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Abstract

A 20-Myr record of creation of oceanic lithosphere is exposed along a segment of the central Mid-Atlantic Ridge on an uplifted sliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry of the exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of approximately 3-4 Myr superimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustal thickness indicates that the mantle is upwelling at an average rate of approximately 25 mm x yr(-1), but this appears to vary through time. Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axis segmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadily hotter over the past 20 Myr, possibly owing to north-south mantle flow.

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... Since then additional topographic data and an ample coverage of seismic reflection and magnetometric profiles have been obtained Fig. 8 The Vema transform region. a Shaded relief image based on a compilation of our multibeam data (60 m grid interval) [43,45,49,187] combined with GEBCO 2021 global bathymetric grid. Mercator projection at 11°N and illumination from the northwest. ...
... Starting about 140 km west of the southern ridge/transform intersection, a prominent transverse ridge (Vema transverse ridge or VTR) rises up to 3 km above the thermal contraction versus age level (Fig. 10). Direct observation and sampling by the submersible Nautile [47] and extensive dredging and geophysical surveys, indicate that the VTR is the exposed edge of a flexured and uplifted slab of oceanic lithosphere ( Fig. 9) generated at an axial ridge segment (EMAR) that today is 80 km long [44,49]. Based on satellite gravimetry this ridge segment started to develop about 40 Ma (Fig. 11) and increased its length at an average rate of 1.6 mm/year [44]. ...
... We defined this as the "Vema Lithospheric Section" (VLS), that represents the edge of a lithospheric sliver created steadily by an 80 km long MAR axial segment (EMAR segment, Fig. 8a). A first study of the VLS detected a steady increase of crustal thickness and of mantle degree of melting from~18 to 5 Ma [49]. Additional sampling of the VLS, carried out in 2005, extended the coverage of the mantle peridotites sampled along the VLS from crustal ages 26-2 Ma [52]. ...
Article
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Half a century ago, our view of the Earth shifted from that of a Planet with fixed con- tinents and ancient stable ocean basins to one with wandering continents and young, active ocean basins, reviving Wegener’s Continental Drift that had rested dormant for years. The lithosphere is the external, mostly solid and relatively rigid layer of the Earth, with thickness and composition different below the oceans and within the con- tinents. We will review the processes leading to the generation and evolution of the Earth’s lithosphere that lies beneath the oceans. We will discuss how the oceanic litho- sphere is generated along mid-ocean ridges due to upwelling of convecting hot mantle. We will consider in particular lithosphere generation occurring along the northern Mid Atlantic Ridge (MAR) from Iceland to the equator, including the formation of trans- form offsets. We will then focus on the Vema fracture zone at 10°–11° N, where a ~ 300 km long uplifted and exposed sliver of lithosphere allows to reconstruct the evolution of lithosphere generation at a segment of the MAR from 25 million years ago to the Present. This axial ridge segment formed 50 million years ago, and reaches today 80 km in length. The degree of melting of the subridge mantle increased from 16 million years ago to today, although with some oscillations. The mantle presently upwelling beneath the MAR becomes colder and/or less fertile going from Iceland to the Equator, with “waves” of hot/fertile mantle migrating southwards from the Azores plume. Scientific revolutions seem to occur periodically in the history of Science; we wonder when the next revolution will take place in the Earth Science, and to what extent our present views will have to be modified.
... The aim of the surveys was to cover the entirety of the VEMA transform fault from the edge of the active transform, already mapped by e.g. Bonatti et al. (2003), east and west to the termination of its topographic expression at 25°W and 50.5°W respectively. Global gravity and bathymetric data (Sandwell et al., 2014) were used to trace the fault off axis. ...
... Given the expected sediment cover here (likely to be upwards of 1 km, e.g. Bonatti et al., 2003 and2005) the fact corrugations are observed is surprising. Even though sedimentation is focused in the valleys and troughs, the sediment cover may be a contributing factor to the low number of core complexes observed in the bathymetric data, although very few of the massive highs displayed a domed profile, the other characteristic feature of a core complex and may instead be inside/outside corner highs, or reflect periods of higher than normal volcanic activity. ...
... The transform itself was surveyed and described by e.g. Cannat et al. (1991), Bonatti et al. (2003) and displays exceptionally steep cliff faces on both the north and south side. The southern uplifted block reaches more than 2000m above the surrounding seafloor and is suggested to be the site of hydrothermal fluid flow (Cannat et al., 1991). ...
Technical Report
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The abyssal seafloor makes up > 60% of our planet´s surface, it is nevertheless largely unexplored. We know very little about how the processes which created it have varied through time, about life on the deep seafloor or about how the hydrosphere, biosphere and lithosphere interact over this vast area. In the Atlantic, transform faults and fracture zones characterize most of the seafloor bathymetry and the volcanic and tectonic process which create and modify the crust can be deduced from their bathymetric signature. During the cruise SO-237 we surveyed and sampled the entire length of one of the major offsets of the Mid-Atlantic Ridge, the Vema Fracture Zone, looking at a history of plate creation and modification over 120Ma. Variations in benthic communities along this transect will be investigated using samples recovered from corers and towed gear as well as detailed photographic mapping of the benthic megafauna using AUV. The results will be used to test the hypothesis that the Mid- Atlantic Ridge serves as a barrier limiting benthic species distribution in the abyssal basins on both sides of the ridge. The Puerto Rico Trench is much deeper than the surrounding abyssal West Atlantic and so we also took samples there to determine whether the biodiversity of its hadal meio-, macro-, and megabenthic fauna differs from that of the abyssal Atlantic due to isolation of the trench. The cruise yielded important information for the Transregio proposal "Maturing oceanic plates: Earth’s hidden reactors" and has provided the first high-resolution bathymetric survey along an entire fracture zone trace and one of the world’s best surveyed seafloor features.
... Oceanic lithosphere forms at mid-ocean ridges 44 (MORs) through a combination of magmatic and tectonic activity (Macdonald et al., 45 1996;Searle, 2013). Variability in the magma supply associated with differences in 46 spreading rate, axial thermal structure, and/or mantle melting processes has been linked 47 to first-order differences in ridge morphology (e.g., Chen basalts (e.g., Bonatti et al., 2003;Behn & Grove, 2015). 51 ...
... For example, Bonatti et al. (2003) found a long-term increase in gravity-derived crustal 54 thickness along the Vema Transform Fault (Guinea Fracture Zone) in the Atlantic, which 55 they hypothesized to result from a long-term (>20 Myr) increase in mantle potential 56 temperature. Previous studies in the Mid-Atlantic also found 2-4 Myr period variations 57 in gravity-derived crustal thickness, which were interpreted as representing changes in 58 mantle upwelling and flow (Bonatti et al., 2003;Pariso et al., 1995) One challenge in deciphering temporal changes in the structure and evolution of 83 oceanic lithosphere is that most previous studies have focused on the formation of young 84 lithosphere at or near the ridge axis. ...
... For example, Bonatti et al. (2003) found a long-term increase in gravity-derived crustal 54 thickness along the Vema Transform Fault (Guinea Fracture Zone) in the Atlantic, which 55 they hypothesized to result from a long-term (>20 Myr) increase in mantle potential 56 temperature. Previous studies in the Mid-Atlantic also found 2-4 Myr period variations 57 in gravity-derived crustal thickness, which were interpreted as representing changes in 58 mantle upwelling and flow (Bonatti et al., 2003;Pariso et al., 1995) One challenge in deciphering temporal changes in the structure and evolution of 83 oceanic lithosphere is that most previous studies have focused on the formation of young 84 lithosphere at or near the ridge axis. For example, to understand the magmatic input to 85 mid-ocean ridges, studies have focused on variations in the chemistry of mid-ocean ridge 86 12 heave recorded by scarp morphology is reduced through sedimentation and mass wasting. ...
... M antle rising beneath the 60,000-km-long mid-ocean ridge system contains, as in a slow-motion movie, a record of ancient upwelling and melting events and of interaction with subduction-or hotspot-derived components. It is difficult to reconstruct temporal records of these ancient events due to lack of suitable samples; however, we were given the opportunity to explore the temporal evolution of the oceanic lithosphere composition and structure at 11° N along the Mid-Atlantic Ridge (MAR) where an uplifted > 300-km-long sliver of lithosphere exposes a basal mantle peridotite unit, lower crustal gabbros, a dyke complex and erupted basalts [1][2][3] . This lithospheric section (Vema lithospheric section (VLS)) was generated at an 80-km-long segment of the MAR (EMAR segment, Supplementary Fig. 1) during a 26 Myr time interval 1,2,4-6 . ...
... Surprisingly, temporal variations of the degree of melting of the mantle estimated from basalt Na 8 (refs 7,8 ) anti-correlate with the degree of melting derived from spinel Cr# (Cr# = Cr/(Cr + Al)) of the peridotites 9,10 , although the two curves converge to a common value in the youngest 3 Myr stretch of the VLS (Fig. 1). Older, isotopically enriched basalts display the lower Na 8 values of the entire VLS, suggesting that they were generated by a higher degree of melting of their mantle source; in contrast, the genetically associated mantle peridotites record a relatively low extent of melting, in agreement with a thinner crust recorded by geophysical data 1 . This anti-correlation contrasts with what is inferred to be the 'normal' signature of partial melting at mid-ocean ridges. ...
... delivered at the ridge axis increased through time, in agreement with gravity profiles running along spreading flow lines ( Supplementary Fig. 1c), revealing that the crustal thickness increased from 4.8 ± 0.2 km in the 22-27 Ma interval to 5.4 ± 0.2 km between 0 and 5 Ma (refs 1,4 ). ...
Article
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After travelling in Earth’s interior for up to billions of years, recycled material once injected at subduction zones can reach a subridge melting region as pyroxenite dispersed in the host peridotitic mantle. Here we study genetically related crustal basalts and mantle peridotites sampled along an uplifted lithospheric section created at a segment of the Mid-Atlantic Ridge through a time interval of 26 million years. The arrival of low-solidus material into the melting region forces the elemental and isotopic imprint of the residual peridotites and of the basalts to diverge with time. We show that a pyroxenite-bearing source entering the subridge melting region induces undercooling of the host peridotitic mantle, due to subtraction of latent heat by melting of the low-T-solidus pyroxenite. Mantle undercooling, in turn, lowers the thermal boundary layer, leading to a deeper cessation of melting. A consequence is to decrease the total amount of extracted melt, and hence the magmatic crustal thickness. The degree of melting undergone by a homogeneous peridotitic mantle is higher than the degree of melting of the same peridotite but veined by pyroxenites. This effect, thermodynamically predicted for a marble-cake-type peridotite–pyroxenite mixed source, implies incomplete homogenization of recycled material in the convective mantle.
... The VLS is the northern wall of the prominent Vema Transverse Ridge (VTR) south of the Vema fracture zone (Fig. 1). This region has been extensively studied over more than 50 years because is one of the rare places on Earth where the oceanic lithosphere is exposed and can be studied over a time interval ranging from the present day back to~30 Ma (Heezen et al., 1964;Van Andel et al., 1971;Bonatti, 1978, Auzende et al., 1989Kastens et al., 1998;Bonatti et al., 2003;Brunelli et al., 2006;Cipriani et al., 2009a;Brunelli et al., 2018). The VTR formed 12-10 Ma ago due to rapid uplift of the southern wall of the fracture zone by elastic rebound during a short lived (~1-2 Ma) counter clockwise migration of the Africa-South America plate rotation pole (Bonatti et al., 2003(Bonatti et al., , 2005. ...
... This region has been extensively studied over more than 50 years because is one of the rare places on Earth where the oceanic lithosphere is exposed and can be studied over a time interval ranging from the present day back to~30 Ma (Heezen et al., 1964;Van Andel et al., 1971;Bonatti, 1978, Auzende et al., 1989Kastens et al., 1998;Bonatti et al., 2003;Brunelli et al., 2006;Cipriani et al., 2009a;Brunelli et al., 2018). The VTR formed 12-10 Ma ago due to rapid uplift of the southern wall of the fracture zone by elastic rebound during a short lived (~1-2 Ma) counter clockwise migration of the Africa-South America plate rotation pole (Bonatti et al., 2003(Bonatti et al., , 2005. The lithosphere was uplifted by 2-4 km over the entire stretch of the transform fault between the eastern and western MAR intersections (E-MAR and W-MAR, Fig. 1), exposing a complete section of oceanic crust and part of the upper mantle below the petrological Moho. ...
... The accessibility to both the residual mantle and its associated erupted MORBs has played a key role in recognizing the temporal evolution of the petrologic, tectonic and thermal interplay during the formation of the oceanic crust at a single ridge segment, i.e. the E-MAR segment (Bonatti et al., 2003;Brunelli et al., 2006Brunelli et al., , 2018Cipriani et al., Lithos 368-369 (2020) 105589 2004, 2009a, 2009b. Geophysical evidence revealed that the magmatic productivity of the E-MAR segment ( Fig. 1) increased over time, as indicated by a gravity-derived crustal thickening of~1 km during the last 26 Ma (Bonatti et al., 2003;Cipriani et al., 2009a). ...
Article
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Oxide gabbros are a minor but diffuse component of the lower oceanic crust. Their presence poses questions on lower crust exhumation processes and magma differentiation at mid ocean ridges because they are systematically associated with shear zones and are hardly explained by classical fractionation and melt migration models. Here, we report on a study of lower-crust gabbros recovered from the Vema Lithospheric Section at 11°N along the Mid Atlantic Ridge, where oxide gabbros are abnormally abundant relative to ridge centred magmatic intrusive and where we found a peculiar lithological occurrence represented by deformed diorites extremely enriched in Fe-Ti-oxides and apatites. Their complex genetic history reveals a hybrid nature consistent with derivation from high pressure injections of Fe-Ti-P saturated nelsonitic melts in a primitive gabbroic groundmass that induced fracturing, de-compaction, mineral resorption and chemical re-equilibration. Melt injections may occur after intense ductile shearing at the edges of the axial magma chamber following lateral differentiation of primitive melts injected at the centre of the ridge axis segment. We propose a regime of lateral, instead of vertical, melt differentiation along the ridge axis and a possible role for melt immiscibility in the formation of Fe-Ti-P melt pockets in oceanic domains.
... A ~5 km deep transform valley is filled by over 1 km thick horizontally stratified turbidite deposits of probable South American derivation (DSDP Sites 216 and 353). A transverse ridge runs parallel to the transform on its southern side ( Figure 2a); it consists of a sliver of flexured and uplifted oceanic lithosphere [Bonatti et al., 2003. Multichannel seismic reflection profiles along the summit of the transverse ridge imaged on its western, shallowest portion a strong horizontal reflector (Reflector B) extending along the crest for roughly 50 km (Figure 3a). ...
... The average subsidence rate of the Vema transverse ridge has been estimated assuming that it subsided as a single block starting soon after the end of the uplift phase at 10 Ma. The spreading half rate decreased from 17.2 mm/a between 26 and 19 Ma to ~16.9 mm/a between 19 and ~10 Ma, and then to ~13.6 mm/a from 10 Ma to present [Bonatti et al., 2003], implying a 23 to 26 Ma crustal age below the carbonate cap. The wave-truncated horizontal surface of oceanic crust at the base of the carbonate platform lies presently 1100 m below sea level, suggesting an average subsidence rate slightly over 0.1 mm/a. ...
... The growth of the platform has been interrupted by a fast sinking in the Late Miocene and today its summit is ~500 m below sea level. Bonatti et al., 2003). Location of dredges along the Vema southern transverse ridge and of the seismic profile VEMA-02; b) Bathymetric 3D map of a portion of the Vema transverse ridge. ...
Thesis
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Oceanic islands can be divided, according to their origin, in volcanic and tectonic. Volcanic islands are due to excess volcanism caused by mantle melting anomalies. Non-volcanic islands, or "tectonic", are formed due to vertical tectonic motions of blocks of oceanic lithosphere along transverse ridges flanking transform faults at slow and ultraslow mid-ocean ridges. Vertical tectonic motions are due to a reorganization of the geometry of the transform plate boundary, with the transition from a transcurrent tectonics to a transtensive and / or transpressive tectonics. The formation of a positive topographic anomaly called "transverse ridge", often strongly asymmetric, may result from the establishment of either a transpressive and transtensive regime. Transverse ridges are formed by uplifted lower oceanic crust and/or upper mantle rocks. When they are at sea level, they form an oceanic non-volcanic island. Tectonic islands can be located also at the ridge – transform intersection, being the “inner corner high”. In this case the uplift is due by the movement of the long-lived detachment faults located along the flanks of the mid-ocean ridges. A modern example of inner corner high near the sea level is the “Anna De Koningh” seamount, located at the intersection between the Southwest Indian Ridge and the Dutoit transform fault (Indian Ocean). Bathymetry data and multichannel seismic reflection profiles have identified four tectonic sunken islands in the equatorial Atlantic. The "Vema" sunken island is at the summit of the transverse ridge adjacent to the Vema transform fault; it is now about 450 m below sea level. It is capped by a carbonate platform about 500 m-thick, 50 km-long and only 5 km-wide. Samples of Vema’s carbonates dated by 87 Sr/ 86 Sr indicate that the formation of the island occurred about 10 Ma. The same age corresponds to a kinematic change of the ridge – transform geometry and the establishment of transtensive tectonics, with flexure of the oceanic lithosphere and uplift of the Vema transverse ridge. However, the discovery of "Miogypsina" in samples dredged at the non-conformity boundary between the basement and the carbonate platform suggest a stage of emergence of the island during Early Miocene, when the island was an inner corner high at the ridge – transform intersection. Three tectonic sunken islands, "Romanche A, B and C", are on the summit of the eastern transverse ridge flanking the Romanche megatrasform; they are now about 1,000 m below sea level. Multichannel seismic reflection profiles show a strong horizontal reflector at a depth of about 1200 m. Above this reflectors we observed stratified seismic units about 250-300 m-thick representing carbonate platforms consisting of shallow-water carbonates dated by 87 Sr/ 86 Sr, between 11 and 6 Ma. A sunken tectonic island, i. e., “Atlantis Bank," today about 700 m below sea level, is located in the South-Western Indian Ridge, along the Atlantis II transform fault, there is the. This island does not have a carbonate platform; it was at sea level when it was located at the ridge-transform intersection. The only modern example of oceanic tectonics island is the Saint Peter - Paul Archipelago (equatorial Atlantic), located along the active zone of the St. Paul transform fault. This archipelago is the top of a peridotitic massif that extends in the direction of the active transform fault and it is now a left overstep undergoing transpression. Markers of sea level dated by 14 C estimate a rate of uplift of the St. Paul Massif of about 1.5 mm/a for the last 6000 years. During my PhD, a multidisciplinary study led to a model to explain the origin and evolution of oceanic tectonic islands: oceanic volcanic islands are characterized by rapid growth and subsequent thermal subsidence and drowning; in contrast, oceanic tectonic islands may have one or more stages of emersion related to vertical tectonic events along the large oceanic fracture zones.
... In particular, we use secondary ion mass spectrometry (SIMS) to measure the in situ volatile contents (in particular H 2 O and F) of opx and cpx in abyssal peridotites from the Vema fracture zone (11°N; Equatorial Atlantic). The Vema lithospheric section (VLS) exposes a >300-km-long, ~1-km-thick segment of lithospheric mantle overlain by oceanic crust made of gabbros, dykes, and basalts (18). The peridotite samples used in this study were collected every 10 to 20 km along the VLS and span crustal ages from 1.5 to 24 million years (Ma) (18,19). ...
... The Vema lithospheric section (VLS) exposes a >300-km-long, ~1-km-thick segment of lithospheric mantle overlain by oceanic crust made of gabbros, dykes, and basalts (18). The peridotite samples used in this study were collected every 10 to 20 km along the VLS and span crustal ages from 1.5 to 24 million years (Ma) (18,19). Crustal ages were estimated from geomagnetic time scales (20) and U-Pb ages in crustal gabbros (21). ...
... Crustal ages were estimated from geomagnetic time scales (20) and U-Pb ages in crustal gabbros (21). The time at which peridotite was emplaced into the lithosphere is estimated using the crustal age and the spreading rate, considering the delay of emplacement between basalts and its parent peridotite (18,19). The degree of melting in peridotites along the VLS increases toward younger crustal ages, from 8 to 14% on average (18,19,22). ...
Article
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The large range of H 2 O contents recorded in minerals from exhumed mantle rocks has been challenging to interpret, as it often records a combination of melting, metasomatism, and diffusional processes in spatially isolated samples. Here, we determine the temporal variations of H 2 O contents in pyroxenes from a 24-Ma time series of abyssal peridotites exposed along the Vema fracture zone (Atlantic Ocean). The H 2 O contents of pyroxenes correlate with both crustal ages and pyroxene chemistry and increase toward younger and more refractory peridotites. These variations are inconsistent with residual values after melting and opposite to trends often observed in mantle xenoliths. Postmelting hydrogen enrichment occurred by ionic diffusion during cryptic metasomatism of peridotite residues by low-degree, volatile-rich melts and was particularly effective in the most depleted peridotites. The presence of hydrous melts under ridges leads to widespread hydrogen incorporation in the oceanic lithosphere, likely lowering mantle viscosity compared to dry models.
... These large oceanic transform domains -also called "megatransforms"show a broad (>100 km) and complex multifaults shear zone similarly to the large continental strike-slip systems. The complexity of the plate boundaries at these locations affects fundamentally their thermal structure that in turn controls mantle melting, melt migration, aggregation and emplacement of the lower crust (Bonatti et al., 2003;Brunelli et al., 2018;Ligi et al., 2005). Several authors (e.g., Ligi et al., 2002Ligi et al., , 2005Maia et al., 2016;Sclater et al., 2005) proposed that an extreme thickness of the lithosphere determines the unusual width and complex geometry of megatransforms. ...
... Basement rocks were collected in 23 dredges, of which 15 contained serpentinized mantle peridotites (Fig. 1b and Supplementary Table 1). They are mostly located along the transform valley, in agreement with evidence of exposure of upper mantle rocks in the large transform systems of the Atlantic, such as the 15.20°N (Kelemen et al., 2004;Pushcharovsky et al., 1988;Suhr et al., 2008), Vema (Bonatti et al., 2003;Brunelli et al., 2006) and Romanche transforms (Bonatti et al., 1996(Bonatti et al., , 2001. We note, however, that peridotites were collected also from the central sector of the active intra-transform ridge segment (ITR-1) at the eastern end of the Doldrums transform, probably due to detachment faulting (Skolotnev et al., 2006(Skolotnev et al., , 2020. ...
Article
The Doldrums transform system offsets the Equatorial Mid Atlantic Ridge by ~630 km at 7–8° N. This transform system consists of four intra-transform spreading centers (ITRs) bounded by five transform faults. The northernmost ITR is linked to the MAR axis by a ~ 180 km-long transform. Here, during two R/V A. N. Strakhov expeditions (S06 and S09), mantle peridotites were dredged along the transverse and median ridge of the transform, across the western flank of the ITR valley. Residual harzburgites were mainly sampled along the northern Doldrums transform valley, whereas plagioclase-bearing peridotites showing evidence for melt-rock interaction characterize the ITR domain. Petrological and geochemical observations reinforced by geochemical modelling are used to define the behaviour of trace elements during melt extraction and melt-rock reaction in our rocks. Results suggest that residual peridotites derive from mantle rocks that have undergone a degree of partial melting up to 12%, with melting likely starting at the transition of garnet-spinel stability fields, whereas peridotites which suffered melt-rock reactions have been divided into two types: (i) pl-impregnated peridotites, formed by migration of melts at high porosity and high melt-rock ratio; and (ii) refertilized peridotites, generated at reduced porosity, when small fractions of the same percolating melt crystallized clinopyroxene and minor plagioclase. We suggest that the refertilizing agent was a melt highly depleted in incompatible trace elements, in turn produced by an ultra-depleted mantle source. This mantle experienced previous degrees of melt extraction at the ridge axis, before being transposed laterally along the transform where it melted a second time during the opening of the intra-transform spreading segment.
... We also discuss several models for the origin of depleted peridotites in the studied area because peridotites with depleted signatures in melt components were recently reported from oceanic core complexes where fertile peridotites are expected to be recovered (Seyler et al. 2007;Tamura et al. 2008;Dick et al. 2010) (Fig. 14.9). Depleted peridotites were also already recovered from other ultraslow to slow-spreading ridges without a hot spot effect (Bonatti et al. 1992(Bonatti et al. , 2003Seyler et al. 2003;Brunelli et al. 2006;Morishita et al. 2007;Godard et al. 2008). These studies indicate that large variations in the melting degree of midocean ridge peridotites are probably not simply related to the spreading rate. ...
... These studies indicate that large variations in the melting degree of midocean ridge peridotites are probably not simply related to the spreading rate. Several models can be applied for the occurrence of the depleted peridotites even where the degree of partial melting is expected to be low: (i) relics of older partial melting (Seyler et al. 2003(Seyler et al. , 2007Liu et al. 2008), (ii) variations of equilibration temperatures probably related to variations of advection velocity below the ridge (Bonatti et al. 2003), and (iii) residue related to the latest melting events, i.e., the present mid-ocean ridge spreading . We cannot examine the second case (ii) based on our samples because further samplings in space and time along the CIR are required. ...
Article
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Peridotites and related gabbroic rocks are widely exposed in the Central Indian Ridge, where the H2-rich-fluid-bearing Kairei hydrothermal field exists. We report on petrological and mineralogical characteristics of peridotites and gabbroic rocks recovered from an oceanic core complex at a latitude of 25° South (25°S OCC) and the Yokoniwa Rise around the Kairei hydrothermal field. Gabbros recovered from the 25°S OCC show a wide range of variations in terms of mineral chemistry and mineral assemblages (olivine-gabbro, gabbronorite to highly evolved oxide gabbro) and are similar to those from the Atlantis Bank of the Southwest Indian Ridge, an ultraslow-spreading ocean ridge. Peridotites recovered from 25°S OCC and the Yokoniwa Rise are generally characterized by moderately to highly depleted melt components. The partial melting of these peridotites is followed by chemical modification through interaction with a wide range of melts from relatively less evolved to highly evolved characteristics. Moderately to highly depleted melt components in the studied peridotites can be explained as being either residue after a relatively high-melt productivity period in intermediate-spreading ridges or a geochemically distinctive domain which has suffered from partial melting in the past rather than partial melting beneath the present mid-ocean ridge systems.
... It follows that the VTR may have risen close to 10 Ma along the entire length of the transform offset. This could be explained as the effect of a major change of the stress regime, which occurred about 10 Ma at the VTR (van Andel et al., 1969;Fox et al., 1969;Bonatti et al., 2003). ...
... Comparison of the Sr isotopic ages of the pelagic limestones encrusting the basement rocks from the lower part of the Vema Lithospheric Section with crustal ages estimated from magnetic anomalies shows limestone ages either close to, or younger than 10-12 Ma even at sites with crustal ages ≫ 10 Ma (Fig. 3). These results are in line with the VTR having risen between 12 and 10 Ma due to a small change in ridge/transform geometry, causing transtension along the transform boundary, with flexure and uplift of a lithospheric sliver on the southern side of the transform ( Bonatti et al., 2003). The change in orientation of the spreading centre is demonstrated by a slight change in the orientation of the oceanic crustal fabric south of the transform boundary ( Fig. 1), close to chron C5 (i.e., 9.74 Ma according to Cande et al., 1988). ...
Article
Transverse ridges are large topographic anomalies running adjacent to slow-slip oceanic transforms. They form due to different processes, including thermal stresses, hydration-dehydration of peridotites, non-linear viscoelastic rheology of the oceanic crust and vertical tectonic motions of lithospheric slivers induced by changes in ridge/transform geometry, causing transpression and/or transtension along the transform boundary. A prominent transverse ridge on the southern side of the Vema transform (Central Atlantic) rose probably between 12 and 10 Ma along the entire length (≈ 320 km) of the transform, exposing a relatively undisturbed section of oceanic lithosphere. We used pelagic limestones encrusting serpentinized peridotites sampled from the lower slopes of the uplifted lithospheric section to date this uplift and define mechanisms of its emplacement. Ages were obtained both by micropaleontology (foraminifera and nannofossils) and by ⁸⁷Sr/⁸⁶Sr isotope ratios. No ages older than ≈ 12 Ma were obtained, even in samples recovered at sites with crustal ages (determined by magnetic anomalies) well over 12 Ma; on the other side, ages as young as 5.6–8.3 Ma were found in clusters of samples collected from the eastern part of the transverse ridge, probably due to mass-wasting episodes that rejuvenated the substratum. These results support the hypothesis that the Vema Transverse Ridge rose between 12 and 10 Ma due to flexural uplift related to transtension along the transform, in line with a general model whereby transverse ridges rise during discrete events as a consequence of changes in ridge-transform geometry.
... The Vema Lithospheric Section (VLS) is a flexured and uplifted sliver of oceanic lithosphere exposed south of the Vema fracture zone at 11 N in the central Atlantic (Fig. 1). The VLS extends for over 300 km along a seafloor spreading flow line and represents more than 26 Myr of accretion at a single 80 km long spreading segment (Fabretti et al., 1998;Bonatti et al., 2003;Cipriani et al., 2009a). Submersible dives and numerous dredge sites have shown that the VLS consists of serpentinized mantle peridotite ($1000 m thick), gabbro ($500 m thick), dolerite dykes (700-1100 m thick) and basaltic lavas ($800 m thick) (Auzende et al., 1989;Bonatti et al., 2003Bonatti et al., , 2005Cipriani et al., 2009a). ...
... The VLS extends for over 300 km along a seafloor spreading flow line and represents more than 26 Myr of accretion at a single 80 km long spreading segment (Fabretti et al., 1998;Bonatti et al., 2003;Cipriani et al., 2009a). Submersible dives and numerous dredge sites have shown that the VLS consists of serpentinized mantle peridotite ($1000 m thick), gabbro ($500 m thick), dolerite dykes (700-1100 m thick) and basaltic lavas ($800 m thick) (Auzende et al., 1989;Bonatti et al., 2003Bonatti et al., , 2005Cipriani et al., 2009a). ...
Article
We present new data on mineral assemblages, mineral chemistry (major and trace element compositions), and fluid inclusions to reconstruct the magmatic to hydrothermal history of gabbroic rocks dredged in association with serpentinized mantle peridotites from the tectonically exposed Vema lithospheric section (central Atlantic). Textural relations and mineral chemistry of the samples indicate a two-stage magmatic evolution, from an early coarse-grained pyroxene–plagioclase cumulate to overprinted assemblages, including interstitial Fe–Ti oxides and titanian hornblende. Titanian hornblende in the gabbros is a product of interaction of crystal aggregates with an evolved residual melt or with a late-magmatic aqueous fluid at 800–900�C. The different contribution of residual melt and magmatic aqueous fluid is inferred from the behaviour of incompatible elements and plagioclase composition. One gabbro sample shows features indicative of a later stage hydrothermal event: formation of Fe- and Cl-rich hornblende sealing fractures in plagioclase, olivine coronas at orthopyroxene–magnetite grain boundaries, plagioclase-hosted high-salinity fluid inclusions associated with local enrichment of plagioclase in anorthite and Sr, and local augite to diopside re-equilibration. These features record the interaction of gabbro with seawater-derived fluid at temperature up to about 600�C. The high-salinity inclusions and reducing hydrothermal reactions in the gabbro indicate that the fluid phase characteristics may have been acquired through a preceding lower-temperature (<500�C) interaction of seawater with mantle peridotite (serpentinization) followed by infiltration of the modified aqueous solution into the higher-temperature (about 600�C) gabbroic zone in a geodynamic environment of a slow-spreading ridge segment end. Compositional features of the hydrothermal fluid were: salinity 20–22 wt % (NaCl eq.), Ca/(CaþNa)�0�12, and log fO2¼–19�6 to –18�8.
... The mechanisms causing the differences in magmatism along the Norwegian Margin, with twice the amount of volume and duration of magmatism on the Vøring Margin than on the Møre and Lofoten margins , are not yet fully understood. However, both temporary variations in magmatic productivity (Bonatti et al., 2003) and variations along a spreading ridge, as observed along the Knipovich Ridge (Kandilarov et al., 2008(Kandilarov et al., , 2010, are expected. It is thus possible that the along strike variation in magmatism observed along the mid-Norwegian Margin is due to processes inherent in normal continental rifting and oceanic spreading processes. ...
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The inner part of the volcanic, passive Møre Margin, mid-Norway, expresses an unusual abrupt thinning from high onshore topography with a thick crust to an offshore basin with thin crystalline crust. Previous P-wave modeling of wide-angle seismic data revealed the presence of a high-velocity (7.7-8.0. km/s) body in the lower crust in this transitional region. These velocities are too high to be readily interpreted as Early Cenozoic intrusions, a model often invoked to explain lower crustal high-velocity bodies in the region. We present a Vp/Vs model, derived from the modeling of wide-angle seismic data, acquired by use of Ocean Bottom Seismograph horizontal components. The modeling suggests dominantly felsic composition of the crust. An average Vp/Vs value for the lower crustal body is modeled at 1.77, which is compatible with a mixture of continental blocks and Caledonian eclogites. The results are compiled with earlier results into a transect extending from onshore Norway to onshore Greenland. Back-stripping of the transect to Early Cenozoic indicates asymmetric conjugate magmatism related to the continental break-up. Further back-stripping to the time when most of the Caledonian mountain range had collapsed indicates that the thinning during the first phase of extension was about 25% higher for proto Norway than proto Greenland.
... The peridotites located between 9.5 and 6 Ma isochrones display a range of 9-13.5% partial melting in their upper part, up to 1 km from their boundary with gabbros. However, the degree of partial melting throughout the older intervals oscillated between 5 and 10% (Bonatti et al., 2003). This range in the degree of partial melting is also displayed by peridotites in the Owen fracture zone and the Romanche fracture zone (Choi et al., 2008). ...
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The mantle is the most voluminous part of the Earth. However, mantle petrologists usually have to rely on indirect geophysical methods or on material found ex situ. In this review paper, we point out the in-situ existence of oceanic core complexes (OCCs), which provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow-spreading ridges. OCCs are a common structure in oceanic crust architecture of slow-spreading ridges. At least 172 OCCs have been identified so far and we can expect to discover hundreds of new OCCs as more detailed mapping takes place. Thirty-two of the thirty-nine OCCs that have been sampled to date contain peridotites. Moreover, peridotites dominate in the plutonic footwall of 77% of OCCs. Massive OCC peridotites come from the very top of the melting column beneath ocean ridges. They are typically spinel harzburgites and show 11.3–18.3% partial melting, generally representing a maximum degree of melting along a segment. Another key feature is the lower frequency of plagioclase-bearing peridotites in the mantle rocks and the lower abundance of plagioclase in the plagioclase-bearing peridotites in comparison to transform peridotites. The presence of plagioclase is usually linked to impregnation with late-stage melt. Based on the above, OCC peridotites away from segment ends and transforms can be treated as a new class of abyssal peridotites that differ from transform peridotites by a higher degree of partial melting and lower interaction with subsequent transient melt.
... As Vr is higher than hS, the transfer rate of successive residues above the melting area is slower than the westward velocity of the melting area itself. Accordingly, mantle exhumed at the bottom of any active axial ridge is always older than the basaltic volcanoes emplaced at the axial ridge ( Bonatti et al., 2003). It is the same for intrusive gabbro bodies which move upwards and laterally, driven by and with the upwelling of the host-mantle: for instance, for hS = 2 cm/yr, Vr = 4 cm/y and a mantle residue initial depth of Di = 40 km, the mantle residue takes t1 2 Ma (t1 = Di/hS) to ascend and to be exhumed at the surface. ...
Article
Combining geophysical, petrological and structural data on oceanic mantle lithosphere, underlying asthenosphere and oceanic basalts, an alternative oceanic plate spreading model is proposed in the framework of the westward migration of oceanic spreading ridges relative to the underlying asthenosphere. This model suggests that evolution of both the composition and internal structure of oceanic plates and underlying upper mantle strongly depends et al.l scales on plate kinematics. We show that the asymmetric features of lithospheric plates and underlying upper asthenosphere on both sides of oceanic spreading ridges, as shown by geophysical data (seismic velocities, density, thickness, and plate geometry), reflect somewhat different mantle compositions, themselves related to various mantle differentiation processes (incipient to high partial melting degree, percolation/reaction and refertilisation) at different depths (down to 300 km) below and laterally to the ridge axis. The fundamental difference between western and eastern plates is linked to the westward ridge migration inducing continuing mantle refertilisation of the western plate by percolation-reaction with ascending melts, whereas the eastern plate preserves a barely refertilized harzburgitic residue. Plate thickness on both sides of the ridge is controlled both by cooling of the asthenospheric residue and by the instability of pargasitic amphibole producing a sharp depression of the water-undersaturated solidus, its intersection with the geotherm at ~ 90 km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the water-undersaturated lherzolite solidus explains the existence of a low-velocity zone (LVZ). As oceanic lithosphere is moving westward relative to asthenospheric mantle, this partially molten upper asthenosphere facilitates the decoupling between lower asthenosphere and lithosphere. Thereby the westward drift of the lithosphere is necessarily slowed down, top to down, inducing a progressive decoupling within the mantle lithosphere itself. This intra-mantle decoupling could be at the origin of asymmetric detachment faults allowing mantle exhumation along slow-spreading ridges. Taking into account the asymmetric features of the LVZ, migration of incipient melt fractions and upwelling paths from the lower asthenosphere through the upper asthenosphere are oblique, upward and eastward. MORB are sourced from an eastward and oblique, near-adiabatic mantle upwelling from the lower asthenosphere. This unidirectional mantle transfer is induced by isostatic suction of the migrating spreading ridge.
... The gabbro sample -L2612-41 (gabbro-41)was collected together with more abundant mantle-derived peridotites from the Vema lithospheric section, Central Atlantic (Auzende et al., 1989;Bonatti et al., 2003Bonatti et al., , 2005 by dredging at 10°42.95′N, 41°34.60′W, 5195-4620 mbsl (Cipriani et al., 2009). ...
Article
Fe-Ti-oxide micro-inclusions in clinopyroxene of oceanic gabbro from the mid Atlantic ridge have been studied using electron backscatter diffraction and electron probe microanalyses. A first generation of Fe-Ti-oxide inclusions occurs as needles or elongated plates lying in the (010) plane of the clinopyroxene host. The inclusions show distinct elongation directions following “irrational” planes either nearly parallel to the “c” or nearly parallel to the “a” axis of the clinopyroxene host. The habit planes correspond to inclusion-host interfaces, where densely packed oxygen layers of the inclusion and host phases are coherent across the interface. Both inclusion types have distinct crystallographic orientation relations to the host, which are determined by the nearly parallel alignment of densely packed oxygen layers in the inclusions and the clinopyroxene host. Based on the angle between the two elongation directions primary formation of the inclusion at 800° to 900 °C was inferred. Initially, homogeneous Ti-bearing magnetite was precipitated together with titanian pargasite lamellae due to reaction of early magmatic clinopyroxene with late magmatic fluid or melt. After cooling below 600 °C the Ti-magnetite decomposed into an oriented magnetite + ulvospinel intergrowth. Late stage hydrothermal alteration leads to the corrosion of the magnetite-ulvospinel inclusions and partial replacement by ilmenite as well as newly precipitated homogeneous ilmenite plates that are closely associated with actinolite lamellae lying in the (100) plane of the clinopyroxene host.
... A second set of observations is concerned with Neogene and Paleogene volcanism and regional epeirogeny. Away from the Reykjanes Ridge with which the buoyant mantle upwelling hypothesis is directly concerned, there is evidence for significant off-axis igneous activity, transient dynamic support of oceanic gateways, and regional epeirogeny cannot easily be accounted for by an axially restrictive model whereby patches of buoyant mantle are envisaged as being confined within a narrow corridor that is <100 km wide ( Barnouin-Jha et al., 1997;Bonatti et al., 2003;Scott & Stevenson, 1989). Since oceanic lithosphere has a small elastic thickness, loading effects generated by cells of buoyant upwelling are unlikely to have regional consequences. ...
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In the North Atlantic Ocean, the geometry of diachronous V-shaped features that straddle the Reykjanes Ridge is often attributed to thermal pulses which advect away from the center of the Iceland plume. Recently, two alternative hypotheses have been proposed: rift propagation and buoyant mantle upwelling. Here, we evaluate these different proposals using basin-wide geophysical and geochemical observations. The centerpiece of our analysis is a pair of seismic reflection profiles oriented parallel to flowlines that span the North Atlantic Ocean. V-shaped ridges and troughs are mapped on both Neogene and Paleogene oceanic crust, enabling a detailed chronology of activity to be established for the last 50 million years. Estimates of the cumulative horizontal displacement across normal faults help to discriminate between brittle and magmatic modes of plate separation, suggesting that crustal architecture is sensitive to the changing planform of the plume. Water-loaded residual depth measurements are used to estimate crustal thickness and to infer mantle potential temperature which varies by 25◦C on timescales of 3–8 Ma. This variation is consistent with the range of temperatures inferred from geochemical modeling of dredged basaltic rocks along the ridge axis itself, from changes in Neogene deep-water circulation, and from the regional record of episodic Cenozoic magmatism. We conclude that radial propagation of transient thermal anomalies within an asthenospheric channel that is 150 50 km thick best accounts for the available geophysical and geochemical observations.
... The crustal pattern (Fig. 18) in the study area shows a complex distribution of thin and thick crust. The thin crust is thought to be the consequence of low melt supply and slow-spreading rate (Kuo and Forsyth 1988;Lin et al. 1990;Langmuir et al. 1992;Hooft et al. 2000;Bonatti et al. 2003;Duo et al. 2020). It is also evident that the thin crust is associated with the accretion zone, which is narrower than the strain zone (Ruedas and Schmeling 2008;Wang et al. 2011;Searle 2013). ...
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The present study deals with mapping of the crustal structure over the Central Indian Ridge (CIR) covering 3°S–11°S using SGG-UGM-2 Free-air gravity data. We have discussed the importance of mantle temperature correction along with Bouguer, sediment thickness, and curvature corrections to the gravity data. We have discussed the influence of mantle temperature correction on gravimetric Moho determination along the ridge axis. We found an erroneous impression of Moho along the ridge axis from the gravity data without thermal correction. The Moho depth near the vicinity of CIR (3°S–16°S) is calculated from the residual mantle Bouguer gravity data using the Parker–Oldenburg algorithm. The inversion-based Moho shows an RMS error of 2.39 km with respect to the CRUST1.0 global Moho model. The isostatic Moho has been calculated using Airy-Heiskanen’s (AH’s) hypothesis to study compensation characteristics. The isostatic Moho exhibit a significant dissimilarity with the inversion-based Moho. The crustal distribution based on the inversion and isostatic-based Moho models were computed by removing the water column and sediment layer. The derived compensation percentage from these two crustal models indicates that the Somalian and Arabian basins are the zone of over-compensation, while an under-compensation zone characterizes the Madagascar basin. We categorized these crustal thicknesses into thin (< 6 km), normal (6–9 km), and thick (> 9 km) crust. Both thin and thick crusts show symmetrical and asymmetrical patterns around the CIR ridge axis. The fracture zone (FZ) and non-transform discontinuities (NTDs) are associated with a thin crust. It is inferred that FZ with a thick crust is perhaps resulted due to the process of serpentinization. We found that the Oceanic core complexes (OCCs) are associated with a thin crust. The OCCs associated with thick crust are perhaps related to the redevelopment of old OCCs near a slow-spreading ridge.
... Reconstruction of the accretion of oceanic lithosphere during the past 20 Myr in the Southern segment of the MAR adjoining the Vema fault from the East implies several pulses of mantle melting beneath the rift valley in the southern MAR segment, given 3-4 Myr oscillations of magmatic activity (Bonatti et al., 2003). ...
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This study presents the results of petrographic, geochemical, and isotope geochronological analyses of rock samples from the Southern flank of the Vema transform fault (Atlantic), which were dredged on cruises 19-th and 22-nd of the R/V Akademik Nikolai Strakhov. The sample suite includes both fresh and metamorphosed gabbros, dolerites, serpentinites, metapyroxenites. Zircons separated from three gabbro samples recovered at three different stations were used for in situ U–Pb dating by LA-ICP-MS. The ages reveal a strong linear relationship with a distance from the axis of the Mid-Atlantic ridge, which allowed us to estimate the rate of spreading in this segment of the Mid-Atlantic Ridge. It can be concluded that the estimated spreading rate of 16.2 ± 0.8 mm/yr was constant over the past 15 Myr. The mutual consistency of all U-Pb zircon and ³⁹Ar–⁴⁰Ar amphibole ages (Cipriani et al., 2009) obtained from the sampled transect suggests the temporal continuity of magmatic events that led to the formation of the original gabbroic rocks and their transformation during subsequent metamorphism. Rb—Sr isotope data show that hydrothermal activity took place in the presence of seawater between 14.7 and 9 Ma in the spreading axis region. Variations in the Nd isotopic composition in the time sequence of magmatic events indicate a high degree of chemical and isotopic heterogeneity of the ascending mantle material which became later entrained in the melting region beneath a spreading zone. Melting of the sources with primitive mantle composition (εNd ~ + 8 to +9) as well as enriched sources took place in the time interval between ~ 17 and 14.7 Ma and at about 8 Ma. The enriched source material is most likely represented by ancient mafic substratum.
... A second set of observations is concerned with Neogene and Paleogene volcanism and regional epeirogeny. Away from the Reykjanes Ridge with which the buoyant mantle upwelling hypothesis is directly concerned, there is evidence for significant off-axis igneous activity, transient dynamic support of oceanic gateways, and regional epeirogeny cannot easily be accounted for by an axially restrictive model whereby patches of buoyant mantle are envisaged as being confined within a narrow corridor that is <100 km wide ( Barnouin-Jha et al., 1997;Bonatti et al., 2003;Scott & Stevenson, 1989). Since oceanic lithosphere has a small elastic thickness, loading effects generated by cells of buoyant upwelling are unlikely to have regional consequences. ...
... On the other hand, small amounts of interstitial melt can favor grain boundary sliding reducing the effective viscosity. Short-term temporal variations of mantle upwelling below a segment of the Mid Atlantic Ridge have been ascribed to active components due to non-uniform mantle rheology, with a sub-ridge low-viscosity zone between layers with higher viscosity due to loss of H 2 O [Bonatti et al., 2003;Cipriani et al., 2009]. The increased viscosity in the upper part of the melting region (where dry melting occurs) induced by dehydration, limits buoyant upwelling; as a consequence, solid flow within the upper layer is mostly driven by plate separation. ...
... Few examples exist of the natural variability of melt supply to mid-ocean ridges. Analysis of dredge samples of exposed mantle rocks along a large equatorial transform of the MAR support an~3-4 Myr long oscillations in the degree of mantle melting, related to upwelling patterns at slow-intermediate spreading rates (Bonatti et al., 2003). Similarly, regional seismic studies mapping relative variations in thickness of fast-spreading crust in the Pacific based on two-way travel time measurements (Ranero et al., 1997) support 2-3 Myr. ...
Article
Crustal structure provides the key to understand the interplay of magmatism and tectonism while oceanic crust is constructed at Mid Ocean Ridges (MOR). At slow spreading rates, magmatic processes dominate central areas of MOR segments, whereas segment ends are highly tectonised. The TAMMAR segment at the Mid-Atlantic Ridge (MAR) between 21°25' N and 22° N is a magmatically active segment. At ~4.5 Ma this segment started to propagate south, causing the termination of the transform fault at 21°40' N. This stopped long-lived detachment faulting and caused the migration of the ridge offset to the south. Here, a segment centre with a high magmatic budget has replaced a transform fault region with limited magma supply. We present results from seismic refraction profiles that mapped the crustal structure across the ridge crest of the TAMMAR segment. Seismic data yield crustal structure changes at the segment centre as a function of melt supply. Seismic Layer 3 underwent profound changes in thickness and became rapidly thicker ~5 Ma. This correlates with the observed “Bull's eye” gravimetric anomaly in that region. Our observations support a temporal change from thick lithosphere with oceanic core complex formation and transform faulting to thin lithosphere with focused mantle upwelling and segment growth. Temporal changes in crustal construction are connected to variations in the underlying mantle. We propose there is a link between the neighbouring segments at a larger scale within the asthenosphere, to form a long, highly magmatically active macro segment, here called the TAMMAR-Kane MacroSegment.
... To determine the timing of magmatism and hydrothermal alteration during formation of the lower crust at a slow-spreading ridge, we applied high-precision geochronology to a hydrothermally metamorphosed gabbro from the Vema lithospheric section at 11°N on the MAR. The Vema lithospheric section is exposed along a~300 km ridge on the southern side of the Vema transform fault (Figure 1) [Auzende et al., 1989;Bonatti et al., 2003;Heezen et al., 1964;Kastens et al., 1998]. The ridge rises 3-4 km above the transform valley, and the faulted northern side exposes a relatively undisturbed cross section through pillow basalts, sheeted dikes, gabbros, and mantle peridotites [Auzende et al., 1989]. ...
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New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid-Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high-temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion-free zircons from the sample yielded Th-corrected ²⁰⁶Pb/²³⁸U dates of 13.528±0.101 to 13.353±0.057Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1-20μm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion-rich zircons suggest that they formed by coupled dissolution-reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th-corrected ²⁰⁶Pb/²³⁸U dates for the inclusion-rich zircons range from 13.598±0.012 to 13.503±0.018Ma and predate crystallization of all but one of the inclusion-free zircons, suggesting that the inclusion-rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction,206Pb/238U dates range from 13.341±0.162 to 12.993±0.055Ma and postdate crystallization of both the inclusion-rich zircons and inclusion-free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge-transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high-precision U-Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid-ocean ridges. Key words: zircon; xenotime; hydrothermal; Mid-Atlantic Ridge; Vema; mid-ocean ridge
... However, gravity minima are also centered above some inner highs such as those located at the eastern RTI of the Arkhangelsky and the Doldrums transforms, probably related to a relative thick crust due to intense basaltic magmatism (Fig. 8). The western inactive part of the Doldrums fracture zone contains topogravity features similar to those observed in other transverse ridges such as that of Vema (Bonatti et al., 2003), although gravity anomalies are not very high on the southern flank of the transform valley. ...
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The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transform domains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs). Among these megatransforms, the Doldrums system (7-8ºN) is arguably the less studied, although it may be considered the most magmatically active. New geophysical data and rock samples were recently collected during the 45th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allow to reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled with extensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km2. The new data clearly indicate that the Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward the central part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminary interpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regime or the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections. We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphere by tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, and the exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys.
... Many studies, especially on slow-spreading ridges, indicate heterogeneity of the oceanic lithosphere not predicted by the Penrose model (Cannat, 1996;Blackman and Collins, 2010). Moreover, a significant reduction of the thickness of the oceanic crust is observed in many locations along slow-spreading ridges (Bonatti et al., 2003;Escartín et al., 2008;Ciazela et al., 2015). Heterogeneous and thinner oceanic crust implies a more important role of melt-rock reaction with respect to fractional crystallization during magma differentiation in the lower oceanic crust (Coogan et al., 2000b;Koepke et al., 2005;Lissenberg and Dick, 2008;Boulanger et al., 2020;Sanfilippo et al., 2020;Ferrando et al., 2021aFerrando et al., , 2021b. ...
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Reactive porous or focused melt flows are common in crystal mushes of mid-ocean ridge magma reservoirs. Although they exert significant control on mid-ocean ridge magmatic differentiation, their role in metal transport between the mantle and the ocean floor remains poorly constrained. Here we aim to improve such knowledge for oceanic crust formed at slow-spreading centers (approximately half of present-day oceanic crust), by focusing on specific igneous features where sulfides are concentrated. International Ocean Discovery Program (IODP) Expedition 360 drilled Hole U1473A 789 m into the lower crust of the Atlantis Bank oceanic core complex, located at the Southwest Indian Ridge. Coarse-grained (5–30 mm) olivine gabbro prevailed throughout the hole, ranging locally from fine- (<1 mm), to very coarse-grained (>30 mm). We studied three distinct intervals of igneous grain size layering at 109.5–110.8, 158.0–158.3, and 593.0–594.4 meters below seafloor to understand the distribution of sulfides. We found that the layer boundaries between the fine- and coarse-grained gabbro were enriched in sulfides and chalcophile elements. On average, sulfide grains throughout the layering were composed of pyrrhotite (81 vol.%; Fe1-xS), chalcopyrite (16 vol.%; CuFeS2), and pentlandite (3 vol.%; [Ni,Fe,Co]9S8), which reflect paragenesis of magmatic origin. The sulfides were most commonly associated with Fe-Ti oxides (titanomagnetites and ilmenites), amphiboles, and apatites located at the interstitial positions between clinopyroxene, plagioclase, and olivine. Pentlandite exsolution textures in pyrrhotite indicate that the sulfides formed from high-temperature sulfide liquid separated from mafic magma that exsolved upon cooling. The relatively homogenous phase proportion within sulfides along with their chemical and isotopic compositions throughout the studied intervals further support the magmatic origin of sulfide enrichment at the layer boundaries. The studied magmatic layers were likely formed as a result of intrusion of more primitive magma (fine-grained gabbro) into the former crystal mush (coarse-grained gabbro). Sulfides from the coarse-grained gabbros are Ir-Platinum Group Element-rich (PGE; i.e., Ir, Os, Ru) but those from the fine-grained gabbros are Pd-PGE-rich (i.e., Pd, Pt, Rh). Notably, the sulfides from the layer boundaries are also enriched in Pd-PGEs, and therefore elevated sulfide contents at the boundaries were likely related to the new intruding melt. Because S concentration at sulfide saturation level is dependent on the Fe content of the melt, sulfide crystallization may have been caused by FeO loss, both via crystallization of late-precipitating oxides at the boundaries, and by exchange of Fe and Mg between melt and Fe-bearing silicates (olivine and clinopyroxene). The increased precipitation of sulfide grains at the layer boundaries might be widespread in the lower oceanic crust, as also observed in the Semail ophiolite and along the Mid-Atlantic Ridge. Therefore, this process might affect the metal budget of the global lower oceanic crust. We estimate that up to ∼20% of the Cu, ∼8% of the S, and ∼84% of the Pb of the oceanic crust inventory is accumulated at the layer boundaries only from the interaction between crystal mush and new magma.
... At fast spreading ridges, the time-scale of recharge of axial magma chambers is typically decadal to centennial (Moore et al., 2014); whereas the residence time of magma in the chamber is centennial to millennial (Sims et al., 2002), and melt-extraction from the magma lens to the surface, can occur over periods as short as a few days (Zellmer et al., 2012). In contrast, source processes vary on longer timescales >1 Myrs (Bonatti et al., 2003;Brandl et al., 2016). These processes include both thermal changes in the mantle and variations of the mantle's composition entering the melting-zone, both of which control the composition of primary melts entering the magma chamber. ...
... Searle (1981) first showed the existence of a N-S trending, ~ 40 km-long intra-transform spreading centre located at a longitude of ~ 31°45' W. The axial part of the ITR has been recently investigated by expeditions CE15007, CE16014 and CE18008 with the R/V Celtic Explorer that confirmed the occurrence of core complexes exhumed probably by low angle normal faults (Georgiopoulou and CE18008 Scientific Party, 2018;. Similar to what was documented in large transforms elsewhere (e.g., Prince and Forsyth, 1988;Bonatti et al., 2003), normal oceanic crust becomes thinner towards the transform, reaching a thickness of 3.5 km within the intra-transform domain Calvert and Whitmarsh, 1986). ...
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The Charlie Gibbs offsetting by ~ 340 km the Mid Atlantic Ridge (MAR) axis at 52°-53° N is one of the main transform systems of the North Atlantic. Located between long mid-ocean ridge segments influenced from the south by the Azores and from the north by the Iceland mantle plumes, this transform system has been active since the early phases of North Atlantic rifting. Object of several surveys in the ’70 and ’80, Charlie Gibbs received great attention for its unique structure characterized by two long-lived right-lateral transform faults linked by a short ~ 40 km-long intra-transform spreading centre (ITR) with parallel fracture zone valleys extending continuously towards the continental margins. In October 2020 expedition S50 of the R/V A.N. Strakhov surveyed an area of 54,552 km2 covering the entire Charlie Gibbs transform system and the adjacent MAR spreading segments. We collected new bathymetric, magnetic and high-resolution single channel seismic data, along with basaltic, gabbroic and mantle rocks from 21 dredges. This work contains preliminary data from cruise S50 and discusses the large-scale architecture of this unique, long-lived transform system.
... It indicates that a high rate of melt supply conditioned by rapid mantle upwelling between fast-diverging plates controls the formation of thick crust formed over the study area. Previous studies argued that variation in the oceanic crust is the consequence of variable spreading rate, upwelling mantle at ridge axes, and hot spots (Searle 2013;Wang et al. 2011;Bonatti et al. 2003;Langmuir et al. 1992;Kuo and Forsyth 1988;Lin et al. 1990;Hooft et al. 2000). Those studies also argued that thick uniform crust is associated with a fast-spreading ridge with uniform melt supply (Wang et al. 2011;Hooft and Detrick 1995;Torné and Banda 1992). ...
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The sophisticated tectonic setting and obscure structural heterogeneity over the Central Indian Ridge (CIR) make it a challenging target for geophysical study. The present study deals with the mapping of crustal structure over the southern part (18° S–25° S) of the CIR using EIGEN6C4 Bouguer gravity data. We have used the Radial Average Power Spectrum (RAPS) technique to estimate Moho depths and compared the results with the CRUST1.0 earth Moho model for validation purposes. We observed the RAPS using spectral window 1° × 1° exhibits a minimum root mean square (RMS) difference of 0.85 km with the CRUST1.0. Furthermore, Moho depths have also been estimated along the IFREMER cruise track (ID: 82001211; 1984) using Parker-Oldenburg’s inversion algorithm for comparison to Moho depths estimated by the RAPS. We observed that the Moho estimated from the two techniques differ with RMS value of 0.92 km. We estimate the crustal thickness over the study area by deducting water depth from the Moho depth derived using the RAPS technique. We defined thin, normal, and thick crust with thickness of < 6 km, 6–8 km, and > 8 km. Thin crust consists of 11.03% of the total area and 9% of the total volume of the crust. Thick crust consists of 18.43% of the total area and 21.87% of the total volume of the crust. Both thin and thick crusts exhibit symmetrical and asymmetrical patterns about the CIR ridge axis. Thin crust over the study area reflects periods and locations of reduced melt supply. Thick crust arranged symmetrically about the ridge crest segments reflects periods of enhanced melt supply to the ridge axis. Thick crust located asymmetrically to the ridge axis is a product of isolated off-axis melt anomalies.
... This can lead to the development of transpressional ridges or transtensional basins as previously strike-slip fault segments along the transform begin to accommodate components of strain across their axes, corrupting the signal of plate motion orientation change (e.g. Bonatti et al., 2003). ...
Chapter
Reconstruction maps and map sequences provide the essential basis and boundary conditions for interpreting ancient Earth system processes. Such maps can be generated using a variety of techniques to reunite formerly conjugate features as interpreted in a range of different data types. The highest confidence is achieved by reuniting large numbers of geographically widespread and precisely located markers that are simply and robustly interpreted. These markers are most widespread in the oceanic lithosphere, because of its relatively simple tectonic history of plate divergence that ends with subduction. The reliability of maps decreases with time in the past as the distribution of markers is first confined to the continental crust and, before this, to geographically ever-smaller and geologically more interpretation-dependent remnants, alongside which the range of suitable reconstruction techniques narrows. Relative reconstructions must be placed in a global reference frame, using any of a variety of techniques that also become smaller with past time. Reconstructions have been used for paleogeographic mapping to provide constraints on studies of regional deformation and to provide constraints on plate motion for geodynamic studies.
... The thickness of the oceanic crust correlates with the pressure of final melting (P f ) (Brunelli et al., 2018;Lambart et al., 2016) and might vary at different geological settings. The pressure of final melting was predicted to be 0.7 GPa according to mantle flow models (Bonatti et al., 2003;Ligi et al., 2008) or 1.1 GPa as the Vema Eastern Mid-Atlantic Ridge axial segment (Brunelli et al., 2018); both conditions can be applied to the low-spreading ridges because the heat conduction to the surface will result in a deep transition from the conductive to the convective thermal regions. For P f = 1.1 GPa and 12% maximum melting fraction of peridotite, the oceanic crustal thickness would be approximately 8.5 km (Fig. 9), which is comparable to the crustal thickness produced by a typical MORB source. ...
Article
Testing the hypothesis that the Hainan mantle plume triggered the opening of the South China Sea (SCS) and interacted with the spreading ridge is important for the purpose of understanding the dynamics of the evolution of the SCS. This study examined the most primitive olivine (Fo = 83.1–87.1) and spinel (Cr# = 28−60 %) in the basalt and glassy samples collected from Hole U1431E of International Ocean Discovery Program (IODP) Expedition 349, with the goal of estimating the crystallization temperature. Spinels mostly enclosed by olivines have low Fe3+ and Ti contents and show compositional equilibrium with the host olivines. The application of the Al-in-olivine thermometer provided crystallization temperatures for the studied samples, which varied from 1208 ± 29 °C to 1287 ± 29 °C. When extrapolating to primitive olivine of Fo = 90, the crystallization temperature of primitive olivine is approximately 1350 °C, which is much higher than that of other primitive mid-ocean ridge basalt (MORB) but comparable to that of Iceland oceanic island basalt (OIB). Therefore, the mantle potential temperature at the extinct ridge of the SCS at ~16 Ma is much higher than that beneath a normal mid-ocean ridge, manifesting the mantle plume-ridge interaction during the last spreading of the SCS. Given the lack of thick oceanic crust, seamount chains or lineaments in the SCS, we propose that this plume impacted on the SCS might be eclogite-rich and of low buoyancy.
... The data analyzed here provide a record of crustal accretion at intermediate 124 spreading rates over a period of ~12 Myr, without the complications of amagmatic 125 spreading that influence findings from the MAR (e.g. Bonatti et al., 2003), and far longer 126 than the ~235 kyr-long record from the EPR (Boulahanis et al., 2020). These data span an 127 along-axis distance of ~255 km over two spreading segments, providing a large number 128 of adjacent profiles that are amenable to stacking. ...
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Our understanding of oceanic crustal formation is mostly limited to observations of young crust formed in the past several million years, due to the thick sediments on older crust, and the remote location of many spreading centers. Here we use 40 m-resolution bathymetric data collected over hundreds of square kilometers during the search for Malaysia Airlines Flight 370 on the flank of the Southeast Indian Ridge, which provides a record of crustal accretion from 11–23 Ma. Spectra calculated from the data show a characteristic timescale of 300¬–400 kyr, and no evidence for periodicity coinciding with glacial cycles. This characteristic timescale could be explained by fluctuations in melt supply and the amount of faulting, leading to variations in crustal thickness. We show that this timescale of variation is consistent with porosity waves observed in a two-phase flow model, which persist over millions of years.
... To determine the timing of magmatism and hydrothermal alteration during formation of the lower crust at a slow-spreading ridge, we applied high-precision geochronology to a hydrothermally metamorphosed gabbro from the Vema lithospheric section at 11°N on the MAR. The Vema lithospheric section is exposed along a~300 km ridge on the southern side of the Vema transform fault (Figure 1) [Auzende et al., 1989;Bonatti et al., 2003;Heezen et al., 1964;Kastens et al., 1998]. The ridge rises 3-4 km above the transform valley, and the faulted northern side exposes a relatively undisturbed cross section through pillow basalts, sheeted dikes, gabbros, and mantle peridotites [Auzende et al., 1989]. ...
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New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation and hydrothermal metamorphism during formation of the lower crust at the Mid-Atlantic Ridge (MAR). The studied sample is an altered gabbro from the Vema lithospheric section (11ºN). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high-temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion-free zircons from the sample yielded Th-corrected 206Pb/238U dates of 13.528 ± 0.101 to 13.353 ± 0.057 Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1–20 µm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion-rich zircons suggest that they formed by coupled dissolution–reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th-corrected 206Pb/238U dates for the inclusion-rich zircons range from 13.598 ± 0.012 to 13.503 ± 0.018 Ma and predate crystallization of all but one of the inclusion-free zircons, suggesting that the inclusion-rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction, 206Pb/238U dates range from 13.341 ± 0.162 to 12.993 ± 0.055 Ma and postdate crystallization of both the inclusion-rich zircons and inclusion-free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge–transform intersection, and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high-precision U-Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid-ocean ridges.
... However, gravity minima are also centered above some inner highs such as those located at the eastern RTI of the Arkhangelsky and the Doldrums transforms, probably related to a relative thick crust due to intense basaltic magmatism (Fig. 8). The western inactive part of the Doldrums fracture zone contains topogravity features similar to those observed in other transverse ridges such as that of Vema (Bonatti et al., 2003), although gravity anomalies are not very high on the southern flank of the transform valley. ...
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The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transform domains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs). Among these megatransforms, the Doldrums system (7-8ºN) is arguably the less studied, although it may be considered the most magmatically active. New geophysical data and rock samples were recently collected during the 45 th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allow to reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled with extensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km 2 . The new data clearly indicate that the Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward the central part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminary interpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regime or the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections. We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphere by tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, and the exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys.
... Numerical modeling efforts also began in the late 1970s. Due to computer limitations, FIGURE 3 | (A) Sketch illustrating the interaction of seismicity, fluid-flow, geochemical reactions, and life at and below the seafloor at MTFFZs inspired by (B) a 3D model of the Vema transform fault in the Equatorial Atlantic (based on multibeam bathymetric data collected during the PRIMAR-S19 Cruise; Fabretti et al., 1998;Bonatti et al., 2003). The large red rectangle indicates a typical location for the area sketched in the upper panel. ...
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Marine transform faults and associated fracture zones (MTFFZs) cover vast stretches of the ocean floor, where they play a key role in plate tectonics, accommodating the lateral movement of tectonic plates and allowing connections between ridges and trenches. Together with the continental counterparts of MTFFZs, these structures also pose a risk to human societies as they can generate high magnitude earthquakes and trigger tsunamis. Historical examples are the Sumatra-Wharton Basin Earthquake in 2012 (M8.6) and the Atlantic Gloria Fault Earthquake in 1941 (M8.4). Earthquakes at MTFFZs furthermore open and sustain pathways for fluid flow triggering reactions with the host rocks that may permanently change the rheological properties of the oceanic lithosphere. In fact, they may act as conduits mediating vertical fluid flow and leading to elemental exchanges between Earth’s mantle and overlying sediments. Chemicals transported upwards in MTFFZs include energy substrates, such as H2 and volatile hydrocarbons, which then sustain chemosythetic, microbial ecosystems at and below the seafloor. Moreover, up- or downwelling of fluids within the complex system of fractures and seismogenic faults along MTFFZs could modify earthquake cycles and/or serve as “detectors” for changes in the stress state during interseismic phases. Despite their likely global importance, the large areas where transform faults and fracture zones occur are still underexplored, as are the coupling mechanisms between seismic activity, fluid flow, and life. This manuscript provides an interdisciplinary review and synthesis of scientific progress at or related to MTFFZs and specifies approaches and strategies to deepen the understanding of processes that trigger, maintain, and control fluid flow at MTFFZs.
... The crustal age of these sites, estimated from magnetic anomalies and basaltic glass 40 Ar/ 39 Ar dating, ranges from 19.2 to 10.2 Ma 8,11,13 . The VLS peridotites chosen for this study have been the object of previous studies 8-12 ; they display protogranular/porphyroclastic textures and their relict mineral phases include orthopyroxene (opx), clinopyroxene (cpx), spinel (sp), and rare olivine 8,9,11 ; they are serpentinized to various extents 14 . They are generally ...
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Global correlations of mid-ocean-ridges basalt chemistry, axial depth and crustal thickness have been ascribed to mantle temperature variations affecting degree of melting. However, mantle H2O content and elemental composition may also play a role. How H2O is distributed in the oceanic upper mantle remains poorly constrained. We tackled this problem by determining the H2O content of orthopyroxenes (opx) and clinopyroxenes (cpx) of peridotites from a continuous lithospheric section created during 26 Ma at a 11°N Mid-Atlantic Ridge segment, and exposed along the Vema Transform. The H2O content of opx ranges from 119 ppm to 383 ppm; that of cpx from 407 ppm to 1072 ppm. We found anomalous H2O-enriched peridotites with their H2O content not correlating inversely with their degree of melting, although H2O is assumed to be incompatible during melting. Inverse correlation of H2O with Ce, another highly incompatible component, suggests post-melting H2O enrichment. We attribute a major role to post-melting temperature-dependent diffusion of hydrogen occurring above the melting region, where water-rich melt flows faster than residual peridotites through dunitic conduits cross-cutting the uprising mantle. Accordingly, estimates of the H2O content of the MORB mantle source based on H2O in abyssal peridotites can be affected by strong uncertainties.
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Comprehensive major-, trace-element and rhenium-osmium (Re-Os) isotope data are presented for abyssal peridotites from Ocean Drilling Program (ODP) Leg 209, in the North Atlantic. The samples are from a single core (Site 1274A) located on the western wall of the axial rift valley of the Mid-Atlantic ridge, and their study allows elemental and isotope information to be precisely related to spatial variations in primary lithology, serpentinisation and seafloor weathering. The harzburgites and dunites at this site are highly serpentinised (with the degree of serpentinisation increasing with depth below the sea floor). Petrographic observations and variations of fluid mobile elements (such as Ba, Sr, U, and Re) are consistent with seawater interaction in the upper part of the core. Nevertheless, major and trace element indicators of the extent of melt depletion indicate extreme melt loss, and suggest that these are amongst the most depleted abyssal peridotites recovered thus far. Despite the evidence for extensive serpentinisation and sea floor weathering all of the samples possess 187Os/188Os isotope compositions that are less radiogenic than estimates for the primitive upper mantle (PUM), lower than any yet reported for abyssal peridotites, and consistent with melt loss over, at least, the past 1.5Ga. Single sulphide Re-Os data show evidence for recent recrystallisation or diffusional modification either due to partial melting or seawater alteration. However, some grains are extremely unradiogenic (187Os/188Os=0.114) providing unequivocal evidence for at least some degree of melt depletion at ca. 2Ga. Taken with recently published data these results suggest that ancient melt depletion may be a widespread feature of the oceanic upper lithosphere, even though evidence for this depleted reservoir has not yet been observed in the Re-Os chemistry of mid-ocean ridge basalts.
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We propose to document the timescale of the magmatic processes occurring beneath the slow-spreading Central Indian Ridge (CIR, 19°2S) through the study of the chemical variations recorded by lavas erupted on-axis. The samples have been collected along a 40 km-long profile transverse to the ridge and running from the ridge axial through up to the Brunhes-Matuyama isochrone (~800 Ky, GIMNAUT cruise). The lavas are greatly homogeneous in term of Mg# but range from N-MORBs to E-MORBs in terms of trace element concentrations (0.6<La/Sm N < 3.4). Their Sr-Nd isotope ratios start from the center of the isotopic field of the CIR lavas and overlap its enriched end. Trace element ratios plotted along the profile describe saw-tooth patterns characterized by enriched spikes regularly and symmetrically distributed on both sides of the ridge axis. Consequently, the lava compositions are an indicator of magmatic processes that have fluctuated periodically through time. Based on the location of the enriched lavas along the profile, the timescale of such magmatic processes has been estimated to 230 Ky. Fractional crystallization can be ruled out to explain the observed chemical variations. Thus, they reflect the variations through time of the primary melt composition, partly governed by mantle source heterogeneity according to the Sr-Nd isotopes. To reproduce the observed periodicity, we propose an interconnecion between the source heterogeneity and the variation of the melting rates. In that case, the most enriched lavas are the products of very low melting of a fertile enriched mantle component. This component may have been specifically introduced in the melting column each ~230 Ky or the melt extraction modalities may have changed with the same timescale. Apart from these enriched episodes, mixing between enriched and more depleted primary melts occurs normally during magma genesis/aggregation and the enriched signatures are diluted into a N-MORB-type composition.
Chapter
Geophysical measurements and models constrain the total rate of production of crustal material and the flux of thermal energy over the global ridge system. Flux estimates based on basin-scale compilations of heat-flow measurements or on 1-dimensional geodynamic models of melt generation and plate cooling provide a useful, but only a partial view of the crustal thermal regime. The rate of heat supply to the crust depends both on complex patterns of flow in the mantle, and on how this flow supplies magma (a major carrier of advective heat flux) to the crust. Much progress is still needed if we are to understand the thermal regime at and close to the crust-mantle boundary, and hence the extent to which segmentation and other variations in crustal structure may be inherited from the mantle. Within the lower and middle crust, the thermal regime is dominated by the presence (or otherwise) of crustal magma chambers. Over the last decade, geophysical data have provided a progressively more sophisticated understanding of these features, at all spreading rates. Correlations and quantitative links between new models of magma chamber structure and what is known from other disciplines about the overlying hydrothermal circulation system remain weak. Significant unknowns also still remain regarding the patterns and pathways of hydrothermal circulation within the crust. High resolution geophysical data are now beginning to provide quantitative constraints on the physical structure (overall porosity, and interconnectedness of pore spaces) of the permeable crust. The same observations and methods are also beginning to allow us to detect in situ variations in the properties of the fluids themselves, to depths equivalent to the base of layer 2, and on horizontal scales of several kilometres. In one case this has provided glimpses of what may be two-layer hydrothermal convection, related to phase separation. How flow patterns are influenced by key tectonic parameters such as spreading rate and ridge morphology remains an open issue. Also unknown is the extent to which shallow circulation may be driven by newly injected dikes, and the spatial and temporal scales of the resultant thermal perturbations. Lastly, we must consider the case where high and low temperature hydrothermal circulation is occurring in the absence of any significant crustal magma body. Are such systems related to the cooling of rocks that have recently crystallized from basaltic magmas? Do serpentinization reactions play a significant role? And how widespread are such circulation regimes?.
Article
Recent studies have proposed that the bathymetric fabric of the seafloor formed at mid-ocean ridges records rapid (23,000 to 100,000 years) fluctuations in ridge magma supply caused by sealevel changes that modulate melt production in the underlying mantle. Using quantitative models of faulting and magma emplacement, we demonstrate that, in fact, seafloor-shaping processes act as a low-pass filter on variations in magma supply, strongly damping fluctuations shorter than about 100,000 years. We show that the systematic decrease in dominant seafloor wavelengths with increasing spreading rate is best explained by a model of fault growth and abandonment under a steady magma input. This provides a robust framework for deciphering the footprint of mantle melting in the fabric of abyssal hills, the most common topographic feature on Earth.
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Bernard Castaing... Examinateur Luce Fleitout... Rapporteur Philippe Lognonné... Examinateur / Président du jury Tony Watts... Rapporteur
Article
Abyssal peridotites recovered directly from mid-ocean ridges (mid-ocean ridge peridotites hereafter) are generally interpreted to be formed as a residue after partial melting and melt extraction in the adiabatically upwelling mantle beneath mid-ocean ridges. Osmium and some other isotopic characteristics of some mid-ocean ridge peridotites, which were formed by partial melting events, are much older than present-day mid-ocean ridge systems (ancient melting residual peridotites), however, suggesting ancient origins. Ancient melting residual peridotites might have been incorporated into the oceanic asthenosphere beneath mid-ocean ridges. If this is the case, the nature of the oceanic plate and its formation processes need to be reconsidered because the asthenospheric mantle, which has abundant ancient melting residual peridotites, is not sufficient to create basaltic oceanic crusts, followed by formation of the serpentinized peridotite crust by hydrothermal circulation along faults developed around the mid-ocean ridges by continuous spreading. Comprehensive geochemical studies on depleted mid-ocean ridge peridotites are required to detect and verify ancient melting residual peridotites from mid-ocean ridge peridotites. It is crucial to examine the effects of minor Os-rich phases on 187Os/188Os isotopic compositions to understand the meaning of their model ages. Precise chemical analyses of fluid mobile elements including light elements, such as H, Li, Be, and B, on clinopyroxene, as well as orthopyroxene, provide clues about differences in geochemical signatures and thermal histories between present-day mid-ocean ridge residual peridotites and ancient melting residual peridotites.
Article
A model for free-convective flows in the asthenosphere beneath the ocean has been derived. The thermophysical model for the asthenosphere beneath a mid-ocean ridge is a horizontal layer being heated from the butt-end (i.e., in the vicinity of the ridge axis) and cooled at the roof, with the sole adiabatic. Laboratory modeling yielded fields of velocity and temperature in the horizontal layer in the boundary layer regime. Requirements for the correct determination of the velocity and temperature fields in the asthenosphere have been defined from the results of thermophysical modeling. The asthenosphere viscosity and maximum temperature gradient in the asthenosphere in the vicinity of the MOR axis have been estimated. Velocity and temperature fields in the asthenosphere layer have been obtained under slow-spreading conditions. On the basis of the experimental field of temperature and streamlines, using the velocity field obtained by laboratory modeling and experimental state diagram of peridotite, stability fields of the main mantle-derived parageneses and a zone of partial melting in the asthenosphere have been established. The width of the partial-melting zone could average 5-7 km (on the one side of the ridge), and its depth, about 80 km. Depthward, the gabbroid associations grade into spinel peridotites, which in turn give way to garnet peridotites. At depths of more than 400 km, olivine grades into ringwoodite.
Article
The morphology of abyssal hills provides important information on the crustal accretion processes acting at mid ocean ridges. Discretely characterizing abyssal-hill shape is challenging, and therefore, the study of abyssal-hill morphology is often approached by examining the average properties of relatively wide regions containing multiple hills. However, this averaging process removes any spatial trends in abyssal-hill shape that may reflect temporal variations in crustal accretion processes. Here we present an automated approach for analyzing the shape of individual abyssal hills, which we detect using the ridgelet transform method of Downey and Clayton (2007). We first analyze seafloor morphologies, as revealed in multibeam bathymetry surveys, across 16 mid-ocean ridge segments with varying spreading rates. The results of this analysis display the known negative correlation between the width and height of abyssal hills and the spreading rate, thus validating our approach. Our results also show that the fraction of inward-facing abyssal hills exhibits a simple linear trend across all spreading rates, suggesting a property that could be used to determine the spreading rate at which the seafloor formed. We then apply our technique to study a flow line transect collected across the fast-spreading 10°30′N segment of the northern East Pacific Rise, revealing temporal changes in the shape of the abyssal hills formed during the last 3.8 Myr. The youngest part of the flow line coincides with a location where basaltic glasses were previously sampled. The abyssal-hill morphology and MgO content of the glass samples share mutual trends (over 10⁵-10⁶ yrs), suggesting that abyssal-hill morphology is sensitive to the rate at which magma is supplied to the ridge axis. Finally, a ∼1 Myr cyclic variation in seafloor morphology is observed. This periodicity is attributed to temporal changes in magma supply at the ridge axis, possibly related to upper mantle dynamics or the presence of chemical heterogeneities.
Article
Crustal features of the Reykjanes Ridge have been attributed to mantle plume flow radiating outward from the Iceland hotspot. This model requires very rapid mantle upwelling and a “rheological boundary” at the solidus to deflect plume material laterally and prevent extreme melting above the plume stem. Here we propose an alternative explanation in which shallow buoyant mantle upwelling instabilities propagate along axis to form the crustal features of the ridge and flanks. As only the locus of buoyant upwelling propagates this mechanism removes the need for rapid mantle plume flow. Based on new geophysical mapping we show that a persistent sub-axial low viscosity channel supporting buoyant mantle upwelling can explain the current oblique geometry of the ridge as a reestablishment of its original configuration following an abrupt change in opening direction. This mechanism further explains the replacement of ridge-orthogonal crustal segmentation with V-shaped crustal ridges and troughs. Our findings indicate that crustal features of the Reykjanes Ridge and flanks are formed by shallow buoyant mantle instabilities, fundamentally like at other slow spreading ridges, and need not reflect deep mantle plume flow.
Article
The Reykjanes Ridge is a key setting to study plate boundary processes overlaid on a regional mantle melting anomaly. The ridge originated as one arm of a ridge-ridge-ridge triple junction that separated Greenland, Eurasia, and North America. It initially formed a linear axis, spreading orthogonally at slow rates without transform faults or orthogonal crustal segmentation. Stable spreading continued in this configuration for ∼18 Myr until Labrador Sea spreading ceased, terminating the triple junction by joining Greenland to North America and causing a rapid ∼30° change in opening direction across the ridge. The ridge abruptly became segmented and offset by a series of transform faults that appear to decrease in length and spacing toward Iceland. Without further changes in opening direction, the ridge promptly began to reassemble its original linear configuration systematically and diachronously from north to south, even though this required the ridge to spread obliquely as it became linear again. Prominent V-shaped crustal ridges spread outward from the axis as the ridge became linear. This reconfiguration is presently nearly complete from Iceland to the Bight transform fault, a distance of nearly 1000 km. Both mantle plume and plate boundary processes have been proposed to control the tectonic reconfigurations and crustal accretion characteristics of the Reykjanes Ridge. Here we review the ridge characteristics and tectonic evolution and various models proposed to influence them.
Article
Oceanic structures, MOR rift zones in particular, where the mantle and lower crustal rocks are exposed on the seafloor are discussed. These complexly built structures, where the normal stratified oceanic crustal sequence does not form, have been called dry spreading zones. Possible formation mechanisms and models of such structures are analyzed. The volume of melts that reaches the crust in dry spreading zones appears to be much smaller than required to form a stratified sequence. The stretching of the previously formed Iithosphere gives rise to numerous tilted fracture zones that accommodate lithospheric blocks movements, rotations, and deformations. The simultaneous upwelling of mantle rocks leads to a complex combination of asynchronous multidirectional stress fields and zones of deformation. As a result, a new crust consisting of tectonically separated, deformed, and combined blocks of different rocks is formed. The extent of chaos peaks near fracture zones. The complex structural relationships between rocks in rift zones are partially concealed by young lavas. The available data suggest that the nonstratified crust is not an accidental phenomenon, but makes up a considerable part of the Atlantic structures. Original Russian Text Copyright © 2004 by Peive. English Translation
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This paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation. Five crossings of the gabbro-peridotite contact along the transform wall show that massive mantle peridotite is intruded by cumulate residues of moderately to highly evolved magmas, few of which could be even close to equilibrium with a primary mantle magma. This contact then does not represent the crust-mantle boundary as envisaged in the ophiolite analog for ocean crust. The residues of the magmas parental to the shallow crust must also lie beneath the center of the complex. This, and the nearly complete absence of dunites in peridotites from the transform wall, shows that melt transport through the shallow lithosphere was largely restricted to the central region of the paleo-ridge segment. There is almost no evidence for a melt lens or high-level storage of primitive melt in the upper 1500 m of Atlantis Bank. Thus, the composition of associated mid-ocean ridge basalt appears largely controlled by fractional crystallization of primitive cumulates at depth, near or at the base of the crust, modified somewhat by melt-rock reaction during transport through the overlying cumulate pile to the seafloor. Inliers of the dike-gabbro transition show that the uppermost gabbros crystallized at depth and were then emplaced upward, as they cooled, into the zone of diking. ODP and IODP drilling along the center of the gabbro massif also found few primitive gabbros that could have been in equilibrium with the original overlying lavas. Evidence of large-scale upward, permeable transport of interstitial melt through the gabbros is ubiquitous. Thus, post-cumulus processes, including extensive reaction, dissolution, and re-precipitation within the cumulate pile have obscured nearly all evidence of earlier primitive origins. We suggest that many of the gabbros may have started as primitive cumulates but were hybridized and transformed by later, migrating melts to evolved compositions, even as they ascended to higher levels, while new primitive cumulates were emplaced near the base of the crust. Mass balance for a likely parental melt intruded from the mantle to form the crust, however, requires that such primitive cumulates must exist at depth beneath Atlantis Bank at the center of the magmatic complex. The Atlantis Bank Gabbro Massif accreted by direct magma intrusion into the lower crust, followed by upward diapiric flow, first as a crystal mush, then by solid-state, crystal-plastic deformation, and finally by detachment faulting to the sea floor. The strongly asymmetric spreading to the south, parallel to the transform, was due to fault capture, with the bounding faults on the northern rift valley wall cut off by the detachment fault, which extended across the zone of intrusion causing rapid migration of the plate boundary to the north.
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Oceanic structures, MOR rift zones in particular, where the mantle and lower crustal rocks аге ехposed оп seafloor, аге discussed. These complexly built structures, where the noгmal stratified oceanic crustal sequence does not fоrm, have Ьееn called dry spreading zones. PossibIe foгmation mechanisms and models of such structures аге analyzed. The volume of melts that reaches the crust in dry spreading zones appears to bе much smaller than required to foгm а stratified sequence. The stretching of the previously foгmed lithosphere gives rise to numerous inclined fracture zones that accommodate lithospheric bIocks movements, rotations, and defoгmation s . The simultaneous upweIling of mantle rocks leads to а соmрlех combination of asynchronous multidirectional stress fields and zones of defoгmation. As а result, а new crust consisting of tectonically separated, defoгmed, and combined bIocks of different rocks is foгmed. The extent of chaos peaks пеаг fracture zones. The complex structural relationships between rocks in rift zones аге partially concealed bу young lavas. The availabIe data suggest that the nonstratified crust is not аn accidental phenomenon but makes uр а considегаblе раrt of the Atlantic structures. В статье рассматриваются океанические структуры, преимущественно рифтовые зоны САХ, где на поверхность выведены глубинные мантийные и нижнекоровые породы. Эти зоны сложного строения , где не формируется нормальный стратифицированный разрез океанической коры, получили название областей " сухого" спрединга. Разбираются возможные механизмы и модели формирования таких структур. Показано, что в областях развития процессов сухого спрединга объем расплавов, достигающих коры , гораздо меньше, чем необходимо для формирования стратифицированного разреза. Растяжение ранее сформированной литосферы сопровождается формированием многочисленных наклонных разломных зон, по которым блоки литосферы смещаются, разворачиваются и деформируются. Одновременный подъем мантийных пород приводит к сложному сочетанию разновременных и разнонаправленных полей напряжений, сопровождаемых образованием зон деформаций. При этом образуется кора, состоящая из тектонически разобщенных, деформированных и перемешанных блоков различных пород. Степень этого хаоса максимальна вблизи разломных зон. В рифтовых зонах молодые лавы частично перекрывают сложные структурные соотношения между породами. Показано, что нестратифицированная кора не является случайным феноменом, а слагает значительную часть структур Атлантики.
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Sea floor rocks can reveal Earth’s history. Based on the various sampling operations conducted by ocean going scientists, it is inferred that the relative distribution of oceanic rocks includes about 50 % basalts/dolerites, 20 % gabbros and 30 % peridotites. Basalt and dolerite are the main volcanic rocks of the upper crust, gabbros are formed during the solidification of a magma chamber or magma conduit, and peridotites are either the heavy mineral residues left within the reservoirs after magma solidification or they could be the remains after a partial melting of mantle material. Basalt is the most common type of volcanic rock found on the sea floor. Basalt and other related rocks have been extruded after partial melting of the Earth’s mantle material. Rocks from the sea floor differ from those encountered on land due to their shape and their chemical composition. The effect of seawater and the pressure it exercises on hot, outpouring lava will fashion the shape of deep-sea volcanic rocks giving the sea floor a different appearance than what we see in subaerial environments. Curved and spherical-shaped pillow lavas are only found on the sea floor due to the fact that seawater pressure is equally applied on all directions of the lava flows and their cooling surfaces. Basalts consist of silicates of magnesium, plus iron and calcium oxides. Less common silica-enriched rocks (SiO2 > 53 %) such as andesites (SiO2 = 53–59 %), rhyolites (>70 %) and trachytes (SiO2 = 59–64 %) are also found on some undersea structures such as domes and seamounts.
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The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid-Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along-axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum crustal thicknesses of 8.1, 6.9, and 6.6+/-0.5km, decreasing from north to south. However, the mean crustal thickness is similar for each segment (5.6+/-0.4, 5.7+/-0.4 and 5.1+/-0.3km). Near the segment ends, crustal thickness is 2.5 to 5+/-0.5km with no systematic variation from north to south. At segment ends, both crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the thickness of the upper crust is relatively uniform along axis (~3 km), although its internal structure is laterally heterogeneous (velocity anomalies of +/-0.6 km s-1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along-axis crustal thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH-1) has an unusually thick crustal root (excess thickness of 2-4 km and along-axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow crustal levels by lateral dike injection. Along this portion of the Mid-Atlantic Ridge, our results suggest that differences in axial morphology, seismic crustal thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH-1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle.
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A systematic study of rare earth and other trace elements in discrete diopsides residual abyssal peridotites sampled from 5000 km of ocean ridge demonstrates that they are the residues of variable degrees of melting in the garnet and spinel peridotite fields. Further, the data clearly demonstrate that the peridotites are the residues of near-fractional melting, not batch melting, and that typical abyssal basalt can evolve from aggregated fractional melts. Ion microprobe analyses of diopsides in abyssal peridotites from fracture zones along the American-Antarctica and Southwest Indian ridges reveal ubiquitous extreme fractionation of rare earth elements (REE) ([Ce/Yb]n=0.002-0.05) depletion of Ti (300-1600 ppm), Zr (0.1-10 ppm), and Sr (0.1-10 ppm); and fractionation of Zr relative to Ti (Ti/Zr=250-4000). Ti and Zr in diopsides decrease with decreasing modal cpx in the peridotites, and samples dredged near hotspots are more depleted in incompatible elements than those dredged away from hotspots, consistent with higher degrees upper mantle-melting in the former. All studied samples exhibit marked negative anomalies in Ti and Zr relative to REE. Incompatible element concentrations in peridotite clinopyroxenes are well modeled by repeated melting and segregation in
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A three-dimensional gravity study of the Mid-Atlantic Ridge near the Atlantis transform was conducted to study the evolution of accretionary processes at this slow-spreading center over the last 10 m.y. Cross sections of crustal thickness extracted along the midpoint traces of paleosegments show that, for a few segments, up to 2 km of gradual crustal thinning is observed. The "apparent' crustal thinning is a result of lateral changes in mantle density associated with buoyant upwelling not predicted by the passive flow model used in the study. Variations in computed crustal thickness are observed across axis in all of the paleosegments in the study area, but are not correlated between individual segments. -from Authors
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Gravity data from the Mid-Atlantic Ridge between latitudes 27°50'N and 30°40'N show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis. Large positive gravity anomalies equivalent to reductions of almost 50% in crustal thickness are observed over non-transform discontinuities bounding spreading segments.
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A high-resolution seafloor spreading history of the South Atlantic since chron C34 is constrained by a combination of Seasat altimeter data and underway marine geophysical data. A set of 45 finite rotation poles defines the relative position of Africa and South America at roughly 2-m.y. intervals. A set of 12 stage poles constrain the relative motion of these two plates at 5- to 10-m.y. intervals. The position of the stage poles continuously migrates, reflecting the continuously changing azimuths of the fracture zones. Major changes in spreading direction are observed in the Late Cretaceous and early Cenozoic as the fracture zones sweep out broad S-shaped curves similar to the pattern seen on the Kane fracture zone in the central Atlantic. Small offset fracture zones were found to be the most accurate recorders of changes in plate motion; large offset fracture zones, such as the Agulhas-Falkland fracture zone, were the least reliable recorders. At 30°S, spreading rates decrease throughout the Late Cretaceous from a high of 75 mm/yr at the end of chron C34 to a low of 30 mm/yr around chron C27. A period of slow spreading between chron C30 and chron C20 corresponds to a zone of fracture zone proliferation, an increase in the amplitude of geoid anomalies over fracture zones, greater relief on topographic profiles, and locally, evidence of intraplate crustal deformation. Spreading rates increase at chron C20 to about 50 mm/yr and then gradually decrease during the late Paleogene and Neogene. A comparison of synthetic fracture zones based on the South Atlantic stage poles to the observed trends of fracture zones in the equatorial Atlantic indicates that the Vema and Marathon fracture zones were generated by South Atlantic spreading, as opposed to central Atlantic spreading, at least as far back as 35 m.y. ago and possibly 50 m.y. ago.
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Shear waves converted from compressional waves at mantle discontinuities near 410- and 660-km depth recorded by two broadband seismic experiments in Iceland reveal that the center of an area of anomalously thin mantle transition zone lies at least 100 km south of the upper-mantle low-velocity anomaly imaged tomographically beneath the hotspot. This offset is evidence for a tilted plume conduit in the upper mantle, the result of either northward flow of the Icelandic asthenosphere or southward flow of the upper part of the lower mantle in a no-net-rotation reference frame.
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The chemical composition of mid-ocean ridge basalt, the most prevalent magma type on the planet, reflects the melt's continuous reequilibration with the surrounding mantle during porous flow. Models of basalt extraction that account for the observed uranium-series disequilibria on the Juan de Fuca ridge constrain both the abundance of melt beneath ridges (0.1 to 0.2 percent) and the style of mantle melting. Unlike models that incorporate near-fractional melts (dynamic melting), mixing of equilibrium porous flow melts derived from heterogeneous source materials quantitatively explains the uranium-series observations.
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High-resolution images covering large areas of the seafloor reveal numerous discontinuities along the mid-ocean ridge. These discontinuities occur at a range of scales (10−1,000 km) and define a fundamental segmentation of seafloor spreading centres. Some are transient; others persist for millions of years, migrating along the mid-ocean ridge and disrupting the structural and geochemical character of approximately 20% of the oceanic lithosphere.
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Understanding the nature and composition of the oceanic crust has been a longstanding goal of Earth scientists. Seismic refraction experiments1–3suggest a simple layered crust made of eruptive basalts underlain by a thick layer of doleritic and gabbroic intrusives and a peridotitic upper mantle. Other evidence comes from ophiolite complexes on land4, although generalizations based on ophiolites are uncertain because they may be dismembered and altered during emplacement, and it is not known whether they represent sections of 'mature' oceanic crust, or crust from very small 'aborted' oceans5, anomalous ocean structures6 or marginal basins. The walls of fracture-zone valleys expose thick sections of oceanic lithosphere which are accessible to in situ observations and sampling7,8, but this approach has been criticized because the pattern of faulting in fracture zones may disrupt the original statigraphy of the crust9, and because the crust near fracture zones is anomalously thin3, 10, 11. Here we report the direct observation and sampling of a section of crust and upper mantle exposed at the Vema fracture zone in the Atlantic, using the French submersibleNautile.
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Closely spaced satellite altimeter profiles collected during the Geosat Geodetic Mission (-6 km) and the ERS 1 Geodetic Phase (8 km) are easily converted to grids of vertical gravity gradient and gravity anomaly. The long-wavelength radial orbit error is suppressed below the noise level of the altimeter by taking the along-track derivative of each profile. Ascending and descending slope profiles are then interpolated onto separate uniform grids. These four grids are combined to form comparable grids of east and north vertical deflection using an iteration scheme that interpolates data gaps with minimum curvature. The vertical gravity gradient is calculated directly from the derivatives of the vertical deflection grids, while Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to a combination of high-density data from the dense mapping phases of Geosat and ERS 1 along with lower-density but higher-accuracy profiles from their repeat orbit phases. A comparison with shipboard gravity data shows the accuracy of the satellite-derived gravity anomaly is about 4-7 mGal for random ship tracks. The accuracy improves to 3 mGal when the ship track follows a Geosat Exact Repeat Mission track line. These data provide the first view of the ocean floor structures in many remote areas of the Earth. Some applications include inertial navigation, prediction of seafloor depth, planning shipboard surveys, plate tectonics, isostasy of volcanoes and spreading ridges, and petroleum exploration.
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Satellite altimetry derived geoid heights and marine gravity anomalies can be combined to determine a detailed gravity fieldover the oceans using the least-squares collocation method and spectral combination techniques. Least-squares collocation,least-squares adjustment in the frequency domain and input-output system theory are employed to determine the gravity field (both geoid and anomalies) and its errors. This paper intercompares these three techniques using simulated data. Simulation studies show that best results are obtained by the input-output system theory among the three prediction methods. The least-squares collocation method gives results which are very close to but a little bit worse than those obtained using input-output system theory. This slightly poorer performance of the least-squares collocation method can be explained by the fact that it uses isotropic structured covariance (thus approximate signal PSD information) while the system theory method uses detailed signal PSD information. The method of least-squares adjustment in the frequency domain gives the poorest results among these three methods because it uses less information than the other two methods (it ignores the signal PSDs). The computations also show that the least-squares collocation and input-output system theory methods are not as sensitive to noise levels as the least-squares adjustment in the frequency domain method is.
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Mid-ocean ridges create oceanic lithosphere consisting normally of basaltic crust a few kilometers thick overlying a peridotitic mantle. However, lithosphere free of basaltic crust formed during the past similar to 30 m.y. at an similar to 50-km-long stretch of Mid-Atlantic Ridge south of the Romanche Fracture Zone, giving rise to a > 500-km-long strip of ocean floor exposing mostly mantle peridotites that have undergone an unusually low (less than or equal to5%) degree of melting, mixed with peridotites that reacted with a small fraction of basaltic melt. This lithosphere contains < 10% of scattered gabbroic pockets, representing melt frozen above 25 km depth within a relatively cold subaxial lithosphere. Numerical modeling excludes dry melting below this crust-free lithosphere, because of the cooling effect of the long-offset Romanche transform combined with a regional mantle thermal minimum; however, modeling allows a limited extent of hydrous melting., This unusual lithosphere, unable to expel the melt fraction, characterizes cold spots along mid-ocean ridges.
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Rocks in the Earth's uppermost sub-oceanic mantle, known as abyssal peridotites, have lost variable but generally large amounts of basaltic melt, which subsequently forms the oceanic crust. This process preferentially removes from the peridotite some major constituents such as aluminium, as well as trace elements that are incompatible in mantle minerals (that is, prefer to enter the basaltic melt), such as the rare-earth elements. A quantitative understanding of this important differentiation process has been hampered by the lack of correlation generally observed between major- and trace-element depletions in such peridotites. Here we show that the heavy rare-earth elements in abyssal clinopyroxenes that are moderately incompatible are highly correlated with the Cr/(Cr + Al) ratios of coexisting spinels. This correlation deteriorates only for the most highly incompatible elements-probably owing to late metasomatic processes. Using electron- and ion-microprobe data from residual abyssal peridotites collected on the central Indian ridge, along with previously published data, we develop a quantitative melting indicator for mantle residues. This procedure should prove useful for relating partial melting in peridotites to geodynamic variables such as spreading rate and mantle temperature.
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A series of high pressure (P), temperature (T) experiments (P = 1.0 to 3.5 GPa, T = 1050-1260 degrees C) were undertaken on Na2O-, TiO2- and pyroxene-rich peri dotite compositions under controlled fO(2) conditions. Compositions of olivine, pyroxene and garnet have been used to evaluate the suitability of existing geothermometer and geobarometer formulations in estimating the equilibration conditions of fertile peridotite. The quality of the mineral chemical data reported here represents an improvement over that in many earlier experimental studies due to: the absence of Fe-loss problems by use of graphite inner capsules; better approach to equilibrium because of increased run times and the presence of a fluid or melt phase; and control of fO(2) at low values so that silicate phases contain negligible Fe3+ contents. Of the geothermometers tested, only those of WELLS (1977) [two-pyroxene] and HARLEY (1984) [garnet-orthopyroxene] reproduce experimental temperatures with any accuracy. For the geobarometers, a version of the WOOD (1974) formulation [garnet-orthopyroxene] gave the most satisfactory results. However, somewhat better P estimates were obtained using a modified version of the NICKEL & GREEN (1985) barometer [NG85mod] in which a Ti correction was applied to the activity term for the Mg-tschermaks component: X-MgTs = (Al-Cr+Ti+Na)/2. To extend the P,T, and compositional range of applicability for two-pyroxene thermometers, a new formulation [TA97] has been devised: T(K) = [24,787 + 678.P(GPa)]/[15.67 + 14.37.Ti-cpx + 3.69.Fe-cpx -3.25 . X-ts + (lnKd)(2)] where: lnKd = ln{a(En)(cpx)} - ln{a(En)(opx)}; X-ts = (Al+Cr-Na)(cpx); and a(En) = (1-Ca-Na).(1-Al-vi-Cr-Ti).(1-Al-iv/2)(2) It satisfactorily reproduces experimental T for the present experiments, as well as those of BREY et al. (1990) and WALTER & PRESNALL (1994). A new thermobarometer based the NG85mod-TA97 pair is recommended for fertile lherzolite and websterite compositions. Application to garnet Iherzolite, websterite, and pyroxenite xenoliths from southeastern Australia suggest that they equilibrated under lower P,T conditions, and originated from restricted depth intervals, compared with previous estimates.
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Based on an analysis of the source of errors in marine gravity measurements, an error model, firstly, is constructed mathematically which can characterize the change of systematic errors and with which a new crossover adjustment model is presented in this paper. Then, two methods of compensating the systematic errors are proposed, i.e., the self-calibrating adjustment and the a-posteriori compensation. Some questions involved in solving the adjustment problem, such as the rank deficiency, the choice of error model, the weighting of model parameters and the significance test of compensation efficiency, etc., are discussed in detail. Finally, a practical survey network is used as a case study to test the efficiency and reliability of the two compensation methods.
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Systematic changes in seafloor depth, crustal structure, and crustal geochemistry can occur within 30 km of a fracture zone. The seafloor gradually deepens roughly 1 km within 30 km from the fracture zone, the crust may be thinner than crust created far from a fracture zone, and systematic compositional differences are observed between basalts erupted near and far from fracture zones. We call these effects the transform fault effect (TFE). To investigate the physical processes responsible for the TFE, a general numerical method is developed to solve for the 1-D flow and thermal structure beneath a mid-ocean spreading center. This model is applied to study an idealized spreading center consisting of a 100 km transform fault offsetting 2 ridge segments spreading at rates of 1, 2, and 4 cm/yr. Present calculations are inconclusive in showing whether the effects of melt production or melt migration dominate at a ridge-transform intersection. -from Authors Scripps Institution of Oceanography, Institute of Geophysics & Planetary Physics, La Jolla, California, USA.
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In simple models of passive flow beneath mid-ocean ridges, fractional melting of the upwelling mantle produces unexpected relationships among crustal thickness, degree of partial melting, and the average depth of melting. If the degree of melting is estimated on the basis of basalt composition, then the average degree of melting is one-third the maximum degree of melting and the average depth of melting is about two-thirds the maximum depth in the simplest models. The crustal thickness is about 1.5 times the product of the average fraction of melting and the height of the melting column. If the average degree of melting represented in mid-ocean ridge basalts were 10 percent and melt production continued to the base of the crust, then only a 40-km-high residual melting column would be required to produce a 6-km-thick crust. In more realistic models, melting beginning at 60 km below the base of the crust and continuing to the top of the mantle would produce a 6-km-thick crust with average degree of melting of 6.67 percent or melting beginning at 67 km and ending at 30 km below the base of the crust would produce a 6-km-thick crust with an average degree of melting of only about 5.4 percent.
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Mantle Bouguer gravity anomalies (MBA) and bathymetry on three profiles covering more than 1000 km along the axis of the Mid-Atlantic Ridge (MAR) are highly correlated, suggesting that along-axis topographic relief is locally compensated by variations in crustal thickness and/or mantle density structure. The paradox is that across-axis profiles show that the median valley is an uncompensated feature, apparently created by a dynamic mechanism. New, extensive off-axis coverage of the MAR from 31° to 36°S shows that the high correlation does not persist outside the axial zone. It is suggested that the on-axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along-axis variations in crustal thickness, both of which contribute to the MBA. -from Authors
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The processes leading to the eruption of mantle-derived magma at mid-ocean ridges were investigated using a quantitative self-consistent fluid dynamical model of melting, magma migration, and buoyancy-driven circulation beneath mid-ocean ridges. The model provides an explanation for the narrowness of the zone of crustal formation at ridges, showing how important are the changes in the intrinsic density of mantle material due to the melting and the distribution of buoyancy beneath the ridge. The model predicts a transition from steady state to episodic crustal formation at low spreading velocities, perhaps giving rise to along-axis variations in the character of seafloor spreading.
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We report the results of a seismic tomography experiment which images the three-dimensional nature of the crustal melt delivery system beneath a segment of the slow-spreading Mid-Atlantic Ridge. In the lower crust ( > 3.5 km depth) near the segment center, inversion of first-arriving crustal P-waves reveals a pair of vertical pipe-like ( < 10-km-diameter) low-velocity anomalies (-0.4 km/s), in the upper crust, these two features, which are physically isolated from each other below 3 km, both connect to a l0-km-wide, 45-km-long, axis-parallel, low-velocity zone (-0.2 km/s). Three higher-amplitude low-velocity anomalies (-0.6 km/s) are observed in the upper crust ( < 2 km depth), and are located directly beneath seafloor volcanic features. We interpret the overall image to represent the thermal/melt signature of a magma feeding system in which focused injections of magma from the mantle travel upward until they intersect the brittle-ductile transition, where they are then diverted along-axis to supply shallow intrusive bodies and seafloor eruptions along much of the ridge segment.
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Motivated to understand the process of melt migration in the earth’s mantle, we have studied a generalised form of Darcy’s law that describes porous flow in a matrix that can deform by creep. We find a remarkable richness of phenomena, including a new class of solitons. These consist of shape-preserving waves of high liquid fraction which buoyantly ascend through a stationary matrix. This may have important implications for the morphology and geochemistry of primary igneous processes, and applicability to other porous flow problems.
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We present the first internally consistent calculation which leads to a narrow ‘conduit’ of rapid vertical advection and melting of mantle under a spreading center. In this model, mantle flow is driven by plate separation and compositional buoyancy. Melt segregation is described as flow through a permeable media. The major new feature is that the viscosity of the mantle is considered to be a strong function of the amount of partial melt present. Experiments show that the bulk viscosity of a partially molten rock is sharply reduced when the melt fraction exceeds a critical value. In the model, the viscosity is reduced as the critical melt fraction is approached. Whether or not a critical melt fraction can be reached under a spreading center depends on the mantle permeability for melt flow. The width of the upwelling area is controlled by the magnitude of the melt related viscosity reduction. Crust should be formed above the focused upwelling. Seismic observations show that the region of crustal accretion is only a few kilometers wide at fast spreading centers. With a viscosity reduction of three orders of magnitude the model predicts a zone of crustal accretion of this width.
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Data on the Vema fracture zone and adjacent areas of the equatorial Atlantic Ocean are presented. Variations in the thickness of the lower crust, gravity anomalies and the presence or absence of seismic layer 3 are explained. -R.E.S.
Article
We conducted a detailed geological-geophysical survey of the west flank of the Mid- Atlantic Ridge between 25ø25'N and 27ø10'N and from the ridge axis out to 29 Ma crust, acquiring Hydrosweep multibeam bathymetry, HAWAII MR1 sidescan-sonar imagery, gravity, magnetics, and single-channel seismic reflection profiles. The survey covered all or part of nine spreading segments bounded by mostly nontransform, right-stepping discontinuities which are subparallel to flow lines but which migrated independently of one another. Some discontinuities alternated between small right- and left-stepping offsets or exhibited zero offset for up to 3-4 m.y. Despite these changes, the spreading segments have been long-lived and extend 20 m.y. or more across isochrons. A large shift (--9 ø) in relative plate motion about 24-22 Ma caused significant changes in segmentation pattern. The nature of this plate-boundary response, together with the persistence of segments through periods of zero offset at their bounding discontinuities, suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt. Within segments, there are distinct differences in seafloor depth, morphology, residual mantle Bouguer gravity anomaly, and apparent crustal thickness between inside-corner and outside-corner crust. This demands fundamentally asymmetric crustal accretion and extension across the ridge axis, which we attribute to low-angle, detachment faulting near segment ends. Cyclic variations in residual gravity over the cross- isochron run of segments also suggest crustal-thickness changes of at least 1-2 km every 2-3 m.y. These are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension, controlled by retention and quasiperiodic release of melt from the upwelling mantle. Detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes, creating crustal domes analogous to "turtleback" metamorphic core complexes that are formed by low-angle, detachment faulting in subaerial extensional environments.
Article
Segmentation and along-axis variations within individual segments indicate the inherently three-dimensional nature of mantle upwelling and melting beneath oceanic spreading centers. Numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center. The experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher-viscosity mantle half-space. A periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets. Buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt. The relative magnitudes of the buoyant and plate-driven components of mantle flow are controlled by the spreading rate and mantle viscosity, with buoyant flow more important at lower spreading rates and viscosities. Buoyant flow beneath the spreading axis amplifies along-axis variations in upwelling near a ridge-transform intersection, and distributes the variations along the entire spreading axis.
Article
To map the lateral distribution of thin crust in a major fracture zone, we have constructed free-air anomaly, bathymetry, and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the Vema fracture zone with the Mid-Atlantic Ridge. Variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the 3-D conduction of heat. The residual gravity anomaly map indicates that there is thin crust along much of the length of the Vema fracture zone. Thin crust is concentrated primarily beneath the walls of the fracture zone, rather than beneath the center of the fracture zone valley, an interpretation confirmed by seismic refraction data. -from Authors
Article
Multibeam bathymetry and gravity data have been obtained along an approximately 800-km-long section of the Mid-Atlantic Ridge from just south of the Hayes fracture zone at 33 deg N to the northern edge of the Azones Platform near 40 deg N. A three-dimensional analysis of these gravity and topography data, combined with results from earlier seismic refraction studies in this area, reveal two different scales of crustal heterogeneity. A systematic, alog-axis, segment-scale (lambda less than 20-100 km) variation in crustal thickness is present with the thickest crust (greater than 8-9 km) near the middle of spreading segments and the thinnest crust (less than 3-4 km) near segment offsets. The magnitude of this along-axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset. There also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs, inferred to be thinner crust, beneath the 'inside corner' highs adjacent to these offsets. This segment-scale crustal heterogeneity is similar to that reported from the Kane-to-Atlantis section of the Mid-Atlantic Ridge and from parts of the intermediate-spreading southern Mid-Atlantic Ridge. It is superimposed on a second, longer wavelength variation in gravity and crustal thickness associated with the Azores hot spot. The most pronounced effect of the Azores hot spot on the Mid-Atlantic Ridge occurs between 38 deg N and 40 deg N where the ridge axis rapidly shoals by more than 1000 m, the crust thickens by over 2 km, and the rift valley largely disappears.
Article
Several analyses of SSH-SH differential travel-times disagree on whether these data contain evidence for azimuthal anistotropy in the North Atlantic upper mantle. To explore possible explanations for these discrepant results, we calculated the azimuthal variation in SSH-SH travel-times predicted by an anisotropic upper mantle model. Theoretical travel-times were obtained by cross-correlating S and SS phases on transverse-component reflectivity synthetic seismograms. We determined that predicted azimuthal travel-time patterns are sensitive to the relative amplitudes of radial and transverse particle motions incident on the anisotropic medium. If a large variation in the relative incident amplitudes exists in a dataset, the overlap of different azimuthal travel-time patterns may obscure anisotropic signatures. We analyzed shear-wave splitting in a dataset of 12 SS and 10 S phase observations to constrain the presence of anisotropy in the North Atlantic upper mantle. A comparison of SS splitting parameters with S and SKS particle motions suggests that shear wave splitting apparent in the SS phases is at least partially due to anisotropy in the vicinity of their bounce-points, although contributions from anisotropy in the source and station regions cannot be ruled out. Observed SS splitting is consistent with an olivine model in which horizontal a-axes are aligned at an average of N24 deg W in the central North Atlantic (15 deg-40 deg N), and at an average of N23 deg E in the northern North Atlantic (40 deg-50 deg N).