Continental Displacements Research Articles

Holocene Sedimentary Record and Coastal Evolution in the Makran Subduction Zone (Iran)

The western Makran coast displays evidence of surface uplift since at least the Late Pleistocene, but it remains uncertain whether this displacement is accommodated by creep on the subduction interface, or in a series of large earthquakes. Here, we address this problem by looking at the short-term (Holocene) history of continental vertical displacements recorded in the geomorphology and sedimentary succession of the Makran beaches. In the region of Chabahar (Southern Iran), we study two bay-beaches through the description, measurement and dating of 13 sedimentary sections with a combination of radiocarbon and Optically Stimulated Luminescence (OSL) dating. Our results show that lagoonal settings dominate the early Holocene of both studied beach sections. A flooding surface associated with the Holocene maximum transgression is followed by a prograding sequence of tidal and beach deposits. Coastal progradation is evidenced in Pozm Bay, where we observe a rapid buildup of the beach ridge succession (3.5 m/years lateral propagation over the last 1950 years). Dating of Beris Beach revealed high rates of uplift, comparable to the rates obtained from the nearby Late Pleistocene marine terraces. A 3150-year-old flooding surface within the sedimentary succession of Chabahar Bay was possibly caused by rapid subsidence during an earthquake. If true, this might indicate that the Western Makran does produce large earthquakes, similar to those that have occurred further east in the Pakistani Makran.

Styles of Thought on the Continental Drift Debate

The continental drift controversy has been deeply analysed in terms of rationalist notions, which seem to find there a unique topic to describe the weight of evidence for reaching consensus. In that sense, many authors suggest that Alfred Wegener’s theory of the original supercontinent Pangea and the subsequent continental displacements finally reached a consensus when irrefutable evidence became available. Therefore, rationalist approaches suggest that evidence can be enough by itself to close scientific controversies. In this article I analyse continental drift debates from a different perspective which is based on styles of thought. I’ll argue that continental drift debate took much longer than it was usually recognized with two styles of thought coexisting for hundreds of years. These were fixism and mobilism and they were always confronting their own evidence and interpretations and functioning as general frameworks for the acceptability of a specific theory. Therefore, this text aims to bring much broader sociological elements than usually involved in the analysis of the continental drift theory.

The Global Species

Postcolonial studies, as Philip Armstrong notes,1 has shown little interest in the fate of the nonhuman animal. The same holds true for other postcolonial life forms, plants and microbes included. 'One reason,' writes Armstrong, 'might be the suspicion that pursuing an interest in the postcolonial animal risks trivializing the suffering of human beings under colonialism'. In this essay I will attempt to demonstrate that the historical process of globalisation - in the long term - can be outlined in terms of the expanding and eventually practically global range not only of our own species, but of several of our affiliated species as well. Human colonisation of planet Earth is essentially ecological not only due to the well-known side-effects it has had, but just as much owing to the schism of nature which we have instated - in part deliberately, in part inadvertently - between favoured and un-favoured species, our friends and our foes. While it is clear that we have been, and are, causing ecological havoc, it has so far not been generally acknowledged that our current ecological predicament is explained not solely by the sudden proliferation of our own species, but also, through our unremitting actions, by the (often forced) proliferation of our affiliated species. Man is not the only animal that alters its environment - but it might be the first to emerge as a global species. Our world-changing inclinations have consequently had global effects long before global warming arose as a phenomenon. The reason why species do not normally have a global range is, first of all, that they tend to acclimatise and adapt to local circumstances. A species attaining global reach would diversify - unless, that is, its sustenance were provided, its reproduction were arranged, and its natural foes were constrained, by a global network of human settlements in a somewhat uniform manner. Adding, through geological eras, to the natural tendency towards homogenisation, is the constant reconfiguration of continents, a process of continental displacements which effectively merge and split land masses, thus not least modifying oceanic circulation and shifting ice masses.2 By establishing a global colonial organism of sorts - to borrow a notion from biology - we have installed an ecological empire, hierarchically organised with Homo sapiens on top and with crop species, pets and livestock in privileged positions. Thereby we have further provided global breeding grounds for other species that might not otherwise have been able to spread at a global scale - from rats and doves to bugs and microbes of various sorts. The flu virus, reports Time,3 'was a tenacious foe from the outset, but once humans invented farming and learned to cultivate animals, we made a bad situation much worse. All at once, chickens, ducks and pigs - which never had much to do with one another - began living cheek to jowl in high numbers and often unsanitary conditions'. Besides its general ecological effects, this developing schism of nature has - no doubt inadvertently - had an effect on human mortality. HW, though not recognised as a life form or as a species - expected (through AIDS) to account for every tenth death globally by 2030 - might serve as an example. Malaria constitutes another. This disease, writes Newsweek* 'can be ascribed almost entirely to human acts of deforestation, which produces stagnant pools of water ... This intensifies the growth of algae and forms the perfect nursery for Anopheles mosquitoes, potent vectors for the malaria parasite. Anopheles barely had a foothold in the ecosystem in its former state'. As we see from this example, in addition to providing certain strings of life (whether benign or malign) with global breeding grounds by way of our bodies and our waste, or those of our affiliates, we have altered landscapes in a manner which has given rise to something akin to a global landscape type, thereby yet again facilitating the geographical spread of selected life forms. …

Heterogeneity and time dependence in 3D spherical mantle convection models with continental drift

Feedback between continents and large-scale mantle flow through thermal blanketing has long been surmised as a mechanism for continental drift and Wilson cycles. Paleomagnetism provides evidence for extensive continental displacements (∼ 10,000 km) on time scales of 100–200 million years, comparable to an intrinsic overturn in whole mantle convection. Here we model continental motions in vigorous 3D spherical convection models, focusing on the effects of continent size, mantle heating mode, and a strong increase in lower mantle viscosity. Continents covering 30%, 10%, and 3% of Earth's surface (representative of the former supercontinent Pangea, present-day Asia, and Antarctica, respectively) are introduced into simple end member mantle convection models characterized by pure core or internal heating, and uniform or layered mantle viscosity. Supercontinents promote temperature anomalies on the largest scales (spherical harmonic degrees 1 and 2), primarily through the organization of the long-wavelength convective planform inherent in models with a high-viscosity lower mantle. Bottom heating can promote long-wavelength heterogeneity by clustering plumes beneath the continent. However, in isoviscous models small-scale structure persists away from the continent regardless of the heating mode. Supercontinents respond to long-wavelength heterogeneity by following great circle paths with variations in velocity on time scales of 1 billion years. Smaller continents are unable to promote long-wavelength structure, and the resulting motions are governed by bursts in velocity on time scales of the order of 100 million years. Continental velocities are roughly a factor of ∼ 3 smaller than those in oceanic regions, an observation that may help explain the observed difference in the speed of predominantly continental or oceanic plates.

The Eotvos force revisited

ABSTRACTThe Eotvos acceleration, too low to be the principal driving force in Wegener's theory of continental drift, where the decoupling surface was placed between crust and mantle, must be re‐evaluated in the context of Plate Tectonics, where the same surface is localized at the lithosphere‐asthenosphere boundary. In this note, taking into account the departure from spherical symmetry of the Earths internal gravitational potential, a value for the Eotvos acceleration is obtained that is one order of magnitude larger than pre‐Plate Tectonics estimates and almost two times greater than more recent results. This force was higher in the geological past because the Earths angular velocity and flattening were greater than today, and thus may be considered in the large‐scale reconstruction of continental displacements.

Aspects of Gondwana paleobotany: gymnosperms of the Paleozoic—Mesozoic transition

During the late Paleozoic and early Mesozoic the Gondwana Supercontinent underwent dramatic geographic and climatic changes. Geologic and biologic factors concurrently played an important role modelling the vegetation of that time. The gymnospermic component of plant assemblages shows significant variations in composition and in the distribution of different taxa. Analysis of the assemblages shows that some plant groups dominated the scenario, such as the pteridosperms, glossopterids, corystosperms and, to a lesser degree, cordaites and conifers. Ginkgophytes, bennettites or cycads were less important in the Paleozoic but their numbers increased in the Triassic. Paleozoic assemblages were extensively dominated by glossopterids that became extinct in the earliest Mesozoic. Pteridosperms crossed the P—M barrier and became dominant during the Triassic, at a time when corystosperms evolved and radiated. Other groups became extinct in the Paleozoic, namely dicranophylls and cordaites. Conifers were represented by different families, restricted either to the Paleozoic or the Mesozoic. They were not conspicuous in the analysed assemblages. In some areas of Gondwana, taxa of the Euramerican alliance are present through a migrational mechanism that occurred during continental displacements which produced global climatic changes. Recent studies have shown that there are far more common elements between Euramerica and Gondwana than suspected up to now. These elements find their distribution especially in the western part of Gondwana (Africa—South America).

Paleomagnetism and the motion of large and small plates

The focus of paleomagnetic research during past decades has varied greatly. The first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements. An enormous effort followed, in the form of individual paleomagnetic studies of cratonic rocks, culminating with the construction of APW (apparent polar wander) paths for the continents which could be used to test paleogeographic reconstructions. Coupled with parallel research in marine geophysics and geology, the refined APWP which result from this effort have helped to develop a reasonable first‐order history of Cenozoic/Mesozoic paleogeography for the major plates.

Comments on: Why do i not accept plate tectonics?

Our emphasis has not been on citing detailed evidence that shows the weakness of Beloussov's position because we believe that such evidence is both ample and familiar. Rather we draw attention to what we see as his philosophical attitude and historical perspective. Beloussov's position, we conclude, derives from those of the German masters of contractional tectonics. Long before the advent of plate tectonics the inadequacies of this approach had been recognized and its place taken by hypotheses involving large‐scale continental displacements in the writing of such tectonicians as Suess, Argand, and duToit.

Changes in the convection pattern in the Earth’s mantle and continental drift: evidence for a cold origin of the Earth

Continental displacements of thousands of kilometres point to flow patterns in the mantle of similar dimensions. As creep depends exponentially on temperature and as it is known that the temperature of the crust increases rapidly with depth, we can in this context suppose the mantle to have a sharp transition between a rigid crust and a fluid mantle at 50 to 100 km depth. Recently the coefficients of the tesseral harmonics of the geopotential from satellite observations have been determined. These departures from hydrostatic equilibrium seem to be caused by flow in the mantle, for from these coefficients can be computed the tractions exerted by the flow on the crust, assuming Navier—Stokes’s equations, and the resulting pattern accords well with the world-wide tectonic features. Rising flow is associated with the East Pacific Ocean Rise, the Mid-Atlantic Ridge and the Mid-Indian Ocean Rise. Descending currents coincide with the Andes, the Alps and the Japan trench. The strong fifth harmonic in the satellite gravity data suggests that a flow pattern of this degree is at present establishing itself. On the other hand, continental reconstructions prior to Wegenerian drift suggest the presence of a strong fourth harmonic in the flow pattern. The theory of marginal stability in thermal convection, discussed by S. Chandrasekhar, shows that convection in a spherical shell under a uniform radial gravitational field gives the critical ratio of the radii of the inner to outer surfaces (tj) at which the fifth order is more likely to develop than the fourth as 0*54. The closeness of this value of to the present one of 0*55 provides a clue to the most puzzling feature of continental drift; that it should have occurred in the last 5 % of the Earth’s history. H. C. Urey suggested that, on the accretion theory of the Earth’s origin, the separation of iron towards the centre might have gradually occurred through the Earth’s life. A growth of the core from 0*54 to 0-55 in the last 100-200 My does not conflict with the known rate of change in the length of the day. This explanation of the flow in terms of convection and changes in the degree of the flow pattern as a result of growth of the core implies earlier epochs of continental displacements, which are identified with peaks in the histograms of radiometric ages in the various continents. The law of growth of the core, so obtained, explains why so few rocks of greater age than 3000 My are found.