Total continental lithosphere extension prior to breakup and sea-floor spreading in the South China Sea (SCS), a marginal ocean basin, ranges from approximately 360 km in the NE to 580 km in the SW. In contrast, total continental lithosphere extension prior to breakup for the Iberia-Newfoundland rifted margins is no more than 180km. SCS extension leading to continental breakup is between x2 and x3 greater than for the Atlantic margin type.In the case of Atlantic type margins, lithosphere deformation transitions from initially wide rifting to more localised stretching and thinning, a process termed necking. The necking domain at rifted continental margins, so produced, typically has crustal thickness of 25 km proximally decreasing to 10 km distally. Further lithosphere stretching and thinning due to hyper-extension and the onset of decompression melting results in the rupture and separation of continental lithosphere, the creation of a divergent plate boundary, and the initiation of sea-floor spreading.The SCS shows very wide domains of thinned continental crust with thicknesses between 25 and 10 km; widths of thinned crust much greater than those of Atlantic type margins. These wide regions of thinned crust on the SCS margin take the form of crustal boudinage with multiple sag basins underlain by highly thinned crust separated by basement highs underlain by less thinned crust.The localisation of lithosphere deformation before breakup, during the formation of Atlantic type margins, is due to failure of the initially strong cold lithospheric mantle lid. The same mechanism of localisation cannot occur to generate necking in the SCS; the SCS was formed by rifting of volcanic arc lithosphere in which the lithospheric mantle was already hot.We attribute the very wide regions of continental crust with thicknesses between 25 and 10 km in the SCS, very much wider than for Atlantic type margins, to a weak inherited lithosphere rheology which favours extensional boudinage of the continental crust rather than crustal rupture and separation, and distributed rather than focused decompression melting of wet mantle from the inherited volcanic arc setting.

Recently published marine geophysical and seafloor drilling data permit a substantive reappraisal of the rifting and spreading that formed the South China Sea (SCS). The SCS rifted margins are different from those of the Atlantic type, having higher strain rates, younger orogenic crust, and distributed syn-rift magmatism. Rifting ~66–11 Ma and spreading 30–14 Ma split a Cretaceous Andean arc and forearc, producing >700 km of seafloor spreading in the east and a ~2000-km-wide rifted margin in the west. Luconia Shoals–Dangerous Grounds–Reed Bank–north Palawan–SW Mindoro were separated from China when the SCS opened. Brittle faulting of the upper crust was decoupled from ductile flow and magmatic intrusion of the lower crust, producing wide rifting with thin spots held together by less extended surrounds. Sediments accumulated in inter-montane lakes. Transform faults formed at/after breakup to link offset spreading segments. Spreading in the eastern subbasin from C11n to C5AD was at rates averaging 62 mm/yr, 30–24 Ma, decreasing to 38.5 mm/yr younger than 23 Ma. Spreading reorganization was common as margin segments broke up to the SW and spreading directions changed from ~N-S before 23 Ma to NW-SE after 17 Ma.

The Bismarck Sea Seismic Lineation (BSSL) is a 1000-km-long band of shallow earthquakes that marks the boundaries between the North and South Bismarck Plates and the Manus Microplate, offshore Papua New Guinea. These tectonic boundaries comprise a series of transform faults separated by spreading ridges, and one extensional transform zone (ETZ). Since it was first identified in 1969, the deformation details of the faults along the BSSL have remained shrouded by the cloud of seismicity surrounding them. Here, we employ a recently published surface wave earthquake relocation algorithm (Howe etl al., 2019) to relocate events along the BSSL and the previously proposed Adelbert block boundary. These relocations show that left-lateral strike-slip seismicity concentrates narrowly along the known transform faults (the Schouten, Willaumez, Djaul, and Weitin) and the western ETZ. Several events that relocate to inside the Manus Microplate may represent distributed deformation by right-lateral bookshelf-style faulting. The spreading ridges are aseismic at our scale of observation, as previously suggested, but we identify clusters of strike-slip events at their terminations. The strike-slip events along the ETZ are at synthetic Riedel shear angles of \textasciitilde10° to those along the Willaumez transform. The earthquake relocations also show that the previously proposed SE boundary of the Adelbert block is not well defined or localized. We carefully selected the best quality relocated focal mechanisms to calculate new best-fit Euler poles for the transform segments. These calculations support earlier indications that the Schouten transform is not well fit by the same Euler pole as the other BSSL transforms, requiring internal deformation of the adjoining plates, for which we present additional evidence.

Bathymetry and acoustic imagery swath mapping, along with observations and samples from four manned submersible and four ROV dives, confirm that a seafloor slope break on the northern approaches to Kaiwi Channel, between the islands of O'ahu and Moloka'i, Hawai'i is a former shoreline, now submerged -800 m below present sea level. Subaerially emplaced, low-relief basaltic lavas above the slope break transition to submarine morphologies below. The entire region has been tilted about 1 degrees to the SSE (150 degrees), and is cut by an 8-15 m-high, north-facing scarp, 100-400 m south of the slope break. The distribution of platy, table-top, and rarer mounded branching corals indicates the former presence of fringing reefs around low-relief paleo-islands. We infer that the regional tilt resulted from loading by younger Hawaiian volcanoes, compounded by flexural uplift and back tilting away from the unloaded footwall of a flank landslide to the north. Basalt samples collected from both above and below the slope break have petrography, chemical composition, and age (1.64-1.80 Ma) indicating correlation with the (late-shield) Lower Member of the East Moloka'i Volcanics, rather than with the more proximal volcano of West Moloka'i. The most likely source of the Kaiwi basalts is a submarine ridge (rift zone) that extends northwest away from 'Ilio Point on West Moloka'i. Although the submarine ridge was previously assumed to be an extension of West Moloka'i's northwest rift, we conclude that regional bathymetry and gravity are consistent with this feature being an extension of the west rift of East Moloka'i. A corallary of this interpretation is that the shoreline slope break (SSB 7 of Taylor, 2019) in this area is distinct from and younger than the southern SSB 7 formed on West Moloka'i volcano (-1.65 Ma vs. -1.8 Ma).

Mapping ocean-continent transitions (OCTs) separating equivocal continental and oceanic crusts is fundamental to investigate breakup processes and define the age and location of initial seafloor spreading. However, proposed limits of OCTs are rarely consistent, do not use uniform criteria, and result in conflicting interpretations as shown for the case of the northern South China Sea (SCS). We review original datasets including reflection and refraction seismic sections, drilling and potential field data with the aim to develop a 'drilling-constrained integrated geological-geophysical' approach to define the OCT along the northern SCS, understand the breakup process, and to compare the OCT in the SCS with those at Atlantic type rifted margins. The result shows a narrow, 5-15 km wide OCT. It separates a segmented margin that rifted a former arc in the west and a forearc in the east, both facing a Penrose oceanic crust that thins from the west towards the east. Seafloor spreading may have first nucleated at two centres during magnetic anomaly C11 in the NE and central subbasins, which then locally propagated both W and E to break through salients and produce full breakup at 29 Ma (anomaly C10r). Breakup at the SCS shows many differences to Atlantic type margins, in part due to inheritance but also due to rift/spreading-related parameters such as strain/spreading rates. A 'drilling-constrained integrated geological-geophysical' approach was developed to define OCTs and explore breakup processes (Figure a). Seafloor spreading had first nucleated in the central and NE subbasins of South China Sea during magnetic anomaly C11, then propagated both W and E and produced full breakup at anomaly C10r (Figure b).image

We study the collision of the Ontong Java Plateau, reversal of the Solomon Islands arc, and subduction of the Woodlark Basin spreading ridge in the southwest Pacific. Double-sided subduction and intra-arc deformation, not just polarity reversal, characterize the rapid Australia-Pacific plate convergence. The rifted margins of the 0-6.2-Ma Woodlark Basin separate its unflexed young lithosphere without a trench from the older flexed lithosphere with >8-km-deep trenches subducting to either side. A gap evident in the seismicity and tomography between the NE-subducted slabs of the Solomon Sea and Australia plates indicates that the Woodlark lithosphere likely melts and/or is quickly (<1 m.y.) resorbed into the mantle. A section of the Pacific Plate remains attached within the gap and provides the slab pull to form the Malaita accretionary prism. Quaternary arc-composition volcanoes that occur on both sides of the Woodlark Basin subduction front are not sourced from a mantle wedge overlying a Wadati-Benioff zone, but come from and through breaks in the young incoming and subducting lithosphere and the overturning mantle beneath. Though the basin is too weak to sustain plate flexure, arc volcanoes on the subducting crust result in locally strong coupling to the forearc, manifest by tsunamigenic earthquakes. At 150-400-km depth, the subducted slabs under the Solomon Islands are near vertical. They are near-horizontal between the 410 and 660-km mantle discontinuities beneath the Woodlark Basin and further south, whereas beneath the Solomon Sea Basin they dip southwest and penetrate the 660-km discontinuity, having been detached by the north-dipping Solomon Sea slab.

An animated 100,000‐year‐interval tectonic reconstruction of the Woodlark Basin in the southwest Pacific illustrates how, at intermediate initial spreading rates, orogenic continents break up (dyke model), spreading segments nucleate, transform faults initiate and ocean basins evolve. We refine the location/timing of initial seafloor spreading and Euler poles of rotation back to 6.2 Ma. In the easternmost basin, where spreading younger than 2.6 Ma is not co‐polar with that to the west, we recognize the formation of a Ghizo microplate and Ranongga Transform Fault at ∼2.6 Ma and a 3‐degree rotational opening of the Itina Trough from 2.6 to 1.0 Ma. Allowing for that motion, we show that the 5.2–2.6 Ma seafloor in the easternmost basin formed co‐polar with that to the west. We also identify a ridge jump reorientation at ∼1.0 Ma that formed the NE‐trending Simbo Spreading Segment, whose neovolcanic zone includes Simbo Island and a submarine edifice to its south. Proposed deterministic models of ridge propagation (due to topographic gradients, mantle flow away from hotspots and/or changing plate motion) are not consistent with those observed; mantle chemical heterogeneities and melting anomalies are a potential cause that remains to be tested. We reconstruct the northern conjugate of the oldest extant oceanic crust and estimate the initiation of its subduction at ∼2.6 Ma, concomitant with observed changes in plate motion and segmentation. Where subducted, the young oceanic lithosphere between the conjugate rifted margins appears to be resorbed into the mantle, leaving a slab window where the Pacific subducted slab remains attached.

In the Solomon Islands, arc magmas are erupting on the subducting Australia Plate. These island (Simbo) and submarine arc volcanoes (Kana Keoki, Coleman and Pavuvu) are about to be recycled by rapid subduction. We identify eight of their former equivalents beneath the forearc by the morphologies and deformation structures that are characteristic of seamount subduction. Tsunamigenic earthquakes recently nucleated just ahead of two of the subducting seamounts. A third (Pavuvu), that has indented the subduction front and uplifted the lower forearc, is associated with a historic earthquake gap. It is positioned such that a rupture there has the potential for tsunami waves to impact the capital, Honiara.

Abstract 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 4.5–4.7 mm/yr with obliquity increasing westwards, and full spreading rates, increasing eastwards along the Woodlark Basin spreading center, of 20–40 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 200 Ka.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

We establish a high‐resolution magnetic isochron pattern in the East Subbasin (ESB) of the South China Sea (SCS) based on recently collected magnetic data, which provides an updated age of seafloor spreading in the ESB and reveals a new type of ridge reorientation. Seafloor spreading in the ESB initiated at Chron 11n.1r (29.7 Ma) and ceased shortly after Chron 5Br (15.6 Ma). Successive ridge jumps occurred between Chrons 9r and 7n, which explains the substantial asymmetric geometry of the ESB. Furthermore, the ridge reorientation associated with ridge jumps highlights a new ridge reorientation model in which the ridge jumps off‐axis and reorients synchronously to adapt to the new direction of seafloor spreading. In the ESB, this type of reorientation responds more rapidly to changes in the direction of plate motion than gradual ridge rotation.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

The Itina Trough is a late Miocene rift graben formed at the beginning of Woodlark Basin opening where the northern Louisiade Plateau micro‐continent had collided with the eastern Pocklington Rise remnant arc. We infer that it was the collisional suture of this Plateau subducted at the Pocklington Trough that either thickened the crust and weakened the lithosphere and/or adjoined the apparently thicker Louisiade micro‐continent to the thinner crust of the eastern Pocklington Rise, and thereby localized the rifting. Other examples of rifting across a remnant arc‐trench system in the SW Pacific include the South Rennell Trough and the Norfolk Basin.

The discovery of large freshwater reservoirs off Hawaii suggests that other volcanic islands may have similar resources, which could help meet water demands amid population growth and climate change.

The plate tectonic revolution and decipherment of magnetic isochrons that followed the pioneering work of Marie Tharp and coworkers in visualizing the seafloor has led to a near-complete understanding of the first-order evolution of global seafloor spreading. However, lagging behind in exploration and understanding are areas of seafloor formed during the Cretaceous Normal Superchron (CNS, 121-83 Ma) when no magnetic reversals were recorded to guide investigators. Thus, for such regions tectonic interpretations are largely driven by mapping and identifying seafloor fabric indicators such as fracture zones, abyssal hills, rift propagators, and extinct spreading centers. Here, we focus on the relict spreading system of the Cretaceous Ellice Basin that was apparently formed by seafloor spreading that split the world’s single largest oceanic plateau Ontong Java Nui, composed of present day Ontong Java, Manihiki and Hikurangi plateaus and other (now subducted) fragments. We examine what was known about this basin from historical single and multibeam bathymetry, what was revealed by the advent of satellite altimetry, and why bathymetric mapping is still required to infer short-length-scale tectonic fabric. High-resolution bathymetric data from the central basin were recently acquired by the University of Hawaii’s vessel R/V Kilo Moana. Evolution of the spreading system is characterized by three main stages of spreading based on directional and morphological analyses of the seafloor fabric indicators identified from bathymetry. Spatially conjugate points symmetric about the spreading central zone were identified at the establishment and cessation of each spreading stage and were assumed to be of the same age to form pseudo-isochrons. Pseudo-isochrons were then utilized in reconstructing the basin through time. The earliest Stage 1 fracture zones trend E-W and consist of multiple closely spaced, parallel fault strands that were indistinguishable in satellite altimetry. A clockwise rotation of the spreading direction led to Stage 2 NW-SE trending fracture zones, which splayed from Stage 1 multistrands. An offset between Stage 2 fracture zones indicates a short-lived late Stage 3 that appears to be the result of a counter-clockwise rotation of the spreading direction shortly before spreading ceased. Seafloor evidence for the initial breakup and rifting between Ontong Java and Manihiki plateaus prior to Stage 1 has yet to be mapped. Basaltic rocks dredged from selected locations along the survey track promise to provide tighter temporal constraints on the evolution of Ellice Basin.

New shipborne surveys provide a closely spaced magnetic anomaly dataset covering the East Subbasin (ESB) of the South China Sea (SCS). Magnetic anomalies of seafloor spreading are identified using the dataset supplemented with previous data and age constraints from recent International Ocean Discovery Program Expeditions 349 and 367/368 holes. We present a high-resolution oceanic crustal age model and associated magnetic lineations of the ESB based on identified magnetic anomaly picks. Seafloor spreading in the ESB initiated at ~30 Ma (C11n) and terminated at ~16 Ma (C5Br). The spreading direction has experienced a gradual counterclockwise rotation between C6Cr and C5Er and a significant counterclockwise rotation at C5Dr. The spreading rotations reorganized the orientation and segmentation of the spreading ridge, resulting in the formation of a series of S-shaped fracture zones. The interpretation of the magnetic lineations reveals that three southward ridge jumps occurred at C9r, C8n, and C7n and a synchronous jump occurred at C5Dr. Three southward ridge jumps contributed to a total difference of ~184 km in the distance between the two flanks and left the paired magnetic lineations C10r–C7r on the present-day north flank. The synchronous jump caused the spreading ridge to rotate rapidly counterclockwise and obliquely intersect the existing seafloor. We postulate that these ridge jumps and rotations are common processes during seafloor spreading reorientation and are dynamic responses to the plate or microplate tectonics around the SCS.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

We announce a new and integrated system for planning and executing marine geophysical surveys and for scrutinizing and visualizing incoming shipboard data. The system incorporates free software designed for use by scientists and shipboard operators and pertains to underway geophysics and multibeam sonar surveys. Regarding underway data, a crucial first step in the approach is to reduce and merge incoming center beam depth, gravity, and towed magnetic data with navigation, then reformat to the standard exchange format. We are then able to apply established quality control methods including along-track and cross-track analyses to identify error sources and to incrementally build the candidate archive file as new data are acquired. Regarding multibeam data, these are subjected to both an automated error removal scheme for quick visualization and to subsequent ping editing in detail. The candidate archive file and sonar data are automatically and periodically updated and adapted for display in Google Earth, wherein survey planning is also carried out. Data layers are also updated automatically in Google Earth, allowing scientists to focus on visual inspection and interpretation of incoming data. By visualizing underway and sonar data together with reference gravity, magnetic, and bathymetry grids in Google Earth, data familiarity is enhanced and the likelihood of noticing extreme errors increased. We hope scientists will embrace these techniques so that each data set being submitted to a data repository is vetted by the seagoing science party.

Slope gradient maps of swath bathymetry around the Hawaiian islands locate 12 shield slope breaks associated with former shorelines that are now submerged, ranging in age back to 5 Ma. The age of their drowning correlates with the waning of tholeiitic shield volcanism that ceases to repave the shoreline sometime between the beginning and the end of the late-shield stage. Slope breaks on Mauna Loa's northeast and southwest rifts are consistent with it being in the waning stages of shield building. Superposition of shorebreaks shows that the Hualalai shield is older than Mauna Kea's. We find evidence for three volcanic shields forming Ka'ena Ridge, for a simultaneous waning of Maui-Lana'i-Kaho'olawe shield building and the initiation of Kohala shield building by 1.3 Ma, and for Mahukona growing to just above sea level about 0.6 Ma. A contiguous shoreline slope break formed similar to 1.8 Ma at the end of the West Moloka'i late-shield stage and the beginning of the East Moloka'i late-shield stage. At its western end the shield dips obliquely away from, rather than toward, this slope break. Similarly, the short Hualalai slope break plunges north, which is opposite to that expected for volcanic loading to the south. Shields locally sloping away from paleoshorelines may be related to landslides causing flexural back tilting away from their unloaded footwall. The Nu'uanu and Wailau slides and Pololu slump were mostly shield flank events that cut marginally into their former shorelines, enough to preferentially nucleate headward erosion by streams.