Geological map 199 A new CGMW map: The structural map of the Indian Ocean: An attempt at ocean cartography

The Commission for the Geological Map of the World (CGMW), an criminate graphically between the Ethiopian and Yemen traps (ca. IUGS affiliated body, has just issued the first edition of the Struc- 30 Ma) and the subsequent rift volcanism. b) Subduction related vol- tural Map of the Indian Ocean* which was presented during the canism exemplified by the Cenozoic Sunda volcanic arc. c) Unspec- 32nd IGC (Florence). During the 1980's, the Commission published ified volcanism, such as the Quaternary volcanoes of SE Australia, the Geological Atlas of the World, which included the maps of the 5 until now unrelated to a clear geodynamic cause. oceans of the globe. Two decades have elapsed and the knowledge of Ophiolites, the result of obduction (in Oman) or of the catch of the sea-floor experienced very significant progress; therefore some 5 ocean lithosphere within suture zones are mapped. An indication of years ago, the Commission decided to launch a new type of ocean the age of the oceanic crust is given, when known. mapping. The Structural Map of the Indian Ocean is the first of this The tectonic features are indicated by faults (normal, strike-slip new series of maps. as e.g. the Semangko fault in Sumatra, and unspecified), and first order thrust like the Himalaya frontal thrust.

How to map an ocean? Antarctic area Antarctica necessitated a specific treatment due to the occur- Working out the map (and not the atlas) of an ocean while attempt- rence of the huge inland ice. Only scattered coastal areas have been ing to provide the most convenient synthesis of its main characteris- "conventionally" mapped (mainly Precambrian formations). Beneath tics in the field of the Earth sciences, is not as obvious an exercise as the ice-cap, the relief of the rocky substratum is suggested with a 500 one could think, if compared with the onshore cartography. The map m-spaced isobath contouring, showing that substantial parts of the geological cannot be a true one, describing the nature of the out- overloaded continent are depressed below the sea-level, particularly cropping formations as for the emerged areas; otherwise it would the Amery graben (ca.70°–75°E) and to the East. Likewise, the edge mainly represent Plio-Quaternary sediments. Neither is it possible to of the continental shelf is not defined, here, by the traditional 200 m consider a tectonic map because of the specific evolution of the isobath (see offshore areas), and located at a deeper level; hence the oceanic lithosphere and the age of the ocean basins, never older than Legend wording: "outer limit of the circum-antarctic platform". The geo- ca. 200 Ma. The map is not the simple cartography of one single true coastline and the edge of the ice-shelf (seaward extension of the physical parameter (seismicity, gravimetry, heat-flow,...). It has also ice-cap) are marked by two different kinds of lines. to include some geological information relevant to the onshore areas, unlike the majority of maps devoted to an oceanic area. It cannot be a simple stacking-up of different types of data, as it would impede an Offshore areas acceptable balance between readability and level of information. For The first pertinent element to consider in the mapping of an all these reasons, we did choose to coin this kind of cartography a ocean is the age of the oceanic crust forming the basement of the sea- "structural map", a term not heavily constrained semantically. floor. Here, we give the chronostratigraphic ages (epochs, with con- Because of legibility constraints, the Map of the Indian Ocean ventional "geological" colors) and not the "geomagnetic" ages, as is composed of 2 sheets at the same scale (1:20 M): Sheet 1 — Phys- usually displayed by the geophysicists, in order to be consistent with iographic Map; Sheet 2 —Structural Map (stricto sensu). geological events. Transform faults and fractures zones, axes of oceanic accretion with full current spreading rates in cm/yr, and occassionaly extinct spreading axes, as in the Somali basin are The "structural" sheet shown. The magnetic anomalies are the key parameter allowing, as just Onshore areas said above, the mapping of the age of the oceanic crust, which was transformed, on the Map, by interpolation (from digitized files by The Indian Ocean is the result of the break-up of the Gondwana Müller et al., 1997) into a chronostratigraphic display. But it was supercontinent from Jurassic onwards, and involves the drift apart of considered of value to show the exact location of the magnetic Africa, India, the Seychelles microcontinent (Precambrian granites), anomaly picks. In the legend, a table provides the list of each stan- Madagascar, Australia, Antarctica and Arabia. The onshore conti- dard anomaly (or chron) and its age, following the geomagnetic time nental areas are very schematically represented by: Archean and scale after Cande & Kent (1995) from chron 1o (0.78 Ma) to chron Proterozoic (or undifferentiated Precambrian) cratonic areas, includ- 33o (79.075 Ma), and after Gradstein et al. (1994) for chron 34 y ing the isolated Seychelles microcontinent; Phanerozoic basins or (83.0 Ma) and older. These picks are represented by a tiny open cir- platforms; Phanerozoic orogenic belts (Paleozoic, e.g. South African cle filled with a specific color for each chron. tip; Mesozoic, e.g. Malaysian peninsula; Cenozoic, e.g. Himalayas), Reliefs corresponding to anomalous volcanic basement considering only the last main orogenic phase when multiple oroge- (seamounts, "aseismic" ridges, oceanic plateaus, or features of nies occurred. uncertain or disputed origin e.g. the Kerguelen Plateau: hotspot The Meso-Cenozoic volcanism which accompanies different induced or continental crust?) are represented by a pale yellowish steps of the evolution of the Indian Ocean, is subdivided into hori- green color. As to the Agulhas Plateau, its nature is still debated. The zontal chronologic units, and into 3 main geodynamic vertical cate- Ninety East Ridge is a remarkable rectilinear morphotectonic feature gories, i.e. a) Rift and/or well-defined hospot volcanism: the Indian which is probably linked to the northward drift of India. Among the traps of Deccan (ca.65 Ma), Rajmahal and Sylhet (ca.120 Ma); in the international deep-sea drilling sites, are only plotted (black stars) case of the East-African/Arabian volcanism, it is still premature, due those having reached the true oceanic crust, or the volcanic basement to the scale of the map and to the state of the regional surveys, to dis- of the seamounts or oceanic plateaus.

*Authors of the Map: J.Ségoufin, M. Munschy, Ph. Bouysse, V. Mendel, with the collaboration of G Grikurov & G. Leitchenkov (Russia), C. Subrahmanyam & S. Chand (India), M. Pubellier (France), and Ph. Patriat (France).

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Between India and Australia, appears a region of deformation mined the decision to produce a companion-sheet to the "structural" (compressional stress) of the oceanic crust joining part of the Central one. Indian (accretionary) Ridge to the Sumatra subduction trench. This On the physiographic background, 4 kinds of data have been area, overprinted with horizontal hatching, is also characterized by added: 1) Selected isobaths drawn with a very fine line with -200, diffuse seismicity; it might correspond to a zone of future definite -1000 m contours, and then every 1000 m. 2) More than 300 active rupture within the Indo-Australian plate, initiating the creation of or Holocene (< 10,000 yrs) volcanoes, marked by a small red trian- two separate plates. gle, all on land and extracted from the Smithsonian online files; they The thickness of the sediments overlying the oceanic basement are particularly concentrated along the Sunda arc, and in Ethiopia. 3) is indicated by the overprint of a greyish shading whose intensity Some 40 astroblemes (or meteoritic impact craters), the majority of increases from 0 to 15 km. The maximum thickness is located in which being located in Australia, selected from the Planetary & front of the Ganges-Brahmaputra delta. Space Science Center of the New Brunswick University; they are The active subduction directed toward the NNE, beneath the marked by a small green inverted triangle. 4) Toponymy, with a Sunda island arc, is marked by a red line with solid triangles. Sub- selection of towns, rivers and islands, and of the main submarine duction vectors show the orientation of the convergence (rate in morphostructures. cm/yr) with respect to the curvature of the arc. A southward incipi- A "Geodynamic sketch", placed as inset on the NE corner of the ent subduction zone in front of the northern coasts of the Lesser sheet, presents an overview of the geodynamic pattern of the whole Sunda archipelago is indicated by a red line with open triangles. To area and, particularly, the extension of each lithospheric plate within the North of the northern tip of Sumatra, the small Andaman mar- the frame of the Map. It shows also the peculiar trail, from about 65 ginal basin is generated by the geometry of the subduction and the Ma (Deccan traps) to Present (La Fournaise shield volcano), of the transform movement linking the Sunda island arc to the Burmese imprint of the deep-rooted Réunion hotspot on the northward mov- strike-slip faulting. This back-arc basin is mapped as an ordinary ing lithosphere, this track being offset by the initiation of the Central oceanic crust of Neogene age, with small spreading axes and trans- Indian spreading ridge in Late Eocene. form faults. The printed version of the Map, of which a reduction of Sheet 2 The Afar triangle and the southern end of the Red Sea is a is presented here, should be followed by a digital interactive one. region of particular interest, linking the rest of the latter to Gulf of This CD-ROM will allow separate access to each of the different Aden. Here, the onshore volcanics correspond actually to oceanic layers of information used for the paper version, for an easy exami- nation of the data set. It must be noted, however, that such a digital crust with active accretion. Nevertheless, they are treated carto- product doesn't make obsolete the printed paper edition which pur- graphically as onshore volcanism, since the oceanic crust underwent, pose is different, i.e. provide a synthetic, comprehensive, direct and in this area, uplift and partial emersion due to a hotspot bulge. How- easy visual impact from the key "geological" (lato sensu) features ever the inferred location of the axes of current accretion is indicated that characterize an ocean. by a green dotted line. The continental margin represents the part of continent covered by the oceans and seas. It includes both the continental shelf — more Philippe Bouysse, former Secretary General, CGMW, Paris, France or less arbitrarily bounded by the 200 m isobath — and the conti- nental slope that meets, at about -3000 m, the seafloor underlain by Véronique Mendel, Geophysicist at The École & Observatoire des the oceanic crust. The shelf is shown in white and the slope in a light Sciences de la Terre (EOST), Strasbourg, France blue. Because island arcs are built up of continentalized crust, their submarine parts receive the same cartographic treatment. Marc Munschy, Senior geophysicist at EOST, Strasbourg, France

Seismicity Jacques Ségoufin, Senior geophysicist at Institut de Physique du Globe, Paris, France Seimicity is a very important geophysical parameter for the cur- rent regional geodynamics; it underlines the limits of the lithospheric plates (spreading axes, transform faults, subduction zones, collision) and also active intraplate rifting. Therefore, earthquakes epicenters References (symbolized by open lozenges) are both plotted onshore and off- shore. The interval of time considered refers to the last 4 decades Cande S.C. and Kent D.V. (1995), revised calibration of the geomagnetic (1959–2002). In order to ensure an acceptable legibility of the back- polarity timescale for the late Cretaceous and Cenozoic. J. Geophys. Res., ground data, only 4 categories of magnitude have been selected, 100, 6093-6095. whose symbol size is increasing accordingly: 5.0–5.9; 6.0–6.9; Gradstein F.M., Agterberg F., Ogg J.G., Hardenbol J., Van Veen p., Thierry 7.0–7.9; 8.0 and greater. The 4 categories of focal depths represented J. and Huang Z. (1994), A Mesozoic time scale. J. Geophys. Res., 99, on the map (0–35; 36–70; 71–300; 301–700 km) are discriminated 24051-24074. Müller R.D., Roest W.R., Royer J.-Y., Gahagan L.M. and Sclater (1997), by the colour of the symbol. The highest density of epicenters is, of Digital isochrones of the Worldís ocean floor. J. Geophys. Res., 102, course, located along the Sunda subduction zone and produces 3211-3214. locally some slight obscuring of the underlying data. It is interesting to note that, offshore and quite unusually in an intraplate context, a swarm of 11 earthquakes is concentrated very close to the McDonald To order this map, contact: & Heard islands, on the Kerguelen Plateau. CGMW – 77, rue Claude-Bernard 75005 Paris, France Fax: +33 1 43 36 95 18 The physiographic sheet [email protected] www.ccgm.org / www.cgmw.net Price for the set of 2 maps: 15 Euro + postage It would have been very difficult to add even a litter more informa- tion on the previous sheet. On the other hand, international geophys- ical data bases allow producing very "fine-grained" computer-gener- ated "physiographic" images of the surface of the Earth, with color- shaded relief based on land and sea-floor elevation models, of great informational and esthetic value. These two considerations deter-

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