Rock Exhumation Rates in Motuo Section of Yarlung Tsangpo River: Evidence from Biotite Ar/Ar Chronology
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摘要: 为揭示东喜马拉雅构造结及其周边区域完整地质演化过程，对采集自雅鲁藏布江墨脱段10块基岩样品进行黑云母40Ar/39Ar测年，并利用“Pecube”软件对年龄代表隆升剥露速率进行定量计算.样品黑云母40Ar/39Ar年龄范围为11.25~24.04 Ma，对应隆升剥露速率范围为0.25~0.51 km/Ma.雅鲁藏布江墨脱段地壳隆升剥露速率存在明显南北差异，北段隆升剥露速率高出约0.2 km/Ma.年代学数据及计算结果表明，与东喜马拉雅构造结内部相比，雅鲁藏布江下游墨脱段为地壳隆升剥露活动相对较弱区域.与喜马拉雅地体向拉萨地体俯冲过程相关北西、北西西走向逆断层活动，不仅在东喜马拉雅构造结内部区域发育，在其东侧雅鲁藏布江墨脱段也可能发育.Abstract: To reveal the tectonic evolution of the eastern Himalayan syntaxis and its surrounding areas, in this paper it reports 10 40Ar/39Ar ages of biotite from Motuo Section of the Yarlung Tsangpo River. It quantitatively interprets the rock's uplift and exhumation rates represented by these ages, using a modeling code "Pecube". Biotite 40Ar/39Ar ages in this paper are in the range of 11.25-24.04 Ma. Corresponding exhumation rates are in the range of 0.25-0.51 km/Ma. The rock exhumation rates in Motuo Section of the Yarlung Tsangpo River have obvious differences between northern and southern, which is characterized by the northern exhumation rates about 0.2 km/Ma higher than the southern. The ages and simulation results show that relative to the inside of the eastern Himalayan syntaxis, the exhumation rates in Motuo Section of the Yarlung Tsangpo River are lower. And the NW/NWW-trending thrust fault zones which were resulted by the collision between Himalayan terrane and Lhasa terrane, not only located in the inside of the eastern Himalayan syntaxis, but also probably located in Motuo Section of the Yarlung Tsangpo River.
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Key words:
- eastern Himalayan syntaxis /
- Yarlung Tsangpo River /
- biotite 40Ar/39Ar /
- exhumation /
- thrust fault /
- geochronology
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图 2 雅鲁藏布江墨脱段流域数字地貌及采样位置
Fig. 2. Digital geomorphological map of the Yarlung Tsangpo River Motuo Section and sample locations
图 3 野外露头照片
a.南段雅鲁藏布江主河段河谷样品MT⁃01采集处弱变形花岗岩；b.南段雅鲁藏布江主河段河谷花岗岩中发育北北东走向近直立脆性破裂面；c.北段雅鲁藏布江支流河谷样品MT⁃12采集处片麻岩，片麻理产状为北西西走向近直立；d.北段雅鲁藏布江支流河谷片麻岩中发育北西西走向近直立脆性破裂面；图中白色宽箭头及其内数字代表照片拍摄方向；红色线段及白色长方形内数字代表破裂面位置及产状
Fig. 3. Photographs of field outcrops
图 6 河流地貌形态样品年龄对比
a.样品实测年龄、模拟年龄、最佳隆升剥露速率对比；b.雅鲁藏布江墨脱段河流纵剖面
Fig. 6. River morphology and age contrast
图 7 东喜马拉雅构造结演化模式
a.8 Ma以前水平面大地构造框架示意图；b.8 Ma以前垂向剖面大地构造框架示意图；c.8 Ma以来水平面大地构造框架示意图；d~f.8 Ma以来垂向剖面大地构造框架示意图；HP.含高压麻粒岩透镜体高级变质杂岩；YCF.雅江峡谷逆冲断裂带；NMLF.那木拉逆冲断裂带；ANQF.阿尼桥正断层
Fig. 7. Cartoon figures of the eastern Himalayan syntaxis tectonic evolution
表 1 样品信息及年龄值
Table 1. Sample information and chronology data
样品名 经度 纬度 海拔(m) 岩性 黑云母40Ar/39Ar
坪年龄(Ma)2σ误差(Ma) MSWD值 MT-01 E95.186° N29.254° 681 黑云角闪花岗岩 20.84 0.61 0.36 MT-02 E95.251° N29.278° 727 黑云角闪花岗
闪长岩17.94 0.57 0.17 MT-03 E95.286° N29.302° 764 黑云角闪花岗
闪长岩20.00 1.10 0.11 MT-05 E95.343° N29.351° 750 黑云母闪长岩 20.16 0.33 0.64 MT-06 E95.355° N29.369° 790 花岗闪长岩 24.04 0.62 0.34 MT-09 E95.448° N29.502° 957 片麻岩 11.25 0.31 0.32 MT-10 E95.458° N29.545° 1 177 片麻岩 15.62 0.20 0.10 MT-11 E95.473° N29.594° 1 440 闪长岩 14.35 0.47 0.13 MT-12-1 E95.483° N29.645° 1 859 片麻岩 14.52 0.67 0.13 MT-12-2 E95.483° N29.645° 1 859 片麻岩 13.46 0.45 0.12 
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表 2 样品实测年龄与模拟计算年龄
Table 2. Samples' observed ages and predicted ages
样品名 实测年龄
(Ma)模拟年龄(Ma) 最佳隆升
剥露速率
(km/Ma)VE=0.2
km/MaVE=0.25
km/MaVE=0.3
km/MaVE=0.35
km/MaVE=0.4
km/MaVE=0.45
km/MaVE=0.5
km/MaVE=0.55
km/MaMT-01 20.84 31.57 25.05 20.66 17.51 15.14 13.28 11.80 10.58 0.30 MT-02 17.94 30.29 24.02 19.81 16.78 14.49 12.71 11.28 10.10 0.33 MT-03 20 30.24 23.98 19.77 16.75 14.47 12.69 11.26 10.08 0.30 MT-05 20.16 30.27 24.01 19.79 16.76 14.48 12.70 11.27 10.09 0.30 MT-06 24.04 30.90 24.51 20.21 17.12 14.80 12.98 11.52 10.32 0.25 MT-09 11.25 30.92 24.52 20.22 17.13 14.80 12.98 11.52 10.32 0.51 MT-10 15.62 31.16 24.70 20.37 17.25 14.90 13.06 11.59 10.38 0.40 MT-11 14.35 31.64 25.08 20.67 17.50 15.11 13.24 11.75 10.52 0.42 MT-12-1 14.52 33.61 26.65 21.97 18.60 16.07 14.09 12.50 11.20 0.45 MT-12-2 13.46 33.61 26.65 21.97 18.60 16.07 14.09 12.50 11.20 0.45
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