Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/78/10/1247/3427797/i0016-7606-78-10-1247.pdf by guest on 30 September 2021 R. L. FISHER Scripps Institution of Oceanography, University of California at San Diego, La Jolla^ California ' ' l^ > Lamont Geological Observatory, Palisades, New ' Mascarene Plateau, Western Indian Ocean Abstract: The submarine Mascarene Plateau, lying ingly developed southeast of the intersection of east and northeast of Madagascar in the west central Rodriguez Ridge with the Mascarene Plateau. Here Indian Ocean, extends as a faulted composite arc for linear, slightly sedimented deeps and narrow ridges 2300 km, from the Precambrian granitic "micro- lie subparallel to Mauritius Trench and to the south- continent" of Seychelles Bank southward through west branch of the Mid-Indian Ocean Ridge. the coral-reef-capped volcanic structures (?) of Except in the northwest sector where northwest Saya de Malha, Nazareth, and Cargados Carajos and north trends lie within the Agalega-Seychelles— banks to the faulted Tertiary-Quaternary oceanic Amirante-Owen Fracture Zone complex, the domi- volcanic island Mauritius. Locally the aseismic pla- nant topographic lineations are northeast to east- teau is buttressed by spurs, is steep-sided and angu- northeast. These are displayed by large features such lar in plan, or is extended, disrupted, or offset by as fractures offsetting the mid-oceanic ridge (for cross-faulting. example, Vema Trench), the Mauritius Trench and Other major structural elements in a sector cen- the linear topography southeast of Mauritius, the tered on the Mascarene Plateau are: (1) the seismi- apparent offsets of the southern half of the Masca- cally active Mid-Indian Ocean Ridge and rift zone rene Plateau, spurs on the west flank of the mid- (including that portion known as the Carlsberg oceanic ridge, and the smaller ridges protruding; Ridge) between the equator and its bifurcation near from the sedimented rise and abyssal plain betweeni 23°S.; (2) the angular and faulted Amirante Trench Tromelin and the Mascarene Plateau. Offsets of the- lying between Seychelles Bank and the Madagascar- Mascarene Plateau along northeast-trending faults- Farquhar complex; (3) the east-west late Tertiary- occurred in Tertiary time as adjustments accompa- Pleistocene Rodriguez Ridge and its possible exten- nying formation of the Mascarene Basin. Activity sion eastward as a fracture zone offsetting the Mid- on several of these faults has continued where they Indian Ocean Ridge; (4) the Mauritius Trench, a intersect the crestal portions of the mid-oceanic 1400-km border deep-trending southwest from east ridge system, but growth of the exceedingly rough, of Mauritius to 28°S., 51°E. Irregular topography, ridge in this region is primarily by addition of igne- attributed to volcanism and block faulting, is strik- ous material along its axis. CONTENTS Introduction 1248 4. Redrawn sounding traces along the western Acknowledgments 1248 slope of Mauritius .......... 1256- Preparation of the bathymetric charts 1249 5. Seismic reflection profiles ......... 1257' The Mascarene Plateau and the surrounding sea 6. Cable failures between Reunion and Rodriguez floor 1250 since 1901 ............. 1258- Mascarene Islands 1252 7. Lineations and inferred structural trends. 1261 Regional geologic setting 1252 Submarine topography of Mauritius 1254 Plate Facing. Submarine topography of Reunion 1255 1. Submarine topography of the Mascarene Pla- Rodriguez Ridge 1255 teau, the Mascarene Basin, and part of the The Mauritius Trench 1259 Mid-Indian Ocean Ridge ....... 1247 Sediment distribution 1260 2. Submarine topography and echo-sounder tapes Discussion of tectonic elements 1260 near Mauritius ............ 1249' Tectonics of the west central Indian Ocean . 1264 3. Three segments of an echo-sounder record References cited 1265 made between Mauritius and Reunion . 1256. 4. Echo-sounder slope tracings, index map, Figure bathymetric profiles, and sediment core 1. Index map: a portion of the Heezen-Tharp log 1260' Physiographic Diagram of the Indian Following. Ocean 1249 5. Bottom photographs from 800-mete depth } 2. Province chart (partly redrawn from Heezen on the west flank of Mauritius ..... andTharp, 1965) 1253 6. Bottom photographs from two stations on the | 3. Index map for Figure 4 and Plate 3 .... 1255 deep-sea floor south of Mauritius . j Geological Society of America Bulletin, v. 78, p. 1247-1266, 7 figs., 6 pis., October 1967 1247 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/78/10/1247/3427797/i0016-7606-78-10-1247.pdf by guest on 30 September 2021 1248 FISHER AND OTHERS—MASCARENE PLATEAU, WESTERN INDIAN OCEAN graphic entities. Rodriguez lies on the essen- INTRODUCTION tially east-west Rodriguez Ridge, which was The Mascarene Plateau forms the foundation discovered in 1901 by the cable ship Anglia for a series of islands and shallow banks that ex- during the laying of the Mauritius-Rodriguez tend in a gentle northwest-to-south crescent cable. This ridge is a major east-west element across the western tropical Indian Ocean. From which intersects and perhaps crosses the dom- north to south, the islands and banks include inant meridional trend of the Mascarene Pla- the Seychelles Archipelago, the Saya de Malha, teau just north of Mauritius. Mauritius lies on Nazareth and Cargados Carajos banks, and the the southern extension of the Mascarene Pla- island of Mauritius. The plateau is over 2300 teau, and Reunion is a separate volcanic pile km long. It ranges from 100 to nearly 400 km 170 km to the southwest. in width at the base and is a major structural The Seychelles Islands lie near the northern element of the Indian Ocean. termination of the Mascavene Plateau. The Hydrographic surveys of the shallow banks Seychelles are the only islands rising from the were made from the mid-nineteenth to the deep-sea floor that are composed of granitic early twentieth centuries, but our knowledge rocks, and recent seismic refraction studies in- of the deeper flanks of the Mascarene Plateau dicate that such granitic material lies beneath is almost entirely a result of the International most of the wide shelf surrounding and linking Indian Ocean Expedition, 1960-1965. Most of the small islands. Radiometric analyses have the recent trans-Indian Ocean expeditions have shown the granites to be Precambrian in age. called at Mauritius, and many have visited the The northern portion of the Mascarene Pla- Seychelles. Co-ordination of programs of the teau appears "continental," suggesting that the Expedition has provided a unique concentra- entire shoal structure might be a "microcon- tion of relatively precise topographic data. tinent." However, the Mascarene Islands, which Slight deviations in the routes of participating lie near and on the southern termination of the oceanographic vessels were made to cover many plateau, are composed of "oceanic-type" ba- critical areas. Earlier, precise but localized in- salts, while seismic refraction observations on vestigations were made by cable survey and re- the intervening Saya de Malha Bank indicate a pair ships. Submarine cables operated between crustal composition intermediate between that Reunion, Mauritius, Rodriguez, and the Sey- of continent and ocean. chelles for the past seventy years have frequent- Thus the shallow Mascarene Plateau, a topo- ly failed due to chafing effects of bottom cur- graphic entity that is complex from the view- rents on the insular slopes and ridges. Ripple points of petrology, age, and crustal structure, marks, scour, and rock outcrops seen on sea- constitutes a major structural element of the floor photographs taken during the Expedition Indian Ocean and must be considered in a dis- constitute further evidence of these bottom cussion of continental drift or the genesis of the currents and of movement of detritus that has Indian Ocean Basin. The present paper, based affected the cables. primarily on recently available topographic in- Mauritius, Rodriguez, and Reunion, the formation supplemented by magnetic, seismic, three Mascarene islands, are wholly volcanic and heat-flow data from the International In- edifices. Documentation for such identification, dian Ocean Expedition, is an attempt to de- and for other statements in this introduction, lineate and interpret structural trends in the is presented in later sections. Rodriguez is a west-central Indian Ocean (Fig. 1). ridge partially bordered by an emerged coral- line platform and surrounded by a 7-to-20-km- ACKNOWLEDGMENTS wide shelf and fringing reef. Recent potassium- Names of vessels which provided the pri- argon dating indicates the rocks exposed on mary sounding data employed in the prepara- Rodriguez are less than 2 m.y. old. Mauritius, tion of the bathymetric charts are listed on an eroded complex volcanic pile surrounded by those figures. In the course of the study, varied a narrow shelf and barrier reefs, appears geo- information was provided by J. Baissac, M. morphically younger but consists largely of vol- Talwani, L. B. Slichter, A. R. Miller, C. canic structures of Tertiary age. Reunion bears Bowin, E. T. Bunce, and I. McDougall. Cable active volcanoes. Though showing a superficial repair information was made available by D. east-west arrangement athwart the Mascarene Scott, Engineer-in-Chief, Cable and Wireless Plateau, the three islands are independent topo- Ltd. Discussions with D. Matthews, T. W. C. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/78/10/1247/3427797/i0016-7606-78-10-1247.pdf by guest on 30 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/78/10/1247/3427797/i0016-7606-78-10-1247.pdf by guest on 30 September 2021 PREPARATION OF THE BATHYMETRIC CHARTS 1249 0" 20° — 30" 70° Figure 1. Index map: a portion of the Heezen-Tharp Physiographic Diagram of the Indian Ocean (Geo- logical Society of America, Inc., 1965). Copyright: 1964, Bruce C. Heezen and Marie Tharp. Note: This figure has not been redrawn to incorporate additional data acquired since mid-1964 that have been used in the preparation of the contoured chart, Plate 1.