Marc Régnier

Origin of the New Caledonian ophiolites based on a French–Australian Seismic Transect

Abstract The origin, age and nature of the New Caledonian ophiolites are still debated. Recently, as part of the joint French–Australian Seismic Transect (FAUST) program, the Australian research vessel Rig-Seismic recorded a series of multichannel deep-seismic profiles between the New Hebrides Arc and the Australian margin. These profiles, which ran south of New Caledonia, image the southern prolongation of the overthrusted ophiolites and allow a new interpretation of the Eocene compressive tectonism along the New Caledonia–Norfolk Ridge. A model involving abduction of the whole oceanic lithosphere of the Loyalty Basin is consistent with the age and origin of the ophiolite. Variations in tectonic style along strike in the north–south-trending part of the Norfolk Ridge have produced ophiolite exposures related to uplifted and partially overthrust upper mantle slivers within the southern part of the Loyalty Basin.

Interseismic and coseismic motions in GPS series related to the Ms 7.3 July 13, 1994, Malekula Earthquake, Central New Hebrides Subduction Zone

On July 13, 1994, an earthquake Ms=7.3 occurred at Malekula, in the New Hebrides archipelago. The GPS data collected across the New Hebrides trench between 1990 and 1996 were processed in order to separate the interseismic and coseismic motions from the drifts related to the convergence with the Australian plate. The GPS-derived coseismic displacements at the GPS site in Malekula are 49 ± 15 mm southward, 230 ± 30 mm westward and 170 ± 37 mm downward, when the CMT-derived displacements are 50 mm southward, 210 mm westward and 150 mm downward. Taking into account the interseismic strain accumulation (25 mm/yr at the source established from historical seismicity, 7.5 mm/yr at the GPS site), the strain-free convergence rate at Malekula is 49 ± 3 mm/yr. Other GPS-derived convergence rates are 95 ± 1 mm/yr at Efate and 37 ± 2 mm/yr at Santo. These rates imply a regional right-lateral motion between the Efate and the Santo-Malekula segments. In contrast, the focal mechanism of the earthquake mostly indicates a left-lateral motion. Therefore, we hypothesize that the earthquake is related to variations in the interplate coupling along the converging boundary of the Santo-Malekula segment.

New insights on the interseismic active deformation along the North Ecuadorian–South Colombian (NESC) margin

[1] The North Ecuadorian–South Colombian subduction zone was the site of the 1906 Mw 8.8 megathrust earthquake. This main shock was followed by three large events in 1942, 1958, and 1979 whose rupture zones were located within the 500 km long 1906 rupture area. Acombined onshore andoffshore temporary seismic network covering from thetrench to the Andes was deployed during 3 months in the area of large earthquakes, in order to obtain a detailed knowledge of the seismic background activity. Resulting earthquakes location and mechanisms bring new insights on interseismic active deformation distribution in the three main tectonic units of the margin, namely, the Interplate Seismogenic Zone, the fore‐arc region which is part of the North Andean Block and the downgoing oceanic Nazca plate. The interplate seismic activity presents along strike variations, suggesting that the seismicity and the associated stress buildup along the plate interface depend on the time elapsed since the last large earthquakes. According to our results, the updip and downdip limits of the seismogenic zone appear to be located at 12 and 30 km depth, respectively. Shallow to intermediate depth seismicity indicates a slab dip angle of ≈25°. North of the Carnegie Ridge, the Wadati‐Benioff plane is defined beneath the fore arc down to ≈100 km depth. Facing the ridge, the Wadati‐Benioff plane extends beneath the Andes, down to ≈140kmdepth.Thisobservationconflictswiththehypothesisofthepresenceofaflatslabat a depth of 100 km facing the ridge. In the overlying fore‐arc region, the crustal seismicity occurs down to 40 km depth and is mainly concentrated in a roughly NW‐SE 100 km wide stripe stretching from the coast, at about 1°N, to the Andes. The location of this active deformation stripe coincides with observed tectonic segmentation of the coastal domain as evidenced by the presence of an uplifting segment to the south and a subsiding segment to the north of the stripe. It also corresponds to a ≈30° change in the trend of the Andes, suggesting that the curvature of the volcanic arc might play an important role in the deformation of the fore‐arc region.

La sismicité du Sud de la Nouvelle-Calédonie: implications structurales

The seismicity of the southern part of New Caledonia stretches in a stripe pattern across the island, trending parallel to its western margin. The seismic activity is mainly concentrated in the peridotite domain and terminates, to the south, with a permanent swarm where the two largest earthquakes (mag 5.6 and 5.1) recorded in New Caledonia are located. This stripe of seismic activity is located at the boundary between two different domains characterized by different average elevations and gravity anomalies. The present-day seismicity is interpreted as the reactivation of a suture zone lying between the old Caledonian substratum and a domain of oceanic origin linked to the Loyalty Basin, accreted during the Eocene obduction.

Transpression active le long de la frontière décrochante Pacifique–Australie : les apports de la cartographie multifaisceaux autour des îles Futuna et Alofi (Pacifique sud-ouest)

Abstract The Futuna–Alofi Ridge is one segment of the active transform boundary between the Australian and the Pacific plates located in a poorly known region of the Melanesian Borderland. The bathymetry and the structure of the ridge have been revealed during a short survey with the R.V. L’Atalante in October 1999 using the EM-12 multibeam echo-sounder system. We show that the ridge is located within a domain showing a typical oceanic fabric and could represent a piece of uplifted oceanic crust such as a wall of a previous fracture zone. The ridge is connected to active transcurrent faults and suffers current compression along its western side and transpression along its eastern-southeastern side. It must be considered as a compressional relay zone along a splay of the major Pacific-Australian transform boundary.

Apports de la croissance des coraux à l'étude sismo-tectonique de Futuna (territoire de Wallis et Futuna, Pacifique Sud-Ouest)

Abstract The morphology and growth analysis of coral microatolls using X-rays indicates that after a subsidence of 3 cm from 1977 to 1993, an uplift of 5 cm occurred in 1993 in the southeast of Futuna (southwest Pacific). These vertical motions are related to the strain accumulation followed by their relaxation during the earthquake (Mw: 6.4) on 12 March 1993. The locations published by the global network are not in agreement with the aftershocks recorded locally and the co-seismic uplifts observed on the reefs. The latter indicate instead that the rupture area is located below the southwest of Futuna at an average depth of 8 km.

Surrections cosismiques et subsidence intersismique enregistrées par des coraux à Mallicolo (Vanuatu, Pacifique sud-ouest)

Evidence of successive periods of uplift and subsidence is present on the western coast of Malekula Island, Vanuatu, in the southwest Pacific. These vertical motions are recorded in coral growth. Corals grew on the outer rim of an ancient reef flat uplifted by the 1965 earthquake and afterwards eroded by sea waves. The occurrence of corals on this reef flat implies that it had been submerged since 1965. The whole unit is now uplifted following the 1994 earthquake. This timing of events suggests that the last submersion of the reef flat resulted from the accumulation of the strain, the relaxation of which caused the 1994 earthquake. Vertical displacements modelled using a dislocation model partly agree with this interpretation, suggesting that some aseismic motions might have occurred.