The strong activities of the Namula fault zone in the eastern Himalayan syntaxis since Pliocene, constraints from thermochronological data
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摘要: 为揭示东喜马拉雅构造结那木拉断裂带上新世以来强烈活动特征，对采集自那木拉断裂带的三件基岩样品进行黑云母40Ar/39Ar、磷灰石裂变径迹两种热年代学方法测年；并利用"Pecube"软件对测得年龄数据及断裂带两侧已发表年龄数据进行定量模拟计算。测试结果显示黑云母40Ar/39Ar年龄范围为4.44±0.71 Ma~3.45±0.24 Ma，磷灰石裂变径迹年龄范围为3.7±0.4 Ma~1.8±0.2 Ma。年龄数据及其模拟计算结果表明，约3 Ma以前那木拉断裂带南侧地壳隆升最快，隆升速率约2.5 km/Ma，断裂带以正断层运动特征为主；约3 Ma以来那木拉断裂带北侧地壳隆升最快，约为1.3 km/Ma，断裂带以逆断层运动特征为主。那木拉断裂带运动特征变化可能与约8 Ma以来东喜马拉雅构造结快速地壳隆升剥露区域由南向北逐渐迁移有关。
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关键词:
- 那木拉断裂带 /
- 东喜马拉雅构造结 /
- 黑云母40Ar/39Ar /
- 磷灰石裂变径迹
Abstract: To reveal the strong activities of the Namula fault zone in the eastern Himalayan syntaxis since Pliocene, this paper reports biotite 40Ar/39Ar and apatite fission track ages of 3 rock samples from the Namula fault zone, and quantitatively interpret these ages and the published ages from the adjacent areas, using a modeling code "Pecube". Biotite 40Ar/39Ar ages range from 4.44±0.71 Ma to 3.45±0.24 Ma, and apatite fission track ages range from 3.7±0.4 Ma to 1.8±0.2 Ma. The ages and simulation results show that, prior to ~3 Ma the highest exhumation rate had been located in the south of the Namula fault zone and was about 2.5 km/Ma, and the Namula fault zone had a motion feature of normal fault; Since ~3 Ma the highest exhumation rate have been located in the north of the Namula fault zone and was about 1.3 km/Ma, and the Namula fault zone had a motion feature of thrust fault. The evolution of the Namula fault zone's motion, probably have resulted from the south to north migration of the fast crust exhumation area in the eastern Himalayan syntaxis since ~8 Ma. -
图 1 东喜马拉雅构造结大地构造位置及构造地质简图
a—青藏高原印度板块大地构造框架图；b—东喜马拉雅构造结地质简图
Figure 1. Generalized tectonic and simplified geologic map of the eastern Himalayan syntaxis. (a) Tectonic diagram of the Tibet and India plates. (b) Geological sketch of the eastern Himalayan syntaxis
图 2 那木拉断裂带区域数字地貌及样品位置图
Figure 2. Digital geomorphological map of the Namula fault zone and sample locations
图 4 那木拉断裂带不同区域热史演化曲线图
正方形—黑云母40Ar/39Ar年龄；圆形—磷灰石裂变径迹年龄；带箭头虚线—热史演化曲线
Figure 4. Thermal history evolution curves in different positions of the Namula fault zone
Squares-Biotite 40Ar/39Ar ages; Rounds-Apatite fission track ages; Dotted lines with arrows-Thermal history evolution curves
图 5 那木拉断裂带热年代学年龄模拟解译结果
a—大于3 Ma时地壳隆升剥露速率分布；b—小于3 Ma时地壳隆升剥露速率分布；c—模拟磷灰石裂变径迹年龄；d—模拟黑云母40Ar/39Ar年龄
Figure 5. Thermochronology simulation results of the Namula fault zone. (a) Crust exhumation rate distribution prior to 3 Ma. (b) Crust exhumation rate distribution after 3 Ma. (c) Predicted apatite fission track ages. (d) Predicted biotite 40Ar/39Ar ages.
图 6 那木拉断裂带实测年龄与模拟年龄对比图
a—黑云母40Ar/39Ar实测结果与模拟结果对比；b—磷灰石裂变径迹年龄实测结果与模拟结果对比
Figure 6. Contrast between observed ages and predicted ages in the Namula fault zone. (a) Predicted and observed biotite 40Ar/39Ar ages. (b) Predicted and observed apatite fission track ages
图 7 那木拉断裂带演化模式图
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—8~3 Ma水平面大地构造框架示意图；d—8~3 Ma垂向剖面大地构造框架示意图；e—3~0 Ma水平面大地构造框架示意图；f—3~0 Ma垂向剖面大地构造框架示意图
Figure 7. Cartoon figures of the Namula fault zone evolution. (a) Horizontal tectonic framework prior to 8 Ma. (b) Vertical profile tectonic framework prior to 8 Ma. (c) Horizontal tectonic framework between 8 Ma and 3 Ma. (d) Vertical profile tectonic framework between 8 Ma and 3 Ma. (e) Horizontal tectonic framework between 3 Ma and 0 Ma. (f) Vertical profile tectonic framework between 3 Ma and 0 Ma.
HP-High-grade metamorphic complex containing high-pressure granulite; NMLF-the Namula trust fault zone; Purple dotted ellipses-Rapid exhumation areas; Red triangles-High-angle thrust faults; Red rounds-High-angle normal faults
表 1 样品信息及年龄结果
Table 1. Sample information and chronology data
样品编号 高程/m 经度E/(°) 纬度N/(°) 岩性 黑云母40Ar/39Ar测试结果 坪年龄/Ma 误差(2σ) MSWD 11DXG-13 4991 94.981 29.551 片麻岩 3.45 0.24 0.65 11DXG-15 4626 94.975 29.553 片麻岩 4.44 0.71 0.11 11DXG-16 4458 94.971 29.554 片麻岩 4.19 0.44 0.3 样品编号 磷灰石裂变径迹测试结果 测试颗粒 ρs (×105 cm-2)
(Ns)ρi (×105 cm-2)
(Ni)ρd (×105 cm-2)
(Nd)Pχ2/% 中值年龄/Ma 误差(1σ) 11DXG-13 34 0.266(93) 32.80(11455) 14.18(8264) 99.96 2.3 0.3 11DXG-15 53 0.286(149) 19.82(10326) 14.00(8264) 99.64 3.7 0.4 11DXG-16 65 0.367(314) 56.02(47891) 13.83(8264) 99.57 1.8 0.2 ρs为自发径迹密度，ρi为诱发径迹密度，ρd为标准径迹密度，Ns为自发径迹数，Ni为诱发径迹数，Nd为标准径迹数，Pχ2为检验概率；Zeta=393.5±27.5 
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表 2 那木拉断裂带区域热年代学年龄统计结果
Table 2. Thermochronology data from the Namula fault zone area
样品编号 经度(E) 纬度(N) 高程/m 黑云母40Ar/39Ar年龄/Ma 磷灰石裂变径迹年龄/Ma 数据来源 坪年龄 2σ误差 中值年龄 1σ误差 DXL-01 94.948° 29.488° 4233 4.5 0.9 3.6 0.4 Gong et al., 2015；Yu et al., 2011 DXL-02 94.956° 29.490° 4073 3.5 0.2 3.9 0.6 Gong et al., 2015；Yu et al., 2011 DXL-03 94.956° 29.486° 4038 3.1 0.2 4.6 0.6 Gong et al., 2015；Yu et al., 2011 11DXG-16 94.971° 29.554° 4458 4.19 0.44 1.8 0.2 文中 11DXG-15 94.985° 29.553° 4626 4.44 0.71 3.7 0.4 文中 11DXG-13 94.981° 29.551° 4991 3.45 0.24 2.3 0.3 文中 NB-14 94.986° 29.578° 4100 4.75 0.71 1.2 0.2 Tu et al., 2015 NB-13 94.996° 29.580° 4400 3.96 0.32 1.5 0.1 Tu et al., 2015 NB-12 94.999° 29.583° 4610 3.77 0.55 1.8 0.2 Tu et al., 2015
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