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1.
Reconstructions to total closure of the Australia-Antarctic continents causes an unacceptable overlap of Broken Ridge and Kerguelen Plateau. This has been partially resolved in the past by supposing that the northern part of Kerguelen, that is principally involved in the overlap, is younger than the remainder. We have revised the early reconstructions using a newly proposed breakup chronology of Australia and Antarctica which suggests that opening began at least 90 m.y. B.P. at an initially slow rate. This eliminates the overlap problem without invoking major age differences within the Kerguelen Plateau. We also suggest that the northeastern flank of Kerguelen Plateau may be underlain by the “missing” westward continuation of the Diamantina Zone. It may have been isolated on the Antarctic plate by a ridge crest jump at about anomaly 24 time that also formed the Ob Trench. 相似文献
2.
A revised model of seafloor spreading between India and Australia from the inception of spreading 125 m.y. to the change to a new system at 90 m.y. stems from the wider recognition of the M-series of magnetic anomalies off the southwestern margin of Australia, from a revised pole of opening between Australia and Antarctica, and by the extension in the central Wharton Basin of the Late Cretaceous set of magnetic anomalies back to 34. The phase of spreading represented by the later anomalies has been extended back to 90 m.y. in order to give a resolved pole that describes the rotation of India from Australia consistent with the M-series anomalies, DSDP site ages, and fracture zone trends. An abandoned spreading ridge in the Cuvier Abyssal Plain indicates a ridge jump within the first ten million years of spreading. Elsewhere, two kinds of ridge jump (one to the continental margin of Australia or India, the other by propagation of the spreading ridge into adjacent compartments thereby causing them to fuse), are postulated to account for other observations. 相似文献
3.
Char-Shine Liu Joseph R. Curray J.M. McDonald 《Earth and Planetary Science Letters》1983,65(2):331-342
From marine magnetic anomaly studies, a fossil spreading ridge is identified beneath the Nicobar Fan in the northwestern Wharton Basin. Several north-south-trending transform faults offset this ridge left-laterally east of the 86°E transform fault. Our findings show that this ridge, which was part of the plate boundary between the Indian and Australian plates, ceased its spreading shortly after formation of magnetic anomaly 20 (～ 45.6m.y. B.P.). Since the breakup of Australia and Antarctica probably occurred sometime between 110 and 90 m.y. B.P., we suggest that the Indian, Australian, and Antarctic plates were moving relative to one another from about 90 to 45 m.y. B.P. A triple junction would have existed in the southeastern Indian Ocean during that period of time. At anomaly 19 time (～ 45m.y. B.P.), the junction became inactive, and Australia and India became a single plate. The northwest-southeast-trending Southeast Indian Ridge was formed by connecting the India-Antarctica spreading center with the Australia-Antarctica spreading center. Its activity has continued to the present time. 相似文献
4.
Reconstructions of the southeastern margin of Gondwanaland require either a separation of East and West Antarctica or movement between the Lord Howe Rise and the Campbell Plateau. Previous plate tectonic reconstructions based on sea-floor spreading data eliminated the Lord Howe Rise-Campbell Plateau separation prior to 36 m.y. ago because of overlap. This conclusion is dependent on the reconstruction of Australia and Antarctica, interpretation of magnetic anomalies between Antarctica and the Campbell Plateau and the nature of the plate boundary in New Zealand. Revised reconstructions of the fit between Australia and Antarctica, and a reinterpretation of the magnetic anomalies between the Campbell Plateau and Antarctica suggest that there is no problem of overlap between the Lord Howe Rise and the Campbell Plateau, and that continued motion between these plates prior to 36 m.y. ago is a more plausible alternative to separation between East and West Antarctica. 相似文献
5.
K.D. Sullivan 《Earth and Planetary Science Letters》1983,62(3):321-339
The ocean-continent boundary in the Newfoundland Basin is defined as the seaward limit of a continental margin magnetic smooth zone. East of the Grand Banks this boundary is marked by a prominent NNE-trending magnetic anomaly that is correlated with the J-Anomaly (115 m.y.). South of Flemish Cap the smooth zone boundary strikes approximately 060° and is approximately 15 m.y. younger. Magnetic anomaly trends suggest two directions of motion during separation of Iberia and North America. The first phase of motion, commencing at J-Anomaly time with a spreading center strike of 015°, produced a rifted margin along the Grand Banks south of the Newfoundland Seamounts. No spreading occurred north of the seamounts during this phase, implying a counter-clockwise rotation of Iberia and no Grand Banks-Galicia Bank separation. The second phase began at about 102 m.y. with a shift of the pole of rotation to a location near Paris, producing a ridge orientation of approximately 060°. This spreading center extended north and east into the northern Newfoundland Basin and Bay of Biscay, producing a rifted margin south of Flemish Cap and opening of Biscay. This ridge geometry produced a component of extension across the Newfoundland Fracture Zone and the southeastward migration of the resultant “leaky” transform fault between 102 m.y. and the next pole shift produced the volcanic edifice of the Southeast Newfoundland Ridge. Fracture zone trends during this phase also exerted strong control on volcanism within the Newfoundland Seamount province; this activity ceased at about 97 m.y. The date at which the second phase ended is not well defined by presently available data. A RRR triple-junction existed in the northeastern Newfoundland Basin-western Biscay region for a short time prior to anomaly33/34 (80 m.y.) which marks the inception of a continuous Mid-Atlantic Ridge spreading center between the Newfoundland and Charlie Gibbs Fracture Zones. 相似文献
6.
Data gathered by recent “Islas Orcadas” cruises reveal the seafloor spreading pattern for a region south of the Agulhas/Falkland fracture zone system. The presence of a magnetic anomaly bight about the Agulhas Plateau indicates that the Agulhas Plateau may have developed at the site of a tectonic plate triple junction during the Late Cretaceous. A westward jump in the seafloor spreading center during the Late Maestrichtian (anomaly 34?31) reduced the offset across the Falkland/Agulhas fracture zone system and resulted in the formation of two conjugate aseismic ridges here described as the Meteor and Islas Orcadas Rises. The magnetic lineation pattern in the Agulhas Basin suggests that a tectonic plate (Malvinas Plate) existed during Campanian to Maestrichtian times. Relative rates of motion are calculated for Antarctica, South America, and Africa for the Late Cretaceous. 相似文献
7.
Rudi G. Markl 《Earth and Planetary Science Letters》1978,39(2):211-225
Magnetic and gravity anomaly data, together with features of the basement topography presented here show that the continental margin of western Australia, including the Naturaliste plateau, was shaped by NE-SW-trending rift segments offset by nearly orthogonal transform faults. A steep landward gradient of the isostatic gravity field and a lineated magnetic anomaly which occur together at the continental slope are interpreted as marking the ocean-continent boundary of the rifted margin off Perth and the sheared margin between Perth and the Wallaby plateaus. Anomalies diagnostic of the ocean-continent boundary are not observed at the margins of the Naturaliste plateau; the geometry of the rift zone here is adduced from the disposition of magnetic lineations, fracture zones, and basement features. A geophysical survey of the Naturaliste fracture zone shows it to be a continuous basement trough extending from the Diamantina fracture zone 800 km northwest to Dirck Hartog ridge. Similar basement troughs west of and orthogonal to the fracture zone imply that the region west/southwest of the Naturaliste plateau was, like the region north of it, formerly occupied by Greater India. Marine magnetic anomaly and basement trends suggest that the oceanic crust between the plateau and Diamantina fracture zone could be substantially older than Paleocene, heretofore the oldest crust identified between Australia and Antarctica. 相似文献
8.
E.S.W. Simpson J.G. Sclater B. Parsons I. Norton L. Meinke 《Earth and Planetary Science Letters》1979,43(2):260-264
East-west-trending Mesozoic magnetic anomalies M2 through M22 have been identified in the northern Mozambique Basin. These anomalies are best matched by sea floor created at 50°S trending N120°E and spreading at a rate of around 1.5 cm/yr. The northward increase in age inferred from the identifications of these anomalies are compatible with observed decrease in the “reliable” heat flow values from 1.4 to 1.1 μcal/cm2 s to the north in the basin. The anomalies terminate in the southern part of the Mozambique Channel against a magnetic quiet zone to the north. Both the Mozambique Basin anomalies and those recently observed off Antarctica are strong evidence in favour of a Gondwanaland reconstruction that places Dronning Maud Land against southern Mozambique, and a late Jurassic or older separation between Africa and Antarctica. 相似文献
9.
Magnetic lineations in the Pacific Jurassic quiet zone 总被引：1，自引：0，他引：1
Steven C. Cande Roger L. Larson John L. LaBrecque 《Earth and Planetary Science Letters》1978,41(4):434-440
Magnetic anomalies of low amplitude (<100 gammas) are present in the Jurassic magnetic quiet zone of the western Pacific Ocean. These small anomalies are lineated and can be correlated among the Phoenix, Hawaiian and Japanese lineation patterns. Thus, they represent seafloor spreading that recorded some sort of magnetic field phenomena prior to magnetic anomaly M25 at 153 m.y. B.P. The most likely possibility is that they represent a series of late Jurassic magnetic field reversals that occurred during a period of anomalously low magnetic field intensity. We propose a time scale of magnetic reversals between 153 and 158 m.y. B.P. to account for these anomalies and suggest that the dipole magnetic field intensity increased by a factor of about four from 160 to 140 m.y. B.P. in the late Jurassic. 相似文献
10.
D.C. Mishra 《Earth and Planetary Science Letters》1984,71(1):173-180
Magnetic anomalies over the continental shelf off the east coast of India (Orissa) suggest the presence of a highly magnetic rock type magnetized with an intensity of 900 nT in a direction, azimuth(A) = 150° and inclination(I) = +65°. This suggest the occurrence of igneous volcanic rocks which is confirmed from samples found below Tertiary sediments from a few boreholes in this region. The depth of this rock type as estimated from magnetic anomalies varies from approximately 1–2 km near the coast to 4–4.5 km towards the shelf margin. This direction of magnetization is the reverse of the reported direction of magnetization for the Rajmahal Traps of the Cretaceous period (100–110 m.y). A small strip of the body near the continental shelf margin appears, however, to possess normal magnetization suggesting the occurrence of normal and reversed polarities side by side, a characteristic typical for oceanic magnetic anomalies. The reversed polarity of the rocks on the continental shelf suggests that they correspond probably to the MO reversal (115 m.y.) on world magnetostratigraphic scale and provide a paleolatitude of 47°S for the land mass of India which agrees with the palaeoreconstruction of India and Antarctica. In this reconstruction, the Mahanadi Gondwana graben on the Indian subcontinent falls into line with the Lambert Rift in Antarctica, suggesting a probable common ancestry. The volcanic rocks on the continental shelf off the east coast of India might represent a missing link, that is, rocks formed between India and Antarctica at the time of the break-up of Gondwanaland. Satellite magnetic anomalies (MAGSAT) recorded over the Indian shield and interpreted in terms of variations in the Curie point geotherm provide a direction of magnetization which also places this continent close to Antarctica. As such MAGSAT anomalies recorded over eastern Antarctica are found compatible with those recorded over the Indian shield. 相似文献
11.
Studies of marine magnetic anomaly data from the Shikoku basin reveal magnetic lineations which strike northwest almost parallel to the trend of the Palau-Kyushu ridge. The lineation pattern is best developed in the western part of the basin and we can confidently identify a sequence of anomalies 7 through 5E between the base of the Palau-Kyushu ridge and the center of the basin. In the eastern part of the basin the basement morphology is rough and complex and magnetic anomalies can not be identified unequivocally. We infer that the Palau-Kyushu ridge and the Izu-Bonin island arc began separating about 27 m.y. B.P. An interval of rapid separation (4.2 cm/yr) occurred between 26 and 22.5 m.y. B.P. which approximately coincides with a period of intense volcanic activity in Japan. The observed magnetic lineation pattern and basement morphology can be best explained if the Shikoku basin formed at a two-limb spreading system during the Late Oligocene to Middle Miocene. Subsequently the eastern half of the basin was disrupted by fractures as the Iwo-Jima ridge collided with the Japanese islands. The accretionary process which formed the crust of the Shikoku marginal basin appears similar to that operating at mid-ocean ridges of the world. 相似文献
12.
C.A. Williams 《Earth and Planetary Science Letters》1975,24(3):440-456
An analysis of the magnetic anomaly profiles in the Bay of Biscay provides evidence for the former existence of an E-W trending sea-floor spreading axis in Biscay. Identification of the magnetic anomalies indicates that the opening of the Bay of Biscay took place during the Cretaceous, between Barremian and Maestrichtian times, and involved the formation of a triple-ridge junction with the Mid-Atlantic Ridge between 80 and 73 m.y. ago. The asymmetric distribution of magnetic anomalies in the Bay of Biscay is confirmed. This evidence, together with a proposed Lower Cretaceous development of the Mid-Atlantic Ridge suggests that Biscay evolved as a result of a three-phase rotation of Iberia. 相似文献
13.
An identification of anomalies 31–34 is presented for the North Atlantic. North of the Azores-Gibraltar Ridge this implies a revision of the identification of the magnetic anomalies older than anomaly 26. DSDP site 10 in the western North Atlantic appears to be located on the old end of anomaly 33. The relative spacings of anomalies 29–34 in the North and South Atlantic, North and South Pacific and Indian Oceans are compared and the estimated relative widths of the magnetic polarity intervals in the Late Cretaceous are revised. 相似文献
14.
This study reports K/Ar ages for basalts from four areas in central Iceland where erosion of structural highs has exposed stratigraphically older levels of the lava pile. The four areas are the Eyjafjördur regional anticline and the Tjörnes horst in north Iceland and the Borgarnes and Hreppar regional anticlines in south Iceland. Three of the areas have their older plateau basalts within the range of at least 8.5–9.5 m.y. old. Only the Hreppar area does not have any exposed rocks much older than about 2.5 m.y. The Tjörnes data confirm that the exposed Husavik faults have played a major role in the transform displacement of the Tjörnes Fracture Zone. The results are further evidence that the spreading axes through Iceland have had a history of shifting their location. Analysis of our results suggests that the regional anticlines of Iceland, a seeming structural anomaly in a spreading regime, have resulted from shifting spreading axes which transitionally coexist and create regional anticlines in between. 相似文献
15.
A magnetic anomaly map of the northern part of the Philippine Sea plate shows two conspicuous north–south rows of long-wavelength anomalies over the Izu–Ogasawara (Bonin) arc, which are slightly oblique to the present volcanic front. These anomalies are enhanced on reduced-to-pole and upward-continued anomaly maps. The east row is associated with frontal arc highs (the Shinkurose Ridge), and the west row is accompanied by the Nishi-Shichito Ridge. Another belt of long-wavelength anomalies very similar to the former two occurs over the Kyushu–Palau Ridge. To explain the similarity of the magnetic anomalies, it is proposed that after the spreading of the Shikoku Basin separated the Izu–Ogasawara arc from the Kyushu–Palau Ridge, another rifting event occurred in the Miocene, which divided the Izu–Ogasawara arc into the Nishi-Shichito and Shinkurose ridges. The occurrence of Miocene rifting has also been suggested from the geology of the collision zone of the Izu–Ogasawara arc against the Southwest Japan arc: the Misaka terrain yields peculiar volcanic rocks suggesting back-arc rifting at ～ 15 Ma. The magnetic anomaly belts over the Izu–Ogasawara arc do not extend south beyond the Sofugan Tectonic Line, suggesting a difference in tectonic history between the northern and southern parts of the Izu–Ogasawara arc. It is estimated that the Miocene extension was directed northeast–southwest, utilizing normal faults originally formed during Oligocene rifting. The direction is close to the final stage of the Shikoku Basin spreading. On a gravity anomaly relief map, northeast–southwest lineaments can be recognized in the Shikoku Basin as well as over the Nishi-Shichito Ridge. We thus consider that lines of structural weakness connected transform faults of the Shikoku Basin spreading system and the transfer faults of the Miocene Izu–Ogasawara arc rifting. Volcanism on the Nishi-Shichito Ridge has continued along the lines of weakness, which could have caused the en echelon arrangement of the volcanoes. 相似文献
16.
The Parece Vela Basin is a back-arc basin. It is approximately 5000 m deep and is divided into two topographic provinces by the north-trending Parece Vela Rift. The western province is thinly sedimented and topographically rough. The eastern province is blanketed by a thick apron of volcaniclastic sediments which were derived from the West Mariana Ridge. The Parece Vela Rift is composed of a series of discrete deeps and troughs with depths commonly of 6 km and locally exceeding 7 km.Petrologic and seismic refraction data indicate that the Parece Vela Basin is of oceanic character.Low-amplitude, nort-trending, lineated magnetic anomalies are present in the basin and appear symmetric about a line near the Parece Vela Rift. In the central latitudes of the basin seafloor spreading anomalies 10 (30 m.y. B.P.) to 5E or 5D (18 or 17 m.y. B.P.) can be identified. The uncertainty in identifying the youngest anomaly may be due to ridge jumps near the end of spreading. Spreading may have started slightly later in the northern end of the basin. Anomalies in the eastern province are disrupted and are difficult to correlate. DSDP results indicate post-spreading volcanism on the eastern side of the basin and this may have degraded the anomalies. The age obtained in the western province of the basin at DSDP Site 449 (～25m.y. B.P.) is in close agreement with that obtained from the magnetic data (～26m.y. B.P.).It is hypothesized that subduction was occurring at a west-dipping subduction zone east of the Palau-Kyushu Ridge in the Early Oligocene. This volcanic arc split about 31 or 32 m.y. ago and interarc spreading was initiated between the Palau-Kyushu Ridge (which then became a remnant arc) and the West Mariana Ridge. The Parece Vela Basin formed between the ridges by two-limb seafloor spreading. Spreading stopped about 17 or 18 m.y. ago.Like certain other marginal basins, the Parece Vela Basin is deeper than predicted from depth vs. age curves. The average heat flow for the Parece Vela Basin is in agreement with that predicted from heat flow vs. age curves.The origin of the Parece Vela Rift is unclear. It may represent the extinct spreading center or may be a postspreading feature. 相似文献
17.
D. VOGELSANG 《Geophysical Prospecting》1972,20(2):317-328
The ground follow-up of a magnetic and radiometric air survey had to cope with the usual dilemma to check a great number of anomalies within a short time. A limited aggregate of magnetic anomalies, expected to correspond to magnetite quartzites was statistically selected for ground identification by this method: the ratios length/width times amplitude were listed for all coherent contours and the calculation of the standard deviation per unit area resulted in different key-numbers for a new lithological “Salem Unit” and for the charnockitic or gneissic environment. The ground work thus directed and reduced by 85% yielded a substantial potential of iron ore. This was supported by by abundant determinations of the magnetic susceptibilities, confirming the sources of anomalies and revealing the amenabilities of iron ores for the magnetic separation process. The lines of truncation of anomalies were found to represent a system of local and regional faults and shear zones, which segregated the area into different tectonic blocks. These sutures have also provided the ways of intrusion for alkaline and basic magmae in the style of a “Rift” structure, housing several carbonatites and impregnations of metal sulphides. The airborne radiometrics obtained many uranium indications by gamma ray spectrometry. However, they led only to disseminated uranium-silicates, associated with syenites, granites and pegmatites, each emanating a characteristic photon energy spectrum. But in general the radiation of thorium prevails, marking northern Madras as a “Thorium Province”. 相似文献
18.
Four high-quality seismic refraction profiles were recorded parallel to the structural grain in the Cuvier Basin and adjacent Wharton Basin to study the nature of the earth's crust in this area. The principal result of this experiment is that this area is generally floored with oceanic crust. No transitional velocity structure exists at the base of the continental slope. Departures from a standard oceanic crustal section are observed on an intermediate profile that are attributed to structural complications on the flank of an abandoned spreading ridge. Additional magnetic anomaly profiles in the eastern Cuvier Basin are used to correlate the lineations in that area with Early Cretaceous reversals M-5 to M-10. This correlation dates the onset of plate separation at 120–125 m.y., essentially contemporaneous with the opening of the Perth Basin to the south. However, it leaves a 220-km gap between M-4 and M-5 in the Cuvier Basin that suggests a ridge jump of that magnitude occurred nominally at 118 m.y. Early Cretaceous magnetic lineations northwest of the Exmouth Plateau suggest that spreading at the seaward edge of the Exmouth Plateau began 120 m.y. ago, while Late Jurassic marine sediments and fault structures landward of the Exmouth Plateau suggest rifting in that area at 155 m.y. 相似文献
19.
Les P. Beard 《Geophysical Prospecting》2000,48(4):745-761
Long, structurally undeformed north–south trending structures show no magnetic anomaly at the magnetic equator, except at the north and south truncations of the structure. However, folding, faulting, differential erosion or other structural deformation can produce detectable magnetic anomalies in a generally north–south trending equatorial structure. Spatial variation in magnetic susceptibility or remanent magnetization can also produce anomalies in equatorial north–south structures. These anomaly patterns are often more complicated than patterns produced by similar structures at high latitudes, but interpretational insight can be gained through numerical modelling of common structures. Reduction-to-pole and analytic signal filters can aid in interpretation of equatorial anomalies, but these must be applied carefully because of instabilities deriving from filter design and noise amplification. 相似文献
20.
Evolution of the Tasman Sea reappraised 总被引：2，自引：0，他引：2
We reexamined available marine magnetics data from the Tasman Sea and reidentified sea-floor spreading anomalies in the southern portion of the basin. From the revised magnetic lineations and fracture zones we calculated new finite rotations which descrobe the evolution of the basin in terms of a simple two-plate spreading system active between about 82 and 60 m.y. ago. Allowing for the probable continental origin of the Dampier ridge, the predicted displacement of the western margin of the Lord Howe rise relative to the eastern Australian margin in the northern Tasman basin is consistent with their observed separation. Thus, the controversial episode of subduction of Tasman basin crust at the east Australian margin proposed in earlier studies is no longer necessary. 相似文献