首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
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.  相似文献   

2.
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.  相似文献   

3.
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   

4.
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.  相似文献   

5.
Earth’s fastest present seafloor spreading occurs along the East Pacific Rise near 31°–32° S. Two of the major hydrothermal plume areas discovered during a 1998 multidisciplinary geophysical/hydrothermal investigation of these mid-ocean ridge axes were explored during a 1999 Alvin expedition. Both occur in recently eruptive areas where shallow collapse structures mark the neovolcanic axis. The 31° S vent area occurs in a broad linear zone of collapses and fractures coalescing into an axial summit trough. The 32° S vent area has been volcanically repaved by a more recent eruption, with non-linear collapses that have not yet coalesced. Both sites occur in highly inflated areas, near local inflation peaks, which is the best segment-scale predictor of hydrothermal activity at these superfast spreading rates (150 mm/yr).  相似文献   

6.
The first map of the acoustic basement and a new map of the C4-C12 chrons are made for the area of the paleospreading West Scotia Ridge. The analysis of the constructed maps and the calculations showed that the bottom growth in the ridge axes began in the southeast in the interval of chron C12r (31.116–33.266 Ma B.P.). In the period of chron C11r (30.217–30.627 Ma), a hundred-kilometer northwestward jump of the spreading axis occurred. The maximal values of the bottom growth (about 6.3 cm/yr) were in the interval of chrons C6–C6B (18.748–22.546 Ma); then, the spreading began to fade. In the time of chron C3n.1r (3.300–4.493 Ma B.P.), the axis of the paleo-mid-oceanic ridge died. The spreading was accompanied with northeastward propagating of the axes, and the propagating proper had an impulse character.  相似文献   

7.
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.  相似文献   

8.
The morphological characteristics of the segmentation of the Central Indian Ridge (CIR) from the Indian Ocean Triple Junction (25°30S) to the Egeria Transform Fault system (20°30S) are analyzed. The compilation of Sea Beam data from R/VSonne cruises SO43 and SO52, and R/VCharcot cruises Rodriguez 1 and 2 provides an almost continuous bathymetric coverage of a 450-km-long section of the ridge axis. The bathymetric data are combined with a GLORIA side-scan sonar swath to visualize the fabric of the ridge and complement the coverage in some areas. This section of the CIR has a full spreading rate of about 50 mm yr–1, increasing slightly from north to south. The morphology of the CIR is generally similar to that of a slow-spreading center, despite an intermediate spreading rate at these latitudes. The axis is marked by an axial valley 5–35 km wide and 500–1800 m deep, sometimes exhibiting a 100–600 m-high neovolcanic ridge. It is offset by only one 40km offset transform fault (at 22°40S), and by nine second-order discontinuities, with offsets varying from 4 to 21 km, separating segments 28 to 85 km long. The bathymetry analysis and an empirical orthogonal function analysis performed on across-axis profiles reveal morphologic variations in the axis and the second-order discontinuities. The ridge axis deepens and the relief across the axial valley increases from north to south. The discontinuities observed south of 22°S all have morphologies similar to those of the slow-spreading Mid-Atlantic Ridge. North of 22°S, two discontinuities have map geometries that have not been observed previously on slow-spreading ridges. The axial valleys overlap, and their tips curve toward the adjacent segment. The overlap distance is 2 to 4 times greater than the offset. Based on these characteristics, these discontinuities resemble overlapping spreading centers (OSCs) described on the fast-spreading EPR. The evolution of one such discontinuity appears to decapitate a nearby segment, as observed for the evolution of some OSCs on the EPR. These morphological variations of the CIR axis may be explained by an increase in the crustal thickness in the north of the study area relative to the Triple Junction area. Variations in crustal thickness could be related to a broad bathymetric anomaly centered at 19°S, 65°E, which probably reflects the effect of the nearby Réunion hotspot, or an anomaly in the composition of the mantle beneath the ridge near 19°S. Other explanations for the morphological variations include the termination of the CIR at the Rodriguez Triple Junction or the kinematic evolution of the triple junction and its resultant lengthening of the CIR. These latter effects are more likely to account for the axial morphology near the Triple Junction than for the long-wavelength morphological variation.  相似文献   

9.
Analysis of Sea Beam bathymetry along the Mid-Atlantic Ridge between 24°00 N and 30°40 N reveals the nature and scale of the segmentation of this slow-spreading center. Except for the Atlantis Transform, there are no transform offsets along this 800-km-long portion of the plate boundary. Instead, the Mid-Atlantic Ridge is offset at intervals of 10–100 km by nontransform discontinuities, usually located at local depth maxima along the rift valley. At these discontinuities, the horizontal shear between offset ridge segments is not accommodated by a narrow, sustained transform-zone. Non-transform discontinuities along the MAR can be classified according to their morphology, which is partly controlled by the distance between the offset neovolcanic zones, and their spatial and temporal stability. Some of the non-transform discontinuities are associated with off-axis basins which integrate spatially to form discordant zones on the flanks of the spreading center. These basins may be the fossil equivalents of the terminal lows which flank the neovolcanic zone at the ends of each segment. The off-axis traces, which do not lie along small circles about the pole of opening of the two plates, reflect the migration of the discontinuities along the spreading center.The spectrum of rift valley morphologies ranges from a narrow, deep, hourglass-shaped valley to a wide valley bounded by low-relief rift mountains. A simple classification of segment morphology involves two types of segments. Long and narrow segments are found preferentially on top of the long-wavelength, along-axis bathymetric high between the Kane and Atlantis Transforms. These segments are associated with circular mantle Bouguer anomalies which are consistent with focused mantle upwelling beneath the segment mid-points. Wide, U-shaped segments in cross-section are preferentially found in the deep part of the long-wavelength, along-axis depth profile. These segments do not appear to be associated with circular mantle Bouguer anomalies, indicating perhaps a more complex pattern of mantle upwelling and/or crustal structure. Thus, the long-recognized bimodal distribution of segment morphology may be associated with different patterns of mantle upwelling and/or crustal structure. We propose that the range of observed, first-order variations in segment morphology reflects differences in the flow pattern, volume and temporal continuity of magmatic upwelling at the segment scale. However, despite large first-order differences, all segments display similar intra-segment, morphotectonic variations. We postulate that the intra-segment variability represents differences in the relative importance of volcanism and tectonism along strike away from a zone of enhanced magma upwelling within each segment. The contribution of volcanism to the morphology will be more important near the shallowest portion of the rift valley within each segment, beneath which we postulate that upwelling of magma is enhanced, than beneath the ends of the segment. Conversely, the contribution of tectonic extension to the morphology will become more important toward the spreading center discontinuities. Variations in magmatic budget along the strike of a segment will result in along-axis variations in crustal structure. Segment mid-points may coincide with regions of highest melt production and thick crust, and non-transform discontinuities with regions of lowest melt production and thin crust. This hypothesis is consistent with available seismic and gravity data.The rift valley of the Mid-Atlantic Ridge is in general an asymmetric feature. Near segment mid-points, the rift valley is usually symmetric but, away from the segment mid-points, one side of the rift valley often consists of a steep, faulted slope while the other side forms a more gradual ramp. These observations suggest that half-grabens, rather than full-grabens, are the fundamental building blocks of the rift valley. They also indicate that the pattern of faulting varies along strike at the segment scale, and may be a consequence of the three-dimensional, thermo-mechanical structure of segments associated with enhanced mantle upwelling beneath their mid-points.  相似文献   

10.
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.  相似文献   

11.
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.  相似文献   

12.
Still photographs and video images collected along the Neovolcanic Zone of the East Pacific Rise from 10°15′N to 11°53′N show that recent volcanic sheet flows, possibly less than 100 years old, are superimposed on an older sediment-laden pillow terrane. This recent activity is restricted to a narrow zone that crosses two topographic highs at 10°55′N and 11°26′N and diminishes along-axis away from these highs. The association of recent sheet flows with older flows and collapse structures on the overlapping spreading centers at 11°45′N supports the evolutionary model for the occurrence and evolution of overlapping spreading centers by MacDonald and others (1986, 1988).  相似文献   

13.
Geophysical data from 900 km of the Southwest Indian Ridge are used todescribe the pattern of evolution of the plate boundary between 61° Eand 70° E over the past 20 million years. The SWIR is anobliquely-opening, ultra slow-spreading axis, and east of61° E comprises a series of ridge sections, each about 100–120 kmin length. The orientation of these sections varies fromsub-orthogonal to oblique to the approximately N–S spreadingdirection. In general, the suborthogonal sections are shallower, commonlysubdivided into an array of discrete axial segments, and carry recognisablecentral magnetic anomalies. The majority of the oblique sections are single,continuous rifts without continuous axial magnetic signatures.Morphotectonics of the Southwest Indian Ridge crust have not previously beenwell constrained off-axis, and we here present sidescan sonar andswath bathymetric data up to 100 km from the ridge to demonstrate the complexities of its spatial and temporal evolution.A model is proposed that the segmentation style correlates with analong-axis variation between: (a) relatively thick crustal sections which overlie mantle sections with higher magmatic supply created in orthogonally-spreading segments and (b) those oblique sections associated with cooler, magmatically-starved mantle and thinner crust. These latter sections are formed at broad offset zones in theplate boundary, more precisely defined on faster-spreading ridges asnontransform discontinuities. The nonsystematic pattern of crustalconstruction, extensional basin formation and the absence of extension-parallel traces of discontinuities off-axis suggest that the oblique spreading sections are not fixed in space or time.  相似文献   

14.
TheSnake Pit Hydrothermal Site lies on the axis of the Mid-Atlantic Ridge at 23°22′ N latitude, about 30 km south of the Kane Transform Intersection. Active ‘black smoker’ vents and a surrounding field of hydrothermal sediment occur at the crest of a laterally extensive neovolcanic ridge. It is one of the first active hydrothermal vent fields to be found on a slow-spreading ridge axis and despite significant differences in its geologic setting from those of the East Pacific Rise, has many similarities to its fast-spreading counterparts. Although preliminary reports have documented many interesting aspects of these vents and their surroundings, new data collected from the manned submersible ALVIN and the deep-towed ANGUS camera system define the regional tectonic setting as well as the local geologic environment of this fascinating area. The Snake Pit vents are located on a local peak of a volcanic constructional ridge at a depth of 3450 m, 700–800 m deeper than vents known from the East Pacific Rise, Galapagos, or Juan de Fuca spreading centers. The vent field is at least 600 m long and up to 200 m wide and is covered by a thick blanket of greenish to yellow-orange hydrothermal sediment. Both active and extinct vents are perched along the crests of steep-sided sulfide mounds that reach heights of over 40 m. High-temperature (350° C) fluids are vented from black smoker chimneys and low-temperature (226° C) fluids seep from sulphide domes and subordinate anhydrite constructions. Water temperatures, flow rates, fluid chemistries, and mineralization are strikingly similar to vents of faster spreading ridge crests; however, a somewhat distinct fauna inhabit the area.  相似文献   

15.
The present morphology and tectonic evolution of more than 1500 kilometres of the Central Indian Ridge are described and discussed following the integration of GLORIA side-scan sonographs with conventional geophysical datasets. Segmentation of the ridge occurs by a series of ridge axis discontinuities ranging in periodicity along strike from 275 km to less than 30 km. These segment boundaries we have classified into two types: first order fracture zones of offsets greater than 50 km which bound five major (mega-) segments, and smaller scale structures of a variety of offset styles and amplitudes which cut four of these segments. We refer to these as ridge-axis discontinuities. The frequent opposite sense of offset identified between the first order structures and the subordinate discontinuities between these major structures is interpreted as resulting from the adjustment to new kinematic parameters after magnetic anomaly 20. As far as our data allows us to determine, the central major segment is not subdivided by minor ridge axis discontinuities, which we suggest is a result of its proximity to the Rodriguez hotspot.  相似文献   

16.
Observations of the median valley within the 24–30° N area ofthe Mid-Atlantic Ridge (MAR), using the IOSDL high resolutionside-scan sonar instrument TOBI, image four separate areas of themedian valley, containing part or all of nine spreading segments, and fivenon-transform discontinuities between spreading segments (NTDs).These high resolution side scan images were interpreted in parallel withmultibeam bathymetry (Purdy et al., 1990), giving a greater degree ofstructural precision than is possible with the multibeam data alone. Threedistinct types of NTD were identified, corresponding in part to typespreviously identified from the multibeam bathymetric survey of the area.Type 1 NTDs are termed septal offsets, and are marked by a topographic ridgeseparating the two spreading segments. The offset between the spreadingsegments ranges from 9 to 14 km. These can be further subdivided into Type1A in which the septa run parallel to the overall trend of the MAR and Type1B in which the septa lie at a high angle to the bulk ridge trend. Type 1ANTDs are characterised by overlap of the neovolcanic zones of the segmentson each side, and strong offaxis traces, while Type 1B NTDs show no overlapof neovolcanic zones, and weak offaxis traces. Type 2 NTDs arebrittle/ductile extensional shear zones, marked by oblique extensionalfractures, and associated with rotation of tectonic and volcanic structuresaway from the overall trend of the MAR. Type 3 NTDs are associated withoffsets of less than 5 km, and show no sign of any accommodating structure.In this type of NTD, the offset zone is covered with undeformed volcanics.The type of NTD developed at any locality along the ridge axis appears todepend on the amount of segment offset and segment overlap, the overalltrend of the mid-ocean ridge, the width of the zone of discontinuity, themedian valley offset and the longevity of the offset. These factorsinfluence the mechanical properties of the lithosphere across thediscontinuity, and ultimately the tectonic style of the NTD that can besupported. Thus brittle/ductile extensional shear zones are long-livedstructures favoured by large segment offsets, and small or negative segmentoverlaps. Septa can be short or long lived, and are associated with largesegment offsets. Segment overlaps vary from negative (an along axis gap) tozero, for Type 1B septal offsets, or positive to zero for Type 1A septaloffsets. Non-tectonised NTDs are generally short lived structures,characterised by small segment offsets and zero or positive overlaps.  相似文献   

17.
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.  相似文献   

18.
The Carlsberg Ridge lies between the equator and the Owen fracture zone. It is the most prominent mid-ocean ridge segment of the western Indian Ocean, which contains a number of earthquake epicenters. Satellite altimetry can be used to infer subsurface geological structures analogous to gravity anomaly maps generated through ship-borne survey. In this study, free-air gravity and its 3D image have been generated over the Carlsberg Ridge using a very high resolution data base, as obtained from Geosat GM, ERS-1, Seasat and TOPEX/POSEIDON altimeter data. As observed in this study, the Carlsberg Ridge shows a slow spreading characteristic with a deep and wide graben (average width ∼15 km). The transform fault spacing confirms variable slow to intermediate characteristics with first and second order discontinuities. The isostatically compensated region of the Carlsberg Ridge could be demarcated with near zero contour values in the free-air gravity anomaly images over and along the Carlsberg Ridge axes and over most of the fracture zone patterns. Few profiles have been generated across the Carlsberg Ridge and the characteristics of slow/intermediate spreading ridge of various orders of discontinuity could be identified. It has also been observed in zero contour image as well as in the characteristics of valley patterns along the ridge from NW to SE that different spreading rates, from slow to intermediate, are occurring in different parts of the Carlsberg ridge. It maintains the morphology of a slow spreading ridge in the NW, where the wide and deep axial valley (∼1.5–3 km) also implies the pattern of a slow spreading ridge. However, a change in the morphology/depth of the axial valley from NW to SE indicates the nature of the Carlsberg Ridge as a slow to intermediate spreading ridge. For the prevailing security restrictions, lat./lon. coordinates have been omitted in few images.  相似文献   

19.
The crust at mid-ocean ridges is formed through a combination of magmatic and tectonic processes. Along slow-spreading ridges, magmatism is inferred to be discontinuous and episodic, and lithospheric faulting may strongly interact with the melt supply system. These interactions can be studied for the first time at the Lucky Strike segment along the Mid-Atlantic Ridge (MAR), where a 3.4 km deep magma chamber (AMC) extending ~6 km along-axis is found at its centre (Singh et al. in Earth Planet Sci Lett 246:353–366, 2006). With an array of ocean bottom seismometers we have detected along this ridge segment approximately 400 microseismic events during a total of 6 days, and located 71 of them, whose local magnitudes ranged from 0.2 to 1.8. While most of the events were concentrated at non-transform offset and inside corners, three events with well-constrained locations were detected beneath the central volcano and at the edges of the AMC. Two of the microearthquakes, which occur in a brittle lithosphere and therefore at temperatures lower than 750°C, are deeper than the AMC and therefore very steep thermal gradients both along- and across-axis. Regionally seismicity deepens from ~6 km at the segment center to >10 km towards the ends.  相似文献   

20.
The first map of the acoustic basement and the new map of chrons C1-C5E for the region of the East Scotia mid-ocean ridge have been made. The analysis of the maps and the calculations have indicated that the sea-floor spreading at the ridge’s flanks started in its southeast in the interval of chrons C5Er-6An (18.52–20.17 Ma BP). The maximal spreading rate (5.3 cm/year) was in the interval of chrons C5Bn-C5Br (14.78–15.97 Ma BP). Then, the spreading rate was slow and increased again from 3–6 Ma BP until the present. The spreading in the last 1–2 Ma was accompanied by the propagating of the axes southwards in the E1, E2, and E4 segments and northwards in the E8 and E9 segments.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号