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Subsiding Sundaland A.-C https://doi.org/10.1130/G45629.1 Manuscript received 24 September 2018 Revised manuscript received 20 November 2018 Manuscript accepted 3 December 2018 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online 4 January 2019 Subsiding Sundaland A.-C. Sarr1,2, L. Husson1, P. Sepulchre2, A-M. Pastier1,3,4, K. Pedoja5, M. Elliot4, C. Arias-Ruiz4, T. Solihuddin6, S. Aribowo7, and Susilohadi8 1ISTerre, Université Grenoble Alpes, CNRS, 38058 Grenoble, France 2Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris Saclay, France 3Géosciences Rennes, Université Rennes-1, 35042 Rennes Cedex, France 4Laboratoire de Planétologie et Géodynamique, Université Nantes, CNRS, 44300 Nantes, France 5Laboratoire de Morphodynamique Continentale et Côtière Caen, Université Caen, CNRS, 14000 Caen, France 6Marine Research Center, Ministry of Marine Affairs and Fisheries, Jakarta, Indonesia 7Indonesian Institute of Sciences (LIPI), Research center for Geotechnology, Bandung, Indonesia 8Marine Geological Institute, Marine Geological Research and Development Centre, Bandung, Indonesia ABSTRACT 10°N Sundaland is the currently partially drowned continental land- A Modern reef Earthquakes mass that encompasses Borneo, Sumatra, Java, and the Malay Penin- Malaysi sula. It has episodically been reclaimed by the sea during successive Sunda Shelf Pacific Ocean a Quaternary glaciations, and is commonly thought to be vertically Sumatra stable. Combining geomorphological observations with numerical 0 Borneo simulations of coral reef growth and shallow seismic stratigraphy, Papua we show that the Sunda shelf is subsiding, and that the intermittent Wallacea regime of transgressions only prevailed over the past 400,000 yr. Prior Java to that time, Sundaland was permanently exposed. We relate these 10°S Sahul shelf drowning events to transient dynamic topography in the Indo-Austra- Belitung Java Trench lian subduction zone. Because the Sunda shelf is very shallow, these Indian Ocean 1,000 km new data provide important insights into Pleistocene paleogeography, 40 km Australia 10°N with implications on the interactions between the solid Earth and B Alluvial plains Mangroves climate, oceanography, and dispersal of species, including hominids. Pleistocene coral reefs Vertical land motion LARGE-SCALE DEFORMATION PATTERN IN SOUTHEAST Sunda S. ASIA 0 Cenozoic Sundaland (the area that encompasses Borneo, Sumatra, Java, the Malay Peninsula, and the Java Sea) paleogeographies are available up to the Early Pliocene (e.g., Hall, 2009) but its more recent paleoge- ography remains largely unknown. The core of Sundaland is nearly tec- 10°S Banda ar Sahul S. tonically quiescent (Fig. 1A), and flooding of its shallow areas (<100 m c depth) is regarded as only contingent on eustatic sea-level changes (Voris, Bathymetry (m) 2001); the Sunda shelf being fully exposed during Quaternary glacial -120 0 1,000 km periods, and partly drowned during interglacials, as it is today. However, 100°E 110°E120°E 130°E 140°E the shelf’s location above the quickly evolving Indo-Australian subduc- Figure 1. Quaternary coastal morphotectonics in Southeast Asia. A: tion zone, where mantle convection efficiently modifies the physiography Earthquakes (M >4 ; U.S. Geological Survey catalog, 1960–2010; https:// (Zahirovic et al., 2016), challenges the default assumption of stability earthquake.usgs.gov/earthquakes/search/) outline the low-seismic- for the shelf. That assumption, which has implications for the Neogene activity area within Sundaland. Inset: Location of dated mid-Holocene reef on Belitung island (red circles). Modern coral reefs are ubiquitous biogeography of Southeast Asia (e.g., Richardson et al., 2013), has not (A) whereas Pleistocene reefs (B) are only found in Wallacea. B: Alluvial yet been properly tested. plains and mangroves prevail in the Sunda and Sahul shelves, and Wallacea (Eastern Indonesia, mostly) actively deforms in response Pleistocene reefs in Wallacea. Vertical land motion (Table DR1 [see to the collision with Australia (e.g., Pownall et al., 2016). Flights of footnote 1]) reveal uplift in Wallacea and subsidence in Sunda and Sahul (white arrows; current study in red). Sundaland encompasses uplifted marine terraces are widespread and attest to an overall Pleisto- the Sunda shelf, Malay Peninsula, Borneo, Java, and Sumatra. Sahul cene uplift of Wallacea (e.g., Pedoja et al., 2018; Table DR1 in the GSA is the shallow north Australian platform, while the deeper Wallacea Data Repository1). Conversely, wide alluvial plains stretch over most lies between Sahul and Sundaland. 1 GSA Data Repository item 2019048, Tables DR1 and DR2 and Figures DR1–DR4, including raw seismic profile and information on the model used for this study, is available online at http://www.geosociety.org/datarepository/2019/, or on request from [email protected]. CITATION: Sarr, A.-C., et al., 2019, Subsiding Sundaland: Geology, v. 47, p. 119–122, https://doi.org/10.1130/G45629.1. Geological Society of America | GEOLOGY | Volume 47 | Number 2 | www.gsapubs.org 119 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/2/119/4617122/119.pdf by California Geological Survey user on 24 June 2019 coastal regions of Sahul and Sunda, and extend hundreds of kilometers A (1) 6.2 - 6.7 kyr B.P Regressive sequence (Methods) inland (Fig. 1B). Onshore marine sediments, and reefs older than Holo- (2) 6.5 - 7.3 kyr B.P x Basement Sand Reef cene age, are never exposed even though reefs bloomed in the region Holocene microatolls Modern microatolls during the late Cenozoic (Wilson and Rosen, 2008) and currently thrive Modern corals 1 Modern SL m in Southeast Asia, including Sunda and Sahul (Fig. 1A). Such contrast- 25 m ing morphologies between Wallacea and surrounding shelves are the x ? x x consequence of different uplift and subsidence histories (Evans et al., x x x x ? x x x x x x x xxx x x x 1936). Late Quaternary subsidence rates for the Sunda shelf are only x x x x x –1 reported on the northern margin (0.13–0.27 mm yr ,Wong et al., 2003; 3,000 m ~1,200 m 0.25 mm yr–1, Hanebuth et al., 2011) or in the Singapore strait (0.06–0.10 mm yr–1, Bird et al., 2006) but are absent from the core of the shelf (Fig. B Modern reef width (km) 1B). Sahul subsides at 0.12–0.45 mm yr–1 (Table DR1) and, based on 0.0 ) 024 6 810 -1 0T its morphological similarity with Sunda (Fig. 1B), we hypothesize that 0T (km)(k m) 10 kmkm rather than being vertically stable, the Sunda shelf may be experiencing mm.yr 1T similar long-wavelength subsidence. While the shelf is currently drowned ( 0.2 1T 1 km at 30 m below sea level (bsl), the absence of marine sediments onshore 1T and the thin layer of alluvia that overlays laterites and substratum (Ben- 2T Avraham and Emery, 1973) attest to continental Cenozoic erosion and 0.4 2T2T 1 km recent inundation (Hall and Morley, 2004). 3T 0 0 0 Subsidence rate Here we use morphological indicators (coastal landforms such as 0 0 0 3T 4 1 km 2 modern and fossil reefs, mangroves, and alluvial plains; Fig. 1B) and 0.6 6000 shallow stratigraphy to capture the pattern of Pleistocene vertical land 0.0 1.0 2.0 Foundation Slope (%) motion in Southeast Asia. We focus on the Tin Belt islands, in the inner 14 Sunda shelf, and combine numerical modeling of reef growth with C C 100 - 0.3 mm.yr - 0.3 mm. - 0.2 mm.y Emersion dating of mid-Holocene reefs to constrain subsidence rates in this region -1 Submersion - 0. r 2 mm.yr yr -1 -1 where almost no data exist, ultimately demonstrating the uniformity of 50 -1 subsidence in the Sunda shelf. 0 SUNDA SHELF SUBSIDENCE 0 mm/yr Karimata strait In order to quantify subsidence rates, we use sequences of coral reefs −50 (Belitung) that fingerprint sea-level oscillations, and ubiquitously stack in flights of sea level (m) Northern marine terraces in the intertropical zone (Pedoja et al., 2014). Those reef −100 0 mm.yr -1-1 shelf sequences are indirect sensors of a variety of parameters, including uplift 0 mm.yr 11 97 5 mis 11 mis 5e or subsidence rates that numerical models can invert for (Toomey et al., −150 2013). We develop an original probabilistic approach, based on a numeri- 800 700 600 500 400 300 200 100 0 cal model that reproduces the inferred architecture of coral reef sequences Time (ka) calibrated by field measurements, to probe Quaternary subsidence rates D W E for the Sunda shelf (Husson et al., 2018; see the Data Repository). Model Incisions Sea floor predictions depend on parameterized reef growth, wave erosion, sedimen- T) Quaternary deposits tation, and relative sea level that follow glacial cycles and vertical land TW motion. This method brackets the parameter space —including subsidence 100 rates—that accurately reproduces observations. Basal uncomformity Our case study is Belitung Island (Fig. 1A, inset), which stands in the depth (ms shallowest part of the Sunda shelf. This area has negligible seismic activity Early Cenozoic Erosion and is distant from the orogenic or sedimentary loads that develop on the 200 250 m edge of the Sunda shelf (e.g., Pubellier and Morley, 2014). Narrow (up to ~1500 m), flourishing fringing reefs surround Belitung and are immune Figure 2. Reef morphologies and subsidence rates in the Sunda shelf, Indonesia. A: Belitung island has a 1200-m-wide modern reef that from terrigenous fluxes that could hamper their development. They are progrades seaward above the granitic basement, as inferred from developing seaward of prograding alluvial plains and mangroves (Fig. 2A; radiocarbon-dated mid-Holocene micro-atolls (1) from the intertidal Fig. DR2), over shallow-dipping basement (<2.5% slope).
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