Search Results (24)

Mylonitic mantle peridotites exposed at the Tosa Megamullion in the Shikoku Basin, Philippine Sea, provide direct evidence for amagmatic ductile shear deformation of the upper mantle beneath a back-arc spreading center. Oceanic core complexes (OCCs), or megamullions, are dome-shaped structures formed by detachment faulting and occur locally along slow-spreading mid-ocean ridges and back-arc basins, where they expose fault rocks derived from ductile shear zones in the lower crust and upper mantle. The Shikoku Basin hosts several OCCs, including the Tosa Megamullion, which formed during the early stage of back-arc spreading. In this study, nine ultramafic rocks were collected from the Tosa Megamullion using the submersible Shinkai6500 during cruise YK23-05S. Although all samples were highly serpentinized, several preserved primary peridotitic textures were composed mainly of olivine, orthopyroxene, with subordinate clinopyroxene, plagioclase, and spinel. Seven samples exhibit well-developed foliation and porphyroclastic textures dominated by orthopyroxene porphyroclasts, ranging from rounded to strongly elongated forms, commonly showing microkinks and undulose extinction. Crystallographic preferred orientations (CPOs) of three representative samples, analyzed using SEM-EBSD, reveal E-type-dominant olivine fabrics characterized by the (001)[100] slip system, with a subordinate contribution from C-type (100)[001] slip. These CPOs suggest deformation under non-dry conditions involving moderate hydration and/or elevated differential stress. These results indicate that the ultramafic rocks from the Tosa Megamullion represent mantle-derived mylonitic peridotites formed by ductile shear beneath the spreading axis and subsequently exhumed under strongly magma-poor, amagmatic conditions. The Tosa Megamullion thus represents an amagmatic end-member of the OCC formation in back-arc basins, dominated by tectonic strain localization rather than by magmatic accretion. Full article

While under-ice submarine hydrothermal systems provide critical insights into extremophile adaptations, the ecological impacts of explosive volcanism on these ecosystems remain poorly constrained. We successfully detected evidence of hydrothermal activities and explosive volcanism at 85° E, the eastern volcanic zone, ultra-slow spreading Gakkel Ridge. Hydrothermal plume, surface sediments, and volcanic glass samples were systematically collected to investigate the diversity of microbial communities. Our results revealed two distinct microbial regimes in hydrothermal plume: (1) chemoautotrophic bacteria (Sulfurimonas and SUP05_cluster), prevalent in global basaltic hydrothermal systems, potentially involved in carbon fixation through the CBB and rTCA cycles and (2) Alcanivorax (up to 82.5%), known for degrading hydrocarbons. Sediment profiles showed a depth-dependent decline of Alcanivorax, tightly coupled with TOC (1.05% to 0.45%, r = 0.75, p < 0.05). Additionally, the Alcanivorax MAGs demonstrated their potential in degrading various types of organic carbon, especially in alkane degradation. Strikingly, this pattern contrasts with hydrothermal plumes from effusive volcanic zones (Aurora and Polaris regions), where Alcanivorax was undetectable. We speculate that the surge of Alcanivorax in the 85° E hydrothermal plume was associated with the violent disturbances caused by explosive volcanism. This mechanism accelerates microbial-mediated carbon turnover rates compared to a stable hydrothermal ecosystem. Full article

This study investigates the volcano–sedimentary processes that occurred in an oceanic branch of the Western Tethys, now part of the Gimigliano–Monte-Reventino metaophiolite Unit, exposed at the southeastern termination of the Sila Piccola Massif, within the northern sector of the Calabria–Peloritani terrane (Calabria, southern Italy). Fieldwork, petrography, and mineralogical analyses on the Gimigliano metaophiolite succession have identified five distinct volcano–sedimentary lithofacies. These lithofacies are characterized by mineral assemblages of epidote, chlorite, quartz, and albite, with minor amounts of muscovite and calcite, resulting from high-pressure–low-temperature (HP-LT) metamorphism followed by low-grade greenschist metamorphism of mid-oceanic ridge basalt (MORB)-type volcanic products. Based on their stratigraphic and textural features, these lithofacies have been interpreted as metabasaltic flow layers emplaced during effusive volcanic eruptions and metahyaloclastic and metavolcaniclastic deposits formed by explosion-driven processes. This lithofacies assemblage suggests that the Gimigliano area likely represented an oceanic sector with high rates of magmatic outflows, where interactions between magma and water facilitated explosive activity and the dispersion of primary volcaniclastic deposits, mainly from the water column, in addition to the emplacement of basaltic lava flow. In contrast, other metaophiolite complexes in the Calabria region, characterized by the presence of pillow basalts, were areas with low effusive rates. The coexistence of these differences, along with the extensive presence of metaultramafites, portrays the Calabrian branch of the Tethys as a slow-spreading oceanic ridge where variations in surficial volcanic processes were controlled by differences in the effusion rates across its structure. This study is a valuable example of how a volcano–sedimentary approach to reconstructing the emplacement mechanisms of metaophiolite successions can provide geodynamic insights into ancient oceanic ridges. Full article

Hainan Island is the only large island located on the northern margin of the South China Sea and is surrounded by Cenozoic graben basins, including the Qiongdongnan, Yinggehai, and Beibuwan basins. The uplift and denudation history of the Jianfeng pluton on southwestern Hainan Island is significant for understanding the formation of the regional geomorphology and adjacent basin evolution. This paper presents apatite and zircon fission-track (FT) analyses conducted on the Jianfeng pluton. The zircon FT (ZFT) ages of the pluton range are from 63 ± 4 to 108 ± 8 Ma, and the apatite FT (AFT) ages are from 19.4 ± 1.8 to 43.9 ± 4.4 Ma. The average confined track lengths in apatite are relatively short (11.9–12.8 μm). An age–elevation plot indicates that two rapid cooling events occurred during 73–63 and 44–40 Ma. Thermal modeling revealed four stages of 73–63 Ma, 44–40 Ma, 40–11 Ma, and 11–0 Ma. From the Late Cretaceous to the middle Eocene (73–40 Ma), the Jianfeng area underwent episodic rapid uplift and denudation. At the end of the Late Cretaceous (73–63 Ma), the area was affected by mid-ocean ridge spreading in the Proto-South China Sea. During the middle Eocene (44–40 Ma), the Yinggehai Basin underwent abrupt expansion and subsidence, which increased the elevation difference between the Jianfeng area and the Yinggehai Basin. From the middle Eocene to the middle Miocene (40–11 Ma), the Jianfeng area underwent slow denudation, and the Yinggehai Basin was rapidly infilled, which eliminated the original elevation difference between the two areas. From the middle Miocene to the present (11–0 Ma), the Jianfeng area has undergone reactivated rapid uplift and denudation, which was driven by the remote effects of the India–Eurasia collision. Full article

Although recent research suggests that the morphology and crustal structure of slow–ultraslow-spreading ridges are mainly controlled by melt supply, there is a lack of quantitative understanding of the effect of systematic changes in melt supply on the seafloor spreading state of mid-ocean ridges. In this study, we used bathymetry, free-air gravity anomaly, and sediment thickness data to calculate the residual bathymetry, mantle Bouguer gravity and crustal thickness of the Reykjanes Ridge. According to the gradient of changes in crustal thickness and residual bathymetry along the axis, the influence of melt supply on the spreading state of the Reykjanes Ridge can be divided into three zones: ultra-strong effect zone (0–160 km), strong effect zone (160–610 km), and weak effect zone (610–930 km). In the ultra-strong effect zone, excess melt supply and a higher melting degree result in a strong upwelling and large melt eruption. The change in relative position between the Reykjanes Ridge and the Iceland hotspot results in the spreading state of the Reykjanes Ridge transforming from asymmetric spreading to symmetric spreading. In the strong effect zone, the decrease in melt supply and melting degree weakens the mantle upwelling and enhances the viscosity of the dehydrated mantle layer. Sufficient viscosity of the dehydrated mantle layer forces asymmetric asthenosphere rise along the sloping boundary of the lithosphere, resulting in symmetric spreading. In the weak effect zone, the pattern of magma upwelling becomes a focused magma supply pattern similar to that of the slow–ultraslow-spreading of the mid-ocean ridge, and tectonics dominate the spreading process. The asymmetry of this weak effect zone may be due to the concentration of tectonic and magmatic activity on one flank of the ridge. Full article

Magmatic and tectonic processes in the formation of oceanic lithosphere at slow–ultraslow-spreading mid-ocean ridges (MORs) are more complicated relative to faster-spreading ridges, as their melt flux is overall low, with highly spatial and temporal variations. Here, we use the teleseismic catalog of magnitudes over 4 between 1995 and 2020 from the International Seismological Center to investigate the characteristics of magmatic and tectonic activities at the ultraslow-spreading Southwest Indian Ridge and Arctic Gakkel Ridge and the slow-spreading North Mid-Atlantic Ridge and Carlsberg Ridge (total length of 14,300 km). Using the single-link cluster analysis technique, we identify 78 seismic swarms (≥8 events), 877 sequences (2–7 events), and 3543 single events. Seismic swarms often occur near the volcanic center of second-order segments, presumably relating to relatively robust magmatism. By comparing the patterns of seismicity between ultraslow- and slow-spreading ridges, and between melt-rich and melt-poor regions of the Southwest Indian Ridge with distinct seafloor morphologies, we demonstrate that a lower spreading rate and a lower melt supply correspond to a higher seismicity rate and a higher potential of large volcano-induced seismic swarms, probably due to a thicker and colder lithosphere with a higher degree of along-axis melt focusing there. Full article

Several Jurassic, ophiolite-hosted Cu-Zn VMS deposits occur in Tuscany. They are hosted by tectonic units of oceanic affinity (Ligurian Units), such as the well-known deposits of nearby Liguria. Industrial production was small and definitively ceased in the 1960s. Locally, massive ore (chalcopyrite-bornite-chalcocite) with an exceptionally high grade was found. The Montecatini Val di Cecina mine exploited the largest “bonanza” and, for few decades in the 19th century, became one of the most profitable copper mines in Europe. This study provides an updated review of these deposits. Tuscan Cu-Zn VMSs mostly occur in proximity of the contact between the serpentinite-gabbro basement and the overlying basalts. Chalcopyrite-pyrite stockworks occur in serpentinite-gabbro cut by dolerite dykes, while the largest massive sulphide bodies are hosted by polymictic-monomictic breccias at the base of pillow basalts. Early chalcopyrite ores were mechanically–chemically reworked and upgraded to bornite-rich nodular ore embedded in a chlorite, calcic amphibole, Fe-rich serpentine, quartz, andradite, ilvaite, and xonotlite assemblage. This bornite-rich ore contains substantial amount of sphalerite and pyrite and ubiquitous grains of clausthalite, hessite, tellurium, and gold. They represent a prime example of the sub-seafloor portion of a hybrid mafic-ultramafic oceanic hydrothermal system formed in an OCC along the slow spreading ridge of the Jurassic Piedmont-Ligurian Ocean. The peculiar mineralogical–textural character of the bornite-rich ore was driven by an interface coupled dissolution–precipitation process mediated by fluids. Full article

Geovisualization tools can supplement the statistical analyses of landscape-level wildfire behavior by enabling the discovery of nuanced information regarding the relationships between fire spread, topography, fuels, and weather. The objectives of this study were to develop and evaluate the effectiveness of geovisualization tools for analyzing wildfire behavior and specifically to apply those tools to study portions of the Thomas and Detwiler wildfire events that occurred in California in 2017. Fire features such as active fire fronts and rate of spread (ROS) vectors derived from repetitive airborne thermal infrared (ATIR) imagery sequences were incorporated into geovisualization tools hosted in a web geographic information systems application. This geovisualization application included ATIR imagery, fire features derived from ATIR imagery (rate of spread vectors and fire front delineations), growth form maps derived from NAIP imagery, and enhanced topographic rasters for visualizing changes in local topography. These tools aided in visualizing and analyzing landscape-level wildfire behavior for study portions of the Thomas and Detwiler fires. The primary components or processes of fire behavior analyzed in this study were ROS, spotting, fire spread impedance, and fire spread over multidirectional slopes. Professionals and researchers specializing in wildfire-related topics provided feedback on the effectiveness and utility of the geovisualization tools. The geovisualization tools were generally effective for visualizing and analyzing (1) fire spread over multidirectional slopes; (2) differences in spread magnitudes within and between sequences over time; and (3) the relative contributions of fuels, slope, and weather at any given point within the sequences. Survey respondents found the tools to be moderately effective, with an average effectiveness score of 6.6 (n = 5) for the visualization tools on a scale of 1 (ineffective) to 10 (effective) for postfire spread analysis and visualizing fire spread over multidirectional slopes. The results of the descriptive analysis indicate that medium- and fine-scale topographic features, roads, and riparian fuels coincided with cases of fire spread impedance and exerted control over fire behavior. Major topographic features such as ridges and valleys slowed, or halted, fire spread consistently between study areas. The relationships between spotting, fuels, and topography were inconclusive. Full article

Space-time adaptive processing (STAP) is a well-known technique for slow-moving target detection in the clutter spreading environment. For an airborne conformal array radar, conventional STAP methods are unable to provide good performance in suppressing clutter because of the geometry-induced range-dependent clutter, non-uniform spatial steering vector, and polarization sensitivity. In this paper, a knowledge aided STAP method based on sparse learning via iterative minimization (SLIM) combined with Laplace distribution is proposed to improve the STAP performance for a conformal array. The proposed method can avoid selecting the user parameter. the proposed method constructs a dictionary matrix that is composed of the space-time steering vector by using the prior knowledge of the range cell under test (CUT) distributed in clutter ridge. Then, the estimated sparse parameters and noise power can be used to calculate a relatively accurate clutter plus noise covariance matrix (CNCM). This method could achieve superior performance of clutter suppression for a conformal array. Simulation results demonstrate the effectiveness of this method. Full article

Back-arc basins in continental settings can develop into oceanic basins, when extension lasts long enough to break up the continental lithosphere and allow mantle melting that generates new oceanic crust. Often, the basement of these basins is not only composed of oceanic crust, but also of exhumed mantle, fragments of continental crust, intrusive magmatic bodies, and a complex mid-ocean ridge system characterised by distinct relocations of the spreading centre. To better understand the dynamics that lead to these characteristic structures in back-arc basins, we performed 2D numerical models of continental extension with asymmetric and time-dependent boundary conditions that simulate episodic trench retreat. We find that, in all models, episodic extension leads to rift and/or ridge jumps. In our parameter space, the length of the jump ranges between 1 and 65 km and the timing necessary to produce a new spreading ridge varies between 0.4 and 7 Myr. With the shortest duration of the first extensional phase, we observe a strong asymmetry in the margins of the basin, with the margin further from trench being characterised by outcropping lithospheric mantle and a long section of thinned continental crust. In other cases, ridge jump creates two consecutive oceanic basins, leaving a continental fragment and exhumed mantle in between the two basins. Finally, when the first extensional phase is long enough to form a well-developed oceanic basin (>35 km long), we observe a very short intra-oceanic ridge jump. Our models are able to reproduce many of the structures observed in back-arc basins today, showing that the transient nature of trench retreat that leads to episodes of fast and slow extension is the cause of ridge jumps, mantle exhumation, and continental fragments formation. Full article

Non-transform discontinuity (NTD) is one category of tectonic units along slow- and ultraslow-spreading ridges. Some NTD-related hydrothermal fields that may reflect different driving mechanisms have been documented along slow-spreading ridges, but the discrete survey strategy makes it hard to evaluate the incidence of hydrothermal activity. On ultraslow-spreading ridges, fewer NTD-related hydrothermal activities were reported. Factors contributing to the occurrence of hydrothermal activities at NTDs and whether they could be potential targets for hydrothermal exploration are poorly known. Combining turbidity and oxidation reduction potential (ORP) sensors with a near-bottom camera, Chinese Dayang cruises from 2014 to 2018 have conducted systematic towed surveys for hydrothermal activity around a large NTD along the ultraslow-spreading Southwest Indian Ridge (SWIR, 48.1–48.7° E). Five new potential hydrothermal anomaly sites (2 inferred and 3 suspected) of high or low temperature and the previously inferred Sudi hydrothermal field occurred in diverse morphotectonic settings along a 78 km long ridge axis. The calculated vent frequency (Fs, sites/100 km) was ~7.7 over the entire study area, higher than the modified value (Fs ≈ 6.5) between 48 and 52° E of SWIR. Even only for the 54 km long large NTD, three hydrothermal anomaly sites yielded an Fs of ~5.6, which is higher than that of most ridge sections and is comparable to some fast-spreading ridges with high-resolution surveys. This indicates that NTDs along ultraslow-spreading ridges could be promising areas in fertilizing hydrothermal activities. Moreover, the deeply penetrating faults on the rift valley walls and strain-focused areas may contribute to the formation of NTD-related hydrothermal circulations. We suggest that NTDs along ultraslow-spreading ridges may be potential targets for further exploration of hydrothermal activities and seafloor sulfide deposits. Full article

Passive seismic interferometry has become very popular in recent years in exploration geophysics. However, it has not been widely applied in marine exploration. The purpose of this study is to investigate the internal structure of a quasi-amagmatic portion of the Southwest Indian Ridge by interferometry and to examine the performance and reliability of interferometry in marine explorations. To reach this goal, continuous vertical component recordings from 43 ocean bottom seismometers were analyzed. The recorded signals from 200 station pairs were cross-correlated in the frequency domain. The Bessel function method was applied to extract phase–velocity dispersion curves from the zero crossings of the cross-correlations. An average of all the dispersion curves was estimated in a period band 1–10 s and inverted through a conditional neighborhood algorithm which led to the final 1D S-wave velocity model of the crust and upper mantle. The obtained S-wave velocity model is in good agreement with previous geological and geophysical studies in the region and also in similar areas. We find an average crustal thickness of 7 km with a shallow layer of low shear velocities and high Vp/Vs ratio. We infer that the uppermost 2 km are highly porous and may be strongly serpentinized. Full article

In 2018, two earthquake swarms occurred along spreading ridge segments of the ultra-slow Southwest Indian Ridge (SWIR). The first swarm was located at the spreading-ridge intersection with the Novara Fracture Zone, comprising 231 events (ISC catalogue) and spanning over 6 days (10 July to 15 July). The second swarm was more of a cluster of events focusing near a discontinuity, 220 km west of the Rodrigues Triple Junction, composed of 92 events and spanning over 31 days (27 September to 27 October). We examined these two swarms using hydroacoustic records from the OHASISBIO network with seven to nine autonomous hydrophones moored on either side of the SWIR. We detected 1109 hydroacoustic events spanning over 13 days (6 July to 18 July) in the first swarm and 4880 events spanning over 33 days in the second swarm (25 September to 27 October). The number of events per day was larger, and the hydroacoustic magnitude (source level) was, on average, smaller during the second swarm than the first. The spatio-temporal distribution of events from both swarms indicates a magmatic origin initiated by dike intrusions and followed by a readjustment of stresses in the surrounding crust. Full article

We present a detailed description of the tectono-stratigraphic architecture of the eclogite-facies Internal Piedmont Zone (IPZ) metaophiolite, exposed in the Lanzo Valleys (Western Alps), which represents the remnant of the Jurassic Alpine Tethys. Seafloor spreading and mantle exhumation processes related to the Alpine Tethys evolution strongly conditioned the intra-oceanic depositional setting, which resulted in an articulated physiography and a heterogeneous stratigraphic succession above the exhumed serpentinized mantle. “Complete” and “reduced” successions were recognized, reflecting deposition in morphological or structural lows and highs, respectively. The “complete” succession consists of quartzite, followed by marble and calcschist. The “reduced” succession differs for the unconformable contact of the calcschist directly above mantle rocks, lacking quartzite and gray marble. The serpentinite at the base of this succession is intruded by metagabbro and characterized at its top by ophicalcite horizons. Mafic metabreccia grading to metasandstone mark the transition between the “complete” and “reduced” successions. The character of the reconstructed succession and basin floor physiography of the IPZ metaophiolite is well comparable with the Middle Jurassic–Late Cretaceous succession of both the Queyras Complex (External Piedmont Zone) and the Internal Ligurian Units (Northern Apennines) and with modern slow-spreading mid-ocean ridges. Full article

The Mid-Atlantic Ridge belongs to slow-spreading ridges. Hannington predicted that there were a large number of mineral resources on slow-spreading ridges; however, seafloor massive sulfide deposits usually develop thousands of meters below the seafloor, which make them extremely difficult to explore. Therefore, it is necessary to use mineral prospectivity mapping to narrow the exploration scope and improve exploration efficiency. Recently, Fang and Shao conducted mineral prospectivity mapping of seafloor massive sulfide on the northern Mid-Atlantic Ridge, but the mineral prospectivity mapping of magmatic-related seafloor massive sulfide on the whole Mid-Atlantic Ridge scale has not yet been carried out. In this study, 11 types of data on magmatic-related seafloor massive sulfide mineralization were collected on the Mid-Atlantic Ridge, namely water depth, slope, oceanic crust thickness, large faults, small faults, ridge, bedrock age, spreading rate, Bouguer gravity, and magnetic and seismic point density. Then, the favorable information was extracted from these data to establish 11 predictive maps and to create a mineral potential model. Finally, the weights-of-evidence method was applied to conduct mineral prospectivity mapping. Weight values indicate that oceanic crust thickness, large faults, and spreading rate are the most important prospecting criteria in the study area, which correspond with important ore-controlling factors of magmatic-related seafloor massive sulfide on slow-spreading ridges. This illustrates that the Mid-Atlantic Ridge is a typical slow-spreading ridge, and the mineral potential model presented in this study can also be used on other typical slow-spreading ridges. Seven zones with high posterior probabilities but without known hydrothermal fields were delineated as prospecting targets. The results are helpful for narrowing the exploration scope on the Mid-Atlantic Ridge and can guide the investigation of seafloor massive sulfide resources efficiently. Full article