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Les pentes externes du récif barrière de la Grande Terre de Nouvelle-Calédonie: morphologie, lithologie, contrôle de la tectonique et de l'eustatisme

Abstract : This thesis is a contribution to the understanding of the New Caledonia ridge's margins structuration and the tectono-eustatic processes which controlled the structuration and the development of New Caledonian barrier reef. This study is based on the one hand on the morpho-structural analysis of the bathymetry of the New Caledonian margins (SIMRAD EM 12 and EM1002 multibeam echosounder data) and, on the other hand, on a textural analysis associated with the determination of biological assemblages realised from samples dredged on the marine terraces located along the outer slopes of the New Caledonian barrier reef.

The morpho-structural analysis reveals the sigmoidal shape of the ridge, the decrease of the slope gradient southward and the presence of landslides probably related to uplift periods of the New Caledonia ridge and/or sea level oscillations. For the first time, it has been highlighted major N110°E trending structures, cross-cutting the margins, overlapping the N135/140°E original trend of the New Caledonia ridge. Other transverse structures have been recognised; they strike N0°E, N70°E and, for a minor part, N90°E. N110°E and N-S trending faults participate actively to the barrier reef segmentation. Marine terraces vertical evolution along the margins seems to be also controlled by these N110°E and N-S trending faults which are, notably, clearly mapped at the vicinity of the “Banc de Coëtlogon” located Southeast of the Main Island. The neotectonic character of both faulting directions is demonstrated by this way.
Two main extensive periods are proposed to explain the post-obduction structuration of the New Caledonian margins. The first one, which probably occurred from the Late Oligocene to the Upper Miocene, is characterised by an extension perpendicular to the New Caledonia ridge axis, accommodated first by N130/150°E and then, by neoformed N0°E and N110°E trending faults. The second extensive period, probably Pliocene to Quaternary in age, may be related to the Vanuatu subduction which initiated at the Upper Miocene. This extension, oblique to the ridge (NNW/SSE), reactivates N110°E and N-S striking normal faults which also present respectively sinistral and dextral components and creates, mainly in the South of the Main Island, N70°E trending normal faults, parallel to the present day convergent vector.
The analysis of the uppermost slopes revealed the presence of five groups of stepped marine terraces. They seem to be better preserved close to passes and on seaward reef segments. These terraces have been interpreted as morphological relics, along the outer barrier reef slopes, of the roof of reef units developed during the Late Quaternary marine highstands, at least since the MIS 15 (610 kyr). The largest and the most exposed all over the margins, located between -70 and -85 m, has been attributed to the MIS 11 (408 kyr). The vertical evolution of the depth of this terrace along the margins is used as a vertical motion indicator. By this way, we determined that the New Caledonian barrier reef is segmented and composed by tilted blocks with independent vertical motion. The calculated mean subsiding rates since 408 kyr range from 0.13 to 0.20 mm/yr. The earthquakes (M ≥ 4), recorded by the International network between 1966 and 2002, should be associated with the reactivation of the N110°E and N70°E trending faults.

This work permitted also to complete our knowledge on the New Caledonian barrier reef development which was based only on the analysis of some drillings. Outer barrier reef slopes surrounding the Main Island are covered by foraminiferal, red algal and more rarely bryozoan crusts and also by material eroded then fallen from upper reef units. A bathymetric distribution of these crusts, variable in nature depending on depth and irradiance, has been established. From the modern sea level to 90 m depth, coralline crusts predominate because brightness is sufficient for their development. Their vertical distribution, well defined, permited us to describe three main coralline assemblages respectively observed between from 0 to -6 m, 0 to -30 m and from -15 to around -90 m. Beneath this limit, we observe a progressive substitution of the coralline crusts by foraminiferal crusts. Observed down to -160 m depth and systematically associated with Melobesoids, Sporolithon sp. and/or Peyssonnelia sp., these foraminiferal crusts are thicker and more abundant downward. Consequently, the percentage of coralline or foraminiferal crust constitutes an excellent paleo-bathymetric tool as observed in this study.
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Submitted on : Monday, October 30, 2006 - 11:19:35 AM
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Benoit Flamand. Les pentes externes du récif barrière de la Grande Terre de Nouvelle-Calédonie: morphologie, lithologie, contrôle de la tectonique et de l'eustatisme. Tectonique. Université de Bretagne occidentale - Brest, 2006. Français. ⟨tel-00110478⟩

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