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A new species, Oocephalus pubescens A.Soares & Harley, from Chapada dos Veadeiros, Goiás, Brazil, is described and illustrated. The characteristics that distinguish it from a similar taxon, Oocephalus foliosus, are listed, and comments on its distribution and an occurrence map are provided.
Dengue is the world's most prevalent mosquito-borne disease, with more than 200 million people each year becoming infected. We used a mechanistic virus transmission model to determine whether climate warming would change dengue transmission in Australia. Using two climate models each with two carbon emission scenarios, we calculated future dengue epidemic potential for the period 2046–2064. Using the ECHAM5 model, decreased dengue transmission was predicted under the A2 carbon emission scenario, whereas some increases are likely under the B1 scenario. Dengue epidemic potential may decrease under climate warming due to mosquito breeding sites becoming drier and mosquito survivorship declining. These results contradict most previous studies that use correlative models to show increased dengue transmission under climate warming. Dengue epidemiology is determined by a complex interplay between climatic, human host, and pathogen factors. It is therefore naive to assume a simple relationship between climate and incidence, and incorrect to state that climate warming will uniformly increase dengue transmission, although in general the health impacts of climate change will be negative.
We aimed to reparameterize and validate an existing dengue model, comprising an entomological component (CIMSiM) and a disease component (DENSiM) for application in Malaysia. With the model we aimed to measure the effect of importation rate on dengue incidence, and to determine the potential impact of moderate climate change (a 1 °C temperature increase) on dengue activity. Dengue models (comprising CIMSiM and DENSiM) were reparameterized for a simulated Malaysian village of 10 000 people, and validated against monthly dengue case data from the district of Petaling Jaya in the state of Selangor. Simulations were also performed for 2008-2012 for variable virus importation rates (ranging from 1 to 25 per week) and dengue incidence determined. Dengue incidence in the period 2010–2012 was modelled, twice, with observed daily weather and with a 1 °C increase, the latter to simulate moderate climate change. Strong concordance between simulated and observed monthly dengue cases was observed (up to r = 0·72). There was a linear relationship between importation and incidence. However, a doubling of dengue importation did not equate to a doubling of dengue activity. The largest individual dengue outbreak was observed with the lowest dengue importation rate. Moderate climate change resulted in an overall decrease in dengue activity over a 3-year period, linked to high human seroprevalence early on in the simulation. Our results suggest that moderate reductions in importation with control programmes may not reduce the frequency of large outbreaks. Moderate increases in temperature do not necessarily lead to greater dengue incidence.
This is a large topic which covers the decay of mountain belts by erosion and more controversially, the possible link explored in Chapter 11 between mountain relief and climate. The topic has already been mentioned in Chapter 5 because it seemed appropriate to discuss the exhumation of the European Alps in the context of the recent dating of the HP metamorphism. A huge amount of literature now exists on the topic, which may be simply set out as the link that mountain building creates topography and this in turn generates precipitation as rain or snow and ice. A more accurate formulation would be that many factors are involved if the effect of topography on climate is to be more than transient. Thus mountains may amplify and modify rather than create precipitation in the first place. The relationship is well shown in the Canary Islands which are in a semi-arid belt. Low islands such as Lanzarote and Fuerteventura are semi-arid. However, on Tenerife there is a volcano of 3700 m altitude, rising straight from sea level. On the windward north side of the volcano the island is very wet, with green pastures on the coastal plain, whereas the leeward side is arid or semi-arid. As a consequence of the precipitation and thus enhanced erosion on the windward side of the island there is also a marked asymmetry in steepness of relief: the wet north side has steep relief and a distinct coastline with high cliffs whereas the leeward south side is more gently sloping. Some global climate models show that mountains, while they do not determine whether or not a monsoon occurs, tend to lengthen and enhance monsoonal periods.
The factors involved in climate include atmospheric circulation and ocean water circulation. If these are not appropriate then the mountains will not create precipitation. The English Lake District and the Scottish Highlands would not be such wet places if the global atmospheric circulation pattern in the form of the Atlantic ‘westerlies’ did not dominate the climate of Britain. Over the past 100 Ma temperatures have decreased globally and this has been attributed to changes in sea-floor spreading rates, or in land–sea distribution, or in the rates of out-gassing of volcanoes.
Previous chapters have outlined the principles underpinning the understanding and analysis of orogenic belts in the modern world, with examples and applications to the Alps, the Himalaya, the Andes and western North America. We have also used examples from earlier times in Earth history to illuminate the issues surrounding, for example, the processes of thrust tectonics, metamorphism at extreme conditions, variability in exhumation rates, and contrasts in the amounts of magmatism in collisional and accretionary orogens. In doing this we have presented evidence from Palaeozoic orogens, such as the Variscan/Hercynian and Caledonian, Neoproterozoic orogens including the ‘Pan African’ belts of former Gondwanan fragments in India, Africa and Antarctica, and Mesoproterozoic orogens such as the Grenville of northeastern America. This begs the simple question – can we look back in time and see similarities in orogeny and the making of mountains throughout Earth history, or has mountain building changed in subtle or even dramatic ways since the earliest records of continental crust, deformation and metamorphism? In other words, has there been secular change in orogeny? Within the context of the recognised diversity in Phanerozoic orogenies, were Archaean and Proterozoic orogenies broadly the same, or different?
The question of secular change in mountain building has been the subject of considerable debate and a burgeoning literature. It forms part of the broader issue of the extent to which uniformitarianism can be applied far back in time – whether ‘the present is the key to the past’. Recent major volumes on the subject include Ancient Orogens and Modern Analogues (Murphy et al., 2009) and When did Plate Tectonics Begin on Planet Earth? (Condie & Pease, 2008). These build on an extensive literature in which the temporal evolution of the Earth's tectonic regime has been assessed or speculated upon using thermal and geodynamic modelling, geological observations from ancient and modern terrains, and geochemical and isotopic constraints on rates of continental growth through time (e.g. Hawkesworth & Kemp, 2006a, b; Kemp et al., 2007) (see Fig. 12.5 later).
Mountains have attracted the attention of mankind at least since Rousseau (or did Petrarch precede him?*) who devoted much thought to nature, perhaps because the height and scale of mountains induced a sense of awe. A love of nature showed itself in the fairly recent desire to get to the top of mountains. George Mallory gave his reason for wanting to climb Everest as “because it is there”, but long before that mountains were important for humankind, because they formed natural barriers for trade and the movement of armies. Perhaps the ancient Egyptians tried to simulate mountains in the pyramids of Giza. The same is true of builders of Gothic cathedrals, which were built ever higher so as to imitate mountains which reach up to heaven. The Greeks worshipped the gods on Mount Olympus, and mountains appeared often in Greek mythology; Prometheus, for example, was chained to a mountain side. The Greeks saw mountains as mysterious and frightening places, and even today for Hindus and Buddhists there are sacred mountains in the Himalaya such as Nanda Devi, Kailas and Everest – Qomolungma, the goddess mother of the Earth. Badrinath near the source of the Ganges in the High Himalaya is the home of the gods and a place of pilgrimage. Moses came down from a mountain bearing his famous tablets. Noah is supposed to have docked his ship on Mount Ararat. The Bible states “the mountains shall melt before the Lord” (Judges 5:5), but perhaps the reference was to volcanoes rather than orogenic mountains.
Many artists, too, have been fascinated by mountains. Leonardo Da Vinci realised that the fossils in the rocks of the Apennines showed that the rocks were once below sea level, and he and other painters used mountain scenes as backgrounds. Cezanne painted many pictures of Mont St. Victoire in Provence.
Orogenesis, the process of mountain building, occurs when two tectonic plates collide – either forcing material upwards to form mountain belts such as the Alps or Himalayas or causing one plate to be subducted below the other, resulting in volcanic mountain chains such as the Andes. Integrating the approaches of structural geology and metamorphism, this book provides an up-to-date overview of orogenic research and an introduction to the physico-chemical properties of mountain belts. Global examples are explored, the interactioning roles of temperature and deformation in the orogenic process are reviewed, and important new concepts such as channel flow are explained. This book provides a valuable introduction to this fast-moving field for advanced undergraduate and graduate students of structural geology, plate tectonics and geodynamics, and will also provide a vital overview of research for academics and researchers working in related fields including petrology geochemistry and sedimentology.
In 1928 Arthur Holmes suggested that the mechanism for continental drift is cells of convection in the mantle. This was a remarkable insight, although many would now question the one-to-one connection between plate motion and mantle convection. So what is the modern view on the driving force for plate movements? There are two models in which the plates drive themselves. The first is called ‘slab pull’, which means that the dense ocean crust exerts a pull on the ocean floor during subduction as it plunges into hot asthenosphere. In contrast, the less dense continental crust is relatively buoyant. Sometimes the subducted slab of ocean crust breaks off and sinks into the hot asthenosphere, but if it survives it will exert a traction and in effect pull the ocean crust away from the Mid Ocean Rise. The opposite view is ‘slab push’, which means that the driving force for the moving ocean floor is situated at the Mid Ocean Rise which is opening under extension to allow in the new ocean crust.
Perhaps it should not be either/or here. Phillip England (1982) calculated the required stresses at the Mid Ocean Rise in the Indian Ocean if slab push were to be responsible for the northward movement of the Indian plate carrying the Indian continent. The forces acting on a plate boundary must do work against gravity during the raising of high mountains and plateaux. The force balance must take into account the Argand number, which expresses the relative magnitudes of the buoyancy forces arising from contrasts in crustal thickness and the forces required to deform the medium. England's results show that the horizontal stress arising from slab push is enough to explain not only the motion of the Indian plate before collision but also the continuation of motion after the India–Asia collision, with the result that India indents Asia, and a wave of deformation has spread across the Asian continent for over 2000 km north of the Himalaya.
The father of geology, James Hutton, in the late eighteenth century provided the insights which led nearly a century later to the first understandings of how mountains are constructed and what causes them. In the nineteenth and early twentieth centuries Lapworth, Peach and Horne, and Clough in Britain, and Argand, Bertrand, Heim and others working in the Swiss Alps, revolutionised our understanding of what the German geologist Kober called ‘orogens’ and of the process of orogenesis, the building of mountains.
To understand the significance of orogens, it is necessary to know something about plate tectonics, which has been remarkably successful in explaining many of the features of the Earth. In particular it deals with large-scale dynamic processes in the planet. Plate tectonics developed from the preceding ideas of continental drift, but in essence originated from an idea put forward in the 1960s by H. H. Hess. Hess postulated a surprising concept: that the ocean floor is in motion and is older as one moves away from submarine mountains known as mid ocean rises: for this reason the model became known as sea-floor spreading. The ocean floor is like a giant conveyor belt, and the interesting question is, where does it go? What this idea meant was that, for the first time in the history of geology, attention was turned on the oceans rather than the continents.
In this chapter we will be looking at the evolution of several types of Phanerozoic orogenic belts. Precambrian orogenesis will be dealt with in Chapter 12. The Himalaya and the Alps are part of a huge belt of Cenozoic age which runs from the Pyrenees through the Balkans into Turkey and on to the Middle East, Pakistan and India into Burma. There is also a leg from the Betic Cordillera to the Rif in North Africa and via Corsica to the Ligurian and Internal Western Alps. These parts were the result of the collision of Gondwanaland (the Late Palaeozoic assemblage of South America, Africa, India and Antarctic) and Eurasia (Europe and Asia). We also consider the Andes and the Caledonides in order to illustrate different types of orogens. For the present, examples are confined to the Cenozoic orogens because, as mentioned above, the younger mountain belts offer a better chance of understanding evolutionary processes in orogenesis than the older deeply eroded belts in which much of the evidence is missing (see Chapter 12).
In the now discarded geosynclinal theory of orogenesis as set out for example in Holmes's Principles of Physical Geology (Holmes, 1944), the pre-orogenic phase was a precursor of the orogenesis because the sedimentary and igneous rocks deposited in the geosyncline were already undergoing compression and so were predestined to become involved in orogeny, the point being reinforced by the postulated downward flow of mantle convection cells which led the whole process. Plate tectonics introduced a paradigm shift which included a denial of any link between the events occurring before orogenesis and the orogeny itself; this is well demonstrated by the Swiss Alps which were undergoing extension not compression before orogeny. The attempt to separate temporally extensional and compressional strain events is much too simple. For example, compressional strain in forearc wedges may be synchronous with extensional strain in the back arc, as for example in South America where overall convergence during the Jurassic–Early Cretaceous between the oceanic and continental plates involved synchronous extensional and compressional strains. This is a common feature around the Pacific where back-arc basins are opening during subduction. In addition, as Royden (1993 a,b) has shown, the roll-back and advance of the subduction zone produces alternations of extension and compression of continental margins.