Kermadec Arc Research Articles

Geophysical events

Tonga Islands eruption(s) and pumice rafts, South Pacific Ocean (all times are local, equal to GMT + 12 hours). The pumice described in last month's Bulletin continued to drift in a generally westward direction in June. In mid‐June, patches of pumice were reported near Vanua Mbalavu, Katafanga, and Yathata, small islands and atolls at the north end of the Lau Ridge (see map). The pumice at Yathata formed a strip 5 km long but only about 5 m wide. Large quantities of pumice were seen during the same period around Late Island (18.81°S, 1 74.65°W), about 400 km ESE of the Lau Group.The source of the pumice remains uncertain. In mid‐May, streaks of pumice extended back to Late Island from the main raft and the island's cone was covered with white material, which was thought to be fresh ash (see last month's Bulletin). However, activity has been reported at 3 other sites along the Tonga Arc in May and June.

Structure of the upper mantle in the convex side of the New Hebrides island arc

Summary S-wave arrival times from 30 earthquakes of the New Hebrides island arc are read from long-period components of the WWSS network, within the distance range 30–80°. S residuals from deep earthquakes of the New Hebrides and Tonga arcs are used to correct the residuals from the lower mantle effect and station corrections. An average delay of 3 to 4 s is found. This shows the existence of a low-velocity zone for S waves in the convex side of the arc. A three-dimensional inversion scheme is used to obtain the velocity structure of this zone. Other seismological data, such as pP attenuation, confirm the results of the inversion: two zones with low shear velocity (5 to 10 per cent lower than the J–B model) are defined. The first one is located beneath the d'Entrecasteaux fracture zone at a depth range of 150 to 350 km and the second one is in the south-west of the arc at the same depth range. Furthermore the deep upper mantle north of 16° S shows a normal S velocity, while south of 16°s it shows a low S velocity. A geodynamical model based on these results is proposed.

On the tectonic regimes of ophiolite genesis

Ophiolites and their associated rocks are examined in terms of field relationships and petrochemistry. Many pre-obduction plagiogranite and dioritic dykes in the ophiolites of southern Quebec, Vourinos, California, Oregon and Newfoundland are judged not to be the end phase of differentiation of tholeiitic magma. Repetitive sedimentary-volcanic formations that overlie many ophiolite suites resemble Cenozoic volcaniclastic sequences of the New Hebrides and Tonga arcs and possibly an analogy may be drawn to the several seismic layers detected in modern marginal basins. Variation diagrams for Ti, Zr, and Cr show that dykes and pillow lavas associated with ophiolite suites from Scotland, Greece, Cyprus and Newfoundland have island arc or calc-alkaline trends. It is difficult to reconcile some of these data with origin at major oceanic ridges. Most evidence indicates proximity to island arcs and origin within marginal basins of western Pacific type. Evolution of such basins was probably initiated by distension along a continental edge resulting in the formation of a graben so that island arcs were formed on break-away continental fragments and the grabens replaced by marginal basins in which komatiitic, calc-alkaline, tholeiitic and other lavas were erupted. The rocks are more diverse than those of major ocean basins due to the interaction of downgoing oceanic plate and adjacent continental margin, the role of subducted sediments and the insulating effect of a thick sedimentary-volcanic cover.

Underwater acoustic signals from a Kermadec Ridge earthquake

Abstract An earthquake of magnitude 6⋅9 Ms occurred on March 24 1976 with its epicentre on the Kermadec Ridge. Shortly after the event large underwater acoustic signals were recorded on a hydrophone suspended in midwater over the South Fiji Basin. The travel times of the signals are related to the crustal structure along the path between the epicentre and the hydrophone. Reverberations from bathymetric features surrounding the South Fiji Basin were observed for over half an hour after the main arrivals and signals attributed to aftershocks were recorded for at least 2½ hours afterward.

ASEISMIC BELT ALONG THE FRONTAL ARC AND PLATE SUBDUCTION IN JAPAN

Along arcs, a belt-like area can be identified where shallow seismicity within the continental plate is extremely low compared with other parts of the plate margin. Examples are in the Tohoku, Hokkaido, Kurile, Kamchatka, Aleutian, Peru-Chile, New Hebrides and Tonga arcs. The present paper proposes to call this inactive (or less-active) area the "aseismic belt, " which seems to be a typical feature of arcs. The aseismic belt is some tens of kilometers wide and is located, generally speaking, along the frontal non-volcanic arc between the volcanic front and the aseismic front. This belt can be explained as a mechanically unstrained area on the basis of a plausible model of plate subduction. Geodetic data and seismological results obtained in Japan are incorporated into this model in the framework of plate tectonics.

Upper mantle structure beneath the Fiji Plateau: seismic observations of second P arrivals from the olivine--spinel phase transition zone

Upper mantle structure beneath the Fiji Plateau: seismic observations of second P arrivals from the olivine--spinel phase transition zone

Seismic moments of earthquakes beneath island arcs, phase changes, and subduction velocities

The analysis of seismic moments of intermediate and deep-focus earthquakes in four island arc regions supports the hypothesis that these shocks are the result of shear stresses caused by phase transitions in the descending slab at depth near 150, 350, and 600 km. Cumulative seismic moments summation M/sub 0/ for earthquakes listed by Gutenberg and Richter were found to exhibit four maxima when they were plotted as a function of depth; the maxima occur within the depth ranges 0 to 100, 100 to 250, 250 to 450, and 450 to 700 km. The uppermost range corresponds to the zone of underthrusting. The seismic moments of earthquakes in the three lower depth ranges ae related to phase transitions in the slab by summation M/sub 0/ = ..mu..LT vertical bar ..delta..V/V vertical bar v/sub sub/, where ..mu.. is the modulus of rigidity, L is the length along strike of the subduction zone, T is the slab thickness, ..delta..V/V is the fractional volume change associated with a particular phase transition, and v/sub sub/ is the subduction velocity. In the range 100 to 250 km the earthquakes result from volume decreases associated with the melting of the oceanic crust. In the depth range 250more » to 450 km the volume change is due mostly to the transformation of olivine to spinel structure with a density increase of about 8%. In the lowest depth range the decrease in volume is due to the change of spinel to postspinel phases with a density increase of about 8%.For any island arc region, up to four independent estimates of v/sub sub/ can be calculated by using Brune's method for the zone of shallow underthrusting and the relationship between summation M/sub 0/ and volume changes for the three deeper zones. As an example, the four estimates of v/sub sub/ for the Tonga arc, in order of increasing depth, are 8.4, 7.9, 7.6, and 8.3 cm/yr.« less

Cenozoic volcanism in Southwestern New Mexico.

Tephrochronology is the study of volcanic ash (tephra) beds for the purpose of correlating and dating volcanic and other geologic events. Large magnitude silicic eruptions can disperse tephra up to thousands of kilometres from the vent, producing a near instantaneous marker horizon. In addition to their geochronological value, tephra beds are a major source of data on the eruption frequency and geochemistry of large rhyolitic volcanoes. The Quaternary Taupo volcanic zone (TVZ) in New Zealand is one of the most frequently active rhyolitic centres on Earth, and for much of the 20th century its tephra beds have been the focus of study. As a result, fingerprinting and dating techniques have been fine-tuned, and tephra beds now under-pin the late Cenozoic chronology for a wide variety of disciplines from volcanology to sequence stratigraphy and archaeology. The key to tephrochronology is the identification and correlation of tephra horizons. In the proximal setting (<50 km from vent), tephra beds can often be identified by their lithology, stratigraphic position and ferromagnesian mineralogy. Farther from source these features became less diagnostic as units become thinner and mineral depleted, and geochemical fingerprinting must then be employed. Grain-specific techniques are commonly required to assess the compositional homogeneity of the tephra, and to avoid xenolithic and detrital contaminants. Criteria that are valuable for identifying both source volcano and individual eruptive events from grain-specific electron microprobe techniques include: glass and Fe–Ti oxide chemistry, and eruption temperature and oxygen fugacity (estimated from oxide equilibrium pairs). Deposits of the contemporaneously active Taupo, Okataina, Maroa and Mayor Island caldera centres in the TVZ can be distinguished on the basis of such criteria. If a tephra lacks contaminants, trace- and rare earth element (REE) compositions of purified glass separates provide additional criteria. Similar approaches can be used on samples of the chilled, nonwelded bases of ignimbrites, while welded zones often display a characteristic thermal remnant magnetism (TRM) direction that can assist in correlation. Late Miocene and Pliocene rhyolitic tephra from the Coromandel region in New Zealand are compositionally indistinguishable from Quaternary TVZ deposits, but deposits from other SW Pacific provinces such as Antarctica and the Tonga–Kermadec arc are compositionally distinct. In the TVZ, rhyolitic tephra erupted during intervals of 10–20 ka from the same volcano are compositionally similar, while over longer periods or following large-volume eruptions the tephra are compositionally and mineralogically distinct. Most TVZ fall deposits are compositionally homogeneous, and there are no temporal compositional trends for individual volcanoes. Tephra beds are also valuable because they can be dated by a variety of methods, and therefore can provide direct age control for sequences that are not normally amenable to radiometric dating. For late Cenozoic tephra beds that are beyond the range of 14C (ca. 40 ka), and are fine-grained and/or lack K-rich phenocrysts, the isothermal plateau fission-track (ITPFT) method using glass can provide an accurate and precise age. 40Ar–39Ar can be employed for high-resolution dating, however this may require single crystal laser fusion to detect relict and partially degassed crystals that are common in pyroclastic deposits, even within single pumice blocks. The New Zealand tephrostratigraphic record is not complete in space or time. Only the post-64 ka record is well established. However, there are chronologically well-constrained tephra beds dating back to 10 Ma. Some tephra beds cover most of the North Island of New Zealand, are found in the South Island, and extend as much as 1400 km from vent into the Pacific Ocean. Widespread and/or stratigraphically important units include: Kaharoa (665 years BP); Kawakawa (22 ka); Rangitawa (0.33 Ma); Potaka (1 Ma) and Pakihikura (1.6 Ma). Ages on these and other tephra beds provide a framework for global climatic and sea-level change recorded in New Zealand, and allow direct correlation between the marine and terrestrial realms.

Geology of Hill End Trough Molong High: The Hill End Trough? An Empirical Approach

The Southernmost parts of the Havre Trough and the region of confluence of the Kermadec arc system with the North Island of New Zealand are a modern analogue for the Late Silurian ? Early Devonian palaeogeography of the Hill End Trough and its southern environs, including the Canberra region.

Composition and age of Lau Basin and Ridge volcanic rocks: Implications for evolution of an interarc basin and remnant arc

Research Article| October 01, 1976 Composition and age of Lau Basin and Ridge volcanic rocks: Implications for evolution of an interarc basin and remnant arc JAMES B. GILL JAMES B. GILL 1Earth Sciences Board and Center for South Pacific Studies, University of California, Santa Cruz, California 95064, and Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (1976) 87 (10): 1384–1395. https://doi.org/10.1130/0016-7606(1976)87<1384:CAAOLB>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JAMES B. GILL; Composition and age of Lau Basin and Ridge volcanic rocks: Implications for evolution of an interarc basin and remnant arc. GSA Bulletin 1976;; 87 (10): 1384–1395. doi: https://doi.org/10.1130/0016-7606(1976)87<1384:CAAOLB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Tonga-Kermadec Ridge, Lau Basin, and Lau-Colville Ridge are, respectively, a frontal arc, interarc basin, and remnant arc at the Australian-Pacific plate boundary. Basement rocks of the Lau-Colville Ridge (Lau Volcanics) are 9- to 6-m.y.-old basaltic andesites to dacites with 55 to 66 percent SiO2, K60 = 1.0 to 1.5 percent, little Fe enrichment, Sr87/Sr86 = 0.7030 to 0.7034, and enrichment in light rare-earth elements. Westward increases in K, Rb, Th, and U suggest that subducted lithosphere was un-derthrust from the east. Variations in rock composition are consistent qualitatively with derivation from basalt by low-pressure crystal-liquid fractionation involving removal of phenocryst phases: plagioclase + clinopyroxene + orthopyroxene + magnetite. Volcaniclastic turbidites of the same age and derived from western sources are found in Tonga. These andesitic vol-canogenic recks are overlain on both ridges by Pliocene limestones, which are capped on the Lau-Colville Ridge by 3.9- to 3.5-m.y.-old olivine + hypersthene normative tholeiites (Korombasanga Volcanics) having minor 56 to 60 percent SiO2 andesitic differentiates. Lau Basin basalts are transitional between ocean-floor and island-arc tholeiites, sharing with the latter their higher Rb, Ba, light rare-earth element, and Sr87 contents and lower Ti, Zr, and Hf contents. These data support Karig's idea that the Lau and Tonga Ridges represent a once-united island arc now dismembered by rifting, which has formed the intervening Lau Basin. This rifting began about 5 m.y. B.P. The change in volcanism on the Lau-Colville Ridge reflects its removal from a subduction site. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Compressional velocity in source regions of deep earthquakes: An application of the master earthquake technique

The technique of earthquake location relative to a master event is used to estimate near-source velocity and take-off angles for rays travelling to selected stations. Computations of a reconnaissance nature were carried out with arrival times of P and pP from deep earthquakes beneath the northwest corner of the Fiji plateau, the Peru-Brazil border region and the basin separating Fiji from the Tonga arc. These data yield estimates of compressional velocity of 11.2 ± 0.4, 11.4 ± 0.7 and 10.7 ± 0.3 km/sec respectively. Each of these velocities and the other parameters of each model space are essentially independent of their starting values. The corresponding depth ranges are 600–660, 580–650 and 540–600 km. These in-situ velocities are 5–10% higher than those of the Helmberger and Wiggins model. To account for such high velocities by a thermal effect alone would require an improbably high thermal contrast of 1000°C between “normal” mantle and the cooler earthquake zones. Spinels of proposed mantle composition would have compressional velocities of about 10.4 km/sec at temperatures that are taken as normal for these depths. If the high values of near-source velocity are explained by the addition of a post-spinel assemblage, then by implication this transformation occurs at shallower depths in those seismic zones than in the “normal” mantle.

Rigid plate accretion in an inter-arc basin: Mariana trough.

. It is verified that an oblique rigid opening results in good fit of the rifted portions of arc on each side of the Mariana trough. The good fit suggests that there has been rigid plate accretion and that a plate 200km wide and 1500 km long can stay undeformed between active consuming and accreting plate boundaries. Small ocean basins, which often exist behind island arcs, have been called basins. KARIG (1 970, 197 1 b) showed that the Lau-Havre trough between the Tonga-Kermadec ridge (frontal arc) and the Lau-Colville ridge (third arc), and the Mariana trough between the Mariana ridge (frontal arc) and the West Mariana ridge (third arc) are the sites of active extension. Using Karig's terminology, a new inter-arc basin is formed by relative migration of the third arc (an inactive rifted portion of the frontal arc) away from the frontal arc-trench system. Some authors such as VAN DER LINDEN (1969) and MILSOM (1970) have implied that the extension results from rigid plate accretion, as on mid-ocean ridges. However, it is now often assumed, following KARIG (1971a) that the process of rifting and creation of new lithosphere in marginal basins is different from the process of rigid plate accretion at mid-ocean ridge crests (PACKHAM and FALVEY, 1971). The actual process of opening is not clearly defined but is assumed by Karig to result from the rise of a hot diapir from the asthenosphere. It is also assumed that, during the opening of the marginal basin, the frontal arc did not remain rigid but was deformed, probably through block faulting, along a series of faults both parallel and perpendicular to the arc (KARIG, 1970; 1971a and b) Westward from the Mariana trench, there is a poorly developed midslope basement-high on the eastern flank of the frontal arc (Mariana ridge) which carries the raised limestone islands: Guam, Saipan (Fig. 1). The back of the frontal arc is defined by a westward facing scarp with 2-3 km of relief t Contribution no 271 du Departement Scientifique du Centre Oceanologique de Bre

Two earthquake swarms on the Kermadec Ridge

Abstract Two earthquake swarms on the Kermadec Ridge, in April 1961 and January 1965, occurred in almost the same place, roughly 4° from the North Island coast. New Zealand seismograph stations recorded about 200 events of magnitude M L ≥ 4 in each swarm. The first swarm lasted 2.3 days and the second 13.5 days. The average values of b are: 1.50 ± 0.19 and 1.34 ± 0.20, respectively. The frequency-magnitude relation varied during the course of the two swarms, but the patterns of changes are different. In the 1961 swarm, the variation in rate of occurrence has two stages, one similar to that in a foreshock sequence, and one like that in an aftershock sequence; the 1965 swarm followed the pattern typical of aftershocks.

Seismic wave attenuation in the upper mantle beneath the southwest Pacific

A detailed study of P and S seismic waves produced by hundreds of earthquakes located along the New Hebrides, Fiji plateau, Solomon, New Britain and New Guinea seismic zones and recorded at stations in Fiji, New Hebrides and New Caledonia shows significant variations in the amplitudes and frequencies of the waves. The data define zones of anomalously high seismic wave attenuation in the uppermost mantle beneath the Fiji plateau and the Woodlark basin. The active volcanoes of the New Hebrides arc seem to mark the western boundary of the high-attenuation zone that exists beneath the Fiji plateau. Our observations are in agreement with the hypothesis that the Fiji plateau and the Woodlark basin are young oceanic features formed by crustal extension. In addition, a zone of high attenuation exists in the uppermost mantle somewhere between the northeastern coast of Australia and the New Hebrides arc; the data further require that this zone be located not closer than about 400 km to the New Hebrides trench. Observations at New Caledonia stations of very low frequency shear waves from intermediate-depth earthquakes in the westward dipping Tonga—Kermadec seismic zone yield an estimate of Q s of about 10 in the zone of extremely high attenuation located beneath the Lau basin, the actively spreading inter-arc basin behind the Tonga arc.

Origin of Lau Basin Rise sediment

The chemistry and mineralogy of sediments in the vicinity of the active rise in the Lau Basin can be accounted for by weathering of tholeiitic basalts exposed in the Basin, the introduction of pumice fragments derived from the Tonga Ridge, precipitation of ferromanganese minerals from sea water and pore water addition.

Relationship between shallow and deep seismicity in the western Pacific region

The two types of relationship between the shallow and deep seismic activities in island-arc regions are described: (1) A great shallow earthquake in a region is preceded and sometimes followed by the marked increase of deep seismic activity in the same down-dip seismic zone perpendicular to the trend of an arc structure. The increase of the deep seismic activity before the great shallow earthquake cannot be understood as an accidental event nor as a mere trigger of the great shallow earthquake, but as an essential forerunning phenomenon of it. This regularity is found in the Kurile-Kamchatka and northern Japan island-arc regions, particularly near the ends of these two arcs and their junctions, where great shallow earthquakes frequently occur. (2) The seismic activity tends to progress in time from a shallow to a deep region within a descending lithosphere. This regularity is found in the Mariana and Tonga arcs, where the seismic activity is nearly continuous from the surface to the deep region and no very great shallow earthquake occurred in the considered period. The rate of migration in these earthquakes along the descending lithosphere is estimated at about 50 km per year, nearly similar to that in shallow earthquakes. These regularities in the space-time distribution of shallow and deep earthquakes in island-arc regions can be explained from the viewpoint that the oceanic lithosphere is pushed downward under the continental lithosphere by external forces, probably due to the mantle convection. The present discussion suggests that the descending lithosphere does not act as a rigid plate but as a visco-elastic body under gradually increasing loading.

Transpacific hydrographic sections at lats. 43°S and 28°S: the SCORPIO expedition—II. deep water

Features of the deep water in the South Pacific are described in terms of the distributions of temperature, salinity, and dissolved oxygen, phosphate, nitrate, and silicate, as measured along transpacific hydrographic sections at Lats. 43°S and 28°S during Eltanin Cruises 28 and 29 (SCORPIO Expedition). These data indicate: a major deep clockwise gyre, with a prominent western boundary current, occupying the Southwest Pacific Basin: considerably less rapid, less discernible mean motion east of the East Pacific Rise, but including a distinct northward flow on the eastern flank of the Rise at Lat. 43°S, which probably, though less evidently, reaches north to Lat. 28°S; and extremely sluggish movements in the deep Tasman Sea. It is speculated, on the basis of property correlations, that some of the homogeneous water below sill depth in the South Fiji Basin may derive from the deep western boundary current. The layer of low oxygen and high phosphate and nitrate concentrations at depths of 1–3 km is interpreted as water moving very much more slowly than that above or below, although regionally varying balances among diffusion, advection, and biological oxidation seem required to account in detail for the oxygen-depth curves. At both latitudes, the northward volume transport of the deep western boundary current, estimated geostrophically with a level of no motion at 2 km, is about 19 × 106m3/sec. Similar calculations for the entire sections east of New Zealand and the Kermadec Ridge indicate a net flow northward below 2 km of 12–15 × 106m3/sec.

Transpacific hydrographic sections at Lats. 43°S and 28°S: the SCORPIO expedition—III. Upper water and a note on southward flow at mid-depth

The characteristics of the upper 2000 m on the SCORPIO plates are dominated by the subtropical anticyclonic gyre which brings shallower waters from higher latitudes equatorward along the eastern boundary. These incoming waters are cold, of low salinity, and high in oxygen and nutrient concentrations. Within the gyre they are warmed, made more saline, and the oxygen and nutrient concentrations modified. Exchange with the inter-tropical waters also modifies the characteristics, and the waters that return poleward near the western boundary are substantially different from those that entered in the east. Along the SCORPIO sections, temperature decreases monotonically downward to great depths, but the other water characteristics have various maxima and minima, the consequence of the different layers that make up the entering water and of the several sorts of modifications and exchanges that take place within the gyre.The principal salinity feature is the immense body of fresher water that enters from the south in the east, flowing at various depths according to the density field. A major subsurface salinity minimum, the Intermediate Water, is obvious on the greater part of both plates, and a shallower but similar minimum lies above it in the east, separated by a more saline layer extending poleward from low latitudes, as part of the eastern-boundary poleward countercurrent.The higher salinities from the great central south Pacific evaporation cell, centered north of the SCORPIO sections, also occur as weak subsurface maxima returning poleward near New Zealand.The oxygen distribution has a slight maximum near 50m to 100m depth, a relic in these (mostly) summer data of the higher concentrations of the winter mixed layer. Beneath this maximum is the decrease in oxygen that is usual in the upper part of the pycnocline. In these sections this decrease creates a shallow (150m to 400m) minimum because the underlying Intermediate Water, which dominates most of the 500-m to 1500-m depth range, is higher in oxygen concentration and accounts for the oxygen maximum immediately above the deepest oxygen minimum. The pycnocline oxygen minimum is intensified in the east by the poleward subsurface flow of water from lower latitudes where the oxygen concentration is very low.The deepest oxygen minimum has its extreme concentrations in two separate cells, in the eastern area and the central area, separated by a slightly greater minimum in the water overlying the East Pacific Rise.The nutrient distributions reflect principally the major oxygen features in the upper layers. The pycnocline oxygen minimum in the east is also a nutrient maximum, so great that the underlying Intermediate Water, even at high concentrations, exists as a nutrient minimum in the eastern parts. Farther west the nutrient concentrations increase monotonically downward through the Intermediate Water oxygen maximum: phosphate and nitrate reach maximum concentrations corresponding closely to the two cells of the deepest oxygen minimum. Silicate, however, has a deep maximum only in the central Pacific Ocean, somewhat deeper than the maximum in the other nutrients. In the east it increases monotonically from the level of the Intermediate Water all the way to the bottom.A comparison of the SCORPIO sections with earlier observations in both the South and North Pacific leads to the suggestion that the separate vertical minimum in oxygen and the associated maximum in phosphate east of the Kermadec Ridge on the SCORPIO sections are a consequence of southward flow at these depths from the North Pacific. There is evidence of an anticyclonic circulation that extends to more than 2000 m depth in these latitudes, confined at depths below 1500 m to the central ocean by the Tonga and Kermadec ridges. The southward-flowing western limb of this anticyclone extends from the intertropical zone to the Antarctic Circumpolar Current. At these depths it may carry water that has entered the North Pacific as dense abyssal water, moderate in oxygen and phosphate, has become nutrient-rich and oxygen-poor as it rises in the North Pacific and mixes with the less dense water, and returns at mid-depth.

PandStraveltimes in the Tonga-Fiji region: A zone of low velocity in the uppermost mantle behind the Tonga Island Arc

The principal purpose of this study is to report the existence of a zone of extremely low compressional-wave velocities in the uppermost mantle beneath most of the Lau basin, an interarc basin located west of or behind the Tonga Island arc. Velocities beneath the basin appear to be as low as 7.1 km/sec. In contrast, times of P and S waves traveling beneath and parallel to the Tonga-Kermadec ridge indicate velocities of 8.45 and 4.75 km/sec, respectively. Although the lateral boundaries of the zone of low velocity beneath the Lau basin are not well defined, they coincide approximately with the boundaries of the zone of high seismic-wave attenuation existing beneath the Lau basin. The large difference (up to 15%) between P wave velocities beneath the Lau basin and those in areas adjacent to it probably requires partial melting in the upper mantle beneath the Lau basin. The P and S velocities measured parallel and approximately perpendicular to the Tonga trench do not differ significantly and hence provide no evidence for anisotropy in the Pacific lithosphere.

Geology, petrography, and geochemistry of the volcanic islands of Tonga

The South Pacific Ocean is an ideal area in which to test some of the compositional implications of plate tectonics and sea-floor spreading. The island arc of Tonga marks an active zone of seismic disturbances and volcanic activity that may be related to thrusting of the oceanic crust beneath the marginal crustal plate on which the islands are constructed. This marginal plate appears to be of oceanic character in the vicinity of Tonga, and the Tongan volcanoes thus provide an opportunity to examine the nature of magmas of a typical circum-Pacific orogenic zone located within an oceanic crustal regime. Volcanic eruptions in the Tonga Islands have averaged about one every four years during the present century, and milder submarine eruptions may have escaped notice. The submarine volcanoes and volcanic islands appear to be located along en-echelon fractures that strike slightly east of the general trend of the islands. These fractures would tend to open under right-lateral shear stress in the sense of the New Zealand Alpine fault and would tend to close under compression directed perpendicular to the island arc from the Pacific side. Thus, the relative amount and direction of movement of crustal plates on either side of the island arc may control the frequency and intensity of volcanic eruptions. The active volcanic islands and sea mounts are along a submarine ridge about 200 km west of the axis of the Tonga trench. The youthful constructional volcanic topography and limited paleomagnetic data indicate a very recent age for the volcanic activity. The rocks of these islands are basaltic andesite, andesite, and dacite, with the exception of Niuafo'ou, from which only basalt has been reported. These rocks differ from most other circum-Pacific andesite-dacite suites in their very low content of alkali, especially low K2O, although similar rocks have been reported from a few other island arcs, such as the Izu peninsula of Japan, the Marianas, and the South Sandwich Islands. The chemical peculiarities are represented modally by exceptionally calcic plagioclase and near absence of alkali feldspar. Pre-late Eocene basalt and uralitized gabbro on Eua Island at the southeast end of the group may represent uplifted pre-arc sea floor. These rocks are high in Al2O3 and are especially poor in K2O. The variation in K2O and total alkalies across the Tonga arc is similar to that observed in other circum-Pacific island arcs. The volcanic rocks show moderately strong iron enrichment, the highest Fe/Mg ratios appearing in the more siliceous andesites and dark dacites. This iron enrichment should not be regarded as evidence for a tholeiitic affinity for these rocks. Trace element abundances are similar to those of many other calcalkaline island arc lavas and apparently are consistent with a genetic evolution of the lavas by fractional crystallization. Bouger gravity anomalies decrease from south to north, from a high of +243 mgal on Eua to a low of +122 mgal on Fonualei. This decrease in gravity is matched by a slight increase in potash content in lavas of otherwise comparable composition, implying a change in the composition of the deep crust or mantle from which the lavas are derived. However, the most important compositional changes along the Tonga-Kermadec-New Zealand volcanic line occur between the Kermadecs and New Zealand, and probably reflect the influence of crustal contamination beneath the North Island.