Morphogenèse tropicale en contexte épirogénique modéré : exemple de la Nouvelle-Calédonie (Pacifique Sud-Ouest)

Résumé : This thesis is a contribution to the understanding of long-term (106 – 107 yr) tropical landform development under the influence of epeirogenic forcing and climate change, with a special reference to mainland New Caledonia. The approach combines geomorphic analysis, GIS-assisted mapping of lateritic landsurfaces, stratigraphic and structural analysis of fluvial sediments as well as the interpretation of geophysical, topographic, and bathymetric data. The results of this multidisciplinary approach allows proposing a morphotectonic development model for the New Caledonia ridge since the end of the Alpine orogeny in the earliest Oligocene. Two main periods are distinguished. The first period records episodic uplift of the rigde, with times of quiescence enhancing lateritic planation, alternating with incision episodes of the lateritic surfaces. Enrichment of the main saprolitic Nickel ore deposits took place during this period, due to a prolonged episode of slow base level lowering, allowing weathered surfaces to evolve towards valley side slopes during continuous lateritization provided by good drainage conditions. Ni enrichment of the downstream lateritics plains (today dissected by erosion) could be contemporaneous- to slightly younger than that of the ore-bearing valley sides. During a second period, a new erosional regime operated, driven by climate change and epeirogenic movements of tectonic or isostatic origin. The beginning of this period is marked, in the Late Oligocene to the earliest Miocene, by the settlement of two aggrading fluvial sedimentary systems following two regional phases of river incision. A system of three stepped, ferricrete-capped, pediments (glacis) of regional extent developed after the abandonment of the first deltaic sequence. They record alternation of phases of drier tropical climate (allowing pedimentation) and returns to humid conditions (allowing ferralitization of the previously settled pediment). In the Late Oligocene / earliest Miocene, the New Caledonia ridge underwent a phase of extensional tectonics characterized by the activation of ridge-parallel faults with a stretching direction normal to the ridge. A regional kinematic change initiated a transtensional regime associated with ridge-oblique extension, compatible with the initiation of subduction of the Australian plate beneath the Pacific plate in the Early/mid Miocene along the Vanuatu trench. Neogene extensional tectonics led to the dissection and collapse of the piedmont and continental platform of the ridge into the adjoining ocean basins as well as the tilting of ridge towards the SW, resulting in the formation of a large coastal escarpment along the NE edge of the island. Tilting induced a phase of regressive erosion on the northeastward flowing drainage of the island whereas a large pediplain developed on the southwestward flowing drainage. The imprints of Quaternary base-level changes appear to have been confined to the coastal regions of the island and its lagoon.
Type de document :
Thèse
Géomorphologie. Université de la Nouvelle-Calédonie, 2005. Français
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Violaine Chevillotte. Morphogenèse tropicale en contexte épirogénique modéré : exemple de la Nouvelle-Calédonie (Pacifique Sud-Ouest). Géomorphologie. Université de la Nouvelle-Calédonie, 2005. Français. 〈tel-01171599〉

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