共查询到20条相似文献，搜索用时 15 毫秒
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G. J. H. McCall R. W. LeMaitre A. Malahoff G. P. Robinson P. J. Stephenson 《Bulletin of Volcanology》1970,34(3):681-696
Ambrym Island has an unusually large, well-preserved basaltic caldera 13 km across. The caldera occurs in the central region of an early broad composite cone, which formed a north-south line with three smailer volcanoes. Alter the caldera was formed volcanism occurred within it and along fissure lines running nearly east-west. Two volcanic cones are active almost continuously and historic fissure cruptions have been recorded. The caldera formed by quiet subsidence, or by subsidence accompanied by eruption of scoria lappili similar to that erupted prior and subsequent to caldera formation. The collapse was at least 600 metres and radiocarbon dating suggests it took place less than 2000 years ago. The caldera is detined by gravity anomalies 10 to 14 milligals lower than those at its rim suggesting predominantly ash infilling. Aeromagnetic anomalies show a prominent. nearly east-west lineation, with normally magnetised bipole anomalies over the centre of the caldera and over fissure lines east of it. The source of the present volcanic activity is believed to be located along dyke fissures, with a perched magma chamber beneath the caldera. The geophysical evidence on Ambrym, together with that of regional east trending magnetic anomalies and recent bathymetric results, suggests that the volcanic activity is localised by the intersection of an east-west fracture zone with the axis of the New Hebrides island are. 相似文献
3.
The central Taupo Volcanic Zone (TVZ) is a region of intense Quaternary rhyolitic volcanism and geothermal activity in the North Island of New Zealand from which about 14,000 km3 of pyroclastics and lavas have been erupted during the last 1.6 Ma. Analysis of aeromagnetic surveys over the TVZ showed the presence of long-wavelength (10 to 25 km) magnetic anomalies which roughly follow the trend of the currently active eastern TVZ, from the north of Lake Taupo to the east of Lake Rotorua. An interpretation of the long-wavelength magnetic anomalies using 3-D magnetic modelling suggests that these anomalies are caused by the magnetic effects of < 3 km thick sequence of volcanic rocks and deeper magnetised bodies within the non-magnetic upper crust (4–7 km depth) beneath the young (age < 0.7 Ma), currently active eastern TVZ. The deep magnetised bodies are interpreted as solidified rhyolitic sub-volcanic plutons that have cooled down to below their Curie temperature.Although the existence of plutonic bodies beneath the TVZ has been postulated prior to this study, this magnetic interpretation result appears to be the first geophysical model of such bodies. 相似文献
4.
B. S. P. Sarma 《Surveys in Geophysics》2008,29(1):51-61
The rifted Eastern Continental Margin of India (ECMI) has evolved as a result of breakup of East Gondwanaland. Previous geophysical
studies of the continental margin have not elucidated upon its volcanic nature. Magnetics plays a useful role in the study
of continental margins, particularly in identifying the volcanic units. The aeromagnetic map of the offshore Mahanadi basin
of ECMI displays a conspicuous linear anomaly along the continental shelf. A comprehensive study of the published aeromagnetic,
marine magnetic and gravity data of the offshore Mahanadi basin reveals the existence of a seaward dipping volcanic unit in
the offshore Mahanadi basin bordering the Hinge zone. This inference suggests that the ECMI is a volcanic rifted margin. The
study further indicates the deepening of the basement towards the sea. In addition, the existing geological studies on the
ECMI demarcated the probable limit of the continental crust by studying the basement detached tectonic style of the sedimentation
in sub-surface configuration of the East coast basins of India. The probable continental crustal limit, the Hinge zone, and
the inner edge of the presently inferred volcanic unit conform to one another spatially in the offshore Mahanadi region. These
features characterize the inferred volcanic body as seaward dipping reflectors (SDRs) that usually occur at the rifted continental
margins. The deepening of the basement towards the sea and the presence of the volcanic body on the continental margin are
indicative of the transitional nature of the crust. It is generally accepted that Antarctica and India were juxtaposed before
the breakup of Gondwanaland. But the microcontinents in the southern Indian Ocean are neglected in the reconstruction of Gondwanaland
continents. The recent studies of the discovery of continental crust within the Elan Bank (EB) microcontinent show that the
EB was contiguous with the East coast of India before the breakup of Gondwanaland. Moreover, it is reported that the upper
igneous crust of the EB consists of a 2–3 km thick layer of accumulated lava flows originating from the Kerguelen hotspot.
An estimate shows that the total volume of volcanic and plutonic component of the Elan Bank is about 0.3 million cubic kilometers.
The present inference of a volcanic body from the offshore Mahanadi basin is in agreement with the above observations of the
juxtaposition of EB with ECMI. 相似文献
5.
Matthias KeyserJoachim R.R. Ritter Michael Jordan 《Earth and Planetary Science Letters》2002,203(1):59-82
The Quaternary Eifel volcanic fields, situated on the Rhenish Massif in Germany, are the focus of a major interdisciplinary project. The aim is a detailed study of the crustal and mantle structure of the intraplate volcanic fields and their deep origin. Recent results from a teleseismic P-wave tomography study reveal a deep low-velocity structure which we infer to be a plume in the upper mantle underneath the volcanic area [J.R.R. Ritter et al., Earth Planet. Sci. Lett. 186 (2001) 7-14]. Here we present a travel-time investigation of 5038 teleseismic shear-wave arrivals in the same region. First, the transverse (T) and radial (R) component travel-time residuals are treated separately to identify possible effects of seismic anisotropy. A comparison of 2044 T- and 2994 R-component residuals demonstrates that anisotropy does not cause any first-order travel-time effects. The data sets reveal a deep-seated low-velocity anomaly beneath the volcanic region, causing a delay for teleseismic shear waves of about 3 s. Using 3773 combined R- and T-component residuals, an isotropic non-linear inversion is calculated. The tomographic images reveal a prominent S-wave velocity reduction in the upper mantle underneath the Eifel region. The anomaly extends down to at least 400 km depth. The velocity contrast to the surrounding mantle is depth-dependent (from −5% at 31-100 km depth to at least −1% at 400 km depth). At about 170-240 km depth the anomaly is nearly absent. The resolution of the data is sufficient to recover the described features, however the anomaly in the lower asthenosphere is underestimated due to smearing and damping. The main anomaly is similar to the P-wave model except the latter lacks the ‘hole’ near 200 km depth, and both are consistent with an upper mantle plume structure. For plausible anhydrous plume material in the uppermost 100 km of the mantle, an excess temperature as great as 200-300 K is estimated from the seismic anomaly. However, 1% partial melt reduces the required temperature anomaly to about 100 K. The temperature anomaly associated with the deeper part of the plume (250 to about 450 km depth) is at least 70 K. However, this estimate is quite uncertain, because the amplitude of the shear-wave anomaly may be larger than the modelled one. Another possibility is water in the upwelling material. The gap at 170-240 km depth could arise from an increase of the shear modulus caused by dehydration processes which would not affect P-wave velocities as much. An interaction of temperature and compositional variations, including melt and possibly water, makes it difficult to differentiate quantitatively between the causes of the deep-seated low-velocity anomaly. 相似文献
6.
Kuju Volcano lies near Aso Caldera at the center of Kyushu Island, western Japan. After a few hundred years of dormancy,
a phreatic explosion accompanied by a small ash eruption occurred on 11 October 1995. This study was undertaken to determine
the subsurface seismic velocity structure associated with the active magmatic regime in the Kuju volcanic region. The three-dimensional,
upper crustal, P-wave velocity structure beneath Kuju Volcano was determined using methods for the simultaneous inversion
of P-wave arrival times from local earthquakes in and around the Kuju volcanic region for velocities and hypocentral parameters.
Results reveal two shallower low-velocity anomalies located in the northern and southern parts of Kuju Volcano, consistent
with the presence of significant negative Bouguer gravity anomalies. In addition, a high-velocity anomaly is located approximately
5 km northwest of Mt. Kuju, one of the domes in Kuju Volcano. Beneath this high-velocity anomaly, a low-velocity anomaly is
present. This velocity structure suggests a magmatic regime that has a lid consisting of cooled solid material overlying a
chamber of partially molten material.
Received: 23 September 1997 / Accepted: 20 June 1998 相似文献
7.
江西省相山火山盆地是我国第一大、世界第三大火山岩型铀矿田,其西部牛头山一带铀矿勘探中发现深部有大垂幅的Pb-Zn-Ag矿化.60多年来,以该矿田为对象开展的研究取得了一系列丰硕成果,但对火山机构的认识仍不确定.我们采集了涵盖该火山盆地主要地质体的1386块钻孔岩芯标本和243块地表岩石标本,开展了电阻率、磁化率、密度等物性参数测量,并在火山盆地中施测了19条MT剖面(2条骨干剖面和17条精细剖面),对3000 m以浅主要地层、岩体和断裂带等目标地质体的三维展布特征进行了解译和三维建模.研究结果表明:(1)相山火山盆地具有变质岩-花岗岩双基底.基底变质岩系顶界面表现为南北分带(三隆间两凹)和东西分块(两垒夹一堑)的三维地质格局;南西部有加里东期花岗岩侵人,具有似层状的空间展布特征;盆地基底变质岩系与上覆火山-沉积岩盖层之间呈连续的水平低阻异常带,不整合界面清晰.(2)打鼓顶组火山岩呈似层状产出,主要分布于盆地西部;在河元背一船坑一杏树下一带识别出近东西走向厚层的流纹英安岩凹槽,相山铀矿田西部探明的主要铀矿床分布在该凹槽内或其边缘.鹅湖岭组火山岩总体形态呈蘑菇状,在盆地中部厚度较大.在相山主峰半径约2 km的范围内,发现自下而上贯通式的低阻异常,推测是鹅湖岭组碎斑熔岩喷发的通道相(火山颈相),其火山颈呈陡立管状,深部向南东倾伏,浅部向南东撒开.后期花岗斑岩呈岩墙-岩床组合状,总体构成一个向西开口的环形岩体.打鼓顶期主要岩浆通道位于相山顶一巴山之间,次岩浆通道位于河元背;鹅湖岭期火山活动主岩浆通道也位于相山顶一巴山之间,次岩浆通道位于河元背、阳家山(芙蓉山)、严坑和柏昌.(3)火山盆地中断裂构造发育,MT测量结果显示存在7条北东向、4条北西向和1条南北向格架性断裂构造(其中一条新发现的北东向断裂隐伏于白垩纪红盆之下),盆地北部发育1条弧形火山塌陷构造,表现为大规模延续的低阻异常带. 相似文献
8.
A detailed gravity survey was carried out on the island of Vulcano, Aeolian Islands, Italy. Gravity was measured on 107 stations and the Bouguer anomalies were computed by assuming geological densities. Aim of this survey was to complete the island structural pattern relatively to the shallower structures. Separation of the gravity anomaly field was carried out by means of data filtering, and two main components were discerned. The λ>2.2 km wavelength component, filtered out of the longer wavelength components, was interpreted quantitatively along a NW profile. The best fitting model consists of an upper layer of recent pyroclastic products (p=2.1 g/cm3) lying upon a highly compacted pyroclastic series or lavas (p=2.4 g/cm3). The shorter wavelength residual gravity field (λ<2.2 km) is characterized by two anomalies, located on Vulcanello and the «Fossa di Vulcano» crater. Vulcanello anomaly could be interpreted, given the geothermal state of the area, as due to an increase of the rock density consequent to propylization processes by high temperature fluids (T>200°C). «Fossa di Vulcano» anomaly is instead attributable to the local volcanic chimney. A schematic comprehensive model of Vulcano is also presented, which accounts for the available main geological and geophysical data. 相似文献
9.
A complex aeromagnetic anomaly in Southern Apennines (Italy) is analysed and interpreted by a multiscale method based on the scaling function. We use multiscale methods allowing analysis of a potential field along ridges, which are lines defined by the position of the extrema of the field at the considered scales. The method developed and applied in this paper is based on the study of the scaling function of the total magnetic field. It allows recovering of source parameters such as depth and structural index. The studied area includes a Pleistocene volcanic structure (Mt. Vulture) whose intense dipolar anomaly is superimposed on a longer wavelength regional anomaly. The interpretation of ridges of the modulus of the analytic signal at different altitude ranges allows recognition of at least three distinct sources between about 5 km and 20 km depth. Their interpretation is discussed in light of borehole data and other geophysical constraints. A reasonable geological model for these sources indicates the presence of intrusions, probably linked to the past activity of Mt. Vulture. 相似文献
10.
Hidden beneath the ~ 2 km thick low-velocity volcaniclastics on the western margin of the Central Volcanic Region, North Island, New Zealand, are two structures that represent the early history of volcanic activity in a continental back-arc. These ~ 20 × 20 km structures, at Tokoroa and Mangakino, form an adjacent gravity high and low, respectively. Interpretations from seismic refraction arrivals and gravity modelling indicate the − 65 mgal Mangakino residual gravity anomaly can be modelled, in part, by two low-density bodies that reach depths of ~ 6.5 km, whereas the Tokoroa gravity anomaly is due to a higher density rock coming, at most, to within ~ 650 m of the surface. The Mangakino anomaly is interpreted to be due to the remnants of magma chambers that fed large ignimbrite eruptions from about 1.2 Ma. An andesite volcano or complex volcanic structure is the preferred interpretation for the Tokoroa gravity high. The size of the putative volcanic structure is comparable to the presently active Tongariro Volcanic Complex in the centre of North Island. 相似文献
11.
Internal structure of Ustica volcanic complex (Italy) based on a 3D inversion of magnetic data 总被引：1，自引：0，他引：1
A ground magnetic study of Ustica Island was performed to provide new insights into subsurface tectonic and volcanic structures.
The total-intensity anomaly field, obtained after a data-reduction procedure, shows the presence of a W–E-striking magnetic
anomaly in the middle of the island and another two intense anomalies, which seem to continue offshore, in the southwestern
and the northeastern sides, respectively. The detected anomalies were analyzed by a quadratic programming (QP) algorithm to
obtain a 3D subsurface magnetization distribution. The volcano magnetization model reveals the presence of intensely magnetized
volumes, interpreted as the feeding systems of the main eruptive centers of the island, which roughly follow the trend of
the main regional structural lineaments. These findings highlight how regional tectonics has strongly affected the structural
and magmatic evolution of the Ustica volcanic complex producing preferential ways for magma ascent. 相似文献
12.
CLAUDE FOURNIER 《Geophysical Prospecting》1989,37(6):647-668
Two geophysical methods [resistivity soundings and spontaneous potentials (SP)] are used to investigate aquifers in the Quaternary volcanic formations of the Chaîne des Puys (France). The interpretation of the resistivity soundings required a specific work of determination of the resistivity of the formations concerned. The basement topography, characterized by axial horst and graben structures and perpendicular palaeovalleys, was revealed. SP was developed experimentally in the field and theoretically. It is shown that the SP anomalies can be considered as double-layer potentials, with their source on the aquifers' interfaces. The most significant of them is the water table, which creates an SP anomaly of similar but inverted form. SP anomalies due to flows in unconfined aquifers, in palaeovalleys, and in volcanic ranges are computed and compared with the observed ones. These two methods have determined the extent and the boundary conditions of the different hydrogeological basins and have determined the major drainage axes and the groundwater flow pattern. 相似文献
13.
普里兹湾位于南极洲东部大陆边缘,其深部地壳结构特征对认识白垩纪冈瓦纳古陆裂解和新生代大陆边缘形成具有重要意义.本文利用重磁、多道反射地震、声纳浮标折射地震和ODP钻井数据对普里兹湾海域的深部地壳结构进行了研究.研究结果显示,普里兹凹陷表现为典型的盆地负重力异常特征,其沉积基底较深,而在四夫人浅滩为高幅重力正异常,其沉积基底普遍抬升.在大陆架中部存在SW-NE向条带状基底的抬升,且呈朝NE向逐渐变深的趋势.在中大陆架外侧,均衡残余重力异常呈V字形负异常条带状分布,其两翼分别与四夫人浅滩和弗拉姆浅滩外的大陆坡相连.该异常带在大陆架中部向陆的偏移可能是由于古大陆架边缘的地形影响,推测其与普里兹冲积扇同属于洋陆过渡带向陆的部分,在重力模拟剖面表现为地壳向海逐渐减薄.普里兹冲积扇的地壳厚度较薄,平均为6 km,最薄处可达4.6 km,并且根据洋陆过渡带向海端的位置,推测可能属于接近洋壳厚度的过渡壳.重力异常分区的走向与兰伯特地堑在普里兹湾的构造走向基本一致,可能主要反映了二叠纪-三叠纪超级地幔柱对普里兹湾的裂谷作用的影响.该区域的自由空间重力异常和均衡残余异常均表现为超过100×10-5m/s2的高幅正异常特征,可能由位于大陆架边缘的巨厚沉积体负载在高强度岩石圈之上的区域挠曲均衡作用所导致,可能与该区域第二期裂谷期之后的沉积间断以及快速进积加厚的演化过程有关.普里兹湾磁力异常的走向与重力异常明显不同,大致可分为东北高幅正异常区和西南低幅异常区.重磁异常在走向上的差异反映高磁异常主要来源于岩浆作用形成的铁镁质火成岩的影响,并且岩浆作用的时代不同于基底隆升的时代,而可能形成于前寒武纪或者南极洲和印度板块裂谷期间(白垩纪). 相似文献
14.
15.
The role of regional-scale faults in controlling a trapdoor caldera,Coromandel Peninsula,New Zealand
《Journal of Volcanology and Geothermal Research》2006,149(3-4):312-328
Regional gravity data from an eroded Miocene to Pliocene volcanic arc exposed in the Coromandel Peninsula, New Zealand, delineate a circular − 26-mGal, 15-km-diameter gravity anomaly. This anomaly, which has steep gradients on its northern and western margins but shallow gradients elsewhere, correlates with relatively young volcanic and volcaniclastic rocks within a broad topographic depression. Gravity modelling, using an exponentially decreasing density contrast with depth profile, requires very low-density rocks (ca. 2280 kg m− 3) in the near-surface to account for the observed anomaly, giving a total depth of ca. 2.8 km for these rocks. The northern and western margins of this body dip steeply inward at 70°, whereas the southern and eastern margins have shallow inward dips (20–30°). The western margin coincides with the regional-scale Mangakino Fault, but the northern margin, recognizable only in the geophysical data (and named here the Ohinemuri Fault), is partially buried under younger volcanic rocks. We interpret these deep and steeply bounded, low-density volcanics in terms of a trapdoor caldera, faulted on its northern and western margins, with its hinge on the southern and eastern margins. Epithermal deposits are spatially associated with the Mangakino and Ohinemuri Faults, suggesting that both structures may have influenced hydrothermal fluid flow. These deposits pre-date caldera fill, indicating that caldera development followed pre-existing regional faults. These results delineate the subsurface geometry of a trapdoor caldera and highlight the role of pre-existing, regional-scale faults in controlling such caldera location and collapse. 相似文献
16.
Detailed total-intensity aeromagnetic surveys of the Kuttyaro and Aso caldera regions, eastern Hokkaido and central Kyushu, were made during early 1964 under the auspices of the U.S.-Japan Co-operative Science Program in conjunction with a project for geophysical studies of calderas in Japan. Each caldera has a maximum diameter of about 22 km; the flights cover a 60 × 60 km rectangular area in each region. The Kuttyaro survey also encompasses the older caldera Akan, south-west of Kuttyaro, and the younger caldera Mashu to the east. All three lie within the Chīshīma (Kurile) volcanic zone. The isomagnetic contour map shows this zone as a belt of short wave-length anomaies which trends east-northeast across the region. Broad wavelength anomalies with trends intersecting the Chīshīma belt at an acute angle probably reflect structural relief on the Neogene volcanic basement concealed beneath Kuttyaro pyroclastic flows. The centre of Kuttyaro caldera coincides with the sharp southern termination of a strong basement high, whereas caldera faults and post-caldera domes have little magnetic expression. Mashu caldera is marked by a minimum in the position of the caldera lake; a symmetrical positive anomaly centering southeast of the caldera suggests either a buried older volcanic edifice or an intrusion. Akan caldera is represented by a magnetic depression encompassing a positive anomaly produced by its central post-caldera cone. The depression extends north of the geologically-deduced boundary of the caldera and may include an earlier collapse structure. Several volcanoes and lava sequences in the region produce negative anomalies due to inverse polarization. The most significant feature of the Aso isomagnetic map is a large, elongate positive anomaly that occupies the southern half of the caldera and extends about one caldera diameter to the south-west along the trend of the Median Tectonic Line of south-west Japan. Whether the anomaly represents the pre-Tertiary basement complex or a younger intrusion perhaps associated with Aso eruptive activity is uncertain. However, the causative body is abruptly truncated within the caldera by a major east-south-east structure passing through the eastern rim and coincident with the approximate locus of resurgent central vent eruptions. The structure may be a fault system that provided egress for the Aso pyroclastic flows. Superimposed on the basement anomaly are the effects of the topography of the caldera, the superficial caldera structure, and the post-caldera cones. An area of intense solfataric activity in the Kuju group of young volcanoes north of Aso has a pronounced negative anomaly. These two surveys illustrate the utility of the magnetic method for investigations of basement structure in caldera regions. They have served as a guide in interpreting reconnaissance aeromagnetic profiles flown concurrently for this project across some 14 other calderas or caldera-like structures in the Japanese islands. 相似文献
17.
M. Fytikas F. Innocenti N. Kolios P. Manetti R. Mazzuoli G. Poli F. Rita L. Villari 《Journal of Volcanology and Geothermal Research》1986,28(3-4)
New geochronological and volcanological data from volcanics of the island group of Milos (South Aegean active volcanic arc), allow four main cycles of volcanic activity to be distinguished, ranging in age between 3.5 and 0.1 m.y. B.P. The oldest volcanic activity consists almost totally of pyroclastics and submarine products, followed by subaerial ones. The eruptive centers migrated in time and concentrated, during the most recent volcanic phase, in the central part of Milos and on the island of Antimilos. The oldest products are dominated by andesites and dacites, while the most recent ones mainly consist of rhyolites. The volcanic evolution and the geochemical characters of the erupted products suggest that the feeding system of the oldest volcanism was located in the deep continental crust, where contamination and fractional crystallization jointly occurred. The younger cycle of volcanic activity was fed from small and relatively shallow magma chambers, where contamination processes played a minor role. These magma bodies are considered to responsible for the shallow thermal anomaly giving rise to the high enthalpy field on Milos. 相似文献
18.
Three-dimensional (3D) magnetotelluric (MT) surveys have been performed in Jeju, the largest volcanic island in Korea to figure out any possible structures or potential anomaly for remnant deep geothermal resources. Various approaches have been applied to interpret MT data observed in Jeju. MT dataset shows generally simple stratigraphy of four layers, though contains the severe static and the sea-effects. In our previous works, the induction vectors and 3D inversion results have commonly indicated the existence of a conductive anomaly in central parts of the island, beneath Mt. Halla. The 3D inversion dealt the static shifts as inversion parameters. The Jeju MT dataset, however, still contains the effect of conductive sea water surrounding the island.The sea-effect on MT impedance can be represented as a distortion tensor and excluded from the Jeju MT dataset by an iterative sea-effect correction. In this study, 3D inversion incorporating static shift parameterization was conducted using MT dataset corrected using 1D resistivity model obtained from the iterative scheme. Reasonably reconstructed images are obtained through the 3D inversion and using the MT dataset with sea-effect correction. The inversion result still shows the conductive anomaly in a similar depth. RMS misfits converged to a lower value than that of inversion using MT data before the sea-effect correction. From the fact, it is highly possible that the conductive anomaly is not an artifact but a real underground structure. Further investigation about the anomaly including exploration drilling is needed to see if it is from a fracture containing conductive sea water or related to the old volcanic activities. 相似文献
19.
An analysis of the relationship between the bathymetry and the gravity field has been carried out both in the frequency domain (admittance technique) and in the space domain over the Del Cano Rise and the Crozet Bank (southern Indian Ocean). A bathymetric map, contoured from original shipboard data and a geoid anomaly map, computed from a merged set of SEASAT and GEOS3 data were used. The inversion of admittances for water depth, crustal thickness, crustal density and elastic plate thickness shows that the isostatic response of the lithosphere supporting the Del Cano Rise differs very much from that of the one situated beneath the Crozet Bank. The Del Cano Rise appears to have been formed near the active Southwest Indian Ridge before Early Eocene and is locally supported by a thickened crust. Shallow thermal processes contribute to the present topography of the Crozet Bank, emplaced further away from the Southeast Indian Ridge on older crust. 相似文献
20.
Gilbert Féraud Janine Gastaud Jean-Marie Auzende Jean-Louis Olivet Guy Cornen 《Earth and Planetary Science Letters》1982,57(1):211-226
Gorringe Bank is situated on the Europe-Africa plate boundary at the eastern end of the Azores-Gibraltar fracture zone. It has two summits, Gettysburg Bank to the Southwest and Ormonde Bank to the northeast.We applied the40Ar/39Ar stepwise heating method to date six samples of the alkaline volcanic rocks, two gabbros from the Ormonde Bank and a dolerite from the Gettysburg Bank. The results that the alkaline volcanism lasted probably for less than 6 Ma(66-60 Ma).Although the nature of this volcanism precludes any subduction feature during its setting, the alkaline volcanism of Ormonde is probably linked to Upper Cretaceous/Eocene compressive tectonic events.The basement rocks of Gorringe Bank reveal distrubed40Ar/39Ar age spectra. One plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 Ma, and related to a variation in the direction of the relative movement between Europe and Africa. The more probable age given by a plagioclase of another gabbro and by a dolerite (110 Ma) corresponds to tilting northeastward of the Gorringe massif. 相似文献