Continent-ocean Transition Research Articles

The continent-to-ocean transition in the Iberia Abyssal Plain

Abstract Conceptual models of magma-poor rifting are strongly based on studies of the nature of the basement in the continent-to-ocean transition of the Iberia Abyssal Plain, and suggest that exhumed mantle abuts extended continental crust. Yet, basement has only been sampled at a few sites, and its regional nature and the transition to seafloor spreading inferred from relatively low-resolution geophysical data are inadequately constrained. This uncertainty has led to a debate about the subcontinental or seafloor-spreading origin of exhumed mantle and the rift-related or oceanic nature of magmatic crust causing the magnetic J anomaly. Different interpretations change the locus of break-up by >100 km and lead to debate of the causative processes. We present the tomographic velocity structure along a 360-km-long seismic profile centered at the J anomaly in the Iberia Abyssal Plain. Rather than delineating an excessive outpouring of magma, the J anomaly occurs over subdued basement. Furthermore, its thin crust shows the characteristic layering of oceanic crust and is juxtaposed to exhumed mantle, marking the onset of magma-starved seafloor spreading, which yields the westward limit of an ~160-km-wide continent–ocean transition zone where continental mantle has been unroofed. This zone is profoundly asymmetric with respect to its conjugate margin, suggesting that the majority of mantle exhumation occurs off Iberia. Because the J anomaly is related to the final break-up and emplacement of oceanic crust, it neither represents synrift magmatism nor defines an isochron, and hence it poorly constrains plate tectonic reconstructions.

Magmatic and rifting-related features of the Lomonosov Ridge, and relationships to the continent–ocean transition zone in the Amundsen Basin, Arctic Ocean

SUMMARY The continental Lomonosov Ridge spans across the Arctic Ocean and was the subject of a geophysical study in 2016 with two seismic reflection lines crossing the ridge in proximity to the North Pole, one of which continues across the continent–ocean transition zone into the Amundsen Basin. One seismic station and 15 sonobuoys were deployed along these two lines to record seismic wide-angle reflections and refractions for development of a crustal-scale velocity model. Its viability is checked using gravity data from the experiment which are also used to interpolate crustal structure in areas with poor seismic constraints. On the line extending into the Amundsen Basin, continental crust composed of two layers with velocities of 6.0 and 6.5 km s–1 is encountered beneath the Lomonosov Ridge where the Moho depth is 21 km based on gravity modelling. The crust is overlain by a 1-km-thick layer with velocities of 4.7 km s–1 coinciding with a zone of positive magnetic anomalies of up to 180 nT. This layer is interpreted to include extrusive volcanic rocks related to the Cretaceous High Arctic Large Igneous Province (HALIP). Within the Amundsen Basin, three distinct crustal domains can be distinguished. Closest to the ridge is a 40-km-wide zone with a crustal thickness around 5 km interpreted as thinned continental crust. Five distinctive faulted basement blocks display high-amplitude reflections along their crests with velocities of 4.6 km s–1, representing the continuation of the magmatic rocks further upslope. Brozena et al. (2003) interpreted magnetic Chron C25 to be located in this zone but our data are not consistent with this being a seafloor spreading anomaly. In the adjacent crustal domain, heading basinward, the basement flattens and two layers with velocities of 5.2 and 6.8 km s–1 can be distinguished, where the upper and lower layer have a thickness of 1.5 and 2.0 km, respectively. The upper layer is interpreted as exhumed and highly serpentinized mantle while the lower layer may be less serpentinized mantle with some gabbroic intrusions. This may explain the high-amplitude reflections within the overlying sediments that are interpreted as sill intrusions. Continuing basinward, the last crustal domain represents 4-to 5-km-thick oceanic crust with a variable basement relief and velocities of 4.8 and 6.5 km s–1 at the top of oceanic layers 2 and 3, respectively. It is within this zone that the first true seafloor spreading anomaly Chron C24 is observed, which argues for a similar breakup age in the Eurasia Basin as in the Northeast Atlantic. On the other profile crossing the Lomonosov Ridge, a 60-km-wide intrusion into the lower crust is observed where velocities are increased to 6.9 km s–1. Gravity modelling supports the interpretation of magmatic underplating beneath the intrusion. Seismic data in this region show that the crust is overlain by a 2-km-thick series of high-amplitude reflections with a velocity of 4.8 km s–1 in a 30-km-wide zone where strong magnetic anomalies (>800 nT) are observed, suggesting a composition of basalt flows. This part of the Lomonosov Ridge appears therefore to have a HALIP-related magmatic overprint at all crustal levels.

A new perspective on the continent–ocean boundary of Palawan continental block identified with geophysical data

The location of the Continent–ocean boundary (COB) between the Palawan continent block (PCB) and the central oceanic basin remains controversial. There are more than 100 km difference among the scholars' view about the specific positions of the COB. To obtain a more reasonable location of the COB in the PCB, a comprehensive interpretation was conducted based on new shipborne geophysical data, including bathymetry, gravity, magnetic and reflection seismic data. In particular, upward continuation was applied to the new stern-towing magnetic data, and analytical signal analysis (ASA) processing was adopted for the total field magnetic anomalies, including both the public and stern-towing magnetic data. The most reasonable COB of PCB is just identified with the geophysical data based on the comprehensive structures' anomalies, such as the foot of massif slope and the magmatic volcanoes in topographic data, and the gentle gradient of the free-air and Bouguer gravity anomalies, and the clear dividing boundary in magnetic ASA sharp anomaly, and the seismic profiles structure's display in the Continent–ocean transition (COT). Meanwhile, structural modeling along the seismic profiles are used to constrained the identification. The refined convex–concave's COB is distributed at the seaward limit of the continental crust along the foot of northwest Palawan slope with a hyper-extended thinner crust, seaward-dipping listric faults, and is reasonably located between the extensive thinned continental crust and expansive subducting oceanic crust.

Vestiges of the Pre-Caledonian Passive Margin of Baltica in the Scandinavian Caledonides: Overview, Revisions and Control on the Structure of the Mountain Belt

The Pre-Caledonian margin of Baltica has been outlined as a tapering wedge with increasing magmatism towards the ocean–continent transition. It is, however, well known that margins are complex, with different and diachronous evolution along and across strike. Baltica’s vestiges in the Scandes have complexities akin to modern margins. It included a microcontinent and magma-poor hyperextended and magma-rich segments. It was probably up to 1500 km wide before distal parts were affected by plate convergence. Characteristic features are exhumed mantle peridotites and their detrital equivalents, some exposed to the seafloor by the pre-orogenic hyperextension. A major change in the architecture of the mountain belt occurred across the NW–SE trending Sveconorwegian front in the Baltican basement. This coincided with the NE termination of the Jotun-Lindås-Dalsfjord basement nappes, the remains of the Jotun Microcontinent (JMC) formed by hyperextension prior to the orogeny. Mantle with ophicalcite breccias exhumed by hyperextension are covered by deep-marine sediments and local conglomerates. Baltican basement slivers are common in the transitional crust basins. Outboard the JMC, the margin was magma-rich. The main break-up magmatism at 605 ± 10 Ma was part of the vast Central Iapetus Magmatic Province. The along-strike heterogeneity of the margin controlled diachronous and contrasting tectonic evolution during the later Caledonian plate convergence and collision.

Stratigraphic record of continental breakup, offshore NW Australia

AbstractContinental breakup involves a transition from rapid, fault‐controlled syn‐rift subsidence to relatively slow, post‐breakup subsidence induced by lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn‐breakup unconformities, indicating that episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth‐dependent extension and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is challenging because uplift‐associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup‐related unconformities and the intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non‐erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra‐Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent–ocean transition zones (COTZ); (ii) the middle unconformity formed between ca. 134 and 133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ca. 132.5 and 131 Ma (i.e. Intra‐Hauterivian), coincident with full continental lithospheric breakup and the onset of seafloor spreading. During unconformity development, uplift was focussed along the continental rift flanks, likely reflecting flexural bending of the crust and landward flow of lower crust and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth‐dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during continent–ocean transition.

Active ocean–continent transform margins: seismic investigation of the Cayman Trough-Swan Island ridge-transform intersection

SUMMARY The southern boundary of the Cayman Trough in the Caribbean is marked by the Swan Islands transform fault (SITF), which also represents the ocean–continent transition of the Honduras continental margin. This is one of the few places globally where a transform continental margin is currently active. The CAYSEIS experiment acquired an ∼165-km-long seismic refraction and gravity profile (P01) running across this transform margin, and along the ridge-axis of the Mid-Cayman Spreading Centre (MCSC) to the north. This profile reveals not only the crustal structure of an actively evolving transform continental margin, that juxtaposes Mesozoic-age continental crust to the south against zero-age ultraslow spread oceanic crust to the north, but also the nature of the crust and uppermost mantle beneath the ridge-transform intersection (RTI). The traveltimes of arrivals recorded by ocean-bottom seismographs (OBSs) deployed along-profile have been inverse and forward modelled, in combination with gravity modelling, to reveal an ∼25-km-thick continental crust that has been continuously thinned over a distance of ∼65 km to ∼10 km adjacent to the SITF, where it is juxtaposed against ∼3–4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are only sparsely observed, and, even then, only by a few OBSs located on the continental margin, the 7.5 km s–1 velocity contour is used as a proxy to locate the crust–mantle boundary along-profile. Along the MCSC, the crust–mantle boundary appears to be a transition zone, at least at the seismic wavelengths used for CAYSEIS data acquisition. Although the traveltime inversion only directly constrains the upper crust at the SITF, gravity modelling suggests that it is underlain by a higher density (>3000 kg m–3) region spanning the width (∼15 km) of its bathymetric expression, that may reflect a broad region of metasomatism, mantle hydration or melt-depleted lithospheric mantle. At the MCSC ridge-axis to the north, the oceanic crust appears to be forming in zones, where each zone is defined by the volume of its magma supply. The ridge tip adjacent to the SITF is currently in a magma rich phase of accretion. However, there is no evidence for melt leakage into the transform zone. The width and crustal structure of the SITF suggests its motion is currently predominantly orthogonal to spreading. Comparison to CAYSEIS Profile P04, located to the west and running across-margin and through 10 Ma MCSC oceanic crust, suggests that, at about this time, motion along the SITF had a left-lateral transtensional component, that accounts for its apparently broad seabed appearance westwards.

Mineral formation conditions in acid magmatic systems related to the formation of massive sulfide deposits of the Urals and Altai-Sayany area

Studies of melt inclusions in quartz indicate the similarity of acid magmatic systems of massive sulfde deposits in the Urals and Altai-Sayany region. The melts of normal alkalinity corresponding to rhyodacite and rhyolite compositions and related to the tholeiitic series are dominant in all the deposits considered. The magmas are characterized by the same type evolution with a decreasing content of main oxides (TiO2, Al2O3, FeO, MgO, CaO, Na2O, K2O) and an increasing SiO2 content. Our results show the accumulation of Cu in relatively low-H2O acidic melts of ancient (Cambrian) deposits of the Altai-Sayany region and low metal contents in the intermediate (Silurian–Devonian) and H2O-saturated magmas of the Urals. The youngest (Devonian) magmas of Siberia evolve simultaneously along these two directions. The analysis of melt inclusions in quartz suggests that the minimum contents of trace and rare earth elements are characteristic of the Silurian-Devonian acid melts of the Urals, with their maximum contents in the youngest (Devonian) magmas and the intermediate contents of ancient (Cambrian) magmatic systems of the Altai-Sayany region. The features of rare and rare earth element patterns in melt inclusions in quartz indicate the similarity of acid magmatic systems of massive sulfde deposits in the Urals and Altai-Sayany region with present-day suprasubduction melts in the ocean-continent transition zones. Computational modeling using data on melt inclusions in quartz confrms our previous conclusions (Simonov, Maslennikov, 2020) that the occurrence of contrasting (basic and felsic) volcanic complexes with massive sulfde deposits in the Urals and Altai-Sayany region is a result of evolution of basaltoid magmas. Keywords: conditions of mineral crystallization, basaltic-rhyolitic complexes, massive sulfde deposits, melt inclusions, quartz, acidic melts.

Rifting evolution of the lithospheric subcontinental mantle: New insights from the External Ligurian ophiolites (Northern Apennine, Italy)

The present study focuses on the petrographic and petrological characteristics of mantle bodies included in Upper Cretaceous sedimentary melanges of the External Ligurian units (Northern Appennine), within the Monte Gavi and Monte Sant'Agostino areas. Two distinct pyroxenite-bearing mantle sections were recognized, mostly based on their plagioclase-facies evolution. The Monte Gavi mantle section is nearly undeformed and records a process of melt infiltration and reaction under plagioclase-facies conditions. The melt-rock interaction event involved both peridotites (mostly harzburgites) and enclosed spinel pyroxenite layers, and is estimated to have occurred at 0.7–0.8 GPa. In the Monte Gavi peridotites and pyroxenites, the spinel-facies clinopyroxene was partially replaced by plagioclase and new orthopyroxene (± secondary clinopyroxene). The reactive melt migration led to relatively high TiO2 contents in relict clinopyroxene and spinel (up to 2.3 wt% and 1.0 wt%, respectively, in the pyroxenites), with the latter also having high Cr# (up to 35 in the peridotites). The Monte Sant'Agostino mantle section displays a widespread ductile shearing and no evidence for melt-rock interaction under plagioclase-facies conditions. The main deformation phase recorded by the Monte Sant'Agostino peridotites (mostly lherzolites) is estimated to have occurred at 750–780 °C and 0.3–0.6 GPa, and gave rise to protomylonitic to ultramylonitic textures characterized by 10–50 μm neoblasts. The enclosed pyroxenite layers yielded relatively high temperature and pressure estimates (870–930 °C and 0.8–0.9 GPa). Presumably, in the Monte Sant'Agostino mantle section, plagioclase crystallization occurred earlier in the pyroxenites than in enclosing lherzolites, thereby enhancing strain localization and formation of mylonite shear zones in the entire mantle section. We propose that subcontinental mantle section from the External Ligurian units consists of three distinct mantle domains, developed in response to the rifting evolution that ultimately formed a Middle Jurassic ocean-continent transition: (1) a spinel tectonite domain that underwent no significant deformation and melt-rock reaction under plagioclase-facies conditions, characterized by static plagioclase development, (2) a plagioclase mylonite domain experiencing melt-absent deformation, and (3) a nearly undeformed domain that underwent melt infiltration and reaction under plagioclase-facies conditions. We relate mantle domains (1,2) to a rifting-driven uplift in the late Triassic accommodated by large-scale shear zones consisting of plagioclase mylonites.

Moho carbonation at an ocean-continent transition

Abstract Carbonation of mantle rocks during mantle exhumation is reported in present-day oceanic settings, both at mid-ocean ridges and ocean-continent transitions (OCTs). However, the hydrothermal conditions of carbonation (i.e., fluid sources, thermal regimes) during mantle exhumation remain poorly constrained. We focus on an exceptionally well-preserved fossil OCT where mantle rocks have been exhumed and carbonated along a detachment fault from underneath the continent to the seafloor along a tectonic Moho. Stable isotope (oxygen and carbon) analyses on calcite indicate that carbonation resulted from the mixing between serpentinization-derived fluids at ~175°C and seawater. Strontium isotope compositions suggest interactions between seawater and the continental crust prior to carbonation. This shows that carbonation along the tectonic Moho occurs below the continental crust and prior to mantle exhumation at the seafloor during continental breakup.

Kinematic Boundary Conditions Favouring Subduction Initiation at Passive Margins Over Subduction at Mid-oceanic Ridges

We perform numerical modelling to simulate the shortening of an oceanic basin and the adjacent continental margins in order to discuss the relationship between compressional stresses acting on the lithosphere and the time dependent strength of the mid-oceanic ridges within the frame of subduction initiation. We focus on the role of stress regulating mechanisms by testing the stress–strain-rate response to convergence rate, and the thermo-tectonic age of oceanic and continental lithospheres. We find that, upon compression, subduction initiation at passive margin is favoured for thermally thin (Palaeozoic or younger) continental lithospheres (<160 km) over cratons (>180 km), and for oceanic basins younger than 60 Myr (after rifting). The results also highlight the importance of convergence rate that controls stress distribution and magnitudes in the oceanic lithosphere. Slow convergence (<0.9 cm/yr) favours strengthening of the ridge and build-up of stress at the ocean-continent transition allowing for subduction initiation at passive margins over subduction at mid-oceanic ridges. The results allow for identifying geodynamic processes that fit conditions for subduction nucleation at passive margins, which is relevant for the unique case of the Alps. We speculate that the slow Africa–Europe convergence between 130 and 85 Ma contributes to the strengthening of the mid-oceanic ridge, leading to subduction initiation at passive margin 60–70 Myr after rifting and passive margin formation.

Formation, segmentation and deep crustal structure variations along the Algerian margin from the SPIRAL seismic experiment

The Algerian continental margin in the western Mediterranean formed as a back-arc basin and is today reactivated by the convergence between the African and Eurasian plates. It is one of the very rare examples of passive margins undergoing inversion expressed in a moderate seismicity of the margin and is possibly giving way for future subduction. With the objective to better know the deep structure of the margin and its associated basin, the origin of its seismicity and to study the mechanism of reactivation, five existing wide-angle seismic profiles along the margin are revisited. They were located offshore Mostaganem, Tipasa, Greater Kabylia, Jijel and Annaba. These profiles show that the basin is underlain by a 5 km thick crust of oceanic magmatic origin, possibly created at non-continuous small accretionary ridge segments. The continent-ocean transition zone is narrow, except at the easternmost profile, possibly due to an opening including a shear movement. No high velocity zone in the lower crust corresponding to mantle rocks has been imaged at the Algerian margin. The continental crust is thinned in a narrow and strongly segmented manner. It is widest (70 km) in the central segment offshore Greater Kabylia where there is a wider zone of distal thinned continental crust than on the other margin segments. The thickest crust detected during this survey corresponds to the African continental crust and the Kabylides blocks and is about 22–25 km thick. This reduced thickness in comparison with unthinned continental crust might be due to the influence of earlier subduction at the margin, in form of erosion by the subducting slab.

NE Greenland Composite Tectono-Sedimentary Element, northern Greenland Sea and Fram Strait

Abstract The NE Greenland margin measures more than 300 km from coast to the continent–ocean transition at its widest part and is in an early state of exploration. It is expected to hold significant petroleum reserves due to a high concentration of large structures and prolific source-rock intervals anticipated in the subsurface. The margin is composed of five tectonic units: (1) the Koldewey Platform; (2) the Danmarkshavn Basin; (3) the Danmarkshavn Ridge; (4) the Thetis Basin; and (5) the Wandel Sea Basin. These units form the NE Greenland Rifted Margin Composite Tectono-Sedimentary Element. Rifting commenced at some time during the Devonian or Carboniferous and recurred during latest Jurassic–Cretaceous time. Major tilted fault blocks and other rift-related structures formed during latest Jurassic–Cretaceous rifting. The structures are typically cored by Triassic–Middle Jurassic sand-prone sequences. Oil-prone Upper Jurassic–lowermost Cretaceous marine source rock caps the structures or is located immediately downdip. A large concentration of structures exists in the eastern Danmarkshavn Basin, the western Thetis Basin and along the edges of the Danmarkshavn Ridge. Other major structures exist in the least data-covered parts of the margin furthest north.

Crustal extension and magmatism along the northeastern margin of the South China Sea: Further insights from shear waves

The northern continental margin of the South China Sea (SCS) is a unique rifted margin that experienced weak magmatism in the syn-rift stage and intense magmatism in the post-rift stage. This area also exhibits a complicated relationship between crustal extension and magmatism. We present the tomographic inversion of seismic P-wave and S-wave velocity models, as well as the Vp/Vs ratio model, in order to delineate the crustal extension and post-rift magmatic features. The velocity structure is created by forward modelling (RayInvr) and travel-time tomographic inversion (Tomo2D). The results suggest that Mesozoic strata near the Dongsha area has a maximum thickness of ~4.6 km with a Vp of 3.5–5.5 km/s, a Vs of 1.9–3.1 km/s, and Vp/Vs ratios of 1.71–1.76, indicating a low porosity and high degree of diagenesis. High-velocity lower crust (HVLC) is imaged in our model, with a Vp of 7.0–7.5 km/s ± 0.05–0.25 km/s and Vp/Vs ratios of 1.70–1.82 ± 0.05. The composition of the HVLC is mainly mafic, we considered it is related to magmatic underplating due to decompression melting caused by crustal extension. The crustal anomalies with high Vp/Vs ratios of 1.80–1.85 ± 0.04 are identified, which are the product of post-rift magmatic intrusions. The seaward increase in continental lower crustal Vp/Vs ratios, suggests an increasing level of mafic intrusion into the continental crust. We attributed crustal hyperextension to increasing fault density, which caused intense magmatic intrusion and thinner HVLC beneath the continent-ocean transition (COT).

Mapping the Moho depth and ocean-continent transition in the South China Sea using gravity inversion

• The lithosphere thermal gravity anomaly in the South China Sea is calculated. • The depth of the Moho surface in the South China Sea is obtained by inversion in spherical coordinates with the thermal gravity anomaly correction. • We delineate the ocean-continent transition (OCT) of the northern South China Sea. In this paper, the depth of the Moho surface in the South China Sea is investigated by the gravity inversion method under the spatial-domain spherical coordinate system based on the global gravity field model WGM2012. The gravity data are corrected for the lithospheric thermal gravity anomaly due to the large negative residual thermal anomaly caused by lithospheric extension in the ocean and continental margins. The calculation results show that the thermal anomaly is mainly concentrated near the residual oceanic ridges in the South China Sea and that the density anomalies caused by the anomaly show a “fusiform” variation with depth; i.e., the anomaly is larger in the middle of the lithosphere and smaller at the top and bottom. The Moho inversion results indicate that the depth of the Moho surface in the South China Sea ranges from 7 to 32 km with an overall northeastward wedge-shaped distribution and is characterized by a shallow basin and a deep perimeter. In addition, a depression of the Moho surface remains under the remnant oceanic ridges, with the depth increasing by 2–5 km, and the depth of the Moho surface on both sides of the basin changes rapidly. Further, we determined the location of the OCT in the South China Sea based on the Moho depth mapped by gravity inversion and combined it with the results of seismic reflection profiles and other relevant geophysical data; its width is approximately 200 km in the east and 70 km in the west.

Passive margin inversion controlled by stability of the mantle lithosphere

Contractional deformation of passive continental margins may resemble early stages of induced subduction initiation. Mechanical instabilities are required to permit underthrusting of the oceanic plate. Therefore, the success of developing a new subduction zone will largely depend on the rheology of the mantle lithosphere. In this physical analogue modelling study, a range of mantle viscosities subject to different convergence rates serve as a proxy to simulate contraction of differently aged passive margins with the purpose of describing and quantifying passive margin deformation and mantle stability using the buoyancy number. The experiments illustrate distinct differences in geometry and length-scale of deformation as a function of lithospheric mantle strengths. The results indicate that deformation occurs at passive margins for intermediate strength lithospheric mantle and high strain rate. In contrast, low and high strength mantle lithospheres lead to dominantly intra-oceanic deformation or long wavelength buckling of the entire model, respectively. Our experiments portray an evolution in which early-stage deformation commences at the ocean-continent transition and is controlled by the ductile lower crust of the continent. In the next stage, shear localization through the formation of a decollement within the ductile passive margin crust favors underthrusting of the oceanic lithosphere, leading to a reduction of the area affected by deformation. Prior to underthrusting, the primary response of the lithosphere to compression is by folding at scaled wavelengths of 100–300 km and 500–1000 km, controlled by the strength of the mantle lithosphere.

Seismic Constraint From Vp/Vs Ratios on the Structure and Composition Across the Continent‐Ocean Transition Zone, South China Sea

AbstractAt non‐volcanic passive continental margins, seismic techniques often failed to uniquely define the nature of crustal domains. Here, we overcome this problem by studying the structure and composition of the continent‐ocean transition (COT) in the Southwest Sub‐basin of the South China Sea, using P and S wave seismic tomography and Vp/Vs ratios, providing unique constraints on lithology. Throughout the image domain, we can rule out large areas of exhumed mantle as Vp/Vs ratios are always <1.9 in the shallow basement layer. Instead, the COT is characterized by extended and fragmented continental crust, and possibly mafic aggregation at the bottom of the crust. In concert with observations from multichannel seismic reflection data, seismic velocities and Vp/Vs ratios suggest that the oldest oceanic crust was formed by starved magmatism, causing rugged basement, thin crust, nearly absent lower crust, and moderately serpentinized mantle below. Our results reveal that rifting occurred without un‐roofing continental mantle.

Opportunistic magnetotelluric transects from CSEM surveys in the Barents Sea

Summary Magnetotelluric (MT) data allow for electrical resistivity probing of the Earth's subsurface. Integration of resistivity models in passive margin studies could help disambiguate non-unique interpretations of crustal composition derived from seismic and potential field data, a recurrent issue in the distal domain. In this contribution, we present the first marine MT data in the Barents Sea, derived from industrial controlled-source electromagnetic (CSEM) surveys. We characterize data quality, dimensionality, depth penetration and elaborate an analysis strategy. The extensive MT database consists of 337 receivers located along 7 regional transects, emanating from ∼70,000 km2 of 3D CSEM surveys acquired for hydrocarbon exploration from 2007 to 2019. High-quality MT data are extracted for periods ranging from 0.5 s to 5000 s. The data show no apparent contamination by the active source nor effects related to large time-gaps in data collection and variable solar activity. Along receiver profiles, abrupt lateral variations of apparent resistivity and phase trends coincide with major structural boundaries and underline the geological information contained in the data. Dimensionality analysis reveals a dichotomy between the western domain of the SW Barents Sea, dominated by a single N-S electromagnetic strike, and the eastern domain, with a two-fold, period-dependent strike. 35 receivers show 3D distortion caused by nearby bathymetric slopes, evidenced by elevated skew values. We delineate geographical areas where the 2D assumption is tenable and lay the foundation for future MT modelling strategies in the SW Barents Sea. We performed 2D MT inversion along one of the regional transects, a ∼220 km-long, E-W profile encompassing a major structural high and sedimentary basin approaching the continent-ocean transition. The resistivity model reveals low crustal resistivity values (1–10 Ω.m) beneath the deep sedimentary basins, in marked contrast with high resistivity values (1000–5000 Ω.m) of the thick crystalline crust on the structural high. We interpret this abrupt lateral resistivity variation as a rapid transition from a thick, dry continental crust to a hyperextended and hydrated crustal domain. Integration of resistivity with seismic velocity, density and magnetic susceptibility models may further refine these structural models and the underlying tectonic processes in the SW Barents Sea margin. Our methodology is applicable globally where 3D CSEM surveys are acquired and has a large potential for harvesting new knowledge on the electrical resistivity properties of the lithosphere.

Modification of Lithospheric Mantle by Melts/Fluids With Different Sulfur Fugacities During the Wilson Cycle: Insights From Lesvos and Global Ophiolitic Peridotites

AbstractThe Lesvos ophiolite, Greece, was recently identified as an analog to the External Liguride Unit formed at an ocean‐continent‐transition (OCT) setting. To further study the evolutionary history of this body, we analyzed highly siderophile element abundances and Re‐Os isotopic compositions of 14 Lesvos peridotites, in combination with petrology, bulk‐rock major, and trace element geochemistry. Almost all the Lesvos peridotites fall within the field of global OCT and mid‐ocean‐ridge (MOR) peridotites, indicating that these rocks have not experienced subduction‐related processes. The near‐horizontal 187Os/188Os versus Al2O3 trend over a wide range of Al2O3 (0.45–3.66 wt. %) may have been caused by recent melt depletion during rifting and thinning of the Lesvos lithosphere. Combining data from available global ophiolitic peridotites, we find that a large proportion of the more Al‐depleted supra‐subduction‐zone (SSZ) peridotites show lower Os/Ir, higher Pd/Ir and remarkably elevated radiogenic Os relative to other tectonic environments (OCT and MOR). By linking this kind of geochemical evolution to the Wilson Cycle, a complete picture emerges: (a) In the OCT to MOR stages, the extensional rifting environment may lead to mild to moderate melt depletion, followed by, or associated with, infiltration of S‐saturated (high fS2) basaltic magmas; (b) when progressing into the SSZ stage, more extreme degrees of mantle melt depletion may be driven by aqueous fluids in the sub‐arc mantle. During this stage, high‐fO2 slab‐derived fluids and/or S‐undersaturated (low fS2) boninitic magmas may infiltrate the sub‐arc mantle, followed by subsequent S‐saturated forearc basaltic magma infiltration.

The Structure of the Continent‐Ocean Transition in the Gulf of Lions From Joint Refraction and Reflection Travel‐Time Tomography

AbstractThe Gulf of Lions—Ligurian basin, in the Western Mediterranean, opened from the end of Oligocene to the Miocene as a back‐arc basin driven by rollback of the Ionian slab. Geophysical surveys across the margin have explored the crustal structure and the continent‐ocean transition (COT), but its location and petrological nature is still a matter of debate. Here, we apply joint refraction and reflection travel‐time tomography that combines travel‐times from multichannel seismic and wide‐angle seismic data to provide improved constraints on the P‐wave velocity structure of the sediment cover and the geometry of the top of the basement with respect to previous studies. Similar to earlier works, the velocity model shows three crustal domains, but their geometry and internal velocity differ. We identify a ∼100 km‐wide domain of oceanic crust with an anomalously high upper crustal velocity possibly caused by porosity‐canceling processes triggered by 7–8 km thick overlying sediment cover. The vertical velocity structure and the thickness of the oceanic crust are similar to back‐arcs in the Pacific and the Western Mediterranean. In contrast to previous interpretations, we propose that this oceanic domain is confined between two domains of ultra‐thin (4–5 km‐thick) continental crust, and that the COT is <10 km wide on either side of the oceanic crust.

FRESH, PSEUDOTACHYLYTE-BEARING MANTLE PERIDOTITES FROM THE LAWSONITE ECLOGITE-FACIES SAN PETRONE UNIT, ALPINE CORSICA

Mantle peridotites exhumed in mountain belts provide important insights on the composition and evolution of the upper mantle, and additionally inform on metamorphic, geochemical, and tectonic processes -including seismic activity- at convergent margins. In this contribution, we present field, microstructural, and mineralogical data of fresh, pseudotachylyte-bearing mantle peridotites from the lawsonite eclogite-facies San Petrone unit, Alpine Corsica, France. The present case study represents the first example of subducted fresh peridotite associated with fresh lawsonite eclogite-facies assemblages. Two bodies of fresh peridotite are embedded in fully serpentinized ultramafic rocks forming the substratum of a subducted ocean-continent transition of the Piemonte-Liguria Basin. Clinopyroxene and spinel mineral chemistry indicates that the investigated peridotite samples were part of a refertilized mantle and, therefore, the San Petrone unit likely belonged to the more distal part of the ocean-continent transition. Because small bodies of fresh peridotite embedded in fully serpentinized rocks can hardly be identified by means of geophysical investigations, this finding suggests that small, yet disseminated bodies of fresh mantle peridotite can potentially be more abundant than previously supposed at ocean-continent transition and, potentially, at mid-ocean ridges and in subduction zones. The preservation of fresh mantle peridotite bodies in subducting slabs is also discussed with respect to its potential implications on intermediate depth seismicity and geochemical cycling -including production of natural energy sources- from rifting to subduction.