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1.
The western part of Yemen is largely covered by Tertiary volcanics and is bounded by volcanic margins to the west (Red Sea) and the south (Gulf of Aden). The Oligo–Miocene evolution of Yemen results from the interaction between the emplacement of the Afar plume, the opening of the Red Sea, and the westward propagation of the Gulf of Aden. Structural and microtectonic analyses of fault slip data collected in the field reveal that the volcanic margins of Yemen are affected by three main extensional tectonic events. The chronological order of these events is as follows: first E–W extension was associated with the emplacement of volcanic traps of Yemen, then NE–SW extension was related to the Red Sea rifting, and finally, the volcanic margin was submitted to N160°E extension, perpendicular to the overall trend of the Gulf of Aden, which we interpret as induced by the westward propagation of the oceanic ridge of the Gulf of Aden.  相似文献   

2.
A number of basins are observed to extend inland from the coasts on both sides of the Gulf of Aden. The basins are orientated at approximately right angles to the spreading direction and intersect the coasts at the meeting of sheared and rifted continental margins. They appear to be grabens, one wall of which is continuous with the half graben of the neighbouring rifted margin. It is suggested that these were once parts of a number of discrete rifts arranged en-echelon along a zone of lithospheric weakness during the early opening of the Gulf of Aden, which became redundant when transform faults formed. The proposed development of rifts and transform faults is similar to that of a spreading centre, transform fault, spreading centre pattern developed in the freezing wax model of Oldenburg and Brune (1975). The Gulf of Suez at the northern end of the Red Sea is interpreted in a similar way since it has a number of features in common with the basins in the continents adjacent to the Gulf of Aden.  相似文献   

3.
Calculation of the downward continuation for the anomalous magnetic field at the Knipovich Ridge showed more complicate segmentation of the spreading oceanic basement than was earlier considered. The structural pattern of the field is evidence that the area consists of no less than four segments separated by transform fracture zones with the azimuth of oceanic crust accretion about 310° and the normal position relative to the rift segments with the azimuth of 40°. The modern location of the axis of the Knipovich Ridge straightens the complicate divergent boundary between the plates in the strike-slip conditions between the spreading centers of the Mohns and Gakkel ridges. The axis is a detachment zone intersecting the oceanic basement having formed from the Late Oligocene. A new magnetoactive layer composed of magmatic products has not yet been formed in this structure.  相似文献   

4.
The results of analysis of the anomalous magnetic field of the Reykjanes Ridge and the adjacent basins are presented, including a new series of detailed reconstructions for magnetic anomalies 1–6 in combination with a summary of the previous geological and geophysical investigations. We furnish evidence for three stages of evolution of the Reykjanes Ridge, each characterized by a special regime of crustal accretion related to the effect of the Iceland hotspot. The time interval of each stage and the causes of the variation in the accretion regime are considered. During the first, Eocene stage (54–40 Ma) and the third, Miocene-Holocene stage (24 Ma-present time at the northern Reykjanes Ridge north of 59° N and 17–11 Ma-present time at the southern Reykjanes Ridge south of 59° N), the spreading axis of the Reykjanes Ridge resembled the present-day configuration, without segmentation, with oblique orientation relative to the direction of ocean floor opening (at the third stage), and directed toward the hotspot. These attributes are consistent with a model that assumes asthenospheric flow from the hotspot toward the ridge axis. Decompression beneath the spreading axis facilitates this flow. Thus, the crustal accretion during the first and the third stages was markedly affected by interaction of the spreading axis with the hotspot. During the second, late Eocene-Oligocene to early Miocene stage (40–24 Ma at the northern Reykjanes Ridge and 40 to 17–11 Ma at the southern Reykjanes Ridge), the ridge axis was broken by numerous transform fracture zones and nontransform offsets into segments 30–80 km long, which were oriented orthogonal to the direction of ocean floor opening, as is typical of many slow-spreading ridges. The plate-tectonic reconstructions of the oceanic floor accommodating magnetic anomalies of the second stage testify to recurrent rearrangements of the ridge axis geometry related to changing kinematics of the adjacent plates. The obvious contrast in the mode of crustal accretion during the second stage in comparison with the first and the third stages is interpreted as evidence for the decreasing effect of the Iceland hotspot on the Reykjanes Ridge, or the complete cessation of this effect. The detailed geochronology of magnetic anomalies 1–6 (from 20 Ma to present) has allowed us to depict with a high accuracy the isochrons of the oceanic bottom spaced at 1 Ma. The variable effect of the hotspot on the accretion of oceanic crust along the axes of the Reykjanes Ridge and the Kolbeinsey and Mid-Atlantic ridges adjoining the former in the north and the south was estimated from the changing obliquity of spreading. The spreading rate tends to increase with reinforcing of the effect of the Iceland hotspot on the Reykjanes Ridge.  相似文献   

5.
The marine magnetic data acquired from offshore Krishna-Godavari (K-G) basin, eastern continental margin of India (ECMI), brought out a prominent NE-SW trending feature, which could be explained by a buried structural high formed by volcanic activity. The magnetic anomaly feature is also associated with a distinct negative gravity anomaly similar to the one associated with 85°E Ridge. The gravity low could be attributed to a flexure at the Moho boundary, which could in turn be filled with the volcanic material. Inversion of the magnetic and gravity anomalies was also carried out to establish the similarity of anomalies of the two geological features (structural high on the margin and the 85°E Ridge) and their interpretations. In both cases, the magnetic anomalies were caused dominantly by the magnetization contrast between the volcanic material and the surrounding oceanic crust, whereas the low gravity anomalies are by the flexures of the order of 3–4 km at Moho boundary beneath them. The analysis suggests that both structural high present in offshore Krishna-Godavari basin and the 85°E Ridge have been emplaced on relatively older oceanic crust by a common volcanic process, but at discrete times, and that several of the gravity lows in the Bay of Bengal can be attributed to flexures on the Moho, each created due to the load of volcanic material.  相似文献   

6.
The available seismic and magnetic data show the Gakkel Ridge rift zone consisting of the Atlantic and Siberian segments divided by a tectonic suture at 70° E. The two segments have had different histories recorded in their sedimentary cover. Apart from the difference in its morphology, the Siberian segment differs from the Atlantic one in the existence of a series of deposition centers, which might represent a vast Paleogenic basin that formed prior to the Gakkel Ridge. The simple model of North Atlantic spreading fails to explain the long and complex history of the Gakkel Ridge rift and the existence of the depocenters. The particular structure of this zone might have resulted from the growth of rift mountains by accretion of magmatic material during the Paleogene, without significant sea floor spreading.  相似文献   

7.
Oblique-shear margins are divergent continental terrains whose breakup and early drift evolution are characterized by significant obliquity in the plate divergence vector relative to the strike of the margin. We focus on the Rio Muni margin, equatorial West Africa, where the ca. 70-km-wide Ascension Fracture Zone (AFZ) exhibits oblique–slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole lithosphere failure and continental breakup (ca. 117 Ma). Oblique extension is recorded also by strike–slip and oblique–slip fault geometry within the AFZ, and buckling of Aptian synrift rocks in response to block rotation and local transpression. Rio Muni shares basic characteristics of both rifted and transform margins, the end members of a spectrum of continental margin kinematics. At transform margins, continental breakup and the onset of oceanic spreading (drifting) are separate episodes recorded by discrete breakup and drift unconformities. Oceanic opening will proceed immediately following breakup on a rifted margin, whereas transform and oblique-shear margins may experience several tens of millennia between breakup and drift. Noncoeval breakup and drift have important consequences for the fit of the equatorial South American and African margins because, in reconstructing the configuration of conjugate continental margins at the time of their breakup, it cannot be assumed that highly segmented margins like the South Atlantic will match each other at their ocean–continent boundaries (OCBs). Well known ‘misfits’ in reconstructions of South Atlantic continental margins may be accounted for by differential timing of breakup and drifting between oblique-shear margins and their adjacent rifted segments.  相似文献   

8.
This paper describes the updated stratigraphy, structural framework and evolution, and hydrocarbon prospectivity of the Paleozoic, Mesozoic and Cenozoic basins of Yemen, depicted also on regional stratigraphic charts. The Paleozoic basins include (1) the Rub’ Al-Khali basin (southern flanks), bounded to the south by the Hadramawt arch (oriented approximately W–E) towards which the Paleozoic and Mesozoic sediments pinch out; (2) the San’a basin, encompassing Paleozoic through Upper Jurassic sediments; and (3) the southern offshore Suqatra (island) basin filled with Permo-Triassic sediments correlatable with that of the Karoo rift in Africa. The Mesozoic rift basins formed due to the breakup of Gondwana and separation of India/Madagascar from Africa–Arabia during the Late Jurassic/Early Cretaceous. The five Mesozoic sedimentary rift basins reflect in their orientation an inheritance from deep-seated, reactivated NW–SE trending Infracambrian Najd fault system. These basins formed sequentially from west to east–southeast, sub-parallel with rift orientations—NNW–SSE for the Siham-Ad-Dali’ basin in the west, NW–SE for the Sab’atayn and Balhaf basins and WNW–ESE for the Say’un-Masilah basin in the centre, and almost E–W for the Jiza’–Qamar basin located in the east of Yemen. The Sab’atayn and Say’un–Masilah basins are the only ones producing oil and gas so far. Petroleum reservoirs in both basins have been charged from Upper Jurassic Madbi shale. The main reservoirs in the Sab’atayn basin include sandstone units in the Sab’atayn Formation (Tithonian), the turbiditic sandstones of the Lam Member (Tithonian) and the Proterozoic fractured basement (upthrown fault block), while the main reservoirs in the Say’un–Masilah basin are sandstones of the Qishn Clastics Member (Hauterivian/Barremian) and the Ghayl Member (Berriasian/Valanginian), and Proterozoic fractured basement. The Cenozoic rift basins are related to the separation of Arabia from Africa by the opening of the Red Sea to the west and the Gulf of Aden to the south of Yemen during the Oligocene-Recent. These basins are filled with up to 3,000 m of sediments showing both lateral and vertical facies changes. The Cenozoic rift basins along the Gulf of Aden include the Mukalla–Sayhut, the Hawrah–Ahwar and the Aden–Abyan basins (all trending ENE–WSW), and have both offshore and onshore sectors as extensional faulting and regional subsidence affected the southern margin of Yemen episodically. Seafloor spreading in the Gulf of Aden dates back to the Early Miocene. Many of the offshore wells drilled in the Mukalla–Sayhut basin have encountered oil shows in the Cretaceous through Neogene layers. Sub-commercial discovery was identified in Sharmah-1 well in the fractured Middle Eocene limestone of the Habshiyah Formation. The Tihamah basin along the NNW–SSE trending Red Sea commenced in Late Oligocene, with oceanic crust formation in the earliest Pliocene. The Late Miocene stratigraphy of the Red Sea offshore Yemen is dominated by salt deformation. Oil and gas seeps are found in the Tihamah basin including the As-Salif peninsula and the onshore Tihamah plain; and oil and gas shows encountered in several onshore and offshore wells indicate the presence of proven source rocks in this basin.  相似文献   

9.
T. V. Gerya 《Petrology》2013,21(6):550-560
This work presents high-resolution 3D numerical model of transform fault initiation at rifted continental margins. Our petrological-thermomechanical visco-plastic model allows for spontaneous nucleation of oceanic spreading process in a continental rift zone and takes into account new oceanic crust growth driven by decompression melting of the asthenospheric mantle. Numerical model predicts that ridge-transform spreading pattern initiate in several subsequent stages: crustal rifting (0–1.5 Myr), spreading centers nucleation and propagation (1.5–3 Myr), proto-transform fault initiation and rotation (3–5 Myr) and mature ridge-transform spreading (> 5 Myr). Comparison of modeling results with the natural data from the Woodlark Basin suggests that the development of this region closely matches numerical predictions. Similarly to the model, the Moresby (proto-) Transform terminates in the oceanic rather than in the continental crust. This fault associates with a notable topographic depression and formed within 0.5–2 Myr while linking two offset overlapping spreading segments. Model reproduces well characteristic “rounded” contours of the spreading centers as well as the presence of a remnant of the broken continental crustal bridge observed in the Woodlark Basin. Proto-transform fault traces and truncated tip of one spreading center present in the model are also documented in nature. Numerical results are in good agreement with the concept of Taylor et al. (2009) which suggests that spreading segments nucleate en echelon in overlapping rift basins and that transform faults develop as or after spreading nucleates. Our experiments also allow to refine this concept in that (proto)-transform faults may also initiate as oblique rather than only spreading-parallel tectonic features. Subsequent rotation of these faults toward the extension-parallel direction is governed by space accommodation during continued oceanic crust growth within offset ridge-transform intersections.  相似文献   

10.
Abstract

The Powell Basin is one of the few present-day examples of a small isolated ocean basin largely surrounded by blocks of continental crust. The continental blocks in this basin result from the fragmentation of the northern Antarctic Peninsula. This basin was created by the eastward motion of the South Orkney microcontinent relative to the Antarctic Peninsula. The axial rift, identified by multichannel seismic profiles obtained during the HESANT 92/93 cruise, and the gravimetric anomalies of the basin plain, together with the transcurrent faults along the northern and southern margins, indicate a predominant WSW-ENE trend of basin extension. The South Orkney microcontinent was incorporated into the Antarctic Plate during the Miocene as a consequence of the end of basin spreading. The eastern and western margins are conjugate and have an intermediate crust in the region of transition to the basin plain. The differences in the basement structure and the architecture of the depositional units suggest that the extensional process was asymmetrical. The southern transtensive margin and the northern transcurrent margin are rectilinear and steep, without any intermediate crust in the narrow fault zone between the base of the continentalblocks slope and the oceanic crust. The multichannel seismic profiles across the central sector of the basin reveal a spreading axis with a double ridge and a central depression filled with sediments. The geometry of the reflectors in this depression indicates that the ponded deposits belong to the early stages of oceaniccrust accretion. This structure is similar to the overlapping spreading centres observed in fast-spreading oceanic axes, where the spreading axis has relay and overlapping segments.

The depositional units of the margins and basin plain have been grouped into four depositional sequences, comprising the classic stages in the formation of an ocean basin: pre-rift (S1), syn-rift (S2), syn-drift (S3), and post-drift (S4). The pre-rift sequence has deformed reflectors and is observed in the southern and eastern margins. The syn-rift sequence, tectonically disrupted, fills depressions bounded by faults and is well-developed in the eastern margin where it is truncated by an erosive surface identified as the break-up unconformity. The syn-drift sequence is wedge-shaped in the basin, thickening towards the margins and having onlap relations on the flanks of the spreading ridge. The post-drift sequence is the thickest unit and is characterised by a cyclic pattern of alternating packages of high-amplitude reflectors, very continuous, and low-amplitude reflectors. Towards the western and eastern margins, the same sequence has channel-levee complexes and channelised, wedged bodies attributed to turbiditic deposits of submarine fans derived from canyons located in the slope and outer shelf. The cyclic nature of this sequence is probably related to advancing and receding grounded ice sheets in the continental shelf since the latest Miocene.  相似文献   

11.
《International Geology Review》2012,54(14):1691-1719
This study investigates the formation of lower oceanic crust and geochemical variations of basalts along the Central Indian Ridge (CIR, lat. 7°45′–17°10′ S). Harzburgites, various gabbroic cumulates, medium- to fine-grained oxide gabbros, diabases, and pillow basalts were recovered by dredging from segment ends such as ridge-transform intersections (RTIs), non-transform discontinuities (NTDs), and transform offset areas. The occurrence of both harzburgites and gabbroic rocks with minor basalts at all segments ends, and leucogabbro intrusive into harzburgite at the 12°45′ S NTD indicates that oceanic crust at segment ends exposes mantle-derived harzburgites and gabbroic intrusions with a thin basaltic cover due to sparse magmatic activity. Basalts collected along the entire ridge show wide compositional variations between N (normal)- and E (enriched)-mid-ocean ridge basalt (MORB). T (transitional)-MORBs with enriched affinities are more prominent than N-MORBs. There is no tendency of enrichment towards specific directions. (La/Sm)N variations in MORB along the CIR (8°–21°S) fluctuates at a regional scale with local high positive anomalies reflecting compositional heterogeneity of the sub-CIR mantle domain.  相似文献   

12.
The Afanasy Nikitin seamount (ANS) is a major structural feature (400 km-long and 150 km-wide) in the Central Indian Basin, situated at the southern end of the so-called 85°E Ridge. Combined analyses of new multibeam bathymetric, seismic reflection and geochronological data together with previously described magnetic data provide new insights into the growth of the ANS through time, and its relationship with the 85°E Ridge. The ANS comprises a main plateau, rising 1200 m above the surrounding ocean floor (4800 m), and secondary elevated seamount highs, two of which (lie at 1600 and 2050 m water depths) have the morphology of a guyot, suggesting that they were formed above or close to sea-level. An unbroken sequence of spreading anomalies 34 through 32n.1 identified over the ANS reveal that the main plateau of the ANS was formed at 80–73 Ma, at around the same time as that of the underlying oceanic crust. The 40Ar/39Ar dates for two basalt samples dredged from the seamount highs are consistent, within error, at 67 Ma. These results, together with published results of late Cretaceous to early Cenozoic Indian Ocean plate reconstructions, indicate that the Conrad Rise hotspot emplaced both the main plateau of the ANS and Conrad Rise (including the Marion Dufresne, Ob and Lena seamounts) at 80–73 Ma, close to the India–Antarctica Ridge system. Subsequently, the seamount highs were formed by late-stage volcanism c. 6–13 Myr after the main constructional phase of the seamount plateau. Flexural analysis indicates that the main plateau and seamount highs of the ANS are consistent with Airy-type isostatic compensation, which suggest emplacement of the entire seamount in a near spreading-center setting. This is contrary to the flexural compensation of the 85°E Ridge further north, which is interpreted as being emplaced in an intraplate setting, i.e., 25–35 Myr later than the underlying oceanic crust. Therefore, we suggest that the ANS and the 85°E Ridge appear to be unrelated as they were formed by different mantle sources, and that the proximity of the southern end of the 85°E Ridge to the ANS is coincidental.  相似文献   

13.
The tectonotype of nonvolcanic passive margins is discussed on the basis of data on the conjugate margins of West Iberia and Newfoundland. Magmatic, structural, and historical aspects are considered. The Late Mesozoic structural elements related to rifting and transition to spreading are considered, as well as the Early Mesozoic sedimentary basins that begin the history of oceanic opening. The problem is set to determine the tectonic conditions of the early opening of the ocean in the framework of the chosen tectonoptype. These conditions are compared with the setting at the volcanic margins. The formation of the conjugate Iberia-Newfoundland margins is reconstructed as an asymmetric rift system developing in an almost amagmatic regime. All three segments of the margins on both sides of the ocean reveal similar features of transverse zoning with zones of the tectonized continental, transitional, and oceanic crust oriented nearly parallel to the margin. Special attention is called to the old age of the continental crust and subcontinental mantle and the absence of newly formed crystalline crust; the stadial tectonic and rheological evolution of the crust and lithospheric mantle; the specific features of the transitional zone; the serpentinization and exhumation of mantle peridotites and their role in the development of detachment at the crust-mantle interface, related listric faults and the Peridotite Ridge, attenuation of the medium, further localization of continental breakup, and the eventual development of asymmetric conjugate margins. Two papers characterizing the tectonotypes of volcanic and nonvolcanic passive margins ([2] and this paper) determine the line of further comparative analysis necessary for insights into the geodynamics of ocean opening.  相似文献   

14.
The walls of the Knipovich Ridge are complicated by normal and reverse faults revealed by a high-frequency profilograph. The map of their spatial distribution shows that the faults are grouped into domains a few tens of kilometers in size and are a result of superposition of several inequivalent geodynamic factors: the shear zone oriented parallel to the Hornsunn Fault and superposed on the typical dynamics of the midocean ridge with offsets along transform fracture zones and rifting along short segments of the Mid-Atlantic Ridge (MAR). According to the anomalous magnetic field, the Knipovich Ridge as a segment of the MAR has formed since the Oligocene including several segments with normal direction of spreading separated by a multitransform system of fracture zones. In the Quaternary, the boundary of plate interaction along the tension crack has been straightened to form the contemporary Knipovich Ridge, which crosses the previously existing magmatic spreading substrate and sedimentary cover at an angle of about 45° relative to the direction of accretion. The sedimentary cover along the walls of the Knipovich is Paleogene in age and has subsided into the rift valley to a depth of 500–1000 m along the normal faults.  相似文献   

15.
The structure of the acoustic basement of the eastern part of the St. Paul multifault transform fracture system hosts rift paleovalleys and a paleonodal depression that mismatch the position of the currently active zones. This displacement zone, which is composed of five fault troughs, is unstable in terms of the position of the rift segments, which jumped according to redistribution of stresses. The St. Paul system is characterized by straightening of the transform transition between two remote segments of the Mid-Atlantic Ridge (MAR). The eastern part of the system contains anomalous bright-spot-like reflectors on the flattened basement, which is a result of atypical magmatism, that forms the standard ridge relief of the acoustic basement. Deformations of the acoustic basement have a presedimentation character. The present-day deformations with lower amplitude in comparison to the basement are accompanied by acoustic brightening of the sedimentary sequence. The axial Bouguer anomalies in the east of the system continue to the north for 120 km from the active segments of the St. Paul system. Currently seismically active segments of the spreading system are characterized by increasing amplitudes of the E–W displacement along the fault troughs. Cross-correlation of the lengths of the active structural elements of the MAR zone (segments of the ridge and transform fracture zones of displacement) indicates that, statistically, the multifault transform fracture system is a specific type of oceanic strike-slip faults.  相似文献   

16.
This article outlines geomorphological and tectonic elements of the Afar Depression, and discusses its evolution. A combination of far-field stress, due to the convergence of the Eurasian and Arabian plates along the Zagros Orogenic Front, and uplift of the Afar Dome due to a rising mantle plume reinforced each other to break the lithosphere of the Arabian–Nubian Shield. Thermal anomalies beneath the Arabian–Nubian Shield in the range of 150 °C–200 °C, induced by a rising plume that mechanically and thermally eroded the base of the mantle lithosphere and generated pulses of prodigious flood basalt since ∼30 Ma. Subsequent to the stretching and thinning the Afar Dome subsided to form the Afar Depression. The fragmentation of the Arabian–Nubian Shield led to the separation of the Nubian, Arabian and Somalian Plates along the Gulf of Aden, the Red Sea and the Main Ethiopian Rift. The rotation of the intervening Danakil, East-Central, and Ali-Sabieh Blocks defined major structural trends in the Afar Depression. The Danakil Block severed from the Nubian plate at ∼20 Ma, rotated anti-clockwise, translated from lower latitude and successively moved north, left-laterally with respect to Nubia. The westward propagating Gulf of Aden rift breached the Danakil Block from the Ali-Sabieh Block at ∼2 Ma and proceeded along the Gulf of Tajura into the Afar Depression. The propagation and overlap of the Red Sea and the Gulf of Aden along the Manda Hararo–Gobaad and Asal–Manda Inakir rifts caused clockwise rotation of the East-Central Block. Faulting and rifting in the southern Red Sea, western Gulf of Aden and northern Main Ethiopian Rift superimposed on Afar. The Afar Depression initiated as diffused extension due to far-field stress and area increase over a dome elevated by a rising plume. With time, the lithospheric extension intensified, nucleated in weak zones, and developed into incipient spreading centers.  相似文献   

17.
A combination of palaeomagnetic, seismological, gravitational, aeromagnetic and geochemical observations, as well as geological and regional considerations are strongly indicative of anticlockwise rotational movements of the Danakil Alps and formation of new oceanic crust in the Northern Afar Triangle. The decreasing amount of spreading in the Southern Red Sea is compensated by en chelon crustal spreading (formation of oceanic crust in a continental environment) in the Danakil-Afar Depression. Here, the geophysical properties are generally intermediate between the more typical continental (Ethiopia) and oceanic (Red Sea, Gulf of Aden) data. Such intermediate type crust is proposed to be caused by “oceanization” of formerly continental crust, i. e. fragmentation and basification through massive dyke injections (mantle diapirism). The structure and evolution of the wider Afar Triangle, East-African Rift System, Red Sea and Gulf of Aden are used to derive a model for possible stages during initial continental break-up and compared with selected, similarly structured parts of the n-Atlantik. The continental break-up probably develops in the following stages: 1. general uplift associated with surface fracturing above an asthenospheric diapir (uplift), 2. development of linear “Scheitel”-Grabensystems along the crest of the uplift or uplift chains (rupture), 3. graben with (contaminated) volcanism stage (volcanism), 4. “oceanization” of the developing depression through fragmentation and basification by massive oceanic and/or contaminated dyke-injections of the former continental crust along several sporadically active lineaments, 5. “crustal spreading” on land or concentration of mantle derived, oceanic crust-injections along one major lineament in a dry, continental environment, 6. “evaporit-stage of sea-floor spreading” with sporadic seawater connections to an open marine basin and 7. “ocean-floor spreading” in the deep-sea environment of advanced oceanic troughs. The derivation of these stages basically involves the addition of “sea-floor spreading” processes (oceanization, crustal-, sea- and ocean-floor spreading) to the well known sequence: Hebung — Spaltung — Vulkanismus (Cloos, 1939) and relate it to mantle-diapirism processes. All the above stages are recognizable along the Afro-Arabian Rifts and seem to have morphological equivalents in the Atlantic.  相似文献   

18.
Investigations of three plausible tectonic settings of the Kerguelen hotspot relative to the Wharton spreading center evoke the on-spreading-axis hotspot volcanism of Paleocene (60-54 Ma) age along the Ninetyeast Ridge. The hypothesis is consistent with magnetic lineations and abandoned spreading centers of the eastern Indian Ocean and seismic structure and radiometric dates of the Ninetyeast Ridge. Furthermore, it is supported by the occurrence of oceanic andesites at Deep Sea Drilling Project (DSDP) Site 214, isotopically heterogeneous basalts at Ocean Drilling Program (ODP) Site 757 of approximately the same age (59-58 Ma) at both sites. Intermix basalts generated by plume-mid-ocean ridge (MOR) interaction, exist between 11° and 17°S along the Ninetyeast Ridge. A comparison of age profile along the Ninetyeast Ridge between ODP Sites 758 (82 Ma) and 756 (43 Ma) with similarly aged oceanic crust in the Central Indian Basin and Wharton Basin reveals the existence of extra oceanic crust spanning 11° latitude beneath the Ninetyeast Ridge. The extra crust is attributed to the transfer of lithospheric blocks from the Antarctic plate to the Indian plate through a series of southward ridge jumps at about 65, 54 and 42 Ma. Emplacement of volcanic rocks on the extra crust resulted from rapid northward motion (absolute) of the Indian plate. The Ninetyeast Ridge was originated when the spreading centers of the Wharton Ridge were absolutely moving northward with respect to a relatively stationary Kerguelen hotspot with multiple southward ridge jumps. In the process, the spreading center coincided with the Kerguelen hotspot and took place on-spreading-axis volcanism along the Ninetyeast Ridge.  相似文献   

19.
Crystalline continental rocks and associated crust‐contaminated basaltic rocks were unexpectedly dredged on the crest and at seamounts of the Rio Grande Rise, South Atlantic. Zircon U–Pb ages of one gabbro (ca. 2,200 Ma) and four granitoids (between ca. 1,430–480 Ma) indicate that the breakup of SW Gondwana left behind continental fragments of dominantly African age. These rocks may have been incorporated into the oceanic lithosphere by complex processes including rifting and interaction of the Tristan‐Gough mantle plume with hyperextended continental margins. Until ca. 80–70 Ma, the Rio Grande Rise and an old portion of the Walvis Ridge formed a conjugate pair of aseismic ridges, and the Tristan‐Gough plume was positioned at the Mid‐Atlantic Ridge. The finding of continental rock fragments in one of these conjugate pairs opens new perspectives on the mechanisms of continental break‐up, the nature of this conjugate pair, and the geodynamic evolution of rifted Gondwana margins in the South Atlantic.  相似文献   

20.
西南印度洋中脊(SWIR)平均扩张速率约为14 mm/yr,是全球洋中脊系统的重要组成端元,因其具有慢速-超慢速扩张特征,引起全球科学家的广泛关注.基于前人对SWIR的综合研究成果,从构造和岩浆作用两个角度出发,系统地回顾了 SWIR的形成和演化历史,探讨了岩浆的分布特征和地幔不均一性成因.SWIR的形成始于冈瓦纳大陆...  相似文献   

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