A revised fit of South America and South Central Africa

A new set of parameters for the total plate tectonic reconstruction of South America and south central Africa is presented: euler pole 46.75°N, 32.65°W; rotation angle 56.40°. This fit is constrained by at least three pre-drift tectonic features crossing from one continent to the other: (1) the geophysically defined eastern and western boundaries of the submarine Jurassic Outeniqua Basin (South Africa) and the Falkland Plateau Basin; (2) the Late Precambrian transcurrent fault and mylonite belts of Pernambuco (Brazil) and Foumban (West Africa); and (3) the Triassic northern tectonic front of the Cape Fold Belt and the major morphological feature on the Falkland Plateau with which it is closely lined up. Isotopic ages of Falkland Plateau gneisses correspond to Cape Pluton and Cape Fold Belt ages, suggesting their palaeoposition was within the realm of the Cape Fold Belt.In addition, the bathymetrically and geophysically defined northeastern apex of the Falkland Plateau fits into the re-entrant angle defined on the South African margin by the steep southeast-facing sheared Agulhas margin and the southern face of the Tugela Cone. Simultaneously known Precambrian outcrops in northeastern Brazil and in the Gulf of Benin area of West Africa are juxtaposed rather than overlapped. Reconstructions producing a closer fit of these cratonic areas are considered untenable.     

Tectonic structure and petrology of the Antarctic plate boundary near the Bouvet triple junction

An en echelon suite of four fracture zones, trending approximately N40°E, has been discovered during a survey of the Southwest Indian Ocean Ridge between Bouvet Island and 14°E. The largest of these fracture zones, the Islas Orcadas and Shaka, are less than 30 km wide, have more than 3 km of vertical relief, and are respectively 100 and 200 km in length. The morphology of these and the Bouvet and Prince Edward fracture zones have been used to compute a pole for the relative motion between Africa and Antarctica. This pole, at 4°S and 32°W, is within the range of previously computed pole positions.Ridge basalts were dredged at three separate locations: at the Conrad fracture zone near 55°40′S and 3°51′W, at the Islas Orcadas fracture zone near 54°5′S and 6°4′E, and at the ridge crest near 11°E. In addition, samples from a probable upper mantle intrusion were recovered from one wall of the Islas Orcadas fracture zone. The opposite wall was very different consisting entirely of normal mid-ocean ridge basalt.     

The single largest oceanic plateau: Ontong Java–Manihiki–Hikurangi

Oceanic plateaus are mafic igneous provinces commonly thought to derive from ascending mantle plumes. By far the largest, the Ontong Java Plateau (OJP) was emplaced ca. 120 Ma, with a much smaller magmatic pulse of ca. 90 Ma. Of similar age and composition, the Manihiki and Hikurangi Plateaus (MP and HP) are separated from the OJP by ocean basins formed during the Cretaceous long normal magnetic period. I present new seafloor fabric data that indicate the three plateaus formed as one (OJMHP). The data support previous interpretations that the Osbourn Trough is the relict of the spreading center that separated the MP and HP but they require a different interpretation than prevailing tectonic models for the Ellice Basin. Closely spaced, large offset, fracture zones in the Ellice Basin bound former right-stepping spreading segments that separated the OJP and MP. The MP was emplaced near the axis of the Pacific–Phoenix ridge and additional plateau fragments formerly bordered its eastern margins. Following OJMHP break-up, seafloor spreading removed these fragments to the east and SSE, together with the symmetric conjugates to the extant Phoenix magnetic lineations.     

Sea-floor spreading and the early opening of the North Atlantic

We have examined available magnetic and gravity data bearing on the initiation of sea-floor spreading in the North Atlantic between Ireland and Newfoundland. The change in character of the magnetic field on the continental margin on either side of the Atlantic from a landward magnetic quiet zone to a seaward “noisy”, magnetic signature is postulated to be related to a change from continental to oceanic crust. Sea-floor spreading between Ireland and Newfoundland was initiated during the long normal geomagnetic polarity interval in the Late Cretaceous. Rockall Trough may have opened at this time. At the end of the normal polarity interval (Late Santonian) the ridge axis jumped westward to bypass Rockall Trough and the related offset initiated the Charlie Gibbs fracture zone.A reconstruction is presented for the relative position between North America and Europe prior to the initiation of sea-floor spreading in the Late Cretaceous.     

St. John's Island (Red Sea): a new geophysical model and its implications for the emplacement of ultramafic rocks in fracture zones and at continental margins

New gravity and magnetic data from the northern Red Sea reveal the extent of the large gravity anomaly (164 mgal) and the presence of significant magnetic anomalies over St. John's Island. Spectral transformation and three-dimensional potential-field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required (1.2 g/cm³) suggests that it is composed of unserpentinised peridotite (density 3.4 g/cm³) to a depth of at least 8 km.St. John's Island is uniquely located, not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation. This precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones.Thermal contraction, serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent-to-continent fracture zone. A post-Mesozoic age of emplacement, associated with the onset of continental rifting and the rejuvenation of a pre-existing continental fracture, seems most probable.     

The memory of the accreting plate boundary and the continuity of fracture zones

A detailed aeromagnetic anomaly map of the Mesozoic seafloor-spreading lineations southwest of Bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation. The magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone (flowline) trends. The linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones. During the period of Mesozoic geomagnetic polarity reversals (when 1200 km of central North Atlantic seafloor formed), the Atlantic accreting boundary consisted of stationary, elongate, spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells. Normal oceanic crustal structure was formed in the spreading center cells, but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries, which parallel flowlines of Mesozoic relative plate motion in the central North Atlantic. We suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest; here, each spreading center cell independently goes through its cylce of stress buildup, stress release, and crustal accretion, after which its memory is refreshed. The temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists.     

Sea-floor spreading in the Bay of Biscay and its relationship to the North Atlantic

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.     

Early spreading history of the Indian Ocean between India and Australia

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.     

Determination of crystallization temperatures in fractional crystallization series by nickel partitioning equations

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.     

Structure and evolution of the sea floor south of South Africa

Geophysical results from the continental margin south of South Africa are consistent with a fault-controlled origin for the steep, linear continental slope. Accepting that the faulting was most probably of the shear type, caused by the transcurrent motion of the Falkland Plateau past South Africa, a model for the evolution of the sea floor adjacent to the margin is proposed. In this model the Agulhas Plateau is considered to be an oceanic feature, possibly an abandoned sea-floor spreading centre.     

SEASAT geoid anomalies and the Macquarie Ridge complex

The seismically active Macquarie Ridge complex forms the Pacific-India plate boundary between New Zealand and the Pacific-Antarctic spreading center. The Late Cenozoic deformation of New Zealand and focal mechanisms of recent large earthquakes in the Macquarie Ridge complex appear consistent with the current plate tectonic models. These models predict a combination of strike-slip and convergent motion in the northern Macquarie Ridge, and strike-slip motion in the southern part. The Hjort trench is the southernmost expression of the Macquarie Ridge complex. Regional considerations of the magnetic lineations imply that some oceanic crust may have been consumed at the Hjort trench. Although this arcuate trench seems inconsistent with the predicted strike-slip setting, a deep trough also occurs in the Romanche fracture zone.Geoid anomalies observed over spreading ridges, subduction zones, and fracture zones are different. Therefore, geoid anomalies may be diagnostic of plate boundary type. We use SEASAT data to examine the Macquarie Ridge complex and find that the geoid anomalies for the northern Hjort trench region are different from the geoid anomalies for the Romanche trough. The Hjort trench region is characterized by an oblique subduction zone geoid anomaly, e.g., the Aleutian-Komandorski region. Also, limited first-motion data for the large 1924 earthquake that occurred in the northern Hjort trench suggest a thrust focal mechanism. We conclude that subduction is occurring at the Hjort trench. The existence of active subduction in this area implies that young oceanic lithosphere can subduct beneath older oceanic lithosphere.     

红河断裂带白垩纪古地磁及青藏高原地质构造演化

红河断裂带两侧古地磁结果表明,羌塘地体与扬子地台至少从早白垩世以来已连接为一个整体。青藏高原是由四个发育历史不同的地体组成的大地构造复合体,在晚古生代它们分别是劳亚古陆、华夏复合古陆和冈瓦纳古陆的组成部分。拉萨地体与羌塘地体碰撞拼合形成欧亚板块构造格局。喜玛拉雅地体、印度板块与欧亚板块碰撞拼合、推挤,使青藏高原隆起,并使欧亚板块的块体沿已存在的断层产生左行走滑,这种作用至今仍在继续     

Mesozoic magnetic lineations and the magnetic quiet zone off northwest Africa

An extensive compilation of recently acquired geophysical reconnaissance data has allowed the Mesozoic magnetic lineations (The Eastern Keathley sequence) to be identified and mapped in detail for the area off northwest Africa lying between Madeira and the Cape Verde Islands. These anomalies were generated as one limb of a symmetric spreading center (Paleo Mid-Atlantic Ridge) from about 107 to 153 m.y.B.P. Offsets in the lineation pattern serve to identify fracture zone traces whose trends are approximately east-west. The seaward boundary of the marginal quiet zone does not precisely define an isochron due to the presence of a variable width transition zone of intermediate amplitude magnetic anomalies. Crust underlying the marginal quiet zone was generated, at least in part, during the Jurassic, Graham normal polarity epoch. The quiet zone boundary is not offset significantly on opposite sides of the Canaries lineament as previously suggested. A possible counterpart of the U.S. east coast magnetic anomaly is observed in some areas near the shelf/slope break of Spanish Sahara and Mauritania. The presence of relatively high-amplitude (but not-correlatable) magnetic anomalies seaward of the Mesozoic sequence and presumably generated during the Cretaceous, Mercanton normal polarity epoch remains a paradox.     

巴布亚新几内亚构造演化与找矿潜力

巴布亚新几内亚在大地构造位置上位于欧亚板块、印度-澳大利亚板块和太平洋板块的结合部位.本文介绍了自晚白垩世以来巴布亚新几内亚经历的复杂地质构造演化过程,不同板块间的汇聚、碰撞、俯冲和拆离、扩张等地质作用形成了以区内南部克拉通、中部褶皱带及北部岛弧带为特点的地质构造单元,在区内形成了具有活动大陆边缘特色的成矿系统,对寻找以斑岩型和浅成低温热液型铜金矿、红土型镍矿为主要成矿类型具有重要意义.     

Late Cretaceous genesis of the Kula plate

We propose that the Pacific-Kula ridge began spreading approximately 85 m.y. B.P., during Late Cretaceous time. Extrapolation of the Great Magnetic Bight backwards in time results in an implausible ridge configuration. This implies that plate velocity vectors for the Pacific, Kula, and Farallon plates were not constant during this interval. Evidence for splitting of the Kula plate from the Pacific plate along the Chinook trough is the relationship between the north-striking Amlia and Adak fracture zones, the Chinook and Emperor troughs, and the magnetic lineations south of the Aleutian trench. If this hypothesis is correct, it will require that Mesozoic reconstructions of the Pacific basin and their relation to Cenozoic reconstructions be re-examined. A previously unrecognized Mesozoic plate may be required. We propose calling this the Izanagi plate.     

西藏高原古地磁及大陆漂移

关于欧亚板块与印度洋扳块的交接地带,根据我们在青藏科学考察中,采集了雅鲁藏布江两侧大量的定向标本:帕里的上新世沉积岩,林周和比如的晚白垩世到古新世的红砂岩,札木的花岗闪长岩,定日白垩纪灰岩,晚侏罗纪的拉萨灰岩和江孜扳岩,土隆三迭纪砂岩,进行了古地磁研究,其结果表明:印度河上游-雅鲁藏布江的北侧属于欧亚板块,南侧属于印度洋板块。 根据古纬度的变化表明:印度洋板块向北漂移,自白垩纪以来,其年平均速率不超过5.5厘米。     

Possible extensions of the Narmada-Son lineament towards Murray Ridge (Arabian Sea) and the eastern syntaxial bend of the Himalayas

Geophysical data contiguous with the Narmada-Son lineament suggests its possible extension westward into the Arabian Sea and eastward up to the Shillong Plateau. The airborne magnetic anomaly map of the north Arabian Sea delineates a linear trend of magnetic anomalies in line with the Narmada-Son lineament. This group of magnetic anomalies, spreading over 20°N to 22°N, starts near the west coast of India at 21°N, 69°30′E and extends to the Murray Ridge. The tectonic feature represented by this group of magnetic anomalies is buried by a thick layer of sediments. This westward extension of the lineament is also reflected in the average Bouguer gravity anomaly map of the Indian Ocean. Towards the east, the gravity and magnetic data delineate a subsurface linear tectonic feature which extends in line with this lineament to the eastern syntaxial bend. These various geophysical signatures further suggest the lineament to be a typical rift-like structure. The tectonic implications of the lineament, which extends from the western to the eastern margins of the Indian plate, is discussed.     

A revised identification of the oldest sea-floor spreading anomalies between Australia and Antarctica

We propose that magnetic anomalies south of Australia and along the conjugate margin of Antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34. The original anomaly identification has two troublesome aspects: (1) it does not account for an “extra” anomaly between anomalies 20 and 21, and (2) it provides no explanation for the rough topography comprising the Diamantina Zone. With our revised identification there is no “extra” anomaly and the Diamantina Zone is attributed to a period of very slow spreading (～4.5mm/yr half rate) between 90 and 43 m.y. The ages bounding the interval of slow spreading (90 and 43 m.y.) correspond to times of global plate reorganizations. Our revised identification opens up the possibility that part of the magnetic quiet zone south of Australia formed during the Cretaceous long normal polarity interval. Breakup of Australia and Antarctica probably occurred sometime between 110 and 90 m.y. B.P. The “breakup unconformity” identified by Falvey in the Otway Basin may correspond to a eustastic sea level change.     

Cenozoic sedimentary records and geochronological constraints of differential uplift of the Qinghai-Tibet Plateau

Geological mapping data (1:250000) in the Qinghai-Tibet Plateau and its adjacent regions reveal the sediment sequences, distribution and tectonic evolution of the 92 Tertiary remnant basins. Southern Tibet and the Yecheng area in Xinjiang, located at southern and northwestern margins of the Qinghai-Tibet Plateau, respectively, were parts of the Neo-Tethys remnant sea in the Paleogene. In southern Tibet, both the subabyssal and abyssal sequences occur at the Gyangze, Saga, Guoyala, and Sangmai areas. The deep-water facies successions outcrop in the west, whereas the shallow-water facies sequences in the east, indicating the east to the west retreat of the Neo-Tethys Ocean. The retreat of the Neo-Tethys Ocean in the east was contributed to the earlier tectonic uplift of the eastern Qinghai-Tibet Plateau. The uplift process of the Plateau from the Late Cretaceous to Pliocene is described as follows: During the Late Cretaceous, tectonic uplift of the Qinghai-Tibet Plateau occurred in the northeastern part and the configuration of the Qinghai-Tibet Plateau was characterized by rise in the northeast and depression in the west. In the Paleocene-Eocene interval, the Tengchong-Baingoin and Kuyake-Golmud areas experienced local tectonic uplifting, the West Kunlun uplift zone broadened easterly, the Qilian uplift zone broadened southerly, and the Songpan-Garzê uplift zone shrank easterly. The Oligocene configuration of the Qinghai-Tibet Plateau was characterized by mountain chains rising along its margins and sedimentary basins in the central part because of tectonic uplifts of the Gangdisê and the Himalaya blocks. Meanwhile, the Kunlun-Altyn-Qilian uplift zones have also broadened southerly and northerly. In contrast, the great uplift zones of the Gangdisê, the Himalaya, the Karakorum, and the Kunlun blocks characterize the paleogeographic contours of the Qinghai-Tibet Plateau during the Miocene-Pliocene. Additionally, the thermochronological data on tectonic uplift events in southern Tibet, West Kunlun Mountains, Altyn Tagh, eastern Tibet, and western Sichuan all suggest that the most intense deformation occurred at 13-8 Ma and since 5 Ma, respectively, corresponding to two great uplift periods in Neogene. As a result, turnover of paleogeographic configuration of the Qinghai-Tibet Plateau occurred during the Neogene, experiencing a change from high contours in the east in the pre-Oligocene to high contours in the west at the end-Pliocene. The uplift of the Qinghai-Tibet Plateau during the Cenozoic was episodic, and the uplifts of various blocks within the Plateau were spatially and chronologically different.     

Breakup of Australia and Antarctica estimated as mid-Cretaceous (95 ± 5 Ma) from magnetic and seismic data at the continental margin

A positive magnetic anomaly marks the seaward edge of the magnetic quiet zone along the southern margin of Australia eastward between 114° and 131°E and along the conjugate Antarctic margin between 105° and 132°E. This anomaly was originally interpreted as the oldest seafloor-spreading anomaly—A22, revised by Cande and Mutter to A34—in the Southeast Indian Ocean, but is better modelled as the edge effect at the continent-ocean boundary (COB) constrained by seismic data. Continental crust abuts the oceanic sequence of normal and reversed spreading blocks, truncated within the Cretaceous normal interval at an extrapolated age of 96 Ma, rounded to 95 ± 5 Ma to take into account the uncertainty of the initial spreading rate and of the location of the COB. The occurrence of the anomaly on both margins defines this as the age of breakup. Farther east between 131° and 139°E on the Australian margin, the COB anomaly is modelled as due to the same kind of effect but with successively younger ages of truncation to 49 Ma, interpreted as indicating the most recent ridge-crest jumping to the Australian COB. The magnetic data from the conjugate sector of Antarctica, albeit scanty, are consistent with this interpretation.The 95 ± 5 Ma age of breakup coincides with that of the breakup unconformity in southern Australia, expressed by a short mid-Cretaceous lacuna in the Otway Basin between faulted Early Cretaceous rift-valley sediments of the Otway Group and the overlying Late Cretaceous Sherbrook Group, and by an unconformity of similar age in the Great Australian Bight Basin.