, Eurrli and Planetary Science Letters, 85 (1987) 427-438 427 Il Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands a Structural trends in the Southern Cook and Austral archipelagoes (South Central Pacific) based on an analysis of SEASAT data: I I geodynamic implications I ~i M. Diament and N. Baudry ‘i2 I ’ Laboratoire de Géopliysique (UA. du C.N.R.S. 730), Bâtiment 509, Universile Paris-Stid, 91405 Orsuy Cédcx (Frunce) I ORSTOM, B.P. AS, Nounieu Cédex (New Ccdedoniu) Received December 22,1986; revised version acceptedJuly 10,1987 I Filtered SEASAT data have been interpreted in ‘an area covering the Cook-‘Austral archipelagoes (South Central Pacific) in order to detect or confirm the existence of structural directions. SEABEAM data recorded by N/O “Jean Charcot” were also interpreted. Additionally to the Austral fracture zone and Cook-Austral archipelagoes trends, the SEASAT data reveal the existence of two directions, the azimuths of which are N150 O and N95 O respectively. The I first, which intersects the Austral archipelago close to the island of Maria corresponds to a long linear topographic bump. It is interpreted as evidence for the existence of an ancient hot spot with a trace copolar to the Emperor chain. This result explains the very important thermal rejuvenation found previously in that area by several authors. It also I explains the very disturbed ages and morphologies of volcanic structures recorded in the Austral archipelago. The I second direction, located south of the Cook archipelago is underlined by geoid signatures of fracture zone type. These postulated linear features are probably of similar origin to the ones detected previously farther east. They are 1 interpreted as a possible consequence of some recent intraplate deformation. ! I 1. Introduction eight with a precision of 15 km. They also gave an estimation of the height of each seamount and of In spite of the effort of many oceanographical their morphological regularity. They confirmed too I institutions during the past decades, the seafloor the result of Lambeck and Coleman [2] who pos- topography is still very poorly known in many tulated that no bathymetric features were present oceanic areas. Due to the lack of data, only very on the charted position of Fabert Bank (Fig. 1). I tentative geodynamic models of evolution of some During the SEAPSO Leg V cruise onboard the l oceanic areas have been proposed. Such a situa- N/O “Jean Charcot” (ORSTOM/IFREMER tion holds for the South Central Pacific [l]. The cruise, January 1986), SEABEAM surveys were recent knowledge of the marine geoid obtained achieved over the Austral archipelago on the l with satellite altimeter data provides a large charted location of Fabert Bank and on the loca- l amount of information in vast areas previously tions of the detected seamounts S6, S5 and S2. unexplored. The high degree of correlation be- The SEABEAM survey confirmed the predictions tween the seafloor topography and the short-wave- of Baudry et al. [7]. The maximum deviation be- length geoid anomalies has been used in numerous l tween the predicted locations and the observed studies in order to detect uncharted features [2-71. ones appeared to be of the order of 12 km [11,12]. l Maps of geoid [8-101 can be studied as well in Similar results were recently (February 1987) ob- I order to analyse structural trends. In a recent tained over the seamounts S3 and S7 (see Fig. 1) study, Baudry et al. [7] presented the results of an during a cruise of the R/V “Sonne” (U. Von investigation of unsurveyed seamounts and their Stackelberg, personal communication). The sea- i precise location in the western part of the Austral mounts detected in the Austral archipelago (S1 to I archipelago, using SEASAT data. They detected SlO) lie on two well defined trends. Some are ten previously unknown seamounts and located located on a northwest extension of the southern 0012-821X/87/$03.50 O 1987 Elsevier Science Publishers B.V. 428 Fig. 1. Bathymetry of thc area under study after Mammerickx et al. [l].SI to*SlO ace the 10 seamounts detected and located by Baudry et al. [7] using SEASAT data. The route of N/O “Jean Charcot” during the SEAPSO Leg V transit is also represented. Magnetic anomalies are also shown [14]. Austral chain, aiid others along a lineation bearing and the Austral fracture zone (Fig. 1). These fea- N150 O which intersects the archipelago (Fig. 1). tures are emplaced on a wide bathymetric swell The purpose of the present paper is to carry out [13]. Identification of sea-floor spreading magnetic a more detailed investigation of these trends in anomalies [14] shows that anomaly 18 is present order to discuss their possible origin from an close to the MacDonald seamount and anomaly analysis of SEASAT data complemented with the 32 south of Raivavae (Fig. 1). Therefore the age of SEABEAM data gathered by N/O “Jean the lithosphere varies from about 42 m.y. close to Charcot”. Our results will then be interpreted in MacDonald to about 70 m.y. close to Rurutu [15], terms of geodynamic evolution of the South this last age being obtained assuming that the Central Pacific. The area under study extends recorded anomaly 32 can be extended to the north. 17OS to 31OS and 199”E to 222”E and therefore More to the west, the age can be only guessed but is larger than the one used for the detection of the crust is probably older than 80 m.y. west of seamounts and covers part of the Society and Maria island. Thus the ocean floor in this area was Tuamotu Islands (see Fig. 1). created at the East Pacific Rise before its 20 m.y. reorganisation [1,16]. The Austral fracture zone is 2. The Southern Cook and Austral chain area the fossil part of a transform fault which offsets the East Pacific Rise before the jump of the accre- Several linear bathymetric features are present tionary center to its new position. The Cook- in the area under study, such as the Cook-Austral Austral chain and the Society Islands are recent chain, the southern part of the Society Islands, volcanic structures [17,18] with an orientation of 429 NllO O which has been interpreted as the traces of cause of the uplifts of Rimatara, Rurutu and a hot spots on the Pacific plate [18,19]. The mor- Tubuai is unkown. phology of the Austral archipelago consists of two Analysis of the mechanical behaviour of the parallel chains, but such a pattern does not seem lithosphere supporting the Southern Cook and to be an exception for mid-plate volcanic chains. Austral islands and seamounts performed with Menard and McNutt [20] pointed out the ex- various techniques [27-301, yields an abnormally istence of features with very variable morphology thin equivalent elastic thickness for such old litho- in the Austral archipelago; for example the Presi- sphere on which recent loads have been emplaced. dent Thiers Bank is equally broad as Raivavae Indeed the various datations [17,18] of the studied island but completely truncated (see Fig. 1). seamounts and islands give an age ranging from O Therefore, from geomorphology, Menard and Mc- m.y. (McDonald seamount) to 18 m.y. (Mangaia). Nutt [20] proposed an older age for this bank. These values, when compared to the ages of the Indeed, the ages of islands and seamounts of the neighbouring lithospheres [16] yield equivalent Austral and Southern Cook archipelagoes as de- elastic thicknesses ranging approximately from 29 duced from K/Ar dating do not show a consistent km to 40 km. The observed ones [27-301 are age progression to the northwest as required by a comprised between 2.5 km to 15 km. Such a single hot spot model [21]. Of the eleven Cook- discrepancy cannot be explained assuming errors Austral islands for which age determinations are in datations, therefore several explanations were presently available, at least four (Aitutaki, Raro- proposed in order to explain why the lithosphere tonga, Atiu and Mauke) have much younger supporting the features of the Cook-Austral ar- volcanism that can be accounted for by a single chipelagoes present a much smaller rigidity than hot spot presently located at McDonald seamount the theoretical one. These low values of the [21]. Some islands present two periods of volcanism equivalent elastic thickness were interpreted by separated by a very long period of quiescence Lambeck [29] as evidence for a viscoelastic be- [21,22]. For example, on Rurutu, volcanism took haviour of the lithosphere. But this thin equivalent place about 12.5 m.y. ago and, more recently, elastic thickness has been interpreted with more about 1.9 to 0.6 m.y. ago. The existence of at least confidence by Menard and McNutt [20] and also three hot spots in this area was then postulated by Calmant and Cazenave [30] as the effect of [21] in order to explain this age pattern. The thermal rejuvenation proposed before by Detrick difficulty to account for the alignment of several and Crough [31]. Yet it is surprising to notice that hot spots along the absolute motion of the Pacific the thinning of the mechanical lithosphere is much plate led Turner and Jarrard [21] to propose the higher for the Cook-Austral chain (almost 20 km) existence of a “hot line”, as previously proposed than for the Hawaiian-Emperor chain (6-10 km by Bonatti and Harrison [23] and Bonatti et al. only) [20]. Various explanations have been pro- [24] on the Nazca plate. This assumed hot line posed.