Scholl D. W. and Vallier, T. L., compilers and editors, 323 1985 ' Geology and offshore resources of Pacific island arcs':Tonga region, Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, v. 2: Houston, Texas, Circum-Paciric Council for Energy and Mineral Resources.

EFFECT OF SUBDUCTION OF THE LOUISVILLE RIDGE ON THE STRUCTURE AND MORPHOLOGY OF THE TONGA ARC

Jacques Dupont Office de la Recherche Scientifique et Technique Outre-Mer (ORSTOM), BP Af Noumea, New Caledonia ·7

Richard H. Herzer New Zealand Geological Survey, DSIR, P.O. Box 30368, Lower Hutt, New Zealand

ABSTRACT

Uplift of the Tonga Arc occurred in the early Pliocene-Pleistocene in response, probably, to (1) thermal expansion prior to rifting of the Lau Basin and (2) subduction of the aseismic Louisville Ridge. A widespread lower Pliocene unconformity in the forearc sequence marks the former event. The western part of the forearc basin has since sub­ sided as the Lau Basin spread, but the eastern part has remained high-standing due to the addition of low-density erustal material to the arc by the subduction and assimilation of the Louisville Ridge. Subduction of the ridge would have begun in the north, sweeping south past Vava'u about 3 Ma and Tongatapu about 1.5 Ma. The uplifted portion consists of domed and tilted fault blocks separated by large normal faults cutting the forearc at right angles to the axis of the arc. The doming and faulting which began in latest Mio­ cene or early Pliocene time, intensified in the late Pliocene-Pleistocene; the latter tim­ ing coincides with subduction of the Louisville Ridge. The profile of the Tonga Arc prior to assimilation of the Louisville Ridge was probably similar to that of the Kermadec Arc to the south which has not yet collided with the Louisville Ridge; that is, the forearc basin would have dipped east towards the trench although it may not have been as deep.

INTRODUCTION turns about 90° and assumes an east-west direction, the second between 24° and 260S The Tonga-Kermadec arc-trench sys­ where it is intersected by the Louisvllle tem is situated in the South Pacific between Ridge south of which the trench is displaced 14°30'S and 360S latitude and between 172°W slightly eastward. The entire bathymetric and 179°E longitudes (Figure 1). The com­ ridge formed by the arc is called the Tonga bined Tonga-Kermadec Arc is more than Ridge north of 260S and the Kermadec Ridge 2,500 km long, virtually linear, and oriented south of 26OS. Between 180S and 260S the north-northeast, There are two changes of forearc region consists ofa high-standing orientation along its length: the first at the plateau informally called the Tonga plat­ northern end of the system where the trench form. 324 DUPONT,HERZER

180' 176'W (Office de la Recherche Scientifique et Technique Outre-Mer) in conjunction with

16'---~""""'~,------+-----J:i:d-----l<:"Il,---I CNEXO (Centre National pour l'Exploitation des Oceans): GEORSTOM III in 1975, A USTRADEC IV in 1976 in conjunction with the Institut Francais du Petrole, and EVA III and VII in 1977 and 1978. Two seismic sur­ veys were carried out by CCOP/SOPAC over the shallower parts of the Tonga Arc (TG-79 ~-.

BASIN a joint United States, AustraUa, New Zealand 24'-----+-~~r______.~ venture for CCOP/SOPAC, has investigated part of the Tonga platform between about 21° and 23OS. Karig (1970), Packham (1978), and Kroenke (in press) have sum marized the tec­ tonic setting of the Tonga Arc as that of a simple island arc with an active backarc 2e------hH--~k:-~~-~--+--1 basin, the Lau Basin, and a composite forearc basin, the present Tonga Ridge. The latter contains the stratigraphie record of a series of arcs that date back to the late Eocene, At least part of the Eocene arc, of which 'Eua is an exposed piece, was carried east by opening of the South Fiji Basin in the Oligo­ 32'----.----_+--++ '--R:c~_+-----_'V\______j S cene. By the Miocene, an arc parallel to the pre.sent one had developed, embodying the Lau Ridge and the Tonga Ridge together. The Lau Ridge comprised the volcanic arc and the Tonga Ridge the forearc basin and frontal arc. Opening of the Lau Basin in the • Pliocene split the arc again, this time separ­ ating the volcanic arc (now the Lau Ridge) Figure 1. Location of the Tonga and Kerma­ from the forearc basin (now the Tonga dec island arcs and of the Louisville Ridge. Ridge). A new volcanic arc, the present Profiles referred to: E = EVA, L = LEE, G = Tofua volcanic chain, has since formed along GEORSTOM. Ridges are shaded, trenche~ the western margin of the forearc. The his­ are hachured, and volcanoes are shown by· tory of the forearc thus involves numerous stars. C.S. = Capricorn Seamount. periods of uplift and subsidence but we will confine our discussion to the rMiocene and younger periods. In additi?n to periods of arc rifting and Serious study of the Tonga-Kermadec backarc spread~ng, the Tonga Arc is assumed Arc began in the 1960's but most geophysical to have undergÔBa collision with the Louis­ data . were gathered from the· 1970's ville Ridge, an aseismic ridge on the subduct­ onwardS. In 1972, the Mobil Oil research ing Pacific plate (Larson and Chase, 1972; vessel Fred H. Moore recorded numerous Isacks and Barazangi, 1977; Dupont, 1979 and multichannel seismic Unes across the arc. 1982). It is assumed that the Louisville Since 1975 four surveys have been carried Ridge was originally longer than it now is, out on the Tonga-Kermadec Arc by ORSTOM and that a considerable part of it has been LOUISVILLE RIDGE SUBDUCTION 325

i t subducted beneath, or accreted onto, the in the Miocene prior to uplift to its present ~ Tonga Arc. height. Perhaps the question of redeposition The history of the sedimentary basin should be investigated in considering the underlying the southern Tonga platform has paleontological evidence here. We propose been worked out in sorne detail by Herzer here that the profile of the Miocene forearc and Exon (this volume). Their work shows may have been similar to that of the Kerma­ that in the Miocene the southern Tonga plat­ dec forearc, that is, eastward dipping, though form was a forearc basin with volcanic cen­ perhaps not as deeply submerged as the Ker­ ters to the west and with the sediment fill madec forearc is now. thinning eastward onto an outer ridge. There is no evidence to confirm either emergence or submergence of the outer ridge at this MORPHOLOGICAL CHARACTERISTICS OF time. In the very late Miocene and early Pli­ THE TONGA AND KERMADEC ARCS ocene there was widespread uplift and the formation of a regional unconformity separa­ Perhaps the most outstanding feature ting Miocene from upper Pliocene sedi­ of the Tonga-Kermadec convergent plate ments. This event was accompanied by the boundary in plan view is the marked kink in formation of a few large, normal fauIts at the volcanic arc and trench where it is joined right angles to the axis of the arc, and rota­ by the Louisville Ridge (Figure 1). North and tion of the fauIt blocks. In the late Pliocene south of this kink there is a profound differ­ and Pleistocene the western side of the fore­ ence in the cross sectional profile of the arc­ arc basin subsided again, presumably as the forearc-trench between what is a typical Lau Basin opened; sedimentation resumed, Kermadec profile to the south and what is a fed presumably by the emerging Tofua Vol­ typical southern and central Tonga profile to canic Arc, burying the unconformity. Nor­ the north. It is the opinion of the authors mal fauIts, downthrown to the west and t.hat the change in the arc-forearc-trench related to rifting and subsidence of the Lau profile at the junction of the Louisville Ridge Basin, developed on the extreme western is not mere coincidence but the resuIt of sub­ margine Elsewhere, fragmentation of the duction of the ridge. forearc basin by large scale, arc-normal The comparative morphologies of the block faulting intensified. The fauIt blocks Tonga and Kermadec Arcs have been des­ themselves were domed and extensively bro­ cribed by Dupont (1982) who noted that ken by minor normal fauIts, apparently in bathymetric profiles across the Tonga-Ker­ Quaternary time. The domes form present­ madec Arc can be grouped into three mor­ day banks, sorne of which are shallow enough phological families, two very different and to havé developed coral reefs and islands. one which seems to be intermediate. One Submarine canyons have developed along the characterizes the Tonga Arc, the other the cross-cutting faults separating the banks, and Kermadec Arc, while the third family is there has been widespread erosion around represented by the profiles crossing Capri­ their heads. corn and Osbourn Seamounts. Osbourn Sea­ On paleontological evidence, Chapro­ mount is the northernmost peak of the Louis­ niere (this volume) and Bukry (this volume) ville Ridge and the closest to the Tonga-Ker­ report that water depths in the late Miocene madec Trench. For the purposes of this and late Pliocene-Pleistocene were little paper we will not consider the Capricorn different from the present range, and Seamount example. The essential features according to Quinterno (this volume) there is are the differences in the distances between little scope for substantially greater water (1) the summit of the ridge and the axis of depths in the early Miocene. Lack of erosion the trench, and (2) the volcanic arc and the until very late in the Miocene, followed by axis of the trench. Between the Kermadec widespread planation, then reef development and Tonga Arcs the distance between the and local intense submarine canyon erosion in volcanic .arc and the trench axis increases the Pliocene-Pleistocene (Herzer and Exon, (Kermadec = 150-180 km; Tonga = 170-210 this volume) lead us to suggest, however, km), and the distance from ridge summit to that the forearc was indeed in deeper water trench axis decreases (Kermadec = 150-180

~------~ 326 DUPONT,HERZER

TRENCH NW ISO/ISO km SE 0 2

KERMADEC FAMILY 4 6 Skm v S 190km W E -"~====~~======I 0 1 .. 165/175 km .r"J'-"--~ _____1 2

CAPRICORN -OSBOURN SEAMOUNTS 4 FAMILY 6 170/210km CO 12 v S Skm NW SE 1-"-==~~~~= 0 • .. 1001155 km 2

TONGA FAMILY 4 6

:/ S .~ 10km 'LEE UNE 11 W VS 16511S0 km E 0 ~~,~_.---~ ~ ...... RC 12 6 scale: 0 50 100 km km , , t ! , ! , vertical exaggeration =6.8 Figure 2. Typical profiles of each morphologic family. characteri~ing the Tonga-Ker~adec.Arc and showing the differences in distance from trench aXIS to volcamc arc and trench aXIS to ridge summit. V =volcanic arc; S =ridge summit.

km; Tonga = 100-155 km). ThE'.se relation­ Osbourn region, the vertical differences ships are clearly seen in Figure 2. between the Kermadec and Tonga Arcs are In the case of the Kermadec Arc, the (Figure 3): ridge summit and the volcanic arc are one and the same while in the Tonga Arc the fol­ From the Kermadec to the Osbourn profile lowing two cases are observed. South of - the seaward trench slope steepens due to 18OS, the volcanic arc is distinctly separate the presence 'of Osbourn Seamount from and lies west of the ridge sum mit; north - the trench becomes shallower (forms a sill) of 18OS, the volcanic arc and the ridge sum­ for the same reason ~. mit again coincide (Figure 2), but we will not - the trench slope break becomes shallower concern ourselves with the northern part just - the distance between the ridge summit and yet. the trench is about the same. Besides these horizontal differences there is an important crustal volume dif­ From the OSbOU~ the Tonga profile ference in the region between the trench and - the trench deepens the summit of the arc. The forearc, which is - the trench slope steepens concave in profile on the Kermadec Arc, is - the forearc is very shal10w and strongly strongly convex on the Tonga Arc. Taking convex, forming a ridge summit which is lnto account the intermediate profile in the nonvolcanic LOUISVILLE RIDGE SUBDUCTION 327

ARC SUMMIT 1 NW TRENCH SE 0 - 2 4

'-----1" 6 Skm

W E 0

2 4 ® ----....._-1- CO 12 6km

SUMMIT SE 0

2 @ 1 4 6 ,/r' S

scale: 0 50 100km :-vI 10 km LEE UNE 11 vertical exaggeration = 6.S

Figure 3. Vertical differences between the Kermadec and Tonga Arcs. The arrows indicate the changes to the profile in passing from (1) the "initial" Kermadec stage through (2) the "transi­ tion" Osbourn stage to (3) the "mature" Tonga stage. V = volcanic arc.

- the ridge summit is closer to the trench DISCUSSION On the basis of these consistent and The Tonga platform was probably profound differences, Dupont (1982) proposed affected by two uplift mechanisms in the Pli­ the following three-stage evolution of arc ocene~Pleistocene: thermal uplift prior to morphology where an aseismic ridge is being opening of the Lau Basin, and subduction or subducted: (1) an "initial stage", before sub­ accretion of the Louisville Ridge. Théopen­ duction of the ridge, typified by the morphol­ ing of the Lau Basin may weIl be linked to ogy of the Kermadec Arc; (2) a "transition the subduction of the Louisville Ridge and stage" where a seamount or ridge approachE'.s therefore the two vertical tectonic events the arc but is not yet subducted, typified by may be difficult to distinguish. Neverthe­ the morphology of the .arc adjacent to less, a feVi general assumptions can be made Osbourn Seamount; and (3) a "final or mature to help distinguish them. stage", where the ridge has been subducted 1. Uplift due to thermal expansion and the forearc greatly uplifted, for which prior to Lau Basin rifting would have been the typical example is the morphology of the greatest along the axis of ultimate rifting, Tonga platform. that is west of the present forearc. Effects

, ------'---~-----'----- 328 DUPONT,HERZER to look for would be (a) a possible unconfor­ Barazangi, 1977; Dupont, 1979 and 1982., mity with strongest development in the west Figure 4). Collision and underthrusting, and (b) evidence for subsidence after rifting. would have occurred in the region of Vava'u 2. Uplift due to ridge subduction would about 3 Ma, reaching Tongatapu about 1.5 Ma not necessarily coincide with the rifting axis ago and sweeping the southern Tonga plat­ and would be more likely to show up closer to form from then to present. Thus, on the the subduction zone, that is, to the east. southern Tonga platform, effects of subduc­ Being a product of collision rather than a tion of the Louisville Ridge should, by and regional thermal event, the uplift would be large, postdate the effects of thermal expan­ expected to be more irregular both in time sion prior to Lau Basin spreading. and space due to the structural and morpho­ According to Herzer and Exon (this vol­ logical complexity of the subducting ridge. ume) there is evidence that in the southern The progress of the subduction of the Tonga platform area, the early Pliocene Louisville Ridge can be predicted from the unconformity (their Horizon A) was well plate tectonic history insofar as it is known developed over both the western and the for the southwest Pacific. Using the pole of eastern sides of the forearc. On the eastern rotation of Minster et al (1974) for the last side of the forearc basin they report sorne 10 m.y. at 59°01'S and 178°E south of New arc-normal faulting which apparently devel­ Zealand, the movement of the Pacific plate oped concurrently with the unconformity. relative to a fixed Australia-India plate is Along the western side, they report exten­ approximately east-southeast with a relative sional faulting which postdates the unconfor­ convergence rate of 7-9 cm/yr in the Tonga mity and locally cuts the present sea bed. region. The picture is complicated by the These observations are consistent with ther­ opening of the Lau Basin with a full spread­ mal uplift in latest Miocene to early Pliocene ing rate of 5-5.6 cm/yr for the last 2 m.y. time prior to rifting of the Lau-Tonga Ridge, years (Cherkis, 1980), increasing the present and later subsidence in late Pliocene to Holo­ convergence rate at the trench to 12-15 cene time as the Lau Basin rifted and cm/yr. Also to be considered is an earlier spread. Faulting of the basin rim in the full spreading rate according to Weissel Holocene may be related to volcanic loading (1977) of 7.6 cm/yr going back to 3.5 Ma. by the young Tofua Arc. The Lau Basin is thought to have begun Their results and· the outcrop evidence spreading in the Pliocene, though the precise on the Tongan islands also show, however, timing is still not certain; Cherkis (1980) that the eastern part of the forearc basin has suggests 4.8 Ma, Weissel (1977), 3.5 Ma, and been high-standing since the late Pliocene Malahoff, Feden, and Fleming (1982), 2.5 and has since been cut by a great many more Ma. Chaproniere (this volume) and Bukry faults. The eastern part of the forearc has, (this volume) report an absence of fauna in in fact, been uplifted relative to the western Blow zones N18 through N20 and Bukry zone side to such an extent that the forearc basin CN10 (upper Miocene and lower Pliocene) in dips westward towards the volcanic arc samples from the Tonga platform, and Scholl, (Figure 5). It appears that the uplift in the Vallier, and Packham (this volume) and Her­ eastern part of the forearc that began in the zer and Exon (this volume) report a weIl early Pliocene or latest Miocene was developed unconformity in this interval in repeated in late Pliocene to Holocene time the seismic stratigraphy. The unconformity when the majority of the cross-cutting faults probably corresponds to the period of ther­ appeared and doming took placé. The domes mal uplift that would have preceded spread­ are extensively broken by very young normal ing of the Lau Basin. Thus, Weissel's (1977) faults which appear to be related to the age of 3.5 Ma or that of Malahoff, Feden, doming event. These observations all suggest and Fleming (1982) of 2.5 Ma best fits the that another more recent tectonic event has new data. affected the eastern part of the forearc, The Louisville Ridge, assuming it was namely, subduction of the Louisville Ridg~, once longer than it now is, has swept from although differential movements could stIll north to south along and under the Tonga be occurring as the region subsides with Ridge (Larson and Chase, 1972; Isacks and spreading of the Lau Basin. LOUISVILLE RIDGE SUBDUCTION 329

170' E 180' The domes form the present day banks while the arc-normal faults are eroded into can­ yons between the banks. Such bank and channel bathymetry persists northward to Vava'u, with the development of many coral islands, and southward to at least latitude 20' 5 --+----'H+--t-----t+-+-=!.,---I--r----J 24OS; and the same pattern of doming and faulting is inferred to be the underlying PAClflC cause. AUSTRALlA­ PLATE -INDIA PLATE The region from Vava'u to 240S is in the

1 "mature" morphologic stage of Dupont (1982) where an aseismic ridge has been subduc­ 30' 5 --~-Bif--1f---+---'------I ted. In this context it is interesting to note the seismological studies of Stein et al (1982) in the Antilles Arc. They have recorded first motions indicating normal faulting oriented at right angles to the arc where it is overrid­ ing an aseismic ridge, and postulate that such normal faulting may be a characteristic fea­ Figure 4. Relative past and present positions ture where aseismic ridges are subducted of the Tonga and Louisville Ridges with res­ beneath arcs. On the Tonga platform we pect to a fixed Pacific plate. Dashed lines may have a trail of such normal faults mark­ are isochrons in m.y.B.P. drawn on the crest ing the southward progress of the point of of the Tonga Ridge (forearc) and Kermadec subduction of the Louisville Ridge. If so, Ridge (volcanic arc). V =Vava'u, T =Tonga­ then large, arc-normal faults began to form tapu, E = 'Eua, R = Raoul Island. Islands in the late Miocene for other reasons and shaded in black are drawn in their present were overtaken by many more such faults in positions. Fiji is also shown rotated back to the late Pliocene-Pleistocene as the ridge its earlier position (white) on the Miocene was subducted. Large cross-cutting faults arc (Falvey, 1978). Black circles represent and domed fault blocks should thus become the Louisville Ridge on the Pacific plate; rarer southward through the transition area open circles represent subducted portions of adjacent to Osbourn Seamount and few the ridge. The heavy black lines represent should be present on the Kermadec forearc. the trench. Arrows indicate the direction of The existing seismic data do not provide the movement of the Australia-India plate rela­ answer and we offer this as an idea to be tive to a fixed Pacific plate. The hachured tested with further seismic work. part of the northern Tonga region identifies If it is accepted that the present the recently formed arc of Louat and Dupont "mature" profile of the Tonga Ridge owes its (1982). Shown on the left is the position of origin to subduction of the Louisville Ridge the Tonga Ridge with respect to the Louis­ then the profile of the Tonga Ridge north of ville Ridge 10 m.y. ago (10 isochron); it was Vava'u (Figure 2, lowest profile) is anomalous then a ttached to the Lau Ridge. Shown on and requires explaining. This northern seg­ the right is the present position of the ridge ment has an "initial" type profile ·similar to (zero isochron). Intermediate positions are that of the Kermadec Ridge where the vol­ plotted from the data of Minster et al (1974), canic arc forms the summit and the forearc Weissel (1977), Cherkis (1980), and Malahoff, basin is depressed. We offer four p6ssible Feden, and Fleming (1982). explanations, aU of which require further investigation. 1. The Louisville Ridge has swept the entire Tongan subduction zone from north to The doming and faulting are clearly south from south of Samoa to its present refiected in the present bathymetry (Figure position. The northern part of the Tonga 6) and in deformation of the basal Miocene Arc, the oldest part affected by the subduc­ surface of Herzer and Exon (this volume). ted ridge, has reverted to its "initial" stage. 330 DUPONT,HERZER

TRENCH NW 1 SE -r------i------y-O (km) 2 4 GEO 320 6 scale: 0 50 100km L' '-, ----JI 8 vertical exaggeration = 6.8 10 1 Figure 5. Comparison between a seismic profile across the Kermadec Arc (GEO 320) (shaded) and a seismic profile across the Tonga Arc (LEE 11). The LEE 11 profile shows the degree of uplift of the Tonga forearc relative to the Kermadec forearc.

This explanation is ooly plausible if one CONCLUSIONS accepts the hypothesis of a complete evolu­ tionary cycle of arc morphology with the arc Subduction of the aseismic Louisville eventually reverting to its original profile as Ridge beneath the Tonga Arc took place the aseismic ridge is "assimilated" by the from 3 or 3.5 Ma to the present, essentially subduction system. after the thermal expansion event that her­ alded rifting of the Lau Basin. The thermal 2. The Louisville Ridge was shorter uplift left its mark in the southern Tonga than previous authors have thought so that region as a regional unconformity of early ridge subduction only began at the latitude of Pliocene age. Spreading of the Lau Basin Vava'u. In this case the whole northern part was accompanied by subsidence, particularly of the Tonga Arc, like the Kermadec Arc, of the western part, of the forearc and col­ would not have undergone collision with an lapse of its rifted margin with down-to-basin aseismic ridge and the morphology would (west-directed) normal faulting. The uncon­ have remained in the "initial" stage. formity is now deeply buried by volcanic sed­ iments derived from the young Tofua Volcan­ 3. Studies of seismicity by Louat and ic Arc. Dupont (1982) sug~est that the northern part The eastern part of the forearc remains of the arc from 18 30'S to 15°8 is much youn­ high standing due to the addition of low den­ ger than the rest of the Tonga Ridge, having sity crustal material to the forearc by the begun to form only 3.5-4 Ma (Figure 4). It subduction of the Louisville Ridge. Struc­ would, therefore, have come into existence tures in this uplifted portion of the arc after the Louisville Ridge had passed through include large arc-normal normal faults which the area and would be simply in the "initial" have broken the platform into a series of stage. large, domed and rotated fault blocks. Each of these domed blocks, which are themselves 4. The segment of the Tonga Arc highly faulted, forms a present day bank or forming the so-called platform from Vava'u coral island pedestal. Although the arc­ to latitude 2~ is underlain by a discrete arc normal faulting began in the latest Miocene fragment, a terrane that has retained its or early Pliocene, it intensified in the late integrity as a relatively high-standing block Pliocene-Pleistocene and much of it may since the late Eocene, and the passage of the reflect structural adjustment to subduction Louisville Ridge bene.ath it has had ooly of the ridge. minor effect. This would require the change . Prior to thermal expansion and to sub­ from "initial" through "transition" to duction of the Louisville Ridge, the profile of "mature" profile in the region adjacent to the the Tonga Arc was probably similar to that Louisville Ridge to be simply coincidental. of the Kermadec Arc; that is, the summit of LOUISVILLE RIDGE SUBDUCTION 331

176· W IS· S

GJ CAPRICORN SEAMOUNT 0 100 200 1 1 1 (km)

, ~

LAU BASIN

22· S

26· S

,.. Figure 6. Bathymetry (from Chase et al, this volume) and main fauIts offsettlng the present surface of the Tonga platform. Two sets of normal fauIts (solid heavy lln~.s) have been dls­ tingulshed by Herzer and Exon (this volume) in the area between 21° and 23OS: a NNE-SSW to NNW-SSE set on the rlm of the Lau Basin (left) and a WSW-ENE to NW-SE set cutting right across the crest of the platform. The cross-cutting, or arc-normal, faults separate a series of structural domes that form the modern banks of the platform. The pattern of fau1t1ng Is extra­ polated northward and southward on the basls of simi1ar morphology (dashed Un~.s). Inset shows location of Unes in the 1982 LEE survey from which the pattern of fau1t1ng was discovered. ------332 DUPONT,HERZER

the arc would have been the volcanic arc and ches and back-arc basins: American Geo­ the forearc basin would have sloped eastward physical Union Maurice Ewing Series 1, p. towards the trench instead of westward as it 99-114. does now. Karig, D. E., 1970, Ridges and basins of the Tonga-Kermadec island arc system: Jour-­ nal of Geophysical Research, v. 75, p. 239­ ACKNOWLEDGEMENTS 254. Kroenke, L. W., in press, Cenozoic tectonie The authors are indebted to Dave development of the southwest Pacific: UN Scholl, U. S. Geological Survey, for his many ESCAP, CCOP/SOPAC Technical Bulletin lucid discussions on the regional tectonics 6. and on arcs in general. We thank L. Pierron Larson, R. L. and Chase, C. G., 1972, Late (ORSTOM) and T. Jaegers (NZGS) for draft­ Mesozoic evolution of the western Pacifie ing the figures, F. Tonks and P. Matthews for Ocean: Geological Society of America typing the manuscripts, and F. Davey (Geo­ Bulletin, v. 93, p. 3627-3644. physics Division, DSIR) and J. Eade (NZ Louat, R. and Dupont, J., 1982, Seismicite de Oceanographic Institute) for critical reading. l'arc des Tonga-Kermadec, in Contribution a l'etude geodynamique du Sud-OUE'.st Pacifique: Paris, ORSTOM, no. 147, p. REFERENCES 299-317. Malahoff, A., Feden, R. H., and Fleming, H. Cherkis, N. Z., 1980, Aeromagnetic investi­ S., 1982, Magnetic anomalies and tectonic gations and sea fioor spreading history in fabric of marginal basins north of New the Lau Basin and northern Fiji Plateau: Zealand: Journal of Geophysical Research, UN ESCAP, CCOP/SOPAC Technical Bul­ v. 87, no. B5, p. 4109-4125. letin 3, p. 37-45. Minster, J. B., Jordan, T. H., Molnar, P., and Dupont, J., 1979, Le systeme d'arc insulaire Haines, E., 1974, Numerical modelling of des Tonga et Kermadec: deux morphologies instantaneous plate tectonics: Geophysical differentes, une seule zone de subduction Journal of the Royal Astronomical Society, (Pacifique Sud): Comptes Rendus de l'Aca­ v. 36, p. 541-576. demie des Sciences Paris, v. 289, serie D, Packham, G. H., 1978, Evolution of a simple p. 245-248. island arc: the Lau-Tonga Ridge: Austra­ Dupont, J., 1982, Morphologie et structures lian Society of Exploration Geophysicists superficielles de l'arc insulaire des Tonga­ Bulletin, v. 9, p. 133-140. Kermadec, in Contribution a l'etude geody­ Stein, S., Engeln, J. F., Weins, D. A., Fujita, namique du Sud-Ouest Pacifique: Paris, K., and Speed, R. C., 1982, Subduction ORSTOM, no. 147, p. 263-282. seismicity and tectonics in the Le.sser Falvey, D., 1978, Analysis of paleomagnetic Antilles arc: Journal of Geophysical data from the New Hebrides: Australian Research, v. 87, no. BIO, p. 8642-8664. Society of Exploration Geophysicists Bulle­ Weissel, J. K., 1977, Evolution of the Lau tin, v. 9, no. 3, p. 117-123. Basin by the growth of small plates, in Tal­ Isacks, B. L. and Barazangi, M., 1977, Geom­ wani, M. and Pitman, W. C., III, eds., Island etry of Benioff zones: Lateral segmenta­ arcs, deep sea trenches and back-arc tion and downwards bending of the subduc­ basins: American Geophysical Union Mau­ ted lithosphere, in Talwani, M. and Pitman, rice Ewing Series 1, p. 429-436. l'" W. C., III, eds., Island arcs, deep ses tren-

- . pmorn"mm 7'$ 5 n ... _ ... _ ~ _.~. - ~ ~ _. Dupont Jacques, Herzer R.H. (1985). Effect of subduction of the Louisville ridge on the structure and morphology of the Tonga arc. In : Scholl D.W. (ed.), Vallier T.L. (ed.) Geology and offshore resources of Pacific island arcs : Tonga region. Houston : Circum-Pacific Council for Energy and Mineral Resources, 2, 323-332. (Earth Science Series - Circum-Pacific Council for Energy and Mineral Resources). ISBN 0-

933687-01-X