Etienne Ruellan

Modes of seafloor generation at a melt-poor ultraslow-spreading ridge

We report on extensive off-axis bathymetry, gravity, and magnetic data that provide a 26-m.y.-long record of axial tectonic and magmatic processes over a 660-km-long and melt-poor portion of the ultraslow Southwest Indian Ridge. We describe a new type of seafloor (the smooth seafloor) that forms at minimal ridge melt supply, with little or no axial volcanism. We propose possible mechanisms leading to this avolcanic or nearly avolcanic mode of spreading, in contradiction with the traditional view of mid-ocean ridges as primarily volcanic systems. We also show evidence for large-offset asymmetric normal faults and detachments at the ridge axis, with asymmetry persisting in some cases for tens of millions of years.

From rifting to active spreading in the Lau Basin – Havre Trough backarc system (SW Pacific): Locking/unlocking induced by seamount chain subduction

Associated with Pacific-Australia plate convergence, the Lau Basin – Havre Trough is an active back-arc basin that has been opened since ?5.5 Ma by rifting and southward propagating oceanic spreading. Current back-arc opening rates decrease from 159 mm yr?1 in the northern Lau Basin to 15 mm y?1 in the southern Havre Trough. Major tectonic changes occur at the transition between Havre Trough rifting and full oceanic spreading of the Eastern Lau Spreading Center (ELSC), where the oblique-to-trench, westward subducting Louisville Seamount Chain (LSC) sweeps southwards along the Tonga trench. New swath bathymetry, seismic reflection data, and limited rock sampling in this area constrain a tectonic and kinematic back-arc model that incorporates the effects of LSC subduction. The ELSC, which extends southward to 24°55?S, forms a deep rift valley propagating southward through older, rifted arc basement. Present-day seismicity and fresh and fractured pillow lavas at 23°42?S are consistent with rift valley neovolcanism. Conversely, the northern Havre Trough has low seismicity and rifted volcanic basement ridges trending 25–45° oblique to the basin axis consistent with low levels of extensional tectonism and volcanism. This latter structural fabric is interpreted as an early stage of rifting that is now “locked” due to compression on the arc exerted by LSC subduction, while in the Lau Basin such effects have passed as the LSC swept along the Tonga Trench. It is proposed that the Lau-Havre back-arc opening is controlled by tectonic constraints exerted at the limits of the system by the LSC subduction, which determines the southward migration of the Tonga Arc pole of rotation and associated Lau Basin opening. A discrete three-stage back-arc opening evolution is proposed, comprising: (1) an initial phase of back-arc rifting along the whole length of the plate boundary, beginning at ?6–5 Ma; (2) a subsequent phase, mostly present in the southern part of the back-arc domain and still active in the Havre Trough, of transpression and transtension, starting at ?4 Ma in the north, as the LSC starts to subduct and sweeps southward along the Tonga trench; and (3) a renewed opening phase in the northern segment of the back-arc domain, with rifting and spreading, starting at ?3.5 Ma, as subduction of the LSC along the northern Tonga trench is progressively completed.

Structure and structural development of the Havre Trough (SW Pacific)

Northeast of New Zealand the Pacific plate is obliquely subducted beneath the overriding Australian plate. Major differences appear in the oblique-convergence-driven structures that occur along the plate boundary. While strike-slip component of convergence is accommodated in the forearc domain to the south at the eastern North Island margin, we here substantiate that at least part of the strike-slip component is accommodated within the back arc Havre Trough to the north offshore New Zealand. New swath bathymetry and structural data collected in the Havre back arc domain show that the basin as a whole displays oblique to basin axis structures that allow considering the basin to be the loci of extensive tectonics associated with dextral displacement of the Kermadec Arc Block relative to the Australian plate. Therefore, where the two plates are oceanic, the oblique convergence between Pacific and Australian plates is partitioned at the subduction trench for its normal-to-trench component and at the rear of the Kermadec Arc Block for part or all of its along-strike component where it is intimately associated with extensional rifting within the back arc domain. The kinematic pattern of Australian-Pacific plate boundary northeast of New Zealand that straddles a limit between continental and oceanic lithosphere allows addressing the specificity of the structural development, the deformation distribution, and the assessment of movement components that occur in a back arc basin associated with oblique convergence.

Active Spreading and Hydrothermalism in North Fiji Basin (SW Pacific). Results of Japanese French Cruise Kaiyo 87

The aim of the Japanese-French Kaiyo 87 cruise was the study of the spreading axis in the North Fiji Basin (SW Pacific). A Seabeam and geophysical survey allowed us to define the detailed structure of the active NS spreading axis between 16° and 22° S and its relationships with the left lateral motion of the North Fiji Fracture Zone. Between 21° S and 18°10′ S, the spreading axis trends NS. From 18°10 S to 16°40 S the orientation of the spreading axis changes from NS to 015°. North of 16°40′ S the spreading axis trends 160°. These two 015° and 160° branches converge with the left lateral North Fiji fracture zone around 16°40′ S to define an RRFZ triple junction. Water sampling, dredging and photo TV deep towing give new information concerning the hydrothermal activity along the spreading axis. The discovery of hydrothermal deposits associated with living communities confirms this activity.

New constraints on the New Zealand–South Fiji Basin continent-back-arc margin

Abstract New cruise results indicate that the New Zealand–South Fiji Basin margin comprises a volcanic plateau, separated from the continent by sedimentary basins (floored by metamorphic rocks and an allochthon containing Cretaceous ophiolites). Seaward lies undisturbed oceanic crust. The plateau began to form in the Early Miocene after the allochthon was obducted (in the Aquitanian). The oceanic crust of the South Fiji basin is contiguous with that of the plateau, which in turn indicates that the South Fiji Basin in this region is of Miocene rather than Oligocene age as commonly thought. The tectonic history is perforce that of a Pacific subduction zone replaced by a back-arc transform margin containing an enormous volume of volcanic material.

The 16°40′ S triple junction in the North Fiji Basin (SW Pacific)

During the last three years, the North Fiji Basin (SW Pacific) has been intensively studied on three oceanographic cruises carried out by French, American and Japanese ships. One of the main goals of these cruises was to study by means of precise SeaBeam, SEAMARC II, seismic and magnetic surveys, the active spreading system and its associated hydrothermal processes. The North Fiji basin, bounded by the major Pacific and Indo-Australian plates, shows a complex polyphased tectonic evolution. One of the last phases of this evolution is the functioning since 3 Ma of a NS spreading center in the axial part of the basin. The tectonic instability of the area resulted in a permanent rearrangement of the ridge axis. Among others, the 16°40′ S triple junction is one of the major manifestations of such an instability. Sinistral strike-slip motion 1 Ma ago, along the North Fiji Fracture Zone induced the change in direction of two segments of the axis from NS to N15 and N160. The first segment is characterized by a typical spreading ridge similar to various parts of the EPR, while the second shows an atypical ‘en echelon’ fan-shape opening. The N15 and N160 ridges converging with the North Fiji Fracture Zone constitute the 16°40′ S Ridge-Ridge-Fracture Zone triple junction. The detailed morphologic and kinematic study of this junction allows us to understand one of the mechanisms of the deformation in the North Fiji basin.

Kinematics of active spreading in the central North Fiji Basin (Southwest Pacific)

Abstract Based on a synthesis of magnetic and bathymetric data, we re-evaluate the kinematics of the recent opening of the central part of the North Fiji Basin (NFB). The westward motion of the Pacific plate along the left-lateral North Fiji Fracture Zone (NFFZ) results in the opening of two N-S trending spreading ridges, located at 173°30′E and 176°E. Both ridges show complex features such as propagating rifts, ridge jumps and overlapping spreading centres. Their spreading rates are similar: 7.6 to 4 cm yr −1 across the western ridge, 5.5 cm yr −1 across the eastern one. While the NFFZ is purely strike-slip to the east of the eastern ridge, it becomes more complex to the west: it changes from transpressional to transtensional, then to purely transform and finally joins the western N-S ridge in a RRR-type triple junction. Our kinematic analysis shows that most of the left-lateral motion along the NFFZ is transferred to the eastern ridge at the RTF-type 176°E triple junction. It suggests that the western N-S ridge is probably connected to the north with another left-lateral transform, possibly the South Pandora “ridge”.

Tectonics, Magmatism, and Evolution of the New Hebrides Backarc Troughs (Southwest Pacific)

In the southwest Pacific, a discontinuous series of narrow and elongated troughs separates the New Hebrides island arc from the adjacent active marginal basin, the North Fiji Basin. This chapter reviews the structural, geophysical, geochronological, and petrological data available for the New Hebrides backarc troughs (NHBAT) and discusses the significance of these structures. A diffuse horst-and-graben morphology, partly obscured in some places by recent volcanic complexes, characterizes the northern Jean-Charcot troughs (JCT). By contrast, the southern Coriolis troughs (CT) show well-developed flat-bottomed grabens. Moreover, no backarc troughs are observed in the central backarc area, adjacent where the d’Entrecasteaux zone collides with the arc. Volcanic rocks dredged in the NHBAT show a wide range of SiO, contents, with highAlzo3 and low-Tio, contents, features typical of their archackarc environments. Trace element analyses indicate a much stronger subduction component in the volcanics of the southern CT than in those of the northern JCT. However, large-ionic-radius-lithophileelement (LILE) (Ba, Rb, Sr) enrichments and high-field-strength-elements (HFSE) (Ta, Nb, Zr, Ti,-Y, Yb) depletions, relative to N-MORB (mid-ocean ridge basalts), are generally observed in most NHBAT volcanics and are features characteristic of island-arc basic and

Propagating rift and overlapping spreading center in the North Fiji Basin

Abstract The present day geometry of the North Fiji marginal basin, at the Pacific and Indo-Australian plate boundary, results from a polyphased tectonic evolution. Since the beginning of the opening, spreading occurred along several ridge axes that are characterized by highly variable trends. From 15°S to 21°40′S, the present day trends of the axis are mainly N20°W, N20°E and N-N5°E. The structural analysis of multi-beam echo sounder and seismic data combined with submersible observations (18 dives in this area) allow a precise study of relationships between the N-N5°E and N20°E axes near 18°30′S. Two main oceanic features occur in this area. The most striking one consists in a broad northward propagating rift of the N-N5°E spreading ridge. The northern tip of the propagator is located at 18°10′S, on 173°30′E and is characterized by a northern V-shaped end and a slight camber. The diving observations corroborate the present day magmatic and tectonic activities along the N-N5°E spreading axis located between 18°10′S and 20°30′S and its northward propagation to the detriment of the N20°E axis which is less active at its southern end. Moreover, the youngest axis trends, inside the N20°E axial domain, confirms this northward propagation of the N-N5°E structural trends. The N-N5°E spreading axis shows very close similarities with those of an intermediate to fast-spreading ridge, either in its morphology or in its spreading rate. On the other hand, the N20°E axis displays a large tectonically active overlapping arm, that is counterposed to the northern end of the N-N5°E propagating system. The transform zone between the two overlapping arms is characterized by typical oblique and curved structural patterns in the sea floor. The whole tectonic system constitutes thus a typical large overlapping spreading center. The average propagating rate is calculated at 5.7 cm/yr that is compatible with backward migration of the N20°E overlapping arm. This study confirms the high instability of the North Fiji Basin spreading system through time and space, due to the regional tectonic stress produced by the plate convergence and the double sphenochasmic opening of the basin.

In situ geological and geochemical study of an active hydrothermal site on the North Fiji Basin ridge

Abstract In June 1989 a diving cruise by the submersibleNautile was carried out on the North Fiji Basin spreading ridge. The objective of the cruise was a geological and geochemical study of the active spreading axis and associated hydrothermal processes. This operation was the third cruise of the French-Japanese Starmer project following the two cruises of the R.V.Kaiyo in 1987 and 1988. Six dives along the spreading axis between 16°58′S and 17°00S show that the axial graben consists of alternating N15—trending horsts and grabens. Extinct hydrothermal sites have been observed all along the graben. These consist of fossil chimneys, oxide staining and dead shells. At 16°59′S an active chimney has been discovered and called the “White Lady” because of its almost exclusively anhydrite composition. This chimney expels a peculiar water characterized by low chlorinity and a 285°C maximum temperature. In the northern extremity of the N15 axis a wide fossil hydrothermal site has been explored and sampled. It is located in an area cut by N15, N140 and N60-trending faults and fissures.