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1.
Norbert J. Schulz Robert S. Detrick Stephen P. Miller 《Marine Geophysical Researches》1988,10(1-2):41-57
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
2.
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. 相似文献
3.
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 相似文献
4.
Gravity data collected at two different levels, sea-surface and seafloor, are compared and interpreted to characterize the effect of hydrothermal circulation on the sediment density in Middle Valley, a deeply sedimented spreading center on the Juan de Fuca Ridge. The sea-surface gravity data constrain density variations at depth beneath the seafloor, whereas sea-bottom measurements are more sensitive to shallow sources. At least two different types of hydrothermal signatures in the sediments can be distinguished from the gravity data: short-wavelength anomalies associated with sulfide deposits and broader anomalies associated with areas of lithified sediments. In Middle Valley, three distinct gravity anomalies were identified. (a) An anomaly over a sulfide mound, south of Bent Hill, shows that the sulfide body extends to depths of 120 to 180 m and has been fed by several near-surface conduits. (b) An anomaly at the base of the fault bordering the valley to the east is interpreted as a regional anomaly combined with the local effect of lithified sediments and possibly sulfide deposits. (c) An anomaly paralleling an intra-valley fault, that limits the deepest part of the graben, is interpreted as indicating lithification of the upper sediment layer. A high heat flow anomaly is located 1 to 2 km east of this fault, suggesting that sediment lithification occurred in a wide band above the fault and eastward to the current high heat flow area, due to the progressive migration of the hot fluid circulation. 相似文献
5.
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. 相似文献
6.
Lodolo Emanuele Coren Franco Schreider Anatoly A. Ceccone Giulio 《Marine Geophysical Researches》1997,19(5):439-450
The southwestern part of the Scotia Sea, at the corner of the Shackleton Fracture Zone with the South Scotia Ridge has been investigated, combining marine magnetic profiles, multichannel seismic reflection data, and satellite-derived gravity anomaly data. From the integrated analysis of data, we identified the presence of the oldest part of the crust in this sector, which tentative age is older than anomaly C10 (28.7 Ma). The area is surrounded by structural features clearly imaged by seismic data, which correspond to gravity lows in the satellite-derived map, and presents a rhomboid-shaped geometry. Along its southern boundary, structural features related to convergence and possible incipient subduction beneath the continental South Scotia Ridge have been evidenced from the seismic profile. We interpret this area, now located at the edge of the south-western Scotia Sea, as a relict of ocean-like crust formed during an earlier, possibly diffuse and disorganized episode of spreading at the first onset of the Drake Passage opening. The successive episode of organized seafloor spreading responsible for the opening of the Drake Passage that definitively separated southern South America from the Antarctic Peninsula, instigated ridge-push forces that can account for the subduction-related structures found along the western part of the South Scotia Ridge. This seafloor accretion phase occurred from 27 to about 10 Ma, when spreading stopped in the western Scotia Sea Ridge, as resulted from the identification of the marine magnetic anomalies. 相似文献
7.
Yoshifumi Nogi Kumiko Nishi Nobukazu Seama Yoichi Fukuda 《Marine Geophysical Researches》2004,25(3-4):221-231
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. 相似文献
8.
G. S. Heinson A. White L. K. Law Y. Hamano H. Utada T. Yukutake J. Segawa H. Toh 《Marine Geophysical Researches》1993,15(2):77-100
From July to November 1988, a major electromagnetic (EM) experiment, known as EMRIDGE, took place over the southern end of the Juan de Fuca Ridge in the northeast Pacific. It was designed to complement the previous EMSLAB experiment which covered the entire Juan de Fuca Plate, from the spreading ridge to subduction zone. The principal objective of EMRIDGE was to use natural sources of EM induction to investigate the processes of ridge accretion. Magnetotelluric (MT) sounding and Geomagnetic Depth Sounding (GDS) are well suited to the study of the migration and accumulation of melt, hydrothermal circulation, and the thermal evolution of dry lithosphere. Eleven magnetometers and two electrometers were deployed on the seafloor for a period of three months. Simultaneous land-based data were made available from the Victoria Magnetic Observatory, B.C., Canada and from a magnetometer sited in Oregon, U.S.A.Changes in seafloor bathymetry have a major influence on seafloor EM observations as shown by the orientation of the real GDS induction arrows away from the ridge axis and towards the deep ocean. Three-dimensional (3D) modelling, using a thin-sheet algorithm, shows that the observed EM signature of the Juan de Fuca Ridge and Blanco Fracture Zone is primarily due to nonuniform EM induction within the ocean, associated with changes in ocean depth. Furthermore, if the influence of the bathymetry is removed from the observations, then no significant conductivity anomaly is required at the ridge axis. The lack of a major anomaly is significant in the light of evidence for almost continuous hydrothermal venting along the neo-volcanic zone of the southern Juan de Fuca Ridge: such magmatic activity may be expected to have a distinct electrical conductivity signature, from high temperatures, hydrothermal fluids and possible melt accumulation in the crust.Estimates of seafloor electrical conductivity are made by the MT method, using electric field records at a site 35 km east of the ridge axis, on lithosphere of age 1.2 Ma, and magnetic field records at other seafloor sites. On rotating the MT impedance tensor to the principal axis orientation, significant anisotropy between the major (TE) and minor (TM) apparent resistivities is evident. Phase angles also differ between the principal axis polarisations, and TM phase are greater than 90° at short periods. Thin-sheet modelling suggests that bathymetric changes accounts for some of the observed 3D induction, but two-dimensional (2D) electrical conductivity structure in the crust and upper mantle, aligned with the ridge axis, may also be present. A one-dimensional (1D) inversion of the MT data suggests that the top 50 km of Earth is electrically resistive, and that there is a rise in conductivity at approximately 300 km. A high conductivity layer at 100 km depth is also a feature of the 1D inversion, but its presence is less well constrained. 相似文献
9.
Resumé Cet article présente des données bathymétriques et magnétiques de la région axiale de la dorsale sud-ouest indienne au voisinage de la zone de fracture majeure Atlantis II. Elles proviennent pricipalement de la campagne MD34 (Marion-Dufresne, 1983).L'axe de la dorsale est défini par la vallée et l'anomalie magnétique qui lui est associée. Le rilief le long de l'axe varie localement très rapidement; A l'ouest de la zone de fracture Atlantis II, le plancher axial présente deux bombements séparés par une dépression importante (4600 m). Cette étude met en évidence la corrélation entre ces hauts bathymétriques, la forme de la vallée et la l'amplitude de l'anomalie magnétique axiale: lorsque la profondeur du plancher axial diminue, la vallée se creuse et son encaissement augmente. On observe ainsi sur les hauts bathymétriques une section d'axe très encaissée, associée à une anomalie magnétique d'amplitude plus importance.L'identification de l'anomalie 5 (10 Ma) sur chaque flanc de la dorsale sud-ouest indienne permet la reconstitution de cette isochrone qui montre clairement une évolution de la géométrie de l'axe: à l'époque de l'anomalie 5, l'axe était composé de segments perpendiculaires à la direction d'expansion, décalés par des failles transformantes, alors qu'il apparait actuellement continu et formé sur les hauts topographiques de courts segments perpendiculaires à la direction d'expansion (et dans les dépressions par des sections d'axe très obliques).La carte bathymétrique met en évidence des lignes de crêtes grossièrement Nord-Sud (007°) dont la direction diffère de la direction d'expansion (357°) déduite des reconstructions, et parallèle à la zone de fracture majeure Atlantis II. Sur les dorsales lentes, les zones de fractures mineures, n'indiqueraient donc pas la véritable direction d'expansion.
The axial region of the Southwest Indian Ridge between 53° E and 59° E: Evolution during the last 10 Ma
An interpretation of bathymetric and magnetic data obtained aboard the R/V Marion Dufresne provides us with new information concerning the evolution of the Southwest Indian Ridge, in the region of the Atlantis II Fracture Zone (57° E), since 10 Ma. On all profiles, the ridge axis and the axial magnetic anomaly have been clearly recognized. Bathymetric data illustrate the rapid variation of depth along the axis. On the western side of the Atlantis II Fracture Zone, the along axis profile is characterized by a succession of two highs, and an important depression between them.Our data show a strong relationship between the regional axial depth, the steep-sidedness of the axial valley and the signature of the central magnetic anomaly. In particular, where the axis is deepest (4500 m), there is a wide, shallow axial valley which is oblique to the spreading direction, and a non-typical central magnetic anomaly signature. In contrast, where the regional axial depth is shallow (3500 m), the axial valley is deep, narrow, perpendicular to the spreading direction, and the central magnetic anomaly is high in amplitude. The ridge axis on the western side of the Atlantis II Fracture Zone appears to consist of short segments located on the axial highs, which are linked by oblique zones. On the eastern side, the ridge axis is continuous, and appears to be oblique to the spreading direction.Clearly lineated magnetic anomalies 3A (5 Ma) and 5 (10 Ma) have been identified and mapped. These magnetic data allow a reconstruction which shows an evolution of the axial geometry since 10 Ma. On the western side of the Atlantis II Fracture Zone, the axis at anomaly 5 time consisted of segments perpendicular to the spreading direction which were offset by transform faults. On the eastern side, the isochron A5 appears to be parallel to the present-day ridge axis. From this plate reconstruction, a spreading direction of 357° was deduced, and appears to be parallel to the Atlantis II Fracture Zone.On each flank of the Suuthwest Indian Ridge, our bathymetric data show elongated ridges, aligned in a north-south direction, which correlate with the axial topographic highs. This direction is not precisely parallel to the spreading direction deduced from plate reconstruction. The differences in these directions suggest that transverse relief on show spreading ridge flanks (which could be interpreted as indicating the location of minor fracture zones) may not be indicative of the seafloor spreading direction.相似文献
10.
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
11.
《Marine and Petroleum Geology》1988,5(4):306-323
A detailed aeromagnetic survey carried out across the northeast Newfoundland margin clearly shows the presence of sea floor spreading anomalies 25 to 34. Correlation of these anomalies with synthetic profiles shows an increase in the rate of spreading soon after anomaly 27 time. Three fracture zones can be identified by dislocations in the magnetic anomalies; their positions are confirmed on the depth to basement map of this region. An eastward extension of the southernmost fracture zone at latitude 49 N matches well with the Faraday Fracture Zone across the Mid Atlantic Ridge, and with a basement ridge known as Pastouret Ridge mapped off Goban Spur. By combining the present survey data with the previously collected shipborne measurements, we have also traced the westward continuation of the Charlie-Gibbs Fracture Zone under the Newfoundland shelf.A large amplitude magnetic anomaly lies along the margin and separates two zones with different magnetic characteristics: long wavelength small amplitude anomalies on the landward side, and quasi lineated anomalies on the seaward side. Seismic data compilations show that this large anomaly coincides with the ocean-continent boundary at most places north of Flemish Cap. Modelling of the magnetic anomalies indicate that the large amplitude anomaly is caused by the juxtaposition of highly magnetized oceanic crust against weakly magnetized continental crust; this situation is similar to that observed across the Goban Spur margin, which is a conjugate of the Flemish Cap margin. The presence of highly magnetized oceanic crust landward of anomaly 34 and within the Cretaceous Magnetic Quiet Zone is attested to by the presence of similar large amplitude anomalies south of the Flemish Cap and Goban Spur regions, but these do not mark the ocean-continent transition. 相似文献
12.
Laura S. L. Kong Robert S. Detrick Paul J. Fox Larry A. Mayer W. B. F. Ryan 《Marine Geophysical Researches》1988,10(1-2):59-90
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
13.
Active oceanic spreading in the northern North Fiji Basin: Results of the NOFI cruise of R/V L'Atalante (newstarmer project) 总被引：1，自引：0，他引：1
Yves Lagabrielle Etienne Ruellan Manabu Tanahashi Jacques Bourgois Georges Buffet Giovanni de Alteriis Jérôme Dyment Jean Goslin Eulàlia Gràcia-Mont Yo Iwabushi Philip Jarvis Masato Joshima Anne-Marie Karpoff Takeshi Matsumoto Hélène Ondréas Bernard Pelletier Olivier Sardou 《Marine Geophysical Researches》1996,18(2-4):225-247
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. 相似文献
14.
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
15.
Large-scale detachment faults on mid-ocean ridges （MORs） provide a window into the deeper earth. They have megamullion on their corrugated surfaces, with exposed lower crustal and upper mantle rocks, rela- tively high residual Bouguer gravity anomaly and P-wave velocity, and are commonly associated with ocean- ic core complex. According to 30 detachment faults identified on MORs, we found that their distances to the axis mostly range from 5 to 50 km, half-spreading rates range from 6.8 to 17 mm/a, and activity time ranges from recent to 3 Ma. Most of the detachment faults are developed on the slow spreading Mid-Atlantic Ridge （MAR） and ultra-slow spreading Southwest Indian Ridge （SWIRl, with the dominant half-spreading rates of 7-13 mm/a, especially 10-13 mm/a. Furthermore, they mostly occur at the inside corner of one segment end and result in an asymmetric seafloor spreading. The detachment faults on MORs are mainly controlled by the tectonism and influenced by the magmatism. Long-lived detachment faults tend to be formed where the ridge magma supply is at a moderate level, although the tectonism is a first-order controlling factor. At the slow spreading ridges, detachment faults tend to occur where local magma supply is relatively low, whilst at the ultra-slow spreading ridges, they normally occur where local magma supply is relatively high. These faults are accompanied by hydrothermal activities, with their relationships being useful in the study of hydrothermal polymetallic sulfides and their origin. 相似文献
16.
New Bathymetry and Magnetic Lineations Identifications in the Northernmost South China Sea and their Tectonic Implications 总被引：12，自引：1，他引：12
Shu-Kun Hsu Yi-ching Yeh Wen-Bin Doo Ching-Hui Tsai 《Marine Geophysical Researches》2004,25(1-2):29-44
The seafloor spreading of the South China Sea (SCS) was previously believed to take place between ca. 32 and 15 Ma (magnetic anomaly C11 to C5c). New magnetic data acquired in the northernmost SCS however suggests the existence of E–W trending magnetic polarity reversal patterns. Magnetic modeling demonstrates that the oldest SCS oceanic crust could be Late Eocene (as old as 37 Ma, magnetic anomaly C17), with a half-spreading rate of 44 mm/yr. The new identified continent–ocean boundary (COB) in the northern SCS generally follows the base of the continental slope. The COB is also marked by the presence of a relatively low magnetization zone, corresponding to the thinned portion of the continental crust. We suggest that the northern extension of the SCS oceanic crust is terminated by an inactive NW–SE trending trench-trench transform fault, called the Luzon–Ryukyu Transform Plate Boundary (LRTPB). The LRTPB is suggested to be a left-lateral transform fault connecting the former southeast-dipping Manila Trench in the south and the northwest-dipping Ryukyu Trench in the north. The existence of the LRTPB is demonstrated by the different patterns of the magnetic anomalies as well as the different seafloor morphology and basement relief on both sides of the LRTPB. Particularly, the northwestern portion of the LRTPB is marked by a steep northeast-dipping escarpment, along which the Formosa Canyon has developed. The LRTPB probably became inactive at ca. 20 Ma while the former Manila Trench prolonged northeastwards and connected to the former Ryukyu Trench by another transform fault. This reorganization of the plate boundaries might cause the southwestern portion of the former Ryukyu Trench to become extinct and a piece of the Philippine Sea Plate was therefore trapped amongst the LRTPB, the Manila Trench and the continental margin. 相似文献
17.
The Clarion-Clipperton Zone (CCZ) of the central Pacific is one of the few regions in the world’s oceans that are still lacking full coverage of reliable identifications of seafloor spreading anomalies. This is mainly due to the geometry of the magnetic lineations’ strike direction sub-parallel to the Earth’s magnetic field vector near the equator resulting in low amplitude magnetic anomalies, and the remoteness of the region which has hindered systematic surveying in the past. Following recently granted research licenses for manganese nodules in the CCZ by the International Seabed Authority, new magnetic data acquired with modern instrumentation became available which combined with older underway data make the identification of seafloor spreading anomalies possible for large parts of the CCZ and adjacent areas. The spreading rates deduced from the seafloor spreading patterns show a sharp increase at the end of Chron 21 (47.5 Ma) which corresponds to the age of the bend in the Hawaii-Emperor seamount chain and an associated plate tectonic reorganisation in the Central Pacific. An accurate map of crustal ages for the central-eastern Pacific based on our anomaly picks may provide a basis for improved plate tectonic reconstructions of the region. 相似文献
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19.
Bideau Daniel Hékinian Roger Sichler Bertrand Gràcia Eulàlia Bollinger Claire Constantin Marc Guivel Christelle 《Marine Geophysical Researches》1998,20(5):425-458
In August–September of 1995, 20 Nautile dives and detailed magnetic surveys (spaced every 1.8 km) were undertaken on two segments of the Mid-Atlantic Ridge between the Oceanographer and Hayes fractures zones. These two segments are only 65 km apart and show strong morphology and gravity contrasts. OH1 is shallower and has a large mantle Bouguer anomaly (MBA) bull's eye, whereas OH3 is deeper and has a smaller MBA bull's eye.Thirteen dives were devoted to segment OH1. The Median Ridge (MR) located on the central high (1700 m deep) is topped by 100 to 300 m high circular volcanoes. The volcanics consists mainly of porphyritic and/or vesicular pillows and volcaniclastics. The NVZ (2200 m deep), located in the valley floor east of the MR, consists of near aphyric fluid lava flows. A chain of off-axis volcanoes, displaying a magnetic continuity with surroundings, extends on both sides of the axis. Three volcanoes on the east side and one on the west side of the axis were explored and sampled by submersible. The off-axis increase of weathering, Fe-Mn coating and magnetic signature suggest that the volcanoes were built at or near the ridge axis. The spacing of NS elongated hills bearing circular volcanoes and separated large magnetic signature (2 to 4 km) depressions suggests that several similar volcanic events occured during the past 2 Ma. The last 1 Ma episode involves (1) the construction of an axial ridge (MR) by fissure eruptions and the formation of circular summit volcanoes by focused volcanism, and (2) the extrusion of fluid magma in the depressions formed by further fissuring and faulting of the MR. 相似文献
20.
Patriat Philippe Sauter Daniel Munschy Marc Parson Lindsay 《Marine Geophysical Researches》1997,19(6):457-480
The study of very low-spreading ridges has become essential to ourunderstanding of the mid-oceanic ridge processes. The Southwest Indian Ridge(SWIR) , a major plate boundary of the world oceans, separating Africa fromAntarctica for more than 100 Ma, has such an ultra slow-spreadingrate. Its other characteristic is the fast lengthening of its axis at bothBouvet and Rodrigues triple junctions. A survey was carried out in thespring of 1993 to complete a multibeam bathymetric coverage of the axisbetween Atlantis II Fracture Zone (57° E) and the Rodrigues triplejunction (70° E). After a review of what is known about the geometry,structure and evolution of the SWIR, we present an analysis of the newalong-axis bathymetric data together with previously acquiredacross-axis profiles. Only three transform faults, represented byAtlantis II FZ, Novara FZ, and Melville FZ, offset this more than 1000 kmlong section of the SWIR, showing that the offsets are more generallyaccommodated by ridge obliquity and non-transform discontinuities. From comparison of the axial geometry, bathymetry, mantle Bouguer anomaly and central magnetic anomaly, three large sections (east of Melville FZ, between Melville FZ and about 65°30 E, and from there to the Rodrigues triple junction) can be distinguished. The central member, east of Melville FZ, does not resemble any other known mid-oceanic ridge section: the classical signs of the accretion (mantle Bouguer anomaly, central magnetic anomaly) are only observed over three very narrow and shallow axis sections. We also apply image processing techniques to the satellite gravity anomaly map of Smith and Sandwell (1995) to determine the off-axis characteristics of the Southwest Indian Ridge domain, more especially the location of the triple junction and discontinuities traces. We conclude that the large-scale segmentation of the axis has been inherited from the evolution of the Rodrigues triple junction. 相似文献