Quantifying subsidence of the Sunda shelf (SE Asia) from coral reefmorphology
Résumé
The vertical motions of the lithosphere have deeply shaped the geography of SE Asia over the last 5 Myrs. Thewide continental Sunda and Sahul platforms have been periodically inundated in the West and Southeast, respec-tively, whereas myriad of islands have emerged in the more central region of "Wallacea". The long wavelengthpattern of vertical motion is mirrored by the coastal geomorphology, which displays a striking bimodal repartitionthroughout the area. Sequences of uplifted terraces, notches and cliffs are prominent in Wallacea - and attest for itsgeneral uplift. Conversely, emerged paleo-reefs are absent above modern reefs and wide alluvial plains dominatecoastal areas surrounding Sunda and Sahul shelves and suggests that subsidence prevails.In order to quantify the subsidence of the Sunda shelf, we used a probabilistic approach based on a numer-ical model that reproduces the development of coral reefs sequences trough time, in response to relative sea levelvariations. The model accounts for growth reef rate, sea level variations, sub-marine erosion and subsequentsedimentation. This method enables to evaluate the field of parameters (slope, vertical rate, reef growth rate) thatsatisfyingly reproduce the observed morphology. Comparison of the predicted and observed morphologies of theisland of Belitung (Sunda shelf) yields short-term subsidence rates ranging from -0.20 to -0.45 mm/yr. Because theshelf is really shallow (typically - 30 m, and up to -120 m), such subsidence rates set the timing of the drowningof Sundaland. It implies that the platform would have been permanently emerged recently, even during period ofhigh sea level stand.The slower, long-term Neogene subsidence of SE Asia, responds to the subduction dynamics of the Sumatra-Javaslab. We tested the possibility that the collision of the Australian continent with the Banda Arc modified thislong-term behavior. Because the transition from oceanic subduction to continental collision modifies the subduc-tion dynamics, subsidence rates should have increased in the Sunda shelf and decreased in Wallacea. Subsidenceover most part of Wallacea came indeed to an end in the last 5 Myrs, when the australian margin collided withthe Banda arc, and uplift has taken over in the most recent period of time. We use three-dimensional subductionnumerical models to show how the Australian collision has modified dynamic topography in the overriding plate,and suggest that the pattern of dynamic topography variations following Australia-Banda collision could havecontributed to differential vertical deformation in SE Asia.