Jurnal Geologi Indonesia, Vol. 5 No. 3 September 2010: 197-207 Indonesian Landforms and Plate Tectonics H. TH. VERSTAPPEN The International Institute for Geo-Information Science and Earth Observation (ITC), Enschede, the Netherlands, Mozartlaan 188, 7522HS ABSTRACT The horizontal configuration and vertical dimension of the landforms occurring in the tectonically unstable parts of Indonesia were resulted in the first place from plate tectonics. Most of them date from the Quaternary and endogenous forces are ongoing. Three major plates – the northward moving Indo- Australian Plate, the south-eastward moving SE-Asian Plate and the westward moving Pacific Plate - meet at a plate triple-junction situated in the south of New Guinea’s Bird’s Head. The narrow North-Moluccan plate is interposed between the Asia and Pacific. It tapers out northward in the Philippine Mobile Belt and is gradually disappearing. The greatest relief amplitudes occur near the plate boundaries: deep ocean trenches are associated with subduction zones and mountain ranges with collision belts. The landforms of the more stable areas of the plates date back to a more remote past and, where emerged, have a more subdued relief that is in the first place related to the resistance of the rocks to humid tropical weathering Rising mountain ranges and emerging island arcs are subjected to rapid humid-tropical river erosions and mass movements. The erosion products accumulate in adjacent sedimentary basins where their increasing weight causes sub- sidence by gravity and isostatic compensations. Living and raised coral reefs, volcanoes, and fault scarps are important geomorphic indicators of active plate tectonics. Compartmental faults may strongly affect island arcs stretching perpendicular to the plate movement. This is the case on Java. Transcurrent faults and related pull-apart basins are a leading factor where plates meet at an angle, such as on Sumatra. The most complicated situation exists near the triple-junction and in the Moluccas. Modern research methods, such as GPS measurements of plate movements and absolute dating of volcanic outbursts and raised coral reefs are important tools. The mega-landforms resulting from the collision of India with the Asian continent, around 50.0 my. ago, and the final collision of Australia with the Pacific, about 5.0 my. ago, also had an important impact on geomorphologic processes and the natural environment of SE-Asia through changes of the monsoonal wind system in the region and of the oceanic thermo-haline circulation in eastern Indonesia between the Pacific and the Indian ocean. In addition the landforms of the region were, of course, affected by the Quaternary global climatic fluctuations and sea level changes. Keywords: mega landforms, morphostructures, plate tectonics, Indonesia SARI Konfigurasi horisontal dan dimensi vertikal bentang alam terjadi di wilayah-wilayah Indonesia, yang secara tektonik tidak stabil terutama sebagai akibat dari pergerakan lempeng tektonik. Sebagian besar bentang alam tersebut berumur Kuarter dan dipengaruhi oleh gaya-gaya endogen. Tiga lempeng utama yaitu Lempeng Indo-Australia yang bergerak ke utara, Lempeng Asia Tenggara yang bergerak ke tenggara, dan Lempeng Pasifik yang bergerak ke arah barat, bertemu pada satu simpang tiga (triple junction) yang terletak di selatan Kepala Burung, Papua. Lempeng Maluku Utara yang sempit menyisip antara Asia dan Pasifik, Lempeng ini miring ke utara pada sabuk Filipina yang mobil dan berangsur menghilang. Amplitudo relief yang terbesar terjadi dekat batas-batas lempeng: parit laut dalam yang berasosiasi dengan zona subduksi dan jajaran pegunungan dengan sabuk tumbukan. Bentang alam daerah-daerah yang lebih stabil terdapat pada lempeng yang berumur jauh lebih tua dan tempat pemunculannya mempunyai relief yang lebih mudah berubah. Hal ini berkaitan dengan resistensi batuan terhadap pelapukan tropis lembab. Jajaran pegunungan dan busur kepulauan yang Naskah diterima 30 Juli 2010, revisi kesatu: 04 Agustus 2010, revisi kedua: 31 Agustus 2010, revisi terakhir: 02 September 2010 197 198 Jurnal Geologi Indonesia, Vol. 5 No. 3 September 2010: 197-207 terbentuk, mengalami erosi yang cepat oleh sungai tropis lembab dan gerakan tanah. Produk erosi menumpuk di cekungan sedimen yang terdekat. Peningkatan penyebaran dan ketebalan sedimen tersebut menyebabkan terjadinya terban oleh gravitasi dan kompensasi isostatik. Terumbu karang yang hidup dan tumbuh, gunung berapi dan gawir sesar merupakan indikator geomorfik penting lempeng tektonik aktif. Bagian-bagian sesar mungkin sangat mempengaruhi busur kepulauan yang meregang tegak lurus terhadap pergerakan lempeng. Kasus ini terjadi di Pulau Jawa. Sesar mendatar dan cekungan-cekungan tarikan adalah faktor penciri adanya pertemuan lempeng yang menyudut satu sama lain, seperti halnya di Sumatra. Situasi paling rumit ditemukan di dekat persimpangan-tiga (triple junction) dan di Maluku. Metode penelitian modern, seperti pengukuran pergerakan lempeng dengan GPS dan pentarikhan absolut produk gunung api dan tumbuhnya terumbu karang merupakan cara yang penting. Mega-bentang alam akibat benturan India dengan Benua Asia, sekitar 50 jtl., dan akhir tumbukan Australia dengan Pasifik, sekitar 5 jtl., juga memiliki dampak penting pada proses geomorfologi dan lingkungan alam di Asia Tenggara melalui perubahan sistem angin musiman di wilayah dan sirkulasi termo-haline samudera di Indonesia timur antara Pasifik dan Samudra Hindia. Selain itu bentang alam wilayah ini, tentu saja, dipengaruhi oleh fluktuasi iklim global Kuarter dan perubahan permukaan air laut. Kata kunci: mega bentang alam, struktur morfo, tektonik lempeng, Indonesia INTRODUCTION patterns of swamps and other characteristics of alluvial plains, coastal configuration, etc. The landforms of Indonesia and adjacent areas The three most important plates affecting the are strongly related to Quaternary Plate tectonics. Indonesian region are the SE-Asian Plate, the Indo- This applies especially to the volcanic- and non- Australian Plate, and the Pacific Plate. They can be volcanic island arcs of the archipelago (Hamilton, subdivided into a number of smaller Plates and meet 1979; Hall and Blundell, 1996; Gupta, 2003). at a triple junction situated south of New Guinea’s Their spatial distribution reveals the location of Bird’s Head. In addition, the narrow North Moluccan subduction- and collision zones caused by the Plate is interposed in the north between the Sulawesi lateral movement of a number of tectonic Plates. Sea Plate, an oceanic associate of the SE-Asian Plate, Most of these subduction- and collision zones are and the Philippine Sea Plate, a forerunner of the active and characterized by great sub-marine and Pacific Plate. They both subduct under the North- sub-aerial relief amplitudes that have strongly Moluccan Plate (Kreemer et al., 2000). This plate increased during the Quaternary. Emerged island tapers out northward in the Philippine Mobile Belt arcs and rising mountain ranges are subjected to that extends up to Taiwan. (Figure 1) Active subduc- intense humid-tropical weathering processes, river tion zones form deep trenches with important nega- erosion and mass movements. The erosion prod- tive gravity anomalies. The three main ones are the ucts accumulate in adjacent sedimentary basins trough situated south of the Sunda Arc and curving where their increasing weight causes subsidence up northward to the east of the Banda Arc, and the by gravity and isostatic compensation. The relief troughs occurring respectively to the east and west of amplitude of the Plates is much less and the land- the North Moluccan Plate / Philippine Mobile Belt. forms of their emerged parts around the Sunda- and Active volcanism is associated with belts of Sahul/ Arafura shelves indicate an advanced stage ongoing Plate subduction. The longest row is that of development related to geological events of a connected with the subduction of the northward more remote past. The landforms in these parts of moving Indian Ocean Plate in Sumatra, Java, Nusa the country now reflect differences in resistance Tenggara and the SE-Moluccas. The volcanic activ- of the rocks to humid tropical weathering in the ity of this row is extinct only on the islands of Alor first place. Several geomorphic indicators exist for and Wetar, where Timor, an outpost of the Austra- detecting zones of neo-tectonics in the lowlands. lian continent, collides with it. Active volcanism They include drainage anomalies, distributional also accompanies the subduction zones situated Indonesian Landforms and Plate Tectonics (H. Th. Verstappen) 199 1000 E 110 0E 1200 E 1300 E 1400 E 1500 E nch N re u T ky Ryu 0 ch 0 20 N n 20 N Tre P h i PHILIPPINE SEA PLATE PHILI PI E MO LE BELlT i p ani TreM ch n la nail n p i i n n SOUTH CHINA P N BI e Bo T SEA PLATE r e n c h ana 0 ari 0 10 N M 10 N PACIFIC PLATE SULU SEA PLATE N- O UC S M L CA SE S AIN PL TE -A SULAWESI SEA PLATE CAROLINE PLATE Kalimantan ALTPE uS am 0 A 0 0 tra Sunda 0 S Belt u rrent n scu d Tran a Tr enc New Guinea h Triple Junction Jawa Ja 0 v Shelf 0 10 S a Tre Sahul - arafura 10 S nch INDO - AUSTRALIAN PLATE 1000 E 110 0E 1200 E 1300 E 1400 E 1500 E Legend: Plate movement direction Figure 1. The plate-tectonic configuration of Indonesia and surroundings (Katili, 1980). at both sides of the North Moluccan Plate, in the left-lateral Sorong - Koor fault zone stretching from Minahasa and northern Halmahera respectively. the north coast of New Guinea to the Moluccas, These two volcanic arcs continue in the two paral- and the left-lateral Philippine fault zone that can be lel and closely spaced rows of active volcanoes in traced from northern Luzon up to Halmahera. The the Philippine Mobile Belt (Javelosa, 1994). Vol- Semangko zone is the scene of a number of pull- canism is extinct in western Sulawesi because the apart basins with related volcanic phenomena. The northward moving Australian Plate blocked Pacific Sorong - Koor fault strongly affects the Mamberamo subduction already in the Tertiary.