Mathilde Cannat

Emplacement of mantle rocks in the seafloor at mid‐ocean ridges

This paper discusses the geological and geophysical data available on mid-ocean ridges with outcrops of serpentinized mantle peridotites, with the objective of better constraining the modes of emplacement of these rocks in the seafloor. Ridges with serpentinized peridotites outcrops are in most cases characterized by slow-spreading rates, and in every case by deep axial valleys. Such deep axial valleys are thought, based on geophysical constraints and on mechanical modelling results, to characterize ridges with a thick axial lithosphere. A predictable effect of a thick axial lithosphere is that it should prevent magmas from pooling at crustal depths in a long-lasting magma chamber: gabbroic magmas should instead form shortlived dike or sill-like intrusions. Samples from axial outcrops of serpentinized peridotites are often cut by dikelets of evolved gabbros which are interpreted as apophyses of such dike and sill-like intrusions. This observation leads to a discontinuous magmatic crust model, in which mantle-derived peridotites form screens for numerous gabbroic intrusions. This discontinuous magmatic crust is expected to form in magma-poor ridge regions, where there is not enough magma to produce a 4-to 7-km-thick magmatic crust, and where the uppermost kilometers of oceanic lithosphere therefore have to be at least partially made of tectonically uplifted mantle material. Because the dimensions of individual mantle-derived ultramafic screens may be smaller than seismic experiments detection limits, the discontinuous magmatic crust model discussed in this paper may produce a layer 3-type seismic signature, even without extensive serpentinization of its ultramafic component. It therefore provides an alternative to Hesss [1962] serpentinite layer 3 model, for the geological interpretation of seismic data from oceanic areas with frequent outcrops of deep crustal and mantle-derived rocks.

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.

How thick is the magmatic crust at slow spreading oceanic ridges

Thermal models based on the geometry of axial segmentation, and mechanical models of axial valley formation, suggest that the axial lithosphere of slow spreading ridges can be thicker than the crust modeled from gravity data, or measured from seismic data. This is particularly the case in thin crust ridge regions that are near axial discontinuities. The temperature field modeled in these thin crust ridge regions is such that some of the melts extracted from the asthenosphere should crystallize in the mantle, before they reach the crust. This prediction is consistent with observations made on gabbroic and ultramafic samples collected in thick lithosphere/thin crust regions of the Mid-Atlantic Ridge, and along the ultra-slow Southwest Indian Ridge. A geological model is proposed for the lithosphere created in these thick lithosphere/thin crust ridge regions. This model suggests that crustal thicknesses measured in seismic surveys of these regions do not directly reflect the melt production in the asthenosphere beneath the ridge: some of the melt gets trapped in the mantle, and part of the crust is made of variably fractured and serpentinized residual ultramafics. Potential consequences on the way we interpret along-axis crustal thickness variations at slow spreading ridges are discussed and evaluated.

Modes of seafloor generation at a melt-poor ultraslow-spreading ridge

We report on extensive off-axis bathymetry, gravity, and magnetic data that provide a 26-m.y.-long record of axial tectonic and magmatic processes over a 660-km-long and melt-poor portion of the ultraslow Southwest Indian Ridge. We describe a new type of seafloor (the smooth seafloor) that forms at minimal ridge melt supply, with little or no axial volcanism. We propose possible mechanisms leading to this avolcanic or nearly avolcanic mode of spreading, in contradiction with the traditional view of mid-ocean ridges as primarily volcanic systems. We also show evidence for large-offset asymmetric normal faults and detachments at the ridge axis, with asymmetry persisting in some cases for tens of millions of years.

Serpentinized peridotites and gabbros in the Mid-Atlantic Ridge axial valley at 15°37′N and 16°52′N

Serpentinized peridotites dredged in the Mid-Atlantic Ridge axial valley at 15°37′N and 16°52′N during theridelente cruise of R.V.Jean Charcot are mantle-derived harzburgites, plastically deformed in low stress/high temperature conditions similar to those inferred for the asthenospheric mantle. This early asthenospheric event was followed by ductile, then brittle, deformation in higher stress and progressively lower temperature conditions. We argue that this deformation occurred during the tectonic uplift of the peridotites throughout the lithosphere of the axial region. The ultramafics studied have mineral compositions similar to those of other mantle-derived Mid-Atlantic Ridge harzburgites. Strongly depleted mineral compositions in samples from the 15°37′N dredge could reflect a high degree of mantle partial melting. Emplacement of peridotites in the seafloor at mid-ocean ridges may not therefore be systematically correlated with low degrees of mantle partial melting. Serpentinized peridotites dredged at 15°37′N were intruded by evolved Na- and Fe-rich apatite and zircon-bearing gabbros. These gabbros were emplaced after the ductile deformation of the peridotites, and prior to their extensive serpentinization. Similar intrusive relationships between extensively fractionated gabbros and mantle peridotites have been observed in samples from the Mid-Atlantic Ridge axial valley south of the Kane transform, in dredged samples from Southwest Indian Ocean fracture zones, and in western Alps ophiolites. We propose that such intrusive relationships are characteristic of magma-starved spreading environments, in which stretching of the axial lithosphere may locally lead to emplacement of peridotites and gabbros in the seafloor.

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.

Ultramafic and gabbroic exposures at the Mid-Atlantic Ridge: geological mapping in the 15°N region

Abstract The outcrops of mantle-derived ultramafic rocks in the 15°N region are the most extensive yet reported for the Mid-Atlantic Ridge. North of the Fifteen Twenty fracture zone, these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall. Ultramafic rocks also crop out extensively south of the Fifteen Twenty fracture zone. Based on dredging and on a morphological analysis of the bathymetric map, we propose that ultramafic outcrops may be common in the crust formed between 14°30′N and 15°50′N during at least the past 2.4 m.y. Moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops. Diving observations also show that the ultramafic rocks are capped, in stratigraphic contact, by a thin layer of basalt. This suggests that these rocks were tectonically emplaced at the axial seafloor, or very close to it, then uplifted in the footwall of the faults that bound the axial valley. The occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region: instead of one master shear zone, there would be a complex array of cross-cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds.

Magnetic properties of variably serpentinized abyssal peridotites

We have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven Deep Sea Drilling Project and Ocean Drilling Program sites. The magnetic susceptibility (K) of these samples does not increase linearly with the degree of serpentinization (S). Instead, K remains modest in partially serpentinized samples (S 75%) peridotites in the oceanic crust. Moderately serpentinized peridotites (S < 75%) have NRM values <5 A/m and K < 0.05 SI. Low-temperature oxidation of magnetite is found to lower the NRM and K values of seafloor samples and of cataclastically deformed drilled samples.

Emplacement of deep crustal and mantle rocks on the west median valley wall of the MARK area (MAR, 23°N)

Abstract Exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow-spreading Mid-Atlantic Ridge. Nine dives of the submersible Nautile explored the western axial valley wall in the northern cell of the MARK area (Mid-Atlantic Ridge/Kane fracture zone), where peridotite and gabbro outcrops had been previously reported, in order to constrain the structure and determine emplacement mechanisms. The ridge/transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl, locally overlain by metabasalts and metadolerites, capped by slightly weathered basalts. The morphology is controlled by ridge-parallel faults, dipping moderately (40–65°) to the east. Transform-parallel scarps, present in the northernmost dives, become rare toward the south. Brittle deformation, along moderately dipping fault scarps, produced a dense microcrack network filled with greenschist facies minerals (chlorite, actinolite, epidote, quartz), locally overprinting high-temperature ductile deformation fabrics. The small hill located at 23°20′ in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl. Above 3100 mbsl, the summit of the hill is composed of pillow-basalts and sediments. The peridotite outerops display a strong schistosity dipping 20–40° to the east, parallel to striated normal fault planes. Some steeper east-facing fault scarps truncate the lower-dipping fault surfaces. The serpentinites, deriving from a harzburgitic protolith, are cut by rare dikelets of highly differentiated gabbro. These new data, combined with previous results of Alvin dives, are used to draw a generalized geological map of the western axial valley wall. This map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the MARK area: the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets. Low-angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south.

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.