Age levels of Precambrian orogenic cycles of India

The paper reviews critically the present status of knowledge on the Precambrian orogenic cycles of India. The Precambrian age data obtained by Pb-U-Th, alpha-helium and Rb-Sr methods are presented cycle-wise, and the degree of dependability of each determination is indicated. The following cycles have so far been delineated — Dharwar (2300±100 M. Y.), Eastern Ghats (1625±75 M. Y.), Mahanadi (1350±200 M. Y.?), Satpura (955±40 M. Y.), Delhi (735±30 M. Y.) and Kishengarh (580±20 M. Y.) and it is shown that these cycles have equivalents in several parts of the world. These results are discussed in relation to the tectonics of India. A comprehensive programme of dating the Precambrian geological formations of India is suggested.     

Paleogeography of the North and South China blocks during the Cambrian

Paleomagnetic results demonstrate that during the Cambrian the South China block was close to the equator. We suggest that it was adjacent to North Australia. This reconstruction juxtaposes Cambrian marine basins in South China and Australia, explaining the affinity between Cambrian trilobites from the two areas, as well as the existence of phosphorite deposits in the Early and Middle Cambrian in Australia and in South China. The stratigraphic similarity between the late Precambrian Sinian System in South China and the Adelaide System in Australia, and the continuing fossil affinities from Cambrian through Ordovician of both areas suggest that the proposed geographic configuration lasted from the late Precambrian (800 Ma) to Early Ordovician (470 Ma). Paleomagnetic results from the Cambrian of North China indicate that it was in the southern hemisphere at that time. Based on the paleontological evidence, we suggest that the North China block was close to Tibet, Iran and northern India during the Paleozoic.     

Proposed relation between anomalous geomagnetic variations and the tectonic history of South India

Anomalous geomagnetic variations have been observed at all geomagnetic stations in South India and they have earlier been attributed to ocean and crustal conductors. The spectral character of induced variations indicate that, in addition to coastal effects and channelling of currents through the Palk Straits, there is a concentration of currents in a conducting body at deeper levels in the crust or mantle.Current thinking on crustal evolution and the influence of Precambrian lineaments on tectonism and mineralisation up to recent times, provides support for the conjecture that zones of high electrical conductivity exist in the crust, which are related to tectonic and mineral influences. The Eastern Ghats and granulite terrain of South India and Sri Lanka are probably associated with such a zone of fundamental weakness in the crust. Extended graphite mineralisation along the Eastern Ghats, occurrence of economic graphite deposits near Trivandrum and in Sri Lanka and the association of alkaline and ultrabasic metallic minerals with deep faults in South India are indications of the possible chemical causes of high electrical conductivity in some regions of the crust.Similar relations between induced anomalous variations and tectonism and mineralisation in the Precambrian crystalline crust have been found in North America and Scotland. However, the regions of high electrical conductivity have not yet been mapped in detail in South India to confirm such a relationship.     

Review of Precambrian paleomagnetic data for Europe

Some 36 paleomagnetic poles are available from Precambrian rock units from Europe, west of the Urals. They allow us to amplify Neuvonen's suggestions, and speculate on the pole path for the interval 1200–2000 my. In order to link younger Precambrian poles with Phanerozoic data, one interpretation is that a closed loop is required during the interval 500–1400 my. By comparison of pole paths for North America, Western Europe, and East and Southern Africa, the paleomagnetic evidence presently available tentatively suggests that these shield areas drifted independently during the later part of the Precambrian.     

Apparent polar wander relative to Australia during the Precambrian

Palaeomagnetic data for Australian Precambrian rocks allow the preparation of a tentative apparent polar wander curve. The proposed curve helps to “date” the magnetic remanence of many rocks whose age of magnetization or remagnetization is imprecise. Using this curve the CRMs of hematite ore deposits can be shown to have been acquired up to hundreds of millions of years after the deposition of the host rocks.Comparison of the Australian Precambrian apparent polar wander curve with proposed curves for Europe, Siberia, North America and Africa provides further evidence for the suggestion of relative continental movement during the Precambrian. If relative continental drift has not (or could not have) taken place, then “true” polar wander apparently varies relative to the continent for which it is determined.     

Influence of Neoproterozoic tectonic fabric on the origin of the Potiguar Basin,northeastern Brazil and its links with West Africa based on gravity and magnetic data

The Potiguar Basin is a ∼6,000 m thick aborted NE-trending rift that was formed during the Cretaceous in the continental margin of northeastern Brazil. Its ∼E–W-trending offshore faults form part of the successful continental margin rift that evolved into the South Atlantic Ocean. The region represents one of the most significant pre-Pangea breakup piercing points between eastern South America and West Africa. We used gravity, aeromagnetic, and geological data to assess the role of reactivated Precambrian shear zones and major terrain boundaries in the development of the Potiguar Basin from the Cretaceous to the Cenozoic. We also looked for possible links between these structures in northeastern Brazil and their continuation in West Africa. Our results indicate that the major fault systems of the Potiguar Basin were superimposed on the Precambrian fabric. Both gravity and magnetic maps show lineaments related to the shear zones and major terrain boundaries in the Precambrian crystalline basement, which also characterize the architecture of the rift. For example, the Carnaubais fault, the master fault of the rift system, represents the reactivation of the Portalegre shear zone, the major tectonic boundary between Precambrian terrains in the crystalline basement. In addition, part of the Moho topography is controlled by these shear zones and developed during the period of main rift extension in the Neocomian. The shear zones bounding the Potiguar rift system continue in West Africa around and underneath the Benue Basin, where fault reactivation also took place.     

Palaeomagnetic evidence for large-scale dextral movement along the Trollfjord-Komagelv Fault,Finnmark, north Norway

Palaeomagnetic data from Late Precambrian dykes from the northern part of Varanger peninsula, north Norway, suggest a two-axis magnetization structure. The dominant component is considered to be syn- to late-tectonic and probably acquired at around 640 m.y. B.P. Superposed on this magnetization is a minor component which is compatible with the relative Lower-Middle Palaeozoic field; i.e. it was most likely imposed during the climax of the Caledonian orogenic movements in north Norway. The estimated relative Late Precambrian palaeopole cannot easily be reconciled with the European Late Precambrian polar path. This disagreement can be accounted for by assuming a post-magnetization dextral megashear, of the order of 500–1000 km, along the Trollfjord-Komagelv fracture zone. This type of displacement is in line with geological evidence and the palaeomagnetic reconstruction supports the long-held view of there having been continuity between the depositional environments of the Varanger Peninsula Barents Sea Group, the Eleonore Bay Group of east Greenland and the Hecla Hoek Formation of east Spitsbergen. The character and age of the horizontal displacement, post-640 to pre-500 m.y. B.P., is seen in conjunction with the opening up of the lapetus Ocean and reactivation of ancient deep-seated fractures during both the spreading and the contraction phases of ocean development.     

Discovery of Precambrian rocks in New Zealand: Age relations of the Greenland Group and Constant Gneiss,West Coast,South Island

Rb-Sr whole-rock analyses yield a Cambro-Ordovician (495 ± 11 m.y.) sedimentation age for the supposed Precambrian Greenland Group and a late Precambrian age, 680 ± 21 m.y., for parts of the Constant Gneiss, the first confirmation of Precambrian rocks in New Zealand. A Precambrian age for the Greenland Group is thus unlikely and the large area of Upper Cambrian-Lower Ordovician rocks now established can be considered as a lateral equivalent of the fossiliferous Lower Palaeozoic succession of northwest Nelson to the east. The Greenland Group, especially in the Paparoa Range has been affected subsequently by a thermal metamorphic overprint about 360 m.y. ago during the Tuhuan Orogeny. Although the Constant Gneiss must form the local basement to the Greenland Group in north Westland, the former does not appear to be the source of the sediments and the true provenance must lie elsewhere.     

Terrestrial heat flow in various parts of India

Results of heat flow studies made in different parts of India including Kolar Gold Field, Cuddapah basin, Singhbhum thrust zone, Aravalli mountain system of Precambrian age, Godavary valley of Mesozoic age and Cambay basin of Cenozoic age are discussed. Heat flow has been found to be low in the southern part of the Preambrian shield. Relatively higher values have been obtained along the northeastern (Singhbhum) and the northwestern parts of the shield (Aravallies). High heat flow has been found along the southeastern part of the Godavary valley and the Cambay basin. The correlation of heat flow with geology and tectonic history in the respective areas is discussed.     

A graphical approach to polar paths: Paleomagnetic cycles and global tectonics

Some problems of the conventional “minimum polar distance” approach to Precambrian pole path construction are discussed. An alternative technique, based on a less restricted approach, is proposed and assessed by using all presently-available Precambrian and Paleozoic data from the world. In the construction of any pole path, assumptions are always made on the shape of the path through regions of apparently missing data. Many different assumptions are possible. Assuming that the Precambrian tectono-stratigraphic record conforms to repeated sequences of geological events suggests we should observe cycles in the paleomagnetic data. Empirically, it is found that the pole paths for the periods Cambrian-Devonian, and 750–1000 Ma, define cycles of pole motion from equator, up to and over the pole, and back down to the equator. Similar cycles of 250-Ma periodicity can be identified in paleomagnetic data back to 2500-Ma. Global wide changes in the phase of the cycles appears to be coincident with the major subdivisions of geological time i.e., Phanerozoic + Hadrynian, Helikian, Aphebian, and Archean. Finally, the geological implications of these cyclic changes in Precambrian pole paths are briefly discussed.     

Intensity of geomagnetic field in the Precambrian and evolution of the Earth’s deep interior

Reliable data on the paleointensity of the geomagnetic field can become an important source of information both about the mechanisms of generation of the field at present and in the past, and about the internal structure of the Earth, especially the structure and evolution of its core. Unfortunately, the reliability of these data remains a serious problem of paleomagnetic research because of the limitations of experimental methods, and the complexity and diversity of rocks and their magnetic carriers. This is true even for relatively “young” Phanerozoic rocks, but investigation of Precambrian rocks is associated with many additional difficulties. As a consequence, our current knowledge of paleointensity, especially in the Precambrian period, is still very limited. The data limitations do not preclude attempts to use the currently available paleointensity results to analyze the evolution and characteristics of the Earth’s internal structure, such as the age of the Earth’s solid inner core or thermal conductivity in the liquid core. However, such attempts require considerable caution in handling data. In particular, it has now been reliably established that some results on the Precambrian paleointensity overestimate the true paleofield strength. When the paleointensity overestimates are excluded from consideration, the range of the field strength changes in the Precambrian does not exceed the range of its variation in the Phanerozoic. This result calls into question recent assertions that the Earth’s inner core formed in the Mesoproterozoic, about 1.3 billion years ago, triggering a statistically significant increase in the long-term average field strength. Instead, our analysis has shown that the quantity and quality of the currently available data on the Precambrian paleointensity are insufficient to estimate the age of the solid inner core and, therefore, cannot be useful for solving the problem of the thermal conductivity of the Earth’s core. The data are consistent with very young or very “old” inner core ages and, correspondingly, with high or low values of core thermal conductivity.     

Trace element geochemistry of some continental tholeiites

Continental tholeiites from four regions (Late Cretaceous to Early Eocene Deccan Trap lavas of central India, Early Mesozoic tholeiites from the Atlantic margins of Northwest Africa-Morocco, and northeastern North America-Nova Scotia, Canada and Precambrian Coppermine River basalts from Northwest Territories, Canada) differ from MORB by higher concentrations of K, Rb, Ba and Th and to a lesser degree light REE. Their chondrite-normalized trace element patterns show negative Nb anomalies. The distribution and variation of trace elements indicate that the rocks from all the areas studied were affected by interaction with the continental crust. It is suggested that continental tholeiites have been generated from a similar source as oceanic tholeiites and many of their geochemical differences are related to crustal contamination.     

A real spectral analysis of the deformation of a homogenous electric field over a thin bed – A Hartley transform approach

Spectral analysis of the deformation of a homogenous electric field caused by a long, thin inclined bed, which is of considerable importance in the exploration of ground water and minerals, is presented using the Hartley transform. The Hartley transform is an alternative and real replacement for the well‐known complex Fourier transform in the field of spectral analysis. The thickness of the bed and the inclination are given as functions of frequency by simple expressions. A theoretical example illustrates the method while the applicability is demonstrated by the field examples from the fractured crystalline basement complex in Burkina Faso, Africa and the Precambrian limestones of the Cuddapah basin, Andhra Pradesh, India. The results obtained by this method agree well with those of the drilling.     

The main old lands in China and assembly of Chinese unified continent

The main old lands in China include the North China Block (NCB), South China Block (SCB) and Tarim Block (TRB), all of which have individual tectonic evolving histories. The NCB experienced complex geological evolution since the early Precambrian onwards, and carries important records from the old continental nuclei, giant crustal growth episode and cratonization (stabilitization), then to the Paleoproterozoic rifting-subduction-accretion-collision with imprints of the Great Oxygen Event (GOE), and to the Late Paleoproterozoic-Neoproterozoic multi-stage rifting representing North China platform tectonic features. The TRB has two-layer basement of the Early Precambrian metamorphic complexes and Neoproterozoic sedimentary sequences. Three till sheets have been reported. The SCB consists of the Yangtze Block (YZB) and Cathaysia Block (CTB) that were cohered in the Neoproterozoic. The YZB recorded tectonic processes of the Early Precambrian crustal growth, 1.0–0.9 Ga and 0.8–0.6 Ga metamorphic-magmatic events, and two Neoproterozoic glaciations. The CTB consists of ca. 1.8 Ga, 1.0 to 0.9 Ga and ca. 0.8 Ga granitic gneisses and metamorphic rocks, indicating there was a vast Precambrian basement. The Neoproterozoic sedimentary rocks overlie partly on the basement. That the YZB and CTB have a Neoproterozoic uniform cover layer illustrates the SCB should form, at least, during 1.0–0.9 Ga, corresponding to the Rodinia Supercontinent. The Central Chinese Orogenic System with high-ultra-high-pressure metamorphic rocks supports a suggestion that the above-mentioned three old lands were collided to assemble a unified Chinese Continent during the Pangea orogenic period.     

Absolute reconstructions of the Paleozoic oceans

The orientation of three intraplate magmatic belts—the Mongolian (from 130 to 280 m.y.), South Siberian (from 320 to 400 m.y.) and Baltic (from 365 to 400 m.y.) belts—which can be considered as hot spot tracks, are used together with apparent paleomagnetic pole wander paths to reconstruct true motions of continents during Paleozoic times. The reconstructions obtained show that the old, Late Precambrian Pangea continued to exist in the Early Paleozoic with an inner arrangement of constituent continents strongly different from that of the Late Paleozoic Pangea. The continents were constantly located in the eastern hemisphere only. The western hemisphere was occupied by the Paleo-Pacific oceans. Four oceans existed in Paleozoic times between the continents in the eastern hemisphere: (1) the Iapetus which originated in the Late Precambrian and closed before Devonian times (400 m.y.), (2) the Asiatic paleo-ocean which originated in the Late Precambrian and closed 450 m.y. ago, (3) the Uralian paleo-ocean which opened around 500 m.y. ago and closed in the latest Permian (240–230 m.y.), (4) the Paleo-Tethys which opened in the Ordovician (480–450 m.y.) and became the Mesozoic Tethys. Life duration of the oceans was 200–400 m.y. The main trend in the Earth's evolution during the Paleozoic was a break up of the old, Precambrian Supercontinent and construction of the new, Late Paleozoic Pangea.     

Variability of Convective Activity over the North Indian Ocean and its Associations with Monsoon Rainfall over India

The present study is an attempt to examine the variability of convective activity over the north Indian Ocean (Bay of Bengal and Arabian Sea) on interannual and longer time scale and its association with the rainfall activity over the four different homogeneous regions of India (viz., northeast India, northwest India, central India and south peninsular India) during the monsoon season from June to September (JJAS) for the 26 year period (1979 to 2004). The monthly mean Outgoing Long-wave Radiation (OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this study and the 26-year period has been divided into two periods of 13 years each with period-i from 1979 to 1991 and period -ii from 1992 to 2004. It is ascertained that the convective activity increases over the Arabian Sea and the Bay of Bengal in the recent period (period -ii; 1992 to 2004) compared to that of the former period (period -i; 1979 to 1991) during JJAS and is associated with a significantly increasing trend (at 95% level) of convective activity over the north Bay of Bengal (NBAY). On a monthly scale, July and August also show increase in convective activity over the Arabian Sea and the Bay of Bengal during the recent period and this is associated with slight changes in the monsoon activity cycle over India. The increase in convective activity particularly over the Arabian Sea during the recent period of June is basically associated with about three days early onset of the monsoon over Delhi and relatively faster progress of the monsoon northward from the southern tip of India. Over the homogeneous regions of India the correlation coefficient (CC) of OLR anomalies over the south Arabian Sea (SARA) is highly significant with the rainfall over central India, south peninsular India and northwest India, and for the north Arabian Sea (NARA), it is significant with northwest India rainfall and south peninsular rainfall. Similarly, the OLR anomalies over the south Bay of Bengal (SBAY) have significant CC with northwest India and south peninsular rainfall, whereas the most active convective region of the NBAY is not significantly correlated with rainfall over India. It is also found that the region over northeastern parts of India and its surroundings has a negative correlation with the OLR anomalies over the NARA and is associated with an anomalous sinking (rising) motion over the northeastern parts of India during the years of increase (decrease) of convective activity over the NARA.     

地球物理数据与区域构造的关系──-对约旦Risha地区构造格架的研究

根据航磁、重力和地震数据以及地质和辅助地球物理资料，对约旦东北部前寒武纪岩石的轮廓和变化及其上覆的沉积岩石的厚度进行了研究．识别出5个具有特定磁性特征的磁场区，每个磁场区都有其特征的样式和突变的边界，每个磁场区的物质组成存在明显差异，其构造边界均与断层相对应．计算表明基底表面有很大起伏，磁化的前寒武纪岩石深度变化范围为-5000m至-10000m，可以识别出由基底下陷相对应的5个盆地或拗陷带，同时可见3个起伏较大的构造隆起．探测结果表明，古生代建造中发育的主要断裂呈N-S与NNE向，而在新生代建造中发育的断裂则呈NE-SW，NW-SE和E-W向．研究区构造发展的第一构造阶段与E-W向张力有关，第二构造阶段的产物明显受到第三构造阶段发生的构造变形的改造，并与阿拉伯板块的逆时针旋转有关．磁场区之间的移位、错断、拖曳和并置被认为剪切断层所造成．剪切形式表明位移是左行的，即北侧的块体向北西方向移动．     

Continental flood basalt magmatism contemporaneous with Deccan traps in the Mannar basin,offshore Sri Lanka

The Gulf of Mannar and adjoining Cauvery basin to the north between India and Sri Lanka are associated with a failed rift, which initiated during the late Jurassic to early Cretaceous as a precursor to the breakup of East Gondwana. Despite the occurrence of igneous rocks that can be noted in seismic profiles, offshore, and deep seated occurrence of those have lead only to the limited understanding of igneous activity in the Mannar basin. Rock cuttings recovered in the Barracuda exploratory well in the Mannar basin shows approximately 700 m thick basalt rock sequence interlayered with sediments at a depth of 3500–4200 m below mean sea level. Here, we analyzed samples recovered from the Barracuda well for major and trace element composition. Major and trace element data suggest that the basalts were crystallized from two different degrees of partial melts from a similar source. Chondrite normalized rare earth element (REE) patterns indicate that the basalts are similar to continental flood basalt, though they show a distinct Ba positive anomaly. Importantly, supported with previously available K–Ar data, we decipher that these basalts are contemporaneous with the Deccan traps. Rifting between Seychelles and India which had occurred at ~62 Ma approximately 3.5 Ma after the main Deccan eruption is synchronous with the Barracuda volcanism suggesting coeval rifting between Seychelles–India and India–Sri Lanka. Thus, our data suggest simultaneous rifting between Seychelles–India and India–Sri Lanka. Large plate reorganizations that took place during this time period in the Indian Ocean have likely caused consequent passive rifting in the Mannar basin.     

全球大地热流-岩石生热率关系综合分析

大地热流分析是研究大地构造地球动力学和地壳化学组成的一个重要手段通过对全球范围大陆地区大地热流和岩石放射性生热率数据的综合统计分析探讨地壳一地慢热流配分统计结果揭示,全球尺度显生亩地质构造区热流与岩石生热率之间不存在简单的线性关系,但两者在隐生宙地盾区却具有明显正相关关系从大地热流-岩石生热率关系在显生亩地区和在隐生宙地区的差异可以大致确定壳-幔热流成分的变化范围取地壳厚度为30－50km,根据前寒武地盾区热流与生热率之间的统计关系可以进一步推测地壳岩石中放射性生热元素的平均丰度范围分别为铀（0．5-1．6）×10－6,钍（1．8－6．1）×10－6,氧化钾0．6％-1．9％．     

Flood traps through space and time and their bearing on some problems of geotectonics

A study of world stratigraphy reveals, somewhat unexpectedly, that there were extensive basalt flows in the early Precambrian, but the Upper Precambrian and the whole of the Palaeozoic Era, with some insignificant exceptions, are practically devoid of flood basalts. In fact there were very few cruptions between about 1000 M.yr. and about 200 M.yr, ago. Since Upper Triassic time, however, basalt floods have come to cover very extensive areas in both the hemispheres, and the activity is continuing almost unabated. Recent researches on the origin and nature of the mid-ocean ridges have indicated that the flood basalts are apparently genetically connected with these ridges which appear to have begun to open the present oceans within the past 150 M.yr. Basalts occur on the opposite coasts where the continents were formerly together, for example, Peninsular India and Eastern Africa. However, along certain coastal areas there are no basalts of appropriate age even though a mid-ocean ridge exists beyond these. This is true, for example, of eastern North America and western Europe. Elsewhere neither basalt flows nor a mid-oceanic ridge exist where the continents are reasonably believed to have been together. These anomalies call for an explanation, and it is suggested that the observed features can be explained by the fact that two distinct phenomena, crustal sliding and global expansion, have been responsible for the present disposition of the continents. The net result of these two activities has been recognised as ‘Continental Drift’.