Basement tectonics and flexural subsidence along western continental margin of India

The Paleocene-recent post-rift subsidence history recorded in the Mumbai Offshore Basin off western continental margin of India is examined. Results obtained through 2-D flexural backstripping modelling of new seismic data reveal considerable thermo-tectonic subsidence over last ca. 56 Myr. Reverse postrift subsidence modelling with variable β stretching factor predicts residual topography of ca. 2000 m to the west of Shelf Margin Basin and fails to restore late Paleocene horizon and the underlying igneous basement to the sea level. This potentially implies that:(1) either the igneous basement formed during the late Cretaceous was emplaced under open marine environs; or(2) a laterally varying cumulative subsidence occurred within Mumbai Offshore Basin(MOB) during ca. 68 to ca. 56 Ma. Pre-depositional topographic variations at ca. 56 Ma across the basin could be attributed to the extensional processes such as varied lower crustal underplating along Western Continental Margin of India(WCMI). Investigations about basement tectonics after unroofing of sediments since late Paleocene from this region support a transitional and heavily stretched nature of crust with high to very high β factors. Computations of past sediment accumulation rates show that the basin sedimentation peaked during late Miocene concurrently with uplift of Himalayan-Tibetan Plateau and intensification of Indian monsoon system. Results from basin subsidence modelling presented here may have significant implications for further studies attempting to explore tectono-climatic interactions in Asia.     

Magnetic anomalies of offshore Krishna-Godavari basin,eastern continental margin of India

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.     

Flexure of the lithosphere and continental margin basins

The accumulation of sediments at an Atlantic-type continental margin constitutes a load on the lithosphere which simply sags due to its weight. Studies of the geometry of deformation suggests the lithosphere will respond to these loads either by local loading of an Airy-type crust or flexural loading of a strong rigid crust. Sediment loading models of either type cannot, however, explain the substantial thicknesses of shallow-water sediments observed in commercial boreholes from Atlantic-type margins. Other factors such as thermal contraction, gravitational outflow of crustal material or deep crustal metamorphism may contribute to the subsidence. We have used biostratigraphic data from commercial boreholes from the Gulf of Lion and the East Coast U.S.A. to quantitatively determine the contribution of sediment loading to the subsidence. From these data we determined sea-floor and basement depths for sequential time intervals during margin development. Using the sediment loading models the sediment layers at each margin were progressively “backstripped” and the depth basement would have been without the sediment load calculated. The computed basement depths indicate there is a recognizable component of the subsidence of these margins which is caused by processes other than adjustments to the weight of the sediment. The nature of this subsidence is discussed and comparisons are made with that which would be expected from thermal-contraction models.     

Tectonics and crustal structure of the Campania continental margin: relationships with volcanism

Summary ¶The crustal structure of the Campania continental margin is synthesized from outcrop, seismic reflection and gravimetric data. Outcrop and subsurface geological data reveal the presence of NE–SW faults, E–W faults and NW–SE faults. An older extensional event occurred along NW–SE faults and was followed by the main extensional event linked to the activity of NE–SW normal faults. The latter were active between 700 and 400ka producing half-grabens filled by more than 5km of Quaternary deposits. The stratigraphic signature of these tectonic events corresponds to a Lower Pleistocene marine unconformity-bounded unit overlain by Middle Pleistocene rocks belonging to a transgressive-regressive cycle. A crustal section of the Campania margin displays an asymmetric linked fault system characterized by a 10–12km-deep main detachment level, listric normal faults and rollover anticlines. Structural and stratigraphic data document that the inception of volcanic activity at Vesuvius occurred at 400ka, just after the main extensional event, and the volcano is located at the margin of a rollover anticline.Received June 26, 2002; revised version accepted November 9, 2002     

Regional tectonic framework,structure and evolution of the western marginal basins of India

The Kutch-Saurashtra, Cambay and Narmada basins are pericontinental rift basins in the western margin of the Indian craton. These basins were formed by rifting along Precambrian tectonic trends. Interplay of three major Precambrian tectonic trends of western India, Dharwar (NNW-SSE), Aravalli-Delhi (NE-SW) and Satpura (ENE-WSW), controlled the tectonic style of the basins. The geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults. The Kutch basin opened up first in the Early Jurassic (rifting was initiated in Late Triassic) along the Delhi trend followed by the Cambay basin in the Early Cretaceous along the Dharwar trend and the Narmada basin in Late Cretaceous time along the Satpura trend. The evolution of the basins took place in four stages. These stages are synchronous with the important events in the evolution of the Indian sub-continent—its breakup from Gondwanaland in the Late Triassic-Early Jurassic, its northward drifting during the Jurassic-Cretaceous and collision with the Asian continent in the Early Tertiary. The most important tectonic events occurred in Late Cretaceous time. The present style of the continental margins of India evolved during Early Tertiary time.The Saurashtra arch, the extension of the Aravalli Range across the western continental shelf, subsided along the eastern margin fault of the Cambay basin during the Early Cretaceous. It formed an extensive depositional platform continuous with the Kutch shelf, for the accumulation of thick deltaic sediments. A part of the Saurashtra arch was uplifted as a horst during the main tectonic phase in the Late Cretaceous.The present high thermal regime of the Cambay-Bombay High region is suggestive of a renewed rifting phase.     

A model for the three-dimensional evolution of continental rift basins,north-east Africa

Large areas of north-east Africa were dominated by regional extension in the Late Phanerozoic. Widespread rifting occurred in the Late Jurassic, with regional extension culminating in the Cretaceous and resulting in the greatest areal extent and degree of interconnection of the west, central and north African rift systems. Basin reactivation continued in the Paleocene and Eocene and new rifts probably formed in the Red Sea and western Kenya. In the Oligocene and Early Miocene, rifts in Kenya, Ethiopia and the Red Sea linked and expanded to form the new east African rift system.This complex history of rifting resulted in failed rift basins with low to high strain geometries, a range of associated volcanism and varying degrees of interaction with older structures. One system, the Red Sea rift, has partially attained active seafloor spreading. From a comparison of these basins, a general model of three-dimensional rift evolution is proposed. Asymmetrical crustal geometries dominated the early phases of these basins, accompanied by low angle normal faulting that has been observed at least locally in outcrop. As rifting progressed, the original fault and basin forms were modified to produce larger, more through-going structures. Some basins were abandoned, others experienced reversals in regional dip and, in general, extension and subsidence became focused along narrower zones near the rift axes. The final transition to oceanic spreading was accomplished in the Red Sea by a change to high angle, planar normal faulting and diffuse dike injection, followed by the organization of an axial magma chamber.     

Relationship of gravity anomalies and tectonics in Assam, India

Gravity data from Assam compiled on Bouguer, Hayford and Airy isostatic anomaly maps have been interpreted in terms of tectonics of the area. The gravity anomalies suggest that the Dauki fault is very deep-seated. A gravity high of about 60 mGal near Haflong is interpreted as being the expression of an intrusive body with a density contrast of about + 0.15 g/cm₃ with respect to the surroundings. From isostatic considerations, approximate crustal thicknesses over the Shillong Plateau, the Upper Assam valley and the Surma valley are estimated to be 40, 29 and 22 km respectively, suggesting a sharp change in crustal thickness from the Shillong Plateau to the Surma valley across the Dauki fault.     

Gravity anomalies and crustal shortening in the eastern Mediterranean

Crustal shortening of the ocean floor in the eastern Mediterranean is recognized by a marked thickening of the sedimentary layer seaward of the Hellenic and Calabrian island arcs. Steep gradients and large negative free-air anomalies in the gravity field along with a highly uniform, low regional heat flow are manifestations of the thickened crust. Bodies of recently deformed sediment in and seaward of the Hellenic Trough reveal the style, polarity, and dynamics of the thickening mechanism.

A linear buried anticlinal structure, inferred from analysis of surface ship gravity profiles, may mark the site of contemporary intrabasinal underthrusting. The distribution of earthquakes beneath the Mediterranean Ridge supports the interpretation that the Anaximander, Ptolomy, and Strabo Mountains are features comparable to large basement nappes. Cyprus is one such structure, offset to the south, where the oceanic crust and part of the upper mantle have been involved in the décollement.     

Gravity field, structure and tectonics of the Eastern Ghats

The Eastern Ghats are a prominent topographic feature on the Indian Peninsula, stretching from the southern tip of the peninsula to near Bhubaneswar (20°N, 86°E) along the east coast. The belt is characterised by occurrences of high grade metamorphic rocks such as pyroxene granulites, sillimanite gneisses, charnockites and gabbro-anorthosite masses. The gravity field over the Eastern Ghats is appreciably positive as compared to the surrounding low grade gneissic terrain.Analysis of the gravity field along the coastal and southern granulite terrain comprising the Eastern Ghats shows that a large number of gravity highs are associated with charnockites of basic and intermediate nature as well as gabbro-anorthosite masses. The lows appear to be associated with acid charnockites, syenite masses or granitic intrusives.The boundary between the Eastern Ghats terrain and the adjoining Dharwar/Bastar cratons appears to be a faulted one. The crust underneath the Eastern Ghats is inferred to be of a higher density than that of the Dharwar/Bastar cratons to its west. The gravity field over the Eastern Ghats is compared to that of similar terrains in other parts of the world. It is inferred that the Eastern Ghats are characterised by a crust of higher than normal density.     

Gravity field and structures of the Rajmahal Hills: Example of the Paleo-Mesozoic continental margin in eastern India

A narrow strip of Gondwana basins separates the Rajmahal traps from the peninsular shield in eastern India. This part of the shield margin is associated with a conspicuous gravity high of 100 km wavelength and 48 mGal amplitude over an area of 25,000 km². Second order residual anomalies due to Gondwana sediments and traps are superposed on this wider gravity high. Gravity interpretation, partly constrained by seismic data, suggests that the wider high is caused by a denser metamorphic layer (amphibolite and granulite) up to 3.5 km thick. The metamorphic layer also extends below the eastern Rajmahal hills where the Gondwanas, traps and younger sediments have covered it. The Gondwanas are downfaulted against the shield edge and are preserved over an irregular basin floor whose deepest part underlies the eastern flank of the Rajmahal hills adjacent to the Bengal basin. It is inferred that the Gondwanas were deposited over a rifted and highly faulted shield margin that was intruded by the Rajmahal traps nearly 100 m.y. ago. High-grade metamorphism along the shield edge presumably preceded the continental rifting, perhaps occurring in the Precambrian as a part of the Eastern Ghats orogeny, along the east coast of India.     

Thrust tectonics along the north-western continental margin of Sabah/Borneo

Widely accepted plate tectonic models suggest that an inactive subduction zone lies along the north-west continental margin of Sabah. In contrast, interpretation of reflection seismic data acquired by BGR shows an autochthonous continental terrane comprising an Oligocene to Early Miocene carbonate platform being progressively overthrust by an allochthonous rock complex. Progressive compression resulted in the development of four structural zones: Imbricated thrust sheets (Zone III); two thrust sheet systems one on top of the other (Zone IV); a complex zone with multiphase deformation (Zone V); and piercement ridges (Zone VI).

---

Zusammenfassung Nach herkömmlichen plattentektonischen Vorstellungen soll eine inaktive Subduktionszone am nordwestlichen Kontinentalrand von Sabah liegen. Reflexionsseismische Meßdaten der BGR zeigen jedoch, daß hier autochthone kontinentale Kruste mit einer oligozänen-frühmiozänen Karbonatplattform progressiv von einem allochthonen Gesteinsverband überschoben wird. Fortschreitender Zusammenschub seit dem frühen Miozän führte zur Anlage von vier Deformationszonen: Tekonische Schuppen (Zone III); zwei übereinander geschobene Verschuppungssysteme (Zone IV); Gürtel mit mehrphasiger Deformation (Zone V) und Durchspießungsstrukturen (Zone VI).

---

Résumé Les modèles géodynamiques que l'on admet habituellement comportent une zone de subduction inactive le long de la marge continentale nord-occidentale de Sabah. Toutefois, des mesures de sismique-réflexion exécutées par le BGR font apparaître qu'à cet endroit, une croûte continentale autochtone, comportant une plateforme carbonatée oligocène à éomiocène, est chevauchée progressivement par un complexe allochtone. La compression, qui s'est manifestée progressivement depuis le Miocène inférieur, a engendré quatre zones structurales: un ensemble d'écailles tectoniques (zone III); deux systèmes de lames tectoniques charriés l'un sur l'autre (zone IV); une zone complexe à déformation multiphasée (zone V); des structures d'extrusion tectonique (zone VI).

---

, - Sabah'a. , - . 4-? : ( 3), ( 4) ( 5) ( 6).

     

Late-Quaternary volcanism and transtensional tectonics in the Bay of Naples, Campanian continental margin, Italy

Summary ¶The Campanian continental margin is characterised by asymmetric half grabens and large-volume volcanic deposits. Vesuvius and Campi Flegrei are active volcanoes located along the coast of Naples Bay along one of these half grabens. The interpretation of an extensive set of seismic reflection data allowed to reconstruct the stratigraphy and structural pattern in Naples Bay and their relationships with volcanism. The stratigraphic succession is characterised by a complex architecture due to the presence of 157Ka old submarine and subaerial vents, pyroclastic flow units associated to the Campanian Ignimbrite and the Neapolitan Yellow Tuff, and numerous tuff cones younger than 12Ka. These volcanic units occur within marine sediments deposited during the late Quaternary sea level oscillations. The structural pattern is characterised by late Quaternary NE- trending normal faults, NW-trending transtensional faults, E–W trending left-lateral faults and WNW-ESE directed folds. The overall fault kinematics and the evaluated very high displacement rates are consistent with an E-trending left-lateral transtensional shear zone located between 41° and 40°N latitude. The relationship between volcanism and tectonics in the Bay of Naples is explained with block rotation associated with this shear zone. Finally, tectonic and stratigraphic data argue against the occurrence of a caldera in the Bay of Naples.Received June 24, 2002; accepted November 8, 2002     

Characterizing slope morphology using multifractal technique: a study from the western continental margin of India

Here, we present the slope configuration of the submarine gullies, ridges and the adjacent slump zone off Goa, along the western continental margin of India utilizing multibeam bathymetric and single-channel seismic data. The fluid flow migration signature in the form of pockmark seepages, traces of mud volcanoes and enhanced reflectors is observed in the area. Altogether thirty-three depth profiles from the gully, ridge and slump areas depict downslope progression in gully incision and varying gradients in the gullies (1.19–4.07°) and ridges (2.13–3.70°), whereas the profiles of the slump zone are comparatively steady (2.25–2.51°). The scatter plot of the three slope characteristics, viz., gradient, mean depth and root mean square relief, characterizes the profiles of the gullies, ridges and slump zone into three distinct clusters. Principal component analysis as well corroborates the categorization. Furthermore, a stochastic multifractal technique has been employed to understand the nature of the fine-scale seafloor processes active in the slope region. The three estimated parameters of the depth profiles, i.e., the degree of multifractality (α), sparseness (C ₁) and the degree of smoothness (H), substantiate a very high degree of multifractality for all the thirty-three bathymetric profiles. Except for the five adjacent profiles (four from the slump zone and one from the ridge), the remaining twenty-eight depth profiles of the gully, ridges and slump zones show negligible difference. Based on the multifractal study, we conclude that the observed discrimination might be due to the significant interaction between the bottom currents off Goa and the varied seafloor morphological aspects with seepages and faults.     

CDP seismic sections of the western Beaufort continental margin

The continental rise, slope, and shelf in the Beaufort Sea off northern Alaska were surveyed with 5600 km of common-depth-point (CDP) seismic data by the U.S. Geological Survey in 1977. The lower continental rise consists of a wedge of at least 4.5 km of low-velocity, generally flat-lying, parallel-bedded sediments. Slump-related diapiric folds, probably cored by shale, occur on the upper rise and lower slope. The observed minimum depth to oceanic basement in the Canada Basin requires an age for this basin of at least 120 m.y., assuming it to be floored by oceanic crust with a subsidence history similar to that of the Atlantic and Pacific oceans.     

Extensional tectonics and sedimentary response of the Early-Middle Cambrian passive continental margin, Tarim Basin, Northwest China

The fact that several half-grabens and normal faults developed in the Lower—Middle Cambrian of Tazhong(central Tarim Basin) and Bachu areas in Tarim Basin,northwest China,indicates that Tarim Basin was under extensional tectonic setting at this time.The half-grabens occur within a linear zone and the normal faults are arranged in en echelon patterns with gradually increasing displacement eastward.Extensional tectonics resulted in the formation of a passive continental margin in the southwest and a cratonic margin depression in the east,and most importantly,influenced the development of a three-pronged rift in the northeast margin of the Tarim Basin.The fault system controlled the development of platform-slope-bathyal facies sedimentation of mainly limestone-dolomite-gypsum rock-saline rock-red beds in the half-grabens.The NW-SE trending half-grabens reflect the distribution of buried basement faults.     

Implications for the lithospheric structure of Cambay rift zone,western India:Inferences from a magnetotelluric study

Broad-band and long-period magnetotelluric(MT) data were acquired along an east-west trending traverse of nearly 200 km across the Kachchh,Cambay rift basins,and Aravalli-Delhi fold belt(ADFB),western India.The regional strike analysis of MT data indicated an approximate N59°E geoelectric strike direction under the traverse and it is in fair agreement with the predominant geological strike in the study area.The decomposed transverse electric(TE)-and transverse magnetic(TM)-data modes were inverted using a nonlinear conjugate gradient algorithm to image the electrical lithospheric structure across the Cambay rift basin and its surrounding regions.These studies show a thick(～1-5 km) layer of conductive Tertiary-Mesozoic sediments beneath the Kachchh and Cambay rift basins.The resistive blocks indicate presence of basic/ultrabasic volcanic intrusives,depleted mantle lithosphere,and different Precambrian structural units.The crustal conductor delineated within the ADFB indicates the presence of fluids within the fault zones,sulfide mineralization within polyphase metamorphic rocks,and/or Aravalli-Delhi sediments/metasediments.The observed conductive anomalies beneath the Cambay rift basin indicate the presence of basaltic underplating,volatile(CO_2,H_2 O) enriched melts and channelization of melt fractions/fluids into crustal depths that occurred due to plume-lithosphere interactions.The variations in electrical resistivity observed across the profile indicate that the impact of Reunion plume on lithospheric structures of the Cambay rift basin is more dominant at western continental margin of India(WCMI) and thus support the hypothesis proposed by Campbell Griffiths about the plume-lithosphere interactions.     

大陆构造、大洋构造和地球构造研究构想

大陆动力学和大洋动力学是当前固体地球科学的前沿领域 ,反映处于中期阶段的板块理论正向更加深入、全面、完善的方向发展 ,并走向统一的地球构造学的趋势。中、新生代造山带构造 ,全球高原构造的比较 ,周边洋底构造对欧亚大陆的动力作用 ,应是大陆动力学中优先研究的问题。对全球洋底构造的继续探测 ,用地震各向异性研究地幔的流动或变形 ,布设海底宽频带地震台阵探测地幔细结构 ,将会提供更多的地球内部过程信息。“地球大系统科学”概念的提出 ,将能推进固、液、气三态地球多球层相互作用的研究 ,例如固体地球微动态、固液气三态球层运动的可比较性、不同球层分区性的比较等 ,都是需要深入探讨的问题 ,代表了从整体地球系统开展学科交叉研究的方向     

Regional and local magmatic anomalies and tectonics of rift zones between the Antarctic and South American plates

The study provides new understanding of magmatism at extinct and modern spreading zones around the western margin of East Antarctica from Bransfield Strait to the Bouvet Triple Junction (BTJ) in the Atlantic Ocean and reveals causes of geochemical heterogeneity of mantle magmatism during the early opening of the Southern Ocean. The results indicate the involvement of an enriched source component in the generation of parental melts, which was formed in several tectonic stages. The enriched (metasomatized) mantle generated at rift zones has geochemical characteristics typical of the western Gondwana lithosphere (with isotopic compositions similar to those inferred for the enriched HIMU and EM-2 sources). This mantle source may have been produced by the thermal erosion of the continental mantle during the early stages of the Karoo–Maud–Ferrar superplume activity. This enriched mantle generated in the apical parts of the plume (sub-oceanic) began to melt during tectonic displacement and fragmentation of Gondwana. The Bouvet Triple Junction, located along modern spreading zones between the Antarctic and South American plate, is characterized by a greater depth of melting and a higher degree of enrichment of primary tholeiitic magmas. The highest enrichment of magmas in this region is controlled by a contribution from a pyroxenite-rich component, which was also identified in the extinct spreading center in Powell Basin.     

Petroliferous basins at continental margins of paleozoic paleoseas: Communication 2. Petroliferous basins at continental margins in the Rheic, Uralian, and Central Asian paleoseas

The continental block of the Earth’s crust was separated in the Paleozoic into two unequal parts: (i) huge supercontinent Gondwana located at high latitudes of the Southern Hemisphere and (ii) several small continents (Laurentia, Baltica, Siberia, Kazakhstan, South Chinese block, and North Chinese blocks) located at low latitudes south and north of the equator. Morphology of the Paleozoic seas between these blocks was subjected to changes (expansion and contraction) with time. Their closure was provoked by several orogenic (Taconian, Caledonian, Acadian, and Hercynian) phases. At present, relicts of these ancient orogenic structures extend as belts along the boundaries of many petroliferous basins and record the position of past seas. One of the oldest oil-and-gas deposition belts, which appeared in southern Iapetus in the Precambrian/Phanerozoic, was confined to a passive margin of Gondwana. In the Early Paleozoic, small blocks of the continental crust (Avalonia, Armorica, Perunica, Iberica, and others) were successively detached from the passive margin. This process was accompanied by the opening of a new deep basin (Rheic Sea or Paleotethys). The Uralian and Central Asian paleoseas were formed approximately at the same time. Many petroliferous basins existing now were located in the Paleozoic at the margins of these paleoseas.     

北美东部被动大陆边缘盆地的分段性与油气勘探

北美东部被动大陆边缘是世界上最古老的完整被动大陆边缘之一,是研究被动大陆边缘发育演化的天然实验室。本文在大量国外研究成果的基础上,应用盆地构造解析方法,深入研究了北美东部被动大陆边缘盆地群的地质结构和构造演化特征,并揭示了盆地群的油气地质规律。研究认为,北美东部盆地群沉积充填和不整合面发育具有明显的分段性和差异性。以区域不整合面为界,不同段盆地可划分为不同的构造层：南段盆地可划分为两套构造层;中段南部盆地可划分为3套构造层;中段北部盆地可划分为4套构造层;而北段盆地可划分为5套构造层。盆地群整体经历了陆内裂谷—陆间裂谷—被动大陆边缘的演化过程,但不同段盆地的构造演化具有明显的分段性和迁移性：晚三叠世沉降中心位于南段盆地;早侏罗世初期迁移至中段盆地,南段大陆开始裂解;中侏罗世逐渐迁移至北段盆地,中段大陆开始裂解;早白垩世晚期,北段大陆开始裂解。受持续的抬升剥蚀及大西洋岩浆活动省的联合作用,南段盆地和中段大多数盆地缺乏油气保存条件;斯科舍盆地和大浅滩盆地是主要的含油气盆地,以上侏罗统烃源岩为主,主要发育断层—背斜圈闭和盐体刺穿圈闭,整体表现为“自生自储”和“下生上储”的特征。