Marc Munschy

MODMAG, a MATLAB program to model marine magnetic anomalies

Identifying marine magnetic anomalies is the most common way to date the ocean floor. Although the technique of magnetic anomaly identification has not changed since the 1960s, a forward modeling software that is easy to use, fast and automatic, without abstruse parameters, was lacking. We present a user-friendly MATLAB-based interface, called MODMAG, which allows one to perform forward modeling of marine magnetic anomalies resulting from several successive spreading periods with different spreading rates and asymmetric spreading possibly alternating with axial jumps. The main advantage of our program is that the management of the magnetized bodies resulting from such successive spreading periods is not the users responsibility. Spreading parameters can be set easily for the picking of the marine magnetic anomalies. Non-specialist geophysicists or geologists can therefore easily identify marine magnetic anomalies with the help of MODMAG.

Structure and evolution of the Kerguelen-Heard Plateau (Indian Ocean) deduced from seismic stratigraphy studies

Abstract The structure and evolution of the Kerguelen-Heard Plateau, the northern domain of the Kerguelen-Gaussberg Ridge, is derived from stratigraphic interpretation of multichannel seismic profiles. The Kerguelen-Heard Plateau was created between 130 and 100 Ma at or near an active spreading center in the gap between the Antarctica-Australia plate and the India plate. From the early Late Cretaceous (about 100 Ma) to the Eocene (42–45 Ma) the Kerguelen-Heard Plateau was a shallow marine structure continuously subsiding at a rate of about 20 m Ma −1 and covered by pelagic and shelf sediments. At 42–45 Ma the Kerguelen-Heard Plateau and Broken Ridge were clearly separated by sea-floor spreading at the Southeast Indian Ridge. From 42–45 Ma to the Miocene, a major gap of sedimentation occurred. Later the Kerguelen-Heard Plateau was covered by pelagic sediments, interbedded with thick clastic sedimentary layers coming essentially from Kerguelen Island.

A wide ocean-continent transition along the south-west Australian margin: first results of the MARGAU/MD110 cruise

Syn-rift exhumation of mantle rocks in a continental breakup zone was highlighted along the present-day west Iberian passive margin [e.g. Boillot et al., 1988, 1995; Whitmarsh et al., 1995, 2001; Beslier et al., 1996; Brun and Beslier, 1996; Boillot and Coulon, 1998; Krawczyk et al., 1996; Girardeau et al., 1998] and along the fossil Tethyan margins [e.g. Froitzheim and Manatschal, 1996; Manatschal and Bernoulli, 1996; Marroni et al., 1998; Muntener et al., 2000; Desmurs et al., 2001]. Along the west Iberian margin, serpentinized peridotite and scarce gabbro and basalt lay directly under the sediments, over a 30 to 130 km-wide transition between the thinned continental crust and the first oceanic crust [Girardeau et al., 1988, 1998; Kornprobst and Tabit, 1988; Boillot et al., 1989; Beslier et al., 1990, 1996; Cornen et al., 1999]. The formation of a wide ocean-continent transition (OCT), mostly controlled by tectonics and associated with an exhumation of deep lithospheric levels, would be an essential stage of continental breakup and a characteristic of magma-poor passive margins. The southwest Australian margin provides an opportunity to test and to generalize the models proposed for the west Iberian margin, as both margins present many analogies. The south Australian margin formed during the Gondwana breakup in the Mesozoic, along a NW-SE oblique extension direction [Willcox and Stagg, 1990]. From north to south, the continental slope is bounded by (1) a magnetic quiet zone (MQZ) where the nature of the basement is ambiguous [Talwani et al., 1979; Tikku and Cande, 1999; Sayers et al., 2001], (2) a zone where the basement shows a rough topography associated with poorly expressed magnetic anomalies [Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999; Sayers et al., 2001], and which is the eastward prolongation of the Diamantina Zone, and (3) an Eocene oceanic domain. The continental breakup zone is believed to be located near or at the southern edge of the MQZ [Cande and Mutter, 1982; Veevers et al., 1990; Sayers et al., 2001]. Breakup is dated at 125 Ma [Stagg and Willcox, 1992], 95 ± 5 Ma [Veevers, 1986] or at 83 Ma [Sayers et al., 2001], and followed by ultra-slow seafloor spreading until the Eocene (43 Ma), and fast spreading afterwards [Weissel and Hayes, 1972; Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999]. The western end of the margin (fig. 1) is starved and bounded in the OCT by basement ridges where peridotite, gabbro and basalt were previously dredged [Nicholls et al., 1981]. Altimetry data [Sandwell and Smith, 1997] show that some of these ridges are continuous over 1500 km along the OCT of the south Australian margin and of the conjugate Antarctic margin. The objectives of the MARGAU/MD110 cruise (May-June 1998; [Royer et al., 1998]; fig. 2) were to define the morpho-structure and the nature and evolution of the basement in the SW Australian OCT. An area of 180 000 km2 was explored with swath bathymetry. Gravimetric data (11382 km) were simultaneously recorded whereas few single channel seismic (1353 km) and magnetic (5387 km) data were obtained due to technical difficulties. Crystalline basement rocks, made of varied and locally well-preserved lithologies, were dredged at 11 sites located on structural highs.

Seismic stratigraphy of the Raggatt Basin, southern Kerguelen Plateau: tectonic and paleoceanographic implications

During Cenozoic and late Mesozoic time, sediment accumulated in the Raggatt Basin on the southern Kerguelen Plateau. We describe the seismic stratigraphy of the Raggatt Basin, utilizing multichannel seismic (MCS) data obtained by the Bureau of Mineral Resources, Geology and Geophysics (Australia) and the Institut de Physique du Globe de Strasbourg (France). Seven major seismic stratigraphic sequences in the Raggatt Basin overlie a basement complex of Early Cretaceous age. The underlying basement complex is characterized by two types of seismic images: acoustic basement and a layered basement which includes dipping reflectors. The seismic stratigraphic sequences include terrestrial and shallow-water units K1 and K2 of Early to Late Cretaceous age, which fill depressions in the basement; a thick unit, K3, of Late Cretaceous to Paleocene age, which is of mixed shallow-water and open-marine facies and in places has a mounded (carbonate) upper surface; a depression-filling pelagic deposit, P1, of Late Cretaceous to Eocene age; a thick pelagic unit, P2, which is mainly Eocene in age; and two post-Eocene pelagic sequences, PN1 and NQ1, which are relatively thin and more limited in areal extent than the underlying sequences. Highlights in the geologic history of the Raggatt Basin include formation and erosion of the basement complex in a subaerial or shallow-water environment in Early Cretaceous time; differential subsidence, probably thermal in origin, and development of carbonate mounds and a major western boundary transform fault in Late Cretaceous time; renewed subsidence near the Cretaceous-Tertiary boundary possibly related to formation of the Labuan Basin/Diamantina Zone by sea-floor spreading; and vigorous Antarctic Circumpolar Current activity beginning by Oligocene time, probably related to the breakup of the northern Kerguelen Plateau and Broken Ridge.

Tectonomagmatic evolution of the final stages of rifting along the deep conjugate Australian-Antarctic magma-poor rifted margins: Constraints from seismic observations

The processes related to hyperextension, exhumed mantle domains, lithospheric breakup, and formation of first unequivocal oceanic crust at magma-poor rifted margins are yet poorly understood. In this paper, we try to bring new constraints and new ideas about these latest deformation stages by studying the most distal Australian-Antarctic rifted margins. We propose a new interpretation, linking the sedimentary architectures to the nature and type of basement units, including hyperextended crust, exhumed mantle, embryonic, and steady state oceanic crusts. One major implication of our study is that terms like prerift, synrift, and postrift cannot be used in such polyphase settings, which also invalidates the concept of breakup unconformity. Integration and correlation of all available data, particular seismic and potential field data, allows us to propose a new model to explain the evolution of magma-poor distal rifted margins involving multiple and complex detachment systems. We propose that lithospheric breakup occurs after a phase of proto-oceanic crust formation, associated with a substantial magma supply. First steady state oceanic crust may therefore not have been emplaced before ~53.3 Ma corresponding to magnetic anomaly C24. Observations of magma amount and its distribution along the margins highlight a close magma-fault relationship during the development of these margins.

Structure and tectonic history of the southern Kerguelen Plateau (Indian Ocean) deduced from seismic reflection data

Early single-channel and recent multichannel seismic reflection data together reveal major tectonic events in the history of the Southern Kerguelen Plateau (SKP). The SKP was created by Early Cretaceous volcanism which was mainly subaerial. We estimate the age of the adjacent ocean basins to be similar to that of the plateau. Following termination of the main volcanism, the SKP began to subside and to accumulate sediment. A lower sedimentary megasequence was deposited at this time, primarily in depressions on the plateau. At about 88 Ma, tectonism affected the eastern Raggatt Basin; this tectonism may be related to faulting which created the steep eastern margin of the SKP. At about 72 Ma, predominant tectonic activity, characterized by widespread extension and uplift forming the tilted block morphology, occurred over large areas of the SKP. The extension centered along several NW-SE trending rift systems, in places creating well-preserved axial rifts. The largest structure of this kind, the Central SKP Uplift, lies along the center of the SKP; its relief increases from the southernmost part of the plateau toward the NW. The northernmost part of this uplift, the Banzare Bank, was elevated during rifting above sea level and was eroded. A second extensional structure, the SW Uplift, lies in the SW corner of the plateau, and other, smaller structures may also be present. The extension appears to have culminated in the initiation of the Southeast Indian Ridge at 43 Ma, but, at least in the Raggatt Basin, the two events were not continuous. Tectonic subsidence that was associated with the extension corresponds to deposition of a second megasequence, estimated to be about 1000 m thick on Banzare Bank. The subparallel trend of the rift systems on the SKP to the Southeast Indian Ridge and their timing suggest that the rearrangement of spreading in the South Indian Ocean at 43 Ma was not solely the result of the collision of India with Asia; it started earlier in association with other plate motions in the area.

Relationship of the Central Indian Ridge segmentation with the evolution of the Rodrigues Triple Junction for the past 8 Myr

Located near 25°33′S, 70°00′E, the Rodrigues Triple Junction is the joining point of the intermediate-spreading Southeast Indian and Central Indian Ridges with the ultraslow spreading Southwest Indian Ridge. Bathymetric data and magnetic anomalies are used to analyze the relationship between the evolution of the Central Indian Ridge segmentation and the evolution of the Rodrigues Triple Junction for the past 8 Myr. The Central Indian Ridge domain exhibits a complex morphotectonic pattern dominated by ridge-normal and oblique bathymetric lows interpreted as the off-axis traces of axial discontinuities. The short-lived nontransform discontinuities as well as the segments that lengthen or shorten along the ridge axis reveal that the Central Indian Ridge segmentation is unstable near the Rodrigues Triple Junction. The combined study of the Central Indian Ridge and Southeast Indian Ridge domains shows that the triple junction evolves between two modes: a continuous mode where the Central Indian Ridge and Southeast Indian Ridge axes are joined and a discontinuous mode where the two ridge axes are offset. Owing to spreading asymmetry, and differences in axis direction or in lengthening rates of the Central Indian and Southeast Indian ridges, the continuous mode is unstable and evolves rapidly (<2 Myr) into a discontinuous mode. This last one is more stable and can evolve into a continuous mode only through the formation of a new Central Indian Ridge segment, which takes place facing the northern Southeast Indian Ridge segment. The evolution of the Rodrigues Triple Junction configuration and the evolution of the Central Indian Ridge segmentation are thus closely related.

Structure and early history of the Labuan Basin South Indian Ocean

Recent multichannel seismic reflection data from the Labuan Basin, in the Southern Indian Ocean, are used to reevaluate older, single channel data from this region. Together, they throw light on the structure and evolution of this basin, situated between the older than 100 Ma Southern Kerguelen Plateau and the younger than 43 Ma Australian-Antarctic Basin. The Labuan Basin is a deep, extensive basement depression, more than 350,000 km2 in area, located adjacent to the eastern margin of the Southern Kerguelen Plateau. In contrast to the nearby Kerguelen Plateau, no prominent reflectors are observed within the basement of the Labuan Basin. This suggests a different mechanism of formation from that of the Kerguelen Plateau, which is believed to have been formed by thick sequences of large lava flows. The basement surface of most of the Labuan Basin is presently quite rough, as the result of a tectonic event which created prominent tilted block structures and turned it into a large northwest-southeast trending syncline. There is a 1–1.5 km elevation difference between the Labuan Basin and the Australian-Antarctic Basin created at the Southeast Indian Ridge, as well as a large difference in sedimentary thickness between them, indicating that the Labuan Basin is significantly older. The boundary of the Labuan Basin with the Kerguelen Plateau is generally a steep and somewhat linear feature which appears to be of tectonic origin: it seems to result from two extensive tectonic episodes, dated at 96 Ma and 75–68 Ma, associated with the prerift phase of plate breakup between Australia and Antarctica. The basement of the Labuan Basin was created 130–100 Ma ago at about the same time than the Kerguelen Plateau. An extensive tectonic episode, to the south of the Labuan Basin, initiated the formation of the boundary between the Southern Kerguelen Plateau and the Labuan Basin. This tectonic episode could be linked to the beginning of seafloor spreading between Australia and Antarctica 96 Ma ago. Sediments were regularly deposited in the basin until 75–68 Ma, at which time an important extensional tectonic event occurred. It involved two large northwest-southeast uplifts; one, centered on the Kerguelen Plateau, affected the western part of the Labuan Basin while the other, of which only the western half is observed, affected the eastern part of the Labuan Basin. This tectonic episode seems to correspond to the prerifting episode leading to the breakup between the Kerguelen Plateau-Labuan Basin and Broken Ridge-Diamantina Zone at 43 Ma. After this tectonic episode, the sediments of the postuplift megasequence filled topographic lows formed by the normal faults and the tilted blocks. The Diamantina Zone, which prior to 43 Ma was continuous with the Labuan Basin, can be expected to have a similar tectonic history. In particular, the rough basement of the Diamantina Zone can be due to postformation tectonism rather than to slow spreading as previously suggested.

Periodicity in the accretion process on the Southeast Indian Ridge at 27°40′S

Abstract Image processing of a Seabeam bathymetric grid on the Southeast Indian Ridge (SEIR) at 27°40′S has been used to analyse the morphotectonics of the ridge flanks. Steep topographic slopes indicating inward-facing or outward-facing faults and those representing tilted blocks were automatically separated. An autocorrelation process allows an estimation of the periodicity of the fault spacing. The outward-facing faults first appear at 16–18 km from the axial valley, are more common on bathymetric highs, and correspond to tilted block zones in adjacent deeper areas. The tilting of the blocks is ~ 4°. The transformation of the rift valley into the smooth undulating relief of the flanks is accomplished by these outward-facing faults and tilted blocks. Inward-facing faults appear either as large faults with major throws or as a series of smaller faults with minor throws. The minor faults are mainly localized on bathymetric highs and are densely organized. The major faults, continuous and symmetrically spaced on each side of the axis zone, are organized with a periodicity of ~ 8.2 km. These faults seem to be old inner walls which have been moved outwards periodically during the spreading process. They alternate with positive magnetic lineaments reflecting old neovolcanic zones, linked to intense magmatic periods. Considering a mean half-spreading rate of 31 km/m.y. during the past million years, this would mean the development of a major fault each ~ 0.26 m.y. We propose a model for the evolution of the SEIR at 27°40′S controlled by a periodic succession of magmatic and tectonic episodes: 1. (1) faulting and volcanism are concentrated in the axial zone above the melt reservoir where the crust is weakened (magmatic period) — outside this zone the external relief is frozen-in, supported by the finite strength of the plate; 2. (2) the axial valley is enlarged, fills up with lavas and bulges due to thermal effects; 3. (3) at the end of the magmatic period, this bulged zone collapses by thermal subsidence along lines of weakness such as the faults forming the inner walls; 4. (4) when the melt reservoir is empty (tectonic period), the processes are no longer concentrated and active faulting extends over tens of kilometres from the axis — the lithosphere is thinned, a new axial valley is created, the flanks are uplifted, and outward-facing faults and block tilting are induced.

STRUCTURE AND MORPHOLOGY OF SUBMARINE VOLCANISM IN THE HOTSPOT REGION AROUND REUNION ISLAND, WESTERN INDIAN OCEAN

The form of the Deccan-Maldives-Mascarene-Reunion hotspot trace suggests that it has, at least in part, been strongly controlled by crustal structures, especially fracture zones. This makes it difficult to assess the present-day or past location of the hotspot, and thus complicates the interpretation of African plate motion reconstruction. We present here results of a cruise to the Reunion area of which the aims were: (a) to determine the extent of present-day volcanism associated with the Reunion hotspot in the region; and (b) to examine the role of pre-existing oceanic crustal structures in controlling the location of present-day volcanism. Additionally, we examined the morphology and geology of the important extinct spreading centre southwest of Reunion abandoned when spreading jumped to separate Seychelles from India during the Deccan flood basalt episode some 60–65 Ma ago. The extensive bathymetrie, seismic and geological investigation shows that significant present-day hotspot volcanism is confined to the Piton de la Fournaise edifice on Reunion Island itself. Apparently, the location of recent Reunion volcanism has not been controlled by a crustal fracture and the major fracture zones on both sides of the island are not acting as magma conduits. For plate motion reconstruction and plume flux calculation purposes, Piton de la Fournaise must be taken as the present location of the Reunion hotspot. Accretion at the extinct spreading centre progressively ceased at the time of anomaly A27 (63 Ma), and was associated with marked propagation of the rift tips.