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3 - EUROPE'S LITHOSPHERE – SEISMIC STRUCTURE
- Edited by D. J. Blundell, R. Freeman, Stephan Mueller
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- Book:
- A Continent Revealed
- Published online:
- 05 November 2009
- Print publication:
- 27 November 1992, pp 33-70
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Summary
SEISMIC METHODS FOR EXPLORING THE CRUST AND UPPER MANTLE OF EUROPE
Methods for exploring the Earth's interior which follow the passage of seismic waves through the ground are adapted according to the scale on which the Earth is viewed. The resolution of the seismic method utilised governs the clarity of the image of the Earth's structure and the extent of information on physical properties. In general, the deeper the investigation, the lower the resolution and the more blurred is the image. Normal incidence seismic reflection techniques provide the best resolution, particularly in a vertical sense, and deep seismic reflection profiles have yielded spectacular views of the detailed structure of the crust and upper mantle to depths of 60 km in recent years. EGT has been able to take advantage of the work of a number of deep seismic reflection profiling programmes, particularly BIRPS (UK), CROP (Italy), DEKORP (Germany), ECORS (France) and NFP 20 (Switzerland). However, this method gives poor information on seismic velocities, for which wide-angle reflection and refraction experiments are much better suited (Giese et al. 1976). These too, provide good resolution and strong control on the properties of the crust and upper mantle but to reach depths of 200 km needed to explore the full thickness of the lithosphere, quite elaborate and large-scale experiments have to be conducted along profiles at least 1500 km in length, firing several tens of explosions with dynamite charges measured in tons into large arrays of seismometers spaced 2–3 km apart along the entire length of the profile.
7 - GEODYNAMICS OF EUROPE
- Edited by D. J. Blundell, R. Freeman, Stephan Mueller
-
- Book:
- A Continent Revealed
- Published online:
- 05 November 2009
- Print publication:
- 27 November 1992, pp 215-232
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Summary
HOW DOES GEOLOGY WORK?
In this book we have endeavoured to present a coherent view of the lithosphere of Europe, brought into focus through the European Geotraverse which provides continuity across the continent on a scale that encompasses the whole lithosphere. We have made use of this to reconstruct the geological evolution of the crustal units that now make up the continent of Europe, within a framework of plate tectonic processes taking place throughout the past 2 Ga of Earth history. But what of the mechanisms? What forces drive these processes? And how do some areas of the continent remain stable over long periods of time whilst others are remobilised again and again? What are the processes that split continents, fragment them into microplates, and what are the processes of terrane accretion and continental growth? Within a continent, deformation and alteration take place across extensive regions so that plate boundaries are not definable as lines of activity separating rigid undeformable plates as is the case in the oceanic regime. This is understandable from the discussion in Chapter 4 because the composition and physical conditions of continental crust are such that the rheology creates a weak zone in the middle crust that gives continental lithosphere a ‘soft centre'. In contrast, the thin crust and basaltic composition of the oceanic lithosphere provide a more uniform and stronger rheological character. But if the continental lithosphere does have a ‘soft centre' and is more readily deformable, why does it not deform more extensively than it does? Why, for example, is the Alpine collision zone so narrow?