Mid-ocean Ridge Research Articles

Late Eocene tectonic-geomorphic transition and its dynamic mechanism: Case study of the Beibuwan Basin, northern South China Sea

The continuous sedimentary records of the Beibuwan Basin preserve vital information on the interaction between Tethys and Pacific tectonic domains. The elusive interaction between the lithosphere and mantle significantly impacts the differential evolution of basins. However, the understanding of surface-mantle dynamic coupling beneath the Beibuwan Basin remains unclear. In this study, we analyze Cenozoic tectonic deformation, sedimentation, and mantle dynamic processes in the Beibuwan Basin in the northern South China Sea to clarify the nature of the tectonic-geomorphic transition during the late Eocene. Results show that the basin underwent late Eocene stratigraphic flexural folding, syn-rift transition from NE to EW orientated, and depocenter migration. The orientation of the primary controlling fault system during this period changed from NE-SW to NEE-SWW, the number of syn-depositional faults decreased from 368 to 172. The thickness of depocenters decreased from 5000 m to 1800 m, and the lacustrine Beibuwan Basin became shallower and wider. A comparison of the tectonic-structural history of the Beibuwan Basin with the history of plate subduction reconstructed from global geodynamic models shows that the late Eocene tectonic transition was closely related to changes in the stress field and the mantle wind related to oceanic plate subduction. We conclude that this enduring surface-mantle interaction associated with subduction of the Izanagi-Pacific mid-ocean ridge resulted in the late Eocene tectonic transition and geomorphic changes in the Beibuwan Basin.

Double-diffusive layering during the evolution of planetary mantles

Summary The degree of layering during planetary mantle evolution has been a key issue and is still heavily debated, especially since the observation of seismic discontinuities within the Earth mantle and their correlation to phase transitions within the mantle mineral assemblage and the detection of geochemical differences in mantle derived basalts (Mid Ocean Ridge Basalts and Ocean Island Basalts). We present the phenomenon of double-diffusive layering in a fluid with strongly temperature dependent viscosity, as relevant for planetary mantle material, to study the influence of an initial compositional gradient with regard to the thermal and chemical evolution of a planet. The numerical experiments show that in a wide parameter range distinct layers are formed self-organized from a continuously stratified state by dynamical fractionation and are thus likely to appear as a generic. Considering this as a plausible model for planetary mantle evolution it provides a dynamical explanation for the existence of distinct chemical reservoirs within the history of a planet's mantle.

Magnetotelluric image of the Patagonian slab window: Constraints on upper mantle physical properties and sources of intraplate magmatism

The Patagonian slab window (PSW) is a region of the Southamerican subduction zone where the absence of subducted slabs is interpreted, due to the subduction of the Chile Mid-Ocean Ridge at the Chile Triple Junction. Here we report the results of a long-period magnetotelluric (MT) study conducted in two 300 km-long trench-parallel transects crossing the northern boundary of the PSW in the proximal backarc. We modeled the MT data using 3-D inversion, obtaining an electrical resistivity model of the continental crust and upper mantle up to a depth of ∼150 km. Our model shows a heterogeneous resistivity structure in the uppermost mantle, dominated by resistivities >300 Ωm below the array of sites even within the PSW, and some low-resistivity zones (LRZs, <10 Ωm) mainly at the edge of the array. Using petrophysical models, we estimated the mantle temperature, water content, melt fraction, and viscosity based on obtained resistivity values and a preexistent model of P-wave velocity (Vp) at 50 km and 100 km depth. These estimates suggest that the uppermost mantle within the PSW region is heterogeneous and dominated by high-viscosity blocks, compatible with the continental mantle lithosphere or even subducted slabs. Based on relatively hot and low-viscosity zones estimated in the periphery of LRZs, we interpret the presence of asthenospheric mantle in areas where LRZs coincide with relatively low Vp. According to this interpretation, asthenospheric upwelling in the study area at depths ≤150 km would be localized rather than ubiquitous over the interpreted extent for the PSW. Such localized asthenosphere upwelling processes in the past could explain the scattered distribution of Neogene basaltic lavas in the southern Patagonia backarc. The continental crust exhibits LRZs in the upper and lower crust. Remarkably, ensembles of LRZs at different crustal depths within the presumable area of the PSW were found below the General Carrera Lake, and towards the North Patagonian Icefield, likely indicating the presence of hidden intraplate magmatic and/or hydrothermal systems.

Glacial Hydro-Isostatic Adjustment at Mid-Ocean Ridges

Summary Recent studies have suggested a link between ice age sea level fluctuations and variations in magma production and crustal faulting along mid-ocean ridges based on the detection of Milankovitch cycle frequencies in topography off several ridges. These fluctuations have also been connected to variability in hydrothermal metal fluxes near ridges. Ice age sea level calculations have shown that the sea level change across glacial cycles will be characterized by significant geographic variability, that is, departures from eustasy, due to the gravitational, deformation and rotational effects of the glacial isostatic adjustment (GIA) process. Using a state-of-the-art GIA simulation that incorporates 3-D variations in Earth viscoelastic structure, including plate boundaries, and updated constraints on the magnitude and geometry of ice mass fluctuations, we predict global sea level changes from Last Glacial Maximum (LGM, 26 ka) to present and from the Penultimate Glacial Maximum (PGM, 143 ka) to the Last Interglacial (LIG, 128 ka). We focus on the results along three ridges: the Mid-Atlantic Ridge, Juan de Fuca Ridge and East Pacific Rise, which are examples of slow, intermediate and fast spreading ridges, respectively. Sea level change across the Mid-Atlantic Ridge shows the greatest variability, ranging from a sea level fall greater than 200 m in Iceland to a maximum rise of ∼150 m in the South Atlantic, with significant non-monotonicity north of the Equator as the ridge weaves across the field of sea level changes. We also calculate changes in crustal normal stress from LGM to present-day across the Mid-Atlantic and Juan de Fuca Ridges and the East Pacific Rise. These results indicate that the contribution from ice mass changes to the crustal stress field can be significant well away from the location of ancient ice complexes. We conclude that any exploration of the hypothesized links to magma production and crustal faulting must consider both ocean and ice loading effects and, more generally, the profound geographic variability of the GIA process.

Chemical and Thermal Changes in Mg3Si2O5 (OH)4 Polymorph Minerals and Importance as an Industrial Material

Serpentine (Mg3Si2O5(OH)4), like quartz, dolomite and magnesite minerals, is a versatile mineral group characterized by silica and magnesium silicate contents with multiple polymorphic phases. Among the phases composed of antigorite, lizardite, and chrysotile, lizardite and chrysotile are the most prevalent phases in the serpentinites studied here. The formation process of serpentinites, which arise from the hydrothermal alteration of peridotites, influences the ratio of light rare earth elements (LREE) to heavy rare earth elements (HREE). In serpentinites, the ratio of light rare earth elements (LREE)/heavy rare earth elements (HREE) provides insights into formation conditions, geochemical evolution, and magmatic processes. The depletion of REE compositions in serpentinites indicates high melting extraction for fore-arc/mantle wedge serpentinites. The studied serpentinites show a depletion in REE concentrations compared to chondrite values, with HREE exhibiting a lesser degree of depletion compared to LREE. The high ΣLREE/ΣHREE ratios of the samples are between 0.16 and 4 ppm. While Ce shows a strong negative anomaly (0.1–12), Eu shows a weak positive anomaly (0.1–0.3). This indicates that fluid interacts significantly with rock during serpentinization, and highly incompatible elements (HIEs) gradually become involved in the serpentinization process. While high REE concentrations indicate mantle wedge serpentinites, REE levels are lower in mid-ocean ridge serpentinites. The enrichment of LREE in the analyzed samples reflects melt/rock interaction with depleted mantle and is consistent with rock–water interaction during serpentinization. The gradual increase in highly incompatible elements (HIEs) suggests that they result from fluid integration into the system and a subduction process. The large differential thermal analysis (DTA) peak at 810–830 °C is an important sign of dehydration, transformation reactions and thermal decomposition, and is compatible with H2O phyllosilicates in the mineral structure losing water at this temperature. In SEM images, chrysotile, which has a fibrous structure, and lizardite, which has a flat appearance, transform into talc as a result of dehydration with increasing temperature. Therefore, the sudden temperature drop observed in DTA graphs is an indicator of crystal form transformation and CO2 loss. In this study, the mineralogical and structural properties and the formation of serpentinites were examined for the first time using thermo-gravimetric analysis methods. In addition, the mineralogical and physical properties of serpentinites can be recommended for industrial use as additives in polymers or in the adsorption of organic pollutants. As a result, the high refractory nature of examined serpentine suggests that it is well-suited for applications involving high temperatures. This includes industries such as metallurgy and steel production, glass manufacturing, ceramic production, and the chemical industry.

Earth’s longest preserved linear volcanic ridge generated by a moving Kerguelen hotspot

Recent seismic tomography unveiled complex mantle plume structures diverging from the originally proposed single, narrow, and vertically-oriented plume conduits, which necessitates new perspectives on the mechanism of hotspot motion. While several recent endeavours have focused on Pacific hotspots’ motion, knowledge of others remains limited. Here we constrain the motions of the Kerguelen hotspot within the Indian Ocean by obtaining robust 40Ar/39Ar ages for the Ninetyeast Ridge, Earth’s longest linear volcanic ridge. These data indicate varying volcanic progression rates along the ridge, contrasting to a constant rate as previously documented. Combined with constraints on the Indian Plate motion and seafloor spreading, we reveal four periods of motions of the hotspot caused by its interactions with the Indian–Antarctic spreading ridge. This suggests that mantle plume lateral flows are susceptible to changes in shallow mantle convection due to the existence of horizontal ponding zones and vertical conduits, especially in the shallow asthenosphere.

Ultraslow spreading ridges: slowest but locally thickest

Ultraslow spreading ridges: slowest but locally thickest

Lithofacies and Microfacies Study of ATE Maastrichtian Reefal Limestones and Comparison with the Aqra Formation, Northeast and Northern Kurdistan Region-Iraq

The Maastrichtian reefal limestone, which developed variably across the northern and northeastern Kurdistan Region of Iraq, includes the Aqra Formation, which formed extensive platforms in the Aqra-Barzan area throughout the Maastrichtian period. Additionally, bars and lenses of reefal limestone in the upper part of the Tanjero Formation created longitudinal submarine ridges in the Arabian-Eurasian Foreland basin. This study selected three sections: the Aqra Formation in the type section and study the reefal limestone lenses in the Upper part of the Tanjero Formation at the Kunamasi and limestone bars in the Balakaity. Focusing on lithofacies and microfacies to determine if these reefal limestones are part of the Aqra Formation. The type section of the Aqra Formation, located in the Aqra area, features significant reefal limestone approximately 700 meters thick, including rudists with bitumen and some dolomite content. This formation is widely deposited in the Aqra and Barzan areas, overlying the Bekhme platform stratigraphically, and extends geographically from Bujel to Chia Gara anticlines. Four submicrofacies identified in this reefal limestone are Miliolids packstone, Rudists floatstone, Peloidal grainstone, and Bioclastic foraminifera grainstone. In the Kunamasi area, the reefal limestone appears as thick deposits associated with marl and sandy claystone beds, approximately 30 meters thick, and situated above the flysch Tanjero Formation. Key submicrofacies here include Omphalocyclus packstone, Rudists floatstone, and Echinoids grainstone. At the Balakaity section, the reefal limestone consists of a single, thick body exceeding 50 meters in thickness, directly overlying the Tanjero Formation and restricted to the Dar-Alsalam area. The submicrofacies in this section include Foraminifera packstone, Omphalocyclus packstone, and Rudists floatstone. Overall, the microfacies interpretation is the Aqra Formation is deposited as a large platform in inner and middle ramp settings. Conversely, the reefal limestone in the Balakaity and Kunamasi areas appears to be part of submerging islands within the flysch marine environment of the Tanjero Formation during the active Arabia-Eurasian Subduction. Based on depositional context and microfacies analysis can be concluded the reefal limestone in these areas is more likely associated with the Tanjero Formation rather than the Aqra Formation.

The Effective Elastic Thickness of the Lithosphere Reveals Tectonic Process and Lithospheric Rheology of the Eurasian Basin

ABSTRACTWe obtained the effective elastic thickness of the lithosphere (Te) in the Eurasian Basin using the fan‐shaped wavelet method. For ultraslow spreading ridges, the magmatic heat input is insufficient to form continuous quasisteady‐state magma lenses along the axis, resulting in significant variability in the rheological strength of the lithosphere. We suggested that the tectonic inheritance resulted in the different lithospheric rheology of WVZ and SMZ, consequently leading to distinct tectonic‐magmatic activities. The SMZ with less magmatic activity has a weaker lithosphere than that of the EVZ with extensive magmatism. We propose that the lithospheric rheology and thermal structure of ultraslow spreading ridge may be influenced not by the amount of magma supply but by the mode of magma emplacement.

Deep mantle plumes feeding periodic alignments of asthenospheric fingers beneath the central and southern Atlantic Ocean

High-resolution full waveform seismic tomography of the Earth's mantle beneath the south and central Atlantic Ocean brings into focus a series of asthenospheric low shear velocity channels, or "fingers" on both sides of the southern and central mid-Atlantic ridge (MAR), elongated in the direction of absolute plate motion with a spacing of [Formula: see text]1,800 to 2,000 km, and associated with bands of shallower residual seafloor depth anomalies that suggest channeled flow over thousands of kilometers. Each of the three most clearly resolved fingers on the African side of the MAR corresponds to a separate group of whole mantle plumes rooted in distinct patches at the core-mantle boundary, feeding hotspots, and volcanic lines with distinct isotopic signatures. Plumes of a given group appear to merge at the top of the lower mantle before separating again, suggesting interaction of deep mantle flow with a more vigorous mesoscale circulation in the upper mantle. The corresponding hotspots are generally offset from the location of the deep mantle plume roots. The distinct isotopic signatures of these hotspot groups are also detected in the mid-ocean ridge basalts at the location where the fingers meet the ridge. Meanwhile, at least some of the variability within each plume group could originate in the upper mantle and extended transition zone where plumes in a given group appear to merge and pond. This study also adds to mounting evidence that the African large low shear velocity province is not a uniform, unbroken pile of dense material rising high above the core-mantle boundary, but rather a collection of mantle plumes rooted in patches of distinct composition.

Late Cenozoic intraplate volcanism as a trigger for hydrothermal venting in the Arctic southwestern Eurasia Basin

Intraplate volcanism has occurred for the last 35 million years within Northeast Atlantic and Arctic margins, including the western Barents Sea, Svalbard, and northern Greenland. Earlier studies have suggested that some of this volcanism might be sourced from nearby mid-ocean ridges. However, legacy data does not reveal correlations between the sporadic volcanism, despite comparable setting, ages, and compositions of basalts across the area. Here, we utilize a compilation of geophysical data to document late Cenozoic intraplate volcanism affecting the northeastern Yermak Plateau and southwestern Eurasia Basin located north of Svalbard. The identified seabed and subsurface features include volcanic (Mound-A) and hydrothermal vent systems (Tayrona Vent) formed approximately 10 and 2.6 million years ago, respectively. These intraplate volcanic products are coincident in age and origin with observed hydrothermal systems on Svalbard and Northeast Atlantic. We propose that these magmatic features are the result of intraplate volcanism associated with seismic and thermal anomalies in the mantle beneath northern Svalbard.

Tracking hydrothermal particles from the ridge axis to the sediment column along the Endeavour segment of the Juan de Fuca Ridge

Elemental fluxes associated with mid-ocean ridge hydrothermal systems are thought to be important in the ocean budgets of many elements but quantitative models of these fluxes, and how they vary in space and time due to different boundary conditions, are in their infancy. This is especially true for non-conservative elements that can be involved in multiple processes transforming them between the dissolved and particulate phases both in the water column and after sedimentation. To help develop a more robust database for parameterizing the processes operating, we undertook a coupled sediment trap and sediment core study of hydrothermal sediments on the west flank of the Endeavour segment of the Juan de Fuca Ridge. Sediment traps deployed in the Main Endeavour Field (MEF) and 3 and 9 km southwest of this field, along the mean flow direction of the hydrothermal plume, show that the rate of change of the concentration of sulfide-associated elements (Cu, Zn, Cd, Pb, Ag, As, Mo) in sediment settling from the plume differs between elements. We interpret this as indicating both different dissolution rates of phases containing these elements and different rates of scavenging of these elements from seawater. Scavenging is most readily tracked using elements for which the scavenged fraction is large relative to the hydrothermally derived fraction. For such elements, the relative efficiency of scavenging from seawater (REE > Cr > Ni > V > P > U) matches that previously reported from the TAG system on the Mid-Atlantic ridge despite different vent fluid and seawater chemistries that are expected to lead to both differing roles of Fe-sulfide and Fe-oxyhydroxides and different Fe oxidation rates. In the sediment trap samples collected 9 km off-axis, but not in those collected 3 km off-axis, the Mn and Ti concentrations correlate strongly despite these samples having much higher Mn concentrations than background sediment. Since Ti is sourced almost entirely from background terrigenous material such a correlation was not expected. The most reasonable explanation for this is that aggregation of hydrothermally derived particles with terrigenous material settling through the water column controls removal of hydrothermally derived particles from the water column. Changes in sediment composition with depth in the off-axis sediment core, along with differences between the composition of material from the sediment core and the off-axis sediment traps, are interpreted as indicating large post-deposition changes in sediment composition. For example, much or all of the Mn, P, As and Mo carried to the sediments with hydrothermally derived particles is released back into the water column during diagenesis. In contrast, V and REE concentrations in the sediment core are higher than those in the off-axis sediment trap samples, which may be due to continued scavenging in a benthic boundary layer post-deposition. Overall these data are interpreted as indicating that the net loss of many elements from the non-buoyant plume may not be a proxy for loss from the ocean, and a better understanding of post-depositional processes in hydrothermal settings may be important in understanding cycling of the aforementioned elements, and likely others, in seawater.

Fore-arc metasomatism by hybrid slab fluids during subduction initiation: Sr–Mg–Ca isotopes of rodingite, western Yarlung Zangbo suture zone

Subduction zone metasomatism is critical for Earth's material exchanges, yet the details of slab dehydration, particularly deserpentinization beneath fore-arcs, remain poorly understood. Here, we present Sr–Mg–Ca isotopic data for Purang rodingites from the western Yarlung Zangbo suture zone (YZSZ), a remnant of the Neo-Tethys Ocean that was subducted during the collision between the Indian and Eurasian Plates. The rodingites, occurring as centimeter- to meter-sized veins and blocks in serpentinized harburgite, are dominated by amphibolite- to greenschist-facies minerals like tremolite, magnesiohornblende, and chlorite. Their cumulate textures and rare earth element patterns resemble troctolite or gabbronorite, presumably formed beneath a seafloor spreading center. The rodingites are enriched in large ion lithophile elements and depleted in high field strength elements, with higher Sr/Nb and Ba/La and lower Nb/U and Ce/Pb ratios than mid-ocean ridge basalts (MORB) and rodingites formed through seawater or serpentinizing fluid alteration of MORB-like protoliths. They also exhibit higher initial 87Sr/86Sr ratios (0.7067–0.7075) and elevated δ26Mg values (−0.22 ± 0.07 ‰ to −0.13 ± 0.02 ‰) compared to unaltered oceanic basalts, while their δ44/40Ca values (0.72 ± 0.02 ‰ to 0.87 ± 0.03 ‰) remain similar to MORB. These signatures point to a metasomatic process beyond seafloor alteration, suggesting modification of a MOR-derived protolith in the fore-arc mantle at slab depths of <40 km, driven by Sr- and Mg-rich fluids from clay-rich sediments and serpentinitized mantle. The measured Sr, Mg and Ca isotope compositions can be reproduced by mixing a MORB-like protolith with hybrid fluids derived from varying proportions of sedimentary clays and serpentinitized peridotite. Combined with previous studies on YZSZ metamorphic soles, we propose that this fore-arc metasomatism occurred during the early Cretaceous, concurrent with the incipient subduction of the Neo-Tethys oceanic rocks. These results highlight the significance of deserpentinization at shallow fore-arc mantle settings during subduction initiation, suggesting that slab dehydration is more complex than previously recognized.

Oceanic geodiversity along back-arc spreading centers reveals analogies with mid-ocean ridges

Oceanic geodiversity provides essential information on the dynamics of the Earth. Here, we focus on the geodiversity of three oceanic back-arc spreading centers: the Mariana Spreading Center, the Central-Southern Lau Basin spreading centers, and the East Scotia Ridge. We defined a method to identify their axial zones, obtaining spreading center depths along the basins. Results improve global plate boundary models and morphology variations, revealing that the average depths along the Mariana, East Scotia, and Lau Basin spreading ridges are 4.5, 3.5, and 2 km, respectively. We also measured new spreading rates based on five magnetic profiles crossing the three back-arc spreading centers, contributing to plate kinematic models. Furthermore, we computed subduction rates, including hinge velocities along the Mariana, South Sandwich, and Tonga Subductions, to understand the existing interactions between the subducting slab hinge motion and the kinematics of their related back-arc spreading centers. Our bathymetric, magnetic, and kinematic data show several differences among the Mariana, the East Scotia, and the Lau spreading centers, stressing the oceanic geodiversity in a similar geodynamic context. Our results also suggest a strong correlation between axial depth and full spreading rates along the back-arc spreading centers, a geological correspondence that allows a similar description of these divergent plate boundaries within the mid-ocean ridge classification. Finally, we show how hinge kinematics affects the relationship between convergence along subduction zones and back-arc spreading rates. All our findings contribute to understand how the oceanic geodiversity is directly related to geodynamic processes, increasing the knowledge of global tectonics.

Assessing the impact of sea level rise on maritime entitlement and delimitation: an interdisciplinary investigation through legal and technical analysis

This paper delves into the complexities of maritime delimitation in the context of sea level rise (SLR) and ice-covered regions, examining several factors and legal implications. Through academic discussion and technical analysis, the adjustments is required in approaches to the Exclusive Economic Zone and the continental shelf boundaries’ delimitation amidst SLR, while the different models adopted by adjacent and opposite states are presenting. As a result, the paper provides a comprehensive overview of common issues surrounding baseline determination, particularly in relation to SLR and the challenges posed by off-shore features. Legal dynamics concerning ‘submarine ridges’ versus ‘oceanic ridges’ are explored, highlighting the complexities inherent in maritime boundary delineation. Additionally, the dynamics of basepoint selection in ice-covered regions is investigated, emphasizing essential criteria for navigation and offering case studies from the Antarctic and Arctic here. Through this exploration, the paper contributes to a deeper understanding of the challenges and considerations involved in maritime delimitation amidst SLR, offering valuable insights from both technical and legal perspectives.

Geochemical constraints on reaction temperature and pressure and heat- and mass-transfer efficiency at Rainbow Hydrothermal Field

Seafloor vent fluids hosted by oceanic core complexes (OCCs) are thought to represent circulation of seawater-derived hydrothermal fluid along deeply penetrating, low-angle detachment faults. However, estimation of the source temperatures and pressures of such fluids has been limited because geochemical methods typically require vent fluid silica concentrations to be buffered by quartz, a condition not often met at oceanic core complexes. Here, we extend the calculation of the Si-Cl geothermobarometer to enable predictions of fluid Si concentrations in equilibrium with any Si-buffering mineral assemblage, rather than being restricted to quartz. We apply this method to Rainbow Hydrothermal Field vent fluid compositions and find that they are consistent with buffering by a plagioclase+talc+chlorite+tremolite mineral assemblage at conditions ranging from (430 °C, 359 bar) to (470 °C, 468 bar), which correspond to depths of 1.3–2.4 km below the seafloor. These estimates agree well with the locations of seismically imaged magma chambers within the Rainbow Massif. Additionally, calculations of fluxibility and Fe solubility support this relatively shallow origin for Rainbow vent fluids and imply relatively efficient heat and Fe extraction from the seafloor. We estimate that only 24–30 % of the heat content and almost no Fe is lost during upflow of Rainbow vent fluids.Compared to other OCC-hosted seafloor vents, the source region of Rainbow vent fluids is anomalously shallow, an observation consistent with geological interpretations of the Rainbow Massif. Vent fluids at Rainbow Hydrothermal Field have exhibited apparently stable and greater-than-seawater salinity for over two decades. We interpret these vent fluids as vapors derived from a higher-salinity source fluid that developed over multiple cycles of magma injection and phase-separation inherent to the formation of oceanic crust along slow-spreading, non-volcanic segments of oceanic spreading centers. Alternatively, higher-salinity source fluids could be derived from mineral hydration reactions associated with serpentinization of ultramafic rocks. The occurrence of greater-than-seawater salinity vent fluids is thus predicted to be a common feature of OCC-hosted vent fields, as indicated by several known examples, including the TAG, Kairei, and Edmond vent fields.

Magmatic and metamorphic evolution of the Late Cretaceous Muslim Bagh Ophiolite, western Pakistan: implications for ridge subduction after subduction initiation

ABSTRACT The magmatic and metamorphic evolutions of the Muslim Bagh Ophiolite (MBO) can help understand the relationship among supra-subduction ophiolite, subduction initiation, and the spreading centre in the forearc. Here, we investigated the whole-rock chemistry, mineral composition, P–T conditions, and K–Ar ages of the MBO and its associated units. The whole-rock and magmatic clinopyroxene compositions indicate that the MBO amphibolites and tholeiitic basalts of the underlying Bagh Complex originated in a supra-subduction setting and mid-ocean ridges, respectively. Late Cretaceous oceanic island basalts found in the Bagh Complex and the Bibai Formation occurred in oceans and near the passive continental margin of western India, respectively. Metamorphic Mg-hornblendes in the MBO amphibolites record low-P/T type upper amphibolite-facies metamorphism (2.4–2.9 kbar, 612–690 °C), indicating a > 50 ºC/km geothermal gradient. The K–Ar hornblende and plagioclase ages suggest contemporaneous low-P/T type metamorphism at ca. 78–71 Ma in the MBO formed in a forearc and tholeiitic ridge-related magmatism of ca. 78–77 Ma in the Bagh Complex, indicating a spreading ridge was active during the subduction. Our results, combined with plate kinematic reconstructions of the Neotethys Ocean in the previous studies, suggest that the abnormally high geothermal gradient in the MBO could be due to orthogonal ridge subduction near the triple junction after the ca. 80 Ma subduction initiation in the forearc. It is also likely that the geothermal gradient in the MBO dropped from >50 °C/km at 78–71 Ma, through 35–45 °C/km, to <25 °C/km at approximately 65 Ma due to gradual steepening of the subducting slab, followed by the obduction of the MBO during the Paleocene to Eocene period.

Spatiotemporal evolution of post-seismic deformation caused by large earthquakes around the Pacific and its impact on plate movement

SUMMARY The fully relaxed deformation caused by an earthquake refers to the total deformation of the earth caused by the earthquake, including the coseismic deformation and the total effect of post-seismic stress relaxation in the mantle on crustal deformation. An in-depth investigation into post-seismic and fully relaxed deformation resulting from large earthquakes is conducive to comprehending the feedback effect of earthquakes on plate motion. In this paper, we first calculated theoretically the coseimic, post-seicmic and fully relaxed deformations caused by the Tohoku-Oki Mw9.0 earthquake using the spherical dislocation theory. The post-seismic displacement caused by the great earthquake leads to the continuous convergence of plates on both sides of the seismogenic fault, which will certainly facilitate the downward insertion of the subduction plate. As time goes by the influence range of seismic deformations becomes larger and larger. The Tohoku-Oki Mw9.0 earthquake can produce &amp;gt;5 cm of cumulative post-seismic horizontal displacement at a far place like the East Pacific Rise over 100 millenniums. Then, we built slip models of 375 earthquakes with Mw7.0 and above in the circum-Pacific seismic belt, calculated the cumulative post-seismic deformations of them, and found that significant post-seismic horizontal displacements covered the entire Pacific Ocean. The post-seismic deformation field caused by the 2011 Tohoku-Oki Mw9.0 earthquake, the 2010 Chile Mw8.8 earthquake, the 1964 Prince William Sound Mw9.1 earthquake and the 1960 Chile Mw(9.3–9.4) earthquake determines the overall distribution pattern of the deformation field in the Pacific region. Those large earthquakes around the Pacific make the total Pacific Plate present a tension strain in the northwest–southeast direction. The one at the East Pacific Rise reaches 300–400 nstr, with orientation of principle strain approximately perpendicular to the Rise. This tensile strain is bound to encourage transverse expansion of the mid-ocean ridge. By a logical extension, the post-seismic stress relaxation in the mantle caused by past earthquakes should be an important driving force for the current plate movement, in addition to the classic driving force like negative buoyancy and plate material phase transition. This study proves theoretically that there is a two-way relationship between great earthquakes and plate movement, and the viscous structure of mantle plays a key role in the relationship.

Petrogenesis and tectonic setting of mafic magmatism within the Laguna Amarga Metamorphic Complex, Andes of Catamarca, Argentina: Insights into the opening of the Cuyania/Precordillera terrane from the Ouachita rift

Neoproterozoic crystalline basement rocks are exposed as fault-bounded blocks over the high Andes of Catamarca. The crystalline basement is stratigraphically grouped into the unique Laguna Amarga Metamorphic Complex and represents the northern extension of the Cuyania/Precordillera terrane. Tabular bodies of meta-mafic rocks are widespread in the basement interspersed within a thick sequence of meta-sedimentary rocks derived from siliciclastic, calc-silicate, and limestone protoliths. Overall, the geochemical characteristics of meta-mafic rocks are in the compositional range of Normal-Mid Ocean Ridge Basalt (Normal-MORB), reflecting a common depleted-mantle source with varying degrees of partial melting. While preserving the typical bulk chemistry of MORB magmatism, some mafic magma underwent differentiation at emplacement, leading to the development of high-Ti mafic rocks. New U-Pb zircon geochronology reveals three distinct age peaks, with two coinciding with ages identified in the metasedimentary host rocks. The dominant Mesoproterozoic age cluster is linked to inherited zircon crystals assimilated within a single meta-mafic rock. In contrast, zircon ages from the late Ordovician to early Devonian are attributed to metamorphic overgrowths. Notably, the third age cluster, delineates a Late Neoproterozoic magmatic event, indicating the temporal span of mafic magmatism. The finding agrees with the best available age (576 ± 17 Ma) for mafic magmatism on the Precordillera Mafic-Ultramafic Belt. Stratigraphic relationships and geochemical fingerprints enable correlation among the meta-mafic rocks from Laguna Amarga, tracing a belt of mafic magmatism with an oceanic affinity that extends southward. Building upon previous works, this study reaffirms that the rift-drift transition of the Cuyania/Precordillera terrane, linked to the Ouachita rift opening from southeastern Grenville, evolved during the latest Neoproterozoic.

Large-scale mantle heterogeneity as a legacy of plate tectonic supercycles

The Earth’s mantle is divided by the circum-Pacific subduction girdle into the African and Pacific domains, each featuring a large low-shear-wave-velocity province (LLSVP) in the lower mantle. However, how this hemispherical-scale mantle structure links to Earth’s plate tectonic evolution remains unclear. Previous geochemical work has suggested the presence of a north–south hemispheric subdivision, with large-scale mantle heterogeneities in the Southern Hemisphere, termed the DUPAL (Dupré and Allegre) anomaly. Here we compile elemental and isotopic data of both shallow-mantle-derived oceanic igneous rocks from mid-ocean ridges and deeper-mantle plume-related samples (ocean islands and oceanic plateaus) and analyse these using supervised machine learning classification methods. Data from both shallow- and deeper-mantle-sourced samples illustrate a consistent chemical dichotomy. Our results indicate that heterogeneities in the present-day shallow and deep mantle are not exclusively controlled by the north–south hemispheric DUPAL anomaly. Instead, they are consistent with a chemical dichotomy between the African and Pacific mantle domains and their associated LLSVPs. These observations can best be explained by tectonic supercycles over the past one billion years involving two supercontinents and two superoceans.