, southwest-directed subduction zone (Kroenke, 1984, cenozoic Tectonic Development). After a relatively quiescent period, convergence ze s umed in a northeast-directed subduction zone about 10 Ma (Kroenke, P.esig and Cooper, this volume). Further complication resulted from subsequent entry of the Woodlark spreading system into the latest subduction zone (Weissel, Taylor and Karner, 1982). At least two small plates are located between the large Pacific and Indo- Australian plates (Fig. 1): The Solomon Sea Plate, which is bounded by the New Britain arc-trench system to the northwest, by the diffuse seismicity of the northern New Guinea coast to the west, and by the Woodlark spreading system, part of which extends into the Papuan peninsula to the south and east; and the Bismarck Sea Plate, bounded by the New Britain trench to the south, New Ireland to the east, the Bismare-k Sea seismic lineation (Denham, 1969) to the north, and the New Guinea Highlands to the west. Whereas the Indo- Australian Plate is currently underthrusting the Pacific Plate in a northeasterly direction at about 11 cm/yr (Luyendyk, Macdonald and Bryan, 1973), the Solomon Sea Plate is now underthrusting the BismarCk Sea Plate in a more northerly direction at about 14 cm/yr (Taylor, 1984). The Bismarck sea Plate, whose relative movement is not as well constrained by data, is apparently moving at about 13 cm/yr in an average direction of N60W(Taylor, 1979) • The main seismic features of the region are as follows: A zone of shallow and intermediate seismicity parallels northern New Guinea. A second zone of shallow seismicity the 'Bismarck sea seismic lineation' of Denham (1969), extends due west from New Ireland to New Guinea. A third zone of shallow seismicity 1s associated with the Woodlark spreading system and extends from the west Woodlark Basin near New Georgia, south through D'Entrecasteaux Islands and into the Papuan Peninsula. Both northerly

-Hawaii Institute of Geophysics Contribution No. 1545

P. Cooper, Kroenke, Resig: Seismicity 95 extension and northwest left-lateral strike-slip focal mechanisms characterize this zone (Rupper, 1980). A zone of shallow, intermediate and deep earthquakes curves along the Solomon Islands arc-trench system and continues to the southwest, paralleling the New Britain Arc. Last, a diffuse zone of shallow and intermediate seismicity which apparently defines a separate tectonic regime, extends north of the Solomon Islands beneath the OJP. of the many studies of the seismicity (Denham, 1969; Hilsom, 1970; CUrtis, 1973, seismicity) and seismotectonics of the Solomon Islands region (Johnson and Molnar, 1972; Krause, 1973; Curtis, 1973, plate tectonics; Taylor, 1975; Pascal, 1979; Weisse1, Taylor and Karner, 1982), few have focused on the southeastern end of the region and none have addressed the implications of the diffuse zone of seismicity extending northeast beneath the Ontong-Java Plateau. This report discusses the relationship between regional structure and seismicity and speculates within the framework of plate tectonic theory on the cause of the earthquakes occurring northeast of the main Solomon Islands arc-trench seismicity.

SEISMICITY

The intensity and distribution of hypocenters along the Solomon Islands arc-trench system vary dramatically at all depths (Fig. 2). Shallow seismicity is most intense in the northwestern Solomon Islands and decreases abruptly just southeast of Bougainville, where an active section of the Woodlark spreading system enters the trench (Weisse 1, Taylor and Karner, 1982). In the vicinity of the New Georgia Group in the central Solomons, shallow activity is much less intense and earthquakes are typically low-to- moderate in magnitude. Generally, earthquake activity associated with the northeast-directed subduction in the area of the Woodlark Basin portion of the Indo-Australian plate is poorly developed. Shallow seismicity is fairly intense in the southeastern Solomon Islands. Several moderate events located far into the OJP are suitable for focal mechanism analysis. Intermediate seismic activity in the northwestern Solomons ceases at about 200-kIn depth, whereas in the central and southeastern Solomons the Wadati-Benioff (W-B) zone extends no deeper than 100 km, The W-B zone is contoured in Fig. 2. Deep seismic activity is observed throughout the Solomons. All deep earthquakes occur in 'pod-shaped' groupings at depths between 380-550 km and are apparently related to subducting slabs detached from and acting independently of the presently subducting lithosphere (Cooper and Taylor, in prep.). The main seismic activity can be attributed to the Solomon Sea Plate underthrusting the northwest Solomon Islands near Bougainville. Beneath the arc the distribution of shallow hypocenters with respect to depth (Fig. 2) indicates a W-Bzone dipping to the northeast; the attitude of the deeper portions of the W-S zone varies from near vertical north of sougainville, to horizontal in the central Solomons and back to vertical in the southern Solomons. Near the New Georgia Group in the central Solomons where the Woodlark Basin is being subducted, the W-B zone is ill defined, there are no intermediate earthquakes, and a roughly inverted V-shaped gap in seismicity is apparent when the W-S zone is viewed as a section parallel to the strike of the trench (Fig. 3e in Cooper and Taylor, in prep.). This gap is offset to the south in the direction of convergence of the Pacific-Indo Australian plates. According to the model of active ridge subduction proposed by Marshak and Karig (1977), as ridge sections are consumed at the trench, a gap forms in the subducting lithosphere and grows in width with continued

P. Cooper, Kroenke, Resig: Seismicity 96 subduction. The gap noted above may be explained by this model. High heat flow values reported by Halunen and Von Herzen (1973) for the Woodlark Basin are indicative of the young, hot lithosphere of that area now being subducted in the central Solomons near NewGeorgia. The positive buoyancy of this newly generated lithosphere may explain the shoaling of the New Britain-San cristobal trench south of NewGeorgia Islands (Delong and Fox, 1977), and the continued uplift of adjacent islands (Hughes, Varol and Dunkley, 1984) and contiguous seafloor (Resig, 1984). The lack of a well-developed w-s zone (Cooper and Taylor, in prep.) and the presence of abnormal island-arc volcanism (Weissel, Taylor and Karner, 1982) may also be explained by the subduction of hot, young, and thin lithosphere. The diffuse zone of seismicity around and extending north of Santa Isabel (Fig. 2) is more difficult to explain. Figure 3 is a cross section of seismicity along approximately the same line as the multichannel reflection profile described by Kroenke (lmJltichannel interpretation, this volume). A northeast-dipping zone of earthquakes extending to about 100 km in depth marks the modern W-S zone associated with the san Cristobal Trench. A southwest- dipping zone of earthquakes extending to 200 km in depth locates another W-B zone (Cooper and Taylor, in prep.). This second feature, which can be associated with the North Solomon Trench, is most evident on profiles orthogonal to the island of Santa- Isabel. It is- believed to represent vestigal motion or a resurgence of motion along the relict North Solomon subduction zone. Scattered hypocenters located elsewhere along the North Solomon trench are insufficient to define this zone. The low incidence of earthquakes with focal nechanisms showing thrusting, however, indicates that the southwest-dipping slab is not vigorously subducting. The seismicity may be due to stress adjustment of the relict slab (Toksoz, 1975), or it may result from the modern, northeast-subducting slab pushing against and depressing the old slab, causing it to settle deeper into the aesthenosphere. we conclude, as did Cooper and Taylor (in prep.), that this southeast-dipping seismicity represents a previously subducted segment of the Pacific plate. One earthquake focal nechanisrn near Malaita, and possibly another near Santa Isabel, indicates thrusting (Fig. 2). In view of seismic reflection profiles that show continuous stratification between the Ontong Java Plateau and the Solomon Islands PaGific Province (Kroenke, this volumel Kroenke, Resig and Cooper, this volume), and paleontological evidence of thousands of meters of uplift of the Pacific Province in late Neogene time (Resig, Cooper and Kroenke, this volume), the mechanisms can be interpreted in terms of OJP crust overthrusting the Solomon Arc. In this interpretation, the crust along the edge of the OJP has detached from the underlying lithosphere and has overriden the North Solomon Arc, whereas the lithosphere, stripped of crust, has descended beneath the old arc along the nOWrelict W-Bzone. The presence of a decollement may be indicated by the increase in depth of hypocenters from 20-25 kIn near the Florida Group to 40-50 km north of Malaita, a depth close to that estimated for the crustal thickness of the OJP (Furumoto et a L, 1970). In contrast, the sparse hypocenters located farther north beneath the OJP are typically 50-100 krn deep.

CONCLUSIONS

Determination of the present active subduction of the Indo-Australian lithosphere beneath the southeastern Solomon Islands region is complicated by

P. Cooper, Kroenke, Resig: Seismicity 97 uplift associated with the subduction of the active Woodlark spreading system, contact of OJP lithosphere with actively subducting Indo-Australian lithosphere, movement or adjustment of the older, southwest-dipping Ontong- Java lithosphere, and finally, by possible continued underthrusting of the southern margin of the OJP by the Solomon Arc. The seismic activity concentrated printarily at the base of the OJP and evidence of recent episodes of uplift and deformation strongly suggest that obduction of the Pacific plate in the vicinity of the OJP is presently taking place. We believe that part of the relative motion between the Indo- Australian and Pacific plates has been, and may still be, accommodated by obduction and shortening of Ontong-Java crust.

ACKNOWLEDGtENTS

We acknoc Ledqe the generosity of the seismology Division, Lamont Doherty Geological Observatory in allowing us access to their film-chip library. We appreciate the assistance there of Margie Yamasaki. We thank Jim Dewey for copies of his earthquake location programs and Selena Billington for helpful discussion regarding their use. We are grateful to D. L. Tiffin and W. T. Coulbourn for critically reviewing the manuscript.

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