岩石学报  2017, Vol. 33 Issue (12): 3989-3998   PDF    
拉萨地块北缘尼玛地区晚白垩世古地磁结果及其构造意义
曹勇1,2 , 孙知明1 , 刘栋梁3 , 张蕾3 , 叶小舟1 , 郑勇3 , 何祥丽2,3     
1. 中国地质科学院地质力学研究所, 国土资源部古地磁与古构造重建重点实验室, 北京 100081;
2. 中国地质大学, 北京 10008;
3. 中国地质科学院地质研究所, 国土资源部大陆动力学重点实验室, 北京 100037
摘要:通过对青藏高原拉萨地块北缘尼玛县城附近晚白垩世红层8个采点的系统古地磁测定,获得了研究区晚白垩世可靠的古地磁数据。通过系统热退磁获得了采样剖面高温特征剩磁方向平均值为:Dg=9.9°,Ig=5.5°,κg=30.3,α95=10.2°,N=8(地理坐标下);Ds=8.2°,Is=27.4°,κs=37.6,α95=9.2°,N=8(层面坐标下);对应古地磁极为:71.2°N,241.9°E,dp/dm=5.5°/10.0°。该高温剩磁分量在地理坐标下与现代地磁场方向有显著差别,具有正、反极性,并通过了倒转检验,由此认为其很可能代表岩石形成时的原生剩磁。本次研究结果表明拉萨地块北缘在晚白垩世位于~14.5°N的古纬度位置(参考点:31.8°N,87.2°E)。对比稳定亚洲大陆参考极显示晚白垩世以来拉萨地块北缘与稳定亚洲大陆之间发生了1200±630km(10.9°±5.7°)的南北向构造缩短,但并未发生明显的相对旋转作用(0.8°±5.9°)。综合地质证据提出印度与亚洲大陆碰撞所造成的亚洲大陆内部南北向构造缩短主要集中在拉萨地块以北的区域。
关键词: 古地理位置     构造缩短     古地磁     晚白垩世     拉萨地块    
Late Cretaceous paleomagnetic results in the Nyima area from the northern margin of the Lhasa block and its tectonic implications
CAO Yong1,2, SUN ZhiMing1, LIU DongLiang3, ZHANG Lei3, YE XiaoZhou1, ZHENG Yong3, HE XiangLi2,3     
1. MLR Key Laboratory of Paleomagnetism and Tectonic Reconstruction, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China;
2. China University of Geosciences, Beijing 10008;
3. MLR Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
Abstract: A paleomagnetic study of the Upper Cretaceous red beds have been carried out near the Nyima County in the northern margin of the Lhasa block. Stepwise thermal demagnetization successfully isolated high temperature characteristic directions from the Upper Cretaceous red beds. The tilt-corrected mean direction for 8 sites is Ds/Is=8.2°/27.4° with α95=9.2°, corresponding to a paleopole at 71.2°N, 241.9°E with dp/dm=5.5°/10.0°. The mean direction of high temperature characteristic directions in geographic coordinates is significantly different from the present geomagnetic field (PGF) direction. Both normal and reversal polarities were observed in the high temperature characteristic direction and it pass reversal tests at the 95% confidence level. These indicate the primary origin for the characteristic remanence of the red beds. The results indicated that the northern margin of the Lhasa block was located at about 14.5°N during the Late Cretaceous (reference point:31.8°N, 87.2°E). Compared with the Late Cretaceous paleopole of the Asian continent, we suggest that significant crustal shortening (1200±630km) and insignificant rotation (0.8°±5.9°) have taken place between the northern margin of the Lhasa block and the stable Asian continent since the Late Cretaceous. The N-S crustal shortening resulted from India-Asia collision was mainly distributed on the north of the Lhasa block.
Key words: Paleogeographic position     Crustal shortening     Paleomagnetism     Late Cretaceous     Lhasa block    
1 引言

青藏高原是由高原上诸地块在不同时期碰撞-拼合到欧亚大陆南缘而形成的(Yin and Harrison, 2000; Metcalfe, 2006, 2013; Wang et al., 2014; Zhu et al., 2016),各地块间的碰撞及其持续效应是研究青藏高原形成和演化的重要科学问题。拉萨地块位于青藏高原南部,是高原主要地块之一。拉萨地块很可能在中生代早期从冈瓦纳大陆裂解向北漂移(Zhu et al., 2011; Li et al., 2016),至白垩纪时期与北边的羌塘地块发生碰撞拼合(Kapp et al., 2007a; Li et al., 2015a; Wang et al., 2016; Zhu et al., 2016; Chen et al., 2017; Huang et al., 2017),成为印度与亚洲大陆碰撞前亚洲大陆最南缘的地块。拉萨地块与羌塘地块碰撞,导致青藏高原中部在印度/欧亚板块碰撞前已有一定的海拔高度(3~4km)(Kapp et al., 2005; Volkmer et al., 2007, 2014),形成白垩纪时期的青藏高原雏形(初始高原)(Wang et al., 2014; Li et al., 2015b)。

拉萨地块是青藏高原形成过程中构造运动最强烈的地区之一,其晚白垩世古地理位置重建对制约拉萨地块与羌塘地块的碰撞拼合和高原形成过程具有重要意义。古地磁学能定量约束板块古纬度,在板块古地理位置重建及构造缩短量的研究上具有独特的优势。前人在拉萨地块开展了大量晚白垩世古地磁学研究,越来越多的古地磁结果显示拉萨地块晚白垩世的古纬度为约10°~15°N(Achache et al., 1984; Sun et al., 2012; 唐祥德等, 2013; Yang et al., 2015; Yi et al., 2015; 马义明, 2016; Cao et al., 2017; 梁雅伦等, 2017; Tong et al., 2017)。然而,早期的古地磁研究认为拉萨地块晚白垩世的古纬度为20°N左右(Pozzi et al., 1982; Westphal and Pozzi, 1983; Lin and Watts, 1988)。Tan et al. (2010)认为林周盆地上白垩统设兴组红层发生了磁倾角浅化现象,因此其对林周盆地设兴组红层进行了数据矫正,获得了与火山岩较一致的结果,并认为拉萨地块晚白垩世的古纬度为~24°N。另一方面,基于古地磁学研究的板块构造重建显示印度与亚洲大陆碰撞导致的构造缩短量为2600~3400km,其中造成亚洲大陆内部的南北向构造缩短为1100~2000km(Achache et al., 1984; Patriat and Achache, 1984; Chen et al., 2010, 2014; Dupont-Nivet et al., 2010; Liebke et al., 2010; Sun et al., 2010; van Hinsbergen et al., 2012; 唐祥德等, 2013; Ma et al., 2014; Yi et al., 2015; Cao et al., 2017; Li et al., 2017; Tong et al., 2017)。而部分古地磁研究却显示印度与亚洲大陆碰撞后亚洲大陆内部的构造缩短仅有几百千米或更小(Tan et al., 2010; Meng et al., 2012; Huang et al., 2013)。

虽然前人在拉萨地块开展了大量的古地磁学研究,但是现有拉萨地块晚白垩世古地磁研究主要分布在拉萨地块南部(Pozzi et al., 1982; Westphal and Pozzi, 1983; Achache et al., 1984; Lin and Watts, 1988; Tan et al., 2010; Sun et al., 2012; Cao et al., 2017)、中部(唐祥德等, 2013; Yang et al., 2015; 马义明, 2016)、西缘(Yi et al., 2015; 马义明, 2016)和东缘(Tong et al., 2017)(图 1)。仅有的拉萨地块北部晚白垩世的古地磁研究中不仅样品数少,而且未经稳定性检验(Lin and Watts, 1988)。为此,本文报道拉萨地块北缘尼玛地区晚白垩世古地磁研究结果,期望为拉萨地块北缘晚白垩世古地理位置及亚洲大陆内部构造缩短量提供可靠的古地磁制约。

图 1 青藏高原及周边地区构造简图(据Yin and Harrison, 2000Tapponnier et al., 2001修改) 五角星代表已有的古地磁研究 Fig. 1 Schematic tectonic map of the Tibetan Plateau and adjacent region (modified after Yin and Harrison, 2000; Tapponnier et al., 2001)
2 地质背景及样品采集

拉萨地块位于青藏高原南部,东西长约2000km,南北宽约300km。该地块北以班公湖-怒江缝合带为界与羌塘地块相邻,南以雅鲁藏布江缝合带为界与喜马拉雅地块接壤(图 1)。研究区位于尼玛县城周边,出露地层主要为下白垩统多尼组(K1d)、郎山组(K1l),上白垩统竟柱山组(K2j)。本次采样剖面位于尼玛县城西北10km(31.8°N、87.2°E),采样地层为上白垩统竟柱山组(K2j)地层,其岩性主要为紫红色砂岩和粉砂质泥岩(图 2b)。研究剖面产丰富的孢粉化石(Coptosporite granulataCoptosporites paradoxaTsugaepollenites ingiculusCedripites arcuatus等),根据化石组合特征确定其时代属晚白垩世早期(贾共祥等, 2007)。利用便携式古地磁钻机共采集了9个采点,每个采点采集古地磁样品8~12块,总计采集87块古地磁定向岩心标本。

图 2 研究区地质简图(a, 据河南省地质调查院,2002修改)和采样位置地质剖面图(b) Fig. 2 Simplified geological map of the study area (a) and measured geological section of the sampling area (b)

① 河南省地质调查院.2002.中华人民共和国1:250000尼玛区幅地质图

3 古地磁测试与分析

全部样品在室内加工成直径为2.54cm、高2.2cm的标准古地磁样品。所有样品均利用美制TD-48大型热退磁炉进行13步以上的系统热退磁处理,热退磁温度在低温段间隔大(50~60℃),高温段则间隔较密(20~30℃),剩磁测量在2G-755R超导磁力仪上进行。为检验岩石中剩磁的稳定性和岩心样品的主要载磁矿物,对代表样品进行了磁化率各向异性(Anisotropy of Magnetic Susceptibility, AMS)和三轴等温剩磁热退磁实验,测试仪器分别为KLY-4卡帕桥和JR-6A旋转磁力仪。以上所有测试均在国土资源部古地磁与古构造重建重点实验室内进行。样品的退磁结果分析利用主向量法(Kirschvink, 1980)或重磁化大圆弧方法(Halls, 1978),部分样品的高温特征剩磁方向的平均使用大圆弧交汇法(McFadden and McElhinny, 1988),最后以采样点为单位进行Fisher统计分析(Fisher, 1953)。

3.1 磁化率各向异性(AMS)结果

选择了42块代表样品进行了岩石磁化率各向异性测试。测试结果表明除了4块样品的磁化率各向异性度>5%以外,其他90%样品的磁化率各向异性度均<5%(图 3c),全部样品的磁化率各向异性度平均值为1.034。磁化率椭球体的形状以压扁状(磁面理>磁线理)为主(图 3b, c),层面校正后的磁化率各向异性椭球体最小轴(k3)近垂直于地层层面(I=88.1°),磁化率各向异性椭球体最大轴(k1)以低角度(I=0.8°)大致呈NWW-SEE优先方向展布(图 3a),其优先方向可能反映了岩石沉积时的水动力方向(Liu et al., 2001; Veloso et al., 2007)。以上岩石磁化率各向异性特征显示了研究剖面明显的沉积压实组构,岩石并未受到后期强烈构造变形作用的影响。

图 3 研究剖面代表样品磁化率各向异性赤平投影图(层面坐标下)(a)、磁面理F与磁线理L关系图(b)和磁化率椭球体形状因子T与磁化率各向异性度P’关系图(c) Fig. 3 Anisotropy of magnetic susceptibility data (in stratigraphic coordinates) (a), Flinn (F-L) diagram (b) and T-P' diagram (c) from the Jingzhushan Formation in studied area
3.2 三轴等温剩磁热退磁结果

对代表样品的Z轴、Y轴和X轴方向依次施加2.5T、0.4T和0.12T直流场,然后对样品进行系统热退磁。三轴等温剩磁热退磁实验结果表明,代表样品的剩磁成份主要由硬磁成份和中间磁成份组成。硬磁成份和中间磁成份的剩磁强度随温度升高逐渐衰减,在约680℃其剩磁强度衰减至零,揭示样品中赤铁矿的解阻,表明样品中主要载磁矿物为赤铁矿(图 4)。另外,中间磁成份在约580℃时的突然减小显示磁铁矿的解阻,表明样品中含有少量的磁铁矿。

图 4 研究剖面代表样品三轴等温剩磁热退磁结果图 Fig. 4 Thermal demagnetization of three components isothermal remnant magnetization of representative samples
3.3 系统热退磁结果

图 5给出了代表样品的系统热退磁矢量正交投影图,系统热退磁结果表明,部分样品存在一低温粘滞剩磁分量外(室温~300℃左右)(图 5a, c, e, g),其高温剩磁分量(300~680℃)随着温度的升高逐渐趋向原点,可以通过主向量分析方法分离出来(图 5a-e)。但部分样品的高温剩磁分量随着温度的升高并不完全趋向原点(图 5f, g)。如样品SK6-5剩磁方向在180~640℃温度段间呈一大圆弧分布,样品SK7-6剩磁方向在500℃以上较为杂乱,但在180~450℃温度段间呈大圆弧分布;对该特征的高温剩磁分量则通过重磁化大圆弧方法分析(Halls, 1978)。

图 5 研究剖面代表样品Z矢量图及等面积投影图(地理坐标下) (a-e)为主向量分析法分析的样品;(f-g)为重磁化大圆弧法分析的样品.实心圆圈和空心圆圈分别代表剩磁方向在水平面和铅垂面上的投影 Fig. 5 Orthogonal demagnetization diagrams and Equal-area plots in situ coordinates of representative rocks from study area The solid and open circles represent vectors endpoints projected onto horizontal and vertical planes, respectively

样品高温特征剩磁分量经分析后,对各采样点统计平均,获得了各采点的平均磁化方向(表 1)。从表 1中可以看出高温特征剩磁方向具有正、反双极性,由于采点SK8的α95值(18.7°)>15°,所以采点SK8的古地磁结果没有参加以采点为单位的最终统计。通过其余8个采点获得了采样剖面晚白垩世高温特征剩磁分量的平均磁化方向:Dg=9.9°,Ig=5.5°,κg=30.3,α95=10.2°(地理坐标下);Ds=8.2°,Is=27.4°,κs=37.6,α95=9.2°(层面坐标下)(表 1图 6)。由于采样剖面为一单斜地层,地层产状变化不大,无法开展古地磁结果的褶皱检验,但对构造校正后的正、反极性高温特征剩磁方向进行了倒转检验,结果表明通过了C级倒转检验(r=13.6°<rc=17.7°)(McFadden and McElhinny, 1990)。

表 1 拉萨地块北缘尼玛县地区晚白垩世古地磁结果 Table 1 Late Cretaceous paleomagnetic results from Nyima area in the northern margin of the Lhasa block

图 6 尼玛县地区上白垩统竟柱山组高温特征剩磁分量赤平投影图 五角星代表样品平均方向的位置;实心圆、空心圆分别代表上、下球面投影 Fig. 6 Equal-area projections of site-mean directions from the Upper Cretaceous Jingzhushan Formation in Nyima area
4 讨论 4.1 拉萨地块北缘晚白垩世古纬度位置重建

目前在拉萨地块获得了大量的晚白垩世古地磁数据(表 2),但是这些古地磁数据大部分来自于拉萨地块南部、中部和西缘(图 1),而在拉萨地块北部仅有两组来自那曲地区安山岩的古地磁结果(Lin and Watts, 1988)(图 1表 2)。但是Lin and Watts (1988)所发表的拉萨地块北部晚白垩世古地磁结果中,不仅样品数量少,而且未经过任何稳定性检验(褶皱检验或倒转检验),在此基础上讨论其古地磁结果的地质意义显然缺少科学性。本次研究剖面位于拉萨地块北缘尼玛地区(图 1),采样地层为上白垩统竟柱山组红色砂岩。虽然一些学者认为拉萨地块白垩纪红层沉积物可能存在因压实作用导致的磁倾角浅化现象(Tan et al., 2010; Lippert et al., 2014),但是根据拉萨地块白垩纪火山岩与沉积岩古地磁数据的对比研究,揭示了拉萨地块白垩纪红层沉积物并不存在明显的磁倾角浅化现象(Sun et al., 2012; Ma et al., 2014; Yang et al., 2015; 马义明, 2016; Cao et al., 2017; Li et al., 2017)。另外,拉萨地块中部措勤地区(Yang et al., 2015; 马义明, 2016)和东部丁青地区(Tong et al., 2017)的古地磁学研究均表明竟柱山组地层未发生明显的磁倾角偏低现象。据此,我们认为本次研究剖面晚白垩世红层的古地磁结果可能并未受磁倾角浅化作用的影响。由于本次研究剖面所获得的晚白垩世高温特征剩磁方向在地理坐标下远离现代地磁场方向(图 6),且这一高温特征剩磁方向具有正、反双极性特征,并通过了倒转检验。据此认为所获得的高温特征剩磁方向很可能代表了岩石形成时的原生剩磁,其晚白垩世的古地磁极位置为:71.2°N/241.9°E,dp/dm=5.5°/10.0°,这一结果表明研究区(拉萨地块北缘)在晚白垩世位于北纬14.5°±7.4°的位置(参考点:31.8°N、87.2°E)。

表 2 拉萨地块晚白垩世古地磁极统计表 Table 2 Late Cretaceous paleomagnetic poles from the Lhasa block

将现有拉萨地块晚白垩古地磁极数据列于表 2图 7。从图 7表 2可以看出,本次研究获得的拉萨地块北缘晚白垩世古纬度与拉萨地块南部(Achache et al., 1984; Lin and Watts, 1988; Tan et al., 2010; Sun et al., 2012; Cao et al., 2017)、西缘(Yi et al., 2015)和东缘(Tong et al., 2017)的古纬度(参考点:31.8°N/87.2°E)在古地磁置信范围内没有差别。这表明拉萨地块的北部、南部、东部和西缘自晚白垩世以来在在动力学意义上为一个整体,但是拉萨地块不同地区之间在晚白垩世以来存在明显的局部水平旋转作用。

图 7 拉萨地块晚白垩世古地磁极赤平投影图 Fig. 7 Equal-area projection of the Late Cretaceous paleomagnetic poles of the Lhasa block
4.2 晚白垩世以来拉萨地块北缘与稳定亚洲大陆之间的构造缩短量

将本次研究获得的拉萨地块北缘晚白垩世古地磁极对比稳定亚洲大陆晚白垩世参考极的平均极(70~100Ma:79.9°N,214.1°E,A95=2.3°)(表 2)(Cogné et al., 2013),表明自晚白垩世以来拉萨地块北缘与稳定亚洲大陆之间发生了1200±630km(古纬度差:10.9°±5.7°)的南北向构造缩短,但并未发生明显的相对旋转作用(0.8°±5.9°)(图 7)。

拉萨地块内部构造缩短发生的时间主要集中在早白垩世(Murphy et al., 1997; Kapp et al., 2007a)。冈底斯弧后冲断带的研究也显示新生代印度与亚洲大陆碰撞以来拉萨地块内部并未发生大规模的南北向构造缩短(Kapp et al., 2007b)。地质资料表明印度与亚洲大陆碰撞以来至少有1400km的南北向构造缩短被喜马拉雅-青藏高原造山带所吸收(Yin and Harrison, 2000),而造成亚洲大陆内部的构造缩短至少有1000km(Yin and Harrison, 2000; Johnson, 2002; Guillot and Replumaz, 2013)。而古地磁证据显示中亚诸块体在印度与亚洲大陆碰撞后并未发生明显的南北向构造缩短(Huang et al., 2006),表明亚洲大陆内部构造缩短主要发生在中亚以南的区域。印度与亚洲大陆碰撞后在羌塘地块至少产生了约120km的南北向构造缩短(Kapp et al., 2005);在松潘-甘孜地块至少产生了200km的南北向构造缩短(Coward et al., 1988; van Hinsbergen et al., 2011);在西昆仑地区南北向构造缩短至少为140~187km(Cowgill et al., 2003);祁连山-南山地区的缩短量约为370km(Yin and Harrison, 2000; Guillot and Replumaz, 2013);柴达木地块的构造缩短量约为170km(Guillot and Replumaz, 2013);阿尔金断裂带缩短量约为97km(Darby et al., 2005; Yue et al., 2005)。此外,印度与亚洲大陆新生代以来的碰撞作用导致高原内部物质向东运移,沿大型走滑断裂向东南侧向滑移约600km(Tapponnier et al., 1990; Yang et al., 2001; Sato et al., 2001; Replumaz and Tapponnier, 2003; Tong et al., 2013)。由此可见,在青藏高原拉萨地块以北地区由于印度与亚洲大陆碰撞所产生的南北向构造缩短量总计>1100km。这一构造缩短量与本次古地磁研究获得的晚白垩世以来拉萨地块北缘与亚洲大陆之间的构造缩短量基本一致,这也表明印度与亚洲大陆碰撞所造成的亚洲大陆内部南北向构造缩短主要集中在拉萨地块以北的区域。

5 结论

本次研究获得拉萨地块北缘晚白垩世古地磁极为:71.2°N,241.9°E,dp/dm=5.5°/10.0°。结果表明拉萨地块北缘在晚白垩世位于~14.5°N的古纬度位置(参考点:31.8°N,87.2°E)。这一结果进一步验证了亚洲大陆最南缘(拉萨地块)在晚白垩世位于北纬~10°~15°的位置。将本次研究获得的拉萨地块晚白垩世古地磁极对比稳定亚洲大陆参考极显示晚白垩世以来拉萨地块北缘与稳定亚洲大陆之间发生了1200±630km(10.9°±5.7°)的南北向构造缩短,并未发生明显的相对旋转作用(0.8°±5.9°)。综合地质资料认为印度与亚洲大陆碰撞所造成的亚洲大陆内部南北向构造缩短主要集中在拉萨地块以北的区域。

致谢 野外工作中得到了吴建国和周伦的大力协助;黄宝春教授和杨天水教授对文稿提出了宝贵意见;在此一并致谢。
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