Christine Deplus

Thin crust, ultramafic exposures, and rugged faulting patterns at the Mid-Atlantic Ridge (22°–24°N)

Off-axis rock sampling in the lat 22°–24° N region of the Mid-Atlantic Ridge shows that the emplacement of mantle-derived rocks in the sea floor has been a common process there for the past few million years. We find a good correlation between domains of positive residual gravity anomalies (inferred to have a thin crust) and the distribution of ultramafic samples. We also find that thin-crust domains have a rugged topography, thought to reflect strong tectonic disruption. We propose that these thin-crust domains are made of tectonically uplifted ultramafic rocks, with gabbroic intrusions and a thin basaltic cover. We also suggest that strong tectonic disruption may be a direct consequence of the lithological and rheological heterogeneity of these thin-crust domains.

Submarine evidence for large-scale debris avalanches in the Lesser Antilles Arc

Results from a recent marine geophysical survey demonstrate the importance of the process of flank collapse in the growth and evolution of volcanoes along an island arc. The Aguadomar cruise, aboard the French R/V L’Atalante, surveyed the flanks of the Lesser Antilles Arc between the islands of Montserrat and St. Lucia. Analysis of the data shows that flank collapse events occurred on active volcanoes all along the arc and resulted in debris avalanches, some of them being of large magnitude. The debris avalanche deposits display hummocky topography on the swath bathymetry, speckled pattern on backscatter images, hyperbolic facies on 3.5 kHz echosounder data and chaotic units on air gun seismic profiles. They extend from horseshoe-shaped structures previously identified on the subaerial part of the volcanoes. In the southern part of the arc, large-scale debris avalanche deposits were identified on the floor of the Grenada Basin west of active volcanoes on Dominica, Martinique and St. Lucia. The extent of debris avalanche deposits off Dominica is about 3500 km2. The debris avalanches have resulted from major flank collapse events which may be mainly controlled by the large-scale structure of the island arc and the presence of the deep Grenada Basin. In the northern part of the arc, several debris avalanche deposits were also identified around the island of Montserrat. With smaller extent (20–120 km2), they are present on the east, south and west submarine flanks of Soufriere Hills volcano which has been erupting since July 1995. Flank collapse is thus a recurrent process in the recent history of this volcano. The marine data are also relevant for a discussion of the transport mechanisms of debris avalanches on the seafloor surrounding a volcanic island arc.

Mid-Atlantic Ridge–Azores hotspot interactions: along-axis migration of a hotspot-derived event of enhanced magmatism 10 to 4 Ma ago

Abstract A recent survey of the Mid-Atlantic Ridge over the southern edge of the Azores Platform shows that two anomalously shallow regions located off-axis on both sides of the ridge are the two flanks of a single rifted volcanic plateau. Crustal thickness over this plateau is up to twice that of surrounding oceanic areas, and original axial depths were near sealevel. The lack of a coherent magnetic anomaly pattern, and the near absence of fault scarps over the plateau suggest that its formation involved outpouring of lava over large distances off-axis. This volcanic plateau formed in Miocene times during an episode of greatly enhanced ridge magmatism caused, as proposed by P.R. Vogt [Geology 7 (1979) 93–98], by the southward propagation of a melting anomaly originated within the Azores hotspot. This melting anomaly could reflect excess temperatures of ∼70°C in the mantle beneath the ridge. It propagated at rates of ∼60 mm/yr and lasted no more than a few million years at any given location along the ridge. Enhanced magmatism due to this melting anomaly played a significant role, some 10 Ma ago, in the construction of the Azores Platform.

Mentawai fault zone off Sumatra: A new key to the geodynamics of western Indonesia

The geodynamic evolution of the western part of the Sunda arc is controlled by the change from frontal subduction of the Indo-Australian plate along Java to oblique subduction along Sumatra. This obliquity gives rise to the Sumatra fault zone that links the accretionary zone of the Andaman Sea to the Sunda Strait. Previous studies have shown a decrease of displacement rate of the movement along the fault zone from north to south. Consequently, it has been proposed that the area between the subduction zone and the fault zone—i.e., the Sumatra sliver platelet—was deformed. An oceanographic cruise on the Indonesian ship R/V Baruna Jaya III was designed to study this area. Seismic reflection data reveal the existence of a major strike-slip fault, parallel to the Sumatra fault zone, that we called the Mentawai fault zone, located in the fore-arc area just east of the Mentawai Islands; it is at least 600 km long. Thus, the Sumatra sliver plate appears to be composed of several strips that move toward the northwest to accommodate the oblique subduction.

Characteristics and evolution of the segmentation of the Mid-Atlantic Ridge between 20°N and 24°N during the last 10 million years

Abstract High-resolution bathymetry and geophysical data collected along the slow-spreading axis and flanks of the Mid-Atlantic Ridge between 20°N and 24°N reveal the characteristics and history of different wavelengths of segmentation during the last 10 m.y. The bathymetric data exhibit a morphotectonic pattern dominated by ridge-normal and oblique bathymetric lows that partition the ridge flanks into rhomb-shaped areas of relatively high elevation. At least four different types of oblique bathymetric lows have been identified which represent the off-axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge-axis segmentation. One group of oblique structures is represented by two deep ridge-normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis. These oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis, while maintaining a constant offset of the latter. Two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge-parallel bathymetric lows symmetric about the ridge axis. Both types of oblique structures frequently change their orientation (from normal to subparallel to the ridge axis) and appear to merge and diverge off-axis. These oblique depressions are characterized by positive filtered mantle Bouguer anomalies, high magnetizations, complex magnetic anomaly patterns, and possible exposure of mantle lithologies. The ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis, with propagation rates varying from 0 to 25 km m.y. −1 . The last and smallest discontinuities observed in this area correspond to small ridge-axis offsets and off-axis traces identified by alignments of the terminations of abutting abyssal hills. The ridge-flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt. The centres of the rhomb-shaped areas correspond to maximum crust production and thin lithosphere, and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites. We propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years.

Direct evidence of active deformation in the eastern Indian oceanic plate

Conventional plate tectonics theory postulates that plates only deform on their boundaries. To the contrary, there is ample evidence of intraplate deformation in the equatorial Indian Ocean, west of the Ninetyeast aseismic ridge. Prior to this study, no direct evidence of deforma- tion east of the Ninetyeast Ridge was available. We present the results of a multipurpose geo- physical cruise showing that intraplate deformation also occurs in this area. Long, at least 1000 km, left-lateral north-south strike-slip faults are active and reactivate fossil fracture zones. This style of deformation is strikingly different from the east-west folds and reverse faults that affect the region west of the Ninetyeast Ridge. Contrasting processes of convergence at the northern plate boundaries can account for the two styles of deformation. West of the Ninetyeast Ridge there is a continent-continent collision, and east of the ridge oceanic lithosphere subducts along the Sumatra trench. The Ninetyeast aseismic ridge therefore appears to be a mechanical border separating two distinct deformed areas.

Geomorphological evolution of Montserrat (West Indies): importance of flank collapse and erosional processes

Analysis of topography and new swath bathymetry as well as geophysical data provides information about aerial and submarine morphological features and mass transfer processes on Montserrat. The island has a characteristic shallow (<100 m) submarine shelf, interpreted as having been formed through erosion with a depth controlled by glacio-eustatic sea-level variation. Several debris avalanche deposits are identified on the lower submarine flanks of Soufrière Hills volcano, and there is evidence of lateral collapses at the older volcanic centres. The morphological evolution of Montserrat is interpreted in terms of three stages. The first stage comprises submarine growth. The second stage, subaerial growth, is represented by the active South Soufrière Hills–Soufrière Hills volcanic centre. During the current eruption of Soufrière Hills volcano (1995–2002) more than half of the lava erupted was transported into the sea. Flank collapses occurred several times during this stage, such as the Englishs Crater event (c. 4000 years ago) or the Boxing Day event during the current eruption (26 December 1997). Montserrats older volcanic centres, the Centre Hills and Silver Hills, illustrate the third stage of evolution, extinction and erosion. Magma production, long-term erosion and total sedimentation rates on Montserrat have been estimated as 0.17 km3 ka−1, 0.0125 km3 ka−1 and 0.11 km3 ka−1 (i.e. 1.1 cm ka−1), respectively.

Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995–2003): What happens when pyroclastic flows enter the ocean?

The Soufriere Hills volcano, Montserrat, West Indies, has undergone a series of dome growth and collapse events since the eruption began in 1995. Over 90% of the pyroclastic material produced has been deposited into the ocean. Sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea. The coarse components (pebbles to boulders) are deposited proximally from dense basal slurries to form steep-sided, near-linear ridges that intercalate to form a submarine fan. The finer ash-grade components are mixed into the overlying water column to form turbidity currents that flow over distances >30 km from the source. The total volume of pyroclastic material off the east coast of Montserrat exceeds 280 × 106 m3, with 65% deposited in proximal lobes and 35% deposited as distal turbidites.

The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3

This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V “Jean Charcot”. The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km2 and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65° oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is offset left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary?

Late Pleistocene tephrochronology of marine sediments adjacent to Montserrat, Lesser Antilles volcanic arc

The recent history of the Soufrière Hills volcano, Montserrat, Lesser Antilles volcanic arc, is deduced using data obtained from a submarine core collected in 2002. The core contains concentrations of ash and several tephra layers, which are identified by the abundance of glass shards, dense and poorly vesiculated particles, and scoria. The tephra layers have been dated using micropalaeontology and stable isotope stratigraphy. Tephra layers in a marine sediment core off the coast of Montserrat record the volcanic history of South Soufrière Hills–Soufrière Hills volcano back to 250 ka. Eight layers are composed of dense juvenile ash related to dome eruptions, five of which can be directly correlated to dated domes or related pyroclastic flow sequences on land. Six layers are composed of pumiceous glassy ash and relate to significant explosive eruptions. A marker sequence of basalt tephra layers is dated at 124–147 ka and is correlated with construction of the South Soufrière Hills basaltic stratocone. Pelagic sediments between the main tephra layers have low abundances of volcanogenic components (<15%) and suggest long periods (c. 104 years) of dormancy or low activity.