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1.
The area reviewed covers the Mid-Norway continental margin between latitudes 62°N and 68°N. Main structural elements, as defined at the base Cretaceous level, are the Tröndelag Platform, underlying the inner shelf, the Möre and Vöring Basins, located beneath the outer shelf and slope, and the Möre Platform and the Outer Vöring Plateau, forming a base of slope trend of highs. Sediments contained in the Mid-Norway Basin range in age from Late Palaeozoic to Cenozoic. The basement was consolidated during the Caledonian orogenic cycle. Devonian and Early Carboniferous wrench movements along the axis of the Arctic-North Atlantic Caledonides are thought to have preceded the Namurian onset of crustal extension. Rifting processes were intermittently active for some 270 My until crustal separation between Greenland and Fennoscandia was achieved during the Early Eocene. During the evolution of the Norwegian-Greenland Sea rift system a stepwise concentration of tectonic activities to its axial zone (the area of subsequent continental separation) is observed. During the Late Palaeozoic to Mid-Jurassic a broad zone was affected by tensional faulting. During the Late Jurassic and Cretaceous the Tröndelag Platform was little affected by faulting whilst major rift systems in the Möre and Vöring Basins subsided rapidly and their shoulders became concomitantly upwarped. During the latest Cretaceous and Early Palaeogene terminal rifting phase only the western Möre and Vöring Basins were affected by intrusive and extrusive igneous activity. Following the Early Eocene crustal separation and the onset of sea floor spreading in the Norwegian-Greenland Sea, the Vöring segment of the Mid-Norway marginal basin subsided less rapidly than the Möre segment. During the Early and Mid Tertiary, minor compressional deformations affected the Vöring Basin and to a lesser degree the Möre Basin. Tensional forces dominated the Late Palaeozoic to Early Cenozoic evolution of the Mid-Norway Basin and effected strain mainly in the area where the crust was weakened by the previous lateral displacements. The lithosphere thinned progressively and the effects of the passively upwelling hot asthenospheric material became more pronounced. Massive dyke invasion of the thinned crust preceded its rupture.  相似文献   

2.
The Kerguelen Province, consisting of two oceanic plateaus (Kerguelen, Broken Ridge) and three basins (Enderby, Labuan and Diamantina), covers a large area of ocean floor in the southeast Indian Ocean. As very few magnetic anomalies have been identified in this area and only a few basement ages from the Kerguelen Plateau are known, reconstruction models of the Kerguelen Province are not well constrained. In an effort to gain more understanding about the evolution of this area, we have used satellite gravity to identify additional fracture zones. As they are likely to be associated with high frequency and low amplitude gravity anomalies, we have computed the vertical derivative map instead of the regular satellite gravity map. Using this approach, we have identified a series of fracture zones in the Enderby Basin, which are aligned with the Mesozoic fracture zones in the Perth Basin and converge to the Kerguelen Fracture Zone. In the conjugate Bay of Bengal, we traced an equivalent pattern of fracture zones which, together, better constrain the early evolution of this part of the Indian Ocean. Synthesis of these images and the other available data from the Kerguelen Province, suggests that the spreading of India from both Australia and Antarctica is closely related. Spreading between the three continents appears to have begun about the same time, in the early Cretaceous and thus, the accretion of some parts of the Kerguelen Province must have occurred before the onset of the quiet magnetic period at 118 Ma. At about 96–99 Ma, when the spreading direction in the Indian Ocean had changed into a N-S direction, it also took place throughout the Kerguelen Province. We find that previously proposed slow spreading in the Diamantina Zone and Labuan Basins, between 96–99 Ma and the initiation of the Southeast Indian Ridge at 43 Ma, could not have taken place. Furthermore, we suggest that there is growing evidence that the same is true for spreading in the eastward continuation of the Diamantina Zone and Labuan Basin, between Australia and Antarctica. Initiation of spreading in this area is likely to be contemporaneous with the spreading in the Kerguelen Province and, thus, older than 96–99 Ma. This revised version was published online in November 2006 with corrections to the Cover Date.  相似文献   

3.
Berndt  C.  Mjelde  R.  Planke  S.  Shimamura  H.  Faleide  J.I. 《Marine Geophysical Researches》2001,22(3):133-152
Ocean bottom seismograph (OBS), multichannel seismic and potential field data reveal the structure of the Vøring Transform Margin (VTM). This transform margin is located at the landward extension of the Jan Mayen Fracture Zone along the southern edge of the Vøring Plateau. The margin consists of two distinctive segments. The northwestern segment is characterized by large amounts of volcanic material. The new OBS data reveal a 30–40 km wide and 17 km thick high-velocity body between underplated continental crust to the northeast and normal oceanic crust in the southwest. The southeastern segment of the mar is similar to transform margins elsewhere. It is characterized by a 20–30 km wide transform margin high and a narrow continent-ocean transition. The volcanic sequences along this margin segment are less than 1 km thick. We conclude from the spatial correspondence of decreased volcanism and the location of the fracture zone, that the amount of volcanism was influenced by the tectonic setting. We propose that (1) lateral heat transport from the oceanic lithosphere to the adjacent continental lithosphere decreased the ambient mantle temperature and melt production along the entire transform margin and (2) that right-stepping of the left-lateral shear zone at the northwestern margin segment caused lithospheric thinning and increased volcanism. The investigated data show no evidence that the breakup volcanism influenced the tectonic development of the southeastern VTM.  相似文献   

4.
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.  相似文献   

5.
An analysis is given of air-gun profiler and magnetic data obtained in the central North Atlantic between 12° and 18°N. Eight fracture zones were crossed, one of which (the 15°20N fracture zone) was traced over a distance of 1500 km. The mode of adjustment of fracture zones to a change in direction of spreading is discussed. It is shown that also if this new direction would lead to an opening of the fracture zone, and adjustment fracture can originate and actually does so in several instances.The about E-W fracture zones dominate the structure of the Ridge province entirely, both with regard to the topography and to the magnetics. A magnetic model is proposed accounting for the different types of anomalies found over fracture zones. No intrusive bodies are needed to explain these anomalies.The origin of fracture zones is related to thermal contraction of a cooling lithosphere while moving from the ridge. Thermal contraction may also explain how the American and the African plates are freed from the grip they are caught in by the fanning of the fracture zones in the central North Atlantic. The fanning of fracture zones has consequences for the determination of the pole of spreading. This pole can only be found as a best fit from a synthesis of the total plate boundary, i.e. from the Azores to Bouvet Island. Local poles have only restricted value, since deviations up to 5 deg occur from a small circle pattern based on existing data.Several huge structures, viz. Researcher Ridge and Royal Trough, are found in the area which seem to parallel the flow lines of the fracture zone system. No adequate explanation exists for these structures.  相似文献   

6.
The Late Cretaceous–Paleocene rifting in the NW Vøring Basin is characterized by four main fault complexes and pronounced upper-crustal structural segmentation. The fault complexes are linked by accommodation zones, which separate fault systems of different polarities and thick from thinner coeval sedimentary successions. Structural and stratigraphic analyses suggest that the early rift phase (∼81 to 65 Ma) was characterized by large-scale normal faulting, along-margin segmentation and varying structural styles; whereas the late rift phase (∼65 to 55 Ma) was associated with continued extension, regional uplift, intrusive igneous activity and subsequent erosion. The rifting ended with breakup at ∼55 Ma accompanied by massive, but gradually waning extrusive igneous activity over the next 3 Myr. The mode of rifting appears to have changed from brittle to more ductile extensional deformation from the early to late rift phase. The changing rift rheology is probably related to the arrival of the Iceland mantle plume and initiation of associated igneous activity. Hence, the NW Vøring Basin provides an example of complex interaction of structural and magmatic relationships during rifting and breakup.  相似文献   

7.
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.  相似文献   

8.
《Marine Geology》2003,193(1-2):93-127
This study is based on detailed investigation of sediment cores and high resolution seismics. We identified and describe five lithofacies on the Vøring Plateau and eight on the mid-Norwegian continental slope. The various lithofacies are mainly related to the fluctuations of the Fennoscandian Ice Sheet and the varying intensity of bottom currents and inflow of Atlantic water masses. Ocean circulation was highly variable between 40 and 22 14C ka BP, being vigorous during interstadials and sluggish during stadials. Between ca 22 and 15 14C ka BP the sedimentary environment was significantly influenced by fluctuations of the Fennoscandian Ice Sheet, repeatedly reaching the outermost shelf. These fluctuations are reflected in the sedimentary record as ice-rafted debris (IRD) accumulation peaks, deposition of stratified diamicton, and glacigenic debris flows on the continental slope. During this period the sediment accumulation rate increased, bottom currents influenced the sedimentary pattern, and surface waters were seasonally ice-free, indicating inflow of Atlantic waters. Subsequent to ca 15 14C ka BP the glacier influence decreased as the margin of the Fennoscandian Ice Sheet retreated to reach the coast before 12.5 14C ka BP. The modern sedimentary environment is characterised by relatively strong bottom current action, causing winnowing or non-deposition down to approximately 1000 m water depth.  相似文献   

9.
We use recently acquired magnetic and SeaBeam bathymetric data to examine the spreading rates and plate boundary geometry of the Mid-Atlantic Ridge 30°–36° S. Using a statistically rigorous estimation of rotation poles we develop a precise spreading history of the African—South American plate boundary. The total opening rate for 1–4.23 Myr (Plio-Pleistocene) is nearly constant at 32.3 ± 1 km Myr–1. The spreading rate apparently is faster in the Late Miocene (7.3-5.3 Myr), though this may reflect inaccuracies in the geomagnetic time scale. The rotation poles enable a plate boundary reconstruction with an accuracy of 2–3 km. The reconstructions also show that the plate boundary geometry underwent several changes since the late Miocene including the growth of one ridge segment from 40 to 105 km in length, and the reorientation of another ridge segment which has spread obliquely from 7 to 1 Myr. Pole calculations using both right- and left-stepping fracture zones show an offset of 1–2 km between the deepest, most linear part of a fracture zone trough and the former plate boundary location. The high-resolution plate kinematics suggests that the plate boundary, as a whole, evolves 2-dimensionally as prescribed by rigid plates. On a local scale, asymmetric accretion, asymmetric extension, small lateral ridge jumps (< 3 km), and intra-segment propagation result in minor plate boundary adjustments and deformation to the rigid plates.  相似文献   

10.
A detailed survey of a 1°×1°-square of seafloor 100 miles south-east of the Azores shows a strong correlation between directions of regional topographic and magnetic lineations. The area is dissected by the East Azores Fracture Zone at 36°55N, identified as the active Eurasian-African plate boundary, and by another large, non-active fracture zone at 36°10N. Both fracture zones strike 265° and are accompanied by large amplitude magnetic anomalies. The general strike in the area in between is 000°–015°. The skewing effect at this magnetic latitude is very sensitive to variations in strike of the magnetic contrasts. This effect was eliminated by a non-linear transformation which also gives the positions of magnetic contrasts. Some N-S contrasts were identified as sea floor spreading polarity contrasts (anomalies 31 and 32). Weak contrasts could be identified as topographic effects and gave a magnetization intensity of 5 A m-1. The identified sea floor spreading anomalies to both sides of the fracture zone at 36°10N agree very well, also quantatively, with a three-dimensional model for the fracture zone anomalies. This model describes the non-linear anomalies as end effects of the magnetic layer which is divided in blocks of alternating polarity.  相似文献   

11.
The ∼400 km-long passive continental margin west of the Lofoten–Vesterålen archipelago, off northern Norway, links the volcanic rifted Vøring margin and the sheared W Barents Sea margin. Multi-channel seismic reflection profiles, supplemented with crustal velocity, gravity and magnetic anomaly data are used to outline the regional setting and main tectono-magmatic features. A well-defined along-strike margin segmentation comprising three segments characterized by distinct crustal properties, structural and magmatic styles, sediment thickness, and post-opening history of vertical motion is revealed. The margin segments are governed by changes in fault polarity on Late Jurassic–Early Cretaceous border faults and are separated by coeval cross-margin transfer zones which acted as persistent barriers to rupture propagation and reflect the trend and character of older structural heterogeneities. The transfer zones spatially correlate to small-offset, early opening oceanic fracture zones, implying a structural inheritance from one rift episode to another culminating with lithospheric breakup at the Paleocene–Eocene transition. The pre-seafloor spreading margin structural evolution is governed by the older, predominantly Late Jurassic–Early Cretaceous structural framework. However, the margin also provides evidence for mid- and Late Cretaceous extension events that are poorly understood elsewhere off Norway. Furthermore, the Lofoten–Vesterålen post-breakup subsidence history contrasts with the adjacent margins reflecting breakup in thicker crust and a diminishing volume of high-velocity lower crust emplaced during breakup.  相似文献   

12.
Sea floor spreading anomalies in the Lofoten-Greenland basins reveal an unstable plate boundary characterized by several small-offset transforms for a period of 4 m.y. after opening. North of the Jan Mayen Fracture Zone, integrated analysis of magnetic and seismic data also document a distinct, persistent magnetic anomaly associated with the continent-ocean boundary and a locally, robust anomaly along the inner boundary of the break-up lavas. These results provide improved constraints on early opening plate reconstructions, which include a new anomaly 23-to-opening pole of rotation yielding more northerly relative motion vectors than previously recognized; and a solution of the enigmatic, azimuthal difference between the conjugate Eocene parts of the Greenland-Senja Fracture Zone if the Greenland Ridge is considered a continental sliver. The results confirm high, 2.36–2.40 cm yr–1, early opening spreading rates, and are consistent with the start of sea floor spreading during Chron 24r. The potential field data along the landward prolongations of the Bivrost Fracture Zone suggest that its location is determined by a Mesozoic transfer system which has acted as a first-order, across-margin tectono-magmatic boundary between the regional Jan Mayen and Greenland-Senja Fracture Zone systems, greatly influencing the pre-, syn- and post-breakup margin development.  相似文献   

13.
A sediment slide complex has been mapped on the West African continental margin north of Dakar, Senegal. Four major slides covering approximately 44,300 km2 were delineated by seismic reflection profiles, 3.5 and 12 kHz echograms and piston cores. Although the slide areas have been altered by later erosion and deposition by turbidity flows, the major components of the slides — slide scar, zones of hummocky and blocky slide material and zones of debris flow — are recognizable. Cores containing flow folds with horizontal axial surfaces substantiate the echogram interpretations of debris flow. Morphology and depositional areas of the slides indicate that several major slide movements have occurred in each of the various slide areas. The triggering mechanism for these slides is perhaps earthquakes associated with the Cape Verde Islands, Cape Verde Plateau, and adjacent fracture zones.  相似文献   

14.
A hypothesis-driven approach was used to examine the relative importance of pelagic and advective organic flux as drivers of macrofaunal community structure at mid-slope (900–1400 m) depths along the eastern Nordic Seas margin. Ice-free stations were sampled on the Vøring Plateau (VP, mainly pelagic flux) and Bear Island Fan (BIF, major downslope advection). Seasonally ice-covered stations with episodic inputs of fresh phytodetritus were located on the Svalbard Margin (SM, major current-driven advective input), and Yermak Plateau (YP, low advective input). Macrofaunal biomass was higher at stations with seasonal ice cover, suggesting a greater supply of bioavailable organic matter in the marginal ice zones. At VP, BIF and SM, box corer and multiple corer samples gave broadly consistent community data. Very large infauna in box corer samples from YP indicated the existence of a faunal component absent or not sampled at the other three stations. This faunal component and higher mean body size for 18 out of 23 major taxa at YP relative to VP was consistent with a hypothesis relating body size to interannual variability of sinking flux, but was confounded by a depth difference between the two stations. Consistent with predictions, taxa associated with subsurface ‘caching’ of fresh organic matter were important community elements at stations experiencing episodic pelagic input, but not at the advection-dominated BIF. Current-driven advective flux was associated with high suspension-feeder biomass at BIF, but not at SM. The predicted high representation of subsurface deposit-feeders at BIF was not found. Sipunculans (Nephasoma sp.) known to rapidly subduct fresh organic matter were found at all stations but were most abundant at VP. Comparison with data from the mid-1980s showed a large difference in estimated macrofaunal abundance on the Vøring Plateau, but good agreement in biomass. Community patterns on the Vøring Plateau were generally consistent with earlier data. Standing stock estimates from the Svalbard Margin and Yermak Plateau also agreed well with previous data. Results suggest that large-scale contrasts in organic flux can be used to predict some aspects of bathyal macrofaunal community composition in the Nordic Seas.  相似文献   

15.
 Continent–ocean fracture zones are the fossil transform offsets located along passive rifted continental margins. Kinematic models identify at least two principal stages in their evolution. During the first stage as rifting proceeds, continent–continent shearing dominates a narrow region in which the transform fault will eventually rupture. High-standing continental marginal ridges 50–100 km wide and bounding deep sedimentary basins, are derived in such settings. In stage two as sea-floor spreading proceeds, the younger oceanic block slides along the active transform, heating the older continental block, and possibly induces thermal uplift and accompanying denudation. Magnetic injection into the continental block at depth may also induce an isostatic uplift. After ridge–transform intersection time, mechanical coupling between the continental and oceanic blocks may influence the stratigraphy and structure of these margins. Received: 12 March 1996 / Revision received: 23 April 1996  相似文献   

16.
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17.
The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.Deceased 11 November 1991  相似文献   

18.
A Seabeam reconnaissance of the 400 km-long fast-slipping (88 mm yr-1) Heezen transform fault zone and the 55 km-long spreading center that links it to Tharp transform defined and bathymetrically described several types of ridges built by tectonic uplift and volcanic construction. Most prominent is an asymmetric transverse ridge, at which abyssal hills adjacent to the fault zone have been raised 2–3 km above normal rise-flank depths. Topographic and petrologic evidence suggests that this uplift, which has produced a 5400 m scarp from the crest of the ridge to the floor of a 10 km-wide transform valley, is caused by rapid serpentinization of upper mantle which has been exposed to hydrothermal circulation by fault-zone fracturing of an unusually thin crust. Transverse ridges have been thought atypical of fast-slipping transforms. One class of volcanic ridge more common at these sites is the overshot ridge, formed by prolongation of spreading-center rift zones obliquely across the transform. Overshot ridges are well developed at Heezen transform, especially at the eastern end where an eruptive rift zone extending 60 km from the southern tip of the East Pacific Rise has built a transform-parallel ridge that fills the eastern transform valley. Obliteration of fault-zone structure by ridges overshooting from the spreading center intersections means that the topography of the aseismic fracture zones is not just inherited from that of the active transform fault zone. The latter has several en echelon and overlapping fault traces, linked by short oblique spreading axes that generally form pull-apart basins rather than volcanic ridges. Interpretation of the origin and pattern of the fault zone's tectonic and volcanic relief requires refinement of the plate geography and history of this part of the Pacific-Antarctic boundary, using new Seabeam and magnetic traverses to supplement and adjust the existing geophysical data base.  相似文献   

19.
SeaMARC II and Sea Beam bathymetric data are combined to create a chart of the East Pacific Rise (EPR) from 8°N to 18°N reaching at least 1 Ma onto the rise flanks in most places. Based on these data as well as SeaMARC II side scan sonar mosaics we offer the following observations and conclusions. The EPR is segmented by ridge axis discontinuities such that the average segment lengths in the area are 360 km for first-order segments, 140 km for second-order segments, 52 km for third-order segments, and 13 km for fourth-order segments. All three first-order discontinuities are transform faults. Where the rise axis is a bathymetric high, second-order discontinuities are overlapping spreading centers (OSCs), usually with a distinctive 3:1 overlap to offset ratio. The off-axis discordant zones created by the OSCs are V-shaped in plan view indicating along axis migration at rates of 40–100 mm yr–1. The discordant zones consist of discrete abandoned ridge tips and overlap basins within a broad wake of anomalously deep bathymetry and high crustal magnetization. The discordant zones indicate that OSCs have commenced at different times and have migrated in different directions. This rules out any linkage between OSCs and a hot spot reference frame. The spacing of abandoned ridges indicates a recurrence interval for ridge abandonment of 20,000–200,000 yrs for OSCs with an average interval of approximately 100,000 yrs. Where the rise axis is a bathymetric low, the only second-order discontinuity mapped is a right-stepping jog in the axial rift valley. The discordant zone consists of a V-shaped wake of elongated deeps and interlocking ridges, similar to the wakes of second-order discontinuities on slow-spreading ridges. At the second-order segment level, long segments tend to lengthen at the expense of neighboring shorter segments. This can be understood if segments can be approximated by cracks, because the propagation force at a crack tip is directly proportional to crack length.There has been a counter-clockwise change in the direction of spreading on the EPR between 8 and 18° N during the last 1 Ma. The cumulative change has been 3°–6°, producing opening across the Orozco and Siqueiros transform faults and closing across the Clipperton transform. The instantaneous present-day Cocos-Pacific pole is located at approximately 38.4° N, 109.5° W with an angular rotation rate of 2.10° m.y.–1 This change in spreading direction explains the predominance of right-stepping discontinuities of orders 2–4 along the Siqueiros-Clipperton and Orozco-Rivera segments, but does not explain other aspects of segmentation which are thought to be linked to patterns of melt supply to the ridge axis.There are 23 significant seamount chains in the mapped area and most are created very near the spreading axis. Nearly all of the seamount chains have trends which fall between the absolute and relative plate motion vectors.  相似文献   

20.
At 11°N latitude, the Mid-Atlantic ridge is offset 300 km by the Vema fracture zone. Between the ridge offset, the fracture consists of an elongate, parallelogram-shaped trough bordered on the north and south by narrow, high walls. The W-E trending valley floor is segmented by basement ridges and troughs which trend W10°N and are deeply buried by sediment. Uniform high heat flow characterizes the valley area. Seismically inactive valleys south of the Vema fracture, also trending W10°N, are interpreted as relict fracture zones. We explain the high heat flow and the shape of the Vema fracture as the results of secondary sea-floor spreading produced by a reorientation of the direction of sea-floor spreading from W10°N to west-east. This reorientation probably began approximately 10 million years ago. Rapid filling of the fracture valley by turbidites from the Demerara Abyssal plain took place during the last million years.The large amount of differential uplift in the Vema fracture is not explained by the reorientation model. Since the spreading rate across the valley is small compared to that across the ridge crest, we suggest that it takes place by intrusion of very thin dikes that cool rapidly and hence have high viscosity. Upwelling in the fracture valley will thus result in cosiderable loss of hydraulic head, according to models by Sleep and Biehler (1970), and recovery of the lost head could produce valley walls higher than the adjacent ridge crest. We further postulate that the spreading takes place along the edges of the fracture zone rather than in the center. This would account for the uniform distribution of heat flow along the fracture valley and for the lack of disturbance of the valley fill. As a consequence, a median ridge should form in the center, where head loss is compensated in the older crust; such a median ridge may be present. The width of the valley should be a function of the angle and time of reorientation, and of the spreading rate; the width so obtained for the Vema fracture is in accordance with the observed width. If this model is correct, the narrowness of the valley walls implies a thin lithosphere of very limited horizontal strength.  相似文献   

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