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Mid-ocean ridge and oceanic transforms are among the most prominent features on the seafloor surface and are crucial for understanding seafloor spreading and plate tectonic dynamics, but the deep structure of the oceanic lithosphere remains poorly understood. The large number of microearthquakes occurring along ridges and transforms provide valuable information for gaining an in-depth view of the underlying detailed seismic structures, contributing to understanding geodynamic processes within the oceanic lithosphere. Previous studies have indicated that the maximum depth of microseismicity is controlled by the 600-°C isotherm. However, this perspective is being challenged due to increasing observations of deep earthquakes that far exceed this suggested isotherm along mid-ocean ridges and oceanic transform faults. Several mechanisms have been proposed to explain these deep events, and we suggest that local geodynamic processes (e.g., magma supply, mylonite shear zone, long-lived faults, hydrothermal vents, etc.) likely play a more important role than previously thought. [ABSTRACT FROM AUTHOR]

Melt supply at the ultraslow‐spreading Southwest Indian Ridge (SWIR) has been shown to vary from nearly amagmatic, leading to ultramafic seafloor, to magmatically robust, producing fully volcanic seafloor. The center of the SWIR 50°28′E segment represents a magmatically robust endmember. High‐resolution bathymetry and backscatter, near‐bottom magnetic data, and visual observations were acquired to infer spatiotemporal variations in upper‐crustal construction over the past 780 kyr. Tectonic strain inferred from observed faults represents <8% of the total plate divergence during this period. Mapped seafloor and lava morphologies show a prevalence of smooth seafloor and smooth hummocky seafloor (64% mapped area), corresponding to a dominance of high‐effusion‐rate, sheet‐lobate lavas, over hummocky seafloor (29%) that mostly comprises low‐effusion‐rate pillow lavas. This prevalence is most pronounced within a ∼5 km‐long domain at the segment center, while hummocky seafloor prevails to its east and west, indicating a substantial along‐axis decrease in average eruption rate. Across‐axis changes of seafloor morphologies and faulting pattern indicate two successive cycles of upper‐crustal construction. These cycles last ∼300 kyr, interpreted as waxing and waning magmatic phases. During waxing phases, shallow axial melt lenses (AMLs) form beneath the segment center, feeding narrow diking systems that build high‐eruption‐rate smooth domal volcanos. During waning phases, lower frequencies of melt replenishment may cause the AML to become deeper and ultimately disappear. This configuration triggers diking and hummocky‐dominated eruptions over a wider axial domain, with more faults. We propose that modes of upper‐crustal construction at slow and ultraslow mid‐ocean ridges are influenced by melt supply more than spreading rate. Plain Language Summary: The ultraslow‐spreading Southwest Indian Ridge (SWIR) is characterized by extremely variable melt supply. Previous studies show that the center of the SWIR 50°28′E segment has a high melt supply with a uniquely 9.5 km‐thick crust and a crustal melt‐mush zone. Seafloor and lava morphologies, fault scarps and fissures, and eruptive units over the past 780 kyr, were mapped in this high melt supply region, using 2‐m resolution bathymetry and backscatter data collected by an autonomous underwater vehicle and seafloor photographs collected by a towed camera. Our geological mapping suggests high eruption rates and low tectonic strain, similar to intermediate‐ and fast‐spreading ridges. It also indicates that the upper crust was constructed with alternating waxing and waning magmatic phases over cycles of ∼300 kyr duration. Waxing phases are characterized by high‐eruption‐rate smooth domal volcanos fed by localized dikes. Based on available geophysical data, waxing phases occur with relatively shallow and frequently replenished axial melt lenses. By contrast, waning phases are dominated by low‐eruption‐rate hummocky ridges fed by delocalized dikes, corresponding to deeper and shorter‐lived axial melt lenses and lower frequency of melt replenishment. We propose that the upper‐crustal geology at slow‐ultraslow ridges is controlled by melt supply more than spreading rate. Key Points: The Southwest Indian Ridge at 50°28′E experienced a high eruption rate and low tectonic strain over the past 780 kyr, despite ultraslow spreading rateSeafloor and lava morphologies indicate two successive cycles of waxing and waning magmatic phases, each lasting 300 kyrUpper‐crustal construction at slow and ultraslow mid‐ocean ridges are controlled by melt supply more than spreading rate [ABSTRACT FROM AUTHOR]

Spreading centers in the proximity of back‐rolling subduction zones constitute an ideal natural laboratory to investigate the interaction of magmatism and tectonism during the early evolution of back‐arc basins. Using 32 days of ocean bottom seismometer data, we located 697 micro‐earthquakes at the southern Fonualei Rift and Spreading Center (S‐FRSC). The majority of epicenters concentrate along the central region of the axial valley, marking the active ridge axis. Only odd events were associated with the prominent faults bounding the axial valley. About 450 events are spatially clustered around 17°42′S and their waveforms show a pronounced similarity. Most of these events are associated with a 138 h lasting earthquake swarm. The tectonic structure of the ridge axis in the S‐FRSC resembles a series of left‐stepping en echelon segments, expressed at the seafloor by numerous volcanic ridges. The recorded earthquake swarm is located at the stepover of two en echelon segments suggesting that the earthquake swarm is mainly tectonically driven. The events directly beneath our seismic network indicate a maximum depth of brittle faulting down to about 14 km below the seafloor. This is within the maximum depth range of brittle faulting at ultraslow mid‐ocean ridges. Since the thickness of the brittle lithosphere is mainly controlled by temperature, our results suggest a sub‐axial thermal structure similar to that of ultraslow mid‐ocean ridges of similar opening rates. Key Points: Microseismicity is focused in the axial valley along the spreading axis and coincides with volcanic ridges and lava flowsAn earthquake swarm is located at the stepover of two en echelon segments. The swarm is mainly controlled by tectonismThe brittle axial lithosphere is about 14 km thick, which is in agreement with mid‐ocean ridges of similar spreading rates [ABSTRACT FROM AUTHOR]

The segmentation of mid‐ocean ridges is a defining characteristic of seafloor spreading, yet some tectonic processes operating at segment boundaries remain poorly understood. Here, we analyze new satellite‐derived vertical gravity gradient (VGG) data, which reveal an abundance of off‐axis seafloor features that are oblique to ridges and transform faults and thus reflect the occurrence of ridge propagation at some segment boundaries. However, unlike many propagating ridges, these features commonly reverse direction leaving W‐shaped signatures in the seafloor which we refer to as "seesaw propagators" (SSPs). Using the VGG, we have documented these globally and find that: (1) SSPs are ubiquitous on seafloor that formed at half spreading rates between 10 and 40 mm year−1 and their total length is about 1/3 that of fracture zones. (2) The lithospheric age offset across SSPs (0–2.5 Ma) is less than transform faults (2–10 Ma), which likely reflects a rheological threshold, whereby only young and weak lithosphere allows for "non‐rigid" SSP behavior. (3) Isostatic modeling of well‐surveyed SSPs confirms that they formed on young and thin (3 km) lithosphere. (4) The directional changes of SSPs in both time and space appear largely uncorrelated and cannot be explained by previous regional‐scale models invoked to explain unidirectional ridge propagation and thus require a different driving force. Key Points: The process of "seesaw" ridge propagation is ubiquitous on seafloor that forms at half spreading rates between 10 and 40 mm yr−1Globally, the total length of seesaw propagators is approximately one‐third that of fracture zonesDriving mechanisms previously proposed to explain unidirectional ridge propagation fail account for the behavior of seesaw propagators [ABSTRACT FROM AUTHOR]

The Charlie Gibbs offsetting by ~ 340 km the Mid Atlantic Ridge (MAR) axis at 52°-53° N is one of the main transform systems of the North Atlantic. Located between long mid-ocean ridge segments influenced from the south by the Azores and from the north by the Iceland mantle plumes, this transform system has been active since the early phases of North Atlantic rifting. Object of several surveys in the '70 and '80, Charlie Gibbs received great attention for its unique structure characterized by two long-lived right-lateral transform faults linked by a short ~ 40 km-long intra-transform spreading centre (ITR) with parallel fracture zone valleys extending continuously towards the continental margins. In October 2020 expedition S50 of the R/V A.N. Strakhov surveyed an area of 54,552 km2 covering the entire Charlie Gibbs transform system and the adjacent MAR spreading segments. We collected new bathymetric, magnetic and high-resolution single channel seismic data, along with basaltic, gabbroic and mantle rocks from 21 dredges. This work contains preliminary data from cruise S50 and discusses the large-scale architecture of this unique, long-lived transform system. [ABSTRACT FROM AUTHOR]

The Papua‐Woodlark region exemplifies many plate tectonic processes, from active continental breakup to double plunging subduction zones. We present a synthesis of geological and geophysical data from the region and interpret these data to characterize the present plate boundaries. A subducted slab shown to underlie the Trobriand forearc is associated with the Papuan volcanic arc and the diapiric emplacement of felsic gneissic domes in the Woodlark rift from previously subducted continental materials in the mantle wedge. In contrast, the Suckling‐Dayman metamorphic core complex formed in the footwall of the Mai'iu extensional detachment, augmented by calc‐alkaline and high‐K magmatic intrusion and uplift. Utilizing Global Positioning System relative velocities, transform fault azimuths, ridge axis azimuths, and two fault slip directions, we derive a plate kinematic model for the Australia, Woodlark, Trobriand, and Solomon Sea plates to describe the neotectonics and provide insight to the past tectonics. We present two cases—with and without subduction at the Trobriand Trough—and favor the former based on our characterization of the plate boundaries. Our model estimates current Trobriand subduction rates of 3–5 mm/yr with obliquity increasing westwards, and full spreading rates, increasing eastwards along the Woodlark Basin spreading center, of 19–35 mm/yr. Based on seafloor morphology, we further estimate the time of the change in the pole of the Woodlark Basin opening at 450 Ka followed by a decrease in spreading rate and complete reorientation of the spreading axis fabric at 225 Ka. Key Points: A four‐plate kinematic model is derived from a characterization of the plate boundaries in the Papua‐Woodlark regionThe Woodlark Basin spreading center recently reoriented (450 Ka) and spreading slowed significantly (225 Ka) to 19–35 mm/yrSubduction at the Trobriand Trough produced a patchily seismogenic slab dipping south to 50–125 km depth and a local tsunami in 1895 [ABSTRACT FROM AUTHOR]

The Macquarie Ridge Complex (MRC) on the Australia‐Pacific plate boundary south of New Zealand is an extinct mid‐ocean ridge that has experienced a complex tectonic history and produced highly heterogeneous mid‐ocean ridge basalts (MORBs). When and how seafloor spreading ceased along the proto‐Macquarie mid‐ocean ridge remain elusive, and it is unclear how the mantle source of MORBs is affected by the gradual cessation of seafloor spreading at mid‐ocean ridges. To constrain the tectonic evolution of the MRC, the mantle source variations for MORBs at dying mid‐ocean ridges, and the mechanisms of mantle enrichment and asthenospheric heterogeneities, we report 11 pyroxene, plagioclase, basaltic glass, groundmass, and sericite 40Ar/39Ar and one zircon U‐Pb ages for the MRC MORBs. Our data reveal that basalts from the MRC seamounts were erupted between 25.9 and 1.6 Ma and Macquarie Island at ∼10 Ma. Combined age and plate reconstruction results reveal that the cessation of seafloor spreading at the MRC generally propagated from south to north along the ridge. Basalts produced by the then dying Macquarie mid‐ocean ridge at different times on different seamounts/island show a large variation in isotopic compositions and there is no clear correlation between ages and isotopic ratios. The heterogeneity of mantle source for MORBs from the proto‐Macquarie mid‐ocean ridge suggests that the upper asthenospheric mantle is heterogeneous, and such heterogeneity becomes most obvious at dying mid‐ocean ridges where the degrees of partial melting are low and a large range of melt compositions are produced. Key Points: New ages of basalts from the Macquarie Ridge Complex (MRC) at the Australia‐Pacific plate boundary are reportedThe cessation of seafloor spreading was progressed from south to north along the Macquarie RidgeUpper mantle heterogeneity becomes obvious at dying mid‐ocean ridges where the melting degrees are low and various melt compositions are produced [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The present-day heat flow increases from the north to south of the northern South China Sea. • In the continental shelf, heat flow increased rapidly twice during the Paleogene syn-rift stage, and followed by continuous thermal subsidence with heat flowdecreased during the Neogene post-rift stage. • The heat flow in the slope continuously increased due to multi-episode seafloor spreading, and gradually reduced after the cessation of the sea floor spreading. • The thermal evolution of the Pearl River Mouth Basin is mainly controlled by tectonic activities. Abstract The Pearl River Mouth Basin (PRMB), the largest basin in the northern continental margin of the South China Sea (SCS), has been one of the key areas to characterize geothermal field and thermal evolution of a passive continental margin. The thermal history of the Pearl River Mouth Basin is related to the seafloor spreading of the SCS in the late Early Oligocene. Heat flow measurements show that the PRMB is characterized by a high background heat flow with average heat flow of 71.8 ± 13.6 mW/m2. The present-day heat flow of the northern SCS increases from the northern shelf with the thick crust to the southern slope with the thinned crust. This study employs forward and inverse modeling to simulate the rift and post-rift processes exampled by four wells at different structural settings. Two thermal evolution models of the PRMB are established for continental shelf and continental slope. In the continental shelf, heat flow increased rapidly twice during the syn-rift stage, and then followed by continuous thermal subsidence with heat flow value decreased during the post-rift stage. While the heat flow in the slope continuously increased due to lithospheric thinning, mantle upwelling and resulting multi-episode seafloor spreading in SCS during the Neogene post-rift stage, which is different from the previously proposed decreasing heat model for the slope during the post-rift. Heat flow gradually reduced after the cessation of the sea floor spreading (10 Ma). The local multi-staged magmatic activities mainly contribute to the high level of maturity in the Liwan Sag located in the lower slope. [ABSTRACT FROM AUTHOR]

Considerable post-breakup extensional deformation is recorded in the continental margins of the South China Sea (SCS). To recognize the nature and origin of the significant deformation during the syn-spreading stage (32-15.5 Ma) in the SCS, we comprehensively analyzed the geometry and kinematics of the faults and contemporaneous magmas in the Baiyun sag, northern margin of the SCS, using high-resolution regional three-dimensional seismic data. The kinematic analyses indicate that the faults in the Baiyun sag are recently formed following the onset of seafloor spreading in the SCS. The faults exhibit multiple episodes of growth history, with three active episodes, 32-29, 23.8-21 and 18.5-16.5 Ma, separated by periods of inactivity. Four volcanic groups comprising 98 volcanic mounds have been identified and described, located separately in the northwestern, the central, the southeastern and the northern slope areas. The occurrence of multiple palaeo-seafloors, complemented by the biostratigraphic and K-Ar dating data, reveals multiple extrusive events of the syn-spreading magmas in the Baiyun sag, with three active periods of 23.8-21, 18.5-17.5 and 17.5-16.5 Ma. This study confirms that the normal faulting has a shared genetic origin with the contemporaneous magmatism during the syn-spreading stage in the deep-offshore Baiyun sag, northern margin of the SCS. The episodic fault growth and magmatic extrusive events reveal that the Baiyun sag has undergone at least three episodic tectonic events during the syn-spreading stage, which evolved in response to the multi-stage seafloor spreading of the SCS. [ABSTRACT FROM AUTHOR]

Abstract The Izu–Bonin–Mariana (IBM) forearc contains a record of a widespread and short-lived subduction initiation event in the early Eocene. We present new high-precision ages to determine the rates and length scales of ocean crust production following subduction initiation along the IBM convergent margin. We also explore the implications of these ages for potential causes of subduction initiation and its far-field effects. Zircons extracted from gabbros collected by Shinkai 6500 diving within the ophiolitic sequence exposed in the outer Bonin and Mariana forearcs give crystallization ages (±2 σ) of 51.94 ± 0.13 Ma and 51.81 ± 0.03 Ma, respectively, consistent with a near-synchronous seafloor spreading after initiation of subduction along the entire length of the IBM system. 40Ar/39Ar ages of forearc basalts (51.34 ± 0.78 Ma) and boninites (51.27 ± 0.09 Ma to 50.33 ± 0.55 Ma) cored upslope of the Bonin sample gabbro during IODP Expedition 352 further reveal that the outer forearc comprises oceanic lithosphere with MORB to boninite compositions created by seafloor spreading during a remarkably short time period of 0.6–2 Myr at a full-rate of c. 70 mm/yr. This time period is consistent with those of some well-dated ophiolites. Thus, near-trench seafloor spreading to form basalt then boninite lithosphere is an extensive, rapid, but short-lived process after subduction initiation. Highlights • Subduction initiation was nearly simultaneous at 51.8–51.9 Ma in the Bonin and Mariana arcs. • Subduction initiation in the western Pacific was part of a regional reconfiguration of plate motions. • The change from decompression melting to flux melting after subduction began took <1.2 Myr. • Near trench sea-floor spreading ceased within ∼2 Myr of subduction initiation. [ABSTRACT FROM AUTHOR]

The kinematics of rift segment linkage in magmatic rifts remain debated. Strain patterns from Afar provide tests of current models of how segmented rifts grow in areas of incipient oceanic spreading. Here, we present a combined analysis of seismicity, interferometric synthetic aperture radar (InSAR), and GPS-derived strain rate maps to reveal that the plate-boundary linkage between the Red Sea and Gulf of Aden rifts of Afar is accommodated primarily by distributed extensional faulting. Large rotations about vertical axes predicted by bookshelf faulting models are not detected. Additionally, models of stress changes and seismicity induced by recent dikes provide poor fits to the observed time-space patterns of strike-slip earthquakes. Instead, we explain these features as resulting from rift-perpendicular shearing at the tips of spreading rifts where extension terminates against less stretched lithosphere. Our results demonstrate that distributed extension drives rift-perpendicular shearing, achieving plate-boundary linkage during incipient seafloor spreading. [ABSTRACT FROM AUTHOR]

The deep structure and sedimentary record of rift basins provide an important insight into understanding the geological processes involved in lithospheric extension. We investigate the crustal structure and large‐scale sedimentary architecture of the southern Porcupine Basin, offshore Ireland along three wide‐angle seismic profiles, supplemented by 13 selected seismic reflection profiles. The seismic velocity and crustal geometry models obtained by joint refraction and reflection traveltime inversion clearly image the deep structure of the basin. Our results suggest the presence of three distinct crustal domains along the rifting axis: (a) continental crust becoming progressively hyperextended from north to south through the basin, (b) a transitional zone of uncertain nature, and (c) a 7‐ to 8‐km‐thick zone of oceanic crust. The latter is overlain by an ~8‐km compacted Upper Paleozoic‐Mesozoic succession and ~ 2 km of Cenozoic strata. Due to the lack of clear magnetic anomalies and in the absence of well control, the precise age of interpreted oceanic crust is unknown. However, we can determine an age range of Late Jurassic to Late Cretaceous from the regional context. We propose a northward propagating rifting process in the Porcupine Basin, resulting in variations in strain along the rift axis. Key Points: New analysis of wide‐angle seismic data from the southern Porcupine BasinEvidence for presence of oceanic crust in the southern Porcupine Basin.Jurassic rifting propagated from south to north, resulting in nonuniform strain when rifting stopped. [ABSTRACT FROM AUTHOR]

The 4 May 2018, MW 6.9 thrust‐faulting earthquake occurred a day after the first fissure eruption in Leilani Estates along the East Rift Zone of Kilauea, Hawaii, relatively early in the 2018 eruptive sequence. The earthquake's location and geodetic deformation pattern are similar to those of the larger 1975 Kalapana earthquake (MW 7.7), which is thought to have occurred on the décollement between the island volcanics and the former Pacific seafloor. The 2018 event has a shallow‐dipping thrust plane and shallow epicenter located just offshore. A finite‐fault kinematic slip model is determined by inversion of teleseismic body waves, strong‐ground motion recordings, and coseismic Global Positioning System offsets along the southeastern coast of Hawaii. Inversions indicate a preferred dip of 7°, slip of up to 3.0 m near 5‐km deep offshore of the coast, and low average rupture speed of ~1 km/s. These are consistent with rupture of a weak décollement. Plain Language Summary: Early in the 2018 eruption sequence at Kilauea volcano in Hawaii, a magnitude 6.9 earthquake occurred just off the coast along the Leilani Estates which had begun to experience fissure eruptions a day earlier. The earthquake was well recorded by global seismic stations and by strong motion instruments on Hawaii, and Global Positioning System offsets during the earthquake were determined along the coastal area. These data are used to determine a space‐time history of the earthquake rupture. The slip is concentrated in a band offshore parallel to the coast, and the fault that ruptured is likely to be the boundary between the volcanic pile of the island and the former Pacific Ocean seafloor. As much as 3 m of slip occurred, with the rupture having behavior similar to slow rupturing tsunami earthquakes in subduction zones, indicating a relatively weak fault. Key Points: Teleseismic, strong‐motion, and GPS displacement data are inverted to determine the spatiotemporal earthquake slip distributionPeak slip is ~3.0 m, average rise time is 4.7 s, rupture speed is 0.7 to 1.6 km/s, and duration is 30 s, indicating low fault strengthThe earthquake likely ruptured the offshore shallow‐dipping décollement between the island volcanics and the former Pacific seafloor [ABSTRACT FROM AUTHOR]

Abstract—: The paper considers geological and geophysical data on the crustal structure of the volcanic Kerguelen Plateau and nearby Indian Ocean, as well as evolution of the Kerguelen Plateau during the Gondwana breakup. It is assumed that three isolated continental blocks (microcontinents) evolved within the Kerguelen Plateau. In order to provide insight into the mechanisms for the microcontinent formation, physical modeling under three different conditions has been carried out. The first experiment simulates a homogeneous lithosphere with two rift fractures propagating toward each one other; in the second experiment, the propagating rift fractures collide with a structural “barrier” having a stronger lithosphere; the third experiment simulated the effect of a local heat source (a hot spot). Based on these experiments and taking into account the available structural and geophysical data, it is suggested that isolation of continental block in the southern Kerguelen Plateau could have begun due to counterpropagation of rifting branches: oceanic from east to west and continental from west to east. The isolation of the southern block with attachment to the Antarctic Plate took place 120 Ma ago after extinction of the spreading ridge in the Princess Elizabeth Trough. Because of the structural inhomogeneity in the prebreakup lithosphere, which is represented by a zone of intracontinental Lambert-Mahanadi rifts, opening of the ocean could have developed in a more complex regime. Under the effect of a hot spot, a jump of the spreading axis occurred at the moment of its emplacement into the lithosphere. [ABSTRACT FROM AUTHOR]

We show that oblique seafloor spreading occurs in several regions where obliquity, α , was not recognized before. These include the slow spreading centers of the Red Sea ( α ≈ 20 ° ), intermediate spreading centers of the Cocos–Nazca plate boundary between 91°W and 94°W ( α ≈ 9 ° ), and superfast spreading centers of the East Pacific Rise at the Nazca–Pacific plate boundary between 29°S and 32°S ( α ≈ 10 ° ) and perhaps between ≈16°S and ≈22°S ( α ≈ 4 ° ). Thus, oblique spreading occurs across slow, intermediate, and superfast spreading centers, but not across fast spreading centers. Across intermediate and superfast spreading centers, oblique spreading does not occur in staircase-mode plate geometries, but does occur in oblique-mode plate geometries. Across slow and intermediate spreading centers, obliquity tends to decrease with increasing spreading rate, while across fast and superfast spreading centers it tends to increase with increasing spreading rate. Oblique spreading at intermediate and superfast spreading centers may be related to magma overpressure or to unusual directions of remote tectonic stress or to ongoing plate boundary reorganizations or to some combination of these. Using prior methods, we estimate the ratio of overpressure to remote tectonic stress for several spreading centers. In particular we show along a segment of the Cocos–Pacific plate boundary that magma overpressure is only one-fourth as large as remote tectonic stress, consistent with a prior inference from other observations. The highest obliquity occurs along ridge segments lying 200 km to 1500 km from a mantle plume, but not all ridge segments near plumes spread obliquely. For one set of estimates of plume fluxes, the rate of plume flux delivered to ridges correlates positively and significantly with spreading rate. [ABSTRACT FROM AUTHOR]

Microcontinents occur outboard of passive margins and stranded in ocean basins. Threedimensional analogue laboratory experiments of continental rifting demonstrate that microcontinent formation at passive margins requires a combination of preexisting linear weaknesses in the lithosphere and rotational extension. Our results suggest that separation of microcontinents from passive margins occurs during the latest stages of continental breakup, before the onset of seafloor spreading, and that preexisting lithospheric weaknesses are a first-order control on where they form. These findings suggest that microcontinent formation may be restricted to localized regions along passive margins associated with zones of lithospheric weakness, providing a new structural and tectonic framework for the interpretation of microcontinents in the geological record. [ABSTRACT FROM AUTHOR]

The article focuses on a study related to anomalous K-Pg–aged seafloor attributed to impact-induced mid-ocean ridge magmatism. Topics discussed include evidence for the triggering of magmatism on a global scale by the Chicxulub meteorite impact at the Cretaceous-Paleogene (K-Pg) boundary; concentrated positive free-air gravity and coincident seafloor topographic anomalies; and seafloor creation at fast-spreading rates.

Despite many of years of mapping effort, only a small fraction of the world ocean's seafloor has been sampled for depth, greatly limiting our ability to explore and understand critical ocean and seafloor processes. Recognizing this poor state of our knowledge of ocean depths and the critical role such knowledge plays in understanding and maintaining our planet, GEBCO and the Nippon Foundation have joined forces to establish the Nippon Foundation GEBCO Seabed 2030 Project, an international effort with the objective of facilitating the complete mapping of the world ocean by 2030. The Seabed 2030 Project will establish globally distributed regional data assembly and coordination centers (RDACCs) that will identify existing data from their assigned regions that are not currently in publicly available databases and seek to make these data available. They will develop protocols for data collection (including resolution goals) and common software and other tools to assemble and attribute appropriate metadata as they assimilate regional grids using standardized techniques. A Global Data Assembly and Coordination Center (GDACC) will integrate the regional grids into a global grid and distribute to users world-wide. The GDACC will also act as the central focal point for the coordination of common data standards and processing tools as well as the outreach coordinator for Seabed 2030 efforts. The GDACC and RDACCs will collaborate with existing data centers and bathymetric compilation efforts. Finally, the Nippon Foundation GEBCO Seabed 2030 Project will encourage and help coordinate and track new survey efforts and facilitate the development of new and innovative technologies that can increase the efficiency of seafloor mapping and thus make the ambitious goals of Seabed 2030 more likely to be achieved. [ABSTRACT FROM AUTHOR]

Glacial geomorphology is used in Antarctica to reconstruct ice advance during the Last Glacial Maximum and subsequent retreat across the continental shelf. Analogous geomorphic assemblages are found in glaciated fjords and are used to interpret the glacial history and glacial dynamics in those areas. In addition, understanding the distribution of submarine landforms in bays and the local controls exerted on ice flow can help improve numerical models by providing constraints through these drainage areas. We present multibeam swath bathymetry from several bays in the South Shetland Islands and the western Antarctic Peninsula. The submarine landforms are described and interpreted in detail. A schematic model was developed showing the features found in the bays: from glacial lineations and moraines in the inner bay to grounding zone wedges and drumlinoid features in the middle bay and streamlined features and meltwater channels in the outer bay areas. In addition, we analysed local variables in the bays and observed the following: (1) the number of landforms found in the bays scales to the size of the bay, but the geometry of the bays dictates the types of features that form; specifically, we observe a correlation between the bay width and the number of transverse features present in the bays. (2) The smaller seafloor features are present only in the smaller glacial systems, indicating that short-lived atmospheric and oceanographic fluctuations, responsible for the formation of these landforms, are only recorded in these smaller systems. (3) Meltwater channels are abundant on the seafloor, but some are subglacial, carved in bedrock, and some are modern erosional features, carved on soft sediment. Lastly, based on geomorphological evidence, we propose the features found in some of the proximal bay areas were formed during a recent glacial advance, likely the Little Ice Age. [ABSTRACT FROM AUTHOR]

We present geochemical data from the first samples of spring fluids from Dorado Outcrop, a basaltic edifice on 23 M.y. old seafloor of the Cocos Plate, eastern Pacific Ocean. These samples were collected from the discharge of a cool hydrothermal system (CHS) on a ridge flank, where typical reaction temperatures in the volcanic crust are low (2–20 °C) and fluid residence times are short. Ridge-flank hydrothermal systems extract 25% of Earth's lithospheric heat, with a global discharge rate equivalent to that of Earth's river discharge to the ocean; CHSs comprise a significant fraction of this global flow. Upper crustal temperatures around Dorado Outcrop are ∼15 °C, the calculated residence time is <3 y, and the composition of discharging fluids is only slightly altered from bottom seawater. Many of the major ions concentrations in spring fluids are indistinguishable from those of bottom seawater; however, concentrations of Rb, Mo, V, U, Mg, phosphate, Si and Li are different. Applying these observed differences to calculated global CHS fluxes results in chemical fluxes for these ions that are ≥15% of riverine fluxes. Fluxes of K and B also may be significant, but better analytical resolution is required to confirm this result. Spring fluids also have ∼50% less dissolved oxygen (DO) than bottom seawater. Calculations of an analytical model suggest that the loss of DO occurs primarily (>80%) within the upper basaltic crust by biotic and/or abiotic consumption. This calculation demonstrates that permeable pathways within the upper crust can support oxic water–rock interactions for millions of years. [ABSTRACT FROM AUTHOR]

In this study we investigate the origin and alteration history of serpentinites in two dredge hauls from the North Wall of the Puerto Rico Trench, where the North American plate subducts beneath the Greater Antilles island arc. Faulting of the North American plate at the North Wall of the Puerto Rico Trench exposes oceanic basement in a nearly spreading-parallel direction, offering a unique perspective on crustal accretion and alteration processes in the Cretaceous Quiet Zone. U-Pb age dating of porous zircon in an altered mafic vein in serpentinite from 20-00'00"N, 66-31'57"W yields an age of 114-8 Ma, suggesting that it originated at the Cretaceous Mid-Atlantic Ridge, c. 2200 km east of its current location. After localized high-temperature (>380-C) hydrothermal alteration to chlorite, antigorite, talc and tremolite, peridotites underwent pervasive serpentinization at lower temperatures (~310-240°C). In addition to lizardite-rich serpentinite, which is prevalent in dredge D10, dredge D2 contains serpentinite dominated by antigorite. Pervasive serpentinization was followed by extensive talc alteration of several antigorite-rich serpentinites from dredge D2, which was accompanied by Fe loss. As the newly formed crust moved off-axis, open-system alteration at low temperatures by seawater (i.e. weathering) modified the mineralogy, chemical composition, and physical properties of the serpentinite. Where present, brucite dissolved and magnetite oxidized to hematite and goethite, which caused a decrease in magnetic susceptibility and an increase in porosity. Weathering led to complete oxidation of several lizardite-serpentinites in dredge D10, whereas talc-altered antigorite serpentinites in dredge D2 were less affected by oxidation. Rare earth element concentrations of lizardite-rich serpentinite in dredge D10 decreased steadily with increasing Fe(III)/Fetot, reflecting simultaneous leaching and oxidation by seawater. These results have implications for our understanding of several processes, including hydrothermal alteration of peridotite at the Cretaceous Mid-Atlantic Ridge, mass transfer and redox reactions between serpentinite and seawater during off-axis weathering, and the introduction of volatiles, oxidizing agents, and antigorite beneath the Greater Antilles Arc. [ABSTRACT FROM AUTHOR]

The evolution of seafloor spreading of Africa, South America, and Antarctica is key to understanding the initial break-up of Gondwana. Vector geomagnetic surveys were conducted in the Natal Valley and Mozambique Ridge, off South Africa. We summarize the nature of the crust using the results of dense vector geomagnetic anomaly data, as well as satellite gravity data. Based on both inversion and forward analytical results, we identified areas of stretched continental crust, with basaltic magma intrusion in parts, as the northern Natal Valley, north part of the Mozambique Ridge, and north part of the southern Natal Valley. Oceanic crust was identified in the south part of the southern Natal Valley and south part of the Mozambique Ridge. Magnetic isochrons M0–M10 were identified in the south part of the southern Natal Valley. Clear magnetic lineations were observed in the south part of the Mozambique Ridge, where some areas were distorted by hotspot volcanism. The location of the continental ocean boundary in the Natal Valley, along with a four-stage model of tectonic evolution of the study area since about 183 Ma, are newly proposed. [ABSTRACT FROM AUTHOR]

Broad areas of subcontinental lithospheric mantle are commonly exposed along ocean-continent transition zones in magma-poor rifts and are thought to be exhumed along lithospheric-scale detachment faults during the final stages of rifting. However, the nature of the transition from final rifting to seafloor spreading is controversial. We present the first high-precision U-Pb zircon geochronologic and Hf isotopic data from gabbros that intrude exhumed mantle at Ocean Drilling Program (ODP) Sites 1070 and 1277 in the Newfoundland-Iberia rift (North Atlantic). The sites are conjugate to one another within crust that is commonly considered to have been emplaced during early seafloor spreading. Magnetic data suggest that crustal accretion occurred at both sites during magnetic polarity chrons M3-M0 (130-126 Ma). However, our data indicate that asthenospheric melts were emplaced over brief intervals (=1 m.y.) prior to or coeval with mantle exhumation at 124 Ma at ODP Site 1070 and 115 Ma at ODP Site 1277. We suggest that this discrepancy is the result of continued mantle exhumation along large, west-dipping detachment faults until lithospheric breakup. The breakup location is likely coincident with the large-amplitude magnetic J anomaly, and our 115 Ma date for magmatism within this anomaly provides the best available age constraint for breakup along the studied transect. [ABSTRACT FROM AUTHOR]

Remote sensing systems are critical tools used for characterizing the geological and ecological composition of the seafloor. However, creating comprehensive and detailed maps of ocean and coastal environments has been hindered by the high cost of operating ship- and aircraft-based sensors. While a number of groups (e.g., academic research, government resource management, and private sector) are engaged in or would benefit from the collection of additional seafloor mapping data, disparate priorities, dauntingly large data gaps, and insufficient funding have confounded strategic planning efforts. In this study, we addressed these challenges by implementing a quantitative, spatial process to facilitate prioritizing seafloor mapping needs in Washington State. The Washington State Prioritization Tool (WASP), a custom web-based mapping tool, was developed to solicit and analyze mapping priorities from each participating group. The process resulted in the identification of several discrete, high priority mapping hotspots. As a result, several of the areas have been or will be subsequently mapped. Furthermore, information captured during the process about the intended application of the mapping data was paramount for identifying the optimum remote sensing sensors and acquisition parameters to use during subsequent mapping surveys. [ABSTRACT FROM AUTHOR]

It has recently been demonstrated that hydrothermal vents are an important source of dissolved Fe (dFe) to the Southern Ocean. The isotopic composition (δ56Fe) of dFe in vent fluids appears to be distinct from other sources of dFe to the deep ocean, but the evolution of δ56Fe during mixing between vent fluids and seawater is poorly constrained. Here we present the evolution of δ56Fe for dFe in hydrothermal fluids and dispersing plumes from two sites in the East Scotia Sea. We show that δ56Fe values in the buoyant plume are distinctly lower (as low as -1.19‰) than the hydrothermal fluids (-0.29‰), attributed to (1) precipitation of Fe sulfides in the early stages of mixing, and (2) partial oxidation of Fe(II) to Fe(III), >55% of which subsequently precipitates as Fe oxyhydroxides. By contrast, the δ56Fe signature of stabilized dFe in the neutrally buoyant plume is -0.3‰ to -0.5‰. This cannot be explained by continued dilution of the buoyant plume with background seawater; rather, we suggest that isotope fractionation of dFe occurs during plume dilution due to Fe ligand complexation and exchange with labile particulate Fe. The δ56Fe signature of stabilized hydrothermal dFe in the East Scotia Sea is distinct from background seawater and may be used to quantify the hydrothermal dFe input to the ocean interior. [ABSTRACT FROM AUTHOR]

We present a joint inversion method for the transmitter navigation and the seafloor resistivity for frequency domain marine controlled-source electromagnetic (CSEM) data. The inversion approach is based on the modified BFGS scheme, which has an advantage that one can update the Hessian matrix by using the BFGS scheme rather than computing the Hessian matrix itself during the inversion process. The partial derivatives of the electromagnetic field responses with respect to both the seafloor resistivity and the transmitter navigation parameters including the azimuth, dip and horizontal positions of the transmitter antenna are analytically calculated. We invert for both the navigation parameters of the towed dipole source (including antenna azimuth, dip, and horizontal positions) and seafloor resistivity by using the whole range of data instead of the near-field data (usually source-receiver offset <1 km). An eigenparameter analysis shows that seafloor resistivities and transmitter navigation parameters can be independently resolved, and a better reconstruction can be obtained with multiple frequency data. The inversions of both the synthetical and field data sets indicate that our inversion method can simultaneously reconstruct seafloor resistivity structures and transmitter navigation parameters. [ABSTRACT FROM AUTHOR]

We use recently published, high-resolution reconstructions of the Southwest Indian Ridge to test whether a previously described systematic difference between Global Positioning System (GPS) and 3.16-Myr-average estimates of seafloor spreading rates between Antarctica and Africa is evidence for a recent slowdown in Southwest Indian Ridge seafloor spreading rates. Along the Nubia-Antarctic segment of the ridge, seafloor opening rates that are estimated with the new, high-resolution reconstructions and corrected for outward displacement agree well with geodetic rate estimates and reduce previously reported, highly significant non-closure of the Nubia-Antarctic-Sur plate circuit. The observations are inconsistent with a slowdown in spreading rates and instead indicate that Nubia-Antarctic plate motion has been steady since at least 5.2 Ma. Lwandle-Antarctic seafloor spreading rates that are estimated from the new highresolution reconstructions differ insignificantly from a GPS estimate, thereby implying steady Lwandle-Antarctic plate motion since 5.2 Ma. Between the Somalia and Antarctic plates, the new Southwest Indian Ridge reconstructions eliminate roughly half of the systematic difference between the GPS and MORVEL spreading rate estimates. We interpret the available observations as evidence that Somalia-Antarctic spreading rates have been steady since at least 5.2 Ma and postulate that the remaining difference is attributable to random and/or systematic errors in the plate kinematic estimates and the combined effects of insufficient geodetic sampling of undeforming areas of the Somalia plate, glacial isostatic adjustment in Antarctica and transient deformation triggered by the 1998 Mw = 8.2 Antarctic earthquake, the 2004 Mw = 9.3 Sumatra earthquake, or possibly other large historic earthquakes. [ABSTRACT FROM AUTHOR]

Terraces on the modern seafloor are defined as topographically flat areas above the active submarine channel thalweg but within the confines of the channel-belt. They have been described from many modern submarine channels, but the controls on terrace distribution, evolution and stacking patterns are not well understood. In this study, we describe the architecture of the Mahin and Avon channel-belts and their associated terraces, located offshore Nigeria towards the northwest of the Niger Delta. The studied channel sections are < 10 km apart up-dip and converge downslope. They are on slopes with similar gradients, yet they have significantly different morphologies indicating that the interplay between sedimentary processes and channel evolution must be different. The surfaces defining the bases of the terrace bodies have been mapped along both channels using high-resolution 3D seismic data. Spectral decomposition of the data reveals subtle variations in seismic character that highlight sedimentological detail that can otherwise not be recognized, allowing us to suggest the processes responsible for terrace formation and terrace body composition. The contrasting evolution of the two channels is reflected in the morphology and architecture of their terraces. While the Mahin terrace bodies show a predictable pattern, typically consisting of stacked channel-fill and overbank deposits in a circular planform shape (shape controlled by cut-off channel bends), the deposits of the Avon terrace bodies up-dip of the confluence with the Mahin are dominated by overbank deposits, with the planform terrace shape heavily controlled by the topography of the underlying channel-belt deposits. This study shows how spatially and temporally associated channels can have markedly different architectures. The evolution of the channel and the abundance and stacking patterns of compositional elements within terrace bodies are shown to be linked. [ABSTRACT FROM AUTHOR]

The beginning of seafloor spreading in the South China Sea (SCS) is now established from IODP drilling Leg 349 at 33 Ma. Chron 12 (32 Ma) is the oldest chron identified in the SCS. The nature of the crust of the northeastern part of the SCS located north of chron C12, where chrons 15 to 17 were previously identified, is not oceanic but thinned continental crust intruded by volcanic elongated features emplaced 17–22 Ma ago. Based on magnetic anomaly identifications, the end of the SCS spreading could be either 15.5, 20.5 Ma (Briais et al., 1993; Barckhausen et al., 2014) or something else. However, as post-spreading magmatic activity (~ 13–3.5 Ma) largely masks the spreading fabric in particular near the axis of the east sub-basin, published locations of the axial magnetic anomaly (extinct spreading axis) and spreading rates are not reliable. A contoured map of the extremely dense set of magnetic data shows that a few magnetic lineations belonging to the magnetic seafloor spreading fabric are still preserved and parallel to the N055° bathymetric seafloor spreading trends identified on swath-bathymetric maps in the central part of the SCS, suggesting that the extinct ridge axis is N055° trending with potential N145° transform faults. Based on published swath-bathymetric data, oceanic domains with different seafloor spreading lineaments have been delimited (N055°, N075° and N085°) and provide important constraints used to propose a kinematic sketch of the SCS opening. As a consequence, the Zhongnan faults zone, located between the east and southeast sub-basins, acted as a major fracture zone system during the SCS opening, with horizontal offsets varying from ~ 50 km to ~ 140 km. The flow-line pattern defined from the seafloor spreading lineaments and the few identified FZs have been used to highlight conjugate segments of continental margins. During the first phases of opening of the SCS, from the fit of continents to chron C10 (30 Ma), the N175° extension observed in the Xisha trough and eventually south of the Macclesfield Bank extends to the Qui Nhon ridge, located along the eastern margin of Vietnam, in the southward prolongation of the Red River fault system. Normal faults curve toward the south with a horsetail geometry interpreted as evidence for a few tens of kilometers of dextral motion along the Qui Nhon ridge. Since chron 10 and until the end of SCS opening, the plate boundary located between the southern South China Sea (SSCS) and EU plates jumped westward several times from the location of the Ulugan fault near Palawan to the western limit of the southwest basin, explaining the progressive formation of the SCS from east to west and giving the characteristic V-shape of the SCS. The opening of the whole SCS is linked and occurred simultaneously with the northward subduction of the proto-SCS whose suture is located south of Palawan and extends westwards in north Borneo. [ABSTRACT FROM AUTHOR]

Systematic differences in mineral composition of mantle peridotites are observed in ophiolites and peridotitic bodies from the Alpine Tethys, the Pyrenean domain, the Dinarides and Hellenides, and the Iberia-Newfoundland rifted margins. These differences can be understood in the context of the evolution of rifted margins and allow the identification of 3 major mantle domains: an inherited domain, a refertilized domain and a depleted domain. Most clinopyroxene from the inherited domain equilibrated in the spinel peridotite field and are too enriched in Na 2 O and Al 2 O 3 to be residues of syn-rift melting. Clinopyroxene from the refertilized domain partially equilibrated with plagioclase and display lower Na 2 O and Al 2 O 3 , and elevated Cr 2 O 3 contents. The refertilized domain is a hybrid zone, which locally preserves remnants from the inherited domain and overlapping chemical compositions. Depleted domains with clinopyroxene similar to abyssal peridotites are rare and Nd-isotopic studies indicate that they represent ancient periods of melting unrelated to the opening of the Jurassic and Cretaceous oceanic basins of the Alpine Tethys and southern North Atlantic. In many studied sections of mantle rocks in exposed ophiolites a systematic spatial distribution of the different domains with respect to the evolution of rifted margins can be identified. This new approach integrates observations from exposed and drilled mantle rocks and proposes that the mantle lithosphere evolved and was modified during an extensional cycle from post-orogenic collapse through several periods of rifting to seafloor spreading. The defined chemical and petrological characteristics of mantle domains based on clinopyroxene and spinel compositions are compiled on present-day and paleogeographic maps of Western and Central Europe. These maps show that the observed distribution of mantle domains are linked to processes related to late post-Variscan extension, rift evolution and refertilization associated to crustal/lithospheric extension, and the development of embryonic oceanic domains. [ABSTRACT FROM AUTHOR]

Seismic velocities determined from 70 sonobuoys widely distributed in Canada Basin were used to discriminate crustal types. Velocities of oceanic layer 3 (6.7–7.1 km/s), transitional (7.2–7.6 km/s) and continental crust (5.5–6.6 km/s) were used to distinguish crustal types. Potential field data supports the distribution of oceanic crust as a polygon with maximum dimensions of ~ 340 km (east–west) by ~ 590 km (north–south) and identification of the ocean–continent boundary (OCB). Paired magnetic anomalies are associated only with crust that has oceanic velocities. Furthermore, the interpreted top of oceanic crust on seismic reflection profiles is more irregular and sometimes shallower than adjacent transitional crust. The northern segment of the narrow Canada Basin Gravity Low (CBGL), often interpreted as a spreading center, bisects this zone of oceanic crust and coincides with the location of a prominent valley in seismic reflection profiles. Data coverage near the southern segment of CBGL is sparse. Velocities typical of transitional crust are determined east of it. Extension in this region, close to the inferred pole of rotation, may have been amagmatic. Offshore Alaska is a wide zone of thinned continental crust up to 300 km across. Published longer offset refraction experiments in the Basin confirm the depth to Moho and the lack of oceanic layer 3 velocities. Further north, toward Alpha Ridge and along Northwind Ridge, transitional crust is interpreted to be underplated or intruded by magmatism related to the emplacement of the High Arctic Large Igneous Province (HALIP). Although a rotational plate tectonic model is consistent with the extent of the conjugate magnetic anomalies that occupy only a portion of Canada Basin, it does not explain the asymmetrical configuration of the oceanic crust in the deep water portion of Canada Basin, and the unequal distribution of transitional and continental crust around the basin. [ABSTRACT FROM AUTHOR]

Near-orthogonal ultra-slow (13.3 mm yr − 1 to 6.5 mm yr − 1 ) sea floor spreading in the absence of large transform faults make the Arctic Gakkel Ridge ideally suited for the study of magmatic processes. To enable this, we generated a three-dimensional gravity model of crustal thickness over the ridge and parts of the adjacent Nansen and Amundsen basins west of 65° E. The model shows that oceanic crust accreted prior to chrons C5/C6 is generally very thin (1–3 km). Magnetic anomalies over this thin crust are highly variable both parallel and perpendicular to the ridge axis. This is the result of amagmatic or weakly volcanic spreading that started with the opening of the basins 56 Ma ago. The separation of Greenland from Svalbard at chron C5/C6 led to the inflow of North Atlantic mantle into the western Eurasia Basin leading to a change in the mantle convection system and the establishment of a magmatic dichotomy along the Gakkel Ridge. Robust magmatism was established in the Western Volcanic Zone (6° 30′ W–3° 30′ E), leading to creation of a 6.6 km thick igneous crust, characterized by a strong positive axial magnetic anomaly, numerous volcanic cones, and widespread thick mid-ocean ridge basalts. The transition to the neighbouring Sparsely Magmatic (3° 30′ E–29° E) and Eastern Volcanic (29° E–85° E) zones is sharp. Peridotites cover the central valley and the inner rift flanks, the central magnetic anomaly vanishes and crustal thickness decreases to 1–4 km. Transverse basement ridges, extending for as much as 100 km into the adjacent basins, intersect the central valley. Although partly of tectonic origin, the transverse ridges are also an expression of long-living magmatic centres, as revealed by increased magnetic anomaly intensities and local thickening of the crust to values as great as 5.9 km. [ABSTRACT FROM AUTHOR]

The Tinjar-West Baram Line is a NW-trending trans-lithospheric fault in northern Kalimantan; its northwestern extension into the South China Sea (SCS) is the West Baram Line. In this paper, we propose that the geodynamic processes of the proto-South China Sea (PSCS) played a key role in the formation and evolution of the Tinjar-West Baram Line, based on previous studies of the strata, crustal thicknesses, gravity anomalies and other characteristics of the blocks adjacent to the Tinjar-West Baram Line and the palaeomagnetic-based plate reconstructions of the PSCS. The Tinjar-West Baram Line has a close link to the subduction of the PSCS. Structural restoration reveals that (1) before 35 Ma, the Tinjar-West Baram Line was a NE-trending transform fault, which is consistent with the NE-trending strike-slip faults widely distributed in the East Asian Continental Margin, in the PSCS. (2) From the perspective of tectonic evolution, the extinction of the PSCS and the spreading of the SCS drove the Luconia Block in the northern SCS to accrete to the western side of the Tinjar-West Baram Line. This process resulted in a contrast of crustal rocks adjacent to the Tinjar-West Baram Line; to the east of this line, the Nansha Trough is oceanic crust, whereas to the west of this line the Luconia Block has an affinity with continental crust. The velocity and thickness of the crust show great differences on either side of the Tinjar-West Baram Line. (3) The kinematic analysis of the Tinjar-West Baram Line reveals that the collisional orogeny between the Luconia and Kalimantan blocks happened at 45-37 Ma on the west of the Tinjar-West Baram Line; on its eastern side, the Nansha Trough was subducting south towards Kalimantan Island. During the interval 35-10 Ma, the kinematics of the Tinjar-West Baram Line shows a feature of dextral strike-slip faulting. (4) The earlier published palaeomagnetic data show that the Kalimantan Block experienced a counterclockwise rotation of about 50° during the period from 25 to 10 Ma. In addition, the exert counterclockwise rotation of about 20° of the Tinjar-West Baram Line happened due to the resistance of the Luconia Block. Therefore, the Tinjar-West Baram Line changed from an early NE-trending to late NW-trending structure which results in its present day tectonic framework. Thus, we suggest that the Tinjar-West Baram Line was originally a NE-trending transform fault of the PSCS extending to the continental crust. Subsequently, the Tinjar-West Baram Line became the western border of the PSCS, accompanying the collisional orogeny between the Luconia and Kalimantan blocks. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The probable N055°-oriented extinct spreading ridge of the East Sub-basin is cut across by the post-spreading volcanic ridge oriented approximately E-W in its western part and called the Zhenbei-Huangyan seamounts chain. A three-dimensional ocean bottom seismometer survey was conducted in 2011 and covered both the extinct spreading ridge and the Zhenbei-Huangyan seamounts chain in the East Sub-basin. The comprehensive seismic record sections of 39 ocean bottom seismometers underline the high-quality data with clear and reliable P-wave seismic first arrivals. The three-dimensional tomographic results show that the crust is oceanic on each side of the Zhenbei-Huangyan seamounts chain. The seafloor spreading lineaments and associated fracture zones pattern provide channels for magma migration and eruption. Based on the ratio of volcanic extrusive and intrusive materials and the shape of the upward convex 6.4 km/s iso-velocity curve beneath the Zhenbei and Huangyan seamounts, we conclude that the Zhenbei and Huangyan seamounts appear to be dominantly formed by intra-crustal intrusion. Combining previous geochemical studies of post-spreading volcanic ridge outcropping samples, the formation mechanism of the seamounts chain might be explained by a buoyancy-driven decompression melting mechanism occurring as separately evolving partial melting bodies beneath each seamount. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

As an important transition domain between the thinnest continental crust and the initial oceanic crust, the ocean-continent transition zone (OCT) preserves a wealth of information on the processes of lithospheric extension and breakup. The northern margin of the South China Sea (SCS) is a non-typical, magma-poor passive continental margin with a particular structural pattern and sedimentary formation. The OCT in the northern margin of the SCS was developed by a series of continental rifting and seafloor spreading. Based on several multi-channel seismic reflection profiles, crustal-scale structural profiles extracted from gravity data and ocean-bottom seismometers (OBS) data, this paper studies the deep crustal structures, deformation and geodynamics of the OCT to further understand the structural patterns and tectonic process from the lithospheric extension to its breakup. The OCT of the northern margin of the SCS is characterized by a crustal thinning and high-velocity layer (HVL) in the lower crust. The OCT has three structural units: rift depression, volcanic zone and tilted fault blocks. The HVL is mainly limited in the eastern portion of the northern SCS. The HVL is distributed essentially within the OCT but does not clearly develop everywhere in the OCT. We propose a hybrid model of rifting to fit characteristics of the northern margin of the SCS. A crustal-scale largely symmetric process predominated at the initial rifting stage. The basin-controlling faults subsequently penetrated the entire crust, resulting in asymmetric tectonic evolution. Then, the OCT occupied this asymmetric location. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Plate tectonic reconstructions assume a major inactive transform fault, the Davie Fracture Zone, in the West Somali Basin, along which Madagascar is thought to have migrated southwards following Gondwana breakup in the Mesozoic. Based on the interpretation of reflection seismic data, we show that the Walu Ridge offshore Kenya and the Kerimbas Basin offshore northern Mozambique are tectonically unrelated to the southward motion of Madagascar and correlate with Late Cretaceous volcanism and inversion in Kenya and the evolution of the East African Rift System respectively. Offshore Tanzania, geophysical data do not show basement structures indicating the presence of a major transform fault. These results challenge the commonly supported transform margin concept and imply a more southerly pre-breakup position of Madagascar within Gondwana. Opening of the West Somali Basin by SW-propagating oblique rifting and seafloor spreading is proposed. [ABSTRACT FROM AUTHOR]

Seafloor massive sulfide (SMS) deposits have significant development potential. In 2011, the China Ocean Mineral Resources Research and Development Association (COMRA) and International Seabed Authority (ISA) signed a contract to explore a 10 000 km 2 region of the seafloor along the Southwest Indian Ridge (SWIR) containing hydrothermal sulfides. As regulated by the contract, China will have to relinquish 50% and 75% of the contract area within 8 and 10 years, respectively. However, exploration for seafloor hydrothermal sulfide deposits in China remains in the initial stage. Based on quantitative prediction theory and the status of exploration for SMS, we assemble factors related to the deposits in terms of topography, geology, geophysics, and several other related aspects along the SWIR and extract the most favorable information to establish a prospecting prediction model for SMS. By employing the weights-of-evidence method, we obtain a weighting for each prediction factor and thereby obtain a posterior probability map for the SWIR. The prediction result suggests that the central region of the SWIR has the highest posterior probability, i.e., it is the most prospective for the formation of hydrothermal vents and related SMS. Known hydrothermal areas such as Mt. Jourdanne, area A and 10°E–16°E are all located in the regions with high posterior probability values. The Chinese contract area (47°–51°E) has the highest posterior probability value and thus can be selected as a reserved region for additional exploration. By narrowing the exploration area and improving exploration accuracy, the predictions made will provide a focus for further exploration of seafloor hydrothermal sulfide resources. [ABSTRACT FROM AUTHOR]

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

Our recent knowledge of the chemistry of the seafloor of the Baltic Sea has been gained during the last century only and most of it during the last three to four decades. When thinking about the start of the industrialization it seems unfortunately late. However, fortunately the seafloor archives all environmental changes in the sedimentary column like pages in a book and they can be later observed and interpreted. The Geological Survey of Finland has performed seabed geochemistry studies for decades. The first studies were performed onboard cruises of the old research vessel Aranda and later on the new Aranda as well as on the surveys on our own vessels Geola, Kaita, and Geomari. A great part of the work has been done as collaboration in international research programmes, but especially the Marine Ecogeological Patrol by the A. P. Karpinsky Russian Geological Research Institute has been of great importance for Gulf of Finland studies and as a trigger for later studies in the Gulf of Finland as well as the Baltic Sea. [ABSTRACT FROM AUTHOR]

AVO inversion is an effective seismic exploration method to predict elastic parameters. In this paper, we review and analyze the linearized AVO inversion method previously published for seafloor elastic parameters, and present a modification strategy. Before the linearized inversion is performed, a proper near-angle range in which the relationship between the reflection coefficient and sine-squared incidence angle is linear needs to be provided. However, the near-angle range is determined by the elastic parameters which are to be estimated by inversion. Therefore, only an approximated value of the near-angle range can be provided for the linearized inversion. Model tests show that a too large near-angle range may cause inversion fault, and a too small near-angle range may cause unreliable estimation. Further analysis shows that the estimation stability can be further improved even though the linearized inversion is performed under an exact near-angle range. To mitigate the strong dependence on the near-angle range, we use the seafloor elastic parameters estimated from the linearized method as the initial model for an unconstrained optimization method. Compared with the previously published method, the modified method is more robust to noisy data and shows less dependence on the near-angle range. [ABSTRACT FROM AUTHOR]

In contrast to the long narrow volcanic chains in the Pacific Ocean, Atlantic hotspot tracks, in particular in the South Atlantic (e.g., Tristan-Gough, Discovery, Shona, and Bouvet), are irregular and, in some cases, diffuse and discontinuous. An important question is whether this irregularity results from tectonic dismemberment of the tracks or if it represents differences in the size, structure, and strength of the melting anomalies. Here we present new age and geochemical data from volcanic samples from Richardson Seamount, Agulhas Ridge along the Agulhas-Falkland Fracture Zone (AFFZ), and Meteor Rise. Six samples yielded ages of 83-72 Ma and are 10-30 m.y. younger than the underlying seafloor, indicating that they are not on-axis seamounts associated with seafloor spreading. The incompatible element and Sr-Nd-Pb-Hf isotopic compositions range from compositions similar to those of the Gough domain of the nearby Tristan-Gough hotspot track to compositions similar to samples from the Shona bathymetric and geochemical anomaly along the southern Mid-Atlantic Ridge (49°-55°S), indicating the existence of a Shona hotspot as much as 84 m.y. ago and its derivation from a source region similar to that of the Tristan-Gough hotspot. Similar morphology, ages, and geochemistry indicate that the Richardson, Meteor, and Orcadas seamounts originally formed as a single volcano that was dissected and displaced 3500 km along the AFFZ, providing a dramatic example of how plate tectonics can dismantle and disseminate a hotspot track across an ocean basin. [ABSTRACT FROM AUTHOR]

A vast ocean basin has spanned the region between the Americas, Asia and Australasia for well over 100 Myr, represented today by the Pacific Ocean . Its evolution includes a number of plate fragmentation and plate capture events, such as the formation of the Vancouver, Nazca, and Cocos plates from the break-up of the Farallon plate, and the incorporation of the Bellingshausen, Kula, and Aluk (Phoenix) plates, which have been studied individually, but never been synthesised into one coherent model of ocean basin evolution. Previous regional tectonic models of the Pacific typically restrict their scope to either the North or South Pacific , and global kinematic models fail to incorporate some of the complexities in the Pacific plate evolution (e.g. the independent motion of the Bellingshausen and Aluk plates), thereby limiting their usefulness for understanding tectonic events and processes occurring in the Pacific Ocean perimeter. We derive relative plate motions (with 95% uncertainties) for the Pacific –Farallon/Vancouver, Kula– Pacific , Bellingshausen– Pacific , and early Pacific –West Antarctic spreading systems, based on recent data including marine gravity anomalies, well-constrained fracture zone traces and a large compilation of magnetic anomaly identifications. We find our well-constrained relative plate motions result in a good match to the fracture zone traces and magnetic anomaly identifications in both the North and South Pacific . In conjunction with recently published and well-constrained relative plate motions for other Pacific spreading systems (e.g. Aluk–West Antarctic, Pacific-Cocos, recent Pacific –West Antarctic spreading), we explore variations in the age of the oceanic crust, seafloor spreading rates and crustal accretion and find considerable refinements have been made in the central and southern Pacific . Asymmetries in crustal accretion within the overall Pacific basin (where both flanks of the spreading system are preserved) have typically deviated less than 5% from symmetry, and large variations in crustal accretion along the southern East Pacific Rise (i.e. Pacific –Nazca/Farallon spreading) appear to be unique to this spreading corridor. Through a relative plate motion circuit, we explore the implied convergence history along the North and South America s, where we find that the inclusion of small tectonic plate fragments such as the Aluk plate are critical for reconciling the history of convergence with onshore geological evidence. [ABSTRACT FROM AUTHOR]

We consider the presence of thick section in the axial trough indicates that the Pliocene-Recent continuous spreading model proposed by Curray (2015) is not appropriate for the Andaman Sea. The high frequency of eustatic sea level changes during the Plio-Pleistocene is incompatible with large, rapid influxes of sediment, widely separated in time but related to eustacy as proposed by Curray (2015). Instead spreading is probably of Miocene age, and was only renewed recently, if at all. The nature of the crust adjacent to the spreading centre remains uncertain. However, industry seismic data from the East Andaman Basin strongly suggests if the western part of the basin is underlain by oceanic crust it must be Oligocene or older, it is not Early Miocene. This in turn suggests the volcanics dredged from the Alcock rise were extruded onto older crust (whether it is oceanic, island arc or continental), and do not represent the age of oceanic crust formation. Gravity and seismic reflection data point to the East Andaman Basin being dominantly underlain by extended, to hyper extended continental crust and transitional crust. The Alcock and Sewell rises are interpreted as representing the western side of the strongly necked zone of continental crust. [ABSTRACT FROM AUTHOR]

Seafloor elastic parameters play an important role in many fields as diverse as marine construction, seabed resources exploration and seafloor acoustics. In order to estimate seafloor elastic parameters, we perform AVO inversion with seafloor reflected seismic data. As a particular reflection interface, the seafloor reflector does not support S-waves and the elastic parameters change dramatically across it. Conventional approximations to the Zoeppritz equations are not applicable for the seafloor situation. In this paper, we perform AVO inversion with the exact Zoeppritz equations through an unconstrained optimization method. Our synthetic study proves that the inversion method does not show strong dependence on the initial model for both unconsolidated and semi-consolidated seabed situations. The inversion uncertainty of the elastic parameters increases with the noise level, and decreases with the incidence angle range. Finally, we perform inversion of data from the South China Sea, and obtain satisfactory results, which are in good agreement with previous research. [ABSTRACT FROM AUTHOR]

The Gop and Laxmi Basins lying off Northwest India have been assigned ambiguous crustal types and evolution mechanisms. The Chagos–Laccadive Ridge (CLR) complex lying along the southwest coast of India has been attributed to different evolutionary processes. Late Cretaceous seafloor spreading between India and Africa formed the Mascarene Basin, and the plate reconstruction models depict unequal crustal accretion in this basin. Re-interpretation of magnetic data in the Gop and Laxmi Basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 Ma at slow half spreading rates (0.6 to 1.5 cm/yr) separating the Seychelles–Laxmi Ridge complex from India. The spreading ridge became extinct at 71 Ma in the central region between 19–20°N and 65.5–67°E, and at 68.7 Ma in the Gop Basin. Extinction progressed southwards with time until 64.1 Ma at ~ 14.5°N in the Laxmi Basin. This spreading probably limited the seafloor spreading in the northern Mascarene Basin, while normal to fast spreading continued in the south. The fracture zone FZ2 acted as a major boundary between the northern and southern Mascarene Basin, and may have also restricted the southward propagation of the Laxmi Basin spreading ridge. Plate reconstruction models generated in this study accommodate several microplates between Madagascar and India, and trace their movements over time within the India–Africa spreading corridor. Furthermore, these models suggest the formation of the Seychelles–Laxmi Ridge microplate by 79 Ma in a complex three plate system. This microplate may have existed up to 64.1 Ma and experienced an anticlockwise rotation of about 50°. Under the influence of the Deccan hotspot activity, the Laxmi Basin spreading ridge jumped southwestward splitting the Seychelles–Laxmi Ridge complex and initiating the Carlsberg Ridge. The presence of about 160 km of oceanic crust in the Gop and Laxmi Basins partially reduces the space problem faced in the earlier plate reconstruction models. [ABSTRACT FROM AUTHOR]

This paper describes the lateral spreading landslides, the weathering processes and the hydrogeological risk patterns of the NW sector of Monte Poro (Calabria, southern Italy), based on the development of a multidisciplinary research. The tectonic uplift of the area combined with the litho-structural features of the sedimentary deposits overlying the intensely weathered granitoid complex produces important gravitational phenomena characterized by lateral spreading kinematics. The contact between deposits with a different behaviour, together with the high slope energy related to the tectonic uplift processes, is the main cause of widespread fracturing and disjoint of the lithologies associated with lateral spreading phenomena. These phenomena are also testified by the various trenches and cracks characterized by a parallel orientation relatively to the main fluvial incisions. Measures of these deformations were obtained with a quantitative result by the use of 'Interferometric Stacking' methods. The experimental results confirm this morphological scenario characterized by strain values ranging between 20 and 25 mm/year relatively to the landslides detected. Geomorphological analysis of the drainage basins in the study area indicates a juvenile stage of the stream networks. This pattern is consistent with the high rate of tectonic uplift of the area, hence the continuous rejuvenation of the stream networks and their low level of hierarchy. These conditions of geomorphic equilibrium of the stream networks, the widespread occurrence of geological instability in continuous evolution, are the main causes of high hydrogeological risk conditions for this coastal area. [ABSTRACT FROM AUTHOR]

Marine debris is a recognized global ecological concern. Little is known about the extent of the problem in the Mediterranean Sea regarding litter distribution and its influence on deep rocky habitats. A quantitative assessment of debris present in the deep seafloor (30–300 m depth) was carried out in 26 areas off the coast of three Italian regions in the Tyrrhenian Sea, using a Remotely Operated Vehicle (ROV). The dominant type of debris (89%) was represented by fishing gears, mainly lines, while plastic objects were recorded only occasionally. Abundant quantities of gears were found on rocky banks in Sicily and Campania (0.09–0.12 debris m −2 ), proving intense fishing activity. Fifty-four percent of the recorded debris directly impacted benthic organisms, primarily gorgonians, followed by black corals and sponges. This work provides a first insight on the impact of marine debris in Mediterranean deep ecosystems and a valuable baseline for future comparisons. [ABSTRACT FROM AUTHOR]