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2003 | Buch

Introduction Kapitel lesen Erstes Kapitel lesen

Geokinematics

Prelude to Geodynamics

verfasst von: Rex H. Pilger

Verlag: Springer Berlin Heidelberg

Enthalten in: Professional Book Archive

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Über dieses Buch

There's something about introductory textbooks that can be fundamentally misleading. As I plowed through freshman Physical Geology some 36 years ago, I couldn't help but acquire the impression that all of the major problems in the earth sciences had been solved. Yes, there was a chapter about Wegener's conti­ nental drift hypothesis along with a few other ideas concerning earth evolution - expansion, contraction - and including Holmes' convection cell idea. Once I began to take the field trips and participate in junior-year field camp, I began to suspect that the textbooks had left something out. It didn't take long, however, to begin hearing about the "New Global Tectonics", at which point I felt that I had been misled by what I had encountered in my first formal contact with geology. All of a sudden it was clear that there was much more to geology and geophysics than a widely used book could convey. Plate Tectonics is now into its fifth decade. I haven't looked at current intro­ ductory textbooks, so I don't know whether they still convey the impression that all of the major problems in the earth sciences have been solved. Clearly, there is still much to do with continuing elaboration of plate tectonics, among all of the other challenges of the geosciences. This monograph introduces a number of new observations, techniques, and hypotheses as extensions to Plate Tectonics.

Inhaltsverzeichnis

Frontmatter
Chapter 1. Introduction
Abstract
For those who entered the earth sciences in the mid-1960s or earlier, the advent of Plate Tectonics was either an extraordinarily upsetting encounter with a seemingly non-geological science or a marvelous revelation that rearranged a couple of centuries of field observations into a high degree of unanticipated coherence. Suddenly, a profound simplicity emerged from apparent complexity — a simplicity that was so clear and transparent that the initial reaction of many was disbelief Indeed, skeptics viewed the New Global Tectonics as a kind of bogus religious experience promulgated by carpet-bagging geophysicists and a few scalawag geologists. The disbelievers for the most part decided to ignore the new “paradigm”; treating Plate Tectonics in the same way that contemporary science philosophers treated Kuhn’s inferred structure of scientific revolutions, of which Plate Tectonics became the latest paradigm.
Rex H. Pilger
Chapter 2. Proper Reference Frames: Hotspots, Paleomagnetism, Paleoclimate
Abstract
One of the more challenging aspects of the doing of science is the apparent failure of communication. Does the paper reporting the results of an investigation get read? Is the paper understood?
Rex H. Pilger
Chapter 3. Plate Kinematic and Reconstruction Fundamentals
Abstract
In the early 1970s, a series of “second-generation” plate tectonics papers appeared in which the reconstruction techniques introduced by Bullard, Everett and Smith (1965) were progressively modified and applied to magnetic isochrons and paleofracture zones. McKenzie and Sclater (1971) attempted the first model of opening of the central Indian Ocean. Their technique involved fitting fracture zones with an instantaneous pole and assuming constant spreading rate to fit segments of the Maldive Ridge (African plate) to the Chagos Ridge (Indian plate).
Rex H. Pilger
Chapter 4. Kinematic and Reconstruction Modeling with Continuously Varying Parameters
Abstract
There is a kind of frustration involved in interpolation of any numerical quantity, particularly if there are uncertainties involved in the input data. Uncertainties have been a passion for many plate tectonic workers; a number of important advances in the estimation of uncertainties in oceanic plate reconstructions and their composition into global reconstruction sets provide some level of confidence in the quality of the reconstructions. However, interpolations between reconstructions and corresponding, derivative kinematics have not been satisfactorily explored. There has been little progress in kinematic inference since the pioneering finite difference calculations of Pitman and Talwani (1972) and Weissel and Hayes (1972).
Rex H. Pilger
Chapter 5. An Inverse Model
Abstract
When Bernard Minster and Tom Jordan, together with Peter Molnar and Ian Haines, first published their inverse plate kinematic model in 1974 (Minster et al. 1974), the affect on the tectonics community was extraordinary. Jason Morgan had already published a global plate tectonics model parameterized in Cartesian vectors (Morgan 1972a), while Clement Chase had similarly produced an inverse model accompanied by a neat stereoscopic pseudovector portrayal of contemporary plate motions. But the Minster et al. model incorporated uncertainties. Interestingly, their application of non-linear least-squares to plate tectonics also stimulated similar applications in non-plate-tectonics applications. Subsequently, Chase introduced uncertainties in his updated formulation using, in my opinion, a somewhat more realistic formulation that included ridge-azimuth as part of the spreading rate datum, an approach also followed by DeMets et al. (1990) in the current contemporary plate kinematic standard model. While uncertainties are an obviously desirable product of such an analysis, the ability to construct a reasonable model that fits the data, even in a subjective analysis, is quite an accomplishment.
Rex H. Pilger
Chapter 6. Hotspot Trace Patterns
Abstract
Data and patterns-so much of what we do in science reduces to that x versus y plot. Sometimes, it’s predicted and observed longitude and latitude (for example, reconstructed magnetic anomalies across a spreading center). When it comes to hotspot traces, it’s age versus distance (but there’s more to it than that). Despite what we may have learned in junior-year geostatistics, the central-limit theorem doesn’t always apply. Yes, experimental dates (or model ages, to the purists) can demonstrate a Gaussian distribution, but there’s more to the tectonic meaning of an age than what emerges from the laboratory. When the 43-Ma age for the Hawaiian-Emperor bend was first pronounced, I had misgivings about its tectonic interpretation, especially since there were no obvious manifestations of tectonic changes of that age elsewhere in the Pacific. A few others shared my doubts (published before and after the 1981 paper I wrote with David Handschumacher, and a 1982 paper), but most discussions of the hotspot traces of the Pacific continued to cite the 43-Ma age almost to the present day.
Rex H. Pilger
Chapter 7. Contemporary Stress Fields
Abstract
Check out the first one or two chapters of a good structural geology or seismology textbook and you’ll find the derivation of the elastic stress tensor. Read further and, while you might find some application of the total tensor; more typically the tensor is quickly reduced to principal stresses. Then, the principal stresses get reduced even more to their components referenced to local earth coordinates.
Rex H. Pilger
Chapter 8. Observed Paleostress Orientations and Predicted Plate-Hotspot Motions
Abstract
As Plate Tectonics matured, a number of workers began to consider philosophical implications of the science. Allan Cox (1973), in particular, found Thomas Kuhn’s “Structure of Scientific Revolutions”; published in the same year as Harry Hess’s “History of ocean basins — an essay in geopoetry”, to be directly applicable to the experience of earth scientists in the 1960s. In a sense, Kuhn’s paradigm hypothesis found a kind of confirmation in this new theory of the Earth. Sometimes earth scientists experience an inferiority complex in relation to the more “basic” sciences — physics and chemistry. Now, geology and geophysics had a revolution of their own to parallel Copernican heliocentricity, Newtonian physics, Darwinian natural selection, Einsteinian relativity, quantum mechanics, and the double helix.
Rex H. Pilger
Chapter 9. Paleomagnetic Reference Frame
Abstract
Paleomagnetism, after morphology and geology, provided key evidence for the great precursor of plate tectonics — continental drift. One of the great hopes with plate tectonics was for paleomagnetism to provide additional kinematic constraints. Alas, the very processes apparently involved in generation of the geomagnetic field conspire against providing the kind of high-resolution information the kinematicist would want. The question of the relation of polar wander curves to hotspot reference frames is a corollary. Even though high resolution is not possible, it is still worthwhile comparing paleomagnetic observations with the hotspot frame.
Rex H. Pilger
Chapter 10. Back-Arc Spreading and Proper Motion
Abstract
Back-arc spreading was one of the big auxiliary surprises of second-generation plate tectonics. Tectonicists had visualized convergence of plates as resulting from converging convection cells with consequent compressive structures. As the geometric implications of plate tectonics began to be elaborated, the idea of convection cells tied to plate geometries evaporated. The documentation of back-arc spreading provided additional support for the decoupling of convection and plate kinematics. However, the mechanism for the origin of back-arc spreading has remained uncertain, although the ideas of Moberly were introduced shortly after spreading was demonstrated.
Rex H. Pilger
Chapter 11. Hotspot Reference Frames: The “Necessity” of Mesoplates
Abstract
Part of the resistance to Plate Tectonics among earth scientists in the 1960s and 1970s involved the concept of rigidity. When advocates of the new global tectonics spoke of rigid plates, they encountered an intuitive misunderstanding of mechanical rigidity rather than kinematic rigidity. The idea of mesoplates as described below is likely to encounter a similar resistance. Is the domain of the mesoplate likely to be rigid? Isn’t it more likely to behave plastically? Again, from a mechanical perspective, mesoplates, as defined below, may well consist of plastically deformable material at the pressures and temperatures at which it exists, if it is subjected to sufficient stresses. The key condition that allows for both lithoplates and mesoplates is the absence of pervasive stresses within their interiors. Deformation is restricted to the margins of plates — both lithoplates (of classic Plate Tectonics) and mesoplates.
Rex H. Pilger
Chapter 12. Mesoplates and the Origin of Hotspots: A Top-Down Model
Abstract
Since Morgan published the first hotspot motion model for the Pacific in 1972, the formal conversation among researchers evolved into a debate as to whether hotspots do indeed define a single, fixed, internal reference frame. However, discrepancies between Atlantic-Indian Ocean and Pacific Ocean sets became the primary focus of attention. In just the past few years, some workers have begun to write in terms of two or three separate reference frames. If there are two or three distinct hotspot reference frames — mesoplates — this presents interesting problems for the deep mantle plume hypothesis. Perhaps it is time to consider the implications of a few distinct hotspot frames for the origin of hotspots.
Rex H. Pilger
Chapter 13. Fracture Reservoirs and Intraplate Stresses
Abstract
Plate Tectonics has been almost completely developed by academic and government geoscientists. However, the largest proportion of geologists and geophysicists is employed in the oil and gas industry. Has Plate Tectonics played a significant role in hydrocarbon exploration over the past 30 years or so? For the most part the answer has to be negative. The origin of continental margins and interior basins in the context of rift tectonics has certainly been informed by plate tectonic modeling, including thermal evolution which probably controls hydrocarbon maturation. The geologic time scale has been refined with incorporation of magnetic reversals from oceanic plates. Perhaps most importantly, sequence stratigraphy has had thermal evolution of sedimentary basin as an implicit or even explicit component, as it has been applied to sedimentary sequence and facies characterization as part of the potential reservoir evaluation process. Nevertheless, it is apparent that Plate Tectonics has, at best, only played a peripheral role in advancing the exploration process.
Rex H. Pilger
Chapter 14. Summary and Conclusion
Abstract
The typical approach to reading a scientific article is to skim the abstract, and, if it is of interest, skip to the last section or two and skim the discussion and conclusion. Since a monograph commonly lacks an abstract, this last chapter may well be where some readers begin.
Rex H. Pilger
Backmatter
Metadaten
Titel
Geokinematics
verfasst von
Rex H. Pilger
Copyright-Jahr
2003
Verlag
Springer Berlin Heidelberg
Electronic ISBN
978-3-662-07439-8
Print ISBN
978-3-642-05608-6
DOI
https://doi.org/10.1007/978-3-662-07439-8