Subsidence Quaternaire en Asie du Sud-Est : de la dynamique du manteau à la circulation atmosphérique - Modélisation géomorphologique, géodynamique et climatique

Abstract : Dynamic topography modulates the extension of inundated areas, at places where elevation is near sea level, by deflecting the surface of the Earth. This phenomenon produces large-scale paleogeography changes, which in turn modify external spheres (atmo-, hydro- and biosphere) by subsequent alteration of atmospheric and oceanic circulations and biodiversity. This inter-disciplinary work illustrates the connection string between Earth mantle dynamics and climate through the study of Quaternary evolution of South East Asia. The insularity of the region and the presence of low bathymetry seas, as the Java sea, enable fast and efficient modifications of land-sea mask and make it an ideal case for studying the connection between geodynamics and climate. Mantle flow, excited by the numerous subduction zones, is vigorously stirred and contributes to surface deformation. In this region, climate dynamics is also tightly related to the peculiar geography of the Indonesian archipelago. Paleogeographic changes are first revealed by coastal morphologies. They show the contrasted pattern of large-scale Quaternary deformation that underlines general uplift within the central-eastern part of the region, namely Wallacea, whereas the continental shelves, to the West and Southeast, are more likely subsiding. The combination of field observations with numerical modeling of coral reef growth is used to quantify vertical deformation. Our method is based on reef morphology (terrace number, depth, modern reef length) that we observed on the Sunda shelf (Western South East Asia) and reef morphologies obtained by numerical modeling, and enable an original quantification of subsidence rates of the platform. The results imply that Sundaland region was entirely and permanently emerged before 400 000 yr and formed at this time a unique continental mass between West Indonesian islands and continental Asia. The causes of paleogeographic changes are explored using modeling of regional geodynamics. A three-dimension subduction numerical model was devised to simulate the dynamical origin of deformation. This model analysis enables us to describe the spatio-temporal evolution of the deformation above a subduction zone in case of perturbation induced by the arrival at the trench of a continental block or oceanic plateau, a simplified case that is similar to SE Asia. Our results show that during a collisional episode, slab tearing generated by the arrival of light material unable to subduct is responsible for changes in mantle convection. Those changes are responsible for dynamic subsidence that followed an uplift event related to the first stages of collision. Inferred deformation rates have an range of magnitude similar to both measured and modeled rates at regional scale. The consequences of paleogeographic changes are studied using general circulation model simulations. Results show that the presence of an emerged Sunda shelf leads to a seasonal increase in precipitation over the Maritime Continent. This increase is related to seasonal increase in large-scale convergence induced by thermal heating of exposed land surfaces, a situation that, as we show, occurred before 400 ka. Sunda shelf exposure is also responsible for changes in horizontal water transport within the Makassar strait that modify sea surface salinities and temperatures at local scale. Our analysis further shows that increased precipitation seasonality is independent on model convection and cloud parameterization
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Anta-Clarisse Sarr. Subsidence Quaternaire en Asie du Sud-Est : de la dynamique du manteau à la circulation atmosphérique - Modélisation géomorphologique, géodynamique et climatique. Sciences de la Terre. Université Grenoble Alpes, 2018. Français. ⟨NNT : 2018GREAU045⟩. ⟨tel-02067253⟩

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