Nd isotopic data reveal the material and tectonic nature of the Main Central Thrust zone in Nepal Himalaya   总被引：1，自引：0，他引：1  

Takeshi Imayama  Kazunori Arita 《Tectonophysics》2008,451(1-4):265-281

The Main Central Thrust (MCT) is a tectono-metamorphic boundary between the Higher Himalayan crystallines (HHC) and Lesser Himalayan metasediments (LHS), reactivated in the Tertiary, but which had already formed as a collisional boundary in the Early Paleozoic. To investigate the nature of the MCT, we analyzed whole-rock Nd isotopic ratios of rocks from the MCT and surrounding zones in the Taplejung–Ilam area of far-eastern Nepal, Annapurna–Galyang area of central Nepal, and Maikot–Barekot area of western Nepal. We define the MCT zone as a ductile–brittle shear zone between the upper MCT (UMCT) and lower MCT (LMCT). The protoliths of the MCT zone may provide critical constraints on the tectonic evolution of the Himalaya. The LHS is lithostratigraphically divided into the upper and lower units. In the Taplejung–Ilam area, different lithologic units and their ε_Nd (0) values are as follows; HHC (− 10.0 to − 18.1), MCT zone (− 18.5 to − 26.2), upper LHS unit (− 17.2), and lower LHS unit (− 22.0 to − 26.9). There is a distinct gap in the ε_Nd (0) values across the UMCT except for the southern frontal edge of the Ilam nappe. In the Annapurna–Galyang and Maikot–Barekot areas, different lithologic units and their ε_Nd (0) values are as follows; HHC (− 13.9 to − 17.7), MCT zone (− 23.8 to − 26.2 except for an outlier of − 12.4), upper LHS unit (− 15.6 to − 26.8), and lower LHS unit (− 24.9 to − 26.8). These isotopic data clearly distinguish the lower LHS unit from the HHC. Combining these data with the previously published data, the lowest ε_Nd (0) value in the HHC is − 19.9. We regard rocks with ε_Nd (0) values below − 20.0 as the LHS. In contrast, rocks with those above − 19.9 are not always the HHC, and some parts of them may belong to the LHS due to the overlapping Nd isotopic ratio between the HHC and LHS. Most rocks of the MCT zone have Nd isotopic ratios similar to those of the LHS, but very different from those of the HHC. The spatial patterns in the distribution of ε_Nd (0) value around the UMCT suggest no substantial structural mixing of the HHC and LHS during the UMCT activities in the Tertiary. A discontinuity in the spatial distribution of ε_Nd (0) values is laterally continuous along the UMCT throughout the Himalayas. These facts support the theory that the UMCT was originally a material boundary between the HHC and LHS, suggesting the MCT zone was mainly developed with undertaking a role of sliding planes during overthrusting of the HHC in the Tertiary.  相似文献   

TeV γ-rays from old supernova remnants     

Ryo Yamazaki  Kazunori Kohri  Aya Bamba  Tatsuo Yoshida  Toru Tsuribe  Fumio Takahara 《Monthly notices of the Royal Astronomical Society》2006,371(4):1975-1982

We study the emission from an old supernova remnant (SNR) with an age of around 10⁵ yr and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 10⁵ yr, proton acceleration is efficient enough to emit TeV γ-rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV γ-rays and X-rays are dominated by hadronic processes,  π⁰  -decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 10⁵ yr, there are few high-energy particles emitting TeV γ-rays because of the energy-loss effect and/or the wave-damping effect occurring at low-velocity isothermal shocks. For old SNRs or SNR–GMC interacting systems capable of generating TeV γ-ray emitting particles, we calculated the ratio of TeV γ-ray (1–10 TeV) to X-ray (2–10 keV) energy flux and found that it can be more than  ∼10²  . Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources.  相似文献   

Cenozoic lithospheric extension induced magmatism in Southwest Japan     

Kozo Uto  Nguyen Hoang  Kazunori Matsui 《Tectonophysics》2004,393(1-4):281

Island chains off western Kyushu are the surface exposure in the northern margin of the Taiwan–Sinzi Folded Zone that spreads along the arc–trench system in the back-arc side from SW Japan to Taiwan. Intermittent igneous activity between the Middle Miocene and Holocene occurred on these islands and widely covered or intruded sedimentary rocks of Early–Middle Miocene. Geochemistry of the volcanic rocks from the Hirado, Ikitsuki and Takushima islands believed to relate to the back-arc opening along the East China and Japan Seas shows a temporal change in source material. Submarine to sub-aerial volcanism occurred on Hirado Island at 15 Ma during the final opening stage of the East China Sea producing tholeiitic basalt and associated andesite–dacite. These eruptives show low incompatible element contents and high FeO*/MgO ratios and reflect a tholeiitic differentiation trend. High Sr and Pb and low Nd isotopic ratios suggest the involvement of EM2-like lithospheric mantle and crustal material in the formation of these syn-opening volcanic rocks. Post-opening alkali basalt volcanism occurred at 9–6 Ma on the islands is characterized by OIB-like higher large ionic lithophile elements (LILE) and high field strength elements (HFSE) compared to 15 Ma basalts in this region and Quaternary basalts along the volcanic front. They have variable range of incompatible element concentrations and ratios along with variable Sr, Pb and Nd isotopic ratios suggesting the involvement of both lithospheric and asthenospheric sources at variable melting degrees (from 4% to less than 15%). The observation that the isotopic compositions of Quaternary alkali basalts south of the studied area are even more depleted suggests an increase in the involvement of asthenospheric source with time.  相似文献   

A reappraisal of regional surface wave tomography   总被引：1，自引：0，他引：1  

B. L. N. Kennett  K. Yoshizawa 《Geophysical Journal International》2002,150(1):37-44

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LiDAR-derived DEM evaluation of deep-seated landslides in a steep and rocky region of Japan   总被引：1，自引：0，他引：1  

Mio Kasai  Manabu Ikeda  Toshihiro Asahina  Kazunori Fujisawa 《Geomorphology》2009,113(1-2):57

In steep and rocky terrains, their rough surfaces make it difficult to create landslide inventories even with detailed maps/images produced from airborne LiDAR data. To provide objective clues in locating deep-seated landslides, the surface textures of a 5 km² steepland area in Japan was investigated using the eigenvalue ratio and slope filters calculated from a very high resolution LiDAR-derived DEM. The range of filter values was determined for each of a number of surface features mapped in the field and these included: cracked bedrock outcrops, coarse colluvial deposits, gently undulating surfaces, and smooth surfaces. Recently active slides commonly contained patches of ground in which deposition and erosion occurred together near the erosion front, or where cracked bedrock outcrops and coarse colluvial deposits coexisted under a gently undulating surface. The characteristic eigenvalue and slope filter values representing this sliding process were applied to maps of the DEM derived filter values to extract potential sites of recent landslide activity. In addition, the relationships between the filter values of deep-seated landslides at various stages of evolution within the field mapped area were extended to the entire study area, to assess the contribution that landslide evolution makes to change in the landscape as a whole. While landslide components made up the steepest as well as the gentlest parts of the landscape depending on their evolutionary stage, landslides were constantly coarsened and steepened by progressive erosion, probably initiated by river bank erosion at the foot of slopes.  相似文献   

Density Current Descending along Continental Slope and the Associated Deep Water Formation: Two-Dimensional Numerical Experiments with a Nonhydrostatic Model     

Kiyoshi Tanaka  Kazunori Akitomo 《Journal of Oceanography》2000,56(2):117-130

Numerical experiments with a two-dimensional nonhydrostatic ocean model have been carried out to investigate the dynamical process of descending density current on a continental slope. The associated deep water formation has been also examined by tracking labeled particles. The descending flow along the continental slope occurs in the bottom Ekman layer. The net pressure gradient determining the volume transport consists of not only the pressure gradient due to density deviation but also the surface pressure gradient due to the depth-mean alongshore flow. Since these constituents have the opposite signs and strengthen each other, the oscillation with an alternation of intense up- and downslope flows appears around the shelf break. This temporal variation of the flow field causes the effective mixing on the slope between descending shelf and interior waters and forms the deep water as a mixture of them at a ratio of about 1:3. The present result is applied to the slope current around Antarctica, using velocity and density fields calculated by an ocean general circulation model. The Ekman volume transport is estimated at 0.97 Sv (1 Sv = 10⁶ m³s^–1) in the Weddell Sea, 0.35 Sv in the Ross Sea, and 1.8 Sv in total. About 70% of them is attributed to the depth-mean alongshore flow, such as the East Wind Drift and the Weddell Gyre driven by the wind. This suggests that the pressure gradient due to other factors than density deviation may play an important role in the deep and bottom water formation in the actual oceans.  相似文献