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The Southeast Indian Ridge Between 127° and 132°40′E: Contrasts In Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 The Southeast Indian Ridge between 127 ~ and 132~ contrasts in segmentation characteristics and implications for crustal accretion JEAN-CHRISTOPHE SEMPt~RI~ 1, BRIAN P. WEST 1, & LOUIS GI~LI 2 ISchool of Oceanography, Box 357940, University of Washington, Seattle, WA 98195- 7940, USA 2Ddpartement de GOosciences Marines, IFREMER, B.P. 70 29280 PlouzanO, France Abstract: The Australian-Antarctic Discordance is an anomalously deep section of the Southeast Indian Ridge which overlies a colder than normal region of the upper mantle. The Southeast Indian Ridge exhibits large contrasts in its geophysical and geochemical characteristics across the eastern boundary of the Discordance. We present new geophysical data collected along the Southeast Indian Ridge between 127~ and 132~ which define the segmentation characteristics of this portion of the Indo-Australian-Antarctic plate boundary. The Southeast Indian Ridge within our survey area can be broken into three first-order segments bounded by one transform fault and two propagating rifts. The transform fault, located at the west end of the study area, forms the eastern boundary of the Australian-Antarctic Discordance. The morphology of the spreading axis is that of an axial high akin to that found along the East Pacific Rise. The non-transform discontinuities found along the axis of accretion are also similar to those encountered at fast-spreading centres. These characteristics are in marked contrast with those of the spreading axis within the Australian-Antarctic Discordance, which resembles a slow-spreading centre despite its intermediate spreading rate (c. 74 mm a-~). We suggest that the geophysical and geochemical transitions that occur along the Southeast Indian Ridge near the eastern boundary of the Australian-Antarctic Discordance may be ascribed to varying mantle temperature and melt production rate along the spreading centre at constant spreading rate. The two propagating rifts, located near 127~ and 131~ are actively migrating toward the Discordance down the regional, along-axis gradient in bathymetry at the rates of 43 and 46 mm a -1, respectively. Propagating rifts play a dominant role in the reorganization of the spreading centre east of the Australian-Antarctic Discordance. Satellite gravity data reveal a number of V-shaped, west-pointing structures on the flanks of the Southeast Indian Ridge. These structures appear to have been spawned near the George V ridge-transform intersection at 139~ Four of these features display a clear association with active propagating rifts. Some of the remaining features may be due to the migration of melting anomalies entrained by asthenospheric flow along the axis of the Southeast Indian Ridge. The Southeast Indian Ridge (SEIR) extends Australian and Antarctic plates, respectively from the Macquarie Triple Junction to the (Hayes 1988; Marks et al. 1990); hence, it Rodriguez Triple Junction. This intermediate- appears to have migrated westward with time spreading centre separates the Indo-Australian with respect to the SEIR (Marks et al. 1990). and Antarctic plates. Between 120~ and 128~ The SEIR within the AAD is associated with a the SEIR traverses the Australian-Antarctic deep rift valley whereas the SEIR east of the Discordance (AAD), one of the most enigmatic AAD corresponds to an axial high (Weissel & features of the ocean basins. The AAD is a Hayes 1974; Anderson et al. 1980; Semp6r6 et al. region of anomalously deep seafloor character- 1991). The existence of a sharp change in axial ized by rough topography and closely-spaced morphology at the eastern boundary of the fracture zones (Hayes & Conolly 1972; Weissel AAD is confirmed by satellite-derived gravity & Hayes 1974; Semp6r6 et al. 1991). Seismic data (Sandwell & Smith 1992). The geophysical studies have shown that the AAD is underlain by contrast across this single transform coincides a mantle which has faster than normal shear with distinct changes in major and trace element wave velocities and therefore which may be geochemistry (Anderson et al. 1975; Christie et unusually cold (Forsyth et al. 1987; Kuo 1993). al. 1990; Klein etal. 1991). The existence of such The residual depth anomaly which characterizes geophysical and geochemical contrasts at con- the AAD extends NNE and SSE across the stant spreading rate provides an opportunity to From: MacLeod, C. J., Tyler, P. A. & Walker, C. L. (eds) 1996, Tectonic, Magmatic, Hydrothermaland Biological Segmentation of Mid-Ocean Ridges, Geological Society Special Publication No. 118, pp. 1-15. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 2 J.-C. SEMPt~RI~ ETAL Fig. 1. The study area is located over the Southeast Indian Ridge between 127~ and 133~ east of the Australian-Antarctic Discordance (see inset). We have acquired SeaMARC II bathymetry and side-scan sonar data over the spreading centre. Ship tracks are overlain on satellite gravity data (Sandwell & Smith 1992). The transform fault located near 127~ at the west edge of the survey area is the eastern boundary of the Australian-Antarctic Discordance (AAD). AUS, Australian plate; ANT, Antarctic plate. examine parameters other than spreading rate structures may be linked to the development of that control crustal accretion. the AAD approximately 30 Ma ago, and to The AAD contains the boundary between the westward asthenospheric flow along the plate Pacific and Indian Ocean isotopic reservoirs boundary. (Klein et al. 1988). Pyle et al. (1992) have In this paper, we present new bathymetric and proposed that this boundary has migrated side-scan sonar data obtained along the axis of westward over the last 30 Ma, and that migration the SEIR immediately east of the AAD between of Pacific mantle may have started when the 127~ and 132~ (Fig. 1). We use these South Tasman Rise separated from Antarctica observations to establish the nature of the about 30 Ma ago. In addition to its association transition in axial topography at the eastern with the migration of the isotopic boundary and boundary of the AAD and to contrast the residual depth anomaly, the AAD is a zone of different segmentation characteristics observed convergence of multiple propagating rifts along at constant spreading rate (c. 74 mm a -1) within the SEIR (Vogt et al. 1984). Satellite-derived this region. Finally, we use satellite gravity data gravity data confirm the existence of propagat- to study the evolution of the segmentation and ing rifts, but indicate the presence of other we speculate on the origin of the numerous V-shaped structures pointing toward the Discor- V-shaped structures present on the flanks of the dance east of the AAD (Fig. 1). Some of these Southeast Indian Ridge east of the AAD. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 THE SE INDIAN RIDGE 3 Fig. 2. (Top) Bathymetry of the Southeast Indian Ridge between 127~ and 132~ (contour interval = 500 m). The large arrows indicate the direction of migration of the two propagating rifts (PR) surveyed. The small arrows show the relative motion across the transform fault at the west end of our coverage. This transform fault marks the eastern boundary of the Australian-Antarctic Discordance. The bold line indicates the inferred location of the spreading axis. (Bottom) The along-axis depth profile shows the regional increase in depth to the west toward the Australian-Antarctic Discordance. The two propagating rifts, as well as minor discontinuities along the plate boundary, correspond to local depth maxima. Data acquisition The Southeast Indian Ridge between 127~ and 132~ The data presented in this paper were acquired using the SeaMARC II side-scan sonar system General characteristics during leg MW8801 on the RV Moana Wave (Fig. 1). SeaMARC II was a 11-12 kHz, shallow- Between 127~ and 132~ the Southeast towed sonar system operated by the University Indian Ridge consists of three first-order seg- of Hawaii and was able to gather bathymetry and ments, spreading at a rate of c. 74mma -1, backscatter information simultaneously (Black- separated by a transform fault and two propa- ington et al. 1983). Resolution of the SeaMARC gating rifts (Fig. 2). The left-slipping transform II side-scan sonar is 5 m and 175 m in the across- fault, which is located near 127~ and strikes and along-track directions, respectively (Tyce 010 ~ forms the eastern boundary of the Aus- 1987). SeaMARC II bathymetry is sufficiently tralian-Antarctic Discordance. The age offset accurate to be contoured with a 100m contour across this 175km long transform is approxi- interval for our survey area. The side-scan sonar mately 3.8Ma (Vogt et al. 1984; Palmer et al. data were edited for bad pings and merged with 1993). The morphology of the SEIR within the corrected navigation. Contrast maps used to AAD west of this transform is discussed in translate the 256 levels of recorded side-scan Palmer et al. (1993). The other two major sonar information into 16 grey shades have been discontinuities along the plate boundary within produced following corrections for changes in our study area are propagating rifts located near system gain. Survey lines were generally run at 127~ and 131~ Based on satellite gravity 45 ~ to the strike of the seafloor fabric to optimize data (Sandwell & Smith 1992), the next major the use of the side-scan sonar. Navigation discontinuity along the SEIR east of our cover- consisted of GPS fixes for about 10 hours per age is a third propagating rift located near 135~ day, and a combination of Transit satellites and Within our survey area, the depth of the dead reckoning when GPS was not available. spreading centre progressively increases by Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 4 J.-C. SEMPERE ET AL Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 THE SE INDIAN RIDGE 5 Fig.
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