Mantle Exhumation Research Articles

Interactions between tectonics and fluid circulations in an inverted hyper-extended basin: Example of mesozoic carbonate rocks of the western North Pyrenean Zone (Chaînons Béarnais, France)

Interactions between fracturing, fluid circulations and fluid chemistry in hyper-extended margins are still poorly described as most of them are located offshore, buried underneath post-rift sediments. The southern Aquitaine basin and the northern Pyrenees constitute an appropriate case study to investigate these interactions since a model of hyper extended margin with mantle exhumation during the Lower Cretaceous subsequently inverted was recently proposed. From a field study, we here describe three main sets of fractures (set 1 to set 3). They are correlated with main stages of the geodynamic evolution of the basin corresponding to the Liassic rifting, the Aptian-Cenomanian hyper-extension, and the Pyrenean compression. Petrographic observations, Raman and micro-thermometry analysis on fluid inclusions, ICP-MS, and isotope analysis permitted to determine chemistries, temperatures, redox conditions, gas compositions, oxygen and carbon isotopic signatures, and REE contents of parent fluids for cements precipitated during each episode. In particular saddle dolomite and chlorite precipitated in set 2 fractures during the hyper-extension corresponding to the thermal peak at temperatures higher than 300 °C. The isotopic signature, the high CO2 content, the occurrence of H2S and the high salinity of parent fluids suggest ascending magmatic fluids percolating across Triassic evaporites. The late and post hyper-extensional phase is characterized by hydraulic brecciation in dolomitic formations, a decrease in temperature and salinity, a decrease in magmatic contribution in parent fluids, a closing of the diagenetic system during burial and a switch to reducing conditions during the precipitation of quartz, pyrite and calcite. The Pyrenean compressive phase associated with the third fracturing stage induced a reopening of the diagenetic system and favored a return to oxidizing conditions and infiltrations of meteoric fluids.

Westward migration of oceanic ridges and related asymmetric upper mantle differentiation

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 at all 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 refertilization) at different depths (down to 300km) 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 refertilization 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 in the mantle solidus as it changes from vapour-undersaturated to vapour-saturated conditions, its intersection with the geotherm at ~90km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the vapour-saturated 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.

Effects of axially variable diking rates on faulting at slow spreading mid-ocean ridges

Magma supply for dike injection can be highly variable within a segment of a slow-spreading mid-ocean ridge but the tectonic impact of this variability is not fully understood. Here, we use three-dimensional numerical models to quantify the effects of variable diking rates on the faulting mode at a 20 km-long slow spreading ridge segment. In addition to end-member faulting modes in which long-lived detachment faults or short-lived normal faults form along the whole segment, we newly identify a transitional mode in which a detachment and a short-lived normal fault form simultaneously but in respective domains separated by a transfer fault. Different faulting modes can be better correlated with the average dike intrusion rate, rather than the highest or lowest rate along the segment. Along-axis stress coupling tends to homogenize fault offset along the segment, inhibiting the domination of a particular faulting mode associated with an extreme local diking rate. This homogenizing effect explains why detachment faults can sometimes form even in the regions previously considered as unfavorable. Our results further suggest that a long (>15 km) and continuous detachment, partially overlain by younger faults, can create an oceanic core complex when faults weaken fast and diking rate is low. When faults weaken slow and diking rate is moderate, however, faulting occurs in the transitional mode, producing a detachment over only a part of the segment length.

From stretching to mantle exhumation in a triangular backarc basin (Vavilov basin, Tyrrhenian Sea, Western Mediterranean)

In this study, we describe the mode of extension of the Vavilov, a fossil backarc basin, triangle-shaped (approximately 240km-wide and 200km-long), located between Sardinia margin to the west and Campania margin to the east. We combine the analysis of recent geophysical and geological data, in order to investigate the relationship between the crustal/sedimentary structure and the tectonic evolution of both apex and bathyal parts of the basin. With this aim, we interpret a large data set of multichannel seismic reflection profiles and several well logs. We observe that the apex basin corresponds to a sediment-balanced basin, with a sedimentary infill recording the episodes of basin evolution. In contrast, the distal basin corresponds to an underfilled basin, characterized by localized volcanic activity and a thin sedimentary succession that covers the exhumed mantle. The basin architecture reveals the occurrence of rift and supradetachment basins in the Vavilov rift zone. We find that the rifting of the Vavilov triangular basin was synchronous from the apex to distal regions around a single Euler pole located in Latium, between 5.1 and 1.8Ma. The kinematic evolution of the Vavilov basin occurred in two stages: initial pure shear mode (5.1–4.0Ma) that produced high-angle normal faults and syn-sedimentary wedges, followed by simple shear mode (4.0–1.8Ma) that caused supradetachment basins filled by a Transgressive-Regressive succession that documents high subsidence rates (1.22mm/y) in the apex region. The final stage of extension in the distal region led to: (i) complete embrittlement of the crust; (ii) direct continuation of crustal faults to upper mantle depth; (iii) serpentinization and mantle exhumation. Based on constraints on the present-day crustal structure of the Vavilov basin, we obtain a stretching value (β=3.5) and extension rates (3cm/y) in the bathyal zone analogous to those reported for magma-poor rifted margins.

Tracing the thermal evolution of the Corsican lower crust during Tethyan rifting

AbstractContinental rifting requires thinning the continental lithosphere from ~120 km to <20 km by a series of processes which each impart a characteristic thermal signature to the extending lithosphere. Here high‐resolution thermochronology is used to trace the synrift thermal evolution within a lower crustal section of an upper plate hyperextended margin sampled in Corsica. Novel zircon, rutile, and apatite 206Pb/238U depth profiling coupled with garnet trace element diffusion modeling provide compelling evidence for rift‐related crustal reheating. A Jurassic thermal pulse is recorded in the footwall of the Belli Piani Shear Zone (BPSZ), where 200–180 Ma zircon 206Pb/238U overgrowth ages on Permian core populations and the preservation of stranded diffusion profiles in resorbed garnets implies the dominant footwall fabric formed as a result of high‐temperature (T ~ 800°C) ductile thinning of the lower crust. Conductive reheating of middle crustal rocks in the immediate BPSZ hanging wall, demonstrated by Jurassic apatite 206Pb/238U ages, was likely achieved by synkinematic juxtaposition against the hot footwall and wholesale conductive steepening of geothermal gradients. Subsequent rapid cooling from 180 to 160 Ma, coeval with extensional unroofing of the footwall, underscores the importance of extreme ductile thinning during crustal hyperextension. The results of this study suggest that early lithospheric‐scale depth‐dependent thinning follows an early phase of diffuse rifting and tectonic subsidence and triggers crustal reheating during early hyperextension. Continued extension results in rapid exhumation and cooling of the lower crust, extreme crustal attenuation, and mantle exhumation followed by relaxation to a steady state thermal field coeval with the start of seafloor spreading.

The Iberian Plate: myth or reality?

The plate tectonics theory generally leads us to consider that Iberia was an independent plate separated from Europe by the North Pyrenean Fault (NPF). The NPF has been commonly interpreted as a transform fault associated with a huge counterclockwise transverse and rotational movement that allowed the opening of the Bay of Biscay and the relative eastward motion of Iberia during the Mesozoic. According to some interpretations, this movement may have generated an interplate gap several hundreds of km wide, which led to the creation of an oceanic crust during the Late Jurassic and Early Cretaceous. However, field studies recently carried out in the Pyrenees do not support these interpretations. The North Pyrenean Fault (NPF) of Tertiary age is observed in the central and eastern Pyrenees, where pioneering researchers defined it as separating the North Pyrenean Zone from the Axial Zone. However, this fault cannot be identified in the western part of the range to the west of the Ossau valley. Consequently, the geodynamic evolution of Iberia has always been dependent on Europe, especially during the failed oceanic rifting in the Mid-Cretaceous. Indeed, during this period, a central zone of crustal thinning occupied by turbiditic basins separated the European from the Iberian continental crust, with a very localized mantle exhumation found only in the Mauleon basin. Therefore, far from being an interplate range, the Pyrenees can neither be considered as an intraplate unit. We can define this orogenic belt as resulting from the “Tertiary tectonic inversion of a Mid-Cretaceous rift system”. According to this new interpretation, Iberia would not have been an isolated plate but represented an unstable, outlying part of Europe. Rather than displaying the features of a rigid lithospheric unit with well-defined boundaries, Iberia grouped together different crustal blocks undergoing specific movements at particular times. During the Mesozoic, normal, reverse or strike-slip displacements along their boundary faults generated several en échelon basins (Bilbao, Logroño, Soria and Maestrazgo in Spain; Parentis, Arzacq and the North Pyrenean Flysch Trough in France) whose diachronous development accounts very well for the opening of the Bay of Biscay and the relative W-E sinistral movement of the Iberian crust with respect to the European crust. In this way, we note a more marked paleogeographic shift in the western Basque region than in eastern Catalonia. Therefore, the Pyrenees were not generated by an “interplate collision”, but merely reflect the confrontation of two distended but continuous continental domains during the Tertiary, leading to the incipient underthrusting of the Iberian crust under the European crust. This zone of confrontation/convergence does not correspond in any way to the NPF, but rather comprises a complex imbricated structure revealed by a jump in the Moho, both in the Pyrenees and in the Cantabrian belt, which has recently become known as the “North Iberian Fault”.

Segment‐scale variations in seafloor volcanic and tectonic processes from multibeam sonar imaging, M id‐ A tlantic R idge Rainbow region (35°45′–36°35′N)

Along-axis variations in melt supply and thermal structure can lead to significant variations in the mode of crustal accretion at mid-ocean ridges. We examine variations in seafloor volcanic and tectonic processes on the scale of individual ridge segments in a region of the slow-spreading Mid-Atlantic Ridge (35o45'-36o35'N) centered on the Rainbow non-transform discontinuity (NTD). We use multibeam sonar backscatter amplitude data, taking advantage of multifold and multi-directional coverage from the MARINER geophysical study to create a gridded compilation of seafloor reflectivity, and interpret the sonar image within the context of other data to examine seafloor properties and identify volcanic flow fields and tectonic features. Along the spreading segments, differences in volcanic productivity, faulting, eruption style and frequency correlate with inferred magma supply. Regions of low magma supply are associated with more widely spaced faults, and larger volcanic flow fields that are more easily identified in the backscatter image. Identified flow fields with the highest backscatter occur near the ends of ridge segments. Their relatively smooth topography contrasts with the more hummocky, cone-dominated terrain that dominates most of the neovolcanic zone. Patches of seafloor with high, moderately high, and low backscatter intensity across the Rainbow massif are spatially correlated with observations of basalt, gabbro and serpentinized peridotite, and sediment respectively. Large detachment faults have repeatedly formed along the inside corners of the Rainbow NTD, producing a series of oceanic core complexes along the wake of the NTD. A new detachment fault is currently forming in the ridge segment just north of the now inactive Rainbow massif. This article is protected by copyright. All rights reserved.

Melt‐rock interactions and fabric development of peridotites from North Pond in the Kane area, Mid‐Atlantic Ridge: Implications of microstructural and petrological analyses of peridotite samples from IODP Hole U1382A

AbstractNorth Pond is an isolated sedimentary pond on the western flank of the Kane area along the Mid‐Atlantic Ridge. Drill‐hole U1382A of IODP Expedition 336 recovered peridotite and gabbro samples from a sedimentary breccia layer in the pond, from which we collected six fresh peridotite samples. The peridotite samples came from the southern slope of the North Pond where an oceanic core complex is currently exposed. The samples were classified as spinel harzburgite, plagioclase‐bearing harzburgite, and a vein‐bearing peridotite that contains tiny gabbroic veins. No obvious macroscopic shear deformation related to the formation of a detachment fault was observed. The spinel harzburgite with a protogranular texture was classified as refractory peridotite. The degree of partial melting of the spinel harzburgite is estimated to be ∼17%, and melt depletion would have occurred at high temperatures in the uppermost mantle beneath the spreading axis. The progressive melt–rock interactions between the depleted spinel harzburgite and the percolating melts of Normal‐Mid Ocean Ridge Basalt (N‐MORB) produced the plagioclase‐bearing harzburgite and the vein‐bearing peridotite at relatively low temperatures. This implies that the subsequent refertilization occurred in an extinct spreading segment of the North Pond after spreading at the axis. Olivine fabrics in the spinel and plagioclase‐bearing harzburgites are of types AG, A, and D, suggesting the remnants of a mantle flow regime beneath the spreading axis. The initial olivine fabrics appear to have been preserved despite the later melt–rock interactions. The peridotite samples noted above preserve evidence of mantle flow and melt–rock interactions beneath a spreading ridge that formed at ∼8 Ma.

Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversion of streamer data and borehole measurements

AbstractSeismic full waveform inversion (FWI) is a promising method for determining the detailed velocity structure of the igneous oceanic crust, especially for locations such as the Mid‐Atlantic Ridge with significant lateral heterogeneity and seafloor topography. We examine the accuracy of FWI by inverting, after downward continuation to datum just above the seafloor, a multichannel seismic (MCS) profile from Atlantis Massif oceanic core complex at 30°N that passes close to Integrated Ocean Drilling Program (IODP) Hole U1309D and comparing the results against borehole measurements and existing on‐bottom refraction data. The comparisons include the results of IODP Expedition 340T, which extended the sonic logging and vertical seismic profiling to the bottom of the borehole at 1400 m below seafloor. Compared to travel time tomography, the refinement in velocity and velocity gradient produced by FWI significantly improves the overall match to the borehole measurements, and allows the multilevel pattern of deformation and alteration of the detachment footwall seen in Hole U1309D to be extrapolated across the Central Dome. Prestack depth migration of the profile using the FWI velocities reveals the top and edges of the high‐velocity, gabbroic core of the massif. It also indicates that the comparatively uniform gabbroic rocks drilled at Hole U1309D extend to ∼2.5 km below seafloor but overlie an extended, ∼2 km thick, mantle transition zone.

Research and outreach at the Mid-Atlantic Ridge

Roger Searle , Christine Peirce and Angela Bentley look at research on oceanic core complexes and detachments at the Mid-Atlantic Ridge, combined with innovative geophysics outreach.

Fault systems at hyper-extended rifted margins and embryonic oceanic crust: Structural style, evolution and relation to magma

The study of seismic sections derived from the magma-poor Australia-Antarctica rifted margins enables to investigate the structural style and evolution of fault systems and their relation to magmatic additions during final rifting and lithospheric breakup. The aim of this study is thus to better characterize fault systems, their organization in time and space as well as their relationship to magmatic systems. Our observations show that extensional detachment faults related to mantle exhumation form out-of-sequence, and initiate as normal upward concave faults. The observations can be best explained by an alternation between a pure shear mode (development of symmetric arrays of normal faults) and a simple shear mode (development of detachment faults), with a continuous delocalization and re-localization of the deformation in the footwall of the previous exhumation fault. This model suggests that the asthenospheric uplift is slow and decoupled from the deformation in the brittle and serpentinized upper basement layer. The presence of such a décollement level, possibly formed by the serpentinisation front, has a major control on the style and organization of faulting. The polyphase, out-of-sequence organization of faults can explain that despite mantle exhumation is accommodated along multiple asymmetric exhumation faults, the overall exhumed mantle domain is mostly symmetric at the scale of both margins.

Constraining lithosphere deformation modes during continental breakup for the Iberia–Newfoundland conjugate rifted margins

A kinematic model of lithosphere and asthenosphere deformation has been used to investigate lithosphere stretching and thinning modes during continental rifting leading to breakup and seafloor spreading. The model has been applied to two conjugate profiles across the Iberia–Newfoundland rifted margins and quantitatively calibrated using observed present-day water loaded subsidence and crustal thickness, together with observed mantle exhumation, subsidence and melting generation histories. The kinematic model uses an evolving prescribed flow-field to deform the lithosphere and asthenosphere leading to lithospheric breakup from which continental crustal thinning, lithosphere thermal evolution, decompression melt initiation and subsidence are predicted. We explore the sensitivity of model predictions to extension rate history, deformation migration and buoyancy induced upwelling. The best fit calibrated models of lithosphere deformation evolution for the Iberia–Newfoundland conjugate margins require; (1) an initial broad region of lithosphere deformation with passive upwelling, (2) lateral migration of deformation, (3) an increase in extension rate with time, (4) focussing of the deformation and (5) buoyancy induced upwelling. The model prediction of exhumed mantle at the Iberia–Newfoundland margins, as observed, requires a critical threshold of melting to be exceeded before melt extraction. The preferred calibrated models predict faster extension rates and earlier continental crustal separation and mantle exhumation for the Iberia Abyssal Plain–Flemish Pass conjugate margin profile than for the Galicia Bank–Flemish Cap profile to the north. The predicted N–S differences in the deformation evolution give insights into the 3D evolution of Iberia–Newfoundland margin crustal separation.

Continental hyperextension, mantle exhumation, and thin oceanic crust at the continent‐ocean transition, West Iberia: New insights from wide‐angle seismic

AbstractHyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent‐ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide‐angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0–7.0 km s−1 velocity contours, corresponding to peridotite serpentinization of 60–30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8–3.5 km, with an average velocity gradient of 1.0 s−1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.

Virtual special issue: Understanding of the largest oceanic core complex on the Earth, Godzilla Megamullion

Virtual special issue: Understanding of the largest oceanic core complex on the Earth, Godzilla Megamullion

Deformation mechanisms in a continental rift up to mantle exhumation. Field evidence from the western Betics, Spain

The identification of the structures and deformation patterns in magma-poor continental rifted margins is essential to characterize the processes of continental lithosphere necking. Brittle faults, often termed mantle detachments, are believed to play an essential role in the rifting processes that lead to mantle exhumation. However, ductile shear zones in the deep crust and mantle are rarely identified and their mechanical role remains to be established. The western Betics (Southern Spain) provide an exceptional exposure of a strongly thinned continental lithosphere, formed in a supra-subduction setting during Oligocene-Lower Miocene. A full section of the entire crust and the upper part of the mantle is investigated. Variations in crustal thickness are used to quantify crustal stretching that may reach values larger than 2000% where the ductile crust almost disappears, defining a stage of hyper-stretching. Opposite senses of shear top-to-W and top-to-E are observed in two extensional shear zones located close to the crust-mantle boundary and along the brittle-ductile transition in the crust, respectively. Where the ductile crust almost disappears, concordant top-to-E-NE senses of shear are observed in both upper crust and serpentinized mantle. Late high-angle normal faults with ages of ca. 21 Ma or older (40Ar/39Ar on white mica) crosscut the previously hyper-stretched domain, involving both crust and mantle in tilted blocks. The western Betics exemplify, probably better than any previous field example, the changes in deformation processes that accommodate the progressive necking of a continental lithosphere. Three successive steps can be identified: i/a mid-crustal shear zone and a crust-mantle shear zone, acting synchronously but with opposite senses of shear, accommodate ductile crust thinning and ascent of subcontinental mantle; ii/hyper-stretching localizes in the neck, leading to an almost disappearance of the ductile crust and bringing the upper crust in contact with the subcontinental mantle, each of them with their already acquired opposite senses of shear; and iii/high-angle normal faulting, cutting through the Moho, with related block tilting, ends the full exhumation of the mantle in the zone of localized stretching. The presence of a high strength sub-Moho mantle is responsible for the change in sense of shear with depth. Whereas mantle exhumation in the western Betics occurred in a backarc setting, this deformation pattern controlled by a high-strength layer at the top of the lithosphere mantle makes it directly comparable to most passive margins whose formation lead to mantle exhumation. This unique field analogue has therefore a strong potential for the seismic interpretation of the so-called “hyper-extended margins”.

Crustal thickness anomalies in the Indian Ocean inferred from gravity analysis

AbstractOceanic crustal thickness is a critical parameter for understanding the magmatic, tectonic and hydrothermal processes at mid‐ocean ridges. Gravity anomalies can be mapped across the oceans through satellite altimetry, and these data hold valuable information about the nature and spatial variation of oceanic crust. Residual mantle Bouguer anomaly (RMBA) reflects variations in crustal thickness and crustal and mantle densities. It was simply treated as an upper mantle gravity component and a crustal gravity component in our model of a lithospheric depth. The upper mantle gravity component results from variations in mantle temperature, mantle porosity and mantle depletion due to partial melting initiating at depths of 80–100 km. The crustal gravity component results from variations in crustal thickness. RMBA contributed by mantle temperature variation of 200 K, mantle depletion of 5% and mantle porosity of 5% are 46, 16 and 21 mGal, respectively. Upper mantle gravity variations do not exceed 46 mGal, and variations in oceanic crustal thickness or deeper mantle effects have to be considered for areas with RMBA variations greater than 46 mGal. Assuming a standard RMBA of a reference crust is 0 mGal, we partitioned the RMBA in the Indian Ocean into three categories: ‘negative’ (less than −46 mGal), ‘normal’ (between −46 and 46 mGal) and ‘positive’ (>46 mGal), corresponding to thick, normal and thin oceanic crust, respectively. Thick oceanic crustal anomalies are associated with hotspots or plumes and are distributed along hotspot tracks. Prominent thin oceanic crustal anomalies are observed along fracture zones, around Rodriguez Triple Junction (RTJ) and Australian‐Antarctic Discordance (AAD). Besides the possible effects of fracture zones, oceanic core complex or some other enigmatic deep mantle factors must be responsible for the positive gravity anomalies and crustal thinning. Copyright © 2016 John Wiley & Sons, Ltd.

Composition and Formation of Gabbro-Peridotite Hosted Seafloor Massive Sulfide Deposits from the Ashadze-1 Hydrothermal Field, Mid-Atlantic Ridge

This paper presents mineralogical and geochemical data on seafloor massive sulfides (SMS) from the Ashadze-1 hydrothermal field at the Mid-Atlantic Ridge (MAR). The Ashadze-1 deposit is associated with the uplifted lower crust and upper mantle (oceanic core complex, OCC) of the MAR segment characterized by asymmetric mode of accretion. The OCC is represented by deep-seated gabbro-peridotite rocks exhumed on the rift valley slope along the detachment fault, during seafloor spreading. Hydrothermal processes in OCC environments result in different deposit composition and morphology compared to basalt-hosted systems. Abundant chimneys and enrichment in particular metals, including copper, zinc, gold, cobalt and tin are typical for this type of SMS deposit. The Ashadze-1 deposit is considered an example of a hydrothermal system in the initial stage of evolution marked by the young age of the sulfides (<7.2 kyr). The mineralogy of Ashadze-1 reflects primary ore-forming processes unaffected by post formation alteration. We propose a model for the primary ore-forming hydrothermal process in an ultramafic-hosted environment on the modern seafloor.

Interpretación geológica y geofísica de la Elevación de Río Grande, margen brasileño suroriental: tectónica extensional y rifting de las cortezas continental y oceánica

This paper discusses the geological and geophysical interpretation of rift structures in the region extending from the Rio Grande Rise, in the Southeastern Brazilian margin, towards the Cabo Frio High, which separates the Campos and Santos basins. We have analysed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins. The Rio Grande Rise shows a relatively negative Bouguer anomaly in an area that corresponds to a major positive bathymetric feature between the Argentine and Brazil basins. North–south propagators related to the early spreading centres of the Atlantic Ocean are observed from Argentina towards the southern Santos Basin, which is characterized by an elevated basement topography relative to the Pelotas Basin. The region adjacent to the Florianopolis Fracture Zone between the Santos and Pelotas basins is also characterized by an elevated basement region aligned in an east–west direction, and locally it is marked by rift structures aligned along a NW–SE direction, forming a lineament or shear zone (Cruzeiro do Sul lineament) that extends from the Cabo Frio High towards the Rio Grande Rise, thus involving both continental and oceanic crusts. The Rio Grande Rise is associated with the east–west-trending fracture zones, which are characterized by several aligned magnetic anomalies in the southern Santos Basin. The Rio Grande Fracture Zone continues landward as the Sao Paulo Ridge, and extends towards the platform as the Florianopolis High. Oceanic propagators are identified from Argentina towards the Pelotas and Santos basins, and locally we observe rupturing of the salt layer by igneous intrusions or possibly by mantle exhumation. The Florianopolis (or Rio Grande) Fracture Zone is marked by an abrupt topographic offset separating the Pelotas Basin from the southern Santos Basin, and the associated volcanic belts limit the southernmost occurrence of the late Aptian evaporate sequence. The evaporite sequence in this segment of the continental margin shows remarkable layering of halite, anhydrite and carnalite. Conjugate to the Rio Grande Rise, the Walvis Ridge, offshore Namibia, is similarly a topographic high, but rift structures as observed in the Brazilian side are apparently unique in the South Atlantic. Alternative interpretations for the origin of the Rio Grande Rise include: a volcanic edifice or plateau rooted in the mantle; an intraplate shear zone affecting both continental and oceanic crust; an oceanic area of igneous over-productivity caused by a hotspot or a thermal anomaly in the mantle; a palaeo-spreading centre in the Cretaceous Atlantic Ocean; an area of excessive volcanic activity resulting from mantle differentiation due to adiabatic decompression; or perhaps an isolated remnant of continental crust left outboard of the Brazilian continental margin during the drifting process.

Very high geothermal gradient during mantle exhumation recorded in mylonitic marbles and carbonate breccias from a Mesozoic Pyrenean palaeomargin (Lherz area, North Pyrenean Zone, France)

Although they are famous among Earth scientists, the Lherz peridotites are exposed within geological formations of the North Pyrenean Zone (NPZ) still lacking detailed investigations. Our study focuses on the metasediments of the Aulus basin hosting the Lherz peridotite body and associated ultramafic fragments of smaller size. The new data set comprises of structural analysis and detailed geological mapping of the massive Mesozoic marbles that form the prerift sequence typical of the NPZ and of the ultramafic-rich clastic breccia formations surrounding the peridotite bodies. The massive marbles display an evolution from hot and ductile to cold and brittle deformation, indicative of an exhumation process ending with the sedimentary reworking of both the deformed Mesozoic metasediments and the exhumed ultramafic rocks. Crystal Preferred Orientations (CPO) measured in the marbles support a deformation mechanism by dislocation creep of calcite, which is dominant between 400°C and 600°C; these deformation temperatures are within the range determined earlier by Clerc et al. (2015), using RSCM (Raman Spectroscopy of Carbonaceous Material) geothermometry. As a consequence, we better describe the transition from ductile to brittle deformation in the prerift marbles and clarify the origin of the syn-rift breccias. Due to continuous exhumation along detachments’ faults, the brecciated metamorphic carbonates of the prerift NPZ sedimentary cover were passively uplifted towards shallower levels and progressively unroofed, while transported passively on the back of the exhumed ultramafic footwall. These results are consistent with the recent interpretations of the North Pyrenean peridotites as remnants of subcontinental mantle rocks exhumed within the pre-Pyrenean rift system. We emphasize the importance of tectonic decoupling between the Mesozoic sedimentary cover and the Palaeozoic basement, which leads to the juxtaposition of metamorphosed and deformed Mesozoic sediments directly on top of exhumed mantle rocks. We favor a model of tectonic denudation of the peridotites below prerift sediments metamorphosed during the extension of the basin floor under high temperatures and in a thermal regime characterized by a very high gradient.

Cretaceous mantle exhumation in the central Pyrenees: New constraints from the peridotites in eastern Ariège (North Pyrenean zone, France)

We provide new constraints on the modes of mantle exhumation and denudation in the pre-orogenic Pyrenees by investigating the poorly known peridotites located east of the Lherz body, in eastern Ariège (Vicdessos, Urs, Bestiac, Caussou and Prades main peridotitic bodies). We show that: (1) numerous peridotite bodies, not described so far, are constituted by decametric to sub-metric outcrops scattered between and around the main outcrops identified by Monchoux (1970); (2) most of the peridotites form blocks and lenses of various sizes, from millimetric monocrystalline grains to hectometric olistoliths, embedded in debris flows involving mainly Mesozoic metasediments; these peridotites can therefore be linked to the S-type peridotites of Lagabrielle et al. (2010), and share similarities with the peridotites of Lherz region (Clerc et al., 2012). All originated from tectono-sedimentary processes associated with mantle exhumation in relation with extreme thinning and breakoff of the continental basement; (3) the various petrographic characters of these peridotites and their heterogeneity in serpentinisation degree imply a peculiar mode of exhumation, which merges mantle fragments inherited from different structural levels; (4) the lack of tectonic structures such as faults and shear planes associated with the ultramafic-rich breccia formation and the presence of neoformation of metamorphic minerals within the matrix preclude their origin as cataclasites related to the Latest Cretaceous and Tertiary Pyrenean contractional deformation. The peridotites of eastern Ariège are indisputable witnesses of the presence of an inverted hyper-extended Cretaceous passive margin forming the current North Pyrenean zone. The geometry of the extensional system was complex and probably associated with strike-slip faults and transcurrent motion.