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The Geology Section (C) of the Leicester Literary and Philosophical Society presents: The Earth’s Crazy Paving:
a 21st Century Perspective on Plate Tectonics Annual Saturday Seminar 13 March 2010, 9.30am - 5.00pm Assemble from 9.00am; Reception to follow the Seminar Lecture Theatre 1, Ken Edwards Building, University of Leicester |
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The modern conception of Plate Tectonics has been in existence for about 50 years, with the ‘plate tectonic revolution’ taking place in the 1960’s when a new generation of researchers, some of whom will be speaking at the Seminar, advanced and developed the theories of Alfred Wegener from 45 years earlier. These pioneers unveiled to a mostly sceptical scientific community the amazing fact that the Earth’s lithosphere is made up of moving plates, whose unpredictable wanderings around the planet over time continually modify the morphology of the Earth’s surface. Our seminar takes a reflective view of those early days, the controversy and disbelief, then focuses on the advancement of plate tectonics in the 21st Century. The story begins with the tectonic setting of the early Earth, moves on to the discoveries and nature of processes impinging on the continents, mountains and oceans, and concludes with the interaction between the processes of plate tectonics and life on the planet. The full day of talks will be presented by Professors Joe Cann, James Jackson, Andreas Rietbrock and Hugh Rollinson, and Drs Dickson Cunningham, Stephen Jones and Alan Owen. |
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Seminar Programme |
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9.00 |
Assemble |
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| 9.30 |
Opening Dr Joanne Norris, Chairman, Geology Section (C), Leicester Literary and Philosophical Society |
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| 9.35 |
Living through the plate tectonic revolution; how the Earth suddenly started to move. Professor Joe Cann (University of Leeds). |
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| 10.20 |
When did Plate Tectonics begin? Professor Hugh Rollinson (University of Derby). . |
11.05 |
Refreshment Break |
| 11.25 |
Dehydration processes in the subducting slab: Seismological observations from the Andean subduction zone. Professor Andreas Rietbrock (University of Liverpool). |
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| 12.10 |
Mountain building in continental interiors: Lessons from central Asia. Dr. Dickson Cunningham (University of Leicester). |
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| 12.55 |
Lunch |
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| 14.00 |
Mountains and shields on the continents. Professor James Jackson (University of Cambridge). |
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| 14.45 |
The link between mantle convection, plate motion and oceanic circulation in the North Atlantic and the onset of the Northern Hemisphere Glaciation. Dr. Stephen Jones (University of Birmingham). |
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| 15.30 |
Refreshment Break |
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| 15.50 |
Plate tectonics and the changing patterns of life in the marine realm. Dr. Alan Owen (University of Glasgow). |
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16.35 |
Discussion and Concluding Remarks |
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17.50 |
Reception. To be held in the Seminar Rooms in the Ken Edwards Building |
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Seminar abstracts |
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Living through the plate tectonic revolution; how the Earth suddenly started to move. Professor Joe Cann University of Leeds. E-mail: j.r.cann@leeds.ac.uk |
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Abstract In one way or other we all know about plate tectonics – “a seismic shift in the tectonic plates of politics”. A simple version of plate tectonics is taught to ten year olds. The idea is used again and again in natural history programmes on television. News reports of earthquakes, tsunamis and volcanic eruptions refer to plate movements without hesitation. But that was not the case before 1960. Then the standard view of the Earth was that continents and oceans had been fixed in place since the earliest times. Sea had flooded the land and land had emerged from the sea repeatedly during hundreds of millions of years, but the size and shape of the deep oceans and the position of the continents relative to one another had not changed. So the transition from this earlier picture to the new one represented a radical shift in our understanding of the Earth – the “plate tectonic revolution” – and that transition not only provided a framework that unified many existing observations and concepts, but also had a great impact on the practical application of earth science to resources, hazards and the environment. I will explore how the transition from older ideas to plate tectonics took place, from its beginnings in the work of Alfred Wegener in 1915 to its virtual completion in about 1976. How did the new ideas evolve? Why was the acceptance of them so slow? Why, even as late as 1967, was the earth science world divided into two opposing camps that apparently could not communicate with each other? I was a young scientist then and happened to be close to the action during that period of change, and could watch with interest (and often amazement) as the rest of the world struggled to come to terms with the new ideas. During that time all earth scientists had access to the same information all of the time, and yet some realised the implications early, while others, equally distinguished as scientists, took decades to understand the implications of this momentous change in the way that we see the Earth. It is a very intriguing story.
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When did Plate Tectonics begin? Professor Hugh Rollinson University of Derby E-mail : h.rollinson@derby.ac.uk |
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Abstract The Plate Tectonic Paradigm has been the principal model for understanding the solid Earth for over forty years. However, although the model is hugely successful there is still great uncertainty as to when the plate tectonic process began. Recent studies have highlighted this difficulty by proposing two very different start times for plate tectonics. One model argues that plate tectonic processes took over from an earlier (unspecified) tectonic regime in a hotter, younger Earth at 1.0 Ga, whereas the other proposes a much earlier start at 4.4-4.5 Ga, and within a 100 Ma of planetary accretion. This talk will review the evidence for the early and late start hypotheses and will argue for a middle position in which plate tectonic processes began during the Archaean before 2.5 Ga. Central to the argument is evidence for the origin and evolution of the Continental crust and how and when the continental crust formed. Reference: Rollinson HR (2007) When did plate tectonics begin? Geology Today, 23, 186-191. |
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Dehydration processes in the subducting slab: Seismological observations from the Andean subduction zone. Professor Andreas Rietbrock University of Liverpool |
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Abstract Subduction zones, the expression of convergent plate boundaries, generate the world's largest and most destructive earthquakes. The Chilean subduction zone is an ideal natural laboratory to study the processes involved in generating these devastating earthquakes. While the upper limit of the seismogenic zone will be soon in reach of scientific drilling projects the lower limit of the seismogenic zones will only be accessible by remote techniques like passive seismological observations. In the last decade several high resolution passive seismic imaging experiments have been carried out to shed light into subduction zone processes, focussing on the lower limit of the seismogenic zone and its transition into intermediate depth seismicity (300 km).
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Mountain building in continental interiors: Lessons from central Asia. Dr Dickson Cunningham University of Leicester |
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Abstract The Indo-Eurasia collisional deformation field provides Earth’s best modern example of how a continental interior region deforms in response to a distant continental collision – a topic relevant to understanding the tectonic history of all continents. Active faulting and mountain building occurs within a vast intraplate region in central Asia that is larger than all of Europe. Although the immediate effects of the collision including the development of the Himalayas and uplift of Tibet are well studied and widely publicised, the more distant effects including the uplift of remote ranges in north-western China, and Mongolia are less well known to the earth science community. These intraplate and intracontinental orogens are also poorly explained by classical plate tectonic theory that suggests that significant mountain building is a plate margin process. In this presentation, I will review what we have learned about the active deformation north of Tibet specifically focusing on fault systems, crustal preconditions and mountain building processes that have led to construction and ongoing deformation in the Gobi Altai, eastern Tien Shan and Beishan mountains. The modern Gobi Altai, easternmost Tien Shan and Beishan can be classified as intracontinental transpressional orogens - a unique class of orogen characterised by oblique deformation in an intraplate setting. These belts occur along the perimeter of the Indo-Eurasia deformation field and between rigid cratonic blocks within central Asia. Late Cenozoic mountain building is fundamentally controlled by the location and orientation of craton boundaries, prevailing structural grain, inherited fault architecture, direction of maximum horizontal stress (SHmax), fault kinematics and slip rates, and erosion rates. Because the ranges form in a continental interior setting, they lack the typical architecture of a telescoped continental margin (e.g. Himalayas), contracted arc and/or backarc (e.g. Andes) shortened accretionary wedge (e.g. Makran), or inverted rift (e.g. Pyrenees). Instead, an intracontinental transpressional orogen is characterised by a basin and range topography containing a diffuse network of linked thrusts and strike-slip faults that generate pure thrust ridges, restraining bends, and other transpressional ridges, many of which are flower structures in cross-section. As the shortening component of deformation increases, parallel flower structures may grow, overlap and coalesce, thus obscuring their individual origins as uplifts that nucleated along a strike-slip fault bend or stepover. In addition, intermontane basins may close by inversion and/or overthrusting from one side or both sides (half-ramp and full-ramp basins). Isolated high massifs may nucleate along major strike-slip systems anywhere within the orogen, thus there is no simple topographic gradient from a low foreland to a higher orogenic hinterland as is typical in purely contractional orogens. Intracontinental transpressional orogens typically lack consistent structural vergence and bilateral thrusting within individual ranges is common. Because range bounding thrusts are observed to link with strike-slip faults that enter the range along strike, it is likely that the root structures for many thrusts are steep-vertical strike-slip faults and not shallow-dipping basal decollements; i.e. there is no regional thrust wedge architecture. Intracontinental transpressional orogens contain similar orogenic and basinal elements to transpressional continental transform boundaries except that they lack a single major fault system (e.g., San Andreas, N. Anatolian Fault) and instead of transferring interplate motion to another plate boundary, they are dead-end zones where intraplate strain is terminally accommodated. It is likely that more ancient Palaeozoic and Precambrian continental collisions also produced oblique deformation belts in rheologically weak regions within or around the perimeter of the deformation field of the indented continent. However, because intracontinental oblique deformation belts generally contain lower mountains, shallower clastic basins, and very little magmatism or metamorphism compared to frontal collision belts, their geological expression is less likely to be preserved in the rock record.
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Mountains and shields on the continents. Professor James Jackson University of Cambridge |
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Abstract The great debates on Continental Drift which dominated the first half of the 20th century were, in the end, resolved by looking at the oceans. The behaviour we now know as Plate Tectonics is the dominant characteristic of the Earth, and describes the motions and evolution of the ocean basins with great simplicity and elegance. But it has never been much help in describing the deformation on continents, where mountains and rift valleys spread out over vast areas, and the whole notion of a plate boundary is meaningless in many places. We need, and now have, other ways of observing and describing what happens where continents collide or rift apart, in which the language of continuum mechanics (or, crudely, ‘flow’) is often more appropriate than that of rigid plates. But even within the continents, contrasts between the ancient Precambrian shields and the young mountain belts are responsible for some of the most obvious variations in the land surface and geological history. The interiors of many continents are flat, relatively featureless areas (‘shields’) that have remained stable for billions of years. Yet when one of these shields, such as India, collides with another younger continent, such as Asia, it is the younger area that is damaged while the older shield that is relatively unaffected. After the India-Asia collision, India is substantially intact, while earthquakes and mountains extend at least 3,000 km north of its borders, into Tibet, China and Mongolia.. What is responsible for such obvious contrasts in strength on the continents? Over the past few years we have made considerable progress in answering this question. The principal new insights are: 1) Strength in continents resides mostly in the crust, rather than in the mantle. Earthquakes in younger continental areas are usually confined to the top half of the crust, where temperatures are less than about 350oC. But in the shields earthquakes can occur throughout the crust, to temperatures of up 600oC, and the probable reason for this is that the lower crust in such shields is anhydrous. The lack of earthquakes in the continental mantle is because it is generally hotter than 600oC, which is the temperature at which they cease to occur in the oceans. 2) This strength contrast is seen also in the way continents bend to support mountains. Just as a thick plank bends on a broader curve than a thin plank, India bends to form the broad Ganges basin in front of the Himalayas, compared to the narrower, more localized bending of the Persian Gulf to support the mountains of SW Iran. 3) The mantle does not need to be strong to support mountains. Mountains are held up by the strong crust beneath their adjacent forelands, and the only way they could collapse is by flowing over the forelands, as honey flows over a glass plate. This type of flow, known as a gravity current, is responsible for the shape of mountain fronts such as the Himalaya that are adjacent to strong shields. 4) Recent advances in seismology have allowed us to see that under many of the ancient continental shields the lithosphere (or ‘plate’) reaches extreme thicknesses of 250 km or more (compared to about 105 km in the oceans). For the first time, we can make maps of these thickness variations, which give many insights into the geological history of the continents. 5) The thick mantle lithosphere of many shields must be buoyant to prevent it delaminating and falling back into the Earth’s interior. The reduction in density was caused by melting during its early history, and occurred prior to its subsequent thickening during continental collision. 6) But the thickening of the lithosphere during collision also thickened the crust, to such an extent that the internal heat generated by radioactivity can cause it to melt. During such melting, granites form and separate into the upper crust, leaving behind a completely dry residue (‘granulite’) that is extremely strong when it cools. This process is, we think, happening today in Tibet. 7) Once the buoyant mantle lithosphere and the strong lower crust are formed, they cannot easily be changed: they are responsible for the stability and survival of the ancient shields over geological time.
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The link between mantle convection, plate motion and oceanic circulation in the North Atlantic and the onset of the Northern Hemisphere Glaciation. Dr Stephen Jones University of Birmingham, |
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Abstract Opening and closing of oceanic gateways between continents can influence global climate. For example, when the Antarctic circum-polar current became established following separation of Antarctica from South America and Australia, the Antarctic ice sheet grew rapidly because the continent was isolated from the influence of warm tropical waters. This talk will look at how a gateway in the North Atlantic ocean near Iceland has affected oceanic circulation. Evolution of the Icelandic gateway influenced the onset of the Northern Hemisphere Glaciation and possibly other periods of global climate change. At the head of the North Atlantic, between Norway and Greenland, lies an important hub in the global oceanic circulation system. Here, warm Gulf Stream water that has flowed north near the ocean surface cools, sinks and returns southward along the seabed. This Atlantic circulation system carries warmth from the tropics to the Arctic, and changes in the circulation system can change the temperature gradient between the equator and the North Pole. The position of the circulation hub near Iceland and the strength of the circulation are both affected by the Greenland-Scotland Ridge, a shallow sill straddling Iceland where the sea floor rises to a depth of only several hundred metres. The elevation of the Greenland-Scotland Ridge has fluctuated over the past 60 million years in response to three controls. First, the ridge is a hotspot track, built from the large volumes of magma formed when unusually hot mantle within the Iceland Mantle Plume rises up beneath the Mid Atlantic Ridge plate spreading axis. Secondly, like all young oceanic plates, the Greenland-Scotland Ridge subsides gradually as it spreads away from the mid-ocean ridge. Finally, the temperature of the Iceland Mantle Plume has fluctuated over time. The consequent waxing and waning of mantle convective support of the Greenland-Scotland Ridge has, from time to time, restricted or even cut off the connection between the main Atlantic and the ocean basin to the north. Oceanic crust south of Iceland preserves an excellent record of these mantle temperature fluctuations in the form of topographic features known as V-Shaped Ridges. Recent research cruises have clarified the long-held notion that V-Shaped Ridges are generated as pulses of hotter and cooler mantle flow outward from Iceland beneath the plates. The Northern Hemisphere Glaciation began during the Pliocene (c. 3 million years ago). The preceding period was the most recent period in Earth’s history in which global average temperatures were similar to those projected for the end of this century; however, state-of-the-art global climate models have great difficulty in reproducing the Pliocene warm period. The new data from the North Atlantic V-Shaped Ridges indicate that the most recent patch of cool mantle within the head of the Iceland Mantle Plume was positioned beneath the Greenland-Scotland Ridge lock-gate during the Pliocene. With cooler mantle beneath, the lock-gate would have been relatively low and allowed the strong Atlantic oceanic circulation that kept the high latitudes warm. As the cool mantle moved towards its present position, the lock-gate rose, oceanic circulation was inhibited and the Arctic ice expanded. It seems likely that the global climate models cannot reproduce pre-Northern Hemisphere Glaciation conditions because they do not yet correctly represent the Icelandic oceanic gateway. |
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Plate tectonics and the changing patterns of life in the marine realm. Dr Alan Owen University of Glasgow |
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Abstract Plate tectonics has had a profound influence on the distribution and diversity of life on Earth. The changing positions of the continents and the opening and closing histories of oceans variously provide habitats for organisms, routes for their dispersal, barriers to their migration and influences on the climate in which they live. A wide range of techniques has been developed for the analysis of the palaeogeographical distribution of organisms and how they have changed through geological time. In recent years, these include the increasing use of Geographical Information Systems, which also provide a link into computerized palaeogeographical packages. In the marine realm, the biogeographical differentiation of organisms can be recognised back at least to the Cambrian and in conjunction with palaeomagnetism and other techniques, it has become a powerful tool in palaeogeographical reconstruction. There is also a strong correlation between biodiversity and the supercontinent cycle. Thus the two major radiations of marine life during the Phanerozoic can be correlated to the break-up of the supercontinents of Rodinia during the latest Proterozoic and early Palaeozoic and of Pangaea in the Mesozoic leading to the creation of more numerous and increasingly widely separated continents and other crustal blocks, new oceans and more opportunities for speciation in isolated populations. Many other aspects of plate tectonics have an influence on marine life, from the production of inorganic nutrients to changes in oceanic circulation patterns that affect both climate and the distribution pathways of organisms. Islands generated by a variety of plate tectonic processes have been described as acting as: stepping stones (for migration), cradles (where new animal or plant taxa originate), museums (refugia for taxa extinct elsewhere), Noah’s Arcs (carrying taxa away from the part of the world where they originated) and Viking Burial Ships (bearing fossil taxa of very different origins from those where the island may eventually collide). Islands that are not lost through subduction eventually become accreted to continental margins and incorporated in orogenic belts. Fossil faunas and floras can play an important role in unravelling the complex origins and histories of such terranes. The evolution and distribution patterns of Phanerozoic marine life are strongly linked to the plate tectonic history of our planet. The details of those links are emerging and continue to form an important focus of investigation of the Earth System. |