The end of ductile shear deformation of Ningshan fault: Constraint from the activity time of Longbozi shear zone
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摘要:
南秦岭宁陕断裂镇安段北缘的龙脖子剪切带记录了宁陕断裂带左行走滑韧性剪切变形过程。带内3类石英脉体和方解石脉体的ESR年龄分别为125.6~88.7Ma、56.7~32.9Ma和19.8~14.6Ma。其中第一类产出于构造片理和A型褶皱核部的石英脉体，代表左行走滑韧性剪切变形结束、脆性构造活跃的时代。研究表明，宁陕左行走滑剪切带在晚三叠世早期开始活动，且可能持续到早—中侏罗世。第一类脉体年龄的确定表明，宁陕断裂带左行走滑韧性剪切变形最晚可持续到早白垩世；晚白垩世—始新世，宁陕断裂带以伸展-走滑脆性或韧-脆性剪切变形为主。因此，早白垩世是宁陕断裂带韧性剪切变形向脆性剪切变形转换的关键时期。宁陕断裂带经历了晚三叠世—中侏罗世晚期快速冷却阶段、晚侏罗世—白垩纪缓慢冷却阶段和古近纪以来快速冷却阶段。宁陕断裂带在缓慢冷却晚期（早白垩世）实现韧性剪切变形向脆性剪切变形转换说明，早白垩世也是秦岭造山带陆内变形机制转变的关键时期。
Abstract:Three groups of ESR dating ages were obtained from different vein materials in Longbozi shear zone, i.e., 125.6~88.7Ma, 56.7~32.9Ma and 19.8~14.6Ma. The first type of vein materials sampled from schistosities and A-type fold cores marked the end of left-lateral shear deformation. It has been confirmed that the ductile deformation of Ningshan fault began from the early Late Triassic, and might have lasted to Early-Middle Jurassic. However, the result of the first type of vein material ESR dating shows that Ningshan fault left-lateral ductile shear effect lasted to Early Cretaceous. So Early Cretaceous was the critical period when the ductile shear deformation transformed to brittleness in Ningshan shear zone. Furthermore, Ningshan fault experienced Late Triassic-Middle Jurassic rapid-cooling phase, Late Jurassic-Cretaceous slow-cooling phase, and Paleocene-Quaternary rapid-cooling phase. The strain translation that occurred in the late slow-cooling phase suggests that Early Cretaceous also was the critical period of intracontinental deformation mechanism changing.
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Keywords:
- Longbozi shear zone /
- Ningshan fault /
- left-lateral shear /
- cleavage /
- A-type fold /
- ESR dating
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致谢: 西北大学石宗培和罗静兰教授在室内镜下分析工作中给予帮助和指导；成都理工大学ESR实验室在实验过程中提供了实验设备；审稿专家对本文提出了宝贵的修改意见，在此深表谢意。
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图 1 秦岭造山带构造及镇安龙脖子地质简图（据参考文献[12]修改）
NCP-华北板块; SCP-华南板块; NQB-北秦岭构造带; SDS-商丹缝合带; MLS-勉略缝合带; NW-NQB-北西-南秦岭构造带; SE-SQB-南东-南秦岭构造带; BT-碧口地体; SGT-松潘-甘孜地体; LMS-龙门山构造带; TBT-桐柏地体; DBT-大别地体; PL-平利地块; WD-武当地块; SX-随县地块; XFB-雪峰山构造带; LLF-洛南-栾川断裂; NSF-宁陕断裂; AKF-安康断裂; ZBF-镇巴断裂; TWF-铁溪-巫溪断裂; CFF-城口-房县断裂; SYF-山阳断裂; QYF-青川-阳平关断裂; SYF-十堰断裂
Figure 1. Schematic geological map of Qinling orogen and the position of Longbozi area, Zhen'an County
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图 2 镇安龙脖子地区基本地质概况图
O-奥陶系; S-志留系; D1-下泥盆统; D2-3-中上泥盆统; D2-中泥盆统; D3-上泥盆统; D3x-上泥盆统星红铺组; D3l-上泥盆统九里坪组; C1-下石炭统; C2-上石炭统; C1-2-石炭系; P1-下二叠统; P1-2-二叠系; T1-下三叠统
Figure 2. Geological map of Longbozi area of Zhen'an County
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图版Ⅰ
a~j.镇安龙脖子九里坪组挤压缩短与左行走滑韧性剪切变形特征; a.野外宏观透入性劈理; b.面理叠加和置换; c.野外劈理折射现象; d.单强硬层褶皱; e.多强硬层褶皱; f.拉伸线理与A型褶皱; g.不对称倾竖褶皱; h.无根倾竖褶皱; i.XZ面上的不对称褶皱; j.左行走滑剪切示意图
图版Ⅰ.
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图 3 镇安龙脖子九里坪组构造要素方位极射赤平投影图
a-构造片理产状极射赤平投影图; b-拉伸线理和A褶皱枢纽产状极射赤平投影图; c-张节理面极射赤平投影图(σ3 < 0 < σ2 < σ1); d-剪节理极射赤平投影图(0 < σ3 < σ2 < σ1)
Figure 3. The stereographic projection diagrams of structural elements for Jiuliping Formation deformations in Longbozi area, Zhen'an County
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图版Ⅱ
a~g.镇安龙脖子九里坪组脆-韧性过渡型剪切变形特征与脆性破裂。a、b.张节理, 充填石英脉; c.韧-脆性右行走滑剪切形成的小型雁列方解石脉; d、e.脆-韧性右行走滑剪切形成的似牵引断层与不连续面; f.剪节理; g.充填方解石脉
图版Ⅱ.
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图 4 龙脖子剪切带实测剖面图及采样位置与构造片理关系
Figure 4. The sketch profile of Longbozi outcrop and the relationship between sampling positions and schistose structure
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图版Ⅲ
a.绿泥石(Chl)与鳞片变晶结构;b.鳞片状黑云母(Bi)与矿物线理;c.石英(Q);d.劈理折射;e.方解石(Cal);f.背斜转折端附近的层间小褶皱;g.长英质层形成的褶皱与透入性劈理;h.面理置换
图版Ⅲ.
表 1 龙脖子剪切带多期脉体ESR测年数据
Table 1 Results of multi-period vein materials ESR dating in Longbozi shear zone
类别 产出状态 样品编号 顺磁中心浓度/
1015Sp/gU当量含量/10-6 年龄/Ma 第一类 A型褶皱核部石英脉 12ZS01/SS9 0.227 0.359 125.6±12.0 平行构造片理石英脉 DL01-Ⅱ/R2 0.486 1.059 91.8±9.0 A型褶皱核部石英脉 12ZS02/SS10 0.248 0.559 88.7±8.0 第二类 斜截构造片理石英脉 DL03-Ⅲ/R7 0.261 0.919 56.7±5.6 斜截构造片理石英脉 DL01-Ⅰ/R1 0.196 0.779 50.3±5.0 斜截构造片理石英脉 DL02-Ⅱ/R4 0.157 0.954 32.9±3.2 第三类 斜截构造片理方解石脉 DL02-Ⅲ/R5 0.105 1.059 19.8±2.0 斜截构造片理方解石脉 DL02-Ⅳ/R6 0.077 1.059 14.6±1.4 
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表 2 宁陕断裂带各测年方法获得的年龄与封闭温度/脉体形成温度数据
Table 2 Ages and corresponding closure (forming) temperatures of different dating approaches in Ningshan fault belt
数据类型 年龄/Ma 测年矿物 计时方法 封闭温度/脉体形成温度/C 统计年龄[14-15] 214.4±1.1 锆石 U-Pb >900 212.8±1.6 186.0±4.0 统计年龄[36] 203.0~197.0 全岩 Rb-Sr 500 统计年龄[12] 169.5±2.3 白云母 Ar-Ar 350~400 统计年龄[12, 15] 165.1±2.6 黑云母 Ar-Ar 350~387 162.1±1.3 黑云母 Ar-Ar 350~387 161.1±9.3 黑云母 Ar-Ar 350~387 160.0±17.0 黑云母 Ar-Ar 350~387 实测年龄 125.6±12.0 高温石英脉 ESR ~300 91.849.0 88.7±8.0 实测年龄 56.7±5.6 低温石英脉 ESR ~200 50.3±5.5 32.9±3.2 实测年龄 19.8±9.0 方解石脉 ESR 150~75 14.6±1.4
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