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

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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.

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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.

Hilde, A. S. Laughton, M. N. Hill, A. E. J. Engel, G. G. Shor, J. B. Hersey, J. R. Conolly, PREPARATION OF THE H. W. Menard, C. J. Shipek, and M. Langseth BATHYMETRIC CHARTS are gratefully acknowledged. Illustrations were Dots representing all soundings used in con- drafted by D. G. Crouch. This work was sup- structing Plate 1 and Plate 2, fig. 2, are shown ported by the Office of Naval Research of the on those plots. Most of the sounding lines were U. S. Navy, National Science Foundation run after 1961 by ships participating in the In- grants G-22255, GP-5538, GA-580, and the ternational Indian Ocean Expedition, 1960- Bell Telephone Laboratories. 1965. Nearly all of these ships were equipped

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with echo-sounders having a recording pre- all soundings, exaggerated plots of profiles, and cision of better than two meters in 5000. Sound- the echo-sounder tapes themselves were con- ings were made at few-second intervals; how- sidered in determining the presence and trend ever, except near shore, the records were read of features such as the "rift valley" commonly either at fixed intervals (commonly about 1.5 found on the mid-oceanic ridge. For areas more km of track) or at significant inflection points. than 4000 m in depth, a 200-m interval was Except for the detailed surveys (as around employed in an attempt to portray the texture Mauritius), soundings were plotted on U. S. and the structural pattern of the deep-sea floor. Navy Oceanographic Office oceanic plotting The availability of precise soundings in the sheets (Mercator, 4 inches equal to 1° of longi- deeper areas permitted an attempt at correla- tude) or on 1:1,000,000 Admiralty Mercator tion of local, narrow, flat-bottomed deeps be- plotting sheets. No slope corrections were ap- tween adjacent sounding tracks; this method plied. Little of the exploration occurred within was used most extensively in the southeast and sight or radar range of shore; practically all the northeast portions of the area shown on Plate 1. ships were dependent on celestial navigation, That area west of the Mascarene Plateau- so their fixes have an accuracy of about +4 km Mauritius Trench (Pi. 1) probably represents under good weather conditions. Navigational the submarine topography fairly accurately. tracks were corrected aboard ship for drift and East of these features, especially on the mid- speed variations where these were known. Small oceanic ridge east of 63° E., the contours shown gaps or large offsets in the sounding tracks oc- are very much less definitive. cur at sampling stations, during trawling op- erations, or occasionally when sounding equip- THE MASCARENE PLATEAU AND ment was inoperative. In the final compilation THE SURROUNDING SEA FLOOR for Plate 1, made on U.S.N.O.O. oceanic-scale In the extended usage in this paper, the plotting sheets, older or less well-controlled Mascarene Plateau includes the Seychelles- tracks were adjusted in position until their Mauritius Ridge (Fairbridge, 1948) and the soundings made good crossings with the ac- Mascarene Ridge (Sewell, 1935; Shor and Pol- cepted lines; doubtful lines were used in part lard, 1963) of various authors as well as the only, or not at all. "Shoals of Capricorn" picturesquely named by Although the soundings recorded are rela- Ommanney (1952). At the 1200-m isobath, the tively precise, they are subject to the usual dif- structure stretches from Seychelles Bank south- ficulties of interpretation of echo-sounding: east and south through 14.5 degrees of latitude side-echo effects, slope corrections, and sweep- to the southern edge of Cargados Carajos Bank. ing of the sound cone as the ship rolls. Never- There, two passages more than 2500 m deep theless, in the regions of gentle slope at any separate the northern banks from similar shoals depth and on the abyssal plains, the discrep- and the high volcanic island of Mauritius. At ancies in depth at crossings were commonly its base the Mascarene Plateau varies in width only 5-30 m. During contouring on steep slopes, from 100 km to nearly 400 km; it is flanked by especially in ridge areas, the discrepancies ac- several spurs, notably just south of the Sey- cepted were considerably greater. Along the chelles (Pi. 1). Despite gaps in continuity, the plateau south of Saya de Malha (PI. 1) and feature is properly called a plateau rather than around the Mascarene Islands, the 100-m con- a ridge, for it has a characteristically wide and tour, as well as the 200-m contours down to level summit, due to extensive growth of cal- 1000 m, were drawn in order to indicate the careous reef organisms, and rather precipitous variations in extent, shape, and depth of the flanks down to depths of nearly 2000 m. Plates shoal features. On Seychelles Bank, on the 1 and 2 show very clearly the extremely steep Amirante Ridge, and on isolated, little-investi- and linear scarps that bound the plateau and gated islands such as Tromelin and Agalega, the the west side of Mauritius. Although the steep 1000-m isobath closely follows the 100-m con- upper flanks of the platforms probably simply tour; intermediate contours were not drawn. reflect the upward growth of coral on a sub- On the chart generally, 1000-m contours were siding foundation, many of the steep scarps at drawn down to 4000 m in order to emphasize greater depth may have a fault origin. Faulting general trends shoaler than the deep-sea floor. is suggested by the blocky outline and angular This procedure necessarily misses much of the intersections of the scarps. However, this relief on the extremely rough median ridge, but oceanic plateau is not marked by current seis-

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mic activity and appears to be a relatively stable tween 8° and 17° S, has been inhibited by the relict structure, except possibly for differential presence there of coralline pinnacles which rise subsidence or drowning of the banks. several meters from the flattish summits or even Recent seismic refraction (Shor and Pollard, reach sea level as sand cays, as in the southeast 1963; Davies and Francis, 1964) and reflection quadrant of Cargados Carajos. However, there studies (Bunce and others, 1966) and magnetic are at least four passes more than 200 m deep and gravity observations (Matthews and between the banks; one of these, near 9°35' S., Davies, 1966) bear out these essentially mor- is deeper than 700 m. Just south of the islets of phological conclusions. Refraction measure- Cargados Carajos, the reef flank drops quite ments on Seychelles Bank (Davies and Francis, steeply down to a gently-sloping flat area that 1964) indicate a "continental" crust more than may be a drowned coralline platform (Pi. 2, 30 km thick. Shor and Pollard (1963) reported fig. 1, F-F'). A deeper crossing just north of that up to several hundred meters of calcareous Nazareth Bank (D-D') shows a less regular sur- sediment and coral thicken away from island face where leveling by coral growth perhaps outcrops of Precambrian granites, and that ma- has not occurred. Profile B-B' just southeast of terial with "granitic velocity" underlies a large Seychelles Bank is similarly rough, with ir- portion, probably all, of Seychelles Bank. In regular, probably rocky, flanks extending near- the deeper portions between Seychelles Bank ly to the 1000 m summit. Two isolated peaks, and Saya de Malha, the shallow crust "is com- reaching 25-m depths, lie just southeast of posed of stratified rocks, probably sedimentary. B-B'; Fortune Bank-Coetivy Island are off the The stratification parallels the sea bottom, ridge on an east-west spur nearby. Similar slopes seaward on the flanks of the ridge, and shoals, as yet undiscovered, may lie on the is nearly horizontal in the central portion. The 1700-1800-m-deep ridge between 6° and 8° S. magnetic anomalies are generally less than 200 In the areas of broadest reef development, as on gammas" (Bowin and others, 1965). Refraction Seychelles Bank (A-A'), at Saya de Malha work of Shor and Pollard (1963) on Saya de Bank (C-C'), and on Nazareth Bank (E-E'), Malha was interpreted as showing lagoonal cal- echograms show ramparts from 5 to 40 km wide careous material from 200 to 500 m thick over- fringing a central, wide lagoon a few meters lying coral rock (about 1 km thick) on a base deeper or, as on Saya de Malha (Pi. 1), several of material displaying seismic velocities (4.4- lagoon systems on a gently tilted base. In these 5.4 km/sec) typical of Pacific volcanic islands. areas the plateau's upper flanks are steep, but Mantle velocities were not observed, but an apparently rather smooth, down to depths of "oceanic" crustal layer (velocity from 6.8 to from 800 to 1200 m. Thicknesses of this magni- 7.0 km/sec) at depths greater than 8 km was tude for coral debris or limestone capping are detected. Bowin and others conclude that on indicated by the seismic results as well. Nazareth and Cargados Carajos banks the flat Throughout the length of the structure, the summits are underlain by limestone, although flanks at depths greater than 2000 m are locally sub-bottom penetration was not observed, and irregular but characteristically more gentle in that strong (400 gammas or greater) magnetic slope than the upper portions. anomalies near the western edge of the struc- West of the Mascarene Plateau, there is con- ture suggest that volcanic rocks underlie a lime- siderable topographic variation. Northwest of stone cap. Immediately north of Mauritius sev- Seychelles Bank, the lower slopes and frontal eral small islands rise from a broad submarine apron are separated from the Somali Abyssal platform. The islands consist of vesicular ba- Plain by a possible southern extension of the salts and dolerites extruded on bedded tuff and Owen Fracture Zone and by the Amirante agglomerate (Walker and Nicolaysen, 1954); Trench-Amirante Ridge complex (Fig. 2). these are vents from which marine erosion has South of 9° S., the east-southeast trend of the removed the greater part of the original cones. Amirante Trench is not well established. How- The proportion of coralline material comprising ever, minor topography at the north edge of the shoals has not been determined. The lim- the Mascarene Basin, as well as the base of ited development of fringing reefs around Agalega, has a marked northwest trend. Be- Mauritius suggests that this bank may well tween the Seychelles and the Amirante-Agalega represent a beveled pyroclastic portion of the "arc" is a sedimented region of intermediate volcanic pile. (3500-4000 m) depth with a few isolated peaks Exploration of the "Shoals of Capricorn" be- and abyssal hills. Within the sector Agalega-

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Fortune Bank-Saya de Malha, the sea floor de Malha to the mid-oceanic ridge and Vema slopes gently and fairly smoothly southward as Trench topography (Heezen and Nafe, 1964). an apron, although here, too, small hills pro- South of these latitudes, the east flank of the trude. Occurrence of an extensive, thinly sedi- Mascarene Plateau steepens and becomes more mented region shoaler than 4000 m between angular in plan. Some of the scarps, developed the Amirantes-Agalega trend and the Mas- to 2000-3000-m depths, strike northeast; the carene Plateau proper suggests the presence topographic pattern suggests en echelon fault- there of intermediate crustal structure. ing. The plateau may be offset between Naz- South of 11° S. and east of the Madagascar- areth and Cargados Carajos banks, with Naz- Farquhar Islands complex, the sea floor is deep. areth Bank displaced east-northeast relative to A very gently sloping apron (at depths of3500- Cargados Carajos. Much more exploration is 4000 m) fronts the Mascarene Plateau as far needed, but it appears that between 10° and south as 15° S., and a narrow plain about 4300 19° S. the southern basin is narrow and only m deep separates the apron from a zone of locally flat-floored, with elongated fault deeps abyssal swales and hills (PI. 1; Fig. 2). The commonly trending subparallel to Vema swales show relief of 100 to 400 m and an elon- Trench, and that near 18° S. hilly topography gation either somewhat east-west or strongly flanking the mid-oceanic ridge extends west- northeast-southwest, the latter being subparal- ward nearly to the base of the plateau. The gen- lel to the Mascarene Plateau. The southwest erally north-trending Mid-Indian Ocean Ridge trend is strikingly developed east and southeast and rift valley seem cross-fractured and offset of Tromelin Island, where ridges protrude as in these latitudes, and ENE. or NE. trends per- much as 1400 m above the deep sedimented sea sist on the ridge, and particularly on its western floor (PI. 1; Fig. 5, upper). Several isolated flank, to the equator. The pattern is developed mountains, of which Tromelin is the largest, especially well between 3° and 11° S.; a series occur in this area. West of Tromelin, the gentle of spurs and intervening flat-floored troughs swale topography terminates at the eastern extends from near 65° E. to, or beyond, the boundary of the Malagasy Abyssal Plain, which "crest" of the mid-oceanic ridge. South of Saya extends southward to 23° S. The absolute de Malha, the trend of the eastern flank of the change in depth from the plain to the swale Mascarene Plateau is similar to that of the mid- topography is from 100 to 200 m, not sufficient oceanic ridge. The most obvious ridge disloca- to be clearly identifiable on the contour map. tion, right-lateral in sense, occurs at the Vema However, on echograms the change is abrupt, Trench; this offset seems reflected by the neck- from essential flatness to irregular low relief. ing and possible displacement of the plateau be- From the latitudes of Cargados Carajos to tween Saya de Malha and Nazareth banks. The the north end of Mauritius, the west flank of structural trend may continue west of the pla- the Mascarene Plateau is irregular in plan and teau in the linear ridges east and southeast of relief, possibly reflecting the intersecting trends Tromelin. of the plateau and the Rodriguez Ridge. How- ever, at Mauritius the west flank is steep, linear, MASCARENE ISLANDS and probably fault-controlled (PI. 2, fig. 2). North of the portion of the Mascarene Pla- Regional Geologic Setting teau between Seychelles Bank and Saya de Geological investigations of nearly all of the Malha, the east flank drops directly and with islands and islets on and adjacent to the Mas- no slope reversals into a fringing apron from carene'! Plateau have been carried out and re- 4000 to 4100 m deep (PI. 1). North of 4° S. ported recently. Baker (1963) did detailed field the flat region deepens northward, to reach work in the Seychelles, Amirante, and Farquhar 4600 m just south of the equator. A low, gen- groups. He described the Precambrian intrusive erally smooth, northwest-trending rise bearing rocks and basaltic dikes, mostly early Tertiary, a few peaks up to 3000 m high is developed near of the granitic islands of Seychelles Bank and 4° S., 60° E. However, the sea floor generally the surficial calcareous deposits of the low sand deepens, as a region of abyssal hills, east to the cays, atolls, and patch reefs where no basement foothills of the mid-oceanic ridge near 65° E. igneous rocks have been obtained. Wiseman and northeast to the troughs and ridges border- (1936) described an (Eocene-Oligocene?) ba- ing the Carlsberg Ridge, the northwest-trend- saltic agglomerate that was dredged off the ing portion of the mid-oceanic ridge. southwestern margin of Providence Reef at a At approximately 9 to 10° S., a low rise depth of 1350 m in 1905. According to Mat- bounding the northern deep area connects Saya thews and Davies (1966), magnetic traverses by

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50' 60°

10' 10'

20° 20'

50° 60* Figure 2. Province chart (partly redrawn from Heezen and Tharp, 1965).

HMS Owen in 1961-1962 suggest the pres- indicate these are composed of beach rock and ence of basaltic foundations at a depth of less coralline sand, with no igneous rocks exposed. than 1 km beneath the atolls of the Amirante Saya de Malha and Nazareth banks are wholly Group. Descriptions of Agalega and the several submerged; in 1964 the Research Vessel Chain reefs and islets on Cargados Carajos Bank in dredged one volcanic(P) pebble from the ridge nautical publications (e.g. Anonymous, 1952) flank northeast of Saya de Malha (Bunce and

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others, 1966). West of the Mascarene Plateau, on a progressive subsidence of the volcanic pile. the isolated island of Tromelin and the sea- The present study indicates that the NNE.- mount Bardin (Belousov, 1965) 140 km north- trending line of weakness extends at least to west of Tromelin are volcanic peaks; Soviet 19° S. and may be expressed in the very deep workers collected basalt on Tromelin in 1964 cleft near 19°20' S., 58° E. (J. Tuzo Wilson, personal commun.). The geology of volcanically active Reunion The Mascarene Islands have been restudied has been studied most extensively by Lacroix rather intensively. Rodriguez is composed of a (1936) and, more recently, by Upton and Wads- uniform series of alkalic olivine basalt flows as worth (1965). Drawing on the former author's the summit of a very large, steep-sided elon- work, Walker and Nicolaysen (1954) compared gated volcanic pile built up from the ocean the islands of Mauritius and Reunion, finding floor (McDougall and others, 1965). The pres- petrological similarities as well as rather im- ent emerged portion, which caps an elliptical, portant differences, particularly in the youngest beveled submarine platform of more than 10 volcanics. The lower (older) part of the Piton times greater extent, has undergone little ero- des Neiges series on Reunion corresponds petro- sion. One flow has been dated at 1.54 ± 0.05 logically to the oldest rocks on Mauritius. How- m.y. by the potassium-argon method, and these ever, intrusions of gabbro, alkali-gabbro and authors conclude that Rodriguez became ex- quartz-syenite are found on Reunion; their ab- tinct in Pliocene-Pleistocene times. A fringing sence on Mauritius might be due to founder- reef well offshore and extensive reef-flats border ing. The latest lavas of Reunion, such as those the southern and western coasts; on the east the from the still-active Piton de la Fournaise, cor- fringing reef reaches the shore. Raised beaches respond to the Pleistocene flows on Mauritius, have been found, and on the southern and east- except that on Reunion the rocks are variations ern coasts cross-bedded calcarenites, of prob- of tholeiite-picrite basalts of explosive erup- able Pleistocene age, occur up to elevations of tion, while on Mauritius the late flows are very 62 m above sea level. However, on the evidence uniform vesicular olivine basalts. It may be of the embayed coastline and fringing reefs, noted here that while Reunion and Mauritius McDougall and others suggest that over-all sub- are separate piles, with deep-sea floor between sidence has been dominant in the recent history them, the vents on Reunion have a north- of Rodriguez. westerly trend, and several well-attested, if iso- Companion papers by Simpson (1950) and lated, shoal soundings west and north of Re- Walker and Nicolaysen (1954) summarize the union indicate that the volcanic activity ex- geology and petrology, respectively, of Mauri- tends well beyond Reunion. tius. These authors postulate that sea-floor fis- In all three of the Mascarene Islands, then, sure eruptions, possibly in Cretaceous time, had the latest volcanics are olivine basalts of oceanic built up by early Tertiary a pile of massive type, dated at Pliocene-Pleistocene on Rod- olivine-rich (picritic) basalt flows, with differ- riguez, Pleistocene on Mauritius, and Recent entiation to trachytes and phonolitic trachytes on Reunion. Coral reef occurrences show a sim- that now appear as jointed plugs. The upper ilar progression; reefs are extensively developed portion of this pile may have been removed by at Rodriguez, less so at Mauritius, and absent explosions or caldera collapse, and the stumps at Reunion. underwent long erosion. A late Tertiary series of somewhat vesicular olivine-bearing tholeiitic Submarine Topography of Mauritius flood basalts covered the southwestern part of Nearshore exploration of Mauritius clearly Mauritius. The final volcanic event was the reveals the broadly indented lobate eastern eruption of vesicular olivine-rich flood basalts slope resulting from volcanic flows and the rela- from a NNE.-trending group of 25 shield vol- tively straight western flank here interpreted canoes. These flows form the present watershed as a fault scarp (PI. 2, fig. 2). On the southern of Mauritius and comprise the small islands to and western coasts the shelf is up to 2 km wide, the north. Since the most recent basalts are except for a 5-km southwestern extension of overlain locally by coral reefs assigned to high Le Morne Brabant. Off the northern coast a Pleistocene sea levels, these authors conclude shallow bank extends for 26 km. To the east, that volcanism on Mauritius ended more than the shelf width varies from 3 to 11 km, with 100,000 years ago and that eustatic submerg- the greatest width along the extension of the ences and emergences have been superimposed early volcanic Bambou Mountains. The prin-

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cipal shelf break is at 80-90 m (PI. 2, fig. 1, G-G'), but terraces occur at depths of about Submarine Topography of Reunion 28, 38, and 48 m. Reunion is the exposed portion of a volcanic The steep western flank and less regular east- pile more than 30,000 km2 in area at the 4000-m ern slope (PL 2, fig. 3) both show pinnacles or contour (PI. 1). Exploration is scanty, but the irregularities at depths greater than 2000 m. flanks appear to be unfaulted. Steep upper At shoaler depths on the east side the echo- slopes decrease in gradient at depth to merge sounder return is sharp and discrete, suggesting with the deep sea floor (Fig. 3; PI. 3, B, C). A the presence there of masking lava flows from seismic reflection profile crossing the base of the the most recent volcanoes. One supposed late slope east of the island (Fig. 5, lower) does not flow is shown in profile on the upper section detect acoustically transparent sediment there (Pi. 2, fig. 3). Inshore lines paralleling the west- on the broad apron. The northern and western ern slope (Fig. 3, 4) show small gullies or can- flanks are less regular. A north-south sounding profile passing 30 km west of Reunion (PI. 4, fig. 1, A) crosses a 50-km-wide plateau lying at a depth of 2000-2300 m. Its surface roughness suggests a collapsed crater, a link to the vol- canism west of Reunion noted above. Peaks to the west and a large caldera(?) on the sub- merged flank of Reunion fit very well with a suggestion of Upton and Wadsworth (1965) who studied Reunion itself. They consider that over-all volcanic activity has steadily shifted southeastward from Piton des Neiges. On this assumption, the submerged caldera is older than some of the late Quaternary flows of oceanic Figure 3. Index map for Figure 4 and Plate 3 basalts from Piton des Neiges. The trend may mark a rift developing on the sea floor in a southeasterly direction, and the possible source yons that have not been traced to the base of of the flood basalts. The trends and extent of the slope. Exploration east of the island has not the several peaks and deeps from 250 to 450 km been done in sufficient detail to determine if southwest of Reunion (PI. 1), and their relation canyons occur there. The large re-entrant off to the development of the Mascarene Plateau, Riviere du Rempart (PI. 2, fig. 2) is a valley be- are not well established. tween volcanic lobes. Neither the faulted Mauritius nor the more West of Mauritius, at depths greater than extensive Reunion is bordered by a moat. Moats 2500-3000 m, an apron extends to the archi- or bordering depressions are variously ascribed pelagic plain which lies at 4362 + 7 m. The to scour, to caldera collapse, or to subsidence surface of this apron (PI. 3, C) is relatively due to loading of the oceanic crust by a vol- gentle but shows some low irregularities; Hee- canic pile. In the case of Reunion, at least, the zen and Tharp (1965) have termed such undu- absence of such a feature may be in accord with lations abyssal swales. East of Mauritius an its relative youth. However, the frequency of apron is well developed in the re-entrant near cable failures on the lower slopes and northern 20° S., but southeast of the island it is sharply apron (Fig. 6) indicates that slumping or dep- terminated by the Mauritius Trench (PI. 1). osition by turbidity currents or both may have Reflection profiles (Fig. 5, upper) show the buried the depression; the small peaks between sediments collected in the deep area east and 22° and 23° S. near 55° E. protrude above a east-northeast of Mauritius. sedimented apron. Magnetic traverses around Mauritius con- firm the conclusion that this island is composed Rodriguez Ridge primarily of basic volcanics extruded as flows The elliptical shape of Rodriguez and its sur- rather than as ash or pumice; the magnetic rounding platform is continued at depth; that traces closely reflect the topographic relief. island is the only emergent portion of a narrow, Gravity measurements made during Argo and nearly east-west ridge that can be traced west- Vema surveys have not been fully interpreted. ward at least to an intersection with the Mas-

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NORTHEAST SOUTHWEST

VERT. EXAGG. 15 1/2 x

VERT. EXAGG. 24 x

VERT. EXAGG. 22 x

VERT. EXAGG. 22 1/2 x

VERT. EXAGG. 18 1/2 x

Figure 4. Redrawn sounding traces along the western slope of Mauritius. See Figure 3 for locations of profiles.

carene Plateau (PI. 1). The two connected extend into the Mascarene Basin. The Rod- shoals between Mauritius and Cargados Carajos, riguez Ridge is most poorly expressed at its separated from the latter features by passes intersection with the "Mauritius Trench" near more than 2500 m deep, apparently belong to 60° E.; soundings to date indicate that the pass this structure rather than to the Mascarene is nearly 4000 m deep and the trench complex Plateau proper, but the westerly trend does not may extend as an ill-defined basin adjacent to

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VI4 37-12 VI4 37-19 BOTTOM PHOTOGRAPHS FROM STATION VI4-37

Bottom photographs from station V14-37 (20°12'S., 57°23'E.) on the western flank of Mauritius, at depths of about 800 m (Pi. 4, fig. 2).

FISHER AND OTHERS, PLATE 5 Geological Society of America Bulletin, volume 78

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-3.500

KILOMETERS

x

,5.750

I. THREE SEGMENTS OF A VEMA-16 PRECISION DEPTH RECORDER RECORD MADE BETWEEN MAURITIUS AND REUNION: A) Insular slope of Reunion. B) Reunion apron. C) Base of slope west of Mauritius. See Figure 3 for location of segments. Vertical scale: reflection time in milliseconds.

FISHER AND OTHERS, PLATE 3 Geological Society of America Bulletin, volume 78

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VI4 36-32

BOTTOM PHOTOGRAPHS FROM STATION VH-36 AND LUSIAD BP-1

Bottom photographs from stations VH-36 (23°06'S., 56°43'E., depth 4800 m) and Lusiad BP-1 (24°33'S., 57°29'E., depth 5050 m) on the deep-sea floor south of Mauritius (PI. 4, fig. 2).

FISHER AND OTHERS, PLATE 6 Geological Society of America Bulletin, volume 78

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s ni,, Srii

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Cargados Carajos. The shoalest portions of the of generally high heat flow with present low ridge are not irregular or conical peaks, but seismicity is puzzling. elongated blocks with terraced, reef-flattened summits. Two banks are shallow enough to sup- THE MAURITIUS TRENCH port living reef-corals. The western portions of Southwest of the intersection of the Rod- the shoals centered at 61°30' E. and at 62°15' riguez Ridge and the Mascarene Plateau, the E. are guyots or drowned atolls, supporting the smooth slopes of the Mascarene Basin-Mas- evidence for subsidence of Rodriguez reported carene Rise are separated from the irregular sea by McDougall and others (1965). floor of the Reunion Basin by a deep trough or Marked shoals occur as far east as 64° E. and trench that is clearly tectonic in origin. This the ridge could continue for several hundred feature, which has been traced southwestward km to the east as a narrow spine, either just for a distance of 1400 km to 28° S., 51° E., is north or just south of 20° S. An interpretation here designated the Mauritius Trench (PL 1; by Heezen and Tharp (1965), shown in Figure Fig. 2). It is distinctively straight, narrow, and 2 of this paper, postulates that the Rodriguez flat-floored over much of its length, and thus Ridge lies on a major fracture zone along which provides a key base for the very rugged linear an apparent 300 km left-lateral offset of the pattern to the east. axis of the mid-oceanic ridge has occurred. Ex- All bathymetric profiles taken within a sector ploration between 19°-23° S. and 65°-75° E. extending from northeast of Mauritius clock- is insufficient to establish that offset or the exact wise to south of that island show extremely ir- nature of the intersection of the southeast and regular topography extending for from 300 to southwest branches of the mid-oceanic ridge 500 km seaward of Mauritius. Plate 4, figure 3 complex. There are no sounding tracks be- (see PL 4, fig. 2, for locations) was constructed tween 20°-21° S. and 66°-70° S. Less direct from soundings from several sources; the ap- evidence than Precision Depth Recorder rec- parently greater irregularity of the peaks, and ords—regional level, seismicity and heat flow flatness of the small deeps along profiles 2, 5, —suggests, however, that Rodriguez Ridge and 6, 8, 9, 10, and 13, as contrasted with profiles its postulated extension eastward as a fracture 1, 3, 4, 11, and 14, are due to closer spacing and zone is a major structural discontinuity. First, greater precision of soundings in the former north of 20° S. there is extensive shoal (<3000 group. East of 60° E. southeast of Mauritius, m) development of the north-northeast-trend- the narrow deeps (>5000 m) do not appear; ing mid-oceanic ridge (PL 1). To the south and the relief is irregular in the range 3400-4600 m east of 21° S., 68°30' E. a comparable shoal out to the southwest-trending mid-oceanic area, and possibly a median valley, trends more ridge near 25° S., 66° E. nearly north-south for more than 400 km, and In a sector south-southeast to south-south- farther south bends toward the southeast. The west of Reunion (PL 1; PL 4, fig. 3: profiles 10- north-south topographic lineations south of the 14), the well-defined Mauritius Trench lies Rodriguez Ridge between the longitudes of north of small linear deeps and several hundred Mauritius and Rodriguez are not obvious north km of deep swale topography within the Re- of 20°-21° S. Second, north of 20° S. the single union Basin. The extremely irregular mid- mid-oceanic ridge is seismically active (see oceanic ridge is reached near 31° S., 57° E. Gutenberg and Richter, 1954). From 20° S. to The depth of the Mauritius Trench is at least the vicinity of Amsterdam-Saint Paul, the in part a function of the amount of sedimentary southeast branch is much less active. Third, fill received from Mauritius and Reunion. Von Herzen and Vacquier (1966) note that Along profile 6, for example, where the trench heat-flow values obtained on the crest, flanks, is 4485 m deep, seismic reflection measurements and lower flanks of the mid-oceanic ridge be- (Fig. 5, lower) show at least an additional 250- tween 11° S. and 28° S. group into two popula- 300 m of sediment without the basement being tions. Measurements north of 20°-21° S. char- detected. Most of the filling must be due to acteristically are low; 21 values average 0.6 X deposition of sediment transported along the 10~6 cals/cm2/sec. Twelve values south of 21° sea floor. Throughout the year, winds and sur- S. average 2.6 X 10~6 cals/cm2/sec. Similarly, face currents are westerly; volcanic ejecta from M. Langseth (personal commun.) found very the islands would move toward the Malagasy high values near 22° S. This evidence is sug- Abyssal Plain. The greatest depth yet found in gestive, but not conclusive, relative to a "Rod- the Mauritius Trench, 5810 m, lies between riguez fracture zone"; however, the association 26° and 28° S., far from emergent volcanoes.

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Most extensive fill occurs at the foot of the the basalts erupted by Piton de Fournaise (D. insular slope of Mauritius, and to the south- Ninkovitch, personal commun.). Brown lutite west, off Reunion, the amount of filling depends is the type of sediment generally found in the on protection afforded by the narrow ridge on isolated deep ocean basins adjoining the islands the northwest side of the trench (PL 4, fig. 3: (PL 4, fig. 4: V16-75, V18-198). Core V16-75 profiles 9, 11). However, it is not clear whether has been dated by the protoactinium-ionium or not this portion of the Mauritius Trench method by T. Ku of Lamont Geological Ob- predated the formation of the large shield servatory. He found a rate of sedimentation of capped by Reunion; bathymetric lines south- 6 mm per 1000 years. From seismic reflection east from Reunion to the trench as drawn are profiles (Fig. 5), the rough topography east of lacking. A small depression in the sediment fill the Mauritius Trench and at the flanks of the near the western side of Mauritius Trench on mid-Indian Ocean Ridge appears to be nearly profiles 6 and 7 (PL 4, fig. 3) suggests that here devoid of sediment. This is borne out for the the trench is still subsiding. most part by bottom sampling and by photo- Deeper, less-sedimented clefts lie subparallel graphs which reveal frequent outcropping of to Mauritius Trench farther to the east and rock and evidence of deep-sea currents. A short southeast. One, near 20° S., 60° E. (Pi. 1), is core (Pi. 4, fig. 4: VI8-199) was taken on the receiving sediment through a gap in the ridge flank of the Mid-Indian Ocean Ridge. The east of Mauritius Trench. The shape of the lower portion of this core consisted almost en- basement in this ridge-and-trough region is not tirely of irregular fragments of serpentinite. known, but the over-all pattern suggests splin- Petrological examination of a sample at 160 cm ter faulting—with closely spaced parallel or sub- below the top revealed a composition of ser- parallel faults that extend several tens of kilo- pentine 70 percent, magnetite 25 percent, and meters along the strike—coupled with volcan- iddingsite 5 percent. A sample at 170 cm con- ism along the ridges and extension in an east- sisted of serpentine 35 percent, carbonate (mag- erly or southeasterly direction south of Rod- nesite) 35 percent, magnetite 25 percent, and riguez Ridge. iddingsite 5 percent. The serpentine, magne- site, and iddingsite are products of the altera- SEDIMENT DISTRIBUTION tion of olivine. The primary rock was composed Seismic reflection measurements were made of olivine 75 percent and very fine-grained and by the Vema during cruises 18 and 20 (1962 dispersed magnetite 25 percent (D. Ninkovitch, and 1964, respectively). A thickness of 100 m personal commun.). of sediment was detected in the smooth topog- Photographs showing rock outcrops and rip- raphy to the northwest of Mauritius (Fig. 5, pled bottom have been taken on the west flank upper). The lack of relief at the base of the zone of Mauritius in depths of about 800 m (PI. 5). suggests it is a sediment-sediment boundary Photographs taken south of Mauritius at a rather than a sediment-basement contact. Re- depth of 4800 m show a current-scoured and flection results gave little indication of the rippled bottom (PL 6: V14-36). Bottom cur- total thickness of sediment in this area. East of rents producing these features must have a con- Mauritius, beneath the Mauritius Trench, more siderable effect on the distribution of sediments. than 200 m of sediment was observed; here Currents are most active on the steep sub- again it is likely that a much greater thickness marine slopes of the islands and over the shoal of sediment than the measurements indicate is portions of the submarine plateaus and ridges, present. The smooth archipelagic apron south but also act at the base of these elements. The of Reunion consists of acoustically opaque sedi- pattern and frequency of failure, mostly ments. One core in the area (PI. 4, fig. 4: V16- through chafe, of submarine telegraph cables 73) is composed entirely of pyroclastic debris. between Reunion and Rodriguez (Fig. 6) gives Sediment cores on the crest of the Mascarene clear evidence of current activity. An undis- Plateau consist primarily of calcareous lutite turbed mud bottom occurs in the swale (PL 4, fig. 4: V14-89, V14-90). Turbidites and topography 450 km south of Mauritius (PL 6: volcanic debris are found on the archipelagic L-BP-1). aprons (PI. 4, fig. 4: VI6-73, VI6-74). Analysis of a pebble from a depth of 500 cm in core DISCUSSION OF TECTONIC V16-73 showed labradorite 45 percent, py- ELEMENTS roxene 45 percent, ore 5 percent, and glass 5 The Mascarene Plateau, the Mid-Indian percent; the composition is similar to that of Ocean Ridge, and the Rodriguez Ridge are

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discrete but interrelated features differing in western flank of Saya de Malha Bank, in water age, crustal structure, stage of development, depths of from 2800 to 3900 m. It is here and nature of fracturing. Some of the fault assumed that this quasi-continental crust un- trends can be traced across both the mid- derlies Coetivy Island-Fortune Bank and ex- oceanic ridge and the Mascarene Plateau (Fig. tends southeast along the western side of the 7). The fractures are still active and apparent Mascarene Plateau perhaps as far as Cargados at the crest of the mid-oceanic ridge, but are Carajos. West of Seychelles Bank it extends to inactive and buried beneath the sedimented the east side of the Amirante Ridge. aprons and plains. The time that has elapsed (4) Three seismic reflection profiles on the since the most recent movement along any of deep (1000-2000 m) portion of the Mascarene the NE.-SW. fractures apparently increases Plateau between the Seychelles and the north- with increasing distance from the crest of the ern edge of Saya de Malha Bank show domi- mid-oceanic ridge, but the date at which the nantly flat-lying sedimentary beds (limestone?). Mascarene Plateau was offset cannot yet be Similar stratification was observed beneath the determined. This variation in time of activity 1000-2000-m bench east of Cargados Carajos on single zones also implies that these are not Bank, but no layering was detected beneath classical strike-slip faults, but that some, at the shoalest and nearly level central portions of least, are transform faults, belonging to a new that bank, where magnetic anomalies of high class of horizontal shear faults recognized by amplitude and short wave length suggest the Wilson (1965 et seq.). The Chagos-Laccadive presence of volcanic rocks at shallow depth Ridge lies just east and northeast of the region (Bunce and others, 1966). here discussed. It is an aseismic north-south (5) The granitic rocks of Seychelles Bank are elevation, composed of coral atolls which are intruded by early Tertiary basaltic dikes underlain by approximately 5 km of material (Baker, 1963) which are expressed by short whose compressional wave velocity ranges from period high-amplitude anomalies on magnetic 3 to 6 km/sec (interpreted as volcanic rock), profiles recorded in the central portion of the and which overlies a thick crustal (6.8 + km/ bank (Matthews and Davies, 1966). Similar sec) layer (Francis and Shor, 1966). Topo- anomalies are found at Cargados Carajos and graphically similar to the Mascarene Plateau, occasionally occur along the western flank of it has not been appreciably offset by the prev- the Plateau but are absent from the few pro- alent northeast-trending fracturing. files run on the other banks (Bunce and others, Any inferred geologic history for this region 1966). From seismic refraction work on Saya de must be extremely speculative at this stage of Malha Bank, Shor and Pollard (1963) reported investigation. Nevertheless, a presentation a thick sequence of 4.4 and 5.4 km/sec material based primarily on topographic evidence, but beneath from 1200 to 1500 m of calcareous with the following assumptions, interpreta- debris and above an "oceanic crustal layer" tions, or observations, will be made: (6.8-7.0 km/sec) whose upper contact lies (1) Madagascar and Precambrian granitic from 8 to 9 km beneath sea level. Drawing an rocks underlying Seychelles Bank were once analogy to similar velocity-structure at Hawaii joined or close together. A linear fault zone, and Eniwetok, these authors suggest that the active since Cretaceous time, lies just seaward base of the Saya de Malha-Mauritius portion of of the east coast of Madagascar (Dixey, 1960, the Mascarene Plateau was formed by out- after Lemoine) and marks a sharp edge of the pourings of lava along a line of weakness in the Precambrian block. ocean floor. However, similar velocity-struc- (2) The presence of basaltic agglomerate of ture was observed in the nonvolcanic Creta- early Tertiary age near Providence Reef, north- ceous and Tertiary limestone-and-marl sequence east of Madagascar (Wiseman, 1936), implies of the Blake Plateau (Sheridan and others, that initial separation had occurred by that 1966). Davies and Francis (1964) report that time. on Seychelles Bank the upper contact of the (3) The granitic basement underlying Sey- 6.8-6.9 km/sec layer lies about 15 km below chelles Bank is sharply bounded on the eastern sea level; thus the crust beneath Seychelles and northern sides of that bank (Francis and Bank has both a continental character and a Shor, 1966; Matthews and Davies, 1966, continental thickness, capped by less than a respectively). Francis and Shor measured a half kilometer of calcareous debris. 10-km-thick layer of 6.03 km/sec material from (6) The narrow, linear Rodriguez Ridge was 100 km east of Agalega to the base of the formed by volcanic eruptions that ended, on

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Rodriguez, about 1.5 m. y. ago. The age of east-trending ridges, probably volcanic, that initial volcanism on this trend is unknown. protrude from the floor of the sedimented It is postulated that late in Mesozoic time a Mascarene Basin south of Tromelin Island. portion of continental crust separated from Along this trend seismic activity is observed Madagascar. Subsequently it has moved north- only where the zone crosses the mid-oceanic east, relative to Madagascar, a distance of from ridge. This implies that the ridge, rather than 1000 to 1200 km. (Alternatively, Madagascar the fault zone, is now the active member. It is may have moved south-to-southwest.) The suggested that the northeast fracture pattern extent of the fragment is not specified, but it predates the development of the mid-oceanic included at least the present Seychelles Bank- ridge, although a rough parallelism of the Fortune Bank complex and possibly the entire eastern side of the Mascarene Plateau and the western flank of the Mascarene Plateau north crest of the ridge south of 10° S. (from to- of Cargados Carajos. Oceanic basaltic material pography, magnetic profiles, and earthquake was emplaced between the separated blocks, epicenters) implies that both plateau and ridge and now floors the intervening oceanic area. have been affected by the major fractures. Between Madasgascar and the Seychelles are Further, the transcurrent system has been ridges, peaks, and troughs formed by faulting active at times since initiation of the ridge, for and volcanism in a tensional environment; the segments of the ridge—if the ridge does Amirante Ridge and trench are products of this represent continuing emplacement of new activity, though of more recent development. crust along a zone thousands of kilometers Their trend may be extended well north of the long (see next section)—would not have been Seychelles. As a consequence of the disruption, born offset. Nevertheless, confinement of a boundary line of weakness developed in a recent earthquake activity to the ridge crest, meridional direction; volcanic rocks now and especially to the areas where the fracture forming much of the Mascarene Plateau from zones cross that crest, suggests ridge-widening Saya de Malha south to Mauritius were ex- and the predominance of transform faulting in truded, commencing in Cretaceous(?)-early the manner proposed by Wilson (1965). On Tertiary time. Limestone was deposited on the such an assumption, igneous material is being ridge as it neared the sea surface and coralline added along the ridge crest; crustal blocks on forms could be supported. Fundamental to this both sides of a fracture zone are moving away argument is a volcanic base underlying the from the crest; actual displacement between banks from Cargados Carajos to Saya de Malha; crestal segments on the Vema Trench fracture recovery of rock samples by intensive dredging zone, for example, is now opposite to the ap- of the steep lower slopes, at depths of from parent right-lateral offset; and the Mascarene 2000 to 3000 m, may be required to establish Plateau, offset earlier, need not be differentially this point. faulted, because movement is limited to por- North and northwest topographic trends tions of the faults lying between crestal occur as major elements in the northwest segments. Earthquake first-motion studies, quadrants of Plate 1 and Figure 7, north of a such as those of Sykes (1967), of this part of the zone extending from Saya de Malha to eastern mid-oceanic ridge system would establish the Madagascar. In the remainder of the area, nature and direction of current faulting; lineations are considered to mark continuing closely spaced magnetic profiles south of 5° N. adjustments of the oceanic crust, with displace- would test the existence of linear anomalies ment and volcanism in a system of subparallel taken to indicate ridge growth (Vine and shears, as a consequence of the separation. From Matthews, 1963). Some well-expressed north- Mauritius north to Saya de Malha Bank, the east lineations are not apparent where they Mascarene Plateau appears to have been would intersect the ridge crest, do not offset it, broken by northeast-southwest-trending faults yet continue northeast of the ridge as well (for with right-lateral offsets. The most obvious example, near the equator, Fig. 7). dislocation occurs between Saya de Malha and The Rodriguez Ridge with its extension east- Nazareth Bank. This faulted zone apparently ward across the mid-oceanic ridge complex was continues northeast and includes the Vema formed late in Tertiary time, but does not Trench, where topography and magnetic pro- extend west of the Mascarene Plateau. Sub- files (Von Herzen and Vacquier, 1966) indicate sequent to the development of Rodriguez right-lateral offset of the crest of the mid- Ridge there has been separation between oceanic ridge, and southwest along the north- Mauritius and Cargados Carajos, and right-

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lateral offset in the Mauritius block. The most relatively old oceanic crust. Similarly, the recent lavas on Mauritius, from vents aligned magnetic lineations north and northeast of the in a NNE. direction, may have been extruded Seychelles (Fig. 7) figured by Matthews (1966) following this adjustment. The northwest- and by Bowin and Vogt (1966) may be due to trending branch of the mid-oceanic ridge ap- ridge topography now buried or thickly sedi- pears offset by the postulated eastward exten- mented. Topographic lineations are not well sion of Rodriguez Ridge. Again, present-day developed in this portion (PI. 1). Nearer to epicenters are confined to the mid-oceanic ridge and on the Carlsberg Ridge, northwest itself. The southwest-trending branch, which topographic trends are apparent. A detailed lacks a well-developed crest within the area survey of the ridge crest centered on 5° 20' N., shown on Plate 1, is the more seismically active 61° 45' E. (Matthews and others, 1965) showed at present; its trend, extended, would not join that topography and magnetic anomalies there the mid-oceanic ridge crest north of Rodriguez are elongated parallel to the ridge. either. The Carlsberg Ridge sector of the mid- The extremely rough, generally unsedi- oceanic ridge was apparently created by the mented, and youthful-appearing topography emplacement of new crust during the separa- between the Mauritius Trench and the south- tion of the Seychelles granitic block from the west branch of the mid-oceanic ridge is at- Indian subcontinent. The fractures which tributed to faulting and volcanism in a tensional transect the Carlsberg Ridge are nearly parallel zone on the deep-sea floor bordering the south- to the NNE.-trending Owen Fracture Zone west branch of the mid-oceanic ridge. A con- (Matthews, 1966) near the western termina- tinuation of this rough zone has not been found tion of the Carlsberg Ridge, but their trend north of Rodriguez Ridge west of Rodriguez. progressively shifts to northeast between the Mauritius Trench, a recently active feature, Owen Fracture Zone and the Vema Trench forms a topographic boundary on the west, and (Heezen and Tharp, 1965, 1966). Throughout may have been the chief locus of crustal dilation the worldwide mid-oceanic ridge system, the and graben formation. Its relief may derive crest of the ridge characteristically lies parallel from recent tectonic activity related to flood or subparallel to the edges of the bordering basalt volcanism at Reunion and Mauritius continental blocks; this is shown most strik- and to formation of deep irregular basins south- ingly in the relatively simpler geometry of the west of Reunion, but its trend, which parallels Atlantic. In the more complex tectonic pattern the mid-oceanic ridge, may be inherited. One exhibited by the central Indian Ocean floor, core (PI. 4, fig. 4: V16-75) from this topo- this mid-oceanic ridge is neither closely parallel graphically irregular region would, from its to, nor exactly in a median position between, length alone, imply sedimentation at least since the major continental blocks of Africa, early Pleistocene time; the upper 25 percent of Australia, Asia, and Antarctica. However, the this 12-m core was deposited in the last 0.5 m.y. Carlsberg Ridge sector of the Mid-Indian Ocean Ridge does lie in a median position TECTONICS OF THE WEST CENTRAL between, and is parallel to, the margin of the INDIAN OCEAN Indian subcontinent on the northeast and the According to recently presented geological- Seychelles granitic block on the southwest. The geophysical evidence for sea-floor spreading, the segment of the Mid-Indian Ocean Ridge be- worldwide mid-oceanic ridge has grown wider tween Vema Trench and the Rodriguez Ridge throughout geologic time (Heezen, 1959). In lies in a median position between, and is ap- the area discussed here this growth is mani- proximately parallel to, the Saya de Malha- fested by a progressive separation of the Mauritius portion of the Mascarene Plateau Mascarene Plateau and the Chagos-Laccadive and the extension southward of the Chagos- Plateau. Such separation is apparently accom- Laccadive Plateau. Thus, in the central Indian plished by addition of crustal material at the Ocean, these microcontinents bear a close crest of the mid-oceanic ridge (Heezen, 1960, geometrical relation to the mid-oceanic ridge 1962; Hess, 1962, 1965; Vine and Matthews, similar to that borne by the major continents 1963; Vine, 1966). The swale topography which bordering the Atlantic. This relation suggests covers large areas south of Reunion and extends that these microcontinents have acted as "con- through the area west of the Mascarene Plateau tinents" during development of the mid- from Reunion to the Seychelles probably oceanic ridge in the central Indian Ocean. represents more thickly sedimented and thus Their function here as "continents" does not

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imply any particular petrologic character of Geophysical exploration to supplement and their foundations or help to discriminate be- extend the topographic information presented tween intrusive, extrusive, or sedimentary here is required to establish the character of the origins, but may simply reflect their extent and Mid-Indian Ocean Ridge between fracture near-continental crustal thickness. However, it zones south of 5° N., and to test the structural does imply that their foundations did exist, in pattern here proposed. Well-controlled mag- approximately their present form and extent, netic surveys, such as that of Matthews and prior to the birth of the Mid-Indian Ocean others (1965) on the Carlsberg Ridge, could Ridge system. demonstrate the existence, width, spacing, and Throughout the world, the mid-oceanic ridge amount of offset of the expected northerly ranges in width from less than 400 km, as in the lineations on the flanks and in deeper areas Norwegian Sea and the Gulf of Aden, to more alongside the mid-oceanic ridge. The meeting than 5000 km in the Southeast Pacific. Minor of three active branches of the Mid-Indian relief of the ridge characteristically consists of Ocean Ridge near Rodriguez Island presents an subparallel ridges and troughs. Major changes intriguing tectonic problem; the area is scarcely in ridge width tend to occur at fracture zones, explored and requires a detailed multiphase which probably mark discontinuities in rate of survey to establish the ridges' trends, relative ridge growth. The 1000-km-wide Carlsberg stage of development, rate of growth, and the Ridge may have grown during the same interval relation of at least the two southern members to as the 300-km-wide section of the Mid-Indian each other and to the east-west Rodriguez Ocean Ridge lying between Nazareth Bank Ridge, and Chagos Bank.

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CONTRIBUTION FROM THE SCRIPPS INSTITUTION OF OCEANOGRAPHY, UNIVERSITY OF CALIFORNIA AT SAN DIEGO LAMONT GEOLOGICAL OBSERVATORY CONTRIBUTION NUMBER 1135 MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 19, 1966

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