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Special issue – Magnetic anomalies
Tectonophysics 478 (2009) 1–2
Contents lists available at ScienceDirect
Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o
Preface
Special issue – Magnetic anomalies (Tectonophysics)
Magnetic anomaly maps derived from aerogeophysical surveys and satellite data have played a key role in understanding and unraveling the structure of the Earth's lithosphere. The papers presented at the Proceedings of IUGG XXIV General Assembly held at Perugia, Italy in 2007 solicited geopotential studies related to modeling and interpretation of new data sets acquired over continents and oceans. The special issue highlights 11 papers from this proceedings. The first three papers discuss the processing and interpretation of airborne magnetic data acquired over young and active volcanic centres. The next two papers focuses on the study of magnetic anomalies around the Transantarctic Mountains, Antarctica. This is followed by four papers on the geomagnetic studies of the Canadian Cordillera; Beattie Magnetic Anomaly (BMA) anomaly in South Africa; Himalayan and Tibetan plateau and the Bay of Bengal region in India. The final two papers respectively, report on magnetic properties of the layered intrusion in Fennoscandian shield, Norway and the correlation of the aeromagnetic anomalies with faults in NW Turkey. Studies of active volcanoes from ground-based and airborne surveys have led to new insights into the composition in particular within top few kilometers of these volcanoes. The results from magnetic inversion of geopotential data suggests that Iwo-jima volcano is mostly composed of trachy andesite with high magnetization (Kubota et al.). The study of Nakatsuka et al. discusses the generalized mis-tie control method to extract errors statistically from aeromagnetic surveys flown at different elevations and times. The method is used to monitor the changes in the magnetic anomaly field over Asama Volcano in Japan, between 1992 and 2005. Yet another application to combine magnetic surveys flown at different times for better understanding of the subsurface structure and compositional variation of Stromboli Volcano in Italy is nicely demonstrated by Okuma et al. They apply an apparent magnetization intensity mapping method to monitor and estimate the lateral changes in magnetization. An obvious magnetization low seen around the summit craters area of Stromboli Volcano is indicative of demagnetization caused by the heat of conduits and/or hydrothermal activity. A similar study (Okubo et al.) investigating the Sakurajima Volcano in Japan from a low-altitude, high-density helicopterborne survey accounts for variation in magnetization due to volcanic processes. Prominent magnetization lows around recent craters can be interpreted to be the effects by thermal alterations of volcanic rocks. Poorly understood, Antarctica has been a target of numerous magnetic surveys for specific geologic interests. The paper by Zunino et al. focuses on delineating the extent and spatial distribution of 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.10.019
Cenozoic magmatism and older basement of Admiralty Block of the Transantarctic Mountains. They show that the Southern Cross and Admiralty blocks are significantly affected by magmatism compared to adjacent blocks indicating major tectono-magmatism segmentation of the Transantacrtic Mountains rift flank. Ferraccioli et al. show the Wilkes Subglacial Basin in East Antarctica to be much more deeper than previously mapped. Interesting comparative studies between Wilkes Subglacial Basin and Cordillera in North America have also been highlighted. The ability of the magnetic method in characterizing the nature of the source magnetization from data at various altitudes is perhaps best demonstrated in the study conducted by Pilkington et al. over Mackenzie River Magnetic Anomaly (MRA) and northwest Canadian Territories. The high-amplitude, long wavelength anomalies were inferred to be Proterozoic magmatic arc complex along the western edge of the North American craton. On a similar footing, the prominent magnetic low over Himalayan-Tibetan plateau as seen in a high resolution MF6 CHAMP lithospheric field model is indicative of non-magnetic lower and much of the middle crust (Singh and Mitchell). They also suggest that the Indian shield region concealed beneath the thick sediment cover of Gangetic plains is weakly magnetic and it does not extend up to the base of the Himalaya. Rajaram et al. produces a comprehensive analysis of satellite gravity data, marine magnetics, bathymetry and seismic profiles over the Bay of Bengal sedimentary basin. The study concludes that 85° E Ridge does not extend to the north of 15° N latitude and supports the sagging of the crust while rejecting the hotspot trace and magmatic under-plating theories. The paper by Quesnel et al. explains the Beattie Magnetic Anomaly (BMA) in South Africa by fitting simple sheet-like prism bodies with induced magnetisation only. The paper discusses the non-uniqueness in interpreting the geological sources for BMA but supports their model from independent magnetotelluric and seismic experiments. It is critical to understand the history of intrusion, magma evolution and fractional crustallization of magnetic minerals like ilmenite, hematite and magnetite while evaluating crustal magnetic anomalies of a region. The paper by McEnroe et al. best illustrates this discussion as they attribute the variations in aeromagnetic and ground magnetic anomaly maps to the variation in the magnetic properties of the intrusion caused by varied magmatic crystallization of opaque minerals. They investigate intrusive history and magma evolution of Bjerkreim-Sokndal (BKS) norite-quartz mangerite layered intrusion in Fennoscandian shield, Norway. The last paper in this special issue highlights an important application of magnetic methods in investigating seismogenic faults and its correlation with observed
2
Preface
aeromagnetic anomalies in the Marmara region of Turkey (Ateş et al.). Using the standard tools like reduction to the pole (RTP) and second vertical derivative (SVD) authors successfully correlate the fault features with SVD alignments and hypothesize that smaller the length of faults, lesser are the chance of producing strong earthquakes. This work definitely needs more data and verification from old records to support their conjectures. The papers in this special issue highlight the state-of-the-art methodology being utilized to enhance our understanding of complex crustal structure. With new advances in instrumentation, navigation and data processing techniques, the two and three-dimensional magnetization distributions derived from magnetic anomaly maps are being increasingly used to provide constraints on spatial and temporal variability in the physiochemical characteristics of crust. This is demonstrated in the study of volcanoes from airborne surveys. The papers also show a trend to integrate magnetic anomaly data with gravity, seismic, magnetotelluric, geologic and laboratory analysis to
facilitate in interpretation. We thank all geophysicists who presented their work at the IUGG XXIV General Assembly in 2007 and particularly those who contributed to this special issue. Kumar H. Singh GEST/UMBC @ Planetary Geodynamics Lab, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Corresponding author. E-mail address: [email protected]. Shigeo Okuma Geological Survey of Japan, AIST, AIST Tsukuba Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan E-mail address: [email protected]. 22 October 2009
Contents lists available at ScienceDirect
Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o
Preface
Special issue – Magnetic anomalies (Tectonophysics)
Magnetic anomaly maps derived from aerogeophysical surveys and satellite data have played a key role in understanding and unraveling the structure of the Earth's lithosphere. The papers presented at the Proceedings of IUGG XXIV General Assembly held at Perugia, Italy in 2007 solicited geopotential studies related to modeling and interpretation of new data sets acquired over continents and oceans. The special issue highlights 11 papers from this proceedings. The first three papers discuss the processing and interpretation of airborne magnetic data acquired over young and active volcanic centres. The next two papers focuses on the study of magnetic anomalies around the Transantarctic Mountains, Antarctica. This is followed by four papers on the geomagnetic studies of the Canadian Cordillera; Beattie Magnetic Anomaly (BMA) anomaly in South Africa; Himalayan and Tibetan plateau and the Bay of Bengal region in India. The final two papers respectively, report on magnetic properties of the layered intrusion in Fennoscandian shield, Norway and the correlation of the aeromagnetic anomalies with faults in NW Turkey. Studies of active volcanoes from ground-based and airborne surveys have led to new insights into the composition in particular within top few kilometers of these volcanoes. The results from magnetic inversion of geopotential data suggests that Iwo-jima volcano is mostly composed of trachy andesite with high magnetization (Kubota et al.). The study of Nakatsuka et al. discusses the generalized mis-tie control method to extract errors statistically from aeromagnetic surveys flown at different elevations and times. The method is used to monitor the changes in the magnetic anomaly field over Asama Volcano in Japan, between 1992 and 2005. Yet another application to combine magnetic surveys flown at different times for better understanding of the subsurface structure and compositional variation of Stromboli Volcano in Italy is nicely demonstrated by Okuma et al. They apply an apparent magnetization intensity mapping method to monitor and estimate the lateral changes in magnetization. An obvious magnetization low seen around the summit craters area of Stromboli Volcano is indicative of demagnetization caused by the heat of conduits and/or hydrothermal activity. A similar study (Okubo et al.) investigating the Sakurajima Volcano in Japan from a low-altitude, high-density helicopterborne survey accounts for variation in magnetization due to volcanic processes. Prominent magnetization lows around recent craters can be interpreted to be the effects by thermal alterations of volcanic rocks. Poorly understood, Antarctica has been a target of numerous magnetic surveys for specific geologic interests. The paper by Zunino et al. focuses on delineating the extent and spatial distribution of 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.10.019
Cenozoic magmatism and older basement of Admiralty Block of the Transantarctic Mountains. They show that the Southern Cross and Admiralty blocks are significantly affected by magmatism compared to adjacent blocks indicating major tectono-magmatism segmentation of the Transantacrtic Mountains rift flank. Ferraccioli et al. show the Wilkes Subglacial Basin in East Antarctica to be much more deeper than previously mapped. Interesting comparative studies between Wilkes Subglacial Basin and Cordillera in North America have also been highlighted. The ability of the magnetic method in characterizing the nature of the source magnetization from data at various altitudes is perhaps best demonstrated in the study conducted by Pilkington et al. over Mackenzie River Magnetic Anomaly (MRA) and northwest Canadian Territories. The high-amplitude, long wavelength anomalies were inferred to be Proterozoic magmatic arc complex along the western edge of the North American craton. On a similar footing, the prominent magnetic low over Himalayan-Tibetan plateau as seen in a high resolution MF6 CHAMP lithospheric field model is indicative of non-magnetic lower and much of the middle crust (Singh and Mitchell). They also suggest that the Indian shield region concealed beneath the thick sediment cover of Gangetic plains is weakly magnetic and it does not extend up to the base of the Himalaya. Rajaram et al. produces a comprehensive analysis of satellite gravity data, marine magnetics, bathymetry and seismic profiles over the Bay of Bengal sedimentary basin. The study concludes that 85° E Ridge does not extend to the north of 15° N latitude and supports the sagging of the crust while rejecting the hotspot trace and magmatic under-plating theories. The paper by Quesnel et al. explains the Beattie Magnetic Anomaly (BMA) in South Africa by fitting simple sheet-like prism bodies with induced magnetisation only. The paper discusses the non-uniqueness in interpreting the geological sources for BMA but supports their model from independent magnetotelluric and seismic experiments. It is critical to understand the history of intrusion, magma evolution and fractional crustallization of magnetic minerals like ilmenite, hematite and magnetite while evaluating crustal magnetic anomalies of a region. The paper by McEnroe et al. best illustrates this discussion as they attribute the variations in aeromagnetic and ground magnetic anomaly maps to the variation in the magnetic properties of the intrusion caused by varied magmatic crystallization of opaque minerals. They investigate intrusive history and magma evolution of Bjerkreim-Sokndal (BKS) norite-quartz mangerite layered intrusion in Fennoscandian shield, Norway. The last paper in this special issue highlights an important application of magnetic methods in investigating seismogenic faults and its correlation with observed
2
Preface
aeromagnetic anomalies in the Marmara region of Turkey (Ateş et al.). Using the standard tools like reduction to the pole (RTP) and second vertical derivative (SVD) authors successfully correlate the fault features with SVD alignments and hypothesize that smaller the length of faults, lesser are the chance of producing strong earthquakes. This work definitely needs more data and verification from old records to support their conjectures. The papers in this special issue highlight the state-of-the-art methodology being utilized to enhance our understanding of complex crustal structure. With new advances in instrumentation, navigation and data processing techniques, the two and three-dimensional magnetization distributions derived from magnetic anomaly maps are being increasingly used to provide constraints on spatial and temporal variability in the physiochemical characteristics of crust. This is demonstrated in the study of volcanoes from airborne surveys. The papers also show a trend to integrate magnetic anomaly data with gravity, seismic, magnetotelluric, geologic and laboratory analysis to
facilitate in interpretation. We thank all geophysicists who presented their work at the IUGG XXIV General Assembly in 2007 and particularly those who contributed to this special issue. Kumar H. Singh GEST/UMBC @ Planetary Geodynamics Lab, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Corresponding author. E-mail address: [email protected]. Shigeo Okuma Geological Survey of Japan, AIST, AIST Tsukuba Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan E-mail address: [email protected]. 22 October 2009