Kohistan Sequence has been considered as island arc formed during the subduction of oceanic lithosphere at the leading edge of northward moving Indian continent.. Sedimentary sequences indicate that formation of the i...Kohistan Sequence has been considered as island arc formed during the subduction of oceanic lithosphere at the leading edge of northward moving Indian continent.. Sedimentary sequences indicate that formation of the intra\|oceanic Kohistan arc began in early Cretaceous time. The isotopic data demonstrate the involvement of enriched, DUPAL type mantle, suggesting that Kohistan arc was formed at or south of the present equator (Khan et al., 1997). The Intra oceanic phase of Kohistan lasted until sometime between 102 and 85 Ma, when Kohistan collided with Asia. From this time until collision with India about 50 Ma ago, Kohistan existed as Andean\|type margin. This paleomagnetic study is from the volcanic and plutonic rocks exposed in Gupis\|Shamran area (west of Gilgit) in northern part of the Kohistan arc. According to geochronological data these rocks were formed 61～55Ma ago (Treloar et al., 1989), when Kohistan was existing as Andean\|type margin. Seven to nine samples were collected from nine sites of Shamran volcanics (58±1)Ma and from five sites of Pingal, Gupis, and Yasin plutons (Ar\|Ar hornblende ages ranges from 61～52Ma). On the basis of one Rb\|Sr age of (59±2)Ma from these plutons, the above\|mentioned Ar/Ar ages may be regarded as reasonable intrusion ages of these plutons (Searle, 1991).展开更多
The Mesozoic—Cenozoic latitudinal displacement amounts of terranes (or blocks) in the Qinghai—Tibet plateau were calculated in paleomagnetism. These terranes (or blocks) include Tarim and Qaidam blocks, East Kunlun,...The Mesozoic—Cenozoic latitudinal displacement amounts of terranes (or blocks) in the Qinghai—Tibet plateau were calculated in paleomagnetism. These terranes (or blocks) include Tarim and Qaidam blocks, East Kunlun, Baryan Har, Qiangtang, Lhasa and Himalaya terranes. The calculated results are listed in table 1. These results show that:(1) There was the latitudinal displacement difference between central area and southwestern area in the Tarim southern margin since the lower Cretaceous. There was a southward latitudinal movement from the beginning of middle Jurassic or upper Jurassic (Zhou Qingjie, 1992). The northward movement amounts of the Tarim northern margin since the Paleocene are greater than that of the Tarim southern margin. Tarim southern margin has moved northward about 1100km since the Paleocene, Tarim northern margin has done about 1700km. Qaidam has moved northward about 3100km since lower\|middle Jurassic. The northward displacement amount of Qaidam since Paleocene is about 810km, near to that of the central area, Tarim southern margin.展开更多
The point at issue:The Kurosegawa Terrane is composed of continental fragments transecting Mesozoic terranes of accretionary complex in Southwest Japan(Fig.1).It is an attenuated tectonic sliver and considered to be a...The point at issue:The Kurosegawa Terrane is composed of continental fragments transecting Mesozoic terranes of accretionary complex in Southwest Japan(Fig.1).It is an attenuated tectonic sliver and considered to be allochthonous with respect to the main part of Southwest Japan.The problem of which continental block in the East Asian continental margin is the source of the Kurosegawa Terrane has puzzled Japanese geologists for many years.Firstly,we try to approach this issue based on the analysis of fusulinacean assemblage in accreted terranes composed of subduction complex in the Pacific Rim.Secondly,by applying the result of this analysis we try to locate the source of the continental fragments of the Kurosegawa Terrane.Thirdly,we try to prove its validity with a new paleomagnetic study.展开更多
Detachment of the sliver-like Cimmerian terrane from eastern Gondwana in the Early Permian triggered mafic volcanism in many parts of the rift zone.To understand this tectonic episode we have carried out paleomagnetic...Detachment of the sliver-like Cimmerian terrane from eastern Gondwana in the Early Permian triggered mafic volcanism in many parts of the rift zone.To understand this tectonic episode we have carried out paleomagnetic investigations on展开更多
As a part of INDEPTH 3/GEODEPTH geological surface investigations were carried out during two field campaigns in 1998 and 1999. The working area covers a roughly N\|S (30～36°N) tren ding, 200km wide belt through...As a part of INDEPTH 3/GEODEPTH geological surface investigations were carried out during two field campaigns in 1998 and 1999. The working area covers a roughly N\|S (30～36°N) tren ding, 200km wide belt through the Lhasa Terrane and Qiangtang Terrane at a longitude of 88～90°E. Field geology was focused on geological mapping, structural measurements, and extensive sampling for the purpose of fission track geochronology. Additionally a total number of 742 orientated palaeomagnetic samples were drilled at 73 sites. Most of the samples were taken in fine\|grained sediments of Cretaceous, Eocene, and Neogene age. Five sites were drilled in acid to intermediate volcanics.A first sequence of measurements has been applied on a set of specimen. Most of the chosen formations (>80%) have acquired stable remanences which are carried by hematite and magnetite. As an example for the characteristic demagnetising behaviour of fine\|grained probable Cretaceous sandstones see Fig.1a. The unblocking temperature of 650℃ allows to identify hematite as remanence carrier.. Although single sites show well\|defined mean directions (e.g. site 76B: α 95 =3 8; k =185 8) the distribution among several site means of one formation could scatter (Fig.1b). It has to be checked carefully if rotations of single crustal elements are responsible for this effect. There is evidence to assume the detected characteristic remanences to be primary—further analysis including fold tests will be performed after the data set has increased.展开更多
The Cimmerian terrane forms an almost unbroken chain stretching 】13,500 km,from central southern Europe to western Indonesia,via SE Europe,the Middle East,Afghanistan,Tibet,SW China and Myanmar.Arguably,it is Earth’...The Cimmerian terrane forms an almost unbroken chain stretching 】13,500 km,from central southern Europe to western Indonesia,via SE Europe,the Middle East,Afghanistan,Tibet,SW China and Myanmar.Arguably,it is Earth’s most spectacular example of a "sliver" terrane,dwarfing in size more recently展开更多
Different attempts have been done to deduce the shortening of the Himalayan belt during the India\|Asia convergence. Dewey et al. (1989) and Le Pichon et al.(1992) calculated an India\|Asia shortening of 2300～2150km ...Different attempts have been done to deduce the shortening of the Himalayan belt during the India\|Asia convergence. Dewey et al. (1989) and Le Pichon et al.(1992) calculated an India\|Asia shortening of 2300～2150km and 2800～3000km in the western and eastern syntaxes, respectively, since the late 45Ma. According to seafloor\|spreading reconstruction, a total shortening of 3000～500km was estimated after the initial contact of the two plates at 55～50Ma (Molnar and Tapponier, 1975 ; Molnar et al., 1988 ; Replumaz, 1999). Since 40Ma, the part of shortening only accommodated by the Himalayan belt was estimated around 470km in the western part (Coward and Butler, 1985) and 550 to 630km to the east (Ratsbacher et al., 1994 ; Replumaz, 1999). In contrast, global plate reconstructions suggest that the shortening in the Himalaya is of about 1250～250km (Achache et al., 1984 ; Powell et al., 1988 ; Dewey et al.,1989 ; Klootwijk et al., 1992 ; Matte et al., 1997). This discrepancy between the amount of shortening estimated by balancing the Himalayan belt and by plate reconstruction favour the existence of a greater India buried up to 1000km north of the present\|day Indus suture zone and subducted before Middle Eocene time (Klootwijk et al., 1979 ; Patriat and Achache, 1984).展开更多
Metasediments from the Tethyan Himalaya (TH) were sampled for paleomagnetic studies in several areas. In this paper, we will present the first results from Carboniferous and Early Triassic marly limestones from Hidden...Metasediments from the Tethyan Himalaya (TH) were sampled for paleomagnetic studies in several areas. In this paper, we will present the first results from Carboniferous and Early Triassic marly limestones from Hidden Valley (Central Nepal).. The paleomagnetic directions reflect a Tertiary overprint probably synchronous with the metamorphism. In this area, the metamorphic conditions reached during Tertiary are poorly constrained. Temperatures are probably in between 300 and 400℃. The age of the thermal event is still debated. No geochronological data is available in this area. Previously published geochronological data from the northern part of TH metasediments in India ranges from 47 to 42Ma (Ar/Ar Illite) after Weissman et al. (1999) and Bonhomme and Garzanti (1991). While in the southern part (close to HHC), biotite Ar/Ar data ranges from 30 to 26Ma in Marsyandi Valley (Coleman and Hodges, 1998) and muscovite Ar/Ar ranges from 18 to 12Ma in the upper Kali Gandaki Valley (Godin et al., 1998).. In this context, the age of the magnetization can′t be defined with precision.展开更多
文摘Kohistan Sequence has been considered as island arc formed during the subduction of oceanic lithosphere at the leading edge of northward moving Indian continent.. Sedimentary sequences indicate that formation of the intra\|oceanic Kohistan arc began in early Cretaceous time. The isotopic data demonstrate the involvement of enriched, DUPAL type mantle, suggesting that Kohistan arc was formed at or south of the present equator (Khan et al., 1997). The Intra oceanic phase of Kohistan lasted until sometime between 102 and 85 Ma, when Kohistan collided with Asia. From this time until collision with India about 50 Ma ago, Kohistan existed as Andean\|type margin. This paleomagnetic study is from the volcanic and plutonic rocks exposed in Gupis\|Shamran area (west of Gilgit) in northern part of the Kohistan arc. According to geochronological data these rocks were formed 61～55Ma ago (Treloar et al., 1989), when Kohistan was existing as Andean\|type margin. Seven to nine samples were collected from nine sites of Shamran volcanics (58±1)Ma and from five sites of Pingal, Gupis, and Yasin plutons (Ar\|Ar hornblende ages ranges from 61～52Ma). On the basis of one Rb\|Sr age of (59±2)Ma from these plutons, the above\|mentioned Ar/Ar ages may be regarded as reasonable intrusion ages of these plutons (Searle, 1991).
文摘The Mesozoic—Cenozoic latitudinal displacement amounts of terranes (or blocks) in the Qinghai—Tibet plateau were calculated in paleomagnetism. These terranes (or blocks) include Tarim and Qaidam blocks, East Kunlun, Baryan Har, Qiangtang, Lhasa and Himalaya terranes. The calculated results are listed in table 1. These results show that:(1) There was the latitudinal displacement difference between central area and southwestern area in the Tarim southern margin since the lower Cretaceous. There was a southward latitudinal movement from the beginning of middle Jurassic or upper Jurassic (Zhou Qingjie, 1992). The northward movement amounts of the Tarim northern margin since the Paleocene are greater than that of the Tarim southern margin. Tarim southern margin has moved northward about 1100km since the Paleocene, Tarim northern margin has done about 1700km. Qaidam has moved northward about 3100km since lower\|middle Jurassic. The northward displacement amount of Qaidam since Paleocene is about 810km, near to that of the central area, Tarim southern margin.
文摘The point at issue:The Kurosegawa Terrane is composed of continental fragments transecting Mesozoic terranes of accretionary complex in Southwest Japan(Fig.1).It is an attenuated tectonic sliver and considered to be allochthonous with respect to the main part of Southwest Japan.The problem of which continental block in the East Asian continental margin is the source of the Kurosegawa Terrane has puzzled Japanese geologists for many years.Firstly,we try to approach this issue based on the analysis of fusulinacean assemblage in accreted terranes composed of subduction complex in the Pacific Rim.Secondly,by applying the result of this analysis we try to locate the source of the continental fragments of the Kurosegawa Terrane.Thirdly,we try to prove its validity with a new paleomagnetic study.
文摘Detachment of the sliver-like Cimmerian terrane from eastern Gondwana in the Early Permian triggered mafic volcanism in many parts of the rift zone.To understand this tectonic episode we have carried out paleomagnetic investigations on
文摘As a part of INDEPTH 3/GEODEPTH geological surface investigations were carried out during two field campaigns in 1998 and 1999. The working area covers a roughly N\|S (30～36°N) tren ding, 200km wide belt through the Lhasa Terrane and Qiangtang Terrane at a longitude of 88～90°E. Field geology was focused on geological mapping, structural measurements, and extensive sampling for the purpose of fission track geochronology. Additionally a total number of 742 orientated palaeomagnetic samples were drilled at 73 sites. Most of the samples were taken in fine\|grained sediments of Cretaceous, Eocene, and Neogene age. Five sites were drilled in acid to intermediate volcanics.A first sequence of measurements has been applied on a set of specimen. Most of the chosen formations (>80%) have acquired stable remanences which are carried by hematite and magnetite. As an example for the characteristic demagnetising behaviour of fine\|grained probable Cretaceous sandstones see Fig.1a. The unblocking temperature of 650℃ allows to identify hematite as remanence carrier.. Although single sites show well\|defined mean directions (e.g. site 76B: α 95 =3 8; k =185 8) the distribution among several site means of one formation could scatter (Fig.1b). It has to be checked carefully if rotations of single crustal elements are responsible for this effect. There is evidence to assume the detected characteristic remanences to be primary—further analysis including fold tests will be performed after the data set has increased.
文摘The Cimmerian terrane forms an almost unbroken chain stretching 】13,500 km,from central southern Europe to western Indonesia,via SE Europe,the Middle East,Afghanistan,Tibet,SW China and Myanmar.Arguably,it is Earth’s most spectacular example of a "sliver" terrane,dwarfing in size more recently
文摘Different attempts have been done to deduce the shortening of the Himalayan belt during the India\|Asia convergence. Dewey et al. (1989) and Le Pichon et al.(1992) calculated an India\|Asia shortening of 2300～2150km and 2800～3000km in the western and eastern syntaxes, respectively, since the late 45Ma. According to seafloor\|spreading reconstruction, a total shortening of 3000～500km was estimated after the initial contact of the two plates at 55～50Ma (Molnar and Tapponier, 1975 ; Molnar et al., 1988 ; Replumaz, 1999). Since 40Ma, the part of shortening only accommodated by the Himalayan belt was estimated around 470km in the western part (Coward and Butler, 1985) and 550 to 630km to the east (Ratsbacher et al., 1994 ; Replumaz, 1999). In contrast, global plate reconstructions suggest that the shortening in the Himalaya is of about 1250～250km (Achache et al., 1984 ; Powell et al., 1988 ; Dewey et al.,1989 ; Klootwijk et al., 1992 ; Matte et al., 1997). This discrepancy between the amount of shortening estimated by balancing the Himalayan belt and by plate reconstruction favour the existence of a greater India buried up to 1000km north of the present\|day Indus suture zone and subducted before Middle Eocene time (Klootwijk et al., 1979 ; Patriat and Achache, 1984).
文摘Metasediments from the Tethyan Himalaya (TH) were sampled for paleomagnetic studies in several areas. In this paper, we will present the first results from Carboniferous and Early Triassic marly limestones from Hidden Valley (Central Nepal).. The paleomagnetic directions reflect a Tertiary overprint probably synchronous with the metamorphism. In this area, the metamorphic conditions reached during Tertiary are poorly constrained. Temperatures are probably in between 300 and 400℃. The age of the thermal event is still debated. No geochronological data is available in this area. Previously published geochronological data from the northern part of TH metasediments in India ranges from 47 to 42Ma (Ar/Ar Illite) after Weissman et al. (1999) and Bonhomme and Garzanti (1991). While in the southern part (close to HHC), biotite Ar/Ar data ranges from 30 to 26Ma in Marsyandi Valley (Coleman and Hodges, 1998) and muscovite Ar/Ar ranges from 18 to 12Ma in the upper Kali Gandaki Valley (Godin et al., 1998).. In this context, the age of the magnetization can′t be defined with precision.
