The base of the Cambrian

The Cambrian System by definition must include the rocks (in the Caledonian geosyncline of Wales) to which this name was first applied. Its type sequence, however, does not define its boundaries. Evidence on the base of the Cambrian can be found in the Caledonian sedimentary areas of Scandinavia and in the adjoining Baltic region where the Blue Clay contains the oldest known Cambrian fauna. Further east, older rocks of similar character are known and there are references to glauconites over 1,200 million years (m.y.) old in these rocks. The base of the Cambrian cannot be extended down to the base of the sedimentary cover of the shelves surrounding the old shields. “Eocambrian” is probably the oldest defined term suitable for late Precambrian rocks which often resemble the Cambrian lithologically. They may contain a similar microflora, though their fauna which is rare and consists mostly of soft‐bodied animals, is different. There are in many areas tillites in rocks which could be Eocambrian, but their correlation is not yet definitely established. The definition of the base of the Cambrian rests therefore on biostratigraphic evidence and on chronological data (Glaessner, 1963).     

The composition of the Earth

Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an Fe---Ni core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements.The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth.The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga).The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an Fe---Ni alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence ( 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition.The Earth has a bulk Fe/Al of 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth ( 14%) and the core ( 86%). Chondritic meteorites display a range of Fe/Al ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.     

The use of the useless

The Human population suffers from a serious and paradoxical misunderstanding of its own place and importance within Nature: Unable or unwilling to control its own growth, being faced by overcrowding with resulting loss of productive land, and lack of food both in quantity and quality, alternative ways of feeding Mankind and his domestic stock have to be found. In the present paper examples and suggestions are given to illustrate how weeds—the generally considered “useless” elements of the Plant Kingdom—could become accepted as alternative food-providers. Thirty spokes share the wheel's hub; It is the center hole that makes it useful. Shape clay into a vessel; It is the space within that makes it useful. Cut doors and windows for a room; It is the holes which make it useful. Therefore profit comes from what is there; Usefulness from what it not there. LAO TSU, in Tao te Ching     

The origin of the Atlantic-Arctic-Ocean

Summary The Atlantic-Arctic Basin is antipodal to the Pacific. Powerful evidence is cited to indicate its development through Continental Drift, as suggested byPickering in 1907. Initiated from the Mesozoic Tethys and progressively enlarged during the Tertiary, its outlines were essentially determined by tensional-rifting oriented mainly N.E and N.W within a zone extending more than half round the circumference of the Earth, from the Antarctic to Alaska. During the Alpine diastrophism fold-linkages, that functioned as land bridges, were pushed up across the Ocean between the West Indies and Eurafrica and subsequently destroyed by the continued westerly drift of the Americas. Crustal stretching was accompanied by widespread volcanicity. The Mid-Atlantic Rise is recent and has an Isostatic basis. The Atlantic-Arctic stretch-basin is largely bordered by Fault-Line Coasts and by downwarped shores that show the marginal, entrenched, terrestially-evolved drainages known as Submarine Canyons.     

The structure of the coronal-streamer belt

We analyze calibrated white-light coronal images from the LASCO-C2/SOHO experiment (processing level L1), focusing on quasistationary events without coronal mass ejections or their manifestations in the solar wind. The previous result that the streamer belt forms a set of rays of increased brightness is confirmed. The cross section of the streamer belt is frequently observed as two closely spaced rays differing in brightness. It is difficult to explain this in terms of ordinary bending of the belt. We suggest that the belt is normally a set of pairs of rays with enhanced brightness (or two close rows of rays). The distance between the rays in each pair is comparable to the ray size. The ray brightnesses in any pair can, in general, be different. The magnetic field has opposite directions in the rays forming a pair, so that the neutral line of the radial component of the solar magnetic field probably runs along the strip between the pairs of rays.     

北京地区的白垩系

<正> 北京地区的白垩系主要出露于西山坨里—大灰厂一带,层序清楚,化石丰富,为华北晚期中生代著名的陆相断陷沉积盆地之一(图1)。坨里—大灰厂白垩系沉积盆地位于华北地台燕山台褶带西山拗陷区之东南缘。盆地北、西缘为著名的八宝山—南大寨与黄庄     

The geodynamics of the Pamir-Punjab syntaxis

The collision of Hindustan with Eurasia in the Oligocene-early Miocene resulted in the rearrangement of the convective system in the upper mantle of the Pamir-Karakoram margin of the Eurasian Plate with subduction of the Hindustan continental lithosphere beneath this margin. The Pamir-Punjab syntaxis was formed in the Miocene as a giant horizontal extrusion (protrusion). Extensive nappes developed in the southern and central Pamirs along with deformation of its outer zone. The Pamir-Punjab syntaxis continued to form in the Pliocene-Quaternary when the deformed Pamirs, which propagated northward, were being transformed into a giant allochthon. A fold-nappe system was formed in the outer zone of the Pamirs at the front of this allochthon. A geodynamic model of syntaxis formation is proposed here.     

The middle mantle of the earth

The middle mantle as a separate geosphere within a depth interval of 840 to 1700 km was recognized in 1995 by Yu.M. Pushcharovsky. The structure, energetics, and tectonics of the middle mantle, as well as phase transformations inherent to this geosphere, are characterized in this paper. The distribution of seismic heterogeneities established by seismic tomography is a definitive attribute of the given geosphere. The middle mantle differs from other geospheres by greater dimensions of seismic heterogeneities, especially as concerns the low- and medium-velocity domains. The high-velocity heterogeneities are round and oval in shape and in some cases reach a few thousand kilometers in size. The distribution of such heterogeneities is nonuniform and varies from one depth level to another. A high lateral contrast of anomalous domains distinguished by elastic wave velocities is expressed in gradient zones hundredths of kilometers wide. Specific general patterns of middle-mantle anomalies in the Pacific and Indian-Atlantic sectors of the Earth reflect their difference in geological history. The consideration of heterogeneities in terms of tectonics leads to the conclusion that role of the tectonic flow of mantle masses in the form of shearing and thrusting is important. The middle mantle is characterized by a special mineral composition with the prevalence of MgSiO₃ crystallized as an orthorhombically distorted perovskite-type structure. The transformations of stishovite into a poststishovite modification at a depth of ～1500 km and of aragonite into the postaragonite phase with an unusual structure at a depth of ～1050 km are inherent to this geosphere. A change of the electronic structure of alkali cations is assumed in the middle mantle. Thus, the recognition of the middle mantle as a special geopshere is emphasized by its crystal chemistry.     

The Grenville-age basement of the Andes

The analysis of the basement of the Andes shows the strong Grenville affinities of most of the inliers exposed in the different terranes from Colombia to Patagonia. The terranes have different histories, but most of them participated in the Rodinia supercontinent amalgamation during the Mesoproterozoic between 1200 and 1000 Ma. After Rodinia break-up some terranes were left in the Laurentian side such as Cuyania and Chilenia, while others stayed in the Gondwanan side. Some of the terranes once collided with the Amazon craton remained attached, experiencing diverse rifting episodes all along the Phanerozoic, as the Arequipa and Pampia terranes. Some other basement inliers were detached in the Neoproterozoic and amalgamated again to Gondwana in the Early Cambrian, Middle Ordovician or Permian times. A few basement inliers with Permian metamorphic ages were transferred to Gondwana after Pangea break-up from the Laurentian side. Some of them were part of the present Middle America terrane. An exceptional case is the Oaxaquia terrane that was detached from the Gondwana margin after the Early Ordovician and is now one of the main Mexican terranes that collided with Laurentia. These displacements, detachments, and amalgamations indicate a complex terrane transfer between Laurentia and Gondwana during Paleozoic times, following plate reorganizations and changes in the absolute motion of Gondwana.     

The differentiation of the Skaergaard intrusion

Conclusions We find no support for the claim that the Skaergaard magma followed the trend of common tholeiitic volcanic magmas, such as those of Iceland and the Scottish Tertiary. The end product of differentiation was not a large mass of rhyolite but an iron-rich, silica-poor liquid not unlike that deduced by Wager in 1960.The proposal that a large mass of rhyolitic liquid occupied the upper levels of the intrusion finds no support in the field. The thick series of ferrogabbos, which became richer in iron and poorer in silica until they reached a field of immiscibility cannot be reconciled with crystallization of a large mass of felsic magma. Mass-balance calculations that indicate otherwise are invalid, because they fail to take into account large volumes of rocks that differ in composition from those assumed in the calculations.While ignoring the existence of major units of the intrusion, Hunter and Sparks propose that lavas in Scotland and Iceland are more relevant to the liquid compositions than rocks that are intimately associated with the intrusion. Their argument that the Skaergaard Intrusion followed a trend of silica enrichment that is universal to tholeiitic magmas is based on an incomplete knowledge of the rocks and faulty calculations of mass-balance relations.We agree that much remains to be learned about the Skaergaard Intrusion and the basic mechanisms of magmatic differentiation. In this case, however, we are ready to hang our case on well-established field relations and a mass of laboratory data for what must be the most intensely studied body of rock on Earth.     

The variscan geology of the Pyrenees

The main outlines of the geology of the Variscan part of the Pyrenees are discussed. Rocks involved in this cycle are high-grade basement gneisses, Palaeozoic sediments and their metamorphic equivalents, late intrusive granodiorites and early, pre-Variscan granites. The main features of the stratigraphy of the Palaeozoic are given.Structures fall into two domains: a low-grade suprastructure, essentially with steep folds and cleavages, and a high-grade infrastructure with dominantly low-dipping foliations. An important phase of early, pre-cleavage folding occurs in low-grade rocks mainly along the southern border of the Axial zone. In high-grade rocks most structures and the metamorphism postdate the main cleavage phase in low-grade rocks. The influence of the Alpine orogeny on the Variscan structures consists mainly of faults, steep, reverse faults in the northern, and south-directed thrusts in the southern part of the Pyrenees. Metamorphism took place under high geothermal gradients and low pressures, as indicated by the abundant occurrence of andalusite and cordierite     

The future of the 'Geological Museum'

The following letter was written for, and has already been published in, the Geologists' Association Circular; but because it represents an important contribution to issues rehearsed in these columns, the editor of the Circular and the author have kindly agreed to its re-publication here.     

The utopia of the binational city

Kerkrade and Herzogenrath, on the German-Dutch border, look back at a common past in the former Land of Rode. This ceased to exist in 1815 when the border was drawn between Prussia and The Netherlands. Since then the people turned their backs to each other more and more and started orientating towards their own nation state. Only after the dramatic nationalism of World War II subsided, did the border loose some of its dividing effects on political and social life. Unification processes of the European Union strengthened this, and the rapprochement between Kerkrade and Herzogenrath has become so intensive that they present themselves as one town: Eurode. The identification of both towns as one territorial, institutional and social entity was and is essential for the success of Eurode. Yet, creating a feeling of 'we-ness' is not enough if the actions and thoughts of the inhabitants do not match this feeling. This article therefore deals with the different stages of cross border integration necessary before one can speak of a 'binational city'. The question remains open however, whether the binational city can ever be more than Utopia. This revised version was published online in July 2006 with corrections to the Cover Date.     

The oceans and the physics of the Earth

The Earth is the only terrestrial planet to have seas, by which it is covered to the extent of seventy percent. The oceans determine the climate of the Earth and it is through them that life and civilization have evolved, but they also have major influence on physical processes in the solid Earth. Tidal friction leads to the Earth's spin slowing down with a consequent recession of the Moon from the Earth, while the presence of the oceans controls the tectonic evolution of the Earth through the formation of oceanic crust, the erosion of land and the accumulation of sediment, the formation of mountains and the establishment of isostatic balance.     

The differentiation of the Skaergaard Intrusion

Previous interpretations of the Skaergaard Intrusion suggested that differentiation involved extreme iron-enrichment but no silica-enrichment until a very late stage. This model is difficult to reconcile with petrological and geochemical evidence, with the behaviour of tholeiitic volcanic suites and with phase equilibria. We propose that the Skaergaard magma evolved on a trend of pronounced silica-enrichment after cumulus magnetite appeared at the top of the Lower Zone. At that stage, the magma was of ferrobasaltic composition with close to 50% SiO₂. The Middle and Upper Zones of the intrusion dominantly represent crystal accumulation during differentiation from ferrobasalt through iron-rich basaltic andesite and icelandite to rhyolite, a fractionation sequence common in tholeiitic volcanic provinces. This interpretation requires re-appraisal of the physical processes responsible for the differentiation. In particular, residual liquids became lower in density with fractionation and would have caused the Skaergaard magma chamber to have become compositionally zoned.     

The P-T-t-D evolution of the Mahabharat,east-central Nepal: The out-of-sequence development of the Himalaya

A garnet-bearing schist from the southernmost such exposure along the Himalaya in east central Nepal records prograde metamorphism at 32.4 ?± ?0.3 ?Ma. Phase equilibria modelling, combined with Ti-in-biotite and quartz c-axis thermometry, outline a tight-to-hairpin pressure-temperature (P-T) path extending from ~515 ?°C and 5.5 ?kbar to peak conditions at ~575 ?°C and 7 ?kbar followed by deformation during the retrograde phase at 480–515 ?°C and 6–7 ?kbar. The new geochronology data place an upper bound on the evolution of metamorphism and deformation in the frontal-most part of the Himalaya, which lasted until 17.5 ?Ma, as indicated by previously published ⁴⁰Ar/³⁹Ar data. The P-T-time data from this part of the Himalaya, as well as that from more hinterland-ward portions of the orogen, outline a progressive, stepwise, commonly out-of-sequence evolution. Further data from along the orogen indicates that this evolution is not a local phenomenon, but instead characterizes the tectonics of this system as a whole.     

The pole of the Mohr diagram

The pole of a Mohr diagram, for the two-dimensional case, is a unique point on the Mohr circle which permits any point on the Mohr circle to be related to the direction in the physical plane associated with that point. A Mohr diagram can be constructed for any second rank tensor. To illustrate the simplicity of this geometrical construction two examples of the use of the pole are presented, one for the strain tensor and the other for the stress tensor.     

The Law of the Repeatability of the Number of Aftershocks

Doklady Earth Sciences - This paper shows that the number of aftershocks with a relative magnitude does not depend on the magnitude of the main shock, and, in global and regional consideration, it...     

The face of the trench

The deep-sea trenches that occur between unstable chains of volanic arcs and the ocean floor attract great interest because they are the most inaccessible areas of our planet. Now, direct observations of trench faces have become possible by using deep-sea submersibles. This article describes features of the Nankai Trough and the Japan and Kuril Trenches at depths of up to 6000 m.