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.     

A free air gravity anomaly contour map of the Irish continental margin

Compilation of currently-available gravity data permits the construction of a free-air anomaly contour map of the continental margin west of Ireland (51–54 N). Major elements in the structure of the margin, previously delineated on the basis of seismic reflection and magnetic surveys, are clearly seen on the FAA contour map, notably the Porcupine Seabight Trough, and Porcupine Ridge. However, contrary to earlier ideas, the gravity data imply that the Seabight Trough extends northwards onto the Slyne Ridge; and the Slyne Trough, formerly regarded as northeasterly prolongation of the Seabight Trough, appears to be a discrete, fault-bounded, feature separated from the latter by a basement ridge. East-west gravity profiles are modelled in terms of thinned crust with the Moho at a minimum depth of 15 km beneath the axis of the Seabight Trough. The models tend to support hypotheses invoking formation of the Seabight Trough by simple westward translation of Porcupine Ridge with respect to the Irish Mainland.     

Lithospheric structure beneath Laxmi Ridge and late Cretaceous geodynamic events

The geophysically unusual Laxmi Ridge (eastern basin, Arabian Sea) is associated with a prominent elongated negative gravity anomaly. A seismically and geodynamically constrained detailed 2D gravity modeling suggests an 11-km-thick normal oceanic crust and an asthenospheric upwarp to a depth of 35 km. We attribute the apparent thickening of the crust to a possible emplacement of an anomalous subcrustal low-density layer between 11 and 19 km depth. We hypothesize that a K-T boundary bolide impact near the Bombay offshore led to several geological events, including eruption of Deccan flood basalts. The spreading Carlsberg Ridge in the Indian Ocean and rifting associated with Deccan volcanism generated the compressive regime, which perhaps originated the Laxmi Ridge.     

Features of the crustal structure of the Arabian Sea

An analysis of the gravity field and geoid heights allowed us to distinguish a third buried basin filled with sediments located in the southwestern part of the sea in the regions adjacent to the Carlsberg Ridge. From the previously known basins, it is separated by saddles. The saddles correspond to a series of faults and are possibly related to the pulse character of the northwestward prograding of the spreading axes of the Carlsberg Ridge. The continental origin of the Laxmi ridge is confirmed. The results of an analysis of the gravity field and its transformants, together with the two-dimensional density modeling, agree with the possibility of the existence of a spreading type of the crust (I) in the region of the Laxmi Basin. An analysis of the geoid height anomalies allows us to suggest that, with respect to the upper layers of the lithosphere, the Laxmi Ridge is not connected with the Chagos-Laccadive Ridge.     

Geophysical investigations over a segment of the Central Indian Ridge, Indian Ocean

 Swath bathymetric, gravity, and magnetic studies were carried out over a 55 km long segment of the Central Indian Ridge. The ridge is characterized by 12 to 15 km wide rift valley bounded by steep walls and prominent volcanic constructional ridges on either side of the central rift valley. A transform fault at 7°45′S displaces the ridge axis. A mantle Bouguer anomaly low of −14 mGals and shallowing of rift valley over the middle of the ridge segment indicate along axis crustal thickness variations. A poorly developed neovolcanic zone on the inner rift valley floor indicate dominance of tectonic extension. The off-axis volcanic ridgs suggest enhanced magmatic activity during the recent past. Received: 24 May 1996 / Rivision received: 13 January 1997     

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.     

Spreading rates,rift propagation,and fracture zone offset histories during the past 5 my on the Mid-Atlantic Ridge; 25°–27°30′ S and 31°–34°30′ S

The southern Mid-Atlantic Ridge (MAR) is spreading at rates (34–38 mm yr⁻¹) that fall within a transitional range between those which characterize slow and intermediate spreading center morphology. To further our understanding of crustal accretion at these transitional spreading rates, we have carried out analysis of magnetic anomaly data from two detailed SeaBeam surveys of the MAR between 25°–27°30′S and 31°–34°30′S. Within these areas, the MAR is subdivided into 9 ridge segments bounded by large- and short-offset discontinuities of the ridge axis. From two-dimensional Fourier inversions of the magnetic anomaly data we establish the history of spreading within each ridge segment for the past 5 my and the evolution of the bounding ridge-axis discontinuities. We see evidence for the initiation and diminishment of small-offset discontinuities, and for the transition of rigid large-offset transform faults to less stable short-offset features. Individual ridge segments display independent spreading histories in terms of both the sense and amount of asymmetric spreading within each which have given rise to changes through time in the lengths of bounding ridge-axis discontinuities. Over the past 3–5 my, the short-offset discontinuities within the area have lengthened/shortened by approximately the same amount (∼ 10 km). During this same time period, larger-offset transform faults have remained comparatively constant in length. A shift in plate motion at anomaly 3 time may have given rise to change in the length of short-offset second-order discontinuities. However, the pattern of lengthening/shortening short-offset discontinuities we see is not simply related to the geometry of the plate boundary in these regions which precludes a simply relationship between plate motion changes and response at the plate boundary. We document a case of rapid (minimum 60 mm yr⁻¹) small-scale rift propagation, occurring between 2.5 and 1.8 my, associated with transition of the Moore transform fault to an oblique-trending ridge-axis discontinuity. Propagation across the Moore discontinuity and similar propagation within the 31°–34°30’S area may be associated with the reduced age contrast in lithosphere across second-order discontinuities. Total opening rates within our northern survey area decreased from anomaly 4′ to 2 time and rates within both areas have increased since the Jaramillo. Total opening rates measured for anomaly intervals differ along the plate boundary significantly, more than expected with changing distance to the pole of rotation. These differences imply a degree of short-term non-rigid plate behaviour which may be associated with ridge segments acting as independent spreading cells. Magnetic polarity transition widths from our inversion studies may be used to infer a zone of crustal accretion which is 3–6 km wide, within the inner floor of the rift valley. A systematic increase of transition width with age would be expected if deeper crustal sources dominate the magnetic signal in older crust but this is not observed. We present results from three-dimensional analysis of magnetic anomaly data which show magnetization highs located at the intersection of the MAR with both large- and short-offset discontinuities. Within the central anomaly the highs exceed 15 A m⁻¹ compared with a background of approximately 8–10 A m⁻¹ and they persist for at least 2.5 my. The highs may be caused by eruption of fractionated strongly magnetized basalts at ridge-axis discontinuities with both large and small offsets.     

Tectonic core of a sedimentary drift: a potential ridge propagation feature beneath the Blake Outer Ridge

The Blake Outer Ridge is a 480–kilometer long linear sedimentary drift ridge striking perpendicular to the North American coastline. By modeling free-air gravity anomalies we tested for the presence of a crustal feature that may control the location and orientation of the Blake Outer Ridge. Most of our crustal density models that match observed gravity anomalies require an increase in oceanic crustal thickness of 1–3 km on the southwest side of the Blake Outer Ridge relative to the northeast side. Most of these models also require 1–4 km of crustal thinning in zone 20–30 km southwest of the crest of the Blake Outer Ridge. Although these features are consistent with the structure of oceanic fracture zones, the Blake Outer Ridge is not parallel to adjacent known fracture zones. Magnetic anomalies suggest that the ocean crust beneath this feature formed during a period of mid-ocean ridge reorganization, and that the Blake Outer Ridge may be built upon the bathymetric expression of an oblique extensional feature associated with ridge propagation. It is likely that the orientation of this trough acted as a catalyst for sediment deposition with the start of the Western Boundary Undercurrent in the mid-Oligocene.     

Active oceanic spreading in the northern North Fiji Basin: Results of the NOFI cruise of R/V L'Atalante (newstarmer project)

The South Pandora and the Tripartite Ridges are active spreading centers located in the northern part of the North Fiji Basin. These spreading centers were surveyed over a distance of 750 km during the NOFI cruise of R/V L'Atalante (August–September 1994) which was conducted in the frame of the french-japanese Newstarmer cooperation project. SIMRAD EM12-dual full coverage swath bathymetric and imagery data as well as airgun 6-channel seismic, magnetics and gravity profiles were recorded along and offaxis from 170°40 E to 178° E. Dredging and piston coring were also performed along and off-axis. The axial domain of the South Pandora Ridge is divided into 5 first-order segments characterized by contrasted morphologies. The average width of the active domain is 20 km and corresponds either to bathymetric highs or to deep elongated grabens. The bathymetric highs are volcanic constructions, locally faulted and rifted, which can obstruct totally the axial valley. The grabens show the typical morphology of slow spreading axes, with two steep walls flanking a deep axial valley. Elongated lateral ridges may be present on both sides of the grabens. Numerous volcanoes, up to several kilometers in diameter, occur on both flanks of the South Pandora Ridge. The Tripartite Ridge consists of three main segments showing a sigmoid shape. Major changes in the direction of the active zones are observed at the segment discontinuities. These discontinuities show various geometrical patterns which suggest complex transform relay zones. Preliminary analysis of seismic reflection profiles suggest that the Tripartite Ridge is a very young feature which propagates into an older oceanic domain characterized by a significant sedimentary cover. By contrast, a very thin to absent sedimentary cover is observed about 100 km on both flanks of the South Pandora Ridge active axis. The magnetic anomaly profiles give evidence of long and continuous lineations, parallel to the South Pandora Ridge spreading axis. According to our preliminary interpretation, the spreading rate would have been very low (8 km/m.y. half rate) during the last 7 Ma. The South Pandora and Tripartite Ridges exhibit characteristics typical of active oceanic ridges: (1) a segmented pattern, with segments ranging from 80 to 100 km in length; (2) an axial tectonic and volcanic zone, 10 to 20 km wide; (3) well-organized magnetic lineations, parallel to the active axis; (4) clear signature on the free-air gravity anomaly map. However, no typical transform fault is observed; instead, complex relay zones are separating first-order segments.     

Crustal analysis of the Ulleung Basin in the East Sea (Japan Sea) from enhanced gravity mapping

To facilitate geological analyses of the Ulleung Basin in the East Sea (Japan Sea) between Korea and Japan, shipborne and satellite altimetry-derived gravity data are combined to derive a regionally coherent anomaly field. The 2-min gridded satellite altimetry-based gravity predicted by Sandwell and Smith [Sandwell DT, Smith WHF (1997) J Geophys Res 102(B5):10,039–10,054] are used for making cross-over adjustments that reduce the errors between track segments and at the cross-over points of shipborne gravity profiles. Relative to the regionally more homogeneous satellite gravity anomalies, the longer wavelength components of the shipborne anomalies are significantly improved with minimal distortion of their shorter wavelength components. The resulting free-air gravity anomaly map yields a more coherent integration of short and long wavelength anomalies compared to that obtained from either the shipborne or satellite data sets separately. The derived free-air anomalies range over about 140 mGals or more in amplitude and regionally correspond with bathymetric undulations in the Ulleung Basin. The gravity lows and highs along the basin’s margin indicate the transition from continental to oceanic crust. However, in the northeastern and central Ulleung Basin, the negative regional correlation between the central gravity high and bathymetric low suggests the presence of shallow denser mantle beneath thinned oceanic crust. A series of gravity highs mark seamounts or volcanic terranes from the Korean Plateau to Oki Island. Gravity modeling suggests underplating by mafic igneous rocks of the northwestern margin of the Ulleung Basin and the transition between continental and oceanic crust. The crust of the central Ulleung Basin is about a 14–15 km thick with a 4–5 km thick sediment cover. It may also include a relatively weakly developed buried fossil spreading ridge with approximately 2 km of relief.     

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.     

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.     

The Tamayo transform fault in the mouth of the Gulf of California

The Tamayo transform fault is located at the north end of the East Pacific Rise where it enters the Gulf of California. This paper presents bathymetric, seismic reflection, magnetic, and gravity data from a detailed survey of the transform fault. The dominant feature of the offset region is a bathymetric ridge trending 120°, parallel to the predicted transform plate boundary. This transform ridge is associated with a large (600 ) positive magnetic anomaly, and a very small positive free-air gravity anomaly. Magnetic and gravity models indicate either a basalt or serpentinite composition for the ridge, but cannot distinguish between these possibilities. At its eastern end, the modern zone of strike-slip motion is in a narrow valley south of the transform ridge. The transform plate margin appears to pass through a saddle in the transform ridge and meet the western spreading center segment in the trough north of the transform ridge. On the basis of this survey and previous work, the history of the Tamayo from continental breakup to the present has been reconstructed. Initial rifting occurred along a trend of 130° at approximately 3.5 m.y.b.p. Once the transform fault was free of the constraints imposed by continent-continent and continent-oceanic lithospheric interaction, the trend of the transform fault rotated counter-clockwise. This rotation resulted in a leaky transform fault and intrusion of a large continuous transform ridge. Further adjustments in the spreading center/transform fault plate boundary configuration have given rise to an incipient zone of rifting cutting across the transform ridge and emplacement of diapiric structures.Contribution of the Scripps Institution of Oceanography, new series.     

The deep structure of the Iceland-Faeroe Ridge

Two long seismic refraction lines along the crest of the Iceland-Faeroe Ridge reveal a layered crust resembling the crust beneath Iceland but differing from normal continental or oceanic crust. The Moho was recognised at the south-eastern end of the lines at an apparent depth of 16–18 km. A refraction line in deeper water west of the ridge and south of Iceland indicates a thin oceanic type crust underlain by a 7.1 km/s layer which may be anomalous upper mantle.An extensive gravity survey of the ridge shows that it is in approximate isostatic equilibrium; the steep gravity gradient between the Norwegian Sea and the ridge indicates that the ridge is supported by a crust thickened to about 20 km rather than by anomalous low density rocks in the underlying upper mantle, in agreement with the seismic results. An increase in Bouguer anomaly of about 140 mgal between the centre of Iceland and the ridge is attributed to lateral variation in upper mantle density from an anomalous low value beneath Iceland to a more normal value beneath the ridge. Local gravity anomalies of medium amplitude which are characteristic of the ridge are caused by sediment troughs and by lateral variations in the upper crust beneath the sediments. A steep drop in Bouguer anomaly of about 80 mgal between the ridge and the Faeroe block is attributed partly to lateral change in crustal density and partly to slight thickening of the crust towards the Faeroe Islands; this crustal boundary may represent an anomalous type of continental margin formed when Greenland started to separate from the Faeroe Islands about 60 million years ago.We conclude that the Iceland-Faeroe Ridge formed during ocean floor spreading by an anomalous hot spot type of differentiation from the upper mantle such as is still active beneath Iceland. This suggests that the ridge may have stood some 2 km higher than at present when it was being formed in the early Tertiary, and that it has subsequently subsided as the spreading centre moved away and the underlying mantle became more normal; this interpretation is supported by recognition of a V-shaped sediment filled trough across the south-eastern end of the ridge, which may be a swamped sub-aerial valley.     

Mantle Bouguer Anomaly Along an Ultra Slow-Spreading Ridge: Implications for Accretionary Processes and Comparison with Results from Central Mid-Atlantic Ridge

A three-dimensional analysis of gravity andbathymetry data has been achieved along the Southwest Indian Ridge (SWIR)between the Rodriguez Triple Junction (RTJ) and the Atlantis II transform,in order to define the morphological and geophysical expression ofsecond-order segmentation along an ultra slow-spreading ridge(spreading rate of 8 mm/yr), and to compare it with awell-studied section along a slow-spreading ridge (spreadingrate of 12.5 mm/yr): the Mid-Atlantic Ridge (MAR) between 28°and 31°30 N.Between the Atlantis II transform and theRTJ, the SWIR axis exhibits a deep axial valley with an 30°oblique trend relative to the north–south spreading direction. Onlythree transform faults offset the axis, so the obliquity has to beaccommodated by the second-order segmentation. Alongslow-spreading ridges such as the MAR, second-order segmentshave been defined as linear features perpendicular to the spreadingdirection, with a shallow axial valley floor at the segment midpoint,deepening to the segment ends, and are associated with Mantle BouguerAnomaly (MBA) lows. Along the SWIR, our gravity study reveals the presenceof circular MBA lows, but they are spaced further apart than expected. Thesegravity lows are systematically centred over narrow bathymetric highs, andinterpreted as the centres of spreading cells. However, along some obliquesections of the axis, the valley floor displays small topographicundulations, which can be interpreted as small accretionary segments frommorphological analysis, but as large discontinuity domains from thegeophysical data. Therefore, both bathymetry and MBA variations have to beused to define the second-order segmentation of an ultraslow-spreading ridge. This segmentation appears to be characterisedby short segments and large oblique discontinuity domains. Analysis of alongaxis bathymetric and gravimetric profiles exhibits three different sectionsthat can be related to the thermal structure of the lithosphere beneath theSWIR axis.The comparison between characteristics of segmentationalong the SWIR and the MAR reveals two major differences: first, the poorcorrelation between MBA and bathymetry variations and second, the largerspacing and amplitude of MBA lows along the SWIR compared to the MAR. Theseobservations seem to be correlated with the spreading rate and the thermalstructure of the ridge. Therefore, the gravity signature of the segmentationand thus the accretionary processes appear to be very different: there areno distinct MBA lows on fast-spreading ridges, adjacent ones on slowspreading ridges and finally separate ones on ultra slow-spreadingridges. The main result of this study is to point out that 2nd ordersegmentation of an ultra slow-spreading ridge is characterised bywide discontinuity domains with very short accretionary segments, suggestingvery focused mantle upwelling, with a limited magma supply through a cold,thick lithosphere. We also emphasise the stronger influence of themechanical lithosphere on accretionary processes along an ultra slow-spreading ridge.     

An Interpretation of the Seafloor Spreading History of the West Enderby Basin between Initial Breakup of Gondwana and Anomaly C34

The seafloor spreading evolution in the Southern Indian Ocean is key to understanding the initial breakup of Gondwana. We summarize the structural lineaments deduced from the GEOSAT 10 Hz sampled raw altimetry data as well as satellite derived gravity anomaly map and the magnetic anomaly lineation trends from vector magnetic anomalies in the West Enderby Basin, the Southern Indian Ocean. The gravity anomaly maps by both Sandwell and Smith 1997, J. Geophys. Res. 102, 10039–10054 and 10 Hz raw altimeter data show almost the same general trends. However, curved structural trends, which turn from NNW–SSE in the south to NNE–SSW in the north, are detected only from gravity anomaly maps by 10 Hz raw altimeter data just to the east of Gunnerus Ridge. NNE–SSW structural trends and magnetic anomaly lineation trends that are perpendicular to them are observed between the Gunnerus Ridge and the Conrad Rise. To the west of Gunnerus Ridge, structural elements trend NNE–SSW and magnetic polarity changes are normal to them. In contrast, almost NNW–SSE structural trends and ENE–WSW magnetic polarity reversal strikes are dominant to the east of Gunnerus Ridge. Curved structural trends, which turn from WNW–ESE direction in the south to NNE–SSW direction in the west, and magnetic polarity reversal strikes that are almost perpendicular to them are observed just south of Conrad Rise. The magnetic polarity reversals may be parts of the Mesozoic magnetic anomaly sequence that formed along side of the structural lineaments before the long Cretaceous normal polarity superchron. Curved structural trends, detected only from gravity anomaly maps by 10 Hz raw altimeter data, most likely indicate slight changes in spreading direction from an initial NNW–SSE direction to NNE–SSW. Our results also suggest that these curved structural trends are fracture zones that formed during initial breakup of Gondwana.     

Structural mapping over the 85°E Ridge and surroundings using EIGEN6C4 high-resolution global combined gravity field model: an integrated approach

A revised gravity anomaly map, generated from the EIGEN6C4 high resolution global gravity model, has been utilized for understanding structure and tectonics over the 85°E Ridge and surroundings. EIGEN6C4 data have been analysed using different derivatives and Analytical Signal techniques for delineation of structural features and comparative analysis with the published results, which shows good correlation. All the structural features observed in two seismic reflection sections over a small part of the 85°E Ridge have also been delineated very well. The lineament trends of N–S, NNW–SSE, ENE–WSW and E–W are consequences of the major changes occurred in the mid-Cretaceous towards the spreading trends from NNW–SSE to N–S and resulted in northward movement of the Indian Plate trailed by interaction with the Asian Plate in the Early Eocene. The lineaments in the eastern side of the ridge have greater circular variances and greater circular standard deviation than those of the western side, which reveals that the eastern side of the ridge has suffered more tectonic activities.     

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.     

The Mid-atlantic Ridge (31°S–34°30′S): Temporal and spatial variations of accretionary processes

The ridge located between 31° S and 34°30′S is spreading at a rate of 35 mm yr⁻¹, a transitional velocity between the very slow (≤20 mm yr⁻¹) opening rates of the North Atlantic and Southwest Indian Oceans, and the intermediate rates (60 mm yr⁻¹) of the northern limb of the East Pacific Rise, and the Galapagos and Juan de Fuca Ridges. A synthesis of multi-narrow beam, magnetics and gravity data document that in this area the ridge represents a dynamically evolving system. Here the ridge is partitioned into an ensemble of six distinct segments of variable lengths (12 to 100 km) by two transform faults (first-order discontinuities) and three small offset (< 30 km) discontinuities (second-order discontinuities) that behave non-rigidly creating complex and heterogeneous morphotectonic patterns that are not parallel to flow lines. The offset magnitudes of both the first and second-order discontinuities change in response to differential asymmetric spreading. In addition, along the fossil trace of second-order discontinuities, the lengths of abyssal hills located to either side of a discordant zone are observed to lengthen and shorten creating a saw-toothed pattern. Although the spreading rate remains the same along the length of the ridge studied, the morphology of the spreading segments varies from a deep median valley with characteristics analogous to the rift segments of the North Atlantic to a gently rifted axial bulge that is indistinguishable from the shape and relief of the intermediate rate spreading centers of the East Pacific Rise (i.e., 21°N). Like other carefully surveyed ridge segments at slow and fast rates of accretion, the along-axis profiles of each ridge segment are distinctly convex upwards, and exhibit along-strike changes in relief of 500m to 1500 between the shallowest portion of the segment (approximate center) and the segment ends. Such spatial variations create marked along-axis changes in the morphology and relief of each segment. A relatively low mantle Bouguer anomaly is known to be associated with the ridge segment characterized by a gently rifted axial bulge and is interpreted to indicate the presence of focused mantle upwelling (Kuo and Forsyth, 1988). Moreover, the terrain at the ends of each segment are known to be highly magnetized compared to the centers of each segment (Carbotte et al, 1990). Taken together, these data clearly establish that these profound spatial variations in ridge segment properties between adjoining segments, and along and across each segment, indicate that the upper mantle processes responsible for the formation of this contrasting architecture are not solely related to passive upwelling of the asthenosphere beneath the ridge axis. Rather, there must be differences in the thermal and mechanical structure of the crust and upper mantle between and along the ridge segments to explain these spatial variations in axial topography, crustal structure and magnetization. These results are consistent with the results of investigations from other parts of the ridge and suggest that the emplacement of magma is highly focused along segments and positioned beneath the depth minimum of a given segment. The profound differences between segments indicate that the processes governing the behavior of upwelling mantle are decoupled and the variations in the patterns of axis flanking morphology and rate of accretion indicate that processes controlling upwelling and melt production vary markedly in time as well. At this spreading rate and in this area, the accretionary processes are clearly three-dimensional. In addition, the morphology of a ridge segment is not governed so much by opening rate as by the thermal structure of the mantle which underlies the segment.     

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.