Seafloor Spreading System Research Articles

A 3D Snapshot of Crustal Breakup Deduced From Seismic Analysis of the Tip of the NW South China Sea

AbstractBased on a high‐resolution seismic reflection data set, we describe and map rift domains at the tip of the V‐shaped NW‐South China Sea (NW‐SCS) Basin. We compare our rift domain map with seismic refraction, magnetic and gravity field data, and previously mapped Continent‐Ocean Boundaries (COB). We also define and map syn‐breakup sedimentary and magmatic sequences across the sediment‐rich NW‐SCS, which enables us to reconstruct the tectono‐stratigraphic evolution of the area. This work enables us to describe and discuss the 3D architecture and nature of the crust at the tip of the NW‐SCS, investigate how extensional and magmatic processes interacted during the breakup, and propose a kinematic model for late rifting, breakup, and finally stalling of the seafloor spreading system. This study provides exceptionally well‐documented insights into the spatial and temporal evolution at the tip of a so‐called “rift propagator,” and enables us to show that breakup in the NW‐SCS may rather correspond to a “retrogrador,” in which the extension zone retreats due to the reorganization of a failing rift system.

The onset of seafloor spreading at the northeastern continent-ocean boundary of the South China Sea

The transition from continental to oceanic crust and the initial formation of typical oceanic crust at passive rifted margin are still poorly understood. In this study, deep crustal structures in the transitional domain of the northeastern South China Sea (SCS) margin are investigated and studied according to newly-acquired high-resolution reflection seismic data and gravity modeling. Three strong reflectors (R1, R2 and R3) are imaged in the narrow transitional domain. About 1 s in the two-way travel time (TWTT) beneath the top basement, R1 shows a similar morphology to the top basement, and quite likely represents a partial serpentinization interface. The presence of R1 suggests a tectonically dominated extensional stage prior to the final lithospheric rupture of the SCS. R2 is from magmatic intrusions or plutons (sills or dikes) from decompression melting. R3 is observed at ~2.5–2.2 s TWTT beneath the top basement but shoals seawards. R3 likely resulted from magmatic relicts in the paleo-melt transfer channels and/or from frozen molten materials in a shear zone of the lithospheric and asthenospheric mantle. The occurrence of R3 indicates the onset of a robust seafloor spreading system. The gradual seaward increase in magmatic activity highlights that, towards the final lithospheric breakup, tectonic extension was overtaken by magmatism. Increased magmatism and partial serpentinization formed a narrow hybrid transitional crust. Nevertheless, the breakup was not an instantaneous event. The last-stage transient magmatism occurred in a very narrow zone and does not rule out that the margin was predominantly amagmatic during the rifting and breakup. Based on reflection seismic and gravity modeling, we propose that the onset of typical seafloor spreading occurred at about 30 Ma around the study area, and the continent-ocean boundary (COB) should be located further seawards in the northeastern SCS margin.

Precambrian zircons in chromitites of the Cretaceous Aladag ophiolite (Turkey) indicate deep crustal recycling in oceanic mantle

Crustal recycling into Earth’s deep mantle has been inferred from both seismic tomography and geochemical observations. As a further line of evidence, we report on zircons having a wide range of ages that were recovered from the Aladag chromitites, providing direct evidence for crustal recycling. Mesozoic zircons represent the earliest stages of Neotethyan seafloor spreading magmatism, whereas Neoproterozoic and Mesoproterozoic–Archean zircons record recycled old crustal material entrained in the ophiolitic mantle melt sources. The incorporation of such old, crustal zircons into the Neotethyan mantle might have followed from previous subduction events, from lithospheric delamination, and/or from the dismantling of West Gondwana in the Permo-Triassic. Subduction–affiliated ophiolitic melts may have picked up these recycled zircons and integrated them into chromitites, which precipitated from peridotite–melt interactions in a mantle wedge beneath the Inner–Tauride (a Neotethyan seaway) seafloor spreading system. Common in ophiolitic chromitites within the Neotethyan realm, such unusually old zircons present unique archives for tracking crustal recycling and mantle processes during rift–drift, seafloor spreading and subduction zone evolution of the oceanic mantle. We conclude that ophiolitic mantle peridotite and chromitite may be an important archive for preserving the recycling history of crustal material back into Earth’s deep mantle.

Tectonic Reconstruction of the Ellice Basin

AbstractA geophysical survey of the Ellice Basin, located in the south equatorial Pacific between the Ontong Java and Manihiki plateaus, revealed evidence for an extinct seafloor spreading system between the Pacific Plate and the Manihiki Plate. The spreading occurred during the Cretaceous Normal Superchron, the longest normal period of magnetic polarity from 121 to 83 Ma, and therefore lacks magnetic‐isochron‐derived age and plate motion constraints. Utilizing high‐resolution bathymetric data acquired on survey KM1609 during December 2016–January 2017, morphological and directional analyses were performed on the seafloor spreading fabric. Plate motion between the Pacific Plate and the Manihiki Plate during the Cretaceous Normal Superchron is described by three main stages of spreading. Stage 1 spreading was generally E‐W until a clockwise rotation of the spreading direction rotated transform faults by ~15° and lengthened spreading ridges to form Stage 2 zed pattern rhomboids. An offset between Stage 2 fracture zones evidences the presence of a complex and short‐lived Stage 3. Reconstructions, with respect to the Pacific Plate, are created for each of the three stages. Our reconstructions show that there was an earlier opening history prior to Stage 1 that ultimately reconstructs the Ontong Java and Manihiki plateaus; its quantification will await high‐resolution multibeam bathymetry data closer to the two plateaus.

What do variable magnetic fabrics in gabbros of the Oman ophiolite reveal about lower oceanic crustal magmatism at fast spreading ridges?

The magmatic processes responsible for accretion of the lower oceanic crust remain one of the least-constrained components of the global seafloor spreading system. Samples of gabbroic rocks recovered by scientific ocean drilling are too limited to allow effective assessment of spatial variations in magmatic flow within in situ lower crust. Extensive exposures of gabbros in ophiolites, on the other hand, provide opportunities to study accretion processes in three dimensions across wide areas and at a resolution that allows variations in magmatic fabrics through the crust to be quantified. Here, we show that magnetic anisotropy provides a reliable proxy for lower-crustal magmatic fabrics in the world’s largest ophiolite in Oman. Important differences in magnetic fabrics are detected that reflect variations in magmatic processes on a range of scales. Fabrics in layered gabbros are aligned with modal layering and display a consistency in the orientation of maximum principal axes of anisotropy between localities at a regional scale. These fabrics are compatible with subhorizontal preferred alignment of crystals orthogonal to the inferred orientation of the Oman spreading axis, resulting from magmatic flow or deformation of melt-rich crystal mushes during spreading. In contrast, magnetic anisotropy in foliated gabbros at the top of the lower crust reveals for the first time distinctly different linear and anastomosing fabric styles between localities sampled at the same pseudostratigraphic level. These differences reflect spatial variations in the style and trajectory of flow in the crystal mush beneath the axial melt lens during upward melt migration at the spreading axis.

Plume-proximal mid-ocean ridge origin of Zhongba mafic rocks in the western Yarlung Zangbo Suture Zone, Southern Tibet

The >2000km-long Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of the Mesozoic Neotethyan oceanic lithosphere, and is divided by the Zhada-Zhongba microcontinent into northern and southern branches in its western segment. Zircon U–Pb dating of a doleritic rock from the northern branch has revealed a concordant age of 160.5±1.3Ma. All of the doleritic samples from the northern branch and the pillow basalt and gabbro samples from the southern branch display consistent REE and trace element patterns similar to those of modern OIB-type rocks. The geochemical and Sr–Nd–Pb isotopic signatures of these OIB-type rocks from the western segment are identical with those of OIB-type and alkaline rocks from other ophiolite massifs along the central and eastern segments of the YZSZ, suggesting a common mantle plume source for their melt evolution. The enriched Sr–Nd–Pb isotopic character of the gabbroic dike rocks from the southern branch points to a mantle plume source, contaminated by subducted oceanic crust or pelagic sediments. We infer that the mafic rock associations exposed along the YZSZ represent the remnants of a Neotethyan oceanic lithosphere, which was developed as part of a plume-proximal seafloor-spreading system, reminiscent of the seamount chains along-across the modern mid-ocean ridges in the Pacific Ocean.

Convective upwelling in the mantle beneath the Gulf of California

In the past six million years, Baja California has rifted obliquely apart from North America, opening up the Gulf of California. Between transform faults, seafloor spreading and rifting is well established in several basins. Other than hotspot-dominated Iceland, the Gulf of California is the only part of the world's seafloor-spreading system that has been surrounded by enough seismometers to provide horizontal resolution of upper-mantle structure at a scale of 100 kilometres over a distance great enough to include several spreading segments. Such resolution is needed to address the long-standing debate about the relative importance of dynamic and passive upwelling in the shallow mantle beneath spreading centres. Here we use Rayleigh-wave tomography to image the shear velocity in the upper 200 kilometres or so of the mantle. Low shear velocities similar to those beneath the East Pacific Rise oceanic spreading centre underlie the entire length of the Gulf, but there are three concentrated locations of anomalously low velocities spaced about 250 kilometres apart. These anomalies are 40 to 90 kilometres beneath the surface, at which depths petrological studies indicate that extensive melting of passively upwelling mantle should begin. We interpret these seismic velocity anomalies as indicating that partial melting triggers dynamic upwelling driven by either the buoyancy of retained melt or by the reduced density of depleted mantle.

Tectonic/volcanic segmentation and controls on hydrothermal venting along Earth's fastest seafloor spreading system, EPR 27°–32°S

We have collected 12 kHz SeaBeam bathymetry and 120 kHz DSL‐120 side‐scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading (∼150 km/Myr) part of the global mid‐ocean ridge system, the southern East Pacific Rise between the Easter and Juan Fernandez microplates. This area is presently reorganizing by large‐scale dueling rift propagation and possible protomicroplate tectonics. Fracture patterns observed in the side‐scan data define structural segmentation scales along these ridge segments. These sometimes, but not always, correlate with linear volcanic systems defining segmentation in the SeaBeam data. Some of the subsegments behave cohesively, with in‐phase tectonic activity, while fundamental discontinuities occur between other subsegments. We also collected hydrothermal plume data using sensors mounted on the DSL‐120 instrument package, as well as CTDO tow‐yos, to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the Pacific‐Nazca axis. Here we report the first rigorous correlation between coregistered hydrothermal plume and high‐resolution marine geophysical data on similar scales and over multisegment distances. Major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low. These correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest, resulting in recent eruptions that have paved over the neovolcanic zone. Areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas. Increased crustal permeability, as gauged by increased fracture density, does not enhance hydrothermal venting in this area. Axial summit troughs and graben are rare, probably because of frequent volcanic resurfacing in this superfast spreading environment, and are not good predictors of hydrothermal activity here. Many of the hydrothermal areas are found in inflated areas near the ends of segments, suggesting that abundant magma is being supplied to these areas.

The tectonic and geomagnetic significance of paleomagnetic observations from volcanic rocks from central Afar, Africa

Deformation throughout Afar over the past 2 myr has been characterized by widespread and intense crustal fragmentation that results from inhomogeneous extension across the region. In eastern Afar, this situation has evolved to localized extension associated with the westward propagation of the Gulf of Aden/Gulf of Tadjurah seafloor spreading system into the Asal–Ghoubbet Rift. During the gradual process of rift propagation and localization, crustal blocks in eastern Afar sustained clockwise rotations of ∼11°. To better understand the processes of rift propagation and localization and how they affect the rest of Afar, we have collected and analyzed over 400 oriented paleomagnetic samples from 67 lava flows from central and southern Afar. Unlike eastern Afar, the mean paleomagnetic direction from central Afar indicates that vertical-axis rotations are statistically insignificant (3.6°±4.4°), though small clockwise rotations (<8°) are permitted. Thus, propagation and localization in central Afar have not had the same influence in causing crustal block rotations or, perhaps more likely, have not reached the same stage of evolution as seen in eastern Afar. In addition, several of the lava flows record intriguing geomagnetic field behavior associated with polarity transitions, excursions, or large secular variation events. Interestingly, the transitional or anomalous virtual geomagnetic poles (VGPs) tend to cluster in two nearly antipodal regions, one in the northern Pacific Ocean and the other in the southwest Indian Ocean. One lava flow has recorded both of the antipodal transitional components, with the two components residing in magnetic minerals with unblocking temperatures above and below ∼500°C, respectively. Reheating and partial remagnetization by the overlying flow cannot explain either of the transitional directions because both differ significantly from that of the reversely magnetized overlying flow. The high-temperature component gives a VGP in the northern Pacific, whereas the lower-temperature component gives a nearly antipodal VGP south of Cape Town, South Africa. Hence, the configuration of the geomagnetic field appears to have jumped nearly instantaneously from a northern-hemisphere transitional state to a southern-hemisphere one during this normal-to-reverse polarity transition.

Structure, petrology and seafloor spreading tectonics of the Kizildag Ophiolite, Turkey

Abstract The Kizildag ophiolite in southern Turkey is a remnant of the Neo-Tethyan oceanic crust and displays well-preserved magmatic and tectonic structures of seafloor spreading origin. The ophiolite consists of two structurally distinct massifs that are separated by the NW-striking high-angle Tahtaköprü fault. The main massif to the west contains a serpentinized peridotite core adjoined on the southeast by the normal fault-bounded plutonic sequence and sheeted dyke complex in a structural graben. The dyke-gabbro boundary within this graben is in places faulted along a low-angle detachment surface and is locally marked by a transition zone with mutual intrusive relations between the dykes and isotropic gabbros and plagiogranites. This igneous boundary contains numerous proto-dyke intrusions marking a well-preserved root zone of the sheeted dyke complex and may represent the roof of a fossil magma chamber. Mineralized oceanic faults within the dyke complex form two major subsets. Dyke-parallel normal faults form horst and graben structures and locally flatten with depth acquiring a listric geometry. Dyke-perpendicular faults display steep dips and subhorizontal slickenside lineations, suggesting their oblique- to strike-slip nature. The graben structure containing the plutonic sequence and the sheeted dyke complex is analogous to those documented in the Troodos ophiolite and may similarly represent a fossil spreading axis. The second massif east of the Tahtaköprü fault consists mainly of serpentinized peridotites directly overlain by lava flows, rotated dyke blocks, and gabbros. Sulfide mineralization along some fault planes in the extrusive rocks indicates that hydrothermal systems were associated and operated synchronously with magmatic and tectonic extensional processes. Stratigraphic relations and the structural architecture in this massif suggest that the Kizildag oceanic crust underwent crustal denudation and unroofing of the upper mantle as a result of tectonic extension at a spreading centre. The Tahtaköprü fault separating the two massifs is an accommodation zone that permitted differential movements between the adjacent ridge segments during generation of the Neo-Tethyan oceanic lithosphere. The general structure of the ophiolite suggests its evolution via seafloor spreading and an asymmetric simple shear extension along a slow-spreading centre. The sheeted dykes and pillow lavas have relatively high SiO 2 and Al 2 O 3 and low FeO and TiO 2 concentrations and show limited FeO and TiO 2 enrichments with decreasing MgO contents. These compositional properties differ markedly from typical tholeiitic suites from the ocean floor and arc settings, but correspond closely to those documented from the sheeted dyke complex and the lower volcanic suite of the Troodos ophiolite. The major element compositions suggest low pressure and high degree of melting from a depleted mantle source. The trace element concentrations are markedly depleted in both high-field strength and rare-earth elements and relatively enriched in largeion-lithophile elements compared to normal MORB. It is inferred that the Kizildag ophiolite formed in a tectonic setting where melting occurred at relatively low pressures within the stability field of spinel and proceeded to high melt fractions possibly by progressive depletion and melt removal. A limited positive correlation between the extent of melting and light rare-earth enrichment can be related to infiltration by fluids or melts derived from a more fertile source or possibly a subducting slab fragment. Comparison of the structure and petrology of the Troodos and Kizildag ophiolites and the regional geology suggest their evolution along a seafloor spreading system within the Southern Neo-Tethys. Spreading probably occurred in short segments and resulted in development of the Cretaceous Neo-Tethyan seaway as a marginal basin between the Tauride platform in the north and Afro-Arabia in the south.

Swath Bathymetry Processing of GLORI-B and SeaBeam 2000

A new bathymetry processing software package has been developed to postprocess new GLORI-B swath bathymetry data using preexisting techniques. GLORI-B bathymetry is calculated using an interferometry (phase delay) method using the modified GLORIA towfish which has parallel rows of transducers on both sides. We describe four types of artifacts observed during the first use of this new system during Legs 5 and 6 of the Gloria Expedition which surveyed the fastest spreading segment of the global seafloor spreading system and the broad chain of volcanoes near Easter Island. These artifacts include cross-track bias, along-track bias, a 'dropped edge' effect, and random noise. We describe and illustrate how we minimize these artifacts. We merge the SeaBeam 2000 bathymetry data with the GLORI-B bathymetry data to produce a final bathymetric mosaic which covers about 243,400 km2 and shows a different style of diffuse widely spread volcanism not previously observed along hotspot chains. The data are used in several studies describing seamount morphology, elastic thickness of the lithosphere, tectonic and geochemical evolution of the area, and mantle flow from a hotspot to a superfast seafloor spreading center.

Late Tertiary tectonic evolution of the seafloor spreading system off the coast of California between the Mendocino and Murray Fracture Zones

A forward modeling program of two‐plate spreading on a sphere was utilized to reconstruct the late Tertiary tectonic evolutionary history of the seafloor spreading system off the coast of California between the Mendocino and Murray fracture zones. This analytical technique was applied to a unique set of magnetic anomaly data consisting principally of a new digital data set: the original Raff‐Mason data from one of the most significant marine geophysical surveys: the 1955–1956 USCGS Pioneer survey. All the data, critical for the development of seafloor spreading and plate tectonic theory, were digitized (32 ° to 52°N); however, only the southern half was analyzed in this study. Investigation of the Raff‐Mason data, supplemented by additional magnetic anomaly data, indicates that the primary mechanism for reorganization of this part of the Pacific‐Farallon ridge, as the spreading center migrated toward and arrived at the North American trench, was rift propagation. The model presented here, containing a total of 21 propagation episodes from 36 Ma to 19.8 Ma (initiation time of the last propagator), shows generally excellent agreement with the isochrons identified from the magnetic anomaly pattern, although minor exceptions exist locally in complex areas. A step‐by‐step evolution of the model is presented in a series of reconstructions documenting the history of spreading between the Pacific plate and five smaller plates created from the breakup of the large ancestral Farallon plate. These plates include the Vancouver plate north of the Pioneer, the Farallon plate existing between the Murray and Pioneer transforms prior to the major reorganization at chron 10 (30.33 Ma), and the Arguello, Monterey, and Reyes microplates with northwest‐southeast spreading existing after chron 10.

Microplate tectonics along a superfast seafloor spreading system near Easter Island

There is a growing Easter microplate between the Pacific and Nazca plates. The eastern and western boundaries of the microplate are actively spreading and propagating north and south, respectively. The northern and southern boundaries are broad transform zones. Most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres.

A Topographic Interpretation of the Mathematician Ridge, Clipperton Ridge, East Pacific Rise System

WITHIN the framework of plate tectonics, dating of the ocean floor has been almost exclusively achieved by studies of magnetic anomalies. But a north–south striking mid-ocean ridge system generating new seafloor in the equatorial region produces magnetic anomalies which are of small amplitude and are frequently distorted and disguised by magnetic “noise”. The correspondence between the absolute depth of oceanic crust and its age1 has provided an alternative method of age dating the oceanic crust anywhere in the world's oceans, provided, of course, that the crust was generated by a seafloor spreading system. Such a correlation tool has been used on the East Pacific Rise (Fig. 1) between 20° N and the equator1 and between 0° and 20° S (ref. 2).