Magmatic processes and mantle heterogeneity beneath the slow-spreading northern Kolbeinsey Ridge segment,North Atlantic

We present new data on mineralogical, major and trace element compositions of lavas from the northernmost segment of the Kolbeinsey Ridge (North Kolbeinsey Ridge, NKR). The incompatible element enriched North Kolbeinsey basalts lie on a crystal fractionation trend which differs from that of the other Kolbeinsey segments, most likely due to higher water contents (~0.2%) in the NKR basalts. The most evolved NKR magmas erupt close to the Jan Mayen Fracture Zone, implying increased cooling and fractionation of the ascending magmas. Mainly incompatible element-enriched basalts, as well as some slightly depleted lavas, erupt on the NKR. They show evidence for mixing between different mantle sources and magma mixing. North Kolbeinsey Ridge magmas probably formed by similar degrees of melting to other Kolbeinsey basalts, implying that no lateral variation in mantle potential temperature occurs on the spreading axis north of the Iceland plume and that the Jan Mayen Fracture Zone does not have a cooling effect on the mantle. Residual garnet from deep melting in garnet peridotite or from enriched garnet pyroxenite veins does not play a role. The incompatible element-enriched source has high Ba/La and Nb/Zr, but must be depleted in iron. The iron-depleted mantle is less dense than surrounding mantle and leads to the formation of the North Kolbeinsey segment and its shallow bathymetry. The enriched NKR source formed from a relatively refractory mantle, enriched by a small degree melt rather than by recycling of enriched basaltic crust. The depleted mantle source resembles the mantle of the Middle Kolbeinsey segment with a depletion in incompatible elements, but a fertile major element composition.     

Rayleigh wave tomography in the North Atlantic: high resolution images of the Iceland, Azores and Eifel mantle plumes

Presented in this paper is a high resolution S_v-wave velocity and azimuthal anisotropy model for the upper mantle beneath the North Atlantic and surrounding region derived from the analysis of about 9000 fundamental and higher-mode Rayleigh waveforms. Much of the dataset comes from global and national digital seismic networks, but to improve the path coverage a number of instruments at coastal sites in northwest Europe, Iceland and eastern Greenland was deployed by us and a number of collaborators. The dense path coverage, the wide azimuthal distribution and the substantial higher-mode content of the dataset, as well as the relatively short path-lengths in the dataset have enabled us to build an upper mantle model with a horizontal resolution of a few hundred kilometers extending to 400 km depth. Low upper mantle velocities exist beneath three major hotspots: Iceland, the Azores and Eifel. The best depth resolution in the model occurs in NW Europe and in this area low S_v-velocities in the vicinity of the Eifel hotspot extend to about 400 km depth. Major negative velocity anomalies exist in the North Atlantic upper mantle beneath both Iceland and the Azores hotspots. Both anomalies are, above 200 km depth, 4–7% slow with respect to PREM and elongated along the mid-Atlantic Ridge. Low velocities extend to the south of Iceland beneath the Reykjanes Ridge where other geophysical and geochemical observations indicate the presence of hot plume material. The low velocities also extend beneath the Kolbeinsey Ridge north of Iceland, where there is also supporting geochemical evidence for the presence of hot plume material. The low-velocity upper mantle beneath the Kolbeinsey Ridge may also be associated with a plume beneath Jan Mayen. The anomaly associated with the Azores extends from about 25°N to 45°N along the ridge axis, which is in agreement with the area influenced by the Azores Plume, predicted from geophysical and geochemical observations. Compared to the anomaly associated with Iceland, the Azores anomaly is elongated further along the ridge, is shallower and decays more rapidly with depth. The fast propagation direction of horizontally propagating S_v-waves in the Atlantic south of Iceland correlates well with the east–west ridge-spreading direction at all depths and changes to a direction close to NS in the vicinity of Iceland.     

Petrology of basalts from the Mohns-Knipovich Ridge; the Norwegian-Greenland Sea

Major element compositions of submarine basalts, quenched glasses, and contained phenocrysts are reported for samples from 25 dredge stations along the Mohns-Knipovich Ridge between the Jan Mayen fracture zone and 77°30N. Most of the basalts collected on the Jan Mayen platform have a subaerial appearance, are nepheline normative, rich in incompatible elements, and have REE-patterns strongly enriched in light-REE. The other basalts (with one exception) are tholeiitic pillow basalts, many of which have fresh quenched glass rims. From the Jan Mayen platform northeastwards the phenocryst assemblage changes from olivine±plagioclase±clinopyroxene±magnetite to olivine +plagioclase±chrome-spinel. This change is accompanied by a progressive decrease in the content of incompatible elements, light-REE enrichments and elevation of the ridge that are similar to those observed south of the Azores and Iceland hotspots. Pillow basalts and glasses collected along the esternmost part of the Mohns Ridge (450 to 675 km east of Jan Mayen) have low K₂O, TiO₂, and P₂O₅ contents, light-REE depleted patterns relative to chondrites, and Mg/(Mg+Fe²⁺) ratios between 0.64 and 0.60. Pillow basalts and glasses from the Knipovich Ridge have similar (Mg/Mg+Fe²⁺) ratios, but along the entire ridge have slightly higher concentrations of incompatible elements and chondritic to slightly light-REE enriched patterns. The incompatible element enrichment increases slightly northward. Plagioclase phenocrysts show normal and reverse zoning on all parts of the ridge whereas olivines are unzoned or show only weak normal zoning. Olivine-liquid equilibrium temperatures are calculated to be in the range of 1,060–1,206° C with a mean around 1,180° C.Rocks and glasses collected on the Jan Mayen Platform are compositionally similar to Jan Mayen volcanic products, suggesting that off-ridge alkali volcanism on the Jan Mayen Platform is more widespread than so far suspected. There is also evidence to suggest that the alkali basalts from the Jan Mayen Platform are derived from deeper levels and by smaller degrees of partial melting of a mantle significantly more enriched in light-REE and other incompatible elements than are the tholeiitic basalts from the Eastern Mohns and Knipovich Ridge. The possibility of the presence of another hitherto unsuspected enriched mantle region north of 77° 30 N is also briefly considered.It remains uncertain whether geochemical gradients revealed in this study reflect: (1) the dynamics of mixing during mantle advection and magma emplacement into the crust along the Mid-Atlantic Ridge (MAR) spreading axis, (e.g. such as in the mantle plume — large-ion-lithophile element depleted asthenosphere mixing model previously proposed); or (2) a horizontal gradation of the mantle beneath the MAR axis similar to that observed in the overlying crust; or (3) a vertical gradation of the mantle in incompatible elements with their contents increasing with depth and derivations of melts from progressively greater depth towards the Jan Mayen Platform.     

Geodynamic evolution of crust accretion at the axis of the Reykjanes Ridge,Atlantic Ocean

The results of analysis of the anomalous magnetic field of the Reykjanes Ridge and the adjacent basins are presented, including a new series of detailed reconstructions for magnetic anomalies 1–6 in combination with a summary of the previous geological and geophysical investigations. We furnish evidence for three stages of evolution of the Reykjanes Ridge, each characterized by a special regime of crustal accretion related to the effect of the Iceland hotspot. The time interval of each stage and the causes of the variation in the accretion regime are considered. During the first, Eocene stage (54–40 Ma) and the third, Miocene-Holocene stage (24 Ma-present time at the northern Reykjanes Ridge north of 59° N and 17–11 Ma-present time at the southern Reykjanes Ridge south of 59° N), the spreading axis of the Reykjanes Ridge resembled the present-day configuration, without segmentation, with oblique orientation relative to the direction of ocean floor opening (at the third stage), and directed toward the hotspot. These attributes are consistent with a model that assumes asthenospheric flow from the hotspot toward the ridge axis. Decompression beneath the spreading axis facilitates this flow. Thus, the crustal accretion during the first and the third stages was markedly affected by interaction of the spreading axis with the hotspot. During the second, late Eocene-Oligocene to early Miocene stage (40–24 Ma at the northern Reykjanes Ridge and 40 to 17–11 Ma at the southern Reykjanes Ridge), the ridge axis was broken by numerous transform fracture zones and nontransform offsets into segments 30–80 km long, which were oriented orthogonal to the direction of ocean floor opening, as is typical of many slow-spreading ridges. The plate-tectonic reconstructions of the oceanic floor accommodating magnetic anomalies of the second stage testify to recurrent rearrangements of the ridge axis geometry related to changing kinematics of the adjacent plates. The obvious contrast in the mode of crustal accretion during the second stage in comparison with the first and the third stages is interpreted as evidence for the decreasing effect of the Iceland hotspot on the Reykjanes Ridge, or the complete cessation of this effect. The detailed geochronology of magnetic anomalies 1–6 (from 20 Ma to present) has allowed us to depict with a high accuracy the isochrons of the oceanic bottom spaced at 1 Ma. The variable effect of the hotspot on the accretion of oceanic crust along the axes of the Reykjanes Ridge and the Kolbeinsey and Mid-Atlantic ridges adjoining the former in the north and the south was estimated from the changing obliquity of spreading. The spreading rate tends to increase with reinforcing of the effect of the Iceland hotspot on the Reykjanes Ridge.     

Asymmetric crustal structure of the ultraslow-spreading Mohns Ridge

ABSTRACT

New analysis of the geophysical data of the ultraslow-spreading Mohns Ridge and its off-axis structure reveals a distinctive asymmetric structure. We calculate residual bathymetry (RB) and residual mantle Bouguer gravity anomaly (RMBA) and decompose the anomalies into symmetric and asymmetric components between the ridge conjugates. The western flank of the Mohns Ridge at crustal age of ~50–15 Ma is characterized by a broad zone of elevated RB and more negative RMBA, which we term the Vesteris Plateau (VP). The VP anomaly has a surface area of ~1.12 × 10⁵ km² and an excess crust volume of ~2.33 × 10⁵ km³, making it a significant anomaly comparable to other anomalies such as the Bermuda Rise. Extending north of the Kolbeinsey Ridge for more than 500 km, the VP lies above an anomalous upper mantle region of low shear-wave seismic velocity, indicating that the VP might represent the northernmost reach of the Iceland-Jan Mayen mantle anomaly. In addition, the western ridge flank of the Mohns Ridge at crustal age of 6–0 Ma is associated with higher RB and more positive RMBA relative to the eastern conjugate, indicating tectonic uplift and associated exposure of lower crust and upper mantle near the ridge axis.     

P-wave velocity structure of the margin of the southeastern Tsushima Basin in the Japan Sea using ocean bottom seismometers and airguns

The Tsushima Basin is located in the southwestern Japan Sea, which is a back-arc basin in the northwestern Pacific. Although some geophysical surveys had been conducted to investigate the formation process of the Tsushima Basin, it remains unclear. In 2000, to clarify the formation process of the Tsushima Basin, the seismic velocity structure survey with ocean bottom seismometers and airguns was carried out at the southeastern Tsushima Basin and its margin, which are presumed to be the transition zone of the crustal structure of the southwestern Japan Island Arc. The crustal thickness under the southeastern Tsushima Basin is about 17 km including a 5 km thick sedimentary layer, and 20 km including a 1.5 km thick sedimentary layer under its margin. The whole crustal thickness and thickness of the upper part of the crust increase towards the southwestern Japan Island Arc. On the other hand, thickness of the lower part of the crust seems more uniform than that of the upper part. The crust in the southeastern Tsushima Basin has about 6 km/s layer with the large velocity gradient. Shallow structures of the continental bank show that the accumulation of the sediments started from lower Miocene in the southeastern Tsushima Basin. The crustal structure in southeastern Tsushima Basin is not the oceanic crust, which is formed ocean floor spreading or affected by mantle plume, but the rifted/extended island arc crust because magnitudes of the whole crustal and the upper part of the crustal thickening are larger than that of the lower part of the crustal thickening towards the southwestern Japan Island Arc. In the margin of the southeastern Tsushima Basin, high velocity material does not exist in the lowermost crust. For that reason, the margin is inferred to be a non-volcanic rifted margin. The asymmetric structure in the both margins of the southeastern and Korean Peninsula of the Tsushima Basin indicates that the formation process of the Tsushima Basin may be simple shear style rather than pure shear style.     

Typification of the Earth’s crust of Central Arctic Uplifts in the Arctic Ocean

The modern views on the structure of the oceanic and continental crust are discussed. The presented geological-geophysical information on the deep structure of the Earth’s crust of the Lomonosov Ridge, Mendeleev Rise, and Alpha Ridge, which make up the province of the Central Arctic Uplifts in the Arctic Ocean, is based on CMP, seismic-reflection, and seismic-refraction data obtained by Russian and Western researchers along geotraverses across the Amerasia Basin. It is established that the crust thickness beneath the Central Arctic Uplifts ranges from 22 to 40 km. Comparison of the obtained velocity sections with standard crust sections of different morphostructures in the World Ocean that are underlain by the typical oceanic crust demonstrates their difference with respect to the crustal structure and to the thickness of the entire crust and its individual layers. Within the continental crust, the supercritical waves reflected from the upper mantle surface play the dominant role. Their amplitude exceeds that of head and refracted waves by one to two orders of magnitude. In contrast, the refracted and, probably, interferential head waves are dominant within the oceanic crust. The Moho discontinuity is the only first-order boundary. In the consolidated oceanic crust, such boundaries are not known. The similarity in the velocity characteristics of the crust of the Alpha Ridge and Mendeleev Rise, on the one hand, and the continental crust beneath the Lomonosov Ridge, on the other, gives grounds to state that the crust of the Mendeleev Rise and Alpha Ridge belongs to the continental type. The interference mosaic pattern of the anomalous magnetic field of the Central Arctic Uplifts is an additional argument in favor of this statement. Such patterns are typical of the continental crust with intense intraplate volcanism. Interpretation of seismic crustal sections of the Central Arctic Uplifts and their comparison with allowance for characteristic features of the continental and oceanic crust indicate that the Earth’s crust of the uplifts has the continental structure.     

Crustal structures of the Canada basin near Alaska, the Lomonosov ridge and adjoining basins near the North Pole

Seismic refraction surveys conducted in 1976 and 1979 over the broken ice surface of the Arctic Ocean, reveal distinctly different crustal structures for the Fram, Makarov and Canada basins. The Canada Basin, characterized by a 2–4 km thick sedimentary layer and a distinct oceanic layer 3B of 7.5 km/s velocity has the thickest crust and is undoubtedly the oldest of the three. The crust of the Makarov Basin has a thin sedimentary layer of less than 1 km and is about 9 km in total thickness. The Fram Basin has a similarly thin sedimentary layer but is 3–4 km thicker than the Makarov as it approaches the Lomonosov Ridge near the North Pole. The ridge itself is cored by material with a velocity of 6.6 km/s and may be a metagabbro similar to oceanic layer 3A. This ridge root material extends to a depth of about 27 km, where a change occurs to upper-mantle material with a velocity of 8.3 km/s. The core is overlain by up to 6 km of material with a velocity of about 4.7 km/s which could be oceanic layer 2A basalts or continental crystalline rocks with some sedimentary material.The Fram Basin probably began to open contemporaneously with the North Atlantic about 70 m.y. ago, by spreading along the Nansen-Gakkel Ridge. Although not yet dated, the Makarov Basin is probably no older than the initiation of the Fram Basin and may be much younger. The Alpha Ridge may once have been part of the Lomonosov Ridge, splitting off to form the Makarov Basin between 70 and 25 m.y. ago and possibly contributing to the Eurekan Orogeny of 25 m.y. ago, evident on Ellesmere Island. In contrast, the likely age of the Canada Basin lies in the 125–190 m.y. range and may have been formed by the counter-clockwise rotation of Alaska and the Northwind Ridge away from the Canadian Arctic Islands. The Lomonosov Ridge emerges from this scenario as a block resulting from a strike-slip shear zone on the European continental shelf, related to the opening of the Canada basin (180-120 my) and then becomes an entity broken from this shelf by the opening of the Eurasia Basin (70-0 m.y.).     

Tectonics and magmatism of ultraslow spreading ridges

The tectonics, structure-forming processes, and magmatism in rift zones of ultraslow spreading ridges are exemplified in the Reykjanes, Kolbeinsey, Mohns, Knipovich, Gakkel, and Southwest Indian ridges. The thermal state of the mantle, the thickness of the brittle lithospheric layer, and spreading obliquety are the most important factors that control the structural pattern of rift zones. For the Reykjanes and Kolbeinsey ridges, the following are crucial factors: variations in the crust thickness; relationships between the thicknesses of its brittle and ductile layers; width of the rift zone; increase in intensity of magma supply approaching the Iceland thermal anomaly; and spreading obliquety. For the Knipovich Ridge, these are its localization in the transitional zone between the Gakkel and Mohns ridges under conditions of shear and tensile stresses and multiple rearrangements of spreading; nonorthogonal spreading; and structural and compositional barrier of thick continental lithosphere at the Barents Sea shelf and Spitsbergen. The Mohns Ridge is characterized by oblique spreading under conditions of a thick cold lithosphere and narrow stable rift zone. The Gakkel and the Southwest Indian ridges are distinguished by the lowest spreading rate under the settings of the along-strike variations in heating of the mantle and of a variable spreading geometry. The intensity of endogenic structure-forming varies along the strike of the ridges. In addition to the prevalence of tectonic factors in the formation of the topography, magmatism and metamorphism locally play an important role.     

Primitive off-rift basalts from Iceland and Jan Mayen: Os-isotopic evidence for a mantle source containing enriched subcontinental lithosphere

New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The ¹⁸⁷Os/¹⁸⁸Os ratios in lavas with <30 ppt Os (n = 4) are elevated compared to ratios in coexisting olivine and appear to be contaminated at a shallow level. The ¹⁸⁷Os/¹⁸⁸Os ratios in the remaining lavas with >30 ppt Os (n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with ¹⁸⁷Os/¹⁸⁸Os of 0.1296). These low ¹⁸⁷Os/¹⁸⁸Os preclude significant shallow-level contamination from oceanic crust. The ¹⁸⁷Os/¹⁸⁸Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between ¹⁴³Nd/¹⁴⁴Nd and ¹⁸⁷Os/¹⁸⁸Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re-Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ³He/⁴He and ¹⁸⁷Os/¹⁸⁸Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic ¹⁸⁷Os/¹⁸⁸Os and ¹⁴³Nd/¹⁴⁴Nd, low ³He/⁴He and high ⁸⁷Sr/⁸⁶Sr, is represented by subcontinental lithospheric mantle stranded and disseminated in the upper mantle during the opening of the Atlantic Ocean. The second end-member corresponds to a hybrid mixture between the depleted-MORB mantle and the enriched Iceland mantle plume, itself resulting from mixing between recycled oceanic crust and depleted lower mantle. This hybrid accounts for the high ³He/⁴He (∼28 Ra), high ¹⁴³Nd/¹⁴⁴Nd (∼0.5132), high ¹⁸⁷Os/¹⁸⁸Os (∼0.14) and low ⁸⁷Sr/⁸⁶Sr (∼0.7026) composition observed in Iceland. Two different models may account for these observed mixing relationships between the end-members. In this first model, the Iceland mantle entrains pristine depleted material when rising in the upper mantle and allows refractory sub-lithospheric fragments to melt because of excess heat derived from the deep plume material. A second model that may better account for the Pb isotopic variations observed, uses the same components but where the depleted-MORB mantle is already polluted by subcontinental lithospheric mantle material before mixing with the Iceland mantle plume. Both cases likely occur. Though only three principal components are required to explain the isotopic variations of the Iceland-Jan Mayen system, the different possible mixing relationships may be accounted for by potentially a greater number of end-members.     

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data: evidence for rifting affected by the hotter than normal mantle

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.     

Paleocene on-spreading-axis hotspot volcanism along the Ninetyeast Ridge: An interaction between the Kerguelen hotspot and the Wharton spreading center

Investigations of three plausible tectonic settings of the Kerguelen hotspot relative to the Wharton spreading center evoke the on-spreading-axis hotspot volcanism of Paleocene (60-54 Ma) age along the Ninetyeast Ridge. The hypothesis is consistent with magnetic lineations and abandoned spreading centers of the eastern Indian Ocean and seismic structure and radiometric dates of the Ninetyeast Ridge. Furthermore, it is supported by the occurrence of oceanic andesites at Deep Sea Drilling Project (DSDP) Site 214, isotopically heterogeneous basalts at Ocean Drilling Program (ODP) Site 757 of approximately the same age (59-58 Ma) at both sites. Intermix basalts generated by plume-mid-ocean ridge (MOR) interaction, exist between 11° and 17°S along the Ninetyeast Ridge. A comparison of age profile along the Ninetyeast Ridge between ODP Sites 758 (82 Ma) and 756 (43 Ma) with similarly aged oceanic crust in the Central Indian Basin and Wharton Basin reveals the existence of extra oceanic crust spanning 11° latitude beneath the Ninetyeast Ridge. The extra crust is attributed to the transfer of lithospheric blocks from the Antarctic plate to the Indian plate through a series of southward ridge jumps at about 65, 54 and 42 Ma. Emplacement of volcanic rocks on the extra crust resulted from rapid northward motion (absolute) of the Indian plate. The Ninetyeast Ridge was originated when the spreading centers of the Wharton Ridge were absolutely moving northward with respect to a relatively stationary Kerguelen hotspot with multiple southward ridge jumps. In the process, the spreading center coincided with the Kerguelen hotspot and took place on-spreading-axis volcanism along the Ninetyeast Ridge.     

Lateral and vertical variability in crustal velocity structure in a small area of the North Atlantic

The detailed velocity structure of a 70 by 35 km area of 6–10 Ma old crust on the flank of the mid Atlantic Ridge at 24°N was studied using 358 explosive charges and several hundred 16.4-1 airgun shots fired into an array of eight ocean bottom hydrophones. Inversion of the first arrival refracted travel times shows that the crust comprises a normal oceanic section about 5 km thick with a steep velocity gradient in the upper crust increasing from about 3.5 km/sec at the seafloor overlying a typical oceanic layer 3 and a probably anisotropic mantle. Delay time function mapping using two datasets containing arrivals from layer 3 and from the mantle show that lateral variability is generally low over most of the survey area, with a small region of high delay times in the northwest corner caused by the presence of abnormal crust probably associated with a minor fracture zone. We find that the topography of the base of layer 2 is similar to that of the top, indicating that the normal faulting which occurs along the margins of the median valley extends down at least into layer 3. Our observations from mantle arrivals are consistent with a much flatter Moho which constrains possible models of crustal formation at the spreading centre.     

Geochemistry of lavas from Mohns Ridge,Norwegian-Greenland Sea: implications for melting conditions and magma sources near Jan Mayen

 Mohns Ridge lavas between 71 and 72°30′N (∼360 km) have heterogeneous compositions varying between alkali basalts and incompatible-element-depleted tholeiites. On a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island Jan Mayen and Mohns Ridge tholeiites. The variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts (∼5%) close to Jan Mayen but also shows its signature much further north on Mohns Ridge. Three lava types with different incompatible element compositions [e.g. chondrite-normalized (La/Sm)_N<1 to >2] occur in the area at 72°N and were generated from this heterogeneous mantle. The relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source. The elements Ba, Rb and K of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite. That such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures, at odds with a hotspot origin for Jan Mayen. Instead we suggest that the melting may be induced by the lowered solidus temperature of a “wet” mantle. Mohns MORB (mid ocean ridge basalt) and Jan Mayen area alkali basalts have high contents of Ba and Rb compared to other incompatible elements (e.g. Ba/La >10). These ratios reflect the signature of the mantle source. Ratios of Ce/Pb and Rb/Cs are normal MORB mantle ratios of 25 and 80, respectively, thus the enrichments of Ba and Rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid, as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts. Geophysical and petrological models suggest that Jan Mayen is not the product of hotspot activity above a mantle plume, and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment, a fracture zone and a spreading axis in this part of the North Atlantic. Received: 3 May 1995 / Accepted: 6 November 1995     

扬马延海脊构造-沉积演化及油气潜力

扬马延海脊位于北大西洋的北极圈附近,东格陵兰板块和挪威板块之间,冰岛东北方向。北极地区地域辽阔,油气资源丰富,但是恶劣的环境一直制约油气的勘探进展。在扬马延海脊的沉积演化过程中,扬马延海脊在第三纪前有着和东格陵兰陆架、挪威陆架相似的沉积序列,其构造演化经历了二叠纪陆内裂谷、三叠纪—侏罗纪同裂谷和微陆块漂移、白垩纪至今热沉降和被动陆缘等3个阶段。结合前人研究成果,对搜集的东格陵兰陆架、挪威陆架的油气地质资料分析,认为扬马延海脊可划分为扬马延盆地、扬马延西部构造带、扬马延中部凸起带、扬马延海槽、扬马延东部斜坡、扬马延南部复杂构造带6个构造单元,在其上发育着2套油气系统。同时扬马延海脊发育有伸展构造圈闭、地垒断块圈闭、构造圈闭和地层圈闭,这些圈闭为油气的赋存提供了良好的环境,也有利于划分有利油气勘探区带。研究结果可为进一步分析扬马延海脊构造特征等方面提供基础信息,同时对我国参与研究开发北极油气资源具有重大意义。     

Tectonics and types of riftogenic basins of the Scotia Sea,South Atlantic

Western, central, and eastern provinces are recognized in the Scotia Sea. They are distinguished by their bottom topography, geophysical characteristics, and crustal structure, which record their different origin and evolution. The western province is characterized by the oceanic crust that formed on the West Scotia Ridge, where active spreading may have ceased as a result of a collision between propagating rift and the structural barrier of the thick continental lithosphere of the Falkland Plateau. The central province is a series of blocks mainly composed of continental crust that subsided to various depths depending on the degree of extension in the course of rifting. These blocks are separated by local areas with oceanic crust formed due to the breakup of the continental crust and diffusive spreading. These areas are characterized by deep bottom and high values of Bouguer anomalies. The southern framework of the central province consists of subsided continental blocks and microcontinents divided by small spreading-type basins formed by lithospheric extension complicated by strike-slip faulting. The eastern province is composed of oceanic crust formed on the backarc spreading East Scotia Ridge. The results of density analysis, analog, and numerical simulations allowed us to explain some features of the structure and evolution of these provinces. The insight into tectonic structure of the provinces and their evolution allowed us to recognize several types of riftogenic basins differing in geodynamics, age, and geological and geophysical characteristics.     

Initiation of transform faults at rifted continental margins: 3D petrological-thermomechanical modeling and comparison to the Woodlark Basin

This work presents high-resolution 3D numerical model of transform fault initiation at rifted continental margins. Our petrological-thermomechanical visco-plastic model allows for spontaneous nucleation of oceanic spreading process in a continental rift zone and takes into account new oceanic crust growth driven by decompression melting of the asthenospheric mantle. Numerical model predicts that ridge-transform spreading pattern initiate in several subsequent stages: crustal rifting (0–1.5 Myr), spreading centers nucleation and propagation (1.5–3 Myr), proto-transform fault initiation and rotation (3–5 Myr) and mature ridge-transform spreading (> 5 Myr). Comparison of modeling results with the natural data from the Woodlark Basin suggests that the development of this region closely matches numerical predictions. Similarly to the model, the Moresby (proto-) Transform terminates in the oceanic rather than in the continental crust. This fault associates with a notable topographic depression and formed within 0.5–2 Myr while linking two offset overlapping spreading segments. Model reproduces well characteristic “rounded” contours of the spreading centers as well as the presence of a remnant of the broken continental crustal bridge observed in the Woodlark Basin. Proto-transform fault traces and truncated tip of one spreading center present in the model are also documented in nature. Numerical results are in good agreement with the concept of Taylor et al. (2009) which suggests that spreading segments nucleate en echelon in overlapping rift basins and that transform faults develop as or after spreading nucleates. Our experiments also allow to refine this concept in that (proto)-transform faults may also initiate as oblique rather than only spreading-parallel tectonic features. Subsequent rotation of these faults toward the extension-parallel direction is governed by space accommodation during continued oceanic crust growth within offset ridge-transform intersections.     

Plume-Ridge Interaction: a Geochemical Perspective from the Reykjanes Ridge

Plume–ridge interaction in the Reykjanes Ridge and Icelandregion is graphically demonstrated by several V-shaped ridgessurrounding the spreading axis, indicating mantle flow awayfrom Iceland. It also has significant geochemical effects. Regionally,incompatible element concentrations increase northwards coincidingwith decreasing depth and increasing crustal thickness, depthof melting and proximity to Iceland. Major and trace elementdata show that isolated magma bodies feed individual volcanicsystems along the ridge. Fractionation within these systemsincreases towards 60–61°N, where it coincides withthe intersection of a V-shaped ridge, thicker crust and moreabundant seamounts. Trace element, Nd and Sr isotopic data revealdynamic melting and mixing within a southward-thinning, heterogeneousmantle wedge beneath the Reykjanes Ridge. Melting is variableand locally enhanced at 58°N, 59°N, 60°N and 61°N.A total of six mantle components are identified. Some are specificto Iceland whereas others are found only beneath the ridge axis.The geographical distribution of these components reflects theirorigin within the deep upper and lower mantle and subsequenttranslation by plume outflow along the entire length of theridge. KEY WORDS: plume–ridge interaction; Iceland; Reykjanes Ridge; dynamic mantle mixing and melting     

Crustal Structure across the Northwestern Margin of South China Sea: Evidence for Magma‐poor Rifting from a Wide‐angle Seismic Profile

We present results from a 484 km wide-angle seismic profile acquired in the northwest part of the South China Sea (SCS) during OBS2006 cruise. The line that runs along a previously acquired multi-channel seismic line (SO49-18) crosses the continental slope of the northern margin, the Northwest Subbasin (NWSB) of the South China Sea, the Zhongsha Massif and partly the oceanic basin of the South China Sea. Seismic sections recorded on 13 ocean-bottom seismometers were used to identify refracted phases from the crustal layer and also reflected phases from the crust-mantle boundary (Moho). Inversion of the traveltimes using a simple start model reveals crustal images in the study area. The velocity model shows that crustal thickness below the continental slope is between 14 and 23 km. The continental part of the line is characterized by gentle landward mantle uplift and an abrupt oceanward one. The velocities in the lower crust do not exceed 6.9 km/s. With the new data we can exclude a high-velocity lower crustal body (velocities above 7.0 km/s) at the location of the line. We conclude that this part of the South China Sea margin developed by a magma-poor rifting. Both, the NWSB and the Southwest Sub-basin (SWSB) reveal velocities typical for oceanic crust with crustal thickness between 5 and 7 km. The Zhongsha Massif in between is extremely stretched with only 6–10 km continental crust left. Crustal velocity is below 6.5 km/s; possibly indicating the absence of the lower crust. Multi-channel seismic profile shows that the Yitongansha Uplift in the slope area and the Zhongsha Massif are only mildly deformed. We considered them as rigid continent blocks which acted as rift shoulders of the main rift subsequently resulting in the formation of the Northwest Sub-basin. The extension was mainly accommodated by a ductile lower crustal flows, which might have been extremely attenuated and flow into the oceanic basin during the spreading stage. We compared the crustal structures along the northern margin and found an east-west thicken trend of the crust below the continent slope. This might be contributed by the east-west sea-floor spreading along the continental margin.     

Transect across the West Antarctic rift system in the Ross Sea,Antarctica

In 1994, the ACRUP (Antarctic Crustal Profile) project recorded a 670-km-long geophysical transect across the southern Ross Sea to study the velocity and density structure of the crust and uppermost mantle of the West Antarctic rift system. Ray-trace modeling of P- and S-waves recorded on 47 ocean bottom seismograph (OBS) records, with strong seismic arrivals from airgun shots to distances of up to 120 km, show that crustal velocities and geometries vary significantly along the transect. The three major sedimentary basins (early-rift grabens), the Victoria Land Basin, the Central Trough and the Eastern Basin are underlain by highly extended crust and shallow mantle (minimum depth of about 16 km). Beneath the adjacent basement highs, Coulman High and Central High, Moho deepens, and lies at a depth of 21 and 24 km, respectively. Crustal layers have P-wave velocities that range from 5.8 to 7.0 km/s and S-wave velocities from 3.6 to 4.2 km/s. A distinct reflection (PiP) is observed on numerous OBS from an intra-crustal boundary between the upper and lower crust at a depth of about 10 to 12 km. Local zones of high velocities and inferred high densities are observed and modeled in the crust under the axes of the three major sedimentary basins. These zones, which are also marked by positive gravity anomalies, may be places where mafic dikes and sills pervade the crust. We postulate that there has been differential crustal extension across the West Antarctic rift system, with greatest extension beneath the early-rift grabens. The large amount of crustal stretching below the major rift basins may reflect the existence of deep crustal suture zones which initiated in an early stage of the rifting, defined areas of crustal weakness and thereby enhanced stress focussing followed by intense crustal thinning in these areas. The ACRUP data are consistent with the prior concept that most extension and basin down-faulting occurred in the Ross Sea during late Mesozoic time, with relatively small extension, concentrated in the western half of the Ross Sea, during Cenozoic time.