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Lithospheric structure of the continents
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Tectonophysics 445 (2007) 1 – 2 www.elsevier.com/locate/tecto
Preface
Lithospheric structure of the continents The word ‘Lithosphere’ has come from the Greek for ‘rocky sphere’. As we all know the lithosphere includes the crust and uppermost mantle and is underlain by asthenosphere. Lithosphere as a strong, rigid outer layer was first proposed by Barrel (1914). The concept was developed based on the prominent gravity anomalies over the continental crust. Barrel also inferred that a weak and fluid layer lies underneath the Lithosphere. These concepts were later enlarged by Daly (1940) and they are now widely accepted. The plate tectonic theory (Wilson, 1965; McKenzie and Parker, 1967) is based on the concept of movement of rigid lithospheric plates over a relatively weak asthenosphere. Lithosphere– asthenosphere boundary can be mapped from the observations of changes in the gradient of seismic velocity or from heat flow. Deep electromagnetic (EM) methods can provide a similar but independent information on the lithosphere–asthenosphere boundary in terms of electrical conductivity. The objective of the present special issue is to bring together the recent theoretical, observational and experimental studies related to deep crust and mantle studies at various active and passive regions of the continents with a focus on the relationships with deep geological processes and tectonic interpretation. The papers considered in this issue are presented in different sessions of electromagnetic induction workshop (IAGA working group I.2) held at National Geophysical Research Institute, Hyderabad during October 18–23, 2004. Variations of electrical conductivity reflects structural and petrophysical properties. An anomalous conductive feature gives clues for the existence of graphite, saline water or partial melt and throw light on the rheological properties and also helps to understand the seismotectonics of the region. Large-scale regional studies derived from geomagnetic and magnetotelluric soundings will constrain the models that lead to new interpretation of the processes. Many experiments have been carried out to understand the deep electrical structure of the lithosphere of 0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.07.001
the Baltic shield. In this issue Elena et al. in their paper on ‘Upper mantle electromagnetic sounding….’ have critically looked into the BEAR EM data for the existence of lithosphere–asthenosphere boundary. The morphology of the source distortions in transfer functions are also presented in the paper. Polag et al. in their paper on EM and geoelectric investigations has made detailed mapping of the Gurinai structure in Inner Mongolia, NW part of China. Three different geophysical techniques have been analyzed namely, direct current (DC) geoelectrical methods, transient electromagnetic (TEM) and magnetotellurics (MT). The study has contributed for better understanding of the Ruoshui basin’s build up and tectonic evolution of the Gurinai structure. Relation between electrical resistivity with seismic activity has been reported in many regions. Aysan Gurer and Murat Bayrak have made concerted efforts to understand this phenomenon from the magnetotelluric data in west Anatolia and Thrace region of Turkey. They observed that large earthquakes have occurred around the areas of high electrical resistivity in the upper crust and small magnitude earthquakes are observed in the conductive lower crust. It is interpreted that fluid migration from the conductive lower crust to resistive upper crust might be the reason for seismicity in resistive zones. Thin layer algorithms are fast and can be approximated to a three-dimensional (3-D) structure. In their paper on ‘fast imaging and inversion….’ Singer and Fainberg have showed the procedures to determine the subsurface structure on land as well as on sea floor. Two specific examples have been tested; one case treats a model with a conductor hidden in a highly conductive media and the other is a model with a thin resistive structure in a conductive environment. The approach developed by them can be used both for natural and controlled source EM data measured either in frequency or in time domain. Large scale flood Basalt regions can be seen in different continents such as Siberian platform, Columbia
2
Preface
River, southern Brazil, peninsular India etc. The study of the subsurface structure helps in understanding the evolution of these provinces. Continental deep geoelectric structure along selected profiles in Deccan Volcanic Province of peninsular India and its relation to seismotectonics have been discussed by Harinarayana et al. in their paper on ‘Regional geoelectric structure….’. Deccan trap thickness map is presented considering various geophysical data sets and the cause of regional seismicity in the region is related to the thickness of the trap cover on the top and the orientation of the subsurface faults are discussed. VIEIRA da Silva et al in their paper on ‘3-D EM imaging….’ based on magnetotelluric data have constructed an image for the plate tectonic boundary between Ossa Morena Zone (OMZ) and South Portuguese zone (SPZ). The paper has brought out new developments on the interpretation proposed for Iberian Reflective Body (IRB). The IRB in Spanish sector has showed a good signature in seismics with 2s thick reflective patterns that are closely correlatable with the conductive band delineated from MT study. These studies have opened up critical questions postulated in geodynamic models. Glaciation of Central Asia and it’s contribution to the cooling of the atmosphere and disturbance of summer monsoon circulation is discussed in a paper on ‘past ice stream network…’ by Mathias Kuhle. Radiographic analyses of the data suggested large thickness of ice
cover of 1200 m over southern Tibet. These observations are based on the presence of moraine at about 450 m on the southern flank of Himalayas and 2300 m on the northern slope of the Tibetan plateau. It is interpreted that such large changes in the ice cover from the past to present might contribute to the various tectonic activities in the region. All these contributions in the special issue have shown the importance of understanding the structure of the Lithosphere in different scales in different parts of the continents which can be further improved again in the years to come. References Barrel, J., 1914. The strength of the Earth's crust. J. Geol. 22, 425–433. Daly, R., 1940. Strength and structure of the Earth. Prentice-Hall, New York. McKenzie, D.P., Parker, R.L., 1967. The North Pacific: An example of tectonics on a sphere. Nature 216, 1276–1280. Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature 207, 343–347.
Guest Editors T. Harinarayana* Hisashi Utada Andreas Junge ⁎Corresponding author. E-mail address: [email protected].
Tectonophysics 445 (2007) 1 – 2 www.elsevier.com/locate/tecto
Preface
Lithospheric structure of the continents The word ‘Lithosphere’ has come from the Greek for ‘rocky sphere’. As we all know the lithosphere includes the crust and uppermost mantle and is underlain by asthenosphere. Lithosphere as a strong, rigid outer layer was first proposed by Barrel (1914). The concept was developed based on the prominent gravity anomalies over the continental crust. Barrel also inferred that a weak and fluid layer lies underneath the Lithosphere. These concepts were later enlarged by Daly (1940) and they are now widely accepted. The plate tectonic theory (Wilson, 1965; McKenzie and Parker, 1967) is based on the concept of movement of rigid lithospheric plates over a relatively weak asthenosphere. Lithosphere– asthenosphere boundary can be mapped from the observations of changes in the gradient of seismic velocity or from heat flow. Deep electromagnetic (EM) methods can provide a similar but independent information on the lithosphere–asthenosphere boundary in terms of electrical conductivity. The objective of the present special issue is to bring together the recent theoretical, observational and experimental studies related to deep crust and mantle studies at various active and passive regions of the continents with a focus on the relationships with deep geological processes and tectonic interpretation. The papers considered in this issue are presented in different sessions of electromagnetic induction workshop (IAGA working group I.2) held at National Geophysical Research Institute, Hyderabad during October 18–23, 2004. Variations of electrical conductivity reflects structural and petrophysical properties. An anomalous conductive feature gives clues for the existence of graphite, saline water or partial melt and throw light on the rheological properties and also helps to understand the seismotectonics of the region. Large-scale regional studies derived from geomagnetic and magnetotelluric soundings will constrain the models that lead to new interpretation of the processes. Many experiments have been carried out to understand the deep electrical structure of the lithosphere of 0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.07.001
the Baltic shield. In this issue Elena et al. in their paper on ‘Upper mantle electromagnetic sounding….’ have critically looked into the BEAR EM data for the existence of lithosphere–asthenosphere boundary. The morphology of the source distortions in transfer functions are also presented in the paper. Polag et al. in their paper on EM and geoelectric investigations has made detailed mapping of the Gurinai structure in Inner Mongolia, NW part of China. Three different geophysical techniques have been analyzed namely, direct current (DC) geoelectrical methods, transient electromagnetic (TEM) and magnetotellurics (MT). The study has contributed for better understanding of the Ruoshui basin’s build up and tectonic evolution of the Gurinai structure. Relation between electrical resistivity with seismic activity has been reported in many regions. Aysan Gurer and Murat Bayrak have made concerted efforts to understand this phenomenon from the magnetotelluric data in west Anatolia and Thrace region of Turkey. They observed that large earthquakes have occurred around the areas of high electrical resistivity in the upper crust and small magnitude earthquakes are observed in the conductive lower crust. It is interpreted that fluid migration from the conductive lower crust to resistive upper crust might be the reason for seismicity in resistive zones. Thin layer algorithms are fast and can be approximated to a three-dimensional (3-D) structure. In their paper on ‘fast imaging and inversion….’ Singer and Fainberg have showed the procedures to determine the subsurface structure on land as well as on sea floor. Two specific examples have been tested; one case treats a model with a conductor hidden in a highly conductive media and the other is a model with a thin resistive structure in a conductive environment. The approach developed by them can be used both for natural and controlled source EM data measured either in frequency or in time domain. Large scale flood Basalt regions can be seen in different continents such as Siberian platform, Columbia
2
Preface
River, southern Brazil, peninsular India etc. The study of the subsurface structure helps in understanding the evolution of these provinces. Continental deep geoelectric structure along selected profiles in Deccan Volcanic Province of peninsular India and its relation to seismotectonics have been discussed by Harinarayana et al. in their paper on ‘Regional geoelectric structure….’. Deccan trap thickness map is presented considering various geophysical data sets and the cause of regional seismicity in the region is related to the thickness of the trap cover on the top and the orientation of the subsurface faults are discussed. VIEIRA da Silva et al in their paper on ‘3-D EM imaging….’ based on magnetotelluric data have constructed an image for the plate tectonic boundary between Ossa Morena Zone (OMZ) and South Portuguese zone (SPZ). The paper has brought out new developments on the interpretation proposed for Iberian Reflective Body (IRB). The IRB in Spanish sector has showed a good signature in seismics with 2s thick reflective patterns that are closely correlatable with the conductive band delineated from MT study. These studies have opened up critical questions postulated in geodynamic models. Glaciation of Central Asia and it’s contribution to the cooling of the atmosphere and disturbance of summer monsoon circulation is discussed in a paper on ‘past ice stream network…’ by Mathias Kuhle. Radiographic analyses of the data suggested large thickness of ice
cover of 1200 m over southern Tibet. These observations are based on the presence of moraine at about 450 m on the southern flank of Himalayas and 2300 m on the northern slope of the Tibetan plateau. It is interpreted that such large changes in the ice cover from the past to present might contribute to the various tectonic activities in the region. All these contributions in the special issue have shown the importance of understanding the structure of the Lithosphere in different scales in different parts of the continents which can be further improved again in the years to come. References Barrel, J., 1914. The strength of the Earth's crust. J. Geol. 22, 425–433. Daly, R., 1940. Strength and structure of the Earth. Prentice-Hall, New York. McKenzie, D.P., Parker, R.L., 1967. The North Pacific: An example of tectonics on a sphere. Nature 216, 1276–1280. Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature 207, 343–347.
Guest Editors T. Harinarayana* Hisashi Utada Andreas Junge ⁎Corresponding author. E-mail address: [email protected].