滇西兰坪盆地白垩纪—早第三纪古地磁结果及其地质意义^*

尹济云　孙知明　杨振宇　梁其中

摘　要　通过对滇西兰坪盆地白垩系地层的古地磁采样和室内退磁研究，揭示出一组高温特征分量，95%正倒转检验和99%置信度下的正褶皱检验，说明这组高温分量很可能代表岩石形成时的原生剩磁．对比国际标准地磁极性年表，发现南新组与阿尔比期、赛诺曼期和土仑期相当，而虎头寺组则相当于晚白垩世的三冬期和康尼阿克期．与中国东部的白垩纪古地磁数据比较，进一步证实了晚白垩世华南与印度支那地块存在明显的纬度差，这一事实说明了印度支那地块在印度板块与欧亚板块的碰撞及进一步挤压下，印度支那地块在早第三纪沿红河大断裂走滑千余公里，并伴随着15°—20°的顺时针旋转．
关键词　兰坪盆地，白垩纪—第三纪，磁性地层，构造滑移．

CRETACEOUS AND EARLY TERTIARY PALEOMAGNETIC RESULTS FROM THE LANPING BASIN AND ITS GEOLOGICAL IMPLICATIONS

YIN JI-YUN¹⁾　SUN ZHI-MING²⁾　YANG ZHEN-YU²⁾　LIANG QI-ZHONG¹⁾
1)Yunnan Institute of Geological Sciences, Kunming 650011, China
2)Institute of Geomechanics, CAGS, Beijing 100081, China

Abstract　After detailed paleomagnetic sampling and thermal demagnetization for Cretaceous and Early Tertiary rocks from the Lanping basin, Western Yunnan, a group of higher temperature characteristic components were revealed. The directions from both the Jinxing and Yunlong formations pass reversal test at 95% confidence level, and a positive fold test is achieved at the 99% confidence for the Nanxin and Futoushi formations. It implies that this group of directions may represent a primary magnetization formed during the formation of the rocks. Comparing with geological time scale, we note that Nanxin formation can be correlated with Cretaceous Albian, Cenomanian and Turonian stages, Futuoshi formation matches with Coniacian and Santonian stages. In comparison with available lower-middle Cretaceous paleomagnetic results from Eastern China, a paleolatitude difference of 11.5°±7.6° is confirmed between Indochina and Easter China. It corresponds a significant southeastward displacement (＞1000km) and accompanied with clockwise rotation of 15°—20° of Indochina relative to Eastern China along the Red River fault since late Cretaceous time, which resulted from the collision and penetration of India into Asia.
Key words　Lanping Basin, Cretaceous and Tertiary, Magnetostratigraphy,
Extrusion.

1　引　言

　　红河西侧兰坪盆地出露的白垩系—早第三系陆相红层碎屑岩，尽管双壳类、介形虫、叶肢介及轮藻等古生物研究获得了许多进展，然而这些红层较准确的地层年代学工作则未得到深入开展，给跨区地层对比和划分带来困难^［1］．因此，开展系统的磁性地层研究是解决这一问题的关键．兰坪盆地地处红河大断裂西侧，是印支地块的北部延伸．出露较完整的中、新生代地层为研究本区的大地构造问题提供了重要的物质基础．Yang和Besse^［2］和杨振宇等^［3］认为红河西侧印支地块在白垩纪后沿着红河剪切大断裂发生近千公里的大型走滑，这一地块的侧向滑移应是印度板块向欧亚大陆俯冲碰撞以及进一步挤压作用的结果，并已初步被野外地质资料证实^［4，5］．前人对中国境内印度支那地块的白垩纪古地磁研究结果，多强调该块体的旋转作用，而未注意到印支地块的纬向变化特征^［6—9］．我们在获得云龙地区可靠的磁性地层资料基础上，同时讨论其大地构造意义．

2　古地磁样品的采集与测试

　　选择云龙县城南江沿岸地质剖面，连续采集了119块古地磁定向样品，基本控制了景星组、南新组、虎头寺组和云龙组下段地层．所有样品的退磁和剩磁测试均在中国科学院地球物理研究所古地磁实验室用2G-55R卧式超导磁力仪进行．

3　测试结果分析

　　所有样品经系统热退磁后，采用主向量分析方法^［10］分离磁性分量，或重磁化弧分析^［11］，样品不同剩磁分量的平均采用McFadden和McElhinny^［11］方法．从图1可见，这些红层的热退磁结果表现出多种磁成分组合类型：单一磁性组分，双磁性组分以及3种磁性分量的组合叠加，图中M为磁化强度．

图1　代表样品的Z矢量图
实心圆和空心圆分别为水平垂直投影，YL8：样品名，IS：地理坐标，TS：层面坐标.
Fig.1　Orthogonal vector projection for representative spacimens from Luanping Baisn solid/open symbols refer to projection on the horizontal/vertical plane, IS:geogrphic, TS:stratigrphic.

　　1. 单一磁分量见图1(f)，从室温至700℃，磁化方向基本保持不变．
　　2. 低温分量200℃或300℃，和高温分量500—680℃或710℃(图1(b)，(c))．
　　3. 低温分量200℃，中温分量(300—500℃)和高温分量(575—680℃或710℃)(图1(a，d，e)．
　　从统计分析可以看出，低温磁分量产状校正后，磁化方向变得更为分散，在地理坐标中的剩磁偏角与倾角的平均值分别为：D＝6.9°，I＝44.4°，置信圆a95＝10°(图2，n＝66块样品)．这一方向与现代地磁场偶极子场(D＝0°，I＝44°)极为相似，所以低温分量很可能是近代地磁场的粘滞剩磁．中温磁分量仅从少量样品中获得，磁化方向极为分散(在地理坐标下：D_g＝329.9°，I_g＝58.3°，a95＝41°；在层面坐标下：D_s＝356°，I_s＝27.8°，a95＝34.6°，n＝7块样品)，所以目前还无法了解该磁成分的含义．高温分量可以从绝大部分样品可获得高温磁分量，对单一磁分量样品，取550—680℃温度段的磁化方向参加统计，其方向与从多磁成分分离出的高温分量非常吻合．

图2　样品低温分量的赤平投影图
(a)地理坐标；(b)层面坐标.
五角星代表样品平均方向的位置，实心圆为上半球，空心圆为下半球，下同.
Fig.2　Equal-area projection of low-temperature vector before (a) and after (b) tectonic tilt correction

　　早白垩世景星组：从23块样品中分离出的高温磁分量具正、反两极性，且呈对趾关系，置信度在95%时通过了McFadden和McElhinny倒转检验^［1］，平均磁化方向在地理坐标下为D_g＝103.2°，I_g＝61.6°，a95＝20.3°(在层面坐标下，D_s＝59.7°，I_s＝41.0°，K_s＝7.4°，a95_s＝111.9°)，与白垩纪以及现代地磁场方向有明显差异(表1，图3)．

图3　早白垩世景星组高温分量的赤平投影图
(a)构造倾角校正前地理坐标下；(b)构造倾角校正后层面坐标下.
实心正方形和空心正方形分别代表正极性样品和负极性样品的平均方向位置，下同.
Fig.3　Equal-area projection of early-Cretaceous sample-mean high-temperature components of Jingxing formation before (a) and after (b) tectonic tilt correction

　　晚白垩世南新组和虎头寺组：从74块样品中分离出的高温分量均为正极性，在99%置信水平上磁化方向通过了McElhinny和McFadden褶皱检验^{［13，14］}(表1，图4)，从区域地质研究发现褶皱期为始新世^［1］，磁化方向应形成于前始新世．

图4　晚白垩世南星组和虎头寺组高温分量的赤平投影图
(a)地理坐标；(b)层面坐标.
Fig.4　Equal-area projection of late-Cretaceous site-mean high-temperature components of Nanxing and Hutoshi formation before (a) and after (b) tectonic tilt correction

　　古新世云龙组：从11块样品中分离出高温磁分量，具正反两极性，且呈明显的对趾关系，在95%置信水平上正、反磁化方向通过了McFadden和McElhinny倒转检验^［12］．平均磁化方向在地理坐标下为D_g＝49.2°，I_g＝－7.6°, a95＝16°，与近代地磁场方向有明显差异(表1，图5)．

图5　古新世云龙组高温分量的赤平投影图
(a)地理坐标；(b)层面坐标.
Fig.5　Equal-area projection of Paleocene site-mean high-temperature components of Yunlong formagion before (a) and after (b) tectonic tilt correction

　　各组高温磁分量的解阻温度，均在680℃左右，磁化方向的剩磁载体应是赤铁矿．从景星组和云龙组样品中分离出的高温磁特征分量通过了倒转检验，且南新组和虎头寺组的特征分量通过了褶皱检验(置信度99%)．表明它们代表原生磁化方向．

4　讨论

4.1　地层年代的确定
　　以白垩纪和古新世地层各层位的位置为纵坐标，样品中分离出的高温特征磁化方向为横坐标，绘制了该剖面的磁极性序列(图6)，可看出南新组和虎头寺组均为正向极性，与白垩世正向极性超带相吻合．南新组中部含少量双壳类及介形虫化石，而虎头寺组则岩性单一，地层划分无生物化石资料．所以南新组与虎头寺组被划为早白垩世^［1］．假定南新组和虎头寺组沉积速率较均一，南新组厚度达1495m，而虎头寺组则仅138m．根据磁性地层的对比，初步确定南新组相当于早白垩世晚期的阿尔比期、晚白垩世的赛诺曼期和土仑期，而虎头寺组则相当于晚白垩世的三冬期和康尼阿克期^［15］．

图6　云龙地区白垩纪和古新世地层的古地磁结果及其磁极性序列(d为地层厚度)
Fig.6　Paleomagnetic result of Cretaceous-Paleocene sediments from Yunlong area and correlations of magnetic polarity sequence

　　南新组整合于早白垩世景星组之上，从景星组中揭示出8个极性带，它很可能与巴雷姆期及戈特里夫期等极性相对应，但目前还难以进行准确对比．云龙组与虎头寺组之间有一沉积间断，给磁极性对比增加了难度，目前还不能确定其年代．
4.2　大地构造意义
　　滇西兰坪和思茅盆地白垩纪古地磁研究表明(表2)，这些地区喜山期构造运动造成明显的顺时针旋转，然而各地区旋转量差异较大^［3］．由于一些客观原因造成古地磁研究的误差较大，使得对其古纬度的估算也存在较大误差，以致于难以确定印度支那块体相对于中国东部是否发生构造滑移．

　　近年来中国东部白垩纪古地磁研究取得了很大进展，从现有的数据可以看出(表3，图7)，早白垩世与晚白垩世极位置在95%误差范围内已无明显的差异，且与欧亚大陆同时期极位置一致^［3］．说明了华南、华北地块以及欧亚大陆已构成一个动力学范畴上的整体板块．印度支那地块是否存在同南部一样大规模的侧向滑移^［2，3］，可以通过对比云龙地区白垩纪时观测的古纬度和据中国东部白垩纪极位置推测的古纬度之差加以验证．

图7　中国东部及印度支那地块白垩纪的级位置图
Fig.7　Cretaceous paleomagentic poles of eastern China and Indochina block

　　由早白垩世古地磁数据推测古纬度差为3.4±8.8°；而从晚白垩世古地磁数据推测的古纬度差为11.5±7.6°．早白垩世古纬度差明显小于晚白垩世的推测值，且从早白垩世获得的纬度差多数在95%误差范围内无显著意义，此结果验证了早白垩世结果^{［6，8，9］}．
　　印度支那地块北部地区早、晚白垩世间古纬度差的差异，一方面由于是早白垩世古地磁研究还不完善(如采样点或样品数偏少)，另一方面也可能反映了印度支那地块从早白垩世开始就与华南地块存在相对(滑移)运动，陈炳尉等^［16］通过对红河走滑断裂的研究，认为燕山期红河存在左行走滑特征．这两种可能性显然需要进一步工作的证实．
　　根据云龙地区晚白垩世的结果推测印度支那地块与中国东部的古纬度差与从泰国呵叻(Khorat)盆地晚白垩世的结果推测值11.5±6.7°明显一致^［2］，说明印度支那地块在印度板块与欧亚板块的碰撞及挤压过程中，向东南滑移近1000km以上^［3］，云龙地区伴随着20.1±8.6°的顺时针旋转，与呵叻盆地的顺时针旋转14.2±7.1°较为一致．

*国家自然科学基金资助项目(49504051)和云南省自然科学研究基金资助项目(96D125M).
作者简介：尹济云，男，1957年11月生，1978年毕业于云南省地矿局职业大学．现为云南省地质矿产研究所工程师，主要从事地质与古地磁学研究工作．
作者单位：尹济云　梁其中　云南省地质矿产研究所，昆明 650011
　　　　　孙知明　杨振宇　中国地质科学院地质力学研究所，北京 100081

参考文献

1　云南省地矿局．云南省区域地质志．北京：地质出版社，1989
　　Bureau of Geology and Mineral Resouces of Yunnan Province, Regional geology of Yunnan province (in Chinese). Beijing: Geological publishing House. 1989
2　Yang Z, Besse J. Paleomagnetic study on Permian and Mesozoic sedimentary rocks from N-orth Thailand supports the extrusion model for Indochina. Earth and Planet Sci. Lett, 1993, 117:525—552
3　杨振宇，Besse J，孙知明等．印度支那地块第三纪构造滑移与青藏高原岩石圈构造演化．地质学报，1998，72(2)：112—125
　　YANG Zhen-Yu, Besse J, SUN Zhi-Ming, et al. Tertiary extrusion of the Indochina block and Lithosepheric tectonic evolution of the Tibetan platean, Acta Geological Sinica (in Chinese), 1998, 72(2):112—125
4　钟大赉，Tapponnier P，吴海威等．大型走滑断裂层——碰撞后陆内变形的重要形式．科学通报，1989，34(7)：526—529
　　ZHONG Da-Lai, Tapponnier P, WU Hai-Wei, et al. Enormous strike-slip faulting: An important style of intercontinental deformation after collision. Chinese Science Bulletin (in Chinese), 1989, 34(7):526—529
5　任纪舜，金小赤．红河断裂的新观察．地质论评，1996，42(5)：439—442
　　REN Ji-Shun, JIN Xiao-Chi. New observations of the Red River fault. Geological Review (in Chinese), 1996, 42(5):439—442
6　Huang K, Opdyke N. Paleomagnetic results from Cretaceous and Jurassic rocks of South and Southest Yunnan: evidence for large clockwise rotations in the Indochina and Shan-Thai-Malay Terranes. Earth Planet.Sci. Lett., 1993, 117:507—524
7　Funahara F, Nishiwaki N, Murata F, et al. Clockise rotation of the Red River fault inferred from paleomagnetic study of Cretaceous rocks in the Shan-Thai-Malay block of estern Yunnan, China. Earth and Planet Sci. Lett. 1993, 117:29—42
8　陈海泓，郝杰，Dobson J P等．云南思茅地区新生代的持续旋转．科学通报，1993，38(24)：2269—2272
　　CHEN Hai-Hong, HAO Jie, Dobson J P, et al. Continuous rotation of Simao area, Yunnan province since Cenozoic. Chinese Science Bulletin(in Chinese), 1993, 38(24):2269—2272
9　Chen H, Dobson H J, Heller F, et al. Paleomagnetic evidence for clockwise rotation of the Simao region since the Cretaceous: A consequence of India-Asia collision. Earth and Planet Sci. Lett, 1995, 134:203—217
10　Kirschvink J L. The least-squares line and plane and the analysis of paleomagnetic data. Geophys. J. R. Astron. Soc. 1980, 62:699—718
11　McFadden P L, McElhinny M W. The combined analysis of remagnetization circle and direct observation in paleomagnetism. Earth Planet. Sci. Lett., 1988, 87:161—172
12　McFadden P L, McElhinny M W. Classification of the reversals test in paleomagnetism. Geophys. J. Int. 1990, 103:725—729
13　McElhinny M W. Statistical significance of the fold test in paleomagnetism. Geophys. J. R. Astron. Soc. 1964, 8:338—340
14　McFadden P L. A new fold test for paleomagnetic studies. Geophys. J. Int., 1990, 103:163—169.
15　Harland W B, Armstrong R, Cox A, et al. A Geologic Time Scale. New York: Cambridge University Press,1989, 263
16　陈炳蔚，曲景川，艾长兴等，三江地区主要大地构造问题及其与成矿的关系．北京：地质出版社，1991
　　CHEN Bing-Wei, CHU Jing-Chuan, AI Chang-Xing et al. The major question of tectonic and its relation with deposits in the Three River area (in Chinese). Beijing: Geological Publishing House, 1991
17　Zheng Z, Kono M, H, Tsunakawa G et al. The Apparent polar wander path for the North China Block since the Jurassic. Geophys. J. Int., 104:29—40, 1991.
18　胡联浩，李普，马醒华．闽西上杭地区白垩纪地层的磁性地层研究．福建地质，1990，1：33—42
　　HU Lian-Hao, LI Pu, MA Xing-Hua. Magnetostratigraphical study on the Cretaceous sediments from Shanghang area, Western Fujian. Fujian Geology (in Chinese), 1990, 1:3—42
19　Kent D V, Zeng, X S, Zhang W Y, et al, Widespread late Mesozoic to recent remagnetization of Paleozoic and lower Triassic sedimentary rocks from south China. Tectonophysics, 1987, 139:133—143
20　Hsu V. Paleomagnetic results from one of the red basin in South China. EOS Trans AGU, 1987, 68:295
21　Enkin R, Courtillot V, Xing L et al. The stationary Cretaceous paleomagnetic pole of Sichuan (South China Block). Tectonics, 1991, 10:547—559
22　朱志文，郝天珧，赵惠生. 攀西及邻区印支燕山期地块构造运动的古地磁考证．地球物理学报，1988，31(4)：420—431
　　ZHU Zhi-Wen, HAO Tian-Yao, ZHAO Hui-Sheng. Paleomagnetic study on the tectonic motion of Pan-Xi block and adjacent area during Yin Zhi-Yanshan period. Chinese T. Geopuys. (in Chinese), 1988, 31(4):420—431
23　Huang K, Opdyke N. Paleomagnetism of Lower Cretaceous to Lower Tertiary rocks from Southwestern Sichuan: a revisit, Earth Planet. Sci. Lett., 1992, 112:29—40
24　Gilder S, Coe R S, Wu H et al. Cretaceous and Tertiary paleomagnetic results from Southeast China and their tectonic implications. Earth Planet. Sci. Lett., 1993, 117:637—652
25　Ma X, Yang Z, Xing L. The lower Cretaceous reference pole from the North China, and its tectonic implications. Geophysical J. Int., 1993, 115:323—331
26　Pruner P. Paleomagnetism and paleogeography of Mongolia from the Carboniferous to the Cretaceous-final report. Physics of the Earth and Planetary Interiors, 1992, 70:169—177
27　Zhao X, Coe R S, Zhou Y, et al, New paleomagnetic results from northern China collision and suturing with Siberia and Kazakhstan. Tectonophysics, 1990, 181:43—81
28　Otofuji Y, Liu Y, Yokoyama M et al. Tectonic deformation of the Southwestern part of the Yangtze craton inferred from Paleomagnetism. Earth Planet. Sci. Lett., 1998, 156,47—60
29　Zhai Y, Seguin M, Zhou Y, et al. Paleomagnetic evidence for the existence of Nanpanjiang Sea between the Yangzi and Huanan blocks, China. EOS Trans. AGU, 1991, 17:105
30　Lin J L, Fuller M, Zhang W. The apparent polar wander paths for the North and South China blocks, Nature, 1985, 313:444—449
31　庄忠海，田端孝，马醒华等，四川盆地雅安地区白垩纪至下第三纪古地磁研究．物探与化探，1988，12(3)：224— 228
　　ZHUANG Zhong-Hai, TIAN Duan-Xiao, MA Xing-Hua, et al. Paleomag netic study on Cretacoeus—late Tertiary sediment, Ya'an area, Sichuan Basin. Geophysical Prospecting and Geochemical Exploration (in Chinese), 1988, 12(3):224—228
32　Chan L. Paleomagnetism of Late Mesozoic granitic instrusions in Hong Kong: implications for Upper Cretaceous reference pole of south China. J. Geophys. Res., 1991, 96:327—335
33　Li Z, Metcalfe I, Wang X F. Vertical-axis block rotation in southeast China: new paleomagnetic results from Hainan Island. Geophys. Res. Lett., 1995, 22:3071—3074
34　吴汉宁，朱日祥，刘椿等．华北地块晚古生代至三叠纪古地磁研究新结果及其构造意义．地球物理学报，1990，33(6)：694—701
　　WU Han-Ning, ZHU Ri-Xiang, LIU Chun, et al. Paleomagnetic observation in North China block: from late Paleozoic to Triassic. 1990, Chinese J. Geophys. (in Chinese) 33(6):694—701
35　Gilder S, Courtillot V. Timing of the North-South China collision from new middle to late Mesozoic Plaeomagnetic data from the North China block. J. Geophys. Res., 1997, 102:17713—17727
36　Yang Z, Besse J, Sutheetorn V, et al. Jurassic and Cretaceous paleomagnetic data for Indochina: Paleogeographic evolution and deformation history of South Eastern Asia. Earth and Planetary Science Letters,1995, 136:325—341