Geology of the Solomon and New Hebrides Islands, as Part of the Melanesian Re-entrant, Southwest Pacific! P. J. COLE MAN2 THE SOLOMON ISLANDSand the N ew Hebrides Company) ; A. A. Day (University of Sydney) ; Archipelago are examples of fractured island J. G. Speight and D. H. Blake (Commonwealth "arcs," autochthonous geological systems, in Scientific and Industrial Research Organisation, which patterns of straight-line fractures and Canberra); and Y. Miezitis (Bureau of Mineral vertical and horizontal movement of blocks are Resources, Canberra). Certain geological and the dominant structural feature . The two groups geophysical information obtained by the recent have similar but yet distinctive geologies. They United Nations Special Project (Director: H. have been studied systematically only over the Winkler) and by expeditions from the Unive r­ last 20 years, so that many of their features are sity of W isconsin and the Hawaii Institute of still not understood, but it is clear that the re­ Geophysics (Director: G. P.W oollard) has sults so far obtained amount to a significant been incorporated. contribution to the understanding of fractured For the New Hebrides the main sources in­ island arcs and, by extrapolation, to the under­ clude, similarly, the work of the Geological standing of the genesis and development of the Survey (D. 1. J. Mallick, Senior Geologist; A. southwestern Pacific. H. G. Mitchell and A. J. W arden , former The aim of this paper is to present a synoptic Senior Geologist); G. P. Robinson (British account of the essential geology of the Solomon Petroleum Company); K. Liggett (New Zea­ and New Hebrides groups. That is, it is the land Geological Survey) ; also publ ished work rock types and successions, their ages and struc­ by Mawson (19 05) , Obelliane ( 1958), de la ture, which are stressed. But certain geophysical Rue ( 1937) , and Sagatzky (1959) . evidence is also incorporated, particularly where I am particularly indebted to the people of the geological implications are clear, even the Geological Surveys, not simply for their in­ though some of this evidence is discussed in formation and encouragement, but also for help detail in the accompanying paper in this issue in the field, often given at the cost of incon­ ( Furumoto et al., 1970) . There is some repe­ venience and personal discomfort. tition, but this is prefe rable to fostering a sense of dichotomy between the geology, sensu stricto, and the geophysics-especially in this REGIO NAL FRAMEWORK area where there is usually a nice correspondence The Solomon Islands and the New Hebrides between geological and geophysical data. Archipelago are prominent elements within the Apart from the writer's own field and labora­ Melanesian Re-entrant. This is one of the more tory studies, the account which follows derives forceful aspects of Carey's Tethyan Tors ion from a variety of major sources. For the Solo­ System (Carey, 1963) . On any recent bathy­ mons they include published and unpublished metric map of the southwest Pacific it shows results from Geological Survey personnel, past as a great angle jutting into the Pacific Basin. and present, in particular : J. D . Bell, J. C. The apex is the area of the Fiji-Tonga groups ; Grover (former Director) , B. D . Hackman, the northern boundary is at first neatly defined P. Pudsey-Dawson, and R.B. Thompson (Di­ by New Ireland and the Solomons, and then by rector); also studies by R. L. Stanton (Univer­ a markedly en echelon arrangement of bathy­ sity of New England), a long-term worker in metric highs and lows which form the northern the area; F. K. Rickwood (British Petroleum edge ( Fairbridge, 1961) of the Fiji Plateau (badly named on many maps as the N orth Fiji 1 Manuscript received Decem ber 2, 1969. 2 Geology Department, Un iversity of Western Aus­ or Pandora Basin) ; the eastern limb is sharp ly tralia, Perth. defined by the line of the Tonga and Kermadec 289 290 PACIFIC SCIENCE , Vol. 24, July 1970 islands, which connect with the north island of echelon disposition (Bougainville-Choiseul­ New Zealand. The New Hebrides form, as it Isabel-Malaita) . A minor deep reflects this ar­ were, a minor re-entrant which at their southern rangement and separates the Solomon Block end swings to the northeast by way of the from the Ontong Java Platform, which supports Hunter Ridge, connecting with Fiji and so con­ Ontong Java and other prominent atolls. This taining the Fiji Plateau (Fig. 1) . platform is a tectonically puzzling feature; it The Melanesian Re-entrant and much of the may be a remnant of the Darwin Rise (Menard, area within it is made of elongated crustal 1964). New Georgia Sound, a severely rhombic blocks which find topographic expression as depression 2,000 meters deep, separates the island chains. New Caledonia, with the Loyalty north and south flanks. The latter (BougainviIle­ Islands, is an advanced example, so that a large New Georgia-Russell-Guadalcanal-San Cristo­ part of the block is now a land mass; the Tonga­ bal) shows some offsetting of the islands but is Kermadec Block is youthful, the islands being not so regularly en echelon as the north flank. relatively small and widely separated ; the Solo­ Over its middle portion, it appears to be linked mons and New Hebrides come between and are by NE-trending horst and graben with the good examples of so-called fractured island arcs. Louisiade-Rennell Ridge (Krause, 1966). The Within the Re-entrant there are rhornbo­ east part of the flank, however, is delineated by chasms (the Coral Sea is a large one, New the South Solomon Trench (about 7,000 me­ Georgia Sound in the Solomons is small but ters) , as is Bougainville, at the other end, by a well defined) , sphenochasms (the Tasman part of the Planet Deep (9 ,000 meters) (Fig. 1) . Sphenochasm, between southeast Australia and The axis of the Solomon Block appears sig­ the Lord Howe Rise, is the most obvious), and moidal, but this is probably accidental and re­ orotaths (the Kermadec-Tonga Ridge is named flects the relative dominance of one or the other as such by Carey) . These are Carey's terms for of two lineament sets with preferred orienta­ features which accompany crustal sundering and tions, NW-SE and W-E, respectively. These spreading (Carey, 1958). sets, with a third subsidiary one trending NE , Emphatic straight-line lineaments are com­ define the outlines and relative positions of the mon ; curved or arcuate lineaments are less so, larger islands. To the west, the Block ends with and some of those shown even on modern maps the Lihir group of small volcanic islands, which may represent the subjective, rounding-off touch are separated by a minor deep from New Ire­ of the cartographer. Others are probably real­ land . This connects with northern coastal New for example, those of the Hebrides-Hunter Guinea by way of the "swirl" of the Bismarck Trench and the markedly rotational, curved fea­ Archipelago (New Britain, New Ireland). At tures of the eastern Fiji Plateau. Horsts and its eastern end, the Block is terminated trans­ graben dominate the structure, even of particu­ versely by the northern extremity of the New ular islands, so that in general the region exem­ Hebrides Block. This "cut-off" relationship is plifies the taphrogenic structural style. Tensional shared by the South Solomon Trough, terminat­ effects are-and appear to have been-the rule. ing against the Torres Trench, and is reflected The rim of the Re-entrant and the New on the northeastern flank by the confused ba­ Hebrides is tectonically unstable and seismically thymetry in the area east of Ulawa, with its active; the New Hebrides are an area of intense pocket depressions (Ulawa Deep) and linear seismicity. The Solomons, New Hebrides, and deeps (Cape Johnson Trough) . other parts of the Re-entrant are areas of mark­ The New Hebrides lie within the Re-entrant. edly anomalous positive gravity (Furumoto et Although not so markedly as the Solomons, the aI., 1970 ; Malahoff and Woollard, 1969) . New Hebrides Archipelago is also in part a The Solomon Islands, which help define the double chain, closed at the north by the Santa northern limb of the Re-entrant, are in part a Cruz group and at the south by a tail of vol­ double chain closed at the northwestern end by canic islands (Epi-Efate-Erromango-Tanna­ Bougainville and at the southeastern end by San Aneityum) (Fig. 2). The axis of the New Cristobal. The northern flank of the group is Hebrides Block is slightly sigmoidal, marked by made up of large islands with a distinct en a line of active and recent volcanoes. As with 0 o $ 180 g2 ~ ~ ..,.,o ~ en o 0" ~ 8 \ o ::J ~<O\ '" ~ E: 0... ' 10 r :r:rt> xr.... 0..: rt> I ~ t""' ttl ~ Z AUSTRALIA 300 \ 140~ -\- F IG. 1. Generalized bathymetric and locality map of the Mel anesian Re-entrant. Bathymetry based on data from N orth American Aviation/ Autonetics; Th e Times Atlas of the World; and H . W . Menard and colleagues. Contours : heavy line, 1,000 meters; open bacbured line, 3,000 meters. Trenches and IV linear deeps : enclosed, close bactnoed line. \0.... 292 PACIFIC SCIENCE, Vol. 24, July 1970 - , LE GE ND • ~ Scbm e r-Ine c o nt o u r , . ,5 0 0 rn. ~ Act ive vo lc a n o e s @ O.RSTO.M. Seismolcgic el st at ion • Supe rf;OaI < 50 Depfh of l oci .. Sh allow <100 in kilometers {v l o-t e r-rn e die-l-e ) 100 <200 ... De ep )200 Magnitude of f• • v ., <5 U.S.C.G.S .