Interpretation of Gravity Data using 3D Euler Deconvolution,Tilt Angle,Horizontal Tilt Angle and Source Edge Approximation of the North-West Himalaya

The collision of the Indian plate and the Eurasian plate created shortening and imbrications with thrusting and faulting which influences northward tectonic movement. This plate movement has divided the Himalaya into four parts, viz. Outer Himalaya, Lesser Himalaya, Greater Himalaya, and Tethys Himalaya. The crystalline basement rock plays an imperative role for structural and tectonic association. The study has been carried out near Rishikesh-Badrinath neighborhood in the northwestern part of the Himalayan girdle with multifarious tectonic set up with thrusted and faulted geological setting. In this study area, 3D Euler deconvolution, horizontal gradient analysis, tilt angle (TILT) and horizontal tilt angle (TDX) analysis have been carried out using gravity data to delineate the subsurface geology and heterogeneity in the northwestern part of Himalaya. The Euler depth solutions suggest the source depth of about 12 km and various derivative analyses suggest the trend of the delineation thrust-fault boundaries along with the dip and strike direction in the study area.     

Interpretation of gravity data to delineate structural features connected to low-temperature geothermal resources at Northeastern Portugal

A great number of low-temperature geothermal fields occur in Northern-Portugal related to fractured rocks. The most important superficial manifestations of these hydrothermal systems appear in pull-apart tectonic basins and are strongly conditioned by the orientation of the main fault systems in the region. This work presents the interpretation of gravity gradient maps and 3D inversion model produced from a regional gravity survey. The horizontal gradients reveal a complex fault system. The obtained 3D model of density contrast puts into evidence the main fault zone in the region and the depth distribution of the granitic bodies. Their relationship with the hydrothermal systems supports the conceptual models elaborated from hydrochemical and isotopic water analyses. This work emphasizes the importance of the role of the gravity method and analysis to better understand the connection between hydrothermal systems and the fractured rock pattern and surrounding geology.     

The Sinai subplate and tectonic evolution of the northern Red Sea region

Although the precise boundaries and kinematics of the Sinai subplate are still doubtful, it has a significant role in the tectonic evolution of the northern Red Sea region. On the basis of earthquake distribution, the Sinai region can be considered as a subplate partially separated from the African plate by the Suez rift. The relative motion between Africa, Sinai and Arabia is the main source generating the present-day earthquake activity in the Gulf of Suez and the Gulf of Aqaba regions.According to geological observations, the southern segment of the Dead Sea fault system can be characterized by a left-lateral displacement of about 107km since the Middle Miocene, in contrast to the northern segment where only 25 to 35km offset can be inferred. We think that along the southern segment the total displacement was 72km until the late Miocene (10Ma). The earthquake activity is strongly reduced along the northern segment of the Dead Sea fault segment. Therefore, we suggest that the northern part (Yammouneh fault) evolves through initial cracking of the crust due to build-up of stress since the Pliocene time (5Ma) and propagates northward into Lebanon and Syria. This last 5 million years is the period when the southern and northern segments became linked and formed a single fault system with a new displacement of 35km.According to the proposed model the predicted opening pole of the Red Sea is near 34.0oN, 22.0oE with an angle of total rotation of 3.4o since the early miocene, providing a 0.82cm/a opening rate in the northern Red Sea. We suggest that the Dead Sea strike-slip fault was active since Middle Miocene time (15Ma) with a slip rate of 0.72cm/a to provide a total displacement of about 107km. This strike slip motion occured about an Euler pole near 33.0oN, 21.0oE with a rotation angle of about 3.0o. It can be inferred from the proximity of the pole and angle of rotations for the Red Sea and Dead Sea fault that more than 85% of the motion has been accommodated on the Gulf of Aqaba and the Dead Sea fault and less than 15% in the Gulf of Suez.This model predicts a normal extensional motion in the Gulf of Suez with a minor left-lateral strike-slip component. We expect the pole of this motion to be at 31.0oN, 29.0oE, offshore of Alamein city about 320 km west of the Nile Delta. The rate of motion through the last 15Ma (Middle Miocene) is about 0.1 cm/a and the angle of rotation is 0.9o. During this period the total opening of the Suez rift is 15 km while the rest of the motion (45 km) occured mainly through the first phase of the development before the Middle Miocene.     

青藏高原东北缘重力异常多尺度横向构造分析

本文研究了青藏高原东北缘地区布格重力异常特征,采用优化滤波法和归一化总水平梯度垂直导数法对研究区重力异常进行多尺度分离和横向构造分析.分离出的多尺度重力异常特征表明:1) 青藏高原东北缘地区大致以东经106°线为界,有一条醒目的重力异常梯级带,即贺兰山-六盘山-川滇南北构造带的北段,其东西两侧布格重力异常特征在形态和走向上截然不同,意味着两侧密度结构和构造特征存在明显差异. 2) 鄂尔多斯地块内部定边以北,重力异常高带走向由北东向转为近南北向,推测定边附近存在一个密度或构造界面,其两侧物质组成和构造特征具有差异,对比大尺度重力异常和中尺度重力异常,表明异常特征的这种差异主要是由上地幔深部结构引起的. 3) 青藏高原东北部各块体深部边界位置与地表构造分布不同,反映出该区构造复杂,深浅构造差异大. 4) 由于印度-欧亚板块碰撞及随后印度板块持续向北的挤压作用,造成青藏高原东北缘中、下地壳物质在巨大的北东向推挤力和鄂尔多斯刚性块体阻挡的共同作用下,沿着相对软弱的秦岭造山带方向蠕动.依据多尺度重力异常及其横向构造特征,综合推断出研究区内五条断裂带,即秦岭地轴北缘断裂带、海原-六盘山断裂带、香山-天景山断裂带、烟筒山断裂带和青铜峡-固原断裂带,并分析了它们在地壳深部的可能展布特征.     

Gravity and magnetic data inversion for 3D topography of the Moho discontinuity in the northern Red Sea area, Egypt

The main goal of our study is to investigate 3D topography of the Moho boundary for the area of the northern Red Sea including Gulf of Suez and Gulf of Aqaba. For potential field data inversion we apply a new method of local corrections. The method is efficient and does not require trial-and-error forward modeling. To separate sources of gravity and magnetic field in depth, a method is suggested, based on upward and downward continuation. Both new methods are applied to isolate the contribution of the Moho interface to the total field and to find its 3D topography. At the first stage, we separate near-surface and deeper sources. According to the obtained field of shallow sources a model of the horizontal layer above the depth of 7 km is suggested, which includes a density interface between light sediments and crystalline basement. Its depressions and uplifts correspond to known geological structures. At the next stage, we isolate the effect of very deep sources (below 100 km) and sources outside the area of investigation. After subtracting this field from the total effect of deeper sources, we obtain the contribution of the Moho interface. We make inversion separately for the area of rifts (Red Sea, Gulf of Suez and Gulf of Aqaba) and for the rest of the area. In the rift area we look for the upper boundary of low-density, heated anomalous upper mantle. In the rest of the area the field is satisfied by means of topography for the interface between lower crust and normal upper mantle. Both algorithms are applied also to the magnetic field. The magnetic model of the Moho boundary is in agreement with the gravitational one.     

The role of slab detachment processes in the opening of the western–central Mediterranean basins: some geological and geophysical evidence

A review of the geological and geophysical data from the central and western Mediterranean region and the present-day upper mantle structure derived from tomographic studies are utilized in order to define the Oligocene–Recent geodynamic evolution for the area. In line with previous work, we suggest that the Miocene–Quaternary opening of the western and central Mediterranean basins is the result of back-arc extension due to the roll back toward the southeast of a northwestward subducting African slab in a geodynamic setting pinned between the Alpine and Betic collisional zones. We find, however, that this general pattern is complicated by four different detachment events which occurred beneath the Alps (Early Oligocene), the Betic chain (Aquitanian), northern Africa (Langhian) and the Apennines (Late Miocene?–Pliocene). We show that each of these events determines a major tectonic reorganization within the European plate.     

Tethyan ophiolites and Pangea break-up

Abstract The break‐up of Pangea began during the Triassic and was preceded by a generalized Permo‐Triassic formation of continental rifts along the future margins between Africa and Europe, between Africa and North America, and between North and South America. During the Middle–Late Triassic, an ocean basin cutting the eastern equatorial portion of the Pangea opened as a prograding branch of the Paleotethys or as a new ocean (the Eastern Tethys); westwards, continental rift basins developed. The Western Tethys and Central Atlantic began to open only during the Middle Jurassic. The timing of the break‐up can be hypothesized from data from the oceanic remnants of the peri‐Mediterranean and peri‐Caribbean regions (the Mesozoic ophiolites) and from the Atlantic ocean crust. In the Eastern Tethys, Middle–Late Triassic mid‐oceanic ridge basalt (MORB) ophiolites, Middle–Upper Jurassic MORB, island arc tholeiite (IAT) supra‐subduction ophiolites and Middle–Upper Jurassic metamorphic soles occur, suggesting that the ocean drifting was active from the Triassic to the Middle Jurassic. The compressive phases, as early as during the Middle Jurassic, were when the drifting was still active and caused the ocean closure at the Jurassic–Cretaceous boundary and, successively, the formation of the orogenic belts. The present scattering of the ophiolites is a consequence of the orogenesis: once the tectonic disturbances are removed, the Eastern Tethys ophiolites constitute a single alignment. In the Western Tethys only Middle–Upper Jurassic MORB ophiolites are present – this was the drifting time. The closure began during the Late Cretaceous and was completed during the Eocene. Along the area linking the Western Tethys to the Central Atlantic, the break‐up was realized through left lateral wrench movements. In the Central Atlantic – the link between the Western Tethys and the Caribbean Tethys – the drifting began at the same time and is still continuing. The Caribbean Tethys opened probably during the Late Jurassic–Early Cretaceous. The general picture rising from the previous data suggest a Pangea break‐up rejuvenating from east to west, from the Middle–Late Triassic to the Late Jurassic–Early Cretaceous.     

Current motion and short-term deformations in the Suez–Sinai area from GPS observations

We analyze observations from eight GPS campaigns carried out between 1997 and 2005 on a network of 13 sites in the Suez–Sinai area, where separation between the African and the Arabian plates takes place. This is the key area to understand if and in which way Sinai behaves like a sub-plate of the African plate and the role played by seismic and geodetic (long-term) deformation release.Our analysis shows that, on average, the Suez–Sinai area motion, in terms of ITRF00 velocities, matches the African plate motion defined by the NNR-NUVEL-1A model.The horizontal principal strain rate axes estimated separately in the Gulf of Suez area and in the northern Sinai vary from compression across the Gulf (−2.2 ± 1.2) × 10⁻⁸ year⁻¹ to NE extension (1.0 ± 1.5) × 10⁻⁸ year⁻¹ in the North, showing the presence of two distinct domains, so that in our opinion Sinai cannot be considered simply a unique rigid block.The analysis of GPS baseline length variations shows short-term deformations across the Gulf of Suez, reaching up a maximum value of more than 1 cm in 8 years.Since current geodynamical models do not predict significant tectonic deformation in this area, we work under the hypothesis that a contribute may be expected by post-seismic relaxation effects. Under this hypothesis, we compare the baselines length variations with the post-seismic relaxation field associated with five major local earthquakes occurred in the area, testing two different viscoelastic models. Our results show that the detected short-term deformations are better modeled for viscosity values of 10¹⁸ Pa s in the lower crust and 10²⁰ Pa s in the asthenosphere. However, since the modeled post-seismic effect results modest and a certain amount of the detected deformation is not accounted for, we think that an improved modeling should take into account the lateral heterogeneities of crust and upper mantle structures.     

Role of southeastern Sanandaj–Sirjan Zone in the tectonic evolution of Zagros Orogenic Belt,Iran

Geological studies indicate that the southeastern Sanandaj–Sirjan Zone, located in the southeastern Zagros Orogenic Belt, is subdivided transversally into the Esfahan–Sirjan Block with typical Central Iranian stratigraphic features and the Shahrekord–Dehsard Terrane consisting of Paleozoic and Lower Mesozoic metamorphic rocks. The Main Deep Fault (Abadeh Fault) is a major lithospheric fault separating the two parts. The purpose of this paper is to clarify the role of the southeastern Sanandaj–Sirjan Zone in the tectonic evolution of the southeastern Zagros Orogenic Belt on the basis of geological evidence. The new model implies that Neo‐Tethys 1 came into being when the Central Iran Microcontinent split from the northeastern margin of Gondwana during the Late Carboniferous to Early Permian. During the Late Triassic a new spreading ridge, Neo‐Tethys 2, was created to separate the Shahrekord–Dehsard Terrane from Afro–Arabian Plate. The Zagros sedimentary basin was formed on a continental passive margin, southwest of Neo‐Tethys 2. The two ophiolitic belts of Naien–Shahrebabak–Baft and Neyriz were developed to the northeast of Neo‐Tethys 1 and southwest of Neo‐Tethys 2 respectively, related to the sinking of the lithosphere of the Neo‐Tethys 1 in the Late Cretaceous. It can be concluded that deposition of the Paleocene conglomerate on the Central Iran Microcontinent and Pliocene conglomerate in the Zagros Sedimentary Basin is directly linked to the uplift generated by collision.     

Seismicity constraints on stress regimes along Sinai subplate boundaries

The relative movement between African, Arabian and Eurasian plates has significantly controlled the tectonic process of Sinai subplate region, although its kinematics and precise boundaries are still doubtful. The respective subplate bounded on both sides by the Aqaba-Dead Sea transform fault to the east and the Gulf of Suez, the only defined part, to the west. Seismicity parameters, moment magnitude relation and fault plane solutions were combined to determine the active tectonics along the aforementioned boundaries. Seven shallow seismogenic zones were defined by the heterogeneity in stress field orientations. Along the eastern boundary, the average fault plane solution obtained from the moment tensor summation (MTS) reveals a sinistral strike-slip faulting mechanism. The corresponding seismic strain rate tensor showed that the present tectonic stress producing earthquakes along the boundary is dominated by both NW-SE compression and NE-SW dilatation. Towards the north, the average focal mechanism showed a normal faulting mechanism of N185°E compression and an N94°E extension in the Carmel Fairi seismic zone. On the other hand, the active crustal deformation along the western boundary (Gulf of Suez region) showed a prevailing tensional stress regime of NE to ENE orientations; producing an average fault plane solution of normal faulting mechanism. The seismic strain rate tensor reveals a dominant stress regime of N58°E extension and N145°E compression in consistence with the general tectonic nature in northeastern Africa. Finally, the extensional to strike-slip stress regimes obtained in the present study emphasize that the deformation accommodated along the Sinai subplate boundaries are in consistence with the kinematics models along the plate boundaries representing the northern extremity part of the Red Sea region.     

Gravity and magnetic field characteristics and hydrocarbon prospects of the Tobago Basin

The Tobago Basin, which is located offshore northern Venezuela with a southern margin close to Trinidad and Tobago, has an area of approximately 59,600 km². The Tobago Basin has relatively favourable hydrocarbon prospects, and to date, exploration work has mainly concentrated on small areas of the southwestern portion of the basin. To conduct a comprehensive study of the structural framework of the basin and the characteristics of the basement in order to identify prospective zones for hydrocarbon exploration, shipborne‐measured and satellite‐measured gravity data, shipborne‐measured magnetic data, and aeromagnetic survey data were analysed. A regularisation filtering method was used to separate and obtain regional and residual gravity and magnetic anomalies. Directional gradients of gravity and magnetic anomalies and the total horizontal gradient and vertical second derivative of gravity anomalies were employed to extract information about fault structures. Regression analysis methods were used to determine the basement depth. The geological significance of the gravity and magnetic fields was examined, the structural framework of the basin was assessed, the basement depth was estimated, and favourable hydrocarbon exploration prospects within the basin were identified. The results show that the Tobago Basin contains complex structures consisting mainly of two groups of faults trending in northeasterly and northwesterly directions and that the major northeasterly trending faults control the main structural configuration and depositional system within the basin. The basement of the Tobago Basin has deep rises and falls. It can be divided into the following four secondary tectonic units: the western sub‐basin, the central uplift area, the southern sub‐basin, and the northeastern sub‐basin. The central uplift area and northeastern sub‐basin are most likely to have developed hydrocarbon accumulations and should be targeted for further exploration.     

Interpretation case study of the Sahl El Qaa area, southern Sinai Peninsula, Egypt

The study area is located in the Sinai Peninsula, which is considered one of the most promising regions for oil resources. Three different tectonic forces affect the area in the triple junction structures associated with the opening of the Gulf of Suez and the strike slip movement along the Gulf of Aqaba. The main goal of this work is to model the structure of the basement rocks in the study area using magnetic methods. To achieve this, a high-resolution land magnetic survey was acquired and the results were combined with existing seismic reflection data. The magnetic interpretation was carried out using the analytical signal, horizontal gradient, Euler and Werner deconvolution and 3D magnetic modelling methods. We concluded that most of the deduced structures are trending in N–S, N35°–N45° west and E–W directions. The Aqaba trend (N15°–N25° east) is barely noticeable. The depth to the basement rocks ranges from 1 km to more than 2 km below sea level and these results are in agreement with the available well log data. In addition, interpretation of seismic reflection sections was carried out and compared with overlapping magnetic profiles interpreted using Euler deconvolution. They show that the sedimentary section was affected by the basement tectonics, with faults extending from the basement upwards through the sedimentary cover. These faults constitute good potential structural traps for oil accumulation.     

郯庐断裂带中段三维密度结构反演与解释

为深入理解研究区的构造运动规律，进一步探求地壳物质可能的运移模式，本文基于Crust1.0地壳结构模型和最新莫霍面深度模型对自由空气重力异常数据进行联合约束，采用三维重力反演技术得到了郯庐断裂带中段的三维密度结构。反演结果显示，研究区的背景场稳定，断裂构造发育部位表现为重力密度低值，特别是营潍断裂带正处于大范围的重力密度低值区并纵贯整个研究区，由此可推断重力密度低值区可指示断裂构造的存在。      

南海北部潮汕坳陷上白垩统盆地原型及其大地构造背景分析

潮汕坳陷MZ-1井揭示的中生界为深入分析南海北部晚中生代的构造演化提供了关键性的资料.基于MZ-1井的标定，开展了系统的地震剖面构造-地层解释，在中生代地层内识别出Tm30区域性不整合面，同位素定年确定该界面发育于早白垩世末至晚白垩世初，落实了潮汕坳陷上白垩统的分布.此外，在研究区西南部识别出大型的兴宁—东沙逆冲推覆带，主要由多条NW—SE向延伸、西倾的叠瓦状逆冲断层及其伴生的不对称褶皱组成，其明显控制了上白垩统厚度分布.由此可见，上白垩统构造层不具有张裂盆地的典型特征，因此南海北部主动陆缘向被动陆缘的转换不会早于晚白垩世末.研究认为，在南海地区特提斯残留洋盆关闭的总背景下，在约80 Ma时期，南海地块与华南陆块强烈碰撞挤压，在靠近碰撞带处的礼乐滩、潮汕坳陷西南部形成褶皱冲断构造体系，进而控制了潮汕坳陷晚白垩世周缘前陆盆地的发育.     

南海重力异常的沉积层密度改正及其对区域构造特征分析的意义

南海位于太平洋板块、印澳板块和欧亚板块交汇处,自晚中生代以来历经张裂作用、海底扩张以及印藏碰撞、菲律宾海板块西向运动等构造事件的叠加改造,不仅形成了复杂多样的构造格局,而且堆积了厚薄不均的沉积层.为了考察沉积层密度改正对利用重力资料分析南海不同尺度构造特征的影响,本文利用南海各区域不同深度沉积层的地震波速度及钻孔密度等数据,建立了沉积层与沉积基底密度差随深度变化的二次函数关系式,并基于该关系式,计算了南海沉积层相对基底密度低而产生的重力异常值.结果显示,南海沉积层的重力异常值在海盆区介于-40~-60 mGal,而在堆积巨厚沉积物的莺歌海盆地可达到-135 mGal;相对于空间重力异常、布格重力异常,经沉积层重力异常改正后的地壳布格重力异常更能突出深部不同尺度的密度结构和莫霍面的起伏特征,其总水平导数模更突显了南海西北部红河断裂带的海上延伸;利用谱分析技术估算岩石圈强度时,经沉积层重力异常改正的地壳布格重力异常数据获得的岩石圈有效弹性厚度值更为符合地质实际,特别是在长条形的巨厚沉积区如莺歌海盆地和马来盆地.分析表明,重力异常的沉积层密度改正对揭示南海构造特征具有重要的意义.     

Paleomagnetic and gravimetrical reconnaissance of Cretaceous volcanic rocks from the Western Colombian Andes: paleogeographic connections with the Caribbean Plate

The reconstruction of the tectonic evolution of the oceanic crust, including the recognition of ancient oceanic plumes and the differentiation between multiple and single oceanic arcs, relies on the paleogeographic analysis of accreted oceanic fragments found in orogenic belts. Here we present paleomagnetic and gravity data from Cretaceous oceanic basaltic and gabbroic rocks, the continental metamorphic basement, and their associated cover from northwestern Colombia. Based on regional scale tectonic reconstructions and geochemical constraints, such rocks have been interpreted as remnants of an oceanic large igneous province formed in southern latitudes, which was accreted to the sialic continental margin during the Late Cretaceous. Gravity analyses suggest the existence of a coherent high density segment separated by major suture zones from a lower density material related to the continental crust and/or thick sedimentary sequences trapped during collision. A characteristic paleomagnetic direction in Early and Late Cretaceous oceanic volcano-plutonic rocks, revealing a southeastern declination (D) and a negative inclination (I), may be interpreted in two different ways: (1a primary magnetization (tilt-corrected direction D = 130.3°, I = -23.3°, k = 23.4, α₉₅ = 26.4°), suggesting clockwise rotation around 130°, and magnetization acquired in southern latitudes (range of 4°S to 21°S); or (2) a remagnetization event during a reverse interval of the Earth’s magnetic field in the Cenozoic (in situ direction D = 128.7°, I = -6.2°, k = 23.1, α₉₅ = 26.1°), suggesting a counter-clockwise rotation around 50°. The first scenario seems more plausible, as it is consistent with previous paleomagnetic studies at other localities; it is compatible with a southern paleogeography for this block, and when integrated with other regional geological and paleomagnetic studies, supports a southern Pacific origin of a major oceanic block, formed as a part of a broader Cretaceous plateau that may have extended south or southwest of Galapagos. After its initial accretion, this block was subsequently fragmented due to the oblique SW-NE approach to the continental margin during the Late Cretaceous.     

Seismicity of the northernmost part of the Red Sea (1995–1999)

Seismicity in the northernmost part of the Red Sea has been studied using data from Hurghada Seismological Network in addition to readings from the existing neighbouring networks. Relocated events in addition to data from national centers are used to obtain a complete and true picture for the seismicity of the area. The spatial distribution of earthquakes defines three earthquake zones in the Gulf of Aqaba and three zones at the entrance of the Gulf of Suez and southern tip of Sinai Peninsula. The thermal activity and the triple junction nature control the activity in this area. The activity defines also an active trend extending from the southern tip of Sinai Peninsula to the median zone of the Red Sea. The seismicity of this trend is probably related to the active spreading zone associated with the opening of the Red Sea. The b-values are derived for the entrance areas of the two gulfs and for Gulf of Aqaba. Values of b are 1.35 for the triple junction region, 1.13 for the activity before the 22 November 1995 Gulf of Aqaba mainshock and 1.25 for the aftershocks of this event.     

Recent tectonic stress field and major earthquakes of the Bohai Sea basin

Introduction The present Bohai Sea is a half-closed shallow one in the continent, located at the northeast to North China, with an area about 7.3104 km2. Geologically, it is situated in the northern North China basin and of a short development history. Previous studies (WANG, LI, 1983; Institute of Oceanology, Chinese Academy of Sciences, 1985; HUANG, et al, 1993) show that the Fu-jian-Lingnan uplift in the East China Sea continental shelf sank gradually into the oceanic bottom, mak…     

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.     

Tilt梯度及其水平导数提取重磁源边界位置

探讨了用于探测异常源边界和形状的Tilt梯度及其水平导数的方法及其性质．通过模型试算,论证了Tilt梯度及其水平导数的有效性,并从理论上给予了证明．Tilt梯度及其水平导数的方法及其性质可广泛用于区域重磁异常的构造解释之中。