ISSN 01458752, Moscow University Geology Bulletin, 2016, Vol. 71, No. 2, pp. 131–136. © Allerton Press, Inc., 2016. Original Russian Text © L.V. Panina, V.A. Zaitsev, 2016, published in Vestnik Moskovskogo Universiteta. Geologiya, 2016, No. 1, pp. 20–26.

The Recent Structure of Mahe Island (the Seychelles Archipelago) L. V. Panina and V. A. Zaitsev Department of Geology, Moscow State University, Moscow, 119991 Russia email: [email protected], [email protected] Received May 5, 2015

This article is devoted to the centenary of the birth of N. P. Kostenko

Abstract—Complex studies were carried out, including structural and geomorphological analyses of space images and statistical processing of their deciphering results, which revealed the blocky pattern of the recent dislocations of Mahe Island, thus defining their relationships with a jointing pattern in granite outcrops. The results were compared with the geological data and instrumental observations of modern movements.

Keywords: neotectonics, structural and geomorphological analysis, interpretation of the space images, the weak zone, the faults, the modern movements DOI: 10.3103/S014587521602006X

INTRODUCTION We should note that the coast in several areas is man made, for example, the eastern reef plateau, which is The recent structure of Mahe Island, which is the located between Victoria and the international airport. largest island of the Seychelles archipelago and is located in the western part of the , is The Seychelles are upraised over the sea bottom almost unstudied. Distinct publications have been areas of the Mascarene (Seychelles) underwater pla devoted mainly to geological reconstructions of the teau, which is part of the Somali Plate. Mahe Island in formation of this island and do not cover the neotec the geological sense is part of a microcontinent that tonic issues. In the article (Korotaev and Nikiforov, separated from the supercontinent, the 1982) a note about the blocky pattern of the island and extension of which started in the Neoproterozoic era. the connection between the configuration of its shore Separation of the islands took place at approximately line and lineaments was made. The objective of the 195 Ma. The modern position of the continental block study was to reveal the recent structure of the island of the Seychelles relative to was estab and the tendencies of its development at the recent lished as approximately 47 Ma. The largest part of the stage in context with the encircling structures of the island is comprised by socalled gray granites and Indian Ocean. graniteporphyries, which are replaced by pink gran ites in the northwest and gneisses and granitediorites in the north. The granites of the Seychelles are analogs GEOLOGICAL–GEOMORPHOLOGICAL of the granites that are found on Madagascar and in CHARACTERISTICS the northwestern part of the India. The age of the granite complexes has been dated back to ~755 Ma, Mahe Island is the largest of the islands of the Sey which relates to the Neoproterozoic era. Granites are chelles archipelago; it is located in the western seg intruded by dikes of the Precambrian, Paleogene, and ment of the Indian Ocean on the plateaulike part of Neogene dolerites. the northwestern margin of the underwater Mascarene ridge. The topography of the island is comprised by a The Seychelles microcontinent, including Mahe series of uplifted and submerged stairstep surfaces Island, has undergone several phases of extension that lower from the most uplifted central part of the accompanied by volcanic activity and intrusive mag island to the coastal plain areas. The greatest value of matism in the Paleozoic and Mesozoic eras. A series of the absolute height is 906 m; this is confined to the elongated grabens formed during the late Paleozoic. central rise, the lowest value is 0 m, which is confined The Mesozoic history of the Seychelles includes three to the area of the coastal plains. The latter are traced at phases of jointing that are connected with the drift of a distance that varies from several tens of meters to the islands, which started in the Middle Cretaceous. several hundreds of meters. Coastal areas or cliffs are These phases included numerous stages of uplift, ero locally absent and granite outcrops project to the sea. sion, and volcanic activity accompanying these pro

131 132 PANINA, ZAITSEV cesses. Early Cretaceous lava outflows and intrusions The blocky type of deformation is characteristic of of dolerite dikes of the early Cretaceous (~135 Ma) age the island. The blocks of relief that are uplifted and and manifestation of magmatism of the late Creta downthrown relative to each other at various heights ceous (~96–84 Ma) are then noted in the Mesozoic. are divided by weak zones (zones of faulting, jointing, The timing of the last event coincides with the rifting rock breaking, and others) of the mainly northwestern between the Seychelles and the Madagascar (Plummer and northeastern strike; the sublatitudinal and sub and Belle, 1995), which was accompanied by the for longitudinal orientations are rarely detected. Zones of mation of the Amirante Basin with the oceanic crust. jointing, rock breaking and faulting with offsets are Rifting between the Seychelles and India and the for called weak zones (Kostenko, 1999). Thus, the tec mation of the Carlsberg ridge occurred during the Late tonic essence of dislocations contributes to this idea. Cretaceous and Paleogene, which correlates with out The term “weak zone” in this sense can be identified flow of basalts in the India (Plummer and Belle, with the term “lineament,” which is widely applied in 1995). Spreading in the area of the Carlsberg ridge publications. These zones are the result of selective with formation of the transform faults of a mainly processes of slope and linear denudation that control northeastern orientation has been occurring. Accord the borders of the recent structural forms. Usually the ing to the recent data, the Seychelles are slowly sub weak zones are developed by large and small rivers, merging into the Indian Ocean at a velocity of greater intermittent water courses, and streams. This is the than 2.5 cm/100 years (Howell, 2014) or are in a rel reason that during the deciphering of the study areas atively stable state. special attention was paid to the configurations of large rivers and their tributaries, small water courses, and other erosional forms. Linearity, recurrence or MATERIALS AND METHODS systematics, and bending are considered as the main A structural–geomorphological analysis was per features that are applicable for the interpretation of the formed to reveal the recent dislocations of the island. weak zones. Distinguishing the linear sections of river It included deciphering of the topographic map of the valleys allows one to reveal the main trends of the weak island at the scale of 1 : 25000, space images, radar zones that form a structural–geomorphological pat images and compilation and interpretation of geomor tern. The recurrence of the trends of the weak zones phological profiles. Deciphering of joints was per increases the degree of reliability and allows one to formed on the basis of highresolution space images of consider them as tectonic dislocations. Bending of the the distinct granite outcrops at the scale of 1 : 1000. river valleys, i.e., changes of their trends, often indi The results of the deciphering were processed statisti cates the presence of a fracture that crosses the river cally. valley. The weak zones that divide one stairstep sur The main statements that were applied for the face with a larger absolute height from the other stair compilation of structuralgeomorphological maps and step surface with a lower amlitude can be considered as geomorphological profiles were given in (Kostenko, recent fractures. Moreover, footsteps and breaks of 1999). We will give a short outline of some peculiarities slopes are of interest. of the method in application to the studied area. This Thus, mapping of the weak zones is used for the method includes quality and quantity analyses of the analysis of the erosional pattern, which is an indicator topography in the horizontal and vertical sections and of the recent tectonic dislocations, mainly of the dis is directed towards revealing the conerosional (from junctive type in application to the study area. However, the moment of the formation of the structural form in it is necessary to consider the fact that the drainage the topography) dislocations and their types, as well as pattern entails not only the recent but also the ancient the establishment of their correlation with the ancient faults and joints and that the river valleys are confined structures. Disjunctive dislocations of various ranks to the weak zones selectively. The structural–geomor are clearly detectable in the topography of Mahe phological analysis allows us to distinguish horizontal Island, as comprised by granites and granite–diorites dislocations in the fracture zones along with the verti dissected by joints and in topographic maps and space cal dislocations. Such zones can be interpreted as images. strike–slip zones. Deciphering of these materials was performed The rank of the weak zones was also considered. using GIS technologies and the ArcMap software The longest weak zones that cross the island are the package. Estimation of the amplitudes of the uplifts borders of the larger blocks, which are structures of the was conducted on the basis of the position of the great first rank. Disjunctions of lower rank are frequently est height of the mapped surfaces, as presented by the expressed in the topography by relatively shorter struc watersheds of ridges and rises and stairstep surfaces tures that reflecting intrablock deformations. They on their slopes and slopes of river valleys. These sur are deciphered inside of the blocks of the higher ranks. faces are shown with various hatching. We distin To increase the degree of the reliability of the guished seven height gradations that were confined to mapped weak zones and the outlining of the recent the interval of absolute heights from several meters to fractures, geomorphological profiles were compiled. more than 700 m (Fig. 1). The amplitudes of the vertical displacement of certain

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S 4°32′30″

4°35′0″ 4

Victoria 4°37′30″

3 GPS “SEY1” 4°40′0″ GPS “DORIS”

4°42′30″ 2

4°45′0″ 1

4°47′30″

2.5 1.25 0 2.5 5 km

55°22′30″ 55°25′0″ 55°27′30″ 55°30′0″ 55°32′30″ E 123456789

Fig. 1. A structural–geomorphological map of Mahe Island: (1) borders of the blocks of the first rank; (2) weak zones (linea ments); (3–9) absolute height of blocks, m: (3) 0–25, (4) 25–50, (5) 50–100, (6) 100–200, (7) 200–500, (8) 500–700, (9) >700. Blocks of the first rank: 1—Lockyer, 2—Lebouli, 3—Central Seychelles, 4—Howard. topographic blocks were defined using these profiles; stages of the evolution of the recent topography. Con this helped to transfer several weak zones or linea sidering the absence of reliable data on the ages of the ments to the category of fracture zones. Profiles were surfaces we used relative age gradation with the highest run through the watershed surfaces transversely or planation surfaces considered to be the most ancient. along the stretching of orographic forms (ridges and The interpretation of geomorphological profiles valleys) and they went through the maximum and allowed us to distinguish the recent fractures and their minimum absolute height marks of the topography. amplitude. The skewness and asymmetry of the river The vertical scale did not exceed the horizontal one. valleys that was caused by uneven uplift of their slopes, Overall, 14 profiles were compiled. The surfaces of various inclinations of blocks and variations of the stairstep levels were correlated using them. According absolute heights of the surfaces with the same age in to our ideas, the surfaces of the stairstep levels have neighboring blocks were considered as evidence in the the same ages as the planation surfaces that reflect the outlining of the fractures.

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А

B Profile 14 S N 1 900 800 2 700 4 600 6 500 6 6 6 6 7 400 8 8 8 8 300 9 200 9 9 100 11 11 0 0 1200 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 m

Fig. 2. The interpretation of the geomorphological profile: A, the position of the profile on the map; B, the interpretation of pro file 14. Explanations are in the text.

The detailed deciphering of space images at the of the island in the same direction. The large river val scale of 1 : 1000 was performed for the areas of the leys radially disperse from the most uplifted blocks in island where the granite massifs are well exposed. The the central part of the island to its periphery, where rose diagrams were created according to the results of blocks are characterized by lower absolute heights. this deciphering. The data were compared with the The next block to the north, the Lebouli block, is results of structural–geomorphological deciphering of bordered from the north and south by long weak zones the area at the scale of 1 : 25000. that cross the entire island from the west to the east. Weak zones with trending azimuths of 330° and 35° THE RESULTS OF THE STRUCTURAL– prevail in the block. These zones border the inner GEOMORPHOLOGICAL ANALYSIS, THEIR blocks with absolute heights of up to 445 m. The COMPARISON WITH GEOLOGICAL DATA, northern limit of the Lebouli block is a sublatitudinal AND MODERN MOVEMENTS weak zone that separates it from the high central Sey chelles block that has the largest area. This block is Structural–geomorphological studies revealed the characterized by the largest variability of trending of blocky character of the dislocations of Mahe Island the weak zones. Here, the zones with a northeastern that result from the combination of the weak zones of trend with an azimuth that changes in wide range, the northwestern, northeastern, and the rarely sub from 20° to 70° (the azimuth of 35° is dominant) are longitudinal and sublatitudinal trending. From the detected on the background of the predominant weak north to the south of the island the azimuths of the zones with the prevailing trend of 330°. The north trends of the weak zones and their combinations in the western trend of the weak zones is predefined by con largest blocks (blocks of the first rank) changed. These figuration of the southeastern and northeastern coasts peculiarities allowed us to distinguish four such blocks of the islands. This block is characterized by the great that differ by their structural–geomorphological pat est absolute heights, which reach 906 m. A decrease of terns and the amplitude of conerosional rises. These the absolute heights of the inner blocks towards the blocks were conditionally named the Lockyer, Leb coast to 75 m is observed. This decrease is less sym ouli, Central Seychelle, and Howard Block (Fig. 1). metrical relative to the central and most uplifted part The Lockyer block is characterized by the combina of the island. Large river valleys radially dispersed from tion of the weak zones of the northwestern (mainly the high central part of the Seychelles block to the low 310°) and the northeastern (mainly 70°) trends that coasts. limit the blocks of the lower rank with absolute heights The northernmost Howard block unites blocks with of 25–225 m. The prevailing trends of the weak zones lower ranks with absolute heights from 400 to 75 m, are northwestern, which coincide with the stretching which are bordered by weak zones of a northwestern

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S N W E

4°35′0″ S Howard block

4°37′30″

Seychelles block 4°40′0″

4°42′30″ Lebouli block

4°45′0″ Lockyer block

4°47′30″

0 1.25 2.5 5 km 4°50′0″ 55°22′30″ 55°25′0″ 55°27′30″ 55°30′0″ 55°32′30″ E 1234

Fig. 3. A scheme that shows the rose diagrams of weak zones (lineaments) and joints of granites: (1) the borders of the blocks of the first rank; (2) weak zones (lineaments); (3) rose diagrams of the trending of weak zones (lineaments) for blocks of the first rank; (4) rose diagrams of the trending of joints in granite outcrops. and northeastern trend (azimuths of 325° and 35° pre The deciphering of the fractures in granite outcrops vail, respectively). Sublongitudinal trends that define was performed for their comparison with the recent the configuration of the western and eastern coasts dislocations that were revealed by the structural–geo emerge (Fig. 1). morphological analyses. Highresolution space images were used for the deciphering. On the basis of Interpretation of the compiled profiles allowed us to the obtained data the rose diagrams of the orientation distinguish 12 levels of planation surfaces. Thus, the fol of the joints in the Lockyer and Seychelles blocks were lowing surfaces are traced confidently with heights (m): created. These data were then compared with the gen 850–900 or more, 2, 700–800; 3, 650–700; 4, 650– eral rose diagrams of the trends of the weak zones in 600; 5, 600–550; 6, 500–450; 7, 400–350; 8, 250–200; these blocks. As a result it was established that the 9, 200–150; 10, 150–100; 11, 100–50; and 12, <50. dominant trend of the weak zones that was revealed by The deformations of these surfaces along the recent the structural–geomorphological analysis on the fractures with the amplitude of vertical displacement whole matches with the trends of the fractures in the up to 50 m, which is shown on sublongitudinal profile granites (Fig. 3). This indicates that the weak zones in the northern part of the island (Fig. 2). The vertical and structures of a higher rank of the recent stage of displacements of the planation surfaces of the sixth development inherit the fractures in the granites. and eighth levels along the recent fractures with ampli Comparison of the results of the structural–geo tudes of 40–50 m are clearly seen. morphological analysis with the geological data on the

MOSCOW UNIVERSITY GEOLOGY BULLETIN Vol. 71 No. 2 2016 136 PANINA, ZAITSEV structures that encircle the island showed that the is emphasized by the pattern of large river valleys. In northeastern trend (one of the dominant trends) of the their turn, dislocations of the inner dislocations of mapped recent dislocations of Mahe Island is also lower rank are distinguished in these blocks. The characteristic of the fracture zones of the oceanic amplitudes of the blocks of the lower rank are greatest Carlsberg ridge, which is located to the northeast of in the central parts of the island and their absolute the island. heights decrease towards the coastlines forming stair The recent blocky tectonics of Mahe Island, which step topographic surfaces. According to the data of were established by the structural–geomorphological geomorphological profiles, the interpretations of many analyses was also reflected in modern movements, of the weak zones (lineaments) are recent fractures with which is confirmed by the data of instrumental obser amplitudes of vertical displacement of up to 50 m. vations that were conducted in 2002–2012. The data The trends of the weak zones as established by the of the DORIS and JGSSEY1 GPSstations, which structural–geomorphological analysis on the whole are located in the northeastern and central parts of coincide with the trends of the joints of the granite the island, showed similar results of the measure outcrops, which indicates the inheritance of the recent ments of the velocity values of the vertical and hori dislocations from the older ones. zontal movements. The meridional component of the The northeastern trends of the weak zones that movement of the crust according to the ULR5 model limit the blocks of various ranks and joints in the gran (Université de La Rochelle, France) after the data of ites of Mahe Island is characteristic of the transform the SEY1 station, which cover the 2002–2012 faults of the Carlsberg ridge that are located to the period, comprises 11.52 ± 0.29 mm/year and the lat northeast from the island. itudinal (eastern) is 24.84 ± 0.08 mm/year, while the The blocky neotectonics of Mahe Island that were vertical is 3.15 ± 0.67 mm/year (subsidence). According revealed by the structural–geomorphological analysis to the model of the Center of analysis of the IGN/JPL is reflected in modern movements, which is confirmed (Institut Géographique National, France and the Jet by the data of GPS observations. Propulsion Laboratory, United States) after the observa tion data of the DORIS system of stations that cover the 2002–2012 period, the meridional component of the REFERENCES movement velocity (to the north) is 11.8 ± 0.29 mm/year DORIS (Dopper Orbitography and Radiopositioning Inte while the latitudinal component is 25.0 ± 0.2 mm/year grated by Satellte, 2014. http://idsdoris.org/network/ids (movement to the east). The vertical component of the stationseries.html#MAHB). Cited August 25, 2014. movement velocity is 0.3 ± 0.03 mm/year (uplift) Howell, P., Seychelles through time, in NOVA Online Web. (DORIS, 2014; Sonel, 2014). January 26, 2014. http://www.pbs.org/wgbh/nova/eden/ Thus, the above data indicate that the tectonic time/html. Cited July 24, 2014. blocks that comprise Mahe Island demonstrate the Korotaev, V.N. and Nikiforov, L.G., Coastal geomorphol tendency to horizontal movement in the east–north ogy and sediments of underwater slope, in Ostrova eastern direction and vertical movements with varying zapadnoi chasti Indiiskogo okeana (Islands in the West signs against the background of the overall uplift. ern Indian Ocean), Moscow, 1982, pp. 67–94. Kostenko, N.P., Geomorfologiya (Geomorphology), Mos cow, 1999. CONCLUSIONS Plummer, P.S. and Belle, E.R., Mesozoic tectonostrati A structural–geomorphological analysis of Mahe graphic evolution of the Seychelles microcontinent, Island revealed the blocky character of the recent dis Sediment. Geol., 1995, vol. 96, nos. 1–2, pp. 73–91. locations. Four large blocks of the first rank were dis Sonel, 2014. http://www.sonel.org/spin.php?page=gps& tinguished according to differences in the trending of idStation=838. Cited August 12, 2014. the weak zones (lineaments). The configuration of the blocks is defined by the trending of the coastlines and Translated by A. Kokhan

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