Ridge Jumps Research Articles

Review of Asymmetric Seafloor Spreading and Oceanic Ridge Jumps in the South China Sea

Seafloor spreading is an important cornerstone of the theory of plate tectonics. Asymmetric seafloor spreading and oceanic ridge jumps are common phenomena in this process and play important roles in controlling oceanic crust accretion, regional tectonics and geological geometric boundaries. As the largest marginal sea in the western Pacific, the South China Sea is an ideal laboratory for dissecting the Wilson cycle of small marginal sea-type ocean basins restricted by surrounding blocks and exploring the deep dynamic processes of confined small ocean basins. In recent years, a lot of research has been conducted on the spreading history of the South China Sea and has achieved fruitful results. However, the detailed dynamic mechanisms of asymmetric seafloor spreading and ridge jumps are still unclear. Therefore, this paper summarizes the basic understanding about the dynamic mechanisms of global asymmetric seafloor spreading and ridge jumps and reviews the related research results of asymmetric seafloor spreading and ridge jumps in the South China Sea. Previous studies have basically confirmed that seafloor spreading in the South China Sea started between ~32 and 34 Ma in the east sub-basin and ended at ~15 Ma in the northwest sub-basin, with at least once oceanic ridge jump in the east sub-basin. The current research mainly focuses on the age of the seafloor spreading in the South China Sea and the location, time and stage of the ridge jumps, but there are relatively few studies on high-resolution lithospheric structure across these ridges and the dynamic mechanism of oceanic ridge jumps. Based on the current research progress, we propose that further studies should focus on the lithosphere–asthenosphere scale in the future, suggesting that marine magnetotelluric and Ocean Bottom Seismometer (OBS) surveys should be conducted across the residual oceanic ridges to perform a detailed analysis of the tectonics magmatism in the east sub-basin to gain insights into the dynamic mechanisms of oceanic ridge jumps and asymmetric seafloor spreading, which can promote understanding of the tectonic evolution of the South China Sea and improve the classical plate tectonics theory that was constructed based on the open ocean basins.

Gravity patterns and crustal architecture of the South-Central Indian Ridge at 22°-17°S: Evidence for the asymmetric ridge accretion

The present study examines the gravity-derived structural fabric and crustal structure beneath the southern portion of the Central Indian Ridge at 22°-17°S. We examined the structures using enhanced edge detection techniques like gradient amplitude of the vertical derivative (HGVD), improved horizontal tilt angle (ITDX), gradient amplitude of the NTilt (HGNTilt) and the fast sigmoid function (FSED) on synthetic examples and the gravity data of the study region. The gravity-derived lineaments describes the lineaments that are parallel to fracture zones trending in ENE-WSW, parallel to segments/major ridge axis, near circular lineaments probably depicting ascending magma sources. We estimated the depth to the gravity-derived lineaments using the tilt_depth technique, which reveals the presence of lineaments with shallow source depth range at 2.5–7 km. The south-Central Indian Ridge is characterized by numerous lineaments with random depths that shows variable spreading rates and continuous ridge jumps. The major lineament depths are rooted into the lower crust beneath the major axis and the upper mantle in the region away from it. In addition, gravity inversion of low-pass filtered Bouguer anomaly data is used to estimate the crustal thickness. The gravity inversion shows that the Moho depth varies from 5 to 14.5 km, which shows thicker crust along the principal ridge axis and thinner in the surrounding regions. The structural fabric in combination with the crustal thickness, depth of the lineaments, age of the ridge crust, earthquake data suggests discrete minor/local ridge jumps along the major ridge axis and asymmetric accretion along the ridge axis.

Lithospheric velocity structure of South China Sea basin from ocean bottom seismometer ambient noise tomography

The South China Sea (SCS) is a typical marginal sea basin formed by the seafloor spreading under the tectonic background of plate convergence. Many crustal-scale studies indicate that the SCS basin has undergone asymmetric spreading, multi-phase ridge jumps, and intense post-spreading volcanic activity. Due to the lack of seismic data in the oceanic basin of the SCS, it remains unclear about the scale and basin control of the Zhongnan fault, the magma source depth of the SCS basin, and the transport channel after the cessation of seafloor spreading. Phase velocity derived from ambient noise surface wave tomography may provide useful information to shed light on the mechanisms of the aforementioned problems. From October 2019 to July 2020, a passive source seismic experiment was carried out with many Ocean Bottom Seismometers (OBS) deployed in a broad area of the SCS. Based on the seismic ambient noise data recorded by 16 OBSs in the SCS basin, we inverted the phase velocity images over a period range of 10–20 s using ambient noise surface wave tomography. Our results indicate that the Zhongnan fault zone is a lithospheric-scale fault, which played a regulating role in the expansion of the SCS basin from the East Subbasin to the Southwest Subbasin. In addition, the low-velocity body in the north flank of the Southwest Subbasin extends from the post-spreading seamounts on the ocean crust to the uppermost mantle (i.e., about 10–30 km), which indicates an oblique magma migration during the post-spreading volcanism.

Tectono-magmatic evolution of the Greenland−Iceland−Norway ridge complex and the Jan Mayen hotspot in the Arctic Atlantic Ocean: Constraints from in situ trace elements and Sr isotopes of minerals

The subarctic North Atlantic Ocean has experienced a complex and gradual magmatic evolution, including continental breakup, mature seafloor spreading, and episodic ridge jumps. Here, we present new in situ major-element, trace-element, and Sr isotopic compositions for minerals (olivine, clinopyroxene, and plagioclase) from the Jan Mayen microcontinent (Integrated Ocean Drilling Program [IODP] Sites 907 and 985), Reykjanes Ridge (Site 983), Greenland Basin (Site 913), and Knipovich Ridge (Site 911) volcanic samples in the Arctic Atlantic Ocean. The Jan Mayen hotspot−type and Iceland plume−type volcanic rocks have a common magma source. The former were likely derived from the latter by further fractional crystallization, resulting in their distinct geochemical features. The in situ incompatible element and isotopic compositions show that the normal mid-ocean-ridge basalt spreading ridge feature for the older (>14 Ma) system and the enriched plume feature for the younger (1.521 Ma and 1.049 Ma) system likely indicate plume-ridge interaction and the evolution from mid-ocean-ridge−type to plume-type magmatism beneath the Norwegian−Greenland Sea. The disequilibrium of Sr isotopes in plagioclase among the groundmass, phenocryst cores, and rims is likely due to heterogeneous compositions beneath the North Atlantic Ocean basin, with the contamination of thick continental crust and the influence of H2O-enriched melts originating from the deep Iceland plume.

Spreading Magnetic Anomalies Separation of the South China Sea Based on the Nested Elliptical Directional Filters

Spreading magnetic anomalies recorded the paleo-geomagnetic field variation that has great significance in the investigation of the extension process of ocean basins. Interpreting spreading magnetic anomalies under complex geological environments is challenging, especially for marginal sea basins. We proposed nested elliptical directional filters to separate the spreading magnetic anomalies of the South China Sea (SCS). The results show that the spreading magnetic anomalies separated by the nested elliptical directional filters depict the expansion process of the oceanic crust, and the interference magnetic anomalies are effectively suppressed. The separated spreading magnetic anomalies indicate that the expansion process of the SCS is affected by the interactions between the surrounding plates. The spreading magnetic anomalies of the SCS are warped, interrupted, and not strictly parallel. The pattern of the spreading magnetic anomalies reflects multiple ridge jumps during the expansion process and the post-spreading magmatic disturbances. The long-wavelength magnetic anomalies indicate lithospheric fractures and Curie surface variations in the SCS, which are affected by the post-spreading magmatic rejuvenation. The magnetic anomalies of the SCS resulted from the superposition of magnetic anomalies in the ocean crust and the uppermost mantle.

Magnetic anomaly lineations in the Northeastern South China Sea and their implications for initial seafloor spreading

Ridge jumps occurred during the spreading process of the South China Sea. Recent research on the lower crustal reflectors event on seismic profiles found at least two southward ridge jumps, but only one has been confirmed by geomagnetic data. Based on magnetic anomalies in the northern South China Sea and the “two ridge jumps” model, this study identified that an early ridge jump occurred at anomaly C10n (28.3 Ma), with a southward jump of 20 km. According to the magnetic lineation distribution in the northern South China Sea, initial spreading was dominated by local punctiform break-up and the oldest anomaly, C12n (∼30.8 Ma), appeared at two turns of COB. In the IODP Expedition 367&368 drilling area, the continuity of magnetic anomalies from Ridge A to Ridge C was enhanced gradually, representing the transition from local magmatism before the final crustal break-up to a stable igneous oceanic crust. The earliest seafloor spreading magnetic lineation in the Northwestern Sub-basin is C12n (∼30.8 Ma) and the magnetic lineation corresponding to the fossil spreading ridge is C10r (∼29 Ma). The average half-spreading rate was ∼27.2 mm/yr. The opening of the Northwestern Sub-basin appears to have been rotated around a fixed point at the west, with a fault at the eastern end formed by the trajectory of the conjugate point moving during seafloor spreading.

Appraisal of gravity-based lineaments around Central Indian Ridge (CIR) in different geological periods: Evidence of frequent ridge jumps in the southern block of CIR

• Application of edge Enhancement to mark the lineaments over the CNIR. • Power spectrum analysis to differentiate long and short lineaments. • Comparison of spreading rate with length of lineaments. • Analysis of lineament orientation and plate movement in different geologic era. • Southern CNIR witnessed frequent ridge jumps for stability of the RTJ . This paper presents an approach for utilizing the balanced edge detection technique in satellite-derived gravity data to study the tectonic movement in past geological periods. The research is initiated by examining the normalized total horizontal derivative (NTHD) edge detection technique based on a moving window. NTHD is found to be effective for both shallow and deep-seated bodies. The noise in the gravity map could be effectively eliminated using a larger window size. The lineaments are marked over the Central Indian Ridge (CIR) using NTHD. Analysis of various geological periods and the lineaments shows that the speed and direction of the tectonic Plate play an essential role in deciding the length and orientation of lineaments formed. The most prominent lineament pattern observed over the CIR is in the NE-SW, ENE-WSW, and NNE-SSW, possibly associated with plate movement during Late Cretaceous to Late Eocene and after the Early Miocene period. In addition, a few lineaments are also found to be oriented in NW-SE, E-W, and N-S. The N-S lineaments might be the consequence of the movement of the Indian Plate in a northward direction for a short time during the Late Eocene and Early Miocene. The E-W lineaments result from the transfer of the Reunion hotspot from the Indian to the African Plate. The high lineament density and circular statistical values indicate left flank of the CIR undergoes rigorous tectonic activity. The high value of circular statistics indicates the chaotic arrangement of lineaments in the southern block of the CIR.

Fractal analysis of lineaments using CryoSat-2 and Jason-1 satellite-derived gravity data: Evidence of a uniform tectonic activity over the middle part of the Central Indian Ridge

The availability of high-accuracy altimetry and satellite gravity data leads to the development of high-resolution global gravity model data. It helps in understanding the structure and evolution of remote oceanic regions. The present study utilizes fractal analysis of structural patterns identified from the Sandwell (2014) gravity data to understand the geological implication over the Central Indian Ridge (CNIR). The normalized standard deviation (NSTD) edge detection technique has been used to identify different possible lineaments/structural patterns. Generally, the Northern, Middle, and Southern blocks of CNIR are characterized by ENE-WSW, NE-SW, NNE-SSW, and NW-SE lineaments. However, the Southern block is additionally characterized by few E-W-oriented lineaments. The lineament density map indicates Southern block exhibit a maximum lineament concentration than other two blocks. The fractal dimensions of lineaments in the Northern, Middle, and Southern blocks are in the order of 1.64, 1.627, and 1.666, respectively. The high fractal dimension and lineament density values of the Southern block indicate a concentrated distribution of lineaments. The low fractal dimension and lineament density values over the Middle block indicate a greater degree of uniformity in the lineament distribution than the other two blocks, representing a relatively stable tectonic setup over the Middle block. The high multifractal width of the Southern block is characterized by variable spreading rates and frequent ridge jumps. The multifractal studies demonstrate the presence of medium-small scale lineaments over the CNIR. The large numbers of small and discontinuous lineaments over the CNIR indicate strong but less frequent tectonic activity along the ridge axis.

Methodology for Evaluating the North Atlantic Igneous Province Reveals Multiple Hotspots at Breakup

AbstractThe North Atlantic Igneous Province (NAIP) has been the focus of numerous studies. However, a systematic integrated methodology to assess the NAIP, has so far been lacking. For this purpose, a synclinal oceanic rift approach is developed from the stratigraphic record. Mechanisms are addressed combining trace element and seismic velocity data as a proxy for excess mantle potential temperature (ΔTp), using simple mantle melting models. Unconformities play a major role in this assessment, providing evidence for a drop in ΔTp to ambient levels (≤50°C). Three phases can be delineated in the stratigraphic record: a (a) Pre‐; (b) Syn‐; and (c) Post‐breakup phase. The Syn‐breakup phase is bound at the base by the Breakup Unconformity and at the top by the Magmatic Unconformity. At breakup, magmatic crustal thickness (H) can be estimated from addition of basalt‐ and Lower Igneous Crust isopachs. These seismic estimations can be further constrained by observations on volcanic transport directions. This allows differentiation between hotspots and Volcanic Passive Margins (VPMs), where hotspots are characterized by continuous ΔTp amplitude variations, whereas VPMs only display declining or absent fluctuations post breakup. On Iceland and the Greenland‐Iceland‐Faroe Ridge, H is primary controlled by ΔTp fluctuations promoting ridge jumps and second, associated fluctuating active upwelling ratio (χ ∼ 2–4). Results suggest that there were at least three hotspots active at breakup. The West Greenland and proto‐Jan Mayen hotspots lost importance in the Oligocene, while Iceland remained and is currently the main hotspot in the North Atlantic.

Ridge Jumps and Mantle Exhumation in Back-Arc Basins

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

Ridge jump reorientation of the South China Sea revealed by high‐resolution magnetic data

AbstractWe 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.

Longest continuously erupting large igneous province driven by plume-ridge interaction

AbstractLarge igneous provinces (LIPs) typically form in one short pulse of ∼1–5 Ma or several punctuated ∼1–5 Ma pulses. Here, our 25 new 40Ar/39Ar plateau ages for the main construct of the Kerguelen LIP—the Cretaceous Southern and Central Kerguelen Plateau, Elan Bank, and Broken Ridge—show continuous volcanic activity from ca. 122 to 90 Ma, a long lifespan of >32 Ma. This suggests that the Kerguelen LIP records the longest, continuous high-magma-flux emplacement interval of any LIP. Distinct from both short-lived and multiple-pulsed LIPs, we propose that Kerguelen is a different type of LIP that formed through long-term interactions between a mantle plume and mid-ocean ridge, which is enabled by multiple ridge jumps, slow spreading, and migration of the ridge. Such processes allow the transport of magma products away from the eruption center and result in long-lived, continuous magmatic activity.

3‐D Analog Modeling Constraints on Rifting in the Afar Region

AbstractRifting in the Afar region is considered to be the only known example of the formation of an incipient divergent triple junction. Taking the Afar region as an example, we use three‐dimensional lithospheric‐scale laboratory experiments to constrain the factors controlling the structure and the tectonic evolution of the area. We systematically evaluate the role of different preexisting heterogeneities, such as those due to plume head impingement or inherited weak structures, and different kinematic boundary conditions, such as orthogonal versus rotational extension. The interaction between these inherited heterogeneities and the tested boundary conditions results in a range of complex rift geometries and structural features, such as rift segmentation and ridge jumps, which are comparable to those observed in the Afar region. At the plate scale, only under rotational extension, consecutive rift zones intersecting at high angles develop akin to the along‐axis segmentation in the Red Sea and the Gulf of Aden. Under orthogonal extension, the dominant rifting trend is perpendicular to the stretching direction. Preexisting weaknesses, including that due to a mantle plume, trigger different rifting styles and influence the geometry of large‐scale continental breakup. When compared to the Afar region, our results suggest that differing the degree of stretching, following the rotation of the Arabian plate since the Oligocene, led to the rifting in the Red Sea and the Gulf of Aden and likely did not cause the formation of the Main Ethiopian Rift.

Seismic Structure of a Postspreading Seamount Emplaced on the Fossil Spreading Center in the Southwest Subbasin of the South China Sea

AbstractPostspreading seamounts are unique seamounts built by volcanism after seafloor spreading had ceased. Limited understanding of their formation processes is mainly due to the lack of investigation. Therefore, we conducted a three‐dimensional wide‐angle seismic experiment in the Southwest Subbasin of the South China Sea and four profiles were acquired within the inactive spreading center over a postspreading seamount. Based on ocean bottom seismometer data, we report two‐dimensional velocity structures of the Longnan seamount. Our results show that a low P wave velocity body (2.9–5.0 km/s, 4 km thick) is observed within the seamount summit, suggesting that material has extensively erupted as volcaniclastic rocks and high‐porosity basalts. The extrusive/intrusive boundary is depressed beneath the seamount, indicating the absence of an intrusive core. Our results further show that the ratio of extrusive to intrusive volume is as high as 3.0 and that the accretion to the oceanic crust occurs in layer 2. Combined with an analysis of velocity structures of seamounts worldwide including seamounts in the South China Sea, we propose that the extrusive/intrusive ratio and type of crustal thickening are related to the magma supply mechanisms, and intrusive cores are more likely to be associated with a hot spot. The above analysis also suggests that the Longnan Seamount is a nonplume origin seamount and structurally different from other seamounts in the South China Sea. Ridge jumps in the South China Sea increase the duration or amount of volcanism, which could explain seamount structural differences.

Zircon xenocryst evidence for crustal recycling at the Mid-Atlantic Ridge

The presence of zircon xenocrysts in Mid Atlantic Ridge (MAR) rocks poses essential questions concerning the path they have followed from their sources, how they have survived transport through the mantle, and why they are much more abundant than in any other ocean. Sixteen out of twenty-five seafloor samples collected between S 21° to N 42° contained xenocrysts, here defined as those grains that coexist with a significantly younger population or are significantly older than the band of the Atlantic seafloor age from which they were collected. The xenocrysts are of two types, young and old. Hf and O isotopes and trace elements indicate that the younger crystals (3.1 Ma to 25.6 Ma) are oceanic, derived from previous MAR rocks, whereas the older crystals (190 Ma to 3.2 Ga) are continental, derived from crust slivers located at the MAR or immediate vicinity. The high ∂18O (~7‰ to 11‰) of many older xenocrysts indicates ultrafast recycling through the mantle; otherwise, the fast diffusivity of oxygen would have driven the xenocrysts ∂18O to that of their mantle hosts, ∂18O ≈ 5.3‰. The incorporation of xenocrysts into MAR magmas mainly occurs after ridge jumps, when hot mafic magma pervades a new crustal section scavenging xenoliths, or locally melts felsic wall-rocks and mixes with the resulting zircon-laden low-T magmas. In either case, the fusible component of the xenocrysts source contaminates the upwelling mafic magma. When the source was oceanic, the resulting chemical and isotopic signatures of the contaminated magma would remain MORB-like, albeit enriched in incompatible elements. When the source was continental, the resulting magma would reflect a perceptible continental influence. Crustal contamination at the ridge explains the abundance of zircon xenocrysts and the distinctive chemistry of the MAR magmas compared with other MORBs. These features are ultimately related to the Atlantic slow-spreading because it promotes ridge jumping and, hence, active crustal recycling.

Evolution of migrating transform faults in anisotropic oceanic crust: examples from Iceland

Major transform fault zones link extensional segments of the North American – Eurasian plate boundary as it transects the Iceland Hotspot. Changes in plate boundary geometry, involving ridge jumps, rift propagation, and related transform fault zone migration, have occurred as the boundary has moved relative to the hotspot. Reconfiguration of transform fault zones occurred at about 6 Ma in northern Iceland and began about 3 Ma in southern Iceland. These systems show a range of different types of transform fault zones, ranging from diffuse, oblique rift zones to narrower, well-defined, transform faults oriented parallel to current plate motions. Crustal deformation structures correlate with the inferred duration and magnitude of strike-slip displacements. Collectively, the different expressions of transform zones may represent different stages of development in an evolutionary sequence that may be relevant for understanding the tectonic history of plate boundaries in Iceland as well as the structure of transform fault zones on more typical parts of the mid-ocean ridge system.

Implications of Updated Magnetic Anomalies for the Late Cretaceous Tectonic Evolution of Walvis Ridge

AbstractIn the South Atlantic, a reorganization of the Mid‐Atlantic Ridge began before anomaly C34n (83.6 Ma) and ended before anomaly C30n (66.4 Ma), complicating tectonics of Rio Grande Rise and older Walvis Ridge (WR), which formed together at the Mid‐Atlantic Ridge. This reorganization is poorly understood because magnetic anomalies C30n‐C34n are poorly defined near WR. We interpreted these anomalies along western WR to improve knowledge of Rio Grande Rise‐WR tectonic development. Anomaly trends indicate that Valdivia Bank has an E‐W age progression, perpendicular to that predicted by hot spot models. Anomaly spacing and width is irregular and anomalous near WR, implying a series of ridge jumps and possibly a microplate between anomalies C34n and C32n. Eastward ridge jumps transferred microplate lithosphere to the South American plate. This study shows that Late Cretaceous tectonic evolution of the Rio Grande Rise‐WR large igneous provinces was more complex than previously understood.

Calcareous nannofossil changes in response to the spreading of the South China Sea basin during Eocene-Oligocene

Numerous models have been proposed to constrain the process of the South China Sea formation, yet the Paleogene paleoenvironmental researches are hindered by the paucity of long, continuous, well-dated geological records. In this work, Eocene to Oligocene nannofossil biostratigraphy and assemblage changes in a sediment section from the northern South China Sea were studied to shed light on these issues. We established an age model to confine the studied section between ~40 and 22 Ma based on 17 calcareous nannofossil datum levels. The age model enabled to date the most prominent seismic reflector T60 centered at ~26.8 Ma, a regional tectonic event interpreted as corresponding to the “Ridge Jump Event” in the early phase of South China Sea spreading. Changes in nannofossil abundance and assemblage compositions imply continuous transgression and deepening of water depth from the Late Eocene to Early Miocene. During the Late Eocene, the studied area was a neritic sea with relatively eutrophic conditions supporting abundant R. lockeri group and C. pelagicus. From ~34 Ma to ~27 Ma, during the Oligocene, the earlier neritic environment was replaced by relative warm and oligotrophic conditions in a shallow sea, as indicated by higher concentration of C. floridanus, Sphenolithus spp. and Helicosphaera spp. Since ~26 Ma, pelagic conditions prevailed, small Reticulofenestra (<5 μm) became dominant, and Discoaster spp. also increased in response to the deepened water depth.

Crustal thickness mapping of the central South Atlantic and the geodynamic development of the Rio Grande Rise and Walvis Ridge

Abstract The origin of the Rio Grande Rise and Walvis Ridge within the central South Atlantic and its implications for the separation of South America and Africa during the Cretaceous are controversial. The recent report of the discovery from submersible sampling of continental material of Proterozoic age on the Rio Grande Rise suggests that the existing explanation involving ocean ridge – mantle plume interaction or simply excess ‘on-ridge’ magmatism for the formation of the Rio Grande Rise and Walvis Ridge needs to be re-examined. We use gravity anomaly inversion to map crustal thickness for the central South Atlantic area encompassing the Rio Grande Rise, Walvis Ridge and adjacent South American and African rifted continental margins. We show that the Rio Grande Rise consists of three distinct bodies of anomalously thick crust (Western, Central and Eastern) separated by normal thickness oceanic crust. The Central Rio Grande Rise forms a large elliptical body with maximum crustal thickness of 25 km. The Walvis Ridge also has a maximum crustal thickness of 25 km but has a narrower and more linear geometry. We use plate reconstructions to restore maps of crustal thickness and magnetic anomaly to Early Cretaceous times to examine the development of the Rio Grande Rise and Walvis Ridge. These restorations together with ages of magmatic addition suggest that the Central Rio Grande Rise and Walvis Ridge formed a single body between 90 and 80 Ma located on the ocean ridge plate boundary similar to Iceland today. On the basis of crustal thickness mapping, the plate restorations and the magmatic ages, we propose that the Rio Grande Rise was fragmented into its 3 parts and separated from Walvis Ridge by at least 4 ocean ridge jumps during the opening of the South Atlantic Ocean between approximately 90 and 50 Ma. Plate reconstructions of crustal thickness showing rotated structural lineaments imply that the separation of Eastern Rio Grande Rise and Walvis Ridge was highly complex involving simultaneous crustal extension and magmatic addition. We propose that the continental material reported on the Rio Grande Rise, if not drop-stones, was isolated from the main continental land-mass and transported into the ocean by these ridge jumps during the Cretaceous formation of the South Atlantic.

Structures within the oceanic crust of the central South China Sea basin and their implications for oceanic accretionary processes

Internal structures in mature oceanic crust can elucidate understanding of the processes and mechanism of crustal accretion. In this study, we present two multi-channel seismic (MCS) transects across the northern flank of the South China Sea basin to reveal the internal structures related to Cenozoic tectono-magmatic processes during seafloor spreading. Bright reflectors within the oceanic crust, including the Moho, upper crustal reflectors, and lower crustal reflectors, are clearly imaged in these two transects. The Moho reflection displays varied character in continuity, shape and amplitude from the continental slope area to the abyssal basin, and becomes absent in the central part of the basin where abundant seamounts and seamount chains formed after the cessation of seafloor spreading. Dipping reflectors are distinct in most parts of the MCS data but generally confined to the lower crust above the Moho reflection. These lower crustal reflectors merge downward into the Moho without offsetting it, probably arising from shear zones between the crust and mantle characterized by interstitial melt, although we cannot exclude other possibilities such as brittle faulting or magmatic layering in the local area. A notable feature of these lower crustal reflector events is their opposite inclinations. We suggest the two groups of conjugate lower crustal reflector events observed between magnetic anomalies C11 and C8 were associated with two unusual accretionary processes arising from plate reorganizations with southward ridge jumps.