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A multidisciplinary update on continental plate tectonics and plate boundary discontinuities Understanding the origin and evolution of the continental crust continues to challenge Earth scientists. Lithospheric Discontinuities offers a multidisciplinary review of fine scale layering within the continental lithosphere to aid the interpretation of geologic layers. Once Earth scientists can accurately decipher the history, internal dynamics, and evolution of the continental lithosphere, we will have a clearer understanding of how the crust formed, how plate tectonics began, and how our continents became habitable. Volume highlights: Theories and observations of the current state of tectonic boundaries and discontinuities Contributions on field observations, laboratory experiments, and geodynamic predictions from leading experts in the field Mantle fabrics in response to various mantle deformation processes Insights on fluid distribution using geophysical observations, and thermal and viscosity constraints from dynamic modeling Discontinuities associated with lithosphere and lithosphere-asthenosphere boundary An integrated study of the evolving physical and chemical processes associated with lithosphere asthenosphere interaction Written for academic and researchgeoscientists, particularly in the field of tectonophysics, geophysicists, geodynamics, seismology, structural geology, environmental geology, and geoengineering, Lithospheric Discontinuities is a valuable resource that sheds light on the origin and evolution of plate interaction processes.
A multidisciplinary update on continental plate tectonics and plate boundary discontinuities Understanding the origin and evolution of the continental crust continues to challenge Earth scientists. Lithospheric Discontinuities offers a multidisciplinary review of fine scale layering within the continental lithosphere to aid the interpretation of geologic layers. Once Earth scientists can accurately decipher the history, internal dynamics, and evolution of the continental lithosphere, we will have a clearer understanding of how the crust formed, how plate tectonics began, and how our continents became habitable. Volume highlights: Theories and observations of the current state of tectonic boundaries and discontinuities Contributions on field observations, laboratory experiments, and geodynamic predictions from leading experts in the field Mantle fabrics in response to various mantle deformation processes Insights on fluid distribution using geophysical observations, and thermal and viscosity constraints from dynamic modeling Discontinuities associated with lithosphere and lithosphere-asthenosphere boundary An integrated study of the evolving physical and chemical processes associated with lithosphere asthenosphere interaction Written for academic and researchgeoscientists, particularly in the field of tectonophysics, geophysicists, geodynamics, seismology, structural geology, environmental geology, and geoengineering, Lithospheric Discontinuities is a valuable resource that sheds light on the origin and evolution of plate interaction processes.
The ensemble of manuscripts presented in this special volume captures the stimulating cross-disciplinary dialogue from the International Symposium on Deep Structure, Composition, and Evolution of Continents, Harvard University, Cambridge, Massachusetts, 15-17 October 1997. It will provide an update on recent research developments and serve as a starting point for research of the many outstanding issues.After its formation at mid-oceanic spreading centers, oceanic lithosphere cools, thickens, and subsides, until it subducts into the deep mantle beneath convergent margins. As a result of this continuous recycling process oceanic lithosphere is typically less than 200 million years old (the global average is about 80 Myr). A comprehensive, multi-disciplinary study of continents involves a wide range of length scales: tiny rock samples and diamond inclusions may yield isotope and trace element signatures diagnostic for the formation age and evolution of (parts of) cratons, while geophysical techniques (e.g., seismic and electromagnetic imaging) constrain variations of elastic and conductive properties over length scales ranging from several to many thousand kilometers. Integrating and reconciling this information is far from trivial and, as several papers in this volume document, the relationships between, for instance, formation age and tectonic behavior on the one hand and the seismic signature, heat flow, and petrology on the other may not be uniform but may vary both within as well as between cratons. These observations complicate attempts to determine the variations of one particular observable (e.g., heat flow, lithosphere thickness) as a function of another (e.g., crustal age) on the basis of global data compilations and tectonic regionalizations.Important conclusions of the work presented here are that (1) continental deformation, for instance shortening, is not restricted to the crust but also involves the lithospheric mantle; (2) the high wavespeed part of continental lithospheric mantle is probably thinner than inferred previously from vertically travelling body waves or form global surface-wave models; and (3) the seismic signature of ancient continents is more complex than expected from a uniform relationship with crustal age.
Traditionally, investigations of the rheology and deformation of the lithosphere (the rigid or mechanically strong outer layer of the Earth, which contains the crust and the uppermost part of the mantle) have taken place at one scale in the laboratory and at an entirely different scale in the field. Laboratory experiments are generally restricted to centimeter-sized samples and day- or year-length times, while geological processes occur over tens to hundreds of kilometers and millions of years. The application of laboratory results to geological systems necessitates extensive extrapolation in both temporal and spatial scales, as well as a detailed understanding of the dominant physical mechanisms. The development of an understanding of large-scale processes requires an integrated approach. This book explores the current cutting-edge interdisciplinary research in lithospheric rheology and provides a broad summary of the rheology and deformation of the continental lithosphere in both extensional and compressional settings. Individual chapters explore contemporary research resulting from laboratory, observational, and theoretical experiments.
" ... developed out of two symposia: 'Deformation at Convergent Margins', convened at the European Union of Geosciences meeting (EUG XI) at Strasbourg in April 2001; and 'Vertical Coupling and Decoupling at Convergent Margins', convened at the AGU Fall meeting in San Francisco in December 2001"--Acknowledgements.
In recent years, high-resolution seismological studies have documented anomalously low shear wave velocity in the middle of the lithosphere, referred to as mid-lithospheric discontinuity (MLD). In contrast to the low seismic velocity, MLD does not show any anomalous electrical conductivity based on available magnetotelluric (MT) observations. This is a perplexing observation since low shear wave velocities are usually associated with partial melts. This plausible inference of partial melts contradicts the observed stability and longevity of cratons. A partially molten layer is rheologically weak and might lead to the erosion of the cratonic root by the convecting mantle over geological timescales. Instead, mantle metasomatism might provide a plausible and alternate explanation for such a low shear wave velocity layer in the middle of the lithosphere. The metasomatized mantle is likely to stabilize secondary mineral phases, such as amphiboles. To evaluate whether mantle metasomatism or the presence of amphiboles can explain the geophysical observations of MLD, we need good constraints on the thermoelasticity and electrical conductivity of amphiboles at conditions relevant to MLD depths. However, pertinent data on the thermoelasticity and transport properties of amphiboles and how they vary as a function of pressure, temperature, and chemistry are currently lacking. To bridge this gap, in my Ph. D. dissertation work, I provided better constraints on the thermoelasticity of amphiboles using first principles simulation. To benchmark the simulation-based data, I calculated fundamental thermodynamics such as heat capacity and compared it with the heat capacity derived from the vibrational spectroscopic data using a Raman spectrometer. The excellent agreement between the theoretically predicted and experimentally derived thermodynamic data validated the simulation methods used in my Ph. D. dissertation. Next, I predicted the thermoelasticity of end-member amphibole tremolite and compared my predicted data with experimental results available at ambient conditions. Again, the agreement between simulations and ambient experimental data was excellent, rendering further confidence in predictions based on first principles simulations. I then explored the thermoelasticity of pargasite amphibole relevant for MLD. In addition, I complied and updated the thermoelastic database of other hydrous minerals that are likely to be stable at MLD, such as chlorite and phlogopite. Based on the modal abundances of mineral assemblages from cratonic xenoliths and experimental petrology, I predicted the aggregate shear wave velocity of the metasomatized cratonic mantle. Our updated thermoelastic database on hydrous phases showed that the hydrous minerals can only explain a fraction of the observed velocity reduction at MLD depths. We found that explaining the 100% of velocity reduction would require an unrealistic amount of hydrous minerals in the metasomatized cratonic mantle. I supplemented my thermoelasticity study with an in-depth examination of the electrical conductivity of an amphibole-bearing rock at MLD conditions. The results from high-pressure and temperature experimental studies showed that the electrical conductivity of amphibole is relatively high compared to volumetrically dominant mantle minerals such as olivine and pyroxene. Our results indicated that the electrical conductivity of amphiboles is quite sensitive to the alkali content. The pargasitic amphibole end-member likely to be stable in the metasomatized mantle tends to have a high alkali content. Thus, based on my results, I anticipated a high electrical conductivity for the alkali-rich amphiboles. However, high-electrical conductivity in MLD regions is not observed, and thus only a small fraction of alkali-bearing amphiboles can be allowed to explain the observed electrical conductivity profiles observed at MLD. Thus, our results indicated that the presence of amphiboles cannot simultaneously explain the seismological and MT observations at MLD. Hence, mantle metasomatism is unlikely to be the sole mechanism to explain MLD. Additional mechanisms need to be explored in addition to mantle metasomatism. These additional mechanisms may include seismic anisotropy and rheological changes, id est, elastically accommodated grain boundary sliding, at MLD depths.