The three components and total intensity of the geomagnetic field were measured using STCM and a ... more The three components and total intensity of the geomagnetic field were measured using STCM and a proton precession magnetometer during the DELP 1987 Ogasawara cruises. The total intensity anomalies are generally negative and small in amplitude in the trough, their lineation pattern can hardly be defined. However, the anomaly patterns of the three vector components of the geomagnetic field show widely distributed lineation features trending north-south over the trough. Two total intensity negative peaks in the northern and southern ends of the trough are located at around the same positions as the free air gravity negative anomaly peaks. 1987年秋のDELP航海により小笠原トラフ周辺海域において地磁気連続測定を行った.測定に於いては船舶用フラックスゲート地磁気三成分測定系ならびに曳航型プロトン磁力計により観測を行ったが,後者については航海中船上測定系に異常が出たことなどもあって完全な三成分対全磁力比較検討の材料とは成らなかったものの,全磁力についてはこの海域に於て地質調査所などから多ぐのデータが提供されているので,解析に支障は無いと思われる.船舶上での三成分磁力測定はかねてから神戸大学理学部により信頼度の高いデータが得られる様に成ってきたが,本観測地域の例にもみられるように低緯度でかつ海底の基盤もまた低緯度で発生したと予測される地域の地磁気観測には三成分磁力計は必要不可欠なものである.この報告にも,従来より明瞭な地磁気縞模様の...
Paleomagnetic samples of Paleocene -Eocene red sandstones were collected at 36 sites from the Jia... more Paleomagnetic samples of Paleocene -Eocene red sandstones were collected at 36 sites from the Jiangdihe-4 and Zhaojiadian formations around the Yongren (26.1jN, 101.7jE) and Dayao areas (25.7jN, 101.3jE). These areas are located in the Chuxiong basin of the Chuan Dian Fragment, southwestern part of the Yangtze block. After stepwise thermal demagnetization, a hightemperature component with unblocking temperature of about 680 jC is isolated from 26 sites. The primary nature of this magnetization is ascertained through positive fold and reversal tests at 95% confidence level. The tilt-corrected mean paleomagnetic directions for the Yongren and Dayao areas are D = 17.2j, I = 26.6j with a 95 = 5.8j and D = 16.5j, I = 31.1j with a 95 = 4.8, respectively. Easterly deflected declinations from this study are consistent with those reported from other areas of the Chuxiong basin, indicating its wide presence in the Cretaceous -Eocene formations of the said basin. Comparison with declination values expected from the Cretaceous -Eocene APWP of Eurasia indicates that the magnitude of clockwise rotation systematically increases toward the southeast within the Chuxiong basin as well as in the Chuan Dian Fragment. This trend of the differential tectonic rotation in the Chuan Dian Fragment is consistent with curvature of the Xianshuihe -Xiojiang fault system. Deformation of the Chuxiong basin can fairly be associated with the formation of eastward bulge in the southern part of the Chuan Dian fragment. During southward displacement, the Chuan Dian Fragment was probably subjected to tectonic stresses as a result interaction with the Yangtze and Indochina blocks, which resulted into east -west extension and north -south shortening. D
Physics of the Earth and Planetary Interiors, 1998
We collected more than 250 samples from 45 sites in lava flows of the three volcanoes on Easter I... more We collected more than 250 samples from 45 sites in lava flows of the three volcanoes on Easter Island. The K–Ar whole rock dating was attempted on 13 sites. All of the ages obtained are younger than 0.8 Ma. Reliable paleomagnetic directions were obtained from 34 sites. The mean VGP position is 89.2°N and 239.1°E with a 95% confidence value of 3.8°. The angular dispersion value of Easter Island VGPs was calculated to be 11.8° with an upper limit of 14.1° and a lower limit of 9.8°. The angular dispersion value is significantly lower than the values predicted by the theoretical secular variation models. The pacific dipole window appears to exist also in the southeast Pacific.
Distinctive fault ruptures, the Nojima Fault and Ogura Fault, appeared along the northwestern coa... more Distinctive fault ruptures, the Nojima Fault and Ogura Fault, appeared along the northwestern coast of Awaji Island at the time of the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake). In order to delineate the shallow resistivity structures around the faults just after they formed, Very Low Frequency Magnetotelluric (VLF-MT) surveys were made at five sites along the Nojima Fault and at one site along the Ogura Fault. Fourteen transects were made at the one site on the Ogura Fault, and another transect covers the area between the two faults. Changes in apparent resistivity or phase, or both, commonly occur when crossing the surface location of one of the faults, except for the northern transects at OGR-0 on the Ogura Fault. Apparent resistivity values of less than 100 Wm were observed for Tertiary and Quaternary sediments and values larger than 200 Wm for granitic rocks. The resistivity structures are related to the morphological characteristics of the fault ruptures. Remarkably conductive zones (less than 10 Wm in apparent resistivity and 30-40 m in width) were found where the surface displacement is distinct and prominent along a single fault plane. If remarkably conductive zones were formed at the time of the 1995 Hyogo-ken Nanbu earthquake, the results provide a good constraint on the dimensions of a conductive zone near the surface that was made by one earthquake. Alternatively, if characteristic resistivity structures existed prior to the earthquake, the conductive zone was probably formed by some tens of earthquakes in relatively modern times. In this case, this phenomenon is inferred to be a concentration of fracturing in a narrow zone and is associated with the formation of clay minerals, which enhance rock conductivity.
In order to describe features of tectonic deformation in the Indochina Peninsula, Early Jurassic ... more In order to describe features of tectonic deformation in the Indochina Peninsula, Early Jurassic to Early Cretaceous red sandstones were sampled at three localities in the Shan-Thai and Indochina blocks. Stepwise thermal treatment of most samples revealed the presence of characteristic remanent magnetization, which is generally unblocked by 680 • C. This component from Phong Saly (21.6 • N, 101.9 • E) and Borikhanxay 18.5 • N, 103.8 • E) localities yield positive fold tests with Late Jurassic-Early Cretaceous directions of Dec/Inc = 28.8 • /32.1 • (k s = 15.4, α 95 = 8.8 • , N = 22) and Dec/Inc = 42.1 • /46.9 • (k s = 20.1, α 95 = 7.9 • , N = 18), respectively. Additionally, a syn-folding mid-Cretaceous characteristic magnetization is observed in the samples of Muang Phin locality (16.5 • N, 106.1 • E), which gave a mean direction of Dec/Inc = 30.8 • /39.9 • , k = 102.6, α 95 = 3.0 • , N = 23. This reliable Late Jurassic to Mid-Cretaceous palaeomagnetic directions from three different localities are incorporated into a palaeomagnetic database for Shan-Thai and Indochina blocks. Based on these compilations, tectonic deformation of the Shan-Thai and Indochina blocks is summarized as follows: (1) the Shan-Thai and Indochina blocks experienced a clockwise rotation of about 10 • as a composite unit in the early stage of India-Asia collision and following this, the Shan-Thai Block underwent an internal tectonic deformation, whereas the Indochina Block behaved as a rigid tectonic unit during the same period. Comparison of our palaeomagnetic results with seismic tomographic images suggests that the strength of continental lithosphere beneath these blocks played an important role in the process of deformation rather than any other tectonic regime. In contrast to the Shan-Thai Block, an existence of continental roots beneath the Indochina Block prevented its internal deformation.
We present palaeomagnetic results from Lower Cretaceous rocks in Bikin area of the Alchan basin (... more We present palaeomagnetic results from Lower Cretaceous rocks in Bikin area of the Alchan basin (46.5 • N, 134.7 • E), Sikhote Alin orogenic belt, Far Eastern Russia. A high-temperature magnetization component with maximum unblocking temperatures at about 590 • C was isolated from six sites of dacite welded tuffs in the Albian Alchanskaya Formation. The fold tests for these six sites are positive, suggesting that primary magnetization is preserved in the studied rocks. The tilt-corrected mean direction of D = 309.3 • , I = 68.7 • (α 95 = 10.1 • ), with a corresponding palaeopole position at 57.0 • N, 76.8 • E (A 95 = 15.1 • ), indicates a counter-clockwise (CCW) rotation for the studied area. CCW rotation is also indicated from west-directed declinations (D = 249.1 • , I = 64.1 • , α 95 = 11.2 • ) obtained from secondary magnetization of the Berriasian Kultukha Formation. Combining with the previously reported studies, the westdirected Cretaceous palaeomagnetic directions cover widely the eastern part of the Mongolia block. Comparison with 100 Ma palaeomagnetic pole for Eurasia shows that the eastern part of the Mongolia block experienced a CCW rotation of over 36 • with respect to the Eurasian continent later than Late Cretaceous. This rotation is ascribed to post-Late Cretaceous extension that affected the continental basins (the Middle Amur, Sanjiang, Razdolnian, Amur-Zeya and Songliao basins) of the northeast Chinese Plain along the eastern margin of the Mongolia block. Contemporaneous with this CCW rotation, similar extension resulted in clockwise rotation of the eastern part of the North China block.
An audio-frequency magnetotelluric (AMT) survey was undertaken at ten sites along a transect acro... more An audio-frequency magnetotelluric (AMT) survey was undertaken at ten sites along a transect across the Hijima fault, a major segment of the Yamasaki fault system, Japan. The data were subjected to dimensionality analysis, following which two-dimensional inversions for the TE and TM modes were carried out. This model is characterized by (1) a clear resistivity boundary that coincides with the downward projection of the surface trace of the Hijima fault, (2) a resistive zone (>500 m) that corresponds to Mesozoic sediment, and (3) shallow and deep two highly conductive zones (30-40 m) along the fault. The shallow conductive zone is a common feature of the Yamasaki fault system, whereas the deep conductor is a newly discovered feature at depths of 800-1,800 m to the southwest of the fault. The conductor is truncated by the Hijima fault to the northeast, and its upper boundary is the resistive zone. Both conductors are interpreted to represent a combination of clay minerals and a fluid network within a fault-related fracture zone. In terms of the development of the fluid networks, the fault core of the Hijima fault and the highly resistive zone may play important roles as barriers to fluid flow on the northeast and upper sides of the conductive zones, respectively.
We present paleomagnetic results from Middle Jurassic red sandstones of the Sangba Formation. Ori... more We present paleomagnetic results from Middle Jurassic red sandstones of the Sangba Formation. Orientated samples were collected at 14 sites on north side of the Basu (30.1°N, 96.9°E), northeastern border of the Lhasa Block. After stepwise thermal demagnetization, high-temperature component with unblocking level of 680°C is isolated from 7 sites. Primary nature of this magnetization is ascertained through positive fold and reversal tests at 95% confidence level. Combined with previously reported paleomagnetic results, tilt-corrected mean direction of D =353.4°, I =15.1°, α 95 =14.1°, N = 8 is calculated, indicating that during Middle Jurassic the study area was located at 7.7°N ± 7.4°. Comparison with the Jurassic paleomagnetic data reported from eastern Tibet indicates that the Lhasa Block was located south of the Qiangtang Block at a latitudinal difference of 31 ± 11°. Compilation of paleomagnetic data further suggest that until the Cretaceous large spatial gap of more than 1200 km remained open between the Lhasa Block and the Qiangtang Block of eastern Himalayan syntaxis and eventually vanished during Tertiary. Cretaceous paleolatitude corresponding to this gap (13.0°N-31.0°N) is almost identical to that reported from Shan-Thai Block (25.4°N ± 5.2°). Furthermore, the Permo-Carboniferous sequences of Gondwanic affinity are distributed in western part of the Shan-Thai block as well as in the Lhasa and Qiangtang blocks. Keeping in view these similarities, we propose a new pre-collision (India vs. Asia) reconstruction model in which the Lhasa, Shan-Thai and Qiangtang blocks are shown in their respective order from south to north along southern margin of the Asian continent. The aftermath of gigantic India-Asia collision however introduced north-south compressional regime in the Asian Continent that eventually played a role in eastward extrusion of the Shan-Thai Block from an area between the Qiangtang and Lhasa blocks.
The three components and total intensity of the geomagnetic field were measured using STCM and a ... more The three components and total intensity of the geomagnetic field were measured using STCM and a proton precession magnetometer during the DELP 1987 Ogasawara cruises. The total intensity anomalies are generally negative and small in amplitude in the trough, their lineation pattern can hardly be defined. However, the anomaly patterns of the three vector components of the geomagnetic field show widely distributed lineation features trending north-south over the trough. Two total intensity negative peaks in the northern and southern ends of the trough are located at around the same positions as the free air gravity negative anomaly peaks. 1987年秋のDELP航海により小笠原トラフ周辺海域において地磁気連続測定を行った.測定に於いては船舶用フラックスゲート地磁気三成分測定系ならびに曳航型プロトン磁力計により観測を行ったが,後者については航海中船上測定系に異常が出たことなどもあって完全な三成分対全磁力比較検討の材料とは成らなかったものの,全磁力についてはこの海域に於て地質調査所などから多ぐのデータが提供されているので,解析に支障は無いと思われる.船舶上での三成分磁力測定はかねてから神戸大学理学部により信頼度の高いデータが得られる様に成ってきたが,本観測地域の例にもみられるように低緯度でかつ海底の基盤もまた低緯度で発生したと予測される地域の地磁気観測には三成分磁力計は必要不可欠なものである.この報告にも,従来より明瞭な地磁気縞模様の...
Paleomagnetic samples of Paleocene -Eocene red sandstones were collected at 36 sites from the Jia... more Paleomagnetic samples of Paleocene -Eocene red sandstones were collected at 36 sites from the Jiangdihe-4 and Zhaojiadian formations around the Yongren (26.1jN, 101.7jE) and Dayao areas (25.7jN, 101.3jE). These areas are located in the Chuxiong basin of the Chuan Dian Fragment, southwestern part of the Yangtze block. After stepwise thermal demagnetization, a hightemperature component with unblocking temperature of about 680 jC is isolated from 26 sites. The primary nature of this magnetization is ascertained through positive fold and reversal tests at 95% confidence level. The tilt-corrected mean paleomagnetic directions for the Yongren and Dayao areas are D = 17.2j, I = 26.6j with a 95 = 5.8j and D = 16.5j, I = 31.1j with a 95 = 4.8, respectively. Easterly deflected declinations from this study are consistent with those reported from other areas of the Chuxiong basin, indicating its wide presence in the Cretaceous -Eocene formations of the said basin. Comparison with declination values expected from the Cretaceous -Eocene APWP of Eurasia indicates that the magnitude of clockwise rotation systematically increases toward the southeast within the Chuxiong basin as well as in the Chuan Dian Fragment. This trend of the differential tectonic rotation in the Chuan Dian Fragment is consistent with curvature of the Xianshuihe -Xiojiang fault system. Deformation of the Chuxiong basin can fairly be associated with the formation of eastward bulge in the southern part of the Chuan Dian fragment. During southward displacement, the Chuan Dian Fragment was probably subjected to tectonic stresses as a result interaction with the Yangtze and Indochina blocks, which resulted into east -west extension and north -south shortening. D
Physics of the Earth and Planetary Interiors, 1998
We collected more than 250 samples from 45 sites in lava flows of the three volcanoes on Easter I... more We collected more than 250 samples from 45 sites in lava flows of the three volcanoes on Easter Island. The K–Ar whole rock dating was attempted on 13 sites. All of the ages obtained are younger than 0.8 Ma. Reliable paleomagnetic directions were obtained from 34 sites. The mean VGP position is 89.2°N and 239.1°E with a 95% confidence value of 3.8°. The angular dispersion value of Easter Island VGPs was calculated to be 11.8° with an upper limit of 14.1° and a lower limit of 9.8°. The angular dispersion value is significantly lower than the values predicted by the theoretical secular variation models. The pacific dipole window appears to exist also in the southeast Pacific.
Distinctive fault ruptures, the Nojima Fault and Ogura Fault, appeared along the northwestern coa... more Distinctive fault ruptures, the Nojima Fault and Ogura Fault, appeared along the northwestern coast of Awaji Island at the time of the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake). In order to delineate the shallow resistivity structures around the faults just after they formed, Very Low Frequency Magnetotelluric (VLF-MT) surveys were made at five sites along the Nojima Fault and at one site along the Ogura Fault. Fourteen transects were made at the one site on the Ogura Fault, and another transect covers the area between the two faults. Changes in apparent resistivity or phase, or both, commonly occur when crossing the surface location of one of the faults, except for the northern transects at OGR-0 on the Ogura Fault. Apparent resistivity values of less than 100 Wm were observed for Tertiary and Quaternary sediments and values larger than 200 Wm for granitic rocks. The resistivity structures are related to the morphological characteristics of the fault ruptures. Remarkably conductive zones (less than 10 Wm in apparent resistivity and 30-40 m in width) were found where the surface displacement is distinct and prominent along a single fault plane. If remarkably conductive zones were formed at the time of the 1995 Hyogo-ken Nanbu earthquake, the results provide a good constraint on the dimensions of a conductive zone near the surface that was made by one earthquake. Alternatively, if characteristic resistivity structures existed prior to the earthquake, the conductive zone was probably formed by some tens of earthquakes in relatively modern times. In this case, this phenomenon is inferred to be a concentration of fracturing in a narrow zone and is associated with the formation of clay minerals, which enhance rock conductivity.
In order to describe features of tectonic deformation in the Indochina Peninsula, Early Jurassic ... more In order to describe features of tectonic deformation in the Indochina Peninsula, Early Jurassic to Early Cretaceous red sandstones were sampled at three localities in the Shan-Thai and Indochina blocks. Stepwise thermal treatment of most samples revealed the presence of characteristic remanent magnetization, which is generally unblocked by 680 • C. This component from Phong Saly (21.6 • N, 101.9 • E) and Borikhanxay 18.5 • N, 103.8 • E) localities yield positive fold tests with Late Jurassic-Early Cretaceous directions of Dec/Inc = 28.8 • /32.1 • (k s = 15.4, α 95 = 8.8 • , N = 22) and Dec/Inc = 42.1 • /46.9 • (k s = 20.1, α 95 = 7.9 • , N = 18), respectively. Additionally, a syn-folding mid-Cretaceous characteristic magnetization is observed in the samples of Muang Phin locality (16.5 • N, 106.1 • E), which gave a mean direction of Dec/Inc = 30.8 • /39.9 • , k = 102.6, α 95 = 3.0 • , N = 23. This reliable Late Jurassic to Mid-Cretaceous palaeomagnetic directions from three different localities are incorporated into a palaeomagnetic database for Shan-Thai and Indochina blocks. Based on these compilations, tectonic deformation of the Shan-Thai and Indochina blocks is summarized as follows: (1) the Shan-Thai and Indochina blocks experienced a clockwise rotation of about 10 • as a composite unit in the early stage of India-Asia collision and following this, the Shan-Thai Block underwent an internal tectonic deformation, whereas the Indochina Block behaved as a rigid tectonic unit during the same period. Comparison of our palaeomagnetic results with seismic tomographic images suggests that the strength of continental lithosphere beneath these blocks played an important role in the process of deformation rather than any other tectonic regime. In contrast to the Shan-Thai Block, an existence of continental roots beneath the Indochina Block prevented its internal deformation.
We present palaeomagnetic results from Lower Cretaceous rocks in Bikin area of the Alchan basin (... more We present palaeomagnetic results from Lower Cretaceous rocks in Bikin area of the Alchan basin (46.5 • N, 134.7 • E), Sikhote Alin orogenic belt, Far Eastern Russia. A high-temperature magnetization component with maximum unblocking temperatures at about 590 • C was isolated from six sites of dacite welded tuffs in the Albian Alchanskaya Formation. The fold tests for these six sites are positive, suggesting that primary magnetization is preserved in the studied rocks. The tilt-corrected mean direction of D = 309.3 • , I = 68.7 • (α 95 = 10.1 • ), with a corresponding palaeopole position at 57.0 • N, 76.8 • E (A 95 = 15.1 • ), indicates a counter-clockwise (CCW) rotation for the studied area. CCW rotation is also indicated from west-directed declinations (D = 249.1 • , I = 64.1 • , α 95 = 11.2 • ) obtained from secondary magnetization of the Berriasian Kultukha Formation. Combining with the previously reported studies, the westdirected Cretaceous palaeomagnetic directions cover widely the eastern part of the Mongolia block. Comparison with 100 Ma palaeomagnetic pole for Eurasia shows that the eastern part of the Mongolia block experienced a CCW rotation of over 36 • with respect to the Eurasian continent later than Late Cretaceous. This rotation is ascribed to post-Late Cretaceous extension that affected the continental basins (the Middle Amur, Sanjiang, Razdolnian, Amur-Zeya and Songliao basins) of the northeast Chinese Plain along the eastern margin of the Mongolia block. Contemporaneous with this CCW rotation, similar extension resulted in clockwise rotation of the eastern part of the North China block.
An audio-frequency magnetotelluric (AMT) survey was undertaken at ten sites along a transect acro... more An audio-frequency magnetotelluric (AMT) survey was undertaken at ten sites along a transect across the Hijima fault, a major segment of the Yamasaki fault system, Japan. The data were subjected to dimensionality analysis, following which two-dimensional inversions for the TE and TM modes were carried out. This model is characterized by (1) a clear resistivity boundary that coincides with the downward projection of the surface trace of the Hijima fault, (2) a resistive zone (>500 m) that corresponds to Mesozoic sediment, and (3) shallow and deep two highly conductive zones (30-40 m) along the fault. The shallow conductive zone is a common feature of the Yamasaki fault system, whereas the deep conductor is a newly discovered feature at depths of 800-1,800 m to the southwest of the fault. The conductor is truncated by the Hijima fault to the northeast, and its upper boundary is the resistive zone. Both conductors are interpreted to represent a combination of clay minerals and a fluid network within a fault-related fracture zone. In terms of the development of the fluid networks, the fault core of the Hijima fault and the highly resistive zone may play important roles as barriers to fluid flow on the northeast and upper sides of the conductive zones, respectively.
We present paleomagnetic results from Middle Jurassic red sandstones of the Sangba Formation. Ori... more We present paleomagnetic results from Middle Jurassic red sandstones of the Sangba Formation. Orientated samples were collected at 14 sites on north side of the Basu (30.1°N, 96.9°E), northeastern border of the Lhasa Block. After stepwise thermal demagnetization, high-temperature component with unblocking level of 680°C is isolated from 7 sites. Primary nature of this magnetization is ascertained through positive fold and reversal tests at 95% confidence level. Combined with previously reported paleomagnetic results, tilt-corrected mean direction of D =353.4°, I =15.1°, α 95 =14.1°, N = 8 is calculated, indicating that during Middle Jurassic the study area was located at 7.7°N ± 7.4°. Comparison with the Jurassic paleomagnetic data reported from eastern Tibet indicates that the Lhasa Block was located south of the Qiangtang Block at a latitudinal difference of 31 ± 11°. Compilation of paleomagnetic data further suggest that until the Cretaceous large spatial gap of more than 1200 km remained open between the Lhasa Block and the Qiangtang Block of eastern Himalayan syntaxis and eventually vanished during Tertiary. Cretaceous paleolatitude corresponding to this gap (13.0°N-31.0°N) is almost identical to that reported from Shan-Thai Block (25.4°N ± 5.2°). Furthermore, the Permo-Carboniferous sequences of Gondwanic affinity are distributed in western part of the Shan-Thai block as well as in the Lhasa and Qiangtang blocks. Keeping in view these similarities, we propose a new pre-collision (India vs. Asia) reconstruction model in which the Lhasa, Shan-Thai and Qiangtang blocks are shown in their respective order from south to north along southern margin of the Asian continent. The aftermath of gigantic India-Asia collision however introduced north-south compressional regime in the Asian Continent that eventually played a role in eastward extrusion of the Shan-Thai Block from an area between the Qiangtang and Lhasa blocks.
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Papers by Hiroo Inokuchi