祁连山中段白垩纪以来阶段性构造抬升过程的磷灰石裂变径迹证据

戚帮申, 胡道功, 杨肖肖, 张耀玲, 谭成轩, 张　鹏, 丰成君

1)中国地质科学院地质力学研究所, 北京 100081;　2)国土资源部新构造与地质灾害重点实验室, 北京 100081; 3)中国科学院地质与地球物理研究所, 北京 100029



祁连山中段白垩纪以来阶段性构造抬升过程的磷灰石裂变径迹证据

戚帮申1, 2), 胡道功1)*, 杨肖肖3), 张耀玲1), 谭成轩1, 2), 张鹏1, 2), 丰成君1, 2)

1)中国地质科学院地质力学研究所, 北京 100081;2)国土资源部新构造与地质灾害重点实验室, 北京 100081; 3)中国科学院地质与地球物理研究所, 北京 100029

摘要:祁连山作为青藏高原的东北边界, 是研究青藏高原隆升和扩展的重要区域, 利用磷灰石裂变径迹分析反映的祁连山地区白垩纪以来阶段性隆升和扩展新认识对理解青藏高原的隆升过程有重要的意义。分别采自南祁连陆块、疏勒南山—拉脊山缝合带、中祁连陆块和北祁连缝合带22个样品的磷灰石裂变径迹年龄介于(124±11) Ma与(13±2) Ma之间, 平均径迹长度介于(13.6±2.3) μm和(10.3±1.8) μm之间。时间-温度反演模拟结果表明祁连山地区至少经历了3个重要构造活动阶段: 1)白垩纪早期(>(129±14)~(115±17) Ma)祁连山隆升, 南祁连陆块和疏勒南山—拉脊山缝合带的冷却速率及剥蚀速率均较大, 并且祁连山南部可能率先抬升而初步构成高原的东北边界; 2)白垩纪中晚期—中新世((115±17)~(25±7) Ma)祁连山构造平静, 南祁连陆块和疏勒南山—拉脊山缝合带冷却速率及剥蚀速率均较低; 3)中新世以来祁连山由南向北逐渐扩展, 构造活动强烈而最终形成盆-山构造地貌格局。祁连山白垩纪早期的快速冷却过程可能是受拉萨地块和羌塘地块碰撞的影响; 中新世以来向北扩展则主要是受印度—欧亚板块碰撞的影响。

关键词:裂变径迹分析; 磷灰石; 白垩纪; 新生代; 祁连山

本文由中国地质调查局天然气水合物资源勘查与试采工程国家专项“祁连山冻土区天然气水合物资源勘查(力学所)”(编号: GZHL20120301)资助。

青藏高原的隆升不仅改变了亚洲大陆的构造地貌格局, 并对南亚乃至全球气候产生了重大影响(England and Houseman, 1988; Prell and Kutzbach, 1992; Raymo and Ruddiman, 1992)。因此, 青藏高原隆升的时间和幅度一直是地学界研究的焦点之一,有些学者认为由于中特提斯洋沿班公—怒江缝合带闭合和拉萨地块与羌塘地块的碰撞影响, 青藏高原可能于印度—欧亚板块碰撞之前便已隆升(Murphy et al., 1997; Schneider et al., 2003; Kapp et al., 2005; Guynn et al., 2006; Otofuji et al., 2007; Volkmer et al., 2007; Li et al., 2013a); 有些学者认为青藏高原开始隆升于印度板块沿雅鲁藏布江缝合带与欧亚板块碰撞时, 主要的隆升时代为古新世—始新世(Yin et al., 2002; Spicer et et al., 2003; Wang et al., 2008); 还有很多学者认为青藏高原的东部和北部直至中新世晚期才快速隆起(Turner et al., 1993; An et al., 1999;万景林等, 2001; 王瑜等, 2002; Zheng et al., 2006;张培震等, 2006), 另有认为直到~4 Ma以来青藏高原才开始整体快速隆起(Li and Fang, 1999)。除了青藏高原隆升时间上的争议, 其隆升空间分布上也存在很多不同的观点, 如由南向北逐步隆升(Meyer et al., 1998; Tapponnier et al., 2001)、青藏高原中部先隆起后向南北两侧扩展(Wang et al., 2008)以及高原南部与北部整体隆升(Yin and Harrison, 2000; Yin et al., 2002)。祁连山处于阿拉善和柴达木地块之间,构成青藏高原的东北缘(图1A, B), 研究其构造地貌演化特征对认识青藏高原隆升和扩展的过程有着重要意义。

祁连山构造带宽约500 km, 整体走向NW—SE,共分为南祁连陆块、蔬勒南山—拉脊山缝合带、中祁连陆块和北祁连缝合带四个构造单元(张雪亭等, 2007)。北祁连缝合带(北祁连新元古代—早古生代缝合带)主体位于托来山南缘并呈北西向分布于中祁连陆块和阿拉善陆块间, 南部大致以中祁连北缘断裂为主断裂构成中祁连陆块与北祁连缝合带的分界线, 北部以祁连山北缘断裂与河西走廊分离(Liu and Gao, 1998; Pan et al., 2013)。中祁连陆块是一个陆块与岩浆弧叠置的构造单位, 夹持于北祁连缝合带与蔬勒南山—拉脊山缝合带之间, 呈北西西向分布于托勒南山—大通山一带。南祁连陆块呈北西西向介于中祁连南缘断裂(疏勒南山—拉脊山缝合带主断裂)与宗务隆—青海南山断裂之间, 沿居洪图—阳康—化隆一带分布(张雪亭等, 2007)。

三叠纪以来, 几个陆块已经拼合到欧亚板块的南缘, 印支期中祁连和南祁连地块的整体抬升, 导致南祁连和中祁连盆地由海相沉积转变为陆相沉积,中三叠世海相地层与晚三叠世阿塔寺组陆相沉积建造多表现为平行不整合接触; 侏罗纪煤系地层与晚三叠世尕勒得寺组陆相地层同样表现为平行不整合接触(张雪亭等, 2007), 表明上侏罗统享堂组沉积之后的燕山运动奠定了祁连山中生代的构造格局, 强烈的构造活动形成中祁连以及南祁连广泛分布的宽缓褶皱和断裂构造; 新生代以来受印度—欧亚板块碰撞的影响, 并在阿尔金走滑断裂和昆仑走滑断裂的控制下, 形成大范围的壳内滑脱构造, 从而导致青藏高原北部的强烈地壳增厚和构造隆升(Meyer et al., 1998; Tapponnier et al., 2001), 祁连山地区受一系列NNW和NWW向的逆冲断裂影响而隆起, 并形成盆-山相间的构造地貌格局。

图1　研究区位置及地质构造简图Fig. 1　Location and simplified geological map of the study areaA-祁连山构造纲要简图(据Vincent and Allen, 1999; Gehrels, 2003; Yue et al., 2005; Bovet et al., 2009; 张雪亭等, 2007修改); B-青藏高原北部构造纲要图及磷灰石裂变径迹研究点分布及结果(断裂分布据Jolivet et al., 2001; Gaudemer et al., 1995; Meyer et al., 1996, 1998; Lasserre et al., 1999); C-研究区地质图(据张雪亭等, 2007修改)及磷灰石采样地点和结果

磷灰石裂变径迹的年龄和径迹长度分布特征为岩石温度冷却贯穿部分退火带(PAZ)提供量化信息, 记录了岩石从温度~110℃到60℃(Gallagher et al., 1998)的热史情况, 而这一过程一般被认为是受地区的隆升或剥蚀的结果, 因此磷灰石裂变径迹分析可以有效地约束地区剥蚀或隆升的热演化历史(Green, 1988; Johnson, 1997; Ventura et al., 2001)。目前, 低温年代学研究对祁连山隆升历史的认识还存在较大争议, 主要的隆升时代可包括: 晚白垩纪(Jolivet et al., 2001; Pan et al., 2013; Li et al., 2013b)、始新世晚期—渐新世(Yin et al., 2002)和中新世(Jolivet et al., 2001; 万景林等, 2001; 王瑜等, 2002;陈正乐等, 2002; Zheng et al., 2006)。以往的青藏高原北部低温年代学研究集中于主要断裂的周围(George et al., 2001; 万景林等, 2001, 2010; Jolivet et al., 2001; 王瑜等, 2002; 陈正乐等, 2002; Wang et al., 2006; 拜永山等, 2008; Zheng et al., 2010; Li et al., 2013b; Pan et al., 2013; 孙岳等, 2014), 特别是针对祁连山地区的研究集中于该地区的东缘、北缘、西缘(图1B), 而祁连山构造带内部的研究较为匮乏。因此, 本文通过对祁连山中段取样进行磷灰石裂变径迹分析, 恢复祁连山地区白垩纪—新生代的热史,为研究祁连山地区构造活动提供科学依据。

1　样品采集和方法

22个裂变径迹样品分布贯穿南祁连陆块、中祁连陆块、疏勒南山—拉脊山缝合带和北祁连缝合带,样品岩性有砂岩、凝灰岩、流纹岩、花岗岩、闪长岩、英安岩等。样品首先经过粉碎、分选和自然晾干, 经传统方法粗选, 再利用电磁选、重液选、介电选等手段, 对矿物颗粒进行单矿物提纯, 分离出磷灰石单矿物颗粒。分别用环氧基树脂和聚四氟乙丙烯透明塑料片将磷灰石固定, 制作成光薄片, 并研磨抛光揭示矿物颗粒内表面。磷灰石样片在恒温25℃的7%的HNO3溶液中蚀刻30 s以揭示自发径迹(Yuan et al., 2003)。将低铀白云母片(<4×10-9)作为外探测器盖在光薄片上, 紧密接触矿粒内表面, 与CN5(磷灰石)标准铀玻璃(Bellemants et al., 1995)一并接受热中子辐照(Yuan et al., 2006)。然后在25℃条件下的40%HF中蚀刻白云母外探测器20 min揭示诱发径迹。最后需要在高精度光学显微镜100倍干物镜下观测统计裂变径迹。应用IUGS推荐的Zeta常数标定法计算出裂变径迹中心年龄。实验中根据标准磷灰石矿物的测定, 加权平均得出Zeta常数值(Hurford and Green, 1983; Hurford, 1990)。由于磷灰石中裂变径迹退火存在各向异性(Green et al., 1986),因此选择平行c轴的柱面来测定水平封闭径迹长度、自发径迹密度和诱发径迹密度。

表1　祁连山磷灰石裂变径迹年龄Table 1　Apatite fission-track data from the Qilian Mountain

2　结果

22个样品的磷灰石裂变径迹年龄介于(124± 11) Ma到(13±2) Ma之间, 平均径迹长度介于(10.3±1.8) μm到(13.6±2.3) μm之间(表1)。仅样品B325-3和B412-1测试的磷灰石颗粒数目小于20,其余样品均超过20粒, 并且大多数样品的围限径迹测试条数超过50条, 数据质量较好。

χ2统计法可判断样品中各单颗粒年龄在多大程度上可作为具有单一平均年龄来看待(Galbraith and Laslett, 1993)。从单颗粒的自发和诱发裂变径迹数可计算出P(χ2), 是单颗粒年龄与所有颗粒的平均年龄符合的几率量度。P(χ2)>5%表示各单颗粒年龄的差别属于统计误差范围, 应作为具有单一平均值看待, FT年龄采用池年龄(Pooled Age); P(χ2)<5%表示各单颗粒年龄确有分散, FT年龄采用中心年龄(Central Age)(Sobel et al., 2006a, b)。统计结果表明共有18个样品通过了χ2检验(图2), 4个样品单颗粒年龄分散程度高于一般范围, 雷达辐射图显示这4个样品的变化范围为±140 ~ ±25 Ma, 和所有通过χ2检验的样品的年龄变化范围相近。裂变径迹年龄分布基本不受岩性的控制, 例如侏罗纪的砂岩和奥陶纪的花岗岩具有相似的裂变径迹年龄, 所有的样品均小于其形成年龄。并且平均径迹长度为(13.6± 2.3) μm 和 (10.3±1.8) μm之间, 指示这些样品均经历形成之后的温度贯穿PAZ的过程(Gleadow et al., 1986; Yuan et al., 2006; Yuan et al., 2007)。

3　磷灰石裂变径迹年龄的意义

总的来看, 裂变径迹年龄和海拔相关关系以及径迹的平均长度与裂变径迹年龄相关关系不强(图3A, B), 但按照SW—NE向从采样所属不同的构造单元位置来看, 可以发现中祁连陆块、疏勒南山—拉脊山缝合带和南祁连陆块的海拔-年龄呈正相关,相关关系较好(图3C)。中祁连陆块裂变径迹年龄为(96±9) Ma和(13±2) Ma之间, 南北差异较大, 南部的裂变径迹年龄明显老于北部地区, 裂变径迹年龄较小((60±5)~(13±2) Ma)的样品主要在中祁连北缘断裂附近(图3C), 这主要是受新生代以来中祁连北缘断裂构造活动的影响。因此, 祁连山地区同一个构造单元具有相似的剥蚀速率, 不受差异热状态的干扰, 样品裂变径迹年龄受样品相对于主要断裂的位置影响(Yuan et al., 2006)。

图2　祁连山磷灰石裂变径迹辐射图Fig. 2　Radial plots of single grain ages for the 22 samples from the Qilian Mountain

虽然磷灰石裂变径迹的退火特征受化学组成(Barbarand et al., 2003)和矿物特征的影响(Carlson et al., 1999)。但这方面的影响相对较弱(Pan et al., 2013), 地壳剥蚀和区域性的构造活动均可能造成年龄-海拔之间的相关性不强(Green et al., 1986)。因此, 不能简单地将平均裂变径迹年龄等同于重要的构造事件, 需要结合热史模拟分析来进一步探讨裂变径迹年龄所代表的岩石热演化过程。

利用AFTSolve软件及Ketcham等(1999)模型进行热史模拟(Ketcham et al., 2003), 模拟次数为10000次, 模拟的评价标准包括K-S检测和GOF检测, 当GOF≥0.05, 模拟的曲线被认为是可以接受的, 当GOF≥0.5, 模拟的曲线被认为是好的模拟曲线(Ketcham, 2005)。每次模拟, 都假设样品实测裂变径迹年龄的1.5倍时地温达200~160℃以致样品完全退火, 在实测裂变径迹年龄的时间样品处于PAZ(110~60℃), 而现今处于地表的~20~0℃地温为另一个限制条件(Pan et al., 2013)。模拟结果见图4,由于北祁连缝合带内仅有样品B054-1, 而单个样品不足以代表区域的热史, 因此本文不对其模拟。

模拟结果显示, 所有样品的K-S检测和GOF检

图3　祁连山磷灰石裂变径迹年龄、平均径迹长度和海拔的关系Fig. 3　Relationship between AFT age, mean track length and elevation

A-裂变径迹年龄和海拔的关系; B-裂变径迹年龄和平均径迹长度的关系; C-样品分布位置、裂变径迹年龄和海拔的关系

A-relationship between AFT age and elevation; B-relationship between AFT age and mean track length; C-plot showing relationship between elevation, main faults, AFT ages and samples’ location, from the South Qilian fold belt to North Qilian suture zone in SW-NE direction测均大于0.5, 模拟质量较高且较为可信。根据所有样品的时间-温度(t-T)最佳的模拟曲线(图4), 每条最佳模拟曲线可以分离出3个限制点, 第一个限制点为岩石降温至~110℃的年龄, 第二个限制点为岩石冷却速率由快转慢, 第三个限制点为岩石降温速率由慢转快。采自不同地点样品的最佳模拟曲线限制点可以构成出3组(图5), 第一组是温度降温至~110℃的时候, 南祁连陆块为(124±7) Ma, 疏勒南山—拉脊山缝合带为(129±14) Ma。第二组限制点为当岩体冷却速度从快转慢的时候, 南祁连陆块为(117±8) Ma, 古地温达(79±15)℃ ; 疏勒南山—拉脊山缝合带内的样品为(115±17) Ma, 古地温为(74±9)℃。第三组限制点为岩体冷却速率从慢再一次转快的时候, 南祁连陆块为(25±7) Ma, 古地温达(54±9)℃ ; 疏勒南山—拉脊山缝合带为(17±9) Ma,古地温为(56±11)℃。中祁连陆块于中新世晚期((10±7) Ma)所有样品可见快速冷却, 中新世之前的t-T模拟曲线比较分散, 可能中祁连地块在中新世之前存在更为复杂的热史, 直到中新世晚期((10±7) Ma)中祁连地块整体出现快速冷却剥蚀。模拟结果中小的冷却事件被忽略(Pan et al., 2013), 因为这些事件可能受退火模型不稳定的影响(Ketcham et al., 2009)。

祁连山的平均地温梯度大致为25℃/km(Hu et al., 2000), 结合时间-温度(t-T)最佳的模拟曲线结果,可估算出每个样品的冷却速率(△温度/△时间)和侵蚀速率(冷却速率/地温梯度)。结果表明, 南祁连陆块白垩纪早期((124±7) Ma至(117±8) Ma), 冷却速率为~3.4℃/Ma, 剥蚀速率达~0.13 mm/a; 白垩纪早期—中新世早期((117±8) Ma至(25±7) Ma), 冷却速率为~0.2℃/Ma, 剥蚀速率为~0.01 mm/a; 中新世以来((25±7) Ma至现今), 冷却速率为~2.0℃/Ma, 剥蚀速率为~0.08 mm/a(图5A)。疏勒南山—拉脊山缝合带白垩纪早期((129±14) Ma至(115±17) Ma), 冷却速率为~2.7℃/Ma, 剥蚀速率达~0.11 mm/a; 白垩纪早期—中新世((115±17) Ma至(17±9) Ma), 冷却速率为~0.1℃/Ma, 剥蚀速率为~0.01 mm/a; 中新世以来((17±7) Ma至现今), 冷却速率为~4.6℃/Ma,剥蚀速率为~0.18 mm/a(图5B)。中祁连陆块中新世晚期以来((10±7) Ma至现今)存在快速冷却剥蚀作用, 冷却速率为~11.9℃/Ma, 剥蚀速率为~0.47 mm/a(图5C)。

4　讨论

磷灰石裂变径迹记录了祁连山的热史情况, 为分析该地区冷却剥蚀历史提供定量信息。结果表明,祁连山白垩纪以来至少经历了3个重要的构造活动阶段: ①白垩纪早期隆升; ②白垩纪中晚期—中新世早期构造平静; ③中新世以来向北逐渐扩展。

图4　祁连山中段热模拟结果(时间-温度模拟曲线和径迹分布)Fig. 4　Modeled inverse t–T paths and length distribution for samples from the middle segment of the Qilian Mountain

图5　祁连山热模拟最佳时间-温度曲线Fig. 5　The best-fit line of the time-temperature modeling for samples from the Qilian MountainA-南祁连陆块热模拟最佳时间-温度曲线; B-疏勒南山—拉脊山缝合带热模拟最佳时间-温度曲线; C-中祁连陆块热模拟最佳时间-温度曲线

4.1祁连山白垩纪早期隆升

裂变径迹结果显示, 祁连山在白垩纪早期((129±14)~(115±17) Ma)南祁连和疏勒南山—拉脊山缝合带均存在快速冷却的过程(图5), 并且大柴旦逆冲断裂带两侧剥蚀速率亦存在明显差异(Jolivet et al., 2001), 低温年代学数据指示祁连山在白垩纪早期很可能已经隆起, 初步构成青藏高原的东北边界。同时, 南祁连陆块的磷灰石裂变径迹年龄明显老于中祁连陆块以及北祁连缝合带(Pan et al., 2013; Li et al., 2013b), 表明祁连山南部可能率先隆起, 从而导致南祁连侏罗系与白垩系仅出露零星露头, 与白垩系主要分布在祁连山北部和东部的特征相一致(张雪亭等, 2007)。

阿尔金断裂东段地区的火山岩主要分布在阿尔金断裂与祁连山西端交汇的山前盆地(酒西盆地)与山间盆地(昌马盆地)中, 均为一套偏碱性基性火山岩, 其Ar-Ar测年结果显示本区岩浆活动分为100~120 Ma和~82 Ma两期岩浆活动(李海兵等, 2004)和阿尔金断裂白垩纪再次强烈走滑活动相一致(金山口北坡糜棱岩化的加里东花岗岩白云母形成时代为89.2 Ma和断层附近侏罗系两个韧性变形样品中云母形成时代91.7 Ma和97.7 Ma; Liu et al., 2001), 并且在祁连山北部山前早白垩地层中出现软沉积变形, 古斜坡指向也反映祁连山的抬升(李海兵等, 2004)。河西走廊地区在白垩纪早期出现类似磨拉石的粗碎屑沉积(Vincent and Allen, 1999)印证了本次构造活动。祁连山白垩纪早期的隆升主要受拉萨地块和欧亚板块碰撞的影响(Vincent and Allen, 1999; Jolivet et al., 2001; 李海兵等, 2004),祁连山很可能已经初步隆起, 构成青藏高原东北边界雏形。

4.2祁连山白垩纪中晚期—中新世早期构造平静

白垩纪中晚期—中新世南祁连陆块和疏勒南山—拉脊山缝合带的冷却速率(~0.2℃/Ma; ~0.1℃/Ma)及剥蚀速率(~0.01 mm/a; ~0.006 mm/a)均比较低, 径迹长度较短以及呈宽缓不对称正态分布的特征均指示该地区长时间处于PAZ(图5)。AFT热年代学证据表明祁连山的东缘于±83 ~ ±24 Ma期间的冷却速率及剥蚀速率(~0.6℃/Ma; ~0.017 mm/a)亦较低(Pan et al., 2013), 本时期祁连山南北两侧的大柴旦地区和河西走廊盆地构造活动由挤压转变为伸展, 沉积速率较低(Vincent and Allen, 1999)。因此,白垩纪中晚期—中新世祁连山地区普遍构造活动较弱。虽然晚始新世—渐新世(距今约35.3~32.6 Ma)河西走廊盆地和中祁连木里盆地内白杨河组(E3b)和火烧沟组(E2-3h)均呈轻微角度不整合接触(戴霜等, 2005; 戚帮申等, 2013), 阿尔金北缘山脉和党河南山在~40 Ma出现快速冷却剥蚀(Jolivet et al., 2001; 孙岳等, 2014)均显示青藏高原北部在始新世晚期—渐新世早期存在构造变形与隆升, 但祁连山构造带内部的磷灰石裂变径迹数据没有显示本次隆升过程, 结合碳氧同位素估算的古近纪的古海拔较低以及火烧沟组和白杨河组均为河湖相的沉积特征(戴霜等, 2005; 戚帮申等, 2013; 戚帮申等, 2015),可以得出祁连山本次隆升的幅度不大。总体上祁连山地区白垩纪晚期—中新世早期构造平静。

4.3祁连山中新世以来向北逐渐扩展

印度板块与欧亚板块碰撞的时间目前还存在很大的争议(Beck et al., 1995; Lee and Lawver., 1995; Patzelt et al., 1996; Searle et al., 1997; Rowley, 1998; Zhang and Scharer, 1999), 首次影响到高原北部导致地壳缩短与增厚的时间也是争论的焦点之一, 如~40 Ma(Jolivet et al., 2001)、~30 Ma(Mock et al., 1999)、25~20 Ma(Sobel and Dumitru, 1997)以及~4 Ma(Li and Fang, 1999; 李吉均等, 2001)。始新世—渐新世高原北部普遍存在构造活动, 但河西走廊盆地和中祁连木里盆地内白杨河组(E3b)和火烧沟组(E2-3h)均呈轻微角度不整合接触(戴霜等, 2005;戚帮申等, 2013), 构造活动的强度可能不大。上新世—第四纪(~3.6 Ma)的隆升证据主要是依靠沉积相变化得出, 然而这也可能是受气候变化因素的影响(Zhang et al., 2001), 缺乏更多构造变形证据的支持(张培震等, 2006)。前人的研究发现祁连山及邻区于中新世中晚期存在“准同期”(~8 Ma)的强烈构造变形, 并通过逆冲断裂和褶皱变形等方式, 使山脉隆升与沉积盆地消亡(Turner et al., 1993; 张培震等, 2006), 同时期红黏土在六盘山地区沉积, 这主要受高原腹地海拔达到一定临界值的影响, 引起高原东北部的气候和环境方面的变化(An et al., 1999; 宋友桂等, 2001), 受此影响祁连山地区新生代河湖相沉积的碳氧同位素(δ13C和δ18O)于中新世中期亦出现明显的变化(Dettman et al., 2003; 戚帮申等, 2015)。自上新世早期以来, 位于祁连山南北两侧的柴达木盆地和河西走廊沉积速率明显加快(Metivier et al., 1998), 由于柴达木盆地和河西走廊地区与其附近的盆地无物质交换, 沉积速率加快指示构造活动加强(Jolivet et al., 2001)。因此, 中新世以来为祁连山地区主要隆升阶段, 并且断裂活动的时间以及区域冷却历史显示中新世以来祁连山具有向北扩展的规律。

祁连山构造带内有若干条北西—北西西向的逆冲断裂带, 自南向北包括柴北缘逆冲断裂带、南山逆冲断裂带、中祁连南缘逆冲断裂带、拉脊山逆冲断裂、中祁连北缘逆冲断裂带和祁连山北缘逆冲断裂带等(Yin et al., 2002; Li et al., 2015)。生物地层学、沉积学以及热年代学研究表明柴达木北缘断裂在~40 Ma之前就已经开始活动(Jolivet et al., 2001; Yin et al., 2002), 南山逆冲断裂带自渐新世(~33 Ma)便已经存在(Wang, 1997; Rumelhat, 1998; Yin et al., 2002), 拉脊山逆冲断裂带却从~22 Ma开始活动(Lease et al., 2011), 北祁连逆冲断裂带则直至8.3~ 0 Ma开始活动(Tapponnier et al., 1990; Yang et al., 2007), 故祁连山构造带内的逆冲断裂活动时间表现由南向北扩展的规律。

区域性冷却历史同样显示祁连山地区中新世以来具有向北扩展的规律。南祁连陆块于(25±7) Ma冷却速率明显加快(图5A), 表明祁连山的南部地区在始新世晚期—渐新世早期存在构造变形与隆升。疏勒南山—拉脊山缝合带于(17±9) Ma开始快速冷却剥蚀(图5B), 而中祁连陆块直至(10±7) Ma冷却速率和剥蚀速率才明显加快, 北祁连亦于±20~ 10 Ma以来转入快速冷却(George et al., 2001; Pan et al., 2013), 而高原北缘直到9~7 Ma发生快速蚀顶过程(George et al., 2001; 万景林等, 2001; 王瑜等, 2002; 陈正乐等, 2002; Zheng et al., 2006, 2010)。由此可见, 中新世以来祁连山的构造变形具有向北逐渐扩展的规律, 这主要受印度—欧亚板块碰撞的影响, 通过一系列的逆冲断裂和下地壳广泛的滑脱作用向北扩展(Bovet et al., 2009)。

5　结论

祁连山构造带内部不同地点的磷灰石裂变径迹分析结果表明, 祁连山白垩纪以来至少经历三个重要的构造活动阶段: 1)白垩纪早期(>(129±14) ~(115±17) Ma)隆升, 祁连山地区受拉萨地块和羌塘地块的碰撞影响而出现隆升剥蚀, 南北磷灰石裂变径迹年龄的差异显示祁连山南部较北部率先隆起,导致祁连山南部出现白垩系沉积间断, 此时的祁连山可能已经构成青藏高原的东北边界; 2)白垩纪中晚期—中新世早期((115±17)~(25±7) Ma)祁连山处于构造平静期, 此时不论是南祁连陆块还是疏勒南山—拉脊山缝合带的样品冷却速率和剥蚀速率都很低; 3)中新世以来(<(25±7) Ma—今)祁连山由南向北逐渐扩展, 祁连山强烈隆起并导致柴达木盆地和河西走廊地区沉积速率加快以及祁连山地区新生代湖相沉积的碳氧同位素变化, 并形成和现代地貌相近的盆-山构造地貌格局。

致谢: 中国地质科学院地质力学研究所吴中海研究员及各位评审专家给予本文的建设性意见和重要指导, 以及中国地质科学院赵珍博士, 中国地质大学(北京)李波硕士、赵钊硕士、高雪咪硕士、于航硕士、田珺硕士, 长江大学徐久晟硕士和李丹江硕士等参与野外取样工作, 磷灰石裂变径迹测试由中国地质大学(北京)袁万明教授协助完成, 谨表谢意。

Acknowledgements:

This study was supported by China Geological Survey (No. GZHL20120301).

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Apatite Fission Track Study of the Cretaceous–Cenozoic Stepwise Uplift of the Middle Segment of the Qilian Mountain

QI Bang-shen1, 2), HU Dao-gong1)*, YANG Xiao-xiao3), ZHANG Yao-ling1), TAN Cheng-xuan1, 2), ZHANG Peng1, 2), FENG Cheng-jun1, 2)
1) Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081; 2) Key Laboratory of Neotectonic Movement & Geohazard, Ministry of Land and Resources, Beijing 100081; 3) Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029

Abstract:The Qilian Mountain constitutes the northeastern margin of the Tibetan Plateau, and hence

characteristics of its tectonic activity recorded by apatite fission track (AFT) analysis play an important role in understanding the uplift and growth of the Tibetan Plateau. 22 samples collected for AFT analysis from the Qilian Mountain belt were located along NS-trending transect across southern Qilian fold belt, Shule Nanshan-Laji Shan suture zone, Central Qilian massif and North Qilian suture zone. AFT ages range from (13±2) Ma to (124±11) Ma, and mean track lengths range from (10.3±1.8) μm to (13.6±2.3) μm. Samples from the same tectonic unit have positive correlation between AFT ages and elevations, whereas samples with younger ages ((60±5) Ma to (13±2) Ma) are clustered around North Central Qilian fault. On the basis of the measured apatite fission track age data, the inversion simulation was used to analyze the thermal history of the Qilian Mountain. The best-fit line ofthe time-temperature modeling results suggest that at least three cooling periods have occurred since early Cretaceous: 1) Rapid cooling in the Qilian Mountain during early Cretaceous (>(129±14) Ma to (115±17) Ma). The cooling rates and exhumation rates of South Qilian fold belt and Shule Nanshan -Laji Shan suture zone were great, suggesting that the Qilian Mountain formed the northeastern margin of the Tibetan Plateau during early Cretaceous; 2) From middle Cretaceous to Miocene ((115±17) Ma to (25±7) Ma), the cooling rates and exhumation rates of South Qilian fold belt and Shule Nanshan-Laji Shan suture zone were quite low, implying that the tectonic activity of the Qilian Mountain was weak during middle Cretaceous to Miocene; 3) Since Miocene time, timing of both thrust activities and regional rapid cooling event shows that the Qilian Mountain experienced north-eastward rise and growth, which is in line with the hypothesis that the Qilian Mountain was formed by thrusting within the Qaidam crust along a large decollement in the lower crust that progressively propagated north-eastward, the Qilian Mountain was uplifted considerably since Miocene, forming basins-mountains tectonic landforms. Early Cretaceous rapid cooling event in the Qilian Mountain probably resulted from the docking of the Lhasa block to the south, and the rapid cooling since Miocene may be the result of the docking of the India-Asia collision, representing the main uplift of the Qilian Mountain.

Key words:fission-track analysis; apatite; Qilian Mountain; Cretaceous; Cenozoic

*通讯作者:胡道功, 男, 1963年生。研究员。主要从事新构造与活动构造研究。E-mail: hudg@263.net。

作者简介:第一 戚帮申, 男, 1988年生。博士研究生。主要从事区域地壳稳定性评价、工程地质和地质灾害研究。

通讯地址:100081, 北京市海淀区民族大学南路11号。E-mail: qibangshen@126.com。

收稿日期:2015-05-24; 改回日期: 2015-09-30。责任编辑: 魏乐军。

中图分类号:P597.3; P542.1

文献标志码:A

doi:10.3975/cagsb.2016.01.05