Crustal thickness variation from Northern Lhasa terrane to Southern Qiangtang terrane revealed by deep seismic reflection data
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摘要:
由多个地体拼接而成的青藏高原，有着世界上最厚的地壳。在高原中部，从拉萨地体到羌塘地体，穿过班公湖-怒江缝合带（BNS）的地壳厚度变化长期存在争议。本文主要论述从拉萨地体北端横跨BNS到达羌塘地体的深地震反射剖面探测的结果。探测发现了清晰的Moho反射，揭示了拉萨地体-羌塘地体Moho深度和地壳厚度的变化。探测结果表明，在BNS下方Moho深度由南至北出现了6.2 km的急剧减小，并且与BNS向北28 km处的羌塘地体南部比较，地壳厚度变浅了12.5 km。否定了前人对BNS下方Moho存在20 km显著变化的认识。
Abstract:The Tibetan Plateau, which is made up of a number of terranes, has the world's most thick crust. There is a long-term dispute concerning crustal thickness across the Bangong-Nujiang suture (BNS) from the Lhasa terrane to the Qiangtang terrane. This paper mainly reports the result of the study of deep seismic reflection profile in this area. A clear reflection of Moho was detected, which reveals the change of crustal thickness from the Lhasa terrane to the Qiangtang terrane. Moho exhibits a sharp decrease of 6.2 km across the BNS and becomes 12.5 km shallower from the northernmost Lhasa terrane to the south Qiangtang terrane at 28 km north of the BNS. The viewpoint based on deep seismic reflection data denies previous understanding of a 20 km offset.
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致谢: 感谢中国石化集团西南石油局第五物探公司的出色的野外工作，以及地科院地质所王海燕研究员、北京派特森科技发展有限公司李兵工程师对于数据处理工作的建议。
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图 1 班公湖怒江缝合带两侧深地震反射剖面及大炮点位置
红色实线示出了地震反射剖面，黑星标记了在BNS两侧激发的药量为1000 kg 的大炮位置，黑色框显示在本文的研究领域。IYS—印度—雅鲁藏布江缝合带；BNS—班公湖怒江缝合带；JS —金沙江缝合带；AKMS—阿尼玛卿—昆仑—慕士塔格缝合带; SQS—南祁连缝合带;KDS—库地缝合带; STDS—藏南拆离系; MCT—主中央冲断裂
Figure 1. Location of the deep seismic reflection profile. BNS-Bangong Nujiang suture. The red solid line shows the seismic reflection profile. The black stars mark the locations of large shots using 1000 kg explosive fired on the both sides of the BNS. The black box shows the study area in this paper.
IYS-Indus-Yarlung Zangbo suture; BNS-Bangong-Nujiang suture; JS-Jinsha River suture; AKMS-Ayimaqin-Kunlun-Mutztagh suture; SQS-South Qilian suture; KDS-Kudi suture; STDS-South Tibet Detachment System; MCT-Main Central Thrust
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图 4 BNS两侧大炮记录振幅分析显示出Moho反射面深度有明显变化
Figure 4. Amplitude analysis for the two large shot gathers across the BNS,showing clear variation of Moho depth
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图 5 跨BNS的莫霍面深度及地壳厚度变化（A）SinoProbe 采集的偏移剖面（B）对偏移剖面的解释结果（C）Hirn 认为跨BNS存在20 km的Moho断距(文献[37]）（D）Haines 认为跨BNS仅存在5 km的Moho断距（文献[33]）
Figure 5. Variation of Moho depth and crustal thickness across BNS. (A) Migration section by SinoProbe (B) Interpretation for the migration section. (C) Hirn’s viewpoint of 20 km Moho offset across BNS (after reference [37]) (D) Haines’viewpoint of 5 km Moho offset across BNS (after reference [33])
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