地球物理学报  2015, Vol. 58 Issue (7): 2413-2423   PDF    
四川盆地东西陆块中下地壳结构存在差异
熊小松1, 高锐1, 张季生1, 王海燕1, 郭良辉1,2,3    
1. 中国地质科学院地质研究所, 国土资源部深部探测与地球动力学重点实验室, 大陆构造与动力学国家重点实验室, 北京 100037;
2. 地下信息探测技术与仪器教育部重点实验室(中国地质大学, 北京), 北京 100083;
3. 中国地质大学(北京)地球物理与信息技术学院, 北京 100083
摘要:四川盆地是中上扬子克拉通的主要组成部分.作为我国三大稳定克拉通之一,扬子克拉通经历了自太古代以来的长期演化,直到新元古代晚期与华夏板块发生碰撞拼合前,一直被认为是一个稳定的统一陆块.基底包括了新太古宙-新元古代岩层,其上广泛被新元古代晚期至显生宙地层覆盖,仅有~2.9—2.95 Ga基底岩石零星出露于四川盆地的西缘、西南缘和三峡地区,使得对于沉积盖层之下的中下地壳的性质和分布规模的认识十分有限.重力异常则能够宏观揭示区域结构特征.本文通过刨除沉积盖层和莫霍面起伏引起的重力异常而获得了中下地壳的重力异常,反映了四川盆地东西陆块中下地壳存在结构差异,结合深地震反射资料、航磁异常和地球化学资料,证实了该分界线位于重庆—华蓥一线,故而推测中上扬子克拉通在太古宙-古元古代可能存在东西两个陆核.
关键词四川盆地     中下地壳     基底     陆核     重力异常    
Differences of structure in mid-lower crust between the eastern and western blocks of the Sichuan basin
XIONG Xiao-Song1, GAO Rui1, ZHANG Ji-Sheng1, WANG Hai-Yan1, GUO Liang-Hui1,2,3    
1. State Key Laboratory of Continental Tectonics and Dynamics, Key Laboratory of Earthprobe and Geodynamics, MLR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. Key Laboratory of Geo-detection (China University of Geosciences, Beijing), Ministry of Education, Beijing 100083, China;
3. School of Geophysics and Information Technology, China University of Geosciences, Beijing 100083, China
Abstract: The Sichuan basin is the main part of the middle-upper Yangtze craton, which has been experienced a long-term tectonic evolution since Archean. It is regarded as a stable block until the collision with the Cathaysia block in late Neoproterozoic. As one of the largest and most complex Archean cratons in China, the Yangtze craton consists of Archean-Paleoproterozoic crystalline basement (mostly covered) outcropped in the western and southwestern margins and the three George area surrounded by late Mesoproterozoic to early Neoproterozoic fold belts, which are locally unconformably overlain by middle Neoproterozoic weakly metamorphosed strata and late Neoproterozoic unmetamorphosed Sinian and Phanerozoic cover. These strata limit the understanding of the nature and distribution characteristics of the middle-lower crust beneath the sedimentary rocks. Gravity anomalies can reveal the overall regional structure. In this paper we obtain the middle-lower crustal gravity anomalies by removing the gravity anomalies induced by the sedimentary rocks and the mantle beneath the Moho.
Gravity anomalies measured on the surface have many possible origins, including deposits, structures, mass excess or defect caused by interfaces in the crust. First, we obtain the Moho depth and sedimentary thickness contours from resampling and interpolating the deep seismic profiles collected. Then we obtain the middle-lower crustal gravity anomalies by removing the gravity anomalies induced by the sedimentary rocks and the mantle beneath the Moho.
The middle-lower crust of the Sichuan basin has gravity anomalies ranging form -32 to 206 mGal, unevenly distributed. It can be divided into western and eastern parts bounded by the Chongqing-Huaying line. In the western part, there are two high gravity anomaly closures, centered at Neijiang-Suining and Nanchong-Dazhou, respectively. The maximum anomaly value reaches 170 mGal. In the eastern part, there is only one anomaly closure with 170 mGal as the highest value. Therefore, the gravity anomaly structures of the Sichuan basin on either side of the Chongqing-Huaying line are different.
Combined with the deep seismic reflection profiling and other geochemical and geophysical data, we propose that the boundary of mid-lower crust between the eastern and western parts is along the Chongqing-Huaying line, which may indicate that there are two continental nucleuses beneath the Yangtze block.
Key words: Sichuan basin     Middle-lower crust     Basement     Continental nucleuse     Gravity anomaly    
1 引言

四川盆地是中上扬子克拉通的主要组成部分,航磁上表现为北东向展布的宽缓正磁异常夹弱磁异常,为扬子克拉通最稳定的区域(包茨等,1985).扬子克拉通经历了自太古代以来的长期演化,直到新元古代晚期与华夏板块发生碰撞拼合前,一直被认为是一个稳定的统一陆块.基底包括了新太古宙-新元古代岩层,其上广泛被新元古代晚期至显生宙地层覆盖,仅有~2.9—2.95 Ga基底岩石零星出露于扬子克拉通的西缘、西南缘和三峡地区(Greentree and Li,2008; Qiu et al.,2000; Wu et al.,2012),使得对于基底的性质和分布规模的认识十分有限,而基底断裂带和结构对于沉积盖层结构和后期构造演化具有重要的控制作用.重力方法是地球物理的主干方法之一,长期在区域地质构造研究发挥着重要作用,是宏观刻画基底结构的有效方法,尤其是经过各项校正得到的布格重力异常不仅包含了地壳内部各种偏离正常密度分布的矿体与构造的影响,也包括了地壳下界面起伏而在横向上相对上地幔质量的巨大亏损或盈余的影响,横向分辨率高,能够用于地壳结构和地质构造的解释研究.对盆地布格重力异常剥离沉积盖层和莫霍面起伏引起的重力异常后,所获得的重力异常能客观地反映沉积盖层以下、莫霍面以上的中下地壳的结构和构造特征.同时,深地震反射剖面是揭示岩石圈精细结构的关键技术,其分辨能力从浅部几米到深部几十米,对于我们认识克拉通的包括基底在内的深部结构提供了其他方法无法比拟的资料.本文以重力资料为主,结合航磁异常和深地震反射剖面资料等其他地球物理和地球化学资料,探讨了四川盆地中下地壳结构存在差异,可能代表了太古代-早元古代以四川盆地为主要组成的中上扬子克拉通存在东西两个陆核.

2 地质背景

四川盆地是中上扬子克拉通的主要组成部分,作为我国最复杂和最大的克拉通之一,扬子克拉通 基底岩石包含太古代-早元古代结晶基底,中元古代 晚期到新元古代早期褶皱带环绕,并被新元古代中期的变质岩层和新元古代晚期未变质的岩层不整合覆盖(Zhao and Cawood,2012),出露的基底岩石主要是元古代岩石,太古代岩石则零星出露于扬子克 拉通的西缘和北缘(图 1).随着年代学分析技术的不断发展,越来越多的太古代物质被确定(Jiao et al.,2009; Qiu et al.,2000; Zheng et al.,2006),可能说明了扬子克拉通广泛存在古老的结晶基底(Wang et al.,2013a; Zhang et al.,2006a; Zheng et al.,2006).冷家溪群和相应的岩群过去常被认为是中元 古代,最新的SHRIMP(Sensitive High-Resolution Ion MicroProbe,敏感高分辨离子探针)定年则限定了其820 Ma的年龄,其上被820—750 Ma的板溪群所覆盖.冷家溪群为一套浅变质、强变形的类复理石沉积组合,以陆源碎屑岩为主,局部夹有少量的火山岩.这套地层被命名为冷家溪群,与江西的双桥山群、皖南的溪口群以及桂北的四堡群、贵州东北部的梵净山群等相当,主要为一套灰色、灰绿色绢云母板岩、条带状板岩、粉砂质板岩与岩屑杂砂岩、凝灰质砂岩组成复理石韵律特征的浅变质岩系,地层普遍发生褶皱变形,褶皱构造主要表现为强烈的紧闭线状褶皱;板溪群高角度不整合覆盖于冷家溪群之上,为一套火山-沉积岩系,几乎没有变质,变形作用微弱,大致相当于贵州地区的下江群、广西地区的丹洲群和皖南地区的历口群,在板溪群的底部,往往发育底砾岩,而且仅在局部发育宽缓的褶皱构造.在岩石组合上,板溪群主要为一套成熟度较高的碎屑岩、局部发育双峰式火山岩及凝灰岩.在晋宁运动之后,扬子克拉通的褶皱基底固结,在其上沉积了厚达10 km的地层覆盖了扬子克拉通的大部分地区,包括南华系(0.75—0.635 Ga),震旦系(0.635—0.542 Ga)和显生宙地层.震旦系是南华系南沱组冰碛岩之上、寒武系牛蹄塘组黑色页岩之下的一套碳酸盐岩和磷块岩层序.寒武纪、奥陶纪和中下志留世为一套海相沉积,早古生代晚期,扬子克拉通经历了隆升剥蚀,致使上志留系和中下泥盆系地层仅零星出露,晚古生代和中生代早期主要为一套滨海相-浅海相沉积,中生代中晚期以来又进入了陆相沉积(图 2).

图 1 中上扬子克拉通地质图(改自于1 ∶ 100万地质图,http://ogc.cgs.gov.cn/地质图) Fig. 1 Geological map of the Sichuan basin and adjacent regions (modified from 1 ∶ 1201507180 geological map of China,http://ogc.cgs.gov.cn)

图 2 上扬子地区地层柱状图(修改自Jia et al.,2006; Ma et al.,2007; Zeng,2010) Fig. 2 Columnar section showing the ages,lithologies,thickness of stratigraphic units preserved in the Sichuan basin(complied from Jia et al.,2006; Ma et al.,2007; Zeng,2010)
3 四川盆地中下地壳重力异常

布格重力异常是地壳密度结构和区域地质构造研究的重要资料,不仅包含了地壳内部各种偏离正常密度分布的矿体与构造的影响,也包括了地壳下界面起伏而在横向上相对上地幔质量的巨大亏损或盈余的影响,横向分辨率高.本文通过对四川盆地及 周缘深地震探测剖面的收集,重新整理并采样插值 后,获得了莫霍面深度和沉积厚度两个主要界面的起伏等值线图,在此基础上,对四川盆地布格重力异常剥离沉积盖层和莫霍面起伏引起的重力异常,获得反映沉积层以下、莫霍面以上的地壳内部密度不均匀体引起的重力异常.

3.1 深地震探测程度

20世纪70年代开始,在四川盆地及周缘开展了大量的深地震探测工作,包括了深地震测深、深地震反射和石油反射剖面采集,为该地区的地壳结构研究提供了基础的研究资料(图 3).其中深地震测深最早开展,中国地震局陆续完成了“唐克—蒲江—阆中”龙门山三角剖面、“奔子栏—唐克”、“巴塘—资中”等跨越龙门山的剖面以及在渝东长江三峡地区完成了“奉节—观音垱”等剖面(陈学波等,1988; 王椿镛等,2003),中国科学院在攀西地区完成了丽江—新市镇、丽江—者海、长河坝—拉鲊和西昌—渡口等剖面(熊绍柏等,1993; 尹周勋和熊绍柏,1992),中国地质科学院则在实施“台湾—阿勒泰”地学断面过程中,完成了贯穿四川盆地及周缘东西向的“花石峡—简阳”“黑水—邵阳”剖面和两条支测线:“南部—富顺”和“金川—唐克”剖面(曹家敏和王有学,1997; 王有学等,2005).

图 3 四川盆地及周缘地震剖面分布图 Fig. 3 The sketch map of distribution of the deep seismic reflection profiles and wide-angle reflection and refraction profiles around the Sichuan basin

近年来,为了研究华南大陆的深部结构和演化 特征,中国地质科学院陆续在四川盆地及周缘完成了多条深地震反射剖面,形成了贯穿该区域的东西向大剖面,以及在四川盆地北缘大巴山褶皱带—秦岭构造带的3条剖面.同时,由于四川盆地是我国南方特别是古生界海相碳酸盐勘探区域的重要组成部 分,经历了50多年的勘探开发,其中石油地震反射 剖面在其中发挥了重要作用.同时,为了研究盆山间的结构关系,前人在龙门山褶皱带、米仓山褶皱带、大巴山褶皱带、川东褶皱带和大娄山褶皱带等四川 盆地不同盆缘分别完成了多条剖面(Burchfiel et al.,1995; Hubbard and Shaw,2009; Hubbard et al.,2010; Jia et al.,2010; Li et al.,2013; Liu et al.,2012; Lu et al.,2012; Plesch et al.,2007; Wang et al.,2013b; Yan et al.,20032009; Zhou et al.,2013).

3.2 莫霍面

本文对宽角反射与折射地震剖面的莫霍面深度间隔5~10 km采样,并在莫霍过渡带位置进行了加密采样,同时对深地震反射剖面资料获得的莫霍面双程走时,根据该地区的平均速度6.2 km·s-1,转换成了深度,在此基础上,利用克里金插值法进行了插值,编制了四川盆地及周缘造山带的莫霍面深度图(图 4),结果显示四川盆地莫霍面平均深度在 40~42 km左右,在其北缘深度略深,可达44~ 46 km,在西缘莫霍面较浅,在 40 km 左右.

图 4 四川盆地及周缘莫霍面深度等值线图 Fig. 4 The Moho depth contour map of the Sichuan basin and adjacent regions
3.3 基底

在前人编制的四川盆地基底顶界面等值线图的基础上(Zhou et al.,2006),我们增补了近年来完成的石油反射剖面获得的基底顶界面深度(Burchfiel et al.,1995; Hubbard and Shaw,2009; Hubbard et al.,2010; Jia et al.,2010; Li et al.,2013; Liu et al.,2012; Lu et al.,2012; Plesch et al.,2007; Wang et al.,2013b; Yan et al.,20032009; Zhou et al.,2013),同样通过克里金插值法获得了四川盆地的沉积厚度图,显示四川盆地存在东西两个沉积中心,沉积厚度可达10~12 km,在西南缘则存在一个基底隆起中心(图 5).

图 5 四川盆地沉积盖层厚度等值线图 Fig. 5 The sedimentary cover thickness contour map of the Sichuan basin
3.4 重力数据来源

本文利用地球重力场模型EGM2008(S and well and Smith,2009)计算了四川盆地自由空气重力异常.EGM2008是由美国国家地理空间情报局(NGA)推出的官方版高阶地球重力场模型,该模型的阶次完全至2159(另外球谐系数的阶扩展至2190,次为2159),空间分辨率约为5′.由于采用了GRACE卫星跟踪数据、卫星测高数据及地面重力 数据等,使得该模型无论在精度还是在分辨率方面 均取得了巨大的进步.该模型在海域的精度可达到1 ∶ 100万比例尺重力勘探的要求,在中国大陆大部分地区精度可达10 mGal,可用于中等比例尺的海洋地质与资源调查和中小比例尺的大陆重力编图与构造研究.

利用EGM2008模型,我们计算了四川盆地自 由空气重力异常(网度为10 km),然后按照区域重 力调查规范作地形校正和布格校正(中间层校正),得到四川盆地布格重力异常数据(网度为10 km),最后利用优化滤波法(Guo et al.,2013; 郭良辉等,2012)有效压制数据中的高频噪声干扰,较好保留有效布格重力异常信息,结果见图 6所示.四川盆地布 格重力异常呈中间高、周围低的特征,走向北东,幅值-299~-80 mGal,同前人的研究结果一致(江为为等,2001).其中盆地中部呈现走向北东的异常高值圈闭,而向盆地边缘异常逐渐降低,并分布有串珠状或团块状的局部异常高,反映盆地中部存在古老基底隆起,及盆地中下地壳隐伏断裂带的存在和重要控制作用.在盆地西北角的龙门山断裂带及西南角邻区异常较低,反映这些地区莫霍面变深、地壳增厚.本文同时用1 ∶ 100万较高精度重力资料进行了检验,两者在数据网格和异常结果与卫星数据一致,证明了本次结果的可靠性.

图 6 四川盆地及周缘布格重力异常等值线图 Fig. 6 The gravity anomaly contour map of the Sichuan basin and adjacent regions
3.5 中下地壳重力异常

根据四川盆地区域岩矿石密度特征(屈燕微,2008),本文取沉积盖层平均密度为2.55 g·cm-3,结晶基岩平均密度取为2.8 g·cm-3,莫霍面以上的下地壳平均密度取为2.9 g·cm-3,上地幔平均密度取为3.3 g·cm-3.然后,根据四川盆地的莫霍面深度信息(图 4)和沉积基底深度信息(图 5),采用频率域正演方法(Parker,1973)分别正演计算盆地的莫霍面重力异常和沉积盖层重力异常.之后,对去噪后的盆地布格重力异常(图 6)减去莫霍面重力异常和沉积盖层重力异常,最终获得的剩余重力异常称之为中下地壳重力异常,见图 7所示,结果显示盆 地中下地壳重力异常呈东西分块的特征,幅值-32~206 mGal. 沿重庆—华蓥一线,西部明显有南北两个高重力异常圈闭,以内江、遂宁、南充和达州为中心,走向北东,异常值可达170 mGal左右,而在东侧则显示了一个高重力异常圈闭,走向北北东,异常达190 mGal 左右(图 7).因此,重庆—华蓥一线东、西两侧异常特征不同,从东向西异常圈闭变窄,轴向特征有差异,反映沿线两侧地壳结构存在差异,而重庆—华蓥一线是两侧高异常圈闭的衔接带,异常相对低些,走向北东,反映沿线深部可能存在隐伏断裂 带,将重庆—华蓥一线两侧分割为两个不同的古陆块.

图 7 四川盆地及周缘中下地壳重力异常等值线图 黑色虚线指示了四川盆地内部中下地壳东西两侧结构不同. Fig. 7 The middle-lower curstal gravity anomaly of the Sichuan basin and adjacent regions The black dash line indicates the distinctive difference between the western and eastern parts.
4 讨论

重力异常结果显示四川盆地内部的中下地壳可能存在东西分块的特征,应该在新元古代南华系沉积之前具有不同的基底物质.同时,航磁异常代表了实测磁场总强度与背景场强度之差,突出了区域异常的分布特征,客观上反映了重要的区域构造轮廓,是了解盆地基底结构、划分构造单元最有效的地球物理方法之一.前人的航磁异常结果显示在重庆—华蓥一线西侧以四川南充和达川为高航磁异常中心,而东侧则以贵州石柱为高航磁异常中心(Zhang et al.,2013; 谷志东和汪泽,2004; 成谷志东等,2014图 8).航磁正异常一般揭示变质结晶基底的区域特征,因此该区域的航磁异常特征指示了四川盆地基底并不统一(谷志东和汪泽成,2014).同时,前人在四川盆地东北缘和西北缘报道的太古宙-古元古代片麻岩的地球化学特征显示(图 9),位于四川盆地东北缘三峡地区出露的崆岭杂岩呈现明显的TTG岩系特征(Gao et al.,1999; Zhang et al.,2006b),而西北缘出露的后河杂岩和鱼洞子杂岩则呈现钙碱性的岛弧特征(Wu et al.,2012; 张宗清等,2001),同样揭示了在四川盆地东西两侧存在不同岩系特征的古陆核.并且近期完成的四川盆地—雪峰山深地震反射剖面在川东褶皱带段中下地壳内显示,在重庆附近发育一系列自西向东自中下地壳延伸至上地幔的反射波组(图 10),一般认为在显生宙和元古代沉积盖层之下的中下地壳到上地幔的倾斜反射体常常是由于从顶部进入到上地幔的榴辉岩化的板片的俯冲作用(Morgan et al.,1994)或者是插入地幔的下部板片导致的逆冲作用所致(Calvert et al.,1995).因此该研究区发现的自西向东倾的中 下地壳-上地幔反射波组可能代表了中上扬子克拉通(以四川盆地为主)在新元古代之前存在一期弧陆碰撞事件,而后四川盆地作为稳定的中上扬子克拉通主体,保留了早期的东西陆核特征和碰撞形迹.

图 8 川东褶皱带及周缘地区航磁异常(Zhang et al.,2013) Fig. 8 Aeromagnetic anomaly in the East Sichuan fold belt and periphery areas(Zhang et al.,2013)

图 9 东西扬子陆块太古代-古元古代杂岩(Gd/Yb)N-Eu/Eu*和Eu/Eu*-Rb/Sr图解 西扬子陆块的鱼洞子杂岩和后河杂岩呈现典型的钙碱性花岗质岩石特征,东扬子陆块的崆岭杂岩则显示典型的TTG岩系特征,鱼洞子杂 岩地球化学数据来源于张宗清等(2001),后河杂岩地球化学数据来源于Wu等(2012),崆岭杂岩地球化学数据来源于Gao等(1999)Zhang等(2006b),全球TTG来源于Kemp和Hawkesworth(2005). Fig. 9 The Eu/Eu* vs.(Gd/Yb)N and Rb/Sr vs. Eu/Eu* diagrams of Archean-Paleoproterozoic complexs in east and west Yangtze blocks The Yudongzi complex and Houhe complex(West part)show TTG characteristics,while Kongling complex(East part)shows typical calk-alkaline granitoids. The Yudongzi complex field is from Zhang et al.(2001)the Houhe complex field is from Wu et al.(2012),the Kongling TTG field is from Gao et al.(1999) and Zhang et al.(2006b),while the global TTG field is from Kemp and Hawkesworth(2005).

图 10 川东褶皱带深地震反射剖面中下地壳主要反射特征(位置见图 1) Fig. 10 The main middle-lower crustal reflectors of the deep seismic profile across the east Sichuan fold belt(location is seen in Fig. 1)
5 结论

本文通过四川盆地及周缘地震资料的采样插值获得了四川盆地的盖层厚度分布图和莫霍面深度图,而后对四川盆地布格重力异常剥离沉积盖层和莫霍面起伏引起的重力异常,所获得的中下地壳重力异常结果显示在四川盆地内部沿重庆—华蓥一线东西两侧存在不同的结构特征,结合航磁异常资料、出露的基底岩石的地球化学特征和深地震反射剖面 显示的精细深部信息,认为四川盆地可能存在东西 两个陆核,具有不同的基底物质.

致谢 论文成稿过程中得到了李秋生、卢占武、李文辉和龚辰等人的帮助,特致谢意.

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