Uplift Movements in New Caledonia-Loyalty Islands Area and Their Plate Tectonics Interpretation

Tectonophysics, 24 (1974) 133-150 0Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands WLIFT MOVEMENTS IN NEW CALEDONIA-LOYALTY ISLANDS AREA AND THEIR PLATE TECTONICS INTERPRETATION JACQUES DUBOIS,-JEAN LAUNAY and JACQUES RECY ! Office de la Recherche Scientifique et Technique Outre-Mer, Noume/a, New Caledonia (Submitted for publication September 21, 1973; accepted April 29, 1974) ABSTRACT Dubois, J., Launay, J. and Recy, J., 1974. Uplift movements in New Caledonia-Loyalty Islands area and their plate tectonics interpretation. Tectonophysics, 24: 133-150. The uplift movements of the New Caledonia and the Loyalty Islands, which are located on the Indian lithospheric plate that descends beneath the New Hebrides arc, have been studied in the geological context. This paper concentrates on the older (since Oligocene) uplift of the ultramafic peneplain in New Caledonia and the younger (since about 2 m.y.) regional uplift of the Loyalty Islands and southern New Caledonia. We computed the amount of uplift and deformation caused by three possible processes: erosional unloading, eustatism, and flexure of the lithosphere as it underthrusts beneath the New Hebrides island arc. The calculations indicate that the older uplift cannot be completely explained by erosional unloading of New Caledonia and may be partially caused by dynamic factors not yet understood. Eustatic uplift of New Caledonia in compensation for the rise in sea- level is also of insufficient amplitude to explain the uplift observed since about the last 2 m.y. Studies of the flexure of the lithosphere caused by subduction beneath the New Heb- rides arc provide an explanation for the observed recent uplift of the emergent atolls of the Loyalty Islands. INTRODUCTION New Caledonia is an island * whose long axis strikes approximately at 310" and it is the emergent part of a submarine structure called the Norfolk ridge. To the east of it lie the Loyalty Islands which are the uplifted parts of a submarine volcanic ridge which is parallel to the strike of New Caledonia (see Fig. 1).Further east the New Hebrides trench is oriented along 340". These three structures bend and converge towards a point, located south of Maré Island where the New Hebrides trench becomes tangential to the southern pro- longation of the Loyalty Islands; beyond this point the Norfolk ridge and Loyalty chain are directed more northsouth, whereas the trench curves to- * New Caledonia is 400 km long, about 45 km wide, surface area 18,000 km2. It is surrounded by a barrier reef which extends 200 km beyond the northern limit of the island. ¡Il3 FE\@- 6.3is s. T;. %Je @qv ?+i Col hdl i a n ($2 R& pi: y;! ï>.E .I , i L. -- DU 2- 2- /' ////, 135 wards the east. Between New Caledonia and the Loyalty Islands lies the flat and regular Loyalty basin. The particular structural arrangement of the New Caledonia-New Hebrides area is distinguished by the dip of the “continental plate” underneath the -I oceanic one and by the existence of partly emerged ridges in situ on the dip- ping lithosphere, near the subduction zone; this provides data for comparison between the theoretical scheme of the bulge and the observed facts, 1 Such a deformation of the lithosphere before its dip has already been intro- duced as a likely hypothesis by Lliboutry (1969) and Hanks (1971), but had never been observed over landmarks as accurate as the uplifted atolls. GEOLOGICAL HISTORY OF NEW CALEDONIA AND LOYALTY ISLANDS Main orogenic phases The geological history of New Caledonia begins with the Permian tuffs (Avias, 1953; Routhier, 1953; Guillon, 1969). From the Permian to the Upper Eocene the formations are volcano-sedimentary and sedimentary rocks (grey- wackes, shales, sandstones) accumulated in a trench called the Melanesian trench. Since the Permo-Trias different orogenic phases have influenced these strata. The last (Le., the Alpine orogeny) begins after the emplacement of Eocene flysch deposits and reaches its highest point at the limit of the Eocene- Oligocene. At the end of the main orogenic phases, the emplacement of peridotites occurred, probably during an Oligocene obduction phase (Guillon and Routhier, 1971). This peridotite emplacement is the last major orogenic oc- currence on the island. Activity on the volcanic Loyalty Islands ceased at about 10 m.y. ago (J.H. Guillon and J. Recy, private communication, 1974). After the last main orogenic phase, New Caledonia and the Loyalty Islands came under the influence of a different kind of tectonics. These consisted of uplift movements of large amplitude. Uplift movements in New Caledonia Four phases have been put forward by Davis (1925), then they were succes- sively precised by Routhier (1953) and Trescases (1969,1973). Phase I: Peneplanation of peridotites ‘, After their emplacement, the peridotites have been affected by an intense .( erosion which began in the Early Miocene (Routhier, 1953). Routhier showed the presence of nickeliferous clays in the Miocene of Nepoui. This weathering d has resulted in a peneplanation, the remains of which are visible as tabular re-. 1 lief. The peridotites, which at present cover 30% of the New Caledonia surface, covered a much greater surface just after their emplacement; the present thick- ness is much less than the original (Routhier, 1953; Guillon and Routhier, - 1971), and therefore a great mass of material of high density has been eroded since the Early Miocene at the latest. 136 Phase II After peneplanation, New Caledonia was uplifted and the peneplaned peridotites eroded. The peneplain remnants can be observed at elevations greater than 1,000, m in the central part of the island. Their altitude is less on the ,- eastern and western borders and on the extremities, especially in the south where they occur below an altitude of 300 m. This is seen on the Kopeto massif: the upper peneplain dips gently to the west coast. The Mue0 Forma- ti tions which are dated as Miocene seem to belong to the same surface of peneplanation (Gonord and Trescases, 1970). Therefore, the upheaval movement must be later than Miocene. The existence of two other surfaces of laterisa- tion below the peneplain indicates interruptions during uplift. The general movement does not seem to be regular in time or in space. Moreover, post- Miocene tectonic activity which was active until fairly recently has cut off the peneplain in blocks of different altitude (Orlof and Gonord, 1968; Gonord and Trescases, 1970). This uplifting phase has continued until recently as is seen in the youth of the river profiles. Its amplitude is small at the present coasts compared with the central parts where it is much greater (e.g., the marine Miocene outcrops only in the middle part of the west coast with a few tens of meters average height). The axis of the bulge is not a straight line because the uplift has been guided by the anterior structural fractures which have regionally modified the effects of the movement. The tilting of the island towards the west during this phase is still an hypothesis and it is at present impossible to determine if the barrier reef of the west coast had already begun to build itself in response to this tilt. We can only note that the northern and southern limits of New Cale- donia, the western slope and the continental platform have well-developed reefs while the east coast has not. Thus, uplift movements during the second phase are asymmetrical across the island. The large quantity of eroded material, the continuity of the movement since the Miocene, the general irregular figure of the residual peneplain and the existence under the central chain of New Caledonia of a 35 km thick crust which thins quickly to 18-20 km (Dubois, 1971) under the present coasts indicate that the phenomena of isostatic equilib- .rium took an important part in the observed uplifting. Phase III The subsidence phase, responsible for the submergence of low valleys and for the indents of the coast appears to be very recent. It is during this period that the great western barrier reefs have been built. Using data on the widths I of the present river mouths, Davis (1925) concluded that the amplitude of t the vertical movement was about 200 m. A drill hole on the islet Tenia on the western barrier reef has penetrated 226 m of coral limestones before reaching k a substratum of phtanite similar to the Eocene one observed on the island i (Coudray, 1971). The growth of the barrier reef in the northwestern extremity, the presence of islands and islets extending north and south of New Caledonia 137 and the distance of the shores from the great barrier reef towards the north and south, could indicate that the longitudinal warping of phase II went on during the subsidence phase. *I Phase IV There are indications of a recent rise in relative sea-level: undercuts, uplifted beaches and reefs (Davis, 1925;Avias, 1949, 1959;Routhier, 1953). This rise has been attributed to a eustatic sea-level change and to tectonic movements for elevated beaches. Some recent works (Baltzer, 1970;Launay and Recy, 1970,1972)have reported radiocarbon and ionum-uranium dating of corals and peats of mangroves. Most of uplifted terraces between 2 and 4 m, above the zero of French Hy- drographical Service, which frequently extend along the present beaches, ap- pear to have been deposited during a high Holocene relative sea-level. On the eastern and western coasts of New Caledonia, some coral reefs were found between 3 aiid 6 m high and show a relative sea-level older than that of the Holocene one. The Isle of Pines southeast of New Caledonia is an almost completely pene- plained lateritized peridotite massif, surrounded by uplifted flat reefs down to sea-level.

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