Havre Trough Research Articles

Deep earthquakes beneath the Fiji Basin, SW Pacific: Earth's most intense deep seismicity in stagnant slabs

Previous work has suggested that many of the deep earthquakes beneath the Fiji Basin occur in slab material that has been detached and foundered to the bottom of the transition zone or has been laid down by trench migration in a similar recumbent position. Since nowhere else in the Earth do so many earthquakes occur in slabs stagnated in the transition zone, these earthquakes merit closer study. Accordingly, we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well-located deep events in the geographic area between the bottoms of the main Vanuatu and Tonga Wadati–Benioff zones. Two regions of deep seismogenesis are recognized there: (i) 163 deep shocks have occurred north of 15°S in the Vityaz Group from 1949 to 1996. These seismological observations and the absence of other features characteristic of active subduction suggest that the Vityaz group represents deep failure in a detached slab that has foundered to a horizontal orientation near the bottom of the transition zone. (ii) Another group of nearly 50 `outboard' deep shocks occur between about 450 and 660 km depth, west of the complexly buckled and offset western edge of the Tonga Wadati–Benioff zone. Their geometry is in the form of two or possibly three small-circle arcs that roughly parallel the inferred motion of Tonga trench migration. Earthquakes in the southernmost of these arcs occur in a recumbent high-seismic-wavespeed slab anomaly that connects both to the main inclined Tonga anomaly to the east and a lower mantle anomaly to the west [Van der Hilst, R., 1995. Complex morphology of subducted lithosphere in the mantle beneath the Tonga trench. Nature, Vol. 374, pp. 154–157.]. Both groups show complexity in their focal mechanisms. The major question raised by these observations is the cause of this apparent temporary arrest in the descent of the Tonga slab into the lower mantle. We approach these questions by considering the effects of buoyant metastable peridotite in cold slab material that was detached and rapidly foundered, or was buckled, segmented and laid out in the transition zone.

Tectonic controls on sedimentation and diagenesis in the Tonga Trench and forearc, southwest Pacific

Sedimentation in the Tonga forearc is dominated by the redeposition of volcaniclastic sediment from the arc volcanic front by mass flows and turbidity currents onto the adjacent Tonga Platform, the shallowest, flattest part of the forearc region. The greatest sediment thicknesses accumulate in debris aprons close to the volcanic front. Collision of seamounts, notably the Capricorn Seamount and the Louisville Ridge, with the forearc radically shortens and steepens the adjacent modern trench slope, allowing sediment to be redeposited deep into the trench. Rotation, usually arcward, of existing basins on the midslope during collision generates angular unconformities, while synchronous uplift of the outer forearc high results in canyon development and downcutting along the eastern edge of the Tonga Platform. Collision also reactivated major across-strike fault zones on the forearc; the zones are subsequently exploited by canyons depositing sediment into the trench. Collapse and renewed extension of the forearc in the wake of collision result in the development of small perched basins, measuring approximately 5 km by 15 km in the midslope area. This morphology is especially developed at 18°30′S to 20°S, implying a 2–3 m.y. interval for their formation following Louisville Ridge collision. Trenchward of these depocenters sedimentation is slow, resulting in manganese crust formation and localized mass wasting along fault scarps. Over longer periods of time (>5 m.y.) tectonic erosion reestablishes a wide, gently sloping forearc into which canyons incise the shallow Tonga Platform by headwall erosion and collapse.

Morphology and Evolution of the Remnant Colville and Active Kermadec Arc Ridges South of 33°30′ S

Swath MR1 data from the remnant Colville and active Kermadec arc margins, south of 33°30′ S (SW Pacific), record the structural morphology and evolution of the rifted, and now separate portions, of the proto-Colville–Kermadec arc flanking the actively widening southern Havre Trough back-arc basin associated with Pacific-Australian plate convergence. Both the remnant Colville and active Kermadec arc margins comprise opposing, asymmetric, partially basement exposed, segmented ridges. Differences in morphology between the two ridges are, however, observed. The single, near linear, border fault system, with relief of ∼1000 m, along the western edge of the Kermadec margin is interpreted to be the exposed fault escarpment of a major, west-dipping, detachment fault. In contrast, two major zig-zag border fault systems along the eastern edge of the Colville Ridge, bounding a back-tilted ridge flank terrace, are interpreted as major antithetic faults between the remnant arc and back-arc region. This contrast in structural morphology coincides with, respectively, lesser and greater degrees of both active tectonism and channel-canyon erosion, on the remnant Colville and active Kermadec margins. These differences are interpreted to reflect the progressive trenchward collapse and associated greater rift flank uplift and incisive erosion of the Kermadec foot-wall contrasting with the non-collapse and relatively lesser rift flank uplift and ridge erosion of the Colville hanging-wall. The data provide further constraints on the early evolution of the Havre Trough in particular, and back-arc basins in general.

Evolution and interaction of migrating cross‐arc volcanism and backarc rifting: An example from the Southern Havre Trough (35°20′–37°S)

The active southern Havre Trough (35°20′ – 37°S) backarc basin is interpreted to form from the evolution and interaction of migrating cross‐arc magmatism, and the progressive development of longitudinal rift grabens. The proposed migration of the proto‐Kermadec arc front from the Colville arc at ∼5 Ma to the active Kermadec arc margin is recorded by the construction of arc edifices trailed across the intervening, and contemporaneously rifting, backarc complex. Migration trails are identified for at least four arc volcano sources. The Rumble V arc migration trail is the most prominent, forming a continuous, high‐standing, magmatic arc ridge. These postulated arc trails segment the backarc region and initially limit rift development to the intervening proto‐arc crust blocks. Early rifting between the arc volcanoes forms fully developed rift grabens which, with progressive basin widening, propagate longitudinally across the trails of migrating constructional arc magmatism. A model of the balance between rates of constructional arc magma production MR and destructive back‐arc (rifting) extension VE is proposed. When VE is high, MR is insufficient to keep abreast of destructional rifting, resulting in small, isolated arc massifs quickly dismembered by rifting. Conversely, when VE is low, MR is sufficiently greater than rifting to produce a continuous, high‐standing cross‐arc ridge which segments longitudinal rift development. Migrating arc magmatism will be best observed, and preserved, in rifting backarc basins when MR is >600 km3 m.y.−1 100 km−1 of plate boundary and VE is <25 mm a−1.

The Lau-Havre-Taupo back-arc basin: A southward-propagating, multi-stage evolution from rifting to spreading

The Lau-Havre-Taupo (LHT) system is an active, 2000-km-long, back-arc basin, associated with Pacific-Australian plate convergence in the SW Pacific. Data from the LHT back-arc system include ODP drillhole, swath bathymetric and side-scan imagery, seismic reflection, rock geochemistry, magnetic anomaly, heat flow, shallow seismicity, gravity and seismic refraction data. The LHT forms a longitudinal transect from southward-propagating true oceanic spreading in the Lau Basin near Tonga, similarly southward-propagating arc rifting in the Havre Trough, and southward-propagating continental rifting in the Taupo Volcanic Zone (TVZ) in northern New Zealand. Four transitional tectonic settings are identified along the LHT, north to south, between the continuum of spreading to rifting and comprise: (1) oceanic spreading of the Central Lau Spreading Centre (CLSC) propagating into oceanic crust formed by the Eastern Lau Spreading Centre (ELSC); (2) oceanic spreading of the ELSC propagating into rifted Lau-Havre arc crust; (3) rifting within Havre arc crust propagating into rifting New Zealand continental crust; and (4) rifting of continental crust propagating into as yet non-rifted, but flexurally subsiding, continental crust. The enhanced spatial and temporal resolution of observed tectonic and magmatic processes along the strike of the LHT, is used to constrain a model for the evolution of back-arc rifting to back-arc spreading. A sequence of five stages is identified and comprise: (1) a combination of incipient simple and pure shear; (2) early, distributed and spatially heterogeneous half-graben rifting with low-angle synthetic and antithetic structures associated with rapid graben sediment infilling, ephemeral early MORB-like rift magmatism, and the initial trenchward migration of the arc front; (3) a latter phase of half-graben rifting mostly confined to a contiguous axial zone, with continued ephemeral MORB-like rift magmatism, and further trenchward migration of the arc front; (4) initial oceanic spreading with evolving “steady-state” MORB magmatism along the axial rift graben system; and (5) the reconfiguration of ridge geometry in response to changes in opening orientation, leading to the development of ridge propagation into newly created oceanic crust. Rifting stages 2 and 3 can produce at least ∼ 120-km-wide back-arc crust by a balance of crustal extension (with an estimated β factor of 3), with rift and constructional arc magmatism. In the LHT system, depending on latitude, rates of widening during stages 2 and 3 vary between 15 and 45 mm yr −1, whilst stages 4 and 5 have full spreading rates of ∼ 90 mm yr −1.

Oblique rifting in the Havre Trough and its propagation into the continental margin of New Zealand: Comparison with analogue experiments

The Havre Trough is opening by oblique back-arc rifting which is propagating into the continental margin of New Zealand at the Taupo Volcanic Zone. Variations of deformational style along the rift axis have been investigated by comparison with analogue experiments which incorporate brittle and ductile rheologies and are scaled for gravity. Based on the results of the analogue experiments, we present a tectonic model for oblique rifting in the Havre Trough, which involves the rheological contrast between oceanic and continental lithosphere and the oblique geometry of the continental margin of New Zealand with respect to the regional rift trend. The model shows that the continental margin, which is weaker than both oceanic and continental lithosphere, cannot support large shear stresses. The two lithospheres can be decoupled during extensional events along the marginal shear and, depending on the continental margin orientation, this shear can modify the regional stress field. A heterogeneous stress field will rotate normal stresses to be perpendicular or parallel to the margin. As the two lithospheres decouple during extension, the rift grabens and internal faults of the oblique rift system propagate normal to the marginal shear. This model explains the oblique trend of the Havre Trough's tectonic fabric and its relationships to the Vening Meinesz Fracture Zone which represents the oceanic/continental lithospheric boundary.

Complex morphology of subducted lithosphere in the mantle beneath the Tonga trench

AT the Tonga trench, old Pacific sea floor subducts at a rapid rate below the Indo-Australia plate, generating most of the world's deep earthquakes (focal depth >300 km)1,2 and producing a deep slab of former oceanic lithosphere. The seismogenic part of the slab has been mapped in detail3,4, but its fate has remained enigmatic. Here I present evidence from seismic tomography that the Pacific plate descends deep into the Earth's mantle along a trajectory that is more complex than previously thought. In the north, the slab deflects in the transition zone (between about 400 and 700 km depth) before continuing into the lower mantle (below 700 km). Further south, penetration into the lower mantle occurs without a kink. The slab morphology can be explained in terms of the recent tectonic evolution of the subduction system, and reconciles pre-existing evidence from this region for both local horizontal flow in the transition zone2–8 and slab penetration into the lower mantle9–12.

伊豆・小笠原，マリアナ前弧の蛇紋岩海山 潜水調査船による観察と陸上蛇紋岩帯との関連

Serpentine seamount was reviewed with recent seven Shinkai 6, 500 dives at Izu-Bonin and Mariana forearcs. The seamounts consist mostly of serpentine flows with huge xenoliths of peridotite, high-pressure type metamorphic rocks such as jadeite-quartz assemblage and aragonite, glaucophane schist as well as MORB like basaltic rocks and chert. Active seamount has the following three characteristic features ; 1) serpentine flow, 2) carbonate chimney, and 3) huge peridotite xenolith in the flow. Serpentine flow shows a lobate structure with several meters width and 2-3 m high, sometimes flows down along the steep cliff. Drilling results indicate that flow unit shows the sedimentary structure similar to the interior structure of the mud diapir. Carbonate chimneys exist as an isolated tree and more than 20 chimneys stand make a line in some cases. Active carbonate chimney has yellowish green bacterial mats on the surface but dead ones are covered with thin black manganese coating. The fate of carbonate chimneys is fell down and dissolves into the sea water when the depth exceeds more than 4, 000 m, becomes a member of sediments when buried and sometimes makes a vein.In the metamorphic belts onland such as Sanbagawa, Kamuikotan metamorphic terrains and Mineoka belt there are serpentinite bodies of both alpine and stratiform types. The former type was thought to be of solid intrusion along fault. These serpentinite bodies yield white sediments as a vein, layer of sediments as well as rodingites. Rodingites always crop out with serpentinites in the Alps and other orogenic belts. Serpentinite bodies have highly sheared and brecciated parts and most geologists thought those parts to be the results of strong shearing when solid intrusion. However, if we look at the body carefully, sedimentary structure and white veins, and sediments will be frequently observed and the body is closely accompanied by rodingites.We strongly insist to think that these white materials are the products of the carbonate chimneys and sedimentary structures are also the product of serpentine flow at the forearc serpentine seamount and emplace into the metamorphic terrains during the secondary processes. During the emplacement, rodingite and magnesite ore would be formed by metasomatism with serpentinites.Most of the serpentinites in the orogenic belt must be of serpentine seamount origin such as in Izu-Bonin, Mariana, Tonga and Middle America Trenches where the crust are thin compared with those of the ordinary island arcs and continental margins.

Petrology and Geochemistry of Boninites from the North Termination of the Tonga Trench: Constraints on the Generation Conditions of Primary High-Ca Boninite Magmas

We report here a detailed mineralogical, geochemical, and experimental study of a high-Ca boninite suite from the northern termination of the Tonga trench. Most samples are strongly olivine porphyritic and show a significant range of phenocryst compositions including a very refractory olivine-spinel assemblage Fo94-CrN = 87. They are also characterized by a wide range of incompatible-element contents, e.g., (La/Yb)y varies from 05 to 16, whereas compatible major-element concentrations (A12O3, FeO, CaO, SiO2, and MgO) remain essentially the same. Primary melt compositions for the suite were established on the basis of an experimental study of melt inclusions in phenocrysts and numerical modelling of the reverse of fractional crystallization. Tongan primary melts are characterized by high MgO contents (22-24 wt.%) and originated in the mantle wedge at pressures of 20-25 kbar and temperatures of 1450-1550 °C. H2O contents in primary melts were estimated from direct measurements of melt inclusions by ion probe, and range from 2-0 to 10 wt.%, and a strong correlation exists between H2O and other incompatible element contents. The primary melts crystallized in the presence of an H2O-rich fluid in the temperature range 1390-1150 °C and pressures of 1-7-O15 kbar. Continuous degassing of melts took place during crystallization. Trace-element concentrations in primary melts were estimated using proton- and ion-probe analyses of melt inclusions in olivine, and whole-rock analyses. Our data suggest that three independent components (D, El, and E2) were involved. Component D was a refractory mantle depleted in incompatible elements, likely to be hot 'dry' lherzolite produced by previous melting within a mantle plume. Component El was an H2O-rich fluid containing LILE and Th, and had an H2O/K2O value of 20; it was probably produced by dehydration of the subducted slab. Component E2 is thought to have been an incompatible- element-enriched silicate melt of plume origin. Formation of high-Ca boninites requires interaction of hot 'dry' residual mantle, associated with plumes, with a subduction-rel ated H2O-bearing component.

Nature and tectonic setting of the southern Kermadec submarine arc volcanoes: An overview

The 1000-km long Kermadec subduction system (northern New Zealand), and adjacent Havre Trough back-arc basin, associated with the Pacific-Australian plate boundary, is an archetypal example of an active “island arc” convergent margin. Although the position and tectonic setting of the frontal arc is critical to understanding tectonism and magmatism within an arc-back-arc system, this has not been, unequivocally established for the Kermadec subduction system. Swath surveys from the 35°–37°S segment of the Kermadec system reveal that the frontal arc comprises seven large arc stratovolcanoes (Rumble II, III, IV, Silent II, Rumble V, Tangaroa, and Clark). The frontal arc continues south with the Whakatane volcano sited at the northern extremity of the Taupo Volcanic Zone. Of these eight volcanoes, the latter four are newly identified, and all have constructional volumes varying between 50 and 260 km 3. All of these arc edifices are sited 15–25 km west of the Kermadec frontal ridge. Bathymetric data similarly indicate that other Kermadec arc volcanoes between 32° and 35°S also lie west of the ridge axis, within the “back-arc” region, while north of 32°S the arc volcanoes lie along the frontal ridge. This apparent variation in the tectonic position of the frontal arc coincides with other changes in the morphology of the subduction system, north and south of 32°S, and as suggested by Pelletier and Dupont (1990), may reflect differences in the length of the underlying subducted slab. The 35°–37°S frontal arc segment is sited within 35–45 km of the present-day back-arc rift axis, and may control the initial loci of rift graben evolution. Sidescan imagery and seafloor photography of the southern Kermadec arc volcanoes reveal a complex volcanic stratigraphy with intertwined pillow lavas, proximal pillow breccias, highly vesicular scoriaceous lapilli, and pervasively mass-wasted volcaniclastic and epiclastic detritus. All volcanic deposits interdigitate with pelagic sediments. A single K-Ar dated andesite from Whakatane volcano suggests that the history of the present frontal arc may extend back to at least 0.77 Ma while Rumble III and V volcanoes are the major, although not exclusive, sites of geologically recent edifice construction. The historical acoustic record of southern Kermadec submarine eruptions since 1958 is now regarded as equivocal, with the July 1986 eruption of Rumble III the only unambiguous volcanic activity recorded in modern times.

Arc and back-arc geochemistry in the southern Kermadec arc-Ngatoro Basin and offshore Taupo Volcanic Zone, SW Pacific

Abstract Back-arc basin basalts from the Ngatoro Basin (the southern end of the Havre Trough) are similar geochemically to, yet subtly distinct from, basalts of the Havre Trough to the north. Whole rock and glass chemistry are consistent with derivation from a fertile mantle source with subsequent evolution by fractionation of olivine (+ Cr-spinel) + plagioclase, and then clinopyroxene. Basalts from the vicinity of Rumble IV seamount at the southern end of the Kermadec island arc, and the eastern Ngatoro rift escarpment, are strongly porphyritic relative to the back-arc basin basalts and show trace element (high LIL abundances and highly depleted HFS abundances) and isotopic signatures of subduction zone basalts. At its southern end, the Ngatoro Basin penetrates the continental crust of New Zealand creating a major, 3000 m deep bathymetric re-entrant in the slope-break; the slope-break marks the transition from oceanic to continental crust. Basalts from the floor of the Ngatoro Basin re-entrant are isotopically distinct from the basalts of the oceanic sector in that they have higher Sr and correspondingly lower Nd isotope ratios and are comparable to basalts of the Taupo Volcanic Zone (TVZ) to the south. In contrast to the basalt-dominated oceanic sector, basalts from the offshore TVZ, a 100 km long area extending roughly NNE from White Island to the submarine Whakatane arc volcano at the edge of the continental slope-break, occur in association with andesites, dacites and rhyolites. These basalts are generally strongly porphyritic (olivine + plagioclase + clinopyroxene) and show trace element abundances typical of suprasubduction zone rocks. However, offshore TVZ basalts show subtle distinctions from onshore TVZ basalts to the south; the former have more radiogenic Sr isotopes. Furthermore, their high field strength element and transition element systematics appear to overlap with those of basalts from the Kermadec arc to the north. The authors attribute these lateral (along arc) and transverse (across arc) variations to source heterogeneity and variable fertility in the sources of the arc and back-arc basin magmas. Sources of the arc-front magmas are more refractory and also more susceptible to contamination by slab-derived fluids than sources for back-arc basin magmatism, reflecting the dynamic nature of flow from the back-arc into the mantle wedge beneath the volcanic front.

Pre-spread rifting and heterogeneous volcanism in the southern Havre Trough back-arc basin

The Havre Trough is an active back-arc basin associated with Pacific—Australian plate convergence. Two segments within the southern Havre Trough (SHT), surveyed with gloria, Sea beam and SYS09 swath data, now provide the first detailed data to reliably document the structure and morphology of this complex back-arc region. At the southern segment (35°40′–37°S), along a strike length of some 120 km, the SHT structure comprises a series of segmented, en echelon, generally flat floored, sediment filled, axial rift grabens, flanked by ridge and basin rift block topography. Prominent fault escarpments, with relief of 500–1000 m and a marked zig-zag arrangement, mark the outer margins of the rift system. Within the rift grabens extensional tectonism is pervasive. Extensive shallow seismicity within the SHT can only partly be associated with the rift graben system at present. Youthful, constructional volcanic terrains within the rift grabens are not common, and invariably have a restricted areal extent and a relief < 400 m. Neovolcanism is fault controlled, being apparently restricted to the rift escarpments. Lavas within the rifts display a marked source heterogeneity as evinced by the eruption of back-arc basin and island arc basalts on the western and eastern escarpments, respectively, even though separated by as little as 14 km. An identical rift morphology has been identified at the northern (33°–34°S) site. Data from both sites show individual rifts are 15–35 km in length, 6–10 km wide, and have axial depths of 3100–4000 m. Apart from at least one 15–20 km wide, cross trough ridge, available data indicate the rift graben system is contiguous between 33°S and 37°S. This rift graben morphology, by analogy with other known rifting back-arc basins, is considered here not to be consistent with true oceanic back-arc spreading. Rather it is argued that the SHT, possibly indeed all of the Havre Trough, is undergoing back-arc rifting prior to true spreading. Further, the rifting tectonic fabric within the SHT, and in the onshore Taupo to the south, extension is consistently dextrally oblique relative to the bounding Colville and Kermadec Ridges. The degree of obliquity increases from south to north. If accepted that the SHT is rifting rather than spreading, magnetic anomaly data within the trough do not record the accretion of new oceanic crust, and consequently can no longer be used to establish the “spreading” history of the SHT. The SHT magnetic anomalies are here interpreted as “pseudo-linear” magnetic anomalies resulting from the generally irregular spatial and temporal emplacement of magnetic sheeted lava and dike intrusives between older, low magnetisation arc basement rift blocks flanking the axial rift. Proxy data from onshore North Island, New Zealand, and offshore SHT suggest that the rate of extension and age of initiation of back-arc rifting are, between 35° and 37°S, respectively, 15–20 mm yr −1 and 5 Ma.

The geochemistry and petrogenesis of basalts from the Taupo Volcanic Zone and Kermadec Island Arc, S.W. Pacific

Basalts from the Taupo Volcanic Zone (TVZ), New Zealand, the Kermadec Island Arc (KA) and its back-arc basin, the Havre Trough show systematic variations in trace-element and isotope geochemistry which are attributed to differences in tectonic setting and source heterogeneity along a more or less continuous plate boundary. Basalts from the Kermadec Arc are characterised by low abundances of high field strength elements (HFSE) such as Ti, Zr, Nb, Ta and Hf and have high ratios of Ti/Zr and low ratios of Ti/Sc and Ti/V relative to typical MORB. Basalts from TVZ also show low abundances of the HFS elements relative to MORB but show lower Ti/Zr, higher Ti/V and Ti/Sc ratios and generally higher Zr abundances than KA most basalts. The Havre Trough basalt is mildly alkaline (< 1% normative nepheline) like many back-arc basin basalts from the Pacific rim, contrasting with the hypersthene normative TVZ and KA rocks. It has higher Zr than most TVZ basalts and all KA basalts. Ratios such as Ti/V, Ti/Sc and Ti/Zr are within the range of TVZ and MORB basalts but distinct from KA basalts. The depleted (relative to MORB) HFSE characteristics of the KA and TVZ basalts are complemented by high abundances of large ion lithophile elements (LIL), such as Ba, Rb and K, when compared to MORB, yielding the distinctive LIL-enriched pattern of subduction related rocks on a normalised multi-element plot. In contrast, the Havre Trough basalt is MORB-like. Chondrite-normalised Rare Earth Element (REE) patterns for the TVZ basalts show a field overlapping with that defined by the southern KA (Rumble Sea Mounts), with light REE enriched patterns (Ce/Yb n = ∼ 1.8−3) and flat heavy REE (Tb-Lu). Basalts from the northern KA are typically light REE depleted (Ce/Yb n = 0.5) or slightly enriched (Ce/Yb n = 1.5). The REE pattern of the Havre Trough basalt is distinctive from both the KA and TVZ fields, being richer in the heavy REE, yet similar to many basalts from back-arc basins. Sr and Nd isotopes show considerable variation from a depleted MORB-like ratio in the Havre Trough basalt ( TVZ-19, 87Sr/ 86Sr = 0.702556, ϵ Nd = +9.3 ) to radiogenic ratios in the Tarawera basalt (TVZ-4, 87Sr/ 86Sr = 0.7052, ϵ Nd = +2.2) from TVZ suggestive of crustal contamination. Between these extreme values the data from the KA and TVZ define distinctive arrays trending towards the evolved andesites, rhyolites and supracrustal basement rocks of central New Zealand. This curvilinear Nd-Sr array of the TVZ data is best explained as a result of crustal contamination. At the unradiogenic-Sr end of the array, the Sr and Nd isotopic ratios of primitive TVZ basalts overlap with those from southern KA suggesting that primitive TVZ basalts were derived from sources that are isotopically similar to those from which the southern KA basalts derived. Basalts from northern KA (Herald, Raoul and Macauley Islands) are appreciably less radiogenic than those from the central part (Curtis and L'Esperance) and the southern (Rumble Seamounts) part of the arc. Using non-slab-derived HFSE ratios and abundances as a guide to relative source depletion, it is inferred that the KA source varies from strongly depleted in the north to less depleted in the TVZ in the south. The KA source has generally experienced more extensive extraction of mafic magmas and hence is impoverished in incompatible HFSE. This leads to high Ti/Zr and low (absolute) Zr, Hf, Ta and Nb abundances. The Havre Trough source is relatively fertile with regard

Late Quaternary tephrostratigraphy from the southern Havre Trough ‐ Bay of Plenty, northeast New Zealand

Abstract A number of late Quaternary subaerial silicic tephra erupted from the Taupo Volcanic Zone are dispersed at least 230 km to the northeast of New Zealand, and are deposited as predominantly airfall vitric ash; other tephra are recognised as reworked ash turbidites. Tephra occurring as both megascopic and nonmegascopic ash within three piston cores from the Bay of Plenty ‐ southern Havre Trough are used to establish a c. 55 ka old marine tephrostratigraphy. On the basis of core stratigraphy, 14C dating, glass chemistry, and ferromagnesian mineralogy, the Kaharoa, Taupo, Mamaku, Waiohau, ?Okareka, Kawakawa, Omataroa, Hauparu, Tahuna, and ?Rotoehu Tephras are identified between 160 and 350 km from source. Peralkaline tephra from previously unrecognised Mayor Island eruptions are identified at c. 30 and 55 ka, and possibly 11.5 ka. Other tephra comprising pumiceous ash‐lapilli, and low SiO2 silicic glass, are also identified, but cannot be reliably correlated to a volcanic centre source. Occasional sha...

Shallow structure and active tectonism of an offshore continental back-arc spreading system: the Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ), the predominant active volcano-tectonic structure within the New Zealand sub-continent, marks the zone of Quaternary back-arc spreading and calc-alkaline volcanism associated with the convergence of the Pacific and Australian plates at the Hikurangi plate-boundary margin, North Island, New Zealand. New bathymetric, shallow seismic reflection, magnetic anomaly, and GLORIA data presented here accurately define for the first time the extent, structure and tectonism of the poorly known offshore portion of the TVZ. These data show that nearly half of the entire ca. 40 km wide TVZ lies offshore, extending northeast for some 150 km seaward to 36°45′S. The southeastern and northwestern boundaries of the offshore TVZ are significant tectonic and bathymetric features, which comprise inward facing normal fault zone (the White Island and Tauranga Fault Zones, respectively). Three, essentially parallel, tectono-morphological units are identified along the length of the offshore TVZ. They comprise (from southeast to northwest) a frontal non-volcanic graben, a volcanic ridge, and a volcanic “back-arc” graben. The spatial position of these units with respect to the Hikurangi subduction margin is not a simple “arc” and “back-arc” relationship. Tectonism within the offshore TVZ, although pervasive, increases from northwest to southeast, so that the area of highest extension and subsidence of the non-volcanic graben occurs in front of the volcanic ridge. Geophysical data also suggest the possible tectonic ablation of blocks of the forearc hinterland into the offshore TVZ along its southeastern margin. The southern Havre Trough and northern TVZ are offset sinistrally by 45–50 km. They overlap by 40 km, and differ in orientation by ca. 7°. Dextral displacement between these two extensional regimes is accomodated by a series of dextrally oblique and en-échelon “bookshelf” faults. The orientation of extensional structures within the offshore TVZ is also dextrally oblique, by 10°–15° to the regional 030° trend of the bounding faults, and agree with axes of maximum extension determined elsewhere from geodetic and seismological data.

Gravity Flows and Rock Recycling on the Tonga Landward Trench Slope: Relation to Trench-Slope Tectonic Processes

Dredge samples collected from the flanks of the frontal-arc platform, from the mid-slope forearc terrace, and from the lower landward slope of the Tonga Trench are dominated by arc-derived volcaniclastic sediment and calcareous ooze. Much of the sediment was delivered by sediment gravity flows. Recycling of older fossils and rocks is very widespread, with evidence for several cycles of erosion and deposition in a single sample. Steep slopes and outcropping rock have prevailed around the frontal-arc platform throughout the late Neogene, probably driven by movement on bounding normal faults. The upper trench slope is dominated by coarse-grained turbidites and debris flows. The mid-slope terrace receives mainly finer-grained turbidites, with fewer coarse turbidites and debris flows. The lower trench slope is dominated by clast-rich debris flows, containing a majority of clasts of volcaniclastic deposits derived from the upper trench slope and mid-slope terrace, mixed with arc-substrate and ocean-plate rocks ...

Structural trends and back‐arc extension in the Havre Trough

The Havre Trough is an active back‐arc ocean btsin located to the west of the Kermadec trench‐arc system in the southwest Pacific. Two recent cruises have provided reconnaissance swaths of Sea Beam bathymetry and GLORIA side‐scan sonar data through much of the Havre Trough as well as a detailed survey at 33.5°S, allowing the overall structural character of the trough and the nature of crustal extension within the detailed survey area to be assessed. The orientation of the tectonic fabric within the Havre Trough is consistently oblique to the basin as a whole, trending 045° to 060° while the bounding Kermadec and Colville Ridges trend 015° to 025°. The oblique internal structure indicates that the Havre Trough has not opened perpendicular to its own trend. Within the detailed survey area a series of short (15–35 km long) en echelon basins are interpreted as an active spreading center; a lack of sediment cover and the dredging of fresh basalt from one of the 3500–4000 m deep basins confirms the youth of these features. No recent extensional features are observed in a GLORIA crossing of the trough at 31.5°S, suggesting that this part of the Havre Trough may currently be inactive.

Some Andisols from Savai'i, Western Samoa

AbstractFive pedons in the lowlands and foothills of Savai'i were studied in detail to determine whether or not they were Andisols. All five met the requirements proposed by the International Committee for the Classification of Andisols (ICOMAND) for the designation of Andisols, as recently accepted into the soil taxonomy. Four of the pedons were Hapludands at the great group level, with subgroup differences due to base status, depth to lithic contact, and water‐retention properties being observed. The fifth pedon was a Thaptic Fulvudand with imogolite present. The results obtained indicate that Andisols are likely to dominate the landscape of Savai'i. This is rather unexpected, as Savai'i is the oldest island of the Samoa chain. Recent volcanism associated with the approach of Savai'i to the edge of the Tonga trench has “rejuvenated” much of the island, leading to the formation of young soils. Since certain soils on Savai'i and the adjacent island of Upolu are mapped as the same series, reexamination of the soils on both islands would appear necessary to determine whether or not the soils are sufficiently different to warrant the establishment of new series.

The petrology, phase relations and tectonic setting of basalts from the taupo volcanic zone, New Zealand and the Kermadec Island arc - havre trough, SW Pacific

Volcanism in the Taupo Volcanic Zone (TVZ) and the Kermadec arc-Havre Trough (KAHT) is related to westward subduction of the Pacific Plate beneath the Indo-Australian Plate. The tectonic setting of the TVZ is continental whereas in KAHT it is oceanic and in these two settings the relative volumes of basalt differ markedly. In TVZ, basalts form a minor proportion (< 1%) of a dominant rhyolite (97%)-andesite association while in KAHT, basalts and basaltic andesites are the major rock types. Neither the convergence rate between the Pacific and Indo-Australian Plates nor the extension rates in the back-arc region or the dip of the Pacific Plate Wadati-Benioff zone differ appreciably between the oceanic and continental segments. The distance between the volcanic front and the axis of the back-arc basin decreases from the Kermadec arc to TVZ and the distance between trench and volcanic front increases from around 200 km in the Kermadec arc to 280 km in TVZ. These factors may prove significant in determining the extent to which arc and backarc volcanism in subduction settings are coupled. All basalts from the Kermadec arc are porphyritic (up to 60% phenocrysts) with assemblages generally dominated by plagioclase but with olivine, clinopyroxene and orthopyroxene. A single dredge sample from the Havre Trough back arc contains olivine and plagioclase microphenocrysts in glassy pillow rind and is mildly alkaline (< 1% normative nepheline) contrasting with the tholeiitic nature of the other basalts. Basalts from the TVZ contain phenocryst assemblages of olivine + plagioclase ± clinopyroxene; orthopyroxene phenocrysts occur only in the most evolved basalts and basaltic andesites from both TVZ and the Kermadec Arc. Sparsely porphyritic primitive compositions (Mg/(Mg+Fe2) > 70) are high in Al2O3 (>16.5%), and project in the olivine volume of the basalt tetrahedron. They contain olivine (Fo87) phenocrysts and plagioclase (> An60) microphenocrysts. These magmas have ratios of CaO/Al2O3, A12O3/TiO2 and CaO/TiO2 in the range of MORB and MORB picrites and can evolve to the low-pressure MORB cotectic by crystallisation of olivine±plagiociase. Such rocks may be the parents of other magmas whose evolutionary pathways are complicated by interaction of crystal fractionation, crystal accumulation and mixing processes and the filtering action of crust of variable density and thickness. The interplay of these processes likely accounts for the scatter of data about the cotectic. More evolved rocks from both TVZ and KAHT contain clinopyroxene and orthopyroxene phenocrysts and their compositions merge with basaltic andesites and andesites. Stepwise least-squares modelling using phenocryst assemblages in proportions observed in the rocks suggest that crystal fractionation and accumulation processes can account for much of the diversity observed in the major-element compositions of all lavas. We conclude that the parental basaltic magmas for volcanism in the TVZ and KAHT segments are similar thereby implying grossly similar source mineralogy. We attribute the diversity to secondary processes influencing liquids as they ascended through complex plumbing systems in the sub arc mantle and cross.

GLORIA survey of the oceanic-continental transition of the Havre-Taupo back-arc basin

A GLORIA (Geological Long-Range Inclined Asdic) side-scan sonar survey, covering 23,000 km2, provides the first complete imagery of an active and contiguous, oceanic to continental back-arc system, namely, the Havre Trough to Taupo Volcanic Zone (TVZ) New Zealand. Havre Trough tectonism and volcanism relates to a series of laterally discontinuous, mutiple spreading rifts which terminate southward at the 3-km-deep Ngatoro Basin. A 45-km sinistral offset attributed toen echelon synthetic shearing separates the basin from the actively spreading TVZ. Sonographs reveal a youthful and complex volcanic seascape with 20 newly discovered seamounts, whereas flanking regions are mantled with a largely featureless mud blanket.