Gravity anomalies of the ridge-transform system in the South Atlantic between 31 and 34.5° S: Upwelling centers and variations in crustal thickness

To decipher the distribution of mass anomalies near the earth's surface and their relation to the major tectonic elements of a spreading plate boundary, we have analyzed shipboard gravity data in the vicinity of the southern Mid-Atlantic Ridge at 31–34.5° S. The area of study covers six ridge segments, two major transforms, the Cox and Meteor, and three small offsets or discordant zones. One of these small offsets is an elongate, deep basin at 33.5° S that strikes at about 45° to the adjoining ridge axes.By subtracting from the free-air anomaly the three-dimensional (3-D) effects of the seafloor topography and Moho relief, assuming constant densities of the crust and mantle and constant crustal thickness, we generate the mantle Bouguer anomaly. The mantle Bouguer anomaly is caused by variations in crustal thickness and the temperature and density structure of the mantle. By subtracting from the mantle Bouguer anomaly the effects of the density variations due to the 3-D thermal structure predicted by a simple model of passive flow in the mantle, we calculate the residual gravity anomalies. We interpret residual gravity anomalies in terms of anomalous crustal thickness variations and/or mantle thermal structures that are not considered in the forward model. As inferred from the residual map, the deep, major fracture zone valleys and the median, rift valleys are not isostatically compensated by thin crust. Thin crust may be associated with the broad, inactive segment of the Meteor fracture zone but is not clearly detected in the narrow, active transform zone. On the other hand, the presence of high residual anomalies along the relict trace of the oblique offset at 33.5° S suggests that thin crust may have been generated at an oblique spreading center which has experienced a restricted magma supply. The two smaller offsets at 31.3° S and 32.5° S also show residual anomalies suggesting thin crust but the anomalies are less pronounced than that at the 33.5° S oblique offset. There is a distinct, circular-shaped mantle Bouguer low centered on the shallowest portion of the ridge segment at about 33° S, which may represent upwelling in the form of a mantle plume beneath this ridge, or the progressive, along-axis crustal thinning caused by a centered, localized magma supply zone. Both mantle Bouguer and residual anomalies show a distinct, local low to the west of the ridge south of the 33.5° S oblique offset and relatively high values at and to the east of this ridge segment. We interpret this pattern as an indication that the upwelling center in the mantle for this ridge is off-axis to the west of the ridge.     

Results from a detailed aeromagnetic survey across the northeast Newfoundland margin,Part I: Spreading anomalies and relationship between magnetic anomalies and the ocean-continent boundary

A detailed aeromagnetic survey carried out across the northeast Newfoundland margin clearly shows the presence of sea floor spreading anomalies 25 to 34. Correlation of these anomalies with synthetic profiles shows an increase in the rate of spreading soon after anomaly 27 time. Three fracture zones can be identified by dislocations in the magnetic anomalies; their positions are confirmed on the depth to basement map of this region. An eastward extension of the southernmost fracture zone at latitude 49 N matches well with the Faraday Fracture Zone across the Mid Atlantic Ridge, and with a basement ridge known as Pastouret Ridge mapped off Goban Spur. By combining the present survey data with the previously collected shipborne measurements, we have also traced the westward continuation of the Charlie-Gibbs Fracture Zone under the Newfoundland shelf.A large amplitude magnetic anomaly lies along the margin and separates two zones with different magnetic characteristics: long wavelength small amplitude anomalies on the landward side, and quasi lineated anomalies on the seaward side. Seismic data compilations show that this large anomaly coincides with the ocean-continent boundary at most places north of Flemish Cap. Modelling of the magnetic anomalies indicate that the large amplitude anomaly is caused by the juxtaposition of highly magnetized oceanic crust against weakly magnetized continental crust; this situation is similar to that observed across the Goban Spur margin, which is a conjugate of the Flemish Cap margin. The presence of highly magnetized oceanic crust landward of anomaly 34 and within the Cretaceous Magnetic Quiet Zone is attested to by the presence of similar large amplitude anomalies south of the Flemish Cap and Goban Spur regions, but these do not mark the ocean-continent transition.     

Asymmetric seafloor spreading and short ridge jumps in the Australian-Antarctic discordance

The crenulated geometry of the Southeast Indian ridge within the Australian-Antarctic discordance is formed by numerous spreading ridge segments that are offset, alternately to the north and south, by transform faults. Suggested causes for these offsets, which largely developed since ~ 20 Ma, include asymmetric seafloor spreading, ridge jumps, and propagating rifts that have transferred seafloor from one flank of the spreading ridge to the other. Each of these processes has operated at different times in different locations of the discordance; here we document an instance where a small (~ 20 km), young (< 0.2 Ma), southward ridge jump has contributed to the observed asymmetry. When aeromagnetic anomalies from the Project Investigator-1 survey are superposed on gravity anomalies computed from Geosat GM and ERM data, we find that in segment B4 of the discordance (between 125° and 126° E), the roughly east-west-trending gravity low, correlated with the axial valley, is 20–25 km south of the ridge axis position inferred from the center of magnetic anomaly 1. Elsewhere in the discordance, the inferred locations of the ridge axis from magnetics and gravity are in excellent agreement. Ship track data confirm these observations: portions of Moana Wave track crossing the ridge in B4 show that a topographic valley correlated with the gravity anomaly low lies south of the center of magnetic anomaly 1; while other ship track data that cross the spreading ridge in segments B3 and B5 demonstrate good agreement between the axial valley, the gravity anomaly low, and the central magnetic anomaly. Based on these observations, we speculate that the ridge axis in B4 has recently jumped to the south, from a ridge location closer to the center of the young normally magnetized crust, to that of the gravity anomaly low. The position of the gravity low essentially at the edge of normally magnetized crust requires a very recent (< 0.2 Ma) arrival of the ridge in this new location. Because this ridge jump is so young, it may be a promising location for future detailed studies of the dynamics, kinematics, and thermal effects of ridge jumps.The U.S. Government right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

The morphology and tectonics of the Mark area from Sea Beam and Sea MARC I observations (Mid-Atlantic Ridge 23° N)

High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40–50 km in length that extends from the eastern Kane ridge-transform intersection south to about 23°12′ N. The rift valley in this area is dominated by a large constructional volcanic ridge that creates 200–500 m of relief and is associated with high-temperature hydrothermal activity. The southern spreading cell is characterized by a NNE-trending band of small (50–200 m high), conical volcanos that are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted. This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells overlap in an anomalous zone between 23°05′ N and 23°17′ N that lacks a well-developed rift valley or neovolcanic zone, and may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10–17 km wide rift valley floor. Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less than 5°), begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the rift valley walls. Features that appear to be constructional volcanic ridges formed in the median valley are preserved largely intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production rates.     

The morphology and structure of the West O’Gorman Fracture Zone

The West O’Gorman Fracture Zone is an unusual feature that lies between the Mathematician Ridge and the East Pacific Rise on crust generated on the East Pacific Rise between 4 and 9 million years ago. We made a reconnaissance gravity, magnetic and Sea Beam study of the zone with particular emphasis on its eastern (youngest) portion. That region is characterized by an elongate main trough, a prominent median ridge and other, smaller ridges and troughs. The structure has the appearance of large-offset fracture zone, possibly in a slow spreading environment. However, magnetic anomalies indicate that the offset, if any, is quite small, and the spreading rate during formation was fast. In addition, the magnetic profiles do not support earlier models for a difference in spreading rate north and south of the fracture. The morphology of the fracture zone suggests that flexure may be responsible for some of the topography; but gravity studies indicate some of the most prominent features of the fracture zone are at least partially compensated. The main trough is underlain by a thin crust (or high density body), similar to large-offset fracture zones in the Atlantic, while the median ridge is underlain by a thickened crust. Sea Beam data does not unambiguously resolve between volcanism or serpentinization of the upper mantle as a mechanism for isostatic compensation. Why the West O’Gorman exists remains enigmatic, but we speculate that the topographic expression of a fracture zone does not require a transform offset during formation. Perhaps the spreading ridge was magma starved for some reason, resulting in a thin crust that allowed water to penetrate and serpentinize portions of the upper mantle.     

南海中部海盆海山磁性反演及初步解析

南海中部海盆分布着众多海山,从实测磁异常反演海山磁性是南海海盆古地磁研究新的重要课题,它有助于解决南海海盆的运动方向和运动形式等问题.对南海海盆16座海山作磁性反演取得了较高的计算精度.对南海中部海盆海山磁性反演表明,海山均为非均匀磁化体,海山形成经过多期火山喷溢叠加.海山磁性差异清晰地显示了海盆分区特征.南海海盆分东部海盆区和西南部海盐区,海盆地壳运动规律差异较大:东部海盆以逆时针旋转由南往北运移;西南海盆先经历了顺时针旋转,后改为逆时针旋转,由北往南运移.     

Two- and three-dimensional inversions of magnetic anomalies in the MARK area (Mid-Atlantic Ridge 23° N)

Magnetic data collected in conjunction with a Sea Beam bathymetric survey of the Mid-Atlantic Ridge south of the Kane Fracture Zone are used to constrain the spreading history of this area over the past 3 Ma. Two-dimensional forward modeling and inversion techniques are carried out, as well as a full three-dimensional inversion of the anomaly field along a 90-km-long section of the rift valley. Our results indicate that this portion of the Mid-Atlantic Ridge, known as the MARK area, consists of two distinct spreading cells separated by a small, zero-offset transform or discordant zone near 23°10′ N, The youngest crust in the median valley is characterized by a series of distinct magnetization highs which coalesce to form two NNE-trending bands of high magnetization, one on the northern ridge segment which coincides with a large constructional volcanic ridge, and one along the southern ridge segment that is associated with a string of small axial volcanos. These two magnetization highs overlap between 23° N and 23°10° N forming a non-transform offset that may be a slow spreading ridge analogue of the small ridge axis discontinuities found on the East Pacific Rise. The crustal magnetizations in this overlap zone are generally low, although an anomalous, ESE-trending magnetization high of unknown origin is also present in this area. The present-day segmentation of spreading in the MARK area was inherited from an earlier ridge-transform-ridge geometry through a series of small (∼ 10 km) eastward ridge jumps. These small ridge jumps were caused by a relocation of the neovolcanic zone within the median valley and have resulted in an overall pattern of asymmetric spreading with faster rates to the west (14 mm yr⁻¹) than to the east (11 mm yr⁻¹). Although the detailed magnetic survey described in this paper extends out to only 3 Ma old crust, a regional compilation of magnetic data from this area by Schoutenet al. (1985) indicates that the relative positions and dimensions of the spreading cells, and the pattern of asymmetric spreading seen in the MARK area during the past 3 Ma, have characterized this part of the Mid-Atlantic Ridge for at least the past 36 Ma.     

The sea-floor spreading history of the eastern Indian Ocean

The geologic history of the eastern Indian Ocean between northwest Australia and the Java Trench is known to involve two separate events of rifting and sea-floor spreading. Late Jurassic spreading in the Argo Abyssal Plain off northwest Australia was followed by Early Cretaceous spreading in the Cuvier and Perth Abyssal Plains off west Australia. However, the evolution and interaction of these events has not been clear. Mesozoic sea-floor spreading anomalies have been identified throughout the Argo Abyssal Plain that define a rifting event and subsequent northward spreading on the northwestern Australian margin at 155 m.y.b.p. Magnetic anomalies northwest of the Argo Abyssal Plain indicate a ridge jump to the south at about 130 m.y.b.p. that is approximately synchronous with east-west rifting along the southwestern Australian margin. The Joey Rise in the Argo Plain was probably formed by volcanism at the intersection of this new rift and the spreading ridge to the north. The southern and northern spreading systems were connected through the Exmouth Plateau which was stretched and faulted as spreading progressed. The RRR triple junction was formed at the intersection of the two spreading systems and appears to have migrated west along the northern edge of the Gascoyne Abyssal Plain. Spreading off northwest Australia cannot be easily related to simultaneous spreading in the west central Pacific via any simple tectonic scheme.     

How much continent under the ocean?

A crustal attenuation model, initially proposed to explain the evolution of the Canadian margin of the Labrador Sea, is applicable to both intra-cratonic rift zones and passive continental margins. Examples examined are the East African rifts and the New Zealand Plateau, S. W. Pacific. In the model continental lithosphere is stretched and rifted over thermally and chemically expanding asthenosphere blisters. Magmas, abundantly generated in the zone of partial melting at the lithosphere-asthenosphere boundary, ascend and intrude into the fracturing crust. Upon cooling, these intrusives produce low amplitude magnetic anomalies. Sea-floor spreading begins when the asthenosphere finally breaks through en masse and it is thus only a late phase in a much longer cycle. Attenuated continental crust is often underlain by low velocity, low density, upper mantle, which is indicative of the blending of the crust and mantle. It is suggested that substantial parts of marginal quiet magnetic zones are continent-ocean transitions that formed by crustal attenuation. The attenuation model is used to explain the evolution of the Arctic region. It should help bridge the gap between fixists and mobilists.     

The role of the Spitsbergen shear zone in determining morphology,segmentation and evolution of the Knipovich Ridge

In 1989–1990 the SeaMARC II side-looking sonar and swath bathymetric system imaged more than 80 000 km² of the seafloor in the Norwegian-Greenland Sea and southern Arctic Ocean. One of our main goals was to investigate the morphotectonic evolution of the ultra-slow spreading Knipovich Ridge from its oblique (115° ) intersection with the Mohns Ridge in the south to its boundary with the Molloy Transform Fault in the north, and to determine whether or not the ancient Spitsbergen Shear Zone continued to play any involvement in the rise axis evolution and segmentation. Structural evidence for ongoing northward rift propagation of the Mohns Ridge into the ancient Spitsbergen Shear Zone (forming the Knipovich Ridge in the process) includes ancient deactivated and migrated transforms, subtle V-shaped-oriented flank faults which have their apex at the present day Molloy Transform, and rift related faults that extend north of the present Molloy Transform Fault. The Knipovich Ridge is segmented into distinct elongate basins; the bathymetric inverse of the very-slow spreading Reykjanes Ridge to the south. Three major fault directions are detected: the N-S oriented rift walls, the highly oblique en-echelon faults, which reside in the rift valley, and the structures, defining the orientation of many of the axial highs, which are oblique to both the rift walls and the faults in the axial rift valley. The segmentation of this slow spreading center is dominated by quasi stationary, focused magma centers creating (axial highs) located between long oblique rift basins. Present day segment discontinuities on the Knipovich Ridge are aligned along highly oblique, probably strike-slip faults, which could have been created in response to rotating shear couples within zones of transtension across the multiple faults of the Spitsbergen Shear Zone. Fault interaction between major strike slip shears may have lead to the formation of en-echelon pull apart basins. The curved stress trajectories create arcuate faults and subsiding elongate basins while focusing most of the volcanism through the boundary faults. As a result, the Knipovich Ridge is characterized by Underlapping magma centers, with long oblique rifts. This style of basin-dominated segmentation probably evolved in a simple shear detachment fault environment which led to the extreme morphotectonic and geophysical asymmetries across the rise axis. The influence of the Spitsbergen Shear Zone on the evolution of the Knipovich Ridge is the primary reason that the segment discontinuities are predominantly volcanic. Fault orientation data suggest that different extension directions along the Knipovich Ridge and Mohns Ridge (280° vs. 330°, respectively) cause the crust on the western side of the intersection of these two ridges to buckle and uplift via compression as is evidenced by the uplifted western wall province and the large 60 mGal free air gravity anomalies in this area. In addition, the structural data suggest that the northwards propagation of the spreading center is ongoing and that a `normal' pure shear spreading regime has not evolved along this ridge. This revised version was published online in November 2006 with corrections to the Cover Date.     

Large-scale oblique features in an active transform fault,the Wilkes fracture zone near 9°S on the East Pacific Rise

The Wilkes fracture zone offsets the East Pacific Rise about 200 km right-laterally near 9°S. The bathymetric expression of the fracture zone ranges from a simple slope or step along its inactive extension to a 100 km wide zone of oblique structural features in the active portion. A low ridge 200 to 300 m high, 5 to 15 km wide and 185 km long is the dominant oblique structure; it trends 23° north of the main transform trend. A high-amplitude magnetic anomaly trends 097° along the southern part of the active portion and apparently marks the main transform direction. The structurally simple, inactive portions of the Wilkes fracture zone trend 105°. Plots of epicenter locations reveal two groupings of earthquakes, one along an 082° trend in the central part of the fracture zone, and a cluster near the southwestern fracture zone — spreading center intersection.Taken together the data suggest that some event, other than a shift in the Nazca-Pacific pole of rotation, occurred 0.9 m.y. ago to change the Wilkes fracture zone from a simple fault to a complex zone of shearing. Since that time the long oblique ridge, probably the surface expression of a Riedel shear, was formed. At present the entire 200 km long, 100 km wide region between the offset axes is seismically active, but transform motion may be largely confined to the southern margin of the active zone, coincident with the high-amplitude magnetic anomaly there.     

Thermal evolution of the western Svalbard margin

The northern Norwegian-Greenland Sea opened up as the Knipovich Ridge propagated from the south into the ancient continental Spitsbergen Shear Zone. Heat flow data suggest that magma was first intruded at a latitude of 75° N around 60 m.y.b.p. By 40–50 m.y.b.p. oceanic crust was forming at a latitude of 78° N. At 12 m.y.b.p. the Hovgård Transform Fault was deactivated during a northwards propagation of the Knipovich Ridge. Spreading is now in its nascent stages along the Molloy Ridge within the trough of the Spitsbergen Fracture Zone. Spreading rates are slower in the north than the south. For the Knipovich Ridge at 78° N they range from 1.5–2.3 mm yr^-1 on the eastern flank to 1.9–3.1 mm yr^-1 on the western flank. At a latitude of 75° N spreading rates increase to 4.3–4.9 mm yr^-1.Thermal profiles reveal regions of off-axial high heat flow. They are located at ages of 14 m.y. west and 13 m.y. east of the northern Knipovich Ridge, and at 36 m.y. on the eastern flank of the southern Knipovich Ridge. These may correspond to episodes of increased magmatic activity; which may be related to times of rapid north-wards rise axis propagation.The fact that the Norwegian-Greenland Sea is almost void of magnetic anomalies may be caused by the chaotic extrusion of basalts from a spreading center trapped within the confines of an ancient continental shear zone. The oblique impact of the propagating rift with the ancient shear zone may have created an unstable state of stress in the region. If so, extension took place preferentially to the northwest, while compression occurred to the southeast between the opening, leaking shear zone and the Svalbard margin. This caused faster spreading rates to the northwest than to the southeast.     

Basin development subsequent to ridge-trench collision: the Jane Basin,Antarctica

The Jane Arc and Basin system is located at the eastern offshore prolongation of the Antarctic Peninsula, along the southern margin of the South Orkney Microcontinent. Three magnetic anomaly profiles orthogonal to the main tectonic and bathymetric trends were recorded during the SCAN97 cruise by the Spanish R/V Hespérides. In our profiles, chron C6n (19.5 Ma) was identified as the youngest oceanic crust of the Northern Weddell Sea, whose northern spreading branch was totally subducted. The profiles from the Jane Basin allow us to date, for the first time, the age of the oceanic crust using linear sea floor magnetic anomalies. The spreading in the Jane Basin began around the age of the oldest magnetic anomaly at 17.6 Ma (chron C5Dn), and ended about 14.4 Ma (chron C5ADn). The distribution of the magnetic anomalies indicate that the mechanism responsible for the development of Jane Basin was the subduction of the Weddell Sea spreading centre below the SE margin of the South Orkney Microcontinent, suggesting a novel mechanism for an extreme case of backarc development.     

An evolving ridge system around the Indian Ocean triple junction

A 2°×2° map of spreading centres and fracture zones surrounding the Indian Ocean RRR triple junction, at 25.5°S, 70°E, is described from a data set of GLORIA side-scan sonar images, bathymetry, magnetic and gravity anomalies. The GLORIA images show a pervasive fabric due to linear abyssal hills oriented parallel to the two medium-spreading ridges (the Central Indian Ridge (CIR) and Southeast Indian Ridge (SEIR)). A cuvature of the fabric occurs along fracture zones, which are also located by lows in the bathymetry and gravity data and by offsets between magnetic anomalies. The magnetic anomalies also record periods of asymmetric spreading marking the development of the fracture zones, including the birth, at anomaly 2A, of a short fracture zone 50 km north of the triple junction on the CIR, and its death near the time of the Jaramillo anomaly. In some localities, a fine-scale fabric corresponds to a coarser fabric on the opposite flank of the CIR, possibly indicating a persistent asymmetry in the faulting at the median valley walls if the fabric has a tectonic and not a volcanic origin. A plate velocity analysis of the triple junction shows that both the CIR and Southwest Indian Ridge (SWIR) are propagating obliquely; the CIR appears to form an oblique trend by segmenting into a series of almost normally-oriented segments separated by short-offset fracture zones. For the last 4 m.y., the abyssal hill lineations indicate that the CIR segment immediately north of the triple junction has been spreading with an average 10° obliquity. The present small 5 km offset of the centres of the CIR and SEIR median valleys (Munschy and Schlich, 1989) is shown to be the result of this obliquity and a 30% spreading asymmetry between anomaly 2 and the Jaramillo on the CIR segment immediately north of the triple junction.     

New Bathymetry and Magnetic Lineations Identifications in the Northernmost South China Sea and their Tectonic Implications

The seafloor spreading of the South China Sea (SCS) was previously believed to take place between ca. 32 and 15 Ma (magnetic anomaly C11 to C5c). New magnetic data acquired in the northernmost SCS however suggests the existence of E–W trending magnetic polarity reversal patterns. Magnetic modeling demonstrates that the oldest SCS oceanic crust could be Late Eocene (as old as 37 Ma, magnetic anomaly C17), with a half-spreading rate of 44 mm/yr. The new identified continent–ocean boundary (COB) in the northern SCS generally follows the base of the continental slope. The COB is also marked by the presence of a relatively low magnetization zone, corresponding to the thinned portion of the continental crust. We suggest that the northern extension of the SCS oceanic crust is terminated by an inactive NW–SE trending trench-trench transform fault, called the Luzon–Ryukyu Transform Plate Boundary (LRTPB). The LRTPB is suggested to be a left-lateral transform fault connecting the former southeast-dipping Manila Trench in the south and the northwest-dipping Ryukyu Trench in the north. The existence of the LRTPB is demonstrated by the different patterns of the magnetic anomalies as well as the different seafloor morphology and basement relief on both sides of the LRTPB. Particularly, the northwestern portion of the LRTPB is marked by a steep northeast-dipping escarpment, along which the Formosa Canyon has developed. The LRTPB probably became inactive at ca. 20 Ma while the former Manila Trench prolonged northeastwards and connected to the former Ryukyu Trench by another transform fault. This reorganization of the plate boundaries might cause the southwestern portion of the former Ryukyu Trench to become extinct and a piece of the Philippine Sea Plate was therefore trapped amongst the LRTPB, the Manila Trench and the continental margin.     

Kinematics and characteristic features of the morphostructural segmentation of the Knipovich Ridge

The rift zone??s relief, the spreading kinematics, and the experimental modeling of the Knipovich Ridge??s formation were analyzed. Its rift zone is formed in a transtension environment. Faulting is predominant in its northern part, while strike-slip is characteristic for the south. A system of short extension basins connected by deep strike-slip U-shaped troughs is observed in the south. A system of volcanic rises connected by short shallow basins is observed in the north. The rift valley is V-shaped. According to the experimental modeling data, these extension kinematics provide the formation of short extension basins connected by strike-slips and transtension faults. Their length and orientation depend on the spreading obliquity of each segment.     

Magnetic anomalies over the Central Levant continental margin

The magnetic field over the central Levant continental margin, off northern Israel and southern Lebanon, and the adjacent Levant Basin has two distinct trends. Mount Carmel and its offshore continuation (Carmel Nose), which are the surface expression of a large subbottom structure that extends from the land area across the continental shelf to the continental slope, form a dividing zone between the two magnetic trends. South of the Carmel structure the magnetic field trends east-west, while north of the Carmel structure it trends northeast and north-northeast.Several pronounced magnetic anomalies exist mainly north of the Carmel structure, the majority of which trend north-northeast and northeast, parallel and sub-parallel to the trend of the magnetic field in this area. Some also trend northwest, perpendicular to the trend of the magnetic field. In several cases the magnetic anomalies indicate large lithological elements which continue from land to sea.Gravity and seismic refraction data show that the two magnetic domains north and south of the Carmel structure are associated with areas of different crustal structure. South of the Carmel structure the continetal-oceanic crustal transition zone is located beyond the continental margin at the base of the continental slope, while north of the Carmel structure it is located under the continental shelf, near the shore. On land, there are also differences in the structure of the crust north and south of the Carmel structure, the crust being much thinner north of the structure than south of it.We suggest that some of the large magnetic anomalies off the Central Levant were formed during the rifting phase of the eastern Mediterranean.     

南海西南次海盆被动陆缘构造单元划分及伸展演化过程

南海西南次海盆被动陆缘洋陆转换带位于陆缘强烈伸展区,蕴含着岩石圈临界伸展破裂和洋盆扩张过程的丰富信息。本文利用多道地震剖面和重力异常数据,对西南次海盆被动陆缘构造单元进行划分,研究陆缘南、北部洋陆转换带结构构造特征,探讨陆缘伸展演化过程。多道地震剖面资料显示,北部洋陆转换带发育有裂陷期断陷和向海倾斜的掀斜断块;南部发育有低角度正断层控制的裂陷期断陷、海底火山以及局部隆起;从陆到洋方向,重力异常值变化明显。根据上述结果南海西南次海盆被动陆缘划分为近端带、洋陆转换带和洋盆三个构造单元,分别对应了其伸展演化过程的三个阶段：前裂谷阶段、陆缘裂陷阶段和海底扩张阶段。     

High resolution statistical estimation of seafloor morphology: Oblique and orthogonal fabric on the flanks of the Mid-Atlantic Ridge, 34°–35.5° S

On the Mid-Atlantic Ridge (MAR) from 34°–35.5° S, three ridge segments span the 108 km distance between the Meteor Fracture Zone (FZ) and the Montevideo FZ. Each of these segments is perpendicular to the adjoining transforms. Magnetic isochrons in the southern half of the region are oblique to the spreading direction and are offset from the morphological expression of the plate boundary, revealing a transition from oblique to orthogonal spreading within the last 750,000 years. Changes in orientation and cross-sectional form of the rift valley, as modified by tectonic processes, are preserved in the off-axis abyssal-hill fabric. We present a new statistical method for describing size and orientation of abyssal hills based on local slopes. For a given offset, the range of sorted slopes from the first to third quartile provides a robust estimate of topographic variability. The variability can be parametrized by azimuthal direction, plan-view aspect ratio, characteristic height and width. We resolve lineation azimuth within 6°, and characteristic height, width and aspect ratio within 20–30%, using 18 by 21 km sample boxes crossed by multiple Sea Beam swaths covering approximately 30% of the box. In the northern portion of the survey, the azimuth is mainly ridge parallel, while in the southern portion, the azimuth rotates 23° clockwise from ridge strike. Characteristic height and width are greater in the southern half than in the northern half, while aspect ratios are lower. The asymmetry of quartiles about the median slope provides evidence that inward-facing normal faults bounding the rift valley are a significant source of topography. Fabric disrupted by migration of small-offset discontinuities has higher than average characteristic height. Characteristic height and width correlate positively with residual gravity, an indicator of crustal thinning. A residual gravity low, possibly the current focus of upwelling, coincides with a newly formed spreading axis. These correlations suggest that evolution of ridge geometry can be controlled by crust and mantle thermal structure. Either variation in magma supply, resulting in changes in stress normal to the ridge axis, or a major realignment of the Montevideo Transform, temporarily resulting in increased shear stress across newly activated faults, may have been responsible for changes in orientation and morphology of the spreading center.     

The western intersection of Oceanographer fracture zone with the mid-Atlantic ridge

A survey across the western intersection of the mid-Atlantic ridge with Oceanographer fracture zone near 35°N shows this intersection to be different in character from its more typical eastern counterpart. At the western junction the transform valley broadens into a parallelogram shaped deep some 46 by 24 km, which extends well across the trace of the active transform. Within 30 km south of the fracture zone the median valley becomes oblique forming a NE trending ridge which is the SE edge of the deep. Magnetic mapping shows the current spreading centre to be adjacent to this ridge.A sequence of evolution for this intersection over the past 0.7 Ma is proposed to explain the features mapped. We suggest that the oblique ridge crest trends extended across the transform trace to form the elongated graben-like deep with its associated faults and sediment slumps. Such complex patterns may occur as plate-wide changes in spreading direction become modified by localised shear stress fields at ridge crest-transform intersections, as have been observed in a number of other cases. The absence of significant tranverse ridges across from the spreading centre at this particular fracture zone intersection, may have temporarily allowed these stress patterns to propagate across the fracture zone.