チベット中央部のナムツォ湖周辺に分布する段丘から制約した高い粘性を持つ中部地殻  [in Japanese] High viscosity mid crust in central Tibet derived from studies of paleoshorelines around Lake Nam Co  [in Japanese]

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チベット高地に点在する湖の周囲には数多くの湖段丘が分布する.湖の水位が下がると地殻を隆起させる浮力が生じる.隆起が起こるために中部地殻における岩石の流動も必要である.流動のタイムスケールは、湖の盆地の幾何学と中部地殻の粘性で決まる.盆地の形は既知であるので,粘性は段丘の隆起量及び隆起のかかった時間から導出できる. 段丘の隆起量は湖の中央が最大であり,一般に同じ段丘は場所によって中央からの距離が異なる.よって,湖の地盤の隆起が起こると段丘は緩やかに傾斜することになる.チベットで最大級の湖であるナムツォ湖周辺の段丘の高さをキネマティックGPSによる精密測量により決定し,段丘に付着したトゥファの14C年代測定を行った.その結果,ナムツォ地域の中部地殻は10^20 Pa s以上の粘性を持つという制約が得られた.一般的に予想されるチベット中部地殻における低粘性層の存在を再検討する必要が出てきた.

Many lakes in Tibet have shorelines developed around them. When water level decreases, a buoyancy force tends to cause uplift. For uplift to occur, flow in the mid crust is required. The time scale for the flow is related to the basin geometry and mid crustal viscosity. The basin shape is known, so the viscosity can be calculated from the amount of uplift and the time taken. Uplift is should be maximum at the lake center and shorelines at different distances from the center along their length will develop a slope as a result of uplift. Heights of shorelines were determined with kinematic GPS around L. Nam Co, one of the largest lakes in Tibet. These were combined with 14C ages of tufa deposits attached to the shorelines to give an estimate of >10^20 Pa s for the viscosity of the mid crust of the area. This value is much higher than that generally considered and suggests that the idea of a weak partially molten mid crust in central Tibet needs revising.

Journal

  • Abstracts for Annual Meeting of Japan Association of Mineralogical Sciences

    Abstracts for Annual Meeting of Japan Association of Mineralogical Sciences 2011(0), 40-40, 2011

    Japan Association of Mineralogical Sciences

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