地球物理学报  2015, Vol. 58 Issue (10): 3671-3686   PDF    
鄂尔多斯盆地深部热结构特征及其对华北克拉通破坏的启示
黄方1,2, 何丽娟2, 吴庆举1    
1. 中国地震局地球物理研究所, 北京 100081;
2. 中国科学院地质与地球物理研究所岩石圈演化国家重点实验室, 北京 100029
摘要: 基于二维稳态热传导方程,利用有限元数值模拟方法,选取东西向横穿鄂尔多斯盆地地质与地球物理解释大剖面进行了深部温度场数值模拟研究,得到了华北克拉通西部的鄂尔多斯盆地下伏岩石圈热结构特征.地幔热流变化范围:21.2~24.5 mW·m-2,体现为东高西低特征.壳幔热流比(Qc/Qm)介于1.51~1.84之间,为"热壳冷幔".与华北东部地幔热流对比表明,西部的鄂尔多斯盆地相对处于稳定的深部动力学环境.在岩石圈热结构研究基础上,对克拉通地震岩石圈与热岩石圈厚度差异进行了对比,研究表明:鄂尔多斯盆地西部地震岩石圈与热岩石圈厚度差异约达140 km,而东部的汾渭地堑,渤海湾盆地二者差异逐渐减小.华北克拉通自西向东,地震岩石圈厚度与热岩石圈厚度差异不断减小,意味着华北克拉通岩石圈下部的软流圈地幔黏性系数自西向东逐渐降低,本文从地热学角度可能印证了太平洋俯冲脱水作用对华北克拉通的影响.
关键词温度场     热结构     热岩石圈     华北克拉通破坏     鄂尔多斯盆地    
Lithospheric thermal structure of the Ordos Basin and its implications to destruction of the North China Craton
HUANG Fang1,2, HE Li-Juan2, WU Qing-Ju1    
1. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China;
2. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Abstract: Recently, the destruction of the North China Craton has become a center of active discussion in earth sciences. While its mechanism remains unclear and debatable. Moreover, geothermal research on this subject is relatively few. The Ordos Basin, located in the west of the North China Craton, is a typical intraplate tectonic unit with stable sedimentation since Paleozoic. Jurassic to Cretaceous is an important period of the formation and evolution of this basin, which were affected by the Tethyan and circum-Pacific tectonic domains. In an attempt to understand the deep dynamics of lithospheric destruction of the North China Craton, the thermal structure of a two-dimensional profile (named AB) across the Ordos Basin from west to east has been studied to provide geothermic evidence to addressing this issue.
Based on the two-dimensional steady-state heat conduction equation and using the finite element algorithm, thermal modeling along the profile(named AB) across the Ordos Basin has been carried out, resulting in the lithospheric thermal structure in the Ordos Basin west of the North China Craton. Furthermore, in the process of simulated calculation, we constantly adjust the heat flow at the bottom of the model to calculate surface heat flow (named calculated surface heat flow), which will be used to fit the measured heat flow on the surface of the earth. So the simulation results are convinced because they are constrained by both thermal physical parameters and the measured surface heat flow.
The results are as follows: (1) The Moho temperature along the profile ranges from 610 ℃ to 700 ℃. The temperature in the east is higher than that in the west. (2) Mantle heat flow values in different tectonic units in the Ordos Basin range from 21.2 to 24.5 mW·m-2. In the eastern Ordos Basin mantle heat flow values are high while the values in the western region are relatively low. But mantle heat flow values are smooth and not high overall, showing a stable deep thermal background in the west of the North China Craton. (3)The heat flow ratios of crust to mantle(Qc/Qm)along the profile are between 1.51 and 1.84, which indicates a thermal state of relatively hotter crust while colder mantle. (4) In the west of Ordos Basin, the thermal lithospheric thickness is about 160km, while in the east it is about 140km. They both indicate that the Ordos Basin has a thick thermal lithospheric root.
By the lithospheric thermal structure study, we find that the Ordos Basin, located in the west of North China Craton, is in a relatively stable deep dynamic environment. Moreover, we focus on the disparity in thickness between the thermal and seismic lithosphere. The difference between seismic and thermal lithosphere thicknesses in the western Ordos Basin is about 140km, which decreases gradually from the Fenwei graben in the eastern Ordos Basin to the Bohai Bay Basin farther east. That is to say the differences between seismic and thermal lithosphere thicknesses decrease gradually from the west to the east of North China Craton. The simulation results imply that viscosity of the asthenosphere under the North China Craton decreases gradually from west to east, confirming that dehydration of the Pacific subduction plate likely has a great effect on the North China Craton. Combining previous results and this study, we suggest that convection erosion and peridotite melting in the big mantle wedge formed by the Pacific subduction under the eastern North China Craton are the dynamic mechanisms of the destruction of the North China Craton.
Key words: Temperature field     Thermal structure     Thermal lithosphere     Destruction of the North China Craton     Ordos Basin    
1 引言

岩石圈热结构主要是指热岩石圈的厚度及温度分布以及该区壳、幔两部分热流配分比及其组构关系.壳幔热流的配分影响到深部温度的分布、地壳及上地幔的活动性.学者们很早就开始关注热结构的问题,但直到20世纪60年代才有所突破.1968年,Birch等提出地表所观测到的热流由两部分构成: 一部分源于地壳浅部放射性元素U、Th、K40衰变所释放的热量;另一部分为该层之下,来自地壳深处和上地幔的热量.后来,Blackwell(1971)正式提出热结构一词.在后续20多年里,人们始终致力于累积不同地质构造单元壳、幔热流的实际资料,而对热结构的内涵未作进一步论述.随着研究的不断深入,汪集旸和汪缉安(1986)认为热结构概念应进行补充和引申:热结构不单指一个地区壳、幔两部分热流的构成和配分,还应当包括地壳内不同岩层之间的热流构成和配分比例,且应当将地壳温度也考虑进去.岩石圈热结构不仅是岩石圈热演化过程的反映,还将对整个岩石圈的演化产生重大影响.岩石圈热状态决定着岩石圈的流变和物理性质,从而影响着构造变形特征和地质演化过程,也影响着地震波传播的速度和衰减及地磁、重力等地球物理场分布(石耀霖,1990).20世纪80年代以来,国内外对岩石圈热 结构的研究更加重视,使其研究已成为地球动力学研究新的生长点(Baumann and Rybach,1991; Čermák and Bodri,1986; Cooper et al.,2004; Gornov et al.,2009; Goutorbe,2010; Green,2006; Leeman et al.,2005; McKenzie et al.,2005; McLennan and Taylor,1996; Pasquale et al.,1990; Rudnick et al.,1998; 陈墨香,1988; 王良书等,2004; 臧绍先等,2002).

晚中生代是整个东亚地区构造体制转折的关键期,最突出的地质事件莫过于“华北克拉通破坏”.华北克拉通破坏研究毋庸置疑已经成为国际地球科学领域的热点.有关华北克拉通破坏及其地球动力学研究已涌现了大量成果.华北岩石圈减薄已从构造地质学、地幔包体和岩浆岩岩石学和地球化学以及 地球物理观测等方面得到了证实(Chen et al.,2009; Gao et al.,2004; Wu et al.,2006; Xu,2007; Zhang et al.,2005; Zheng et al.,2007; 朱日祥等,20112012),但对于岩石圈减薄发生的时间、机制等依旧存在争议(吴福元等,20032008).然而,对华北克拉通破坏的研究来自地热学方面的贡献相对较少.事实上,地热在大陆克拉通演化、破坏研究中具有重要的地位和意义(Grove and Parman,2004; Jaupart and Mareschal,1999; Michaut et al.,2009; Pollack,1986; Sleep,2003),大陆岩石圈的热结构及流变结构对岩石圈动力学过程有很大的影响.岩石圈热状态以及热岩石圈厚度的变化是克拉通破坏的重要表现或证据之一.目前关于华北克拉通破坏机制很多,如拆沉、热侵蚀、橄榄岩-熔体相互作用等模型,其中 这些模型大多都与热作用密切相关.尤其是沉积盆地作为岩石圈上部局限分布的薄层状地质单元,其形成和演化是深部地球动力学过程的浅部响应(McKenzie,1978; Wernicke,1981; Ziegler and Cloetingh,2004). 渤海湾盆地是分布在华北克拉通东部主要沉积盆地,并且其深部作为华北克拉通破坏的典型,在各学科以及地热学方面等已经进行了重点研究.而位于其西部的鄂尔多斯盆地,关于其地热学研究,前人对鄂尔多斯盆地古地温研究相对较多(任战利等,1994),大部分研究主要集中在古温标Ro和磷灰石裂变径迹(赵孟为 and Behr,1996),有的还利用流体包裹体来研究古地温,以及其热演化史与油气成藏、成矿关系等研究(任战利等,199620062007),而对鄂尔多斯盆地构造热演化或二维岩石圈热结构研究相当欠缺,鄂尔多斯盆地的二维岩石圈热结构与华北克拉通破坏相关的地球动力学研究几乎为空白.

热岩石圈是地球最外面的热传导层(Morgan,1984);地震岩石圈则指的是位于低速软流圈之上的高速盖层(Anderson,1995).在众多定义的岩石圈中,热岩石圈与地震岩石圈常常用来互相对比.就全球范围而言,二者在某些地区吻合得较好,譬如南非Kalahari克拉通;而在某些地区则差异较大,如东欧地台和北美的Wabigoom.尽管许多学者试图将二者统一起来,但比较困难.这种差异蕴含着何种丰富的地球动力学信息值得我们深入探讨.其次,关于华北克拉通热结构研究,主要集中在华北克拉通破坏中心区的渤海湾盆地以及华北东部地区(龚育龄等,2005; 何丽娟等,2001; 刘绍文等,2005; 汪洋和程素华,2011; 左银辉等,2013),而对其西部的鄂尔多斯盆地深部热结构研究较少.再次,前人对鄂尔多斯盆地现今岩石圈厚度的认识仍然存在有较大的分歧,一些学者认为其现今仍保持着200 km;巨厚的“岩石圈根”(朱日祥等,2011);也有一些学者认为 鄂尔多斯盆地现今“热”岩石圈的厚度为78~140 km(Wang et al.,1996; 任战利,1998),这与东部渤海湾盆地的60~100 km(付明希等,2004; 朱日祥等,2011)的岩石圈厚度相差并不大.焦亚先等(2013)通过镜质体反射率古温标模拟了鄂尔多斯盆地7口典型井的热历史,并计算了其深部一维温度场,认为鄂尔多斯盆地现今“热”岩石圈厚度为125 km,也与东部渤海湾盆地相近.

故本文将把位于华北克拉通西部在鄂尔多斯地块基础上发育的大型沉积盆地-鄂尔多斯盆地,作为本文的研究对象,主要基于精选的东西向横跨鄂尔多斯盆地的二维大剖面,通过收集的最新大地热流数据,拟采用有限元数值模拟方法,对横跨鄂尔多斯盆地的东西向AB地质地球物理解释剖面(其平面位置见图 1图 2),结合实测的热物性参数及前人研究成果,对该剖面进行了二维温度场数值模拟研究,获得其深部岩石圈热结构特征;本文作者也进一步探讨地震岩石圈厚度与热岩石圈厚度差异所蕴含的地球动力学意义,试图从地热学角度,对华北克拉通破坏研究补充来自地热学的认识.

图 1 华北克拉通大地构造背景及大地热流分布特征(构造分区根据Zhao et al., 2001)Fig. 1 Tectonic background and heat flow distribution of the North China Craton (Tectonic divisions from Zhao et al., 2001)

图 2 鄂尔多斯盆地构造单元划分、剖面位置大地热流分布Fig. 2 Divisions of tectonic units、the location of AB profile and heat flow distribution of the Ordos Basin
2 区域地质概况及大地热流特征

鄂尔多斯盆地是一个多构造体制、多演化阶段、多沉积体系、古生代地台与中-新生代台内坳陷叠合的克拉通盆地,位于华北克拉通的西段(杨俊杰,2002).其北部为兴蒙造山带,南部为秦岭大别造山带,西界为贺兰山—六盘山,东临吕梁山.阴山—燕山造山带横亘华北克拉通北缘,在近东西向绵延千余公里,该造山带东部以吕梁山、太行山为界,将华北克拉通分隔为西部鄂尔多斯盆地,东部渤海湾盆地和中部造山带三大部分(见图 1),共同构成了华北克拉通(滕吉文等,2010).鄂尔多斯(陆块)盆地沉积演化-改造主要经历了3个阶段,早古生代-晚古生代陆表海-滨浅海沉积沉降阶段、中生代三叠纪-早白垩世的内陆河湖相沉积沉降阶段、晚白垩世以来盆地整体抬升-剥蚀改造阶段(杨俊杰,2002).其中,中生代三叠纪之前的华北陆表海-滨浅海沉积主体受控于古生代板块构造环境下,并作为华北克拉通陆块的一部分.而它成为独立的沉积盆地主要发生在中生代-新生代大陆动力学构造-演化背景下,经历了多阶段沉积沉降与多旋回抬升改造,并与之相应伴生了多种矿产耦合成藏(矿)与最终定位.因此,鄂尔多斯盆地中-新生代的多旋回沉积与改造最终形成了现今盆地的6个一级构造单元,根据地 质演化历史及其中生界地质构造特征的差异性,盆地内部可划为: 伊盟隆起、渭北隆起、晋西挠褶带、陕北斜坡、天环坳陷、西缘掩冲构造带等6个区域构造单元(见图 2).

华北克拉通大地热流测点及热流值大小分布特征显示(见图 1),大地热流高值主要集中在华北克拉通东部,特别是大于85 mW·m-2大地热流值几乎全部集中在华北克拉通中东部,且东部最多,其次为中间过渡带,而华北克拉通西部仅在汾渭地堑有一个大于85 mW·m-2热流高值;而热流低值则主要集中在华北克拉通西部的鄂尔多斯盆地以及中间过渡带地区.75~85 mW·m-2的热流值也主要集中在华北克拉通中东部.华北克拉通地表热流高值分布自东向西不断减少,从某种程度上,此热流分布特征可能是受到西太平洋板块俯冲作用的影响.而由鄂 尔多斯盆地热流点的分布知(见图 2),鄂尔多斯盆地热流东高西低,南高北低.横向上,自东向西分布的晋西挠褶带热流平均值: 68.2±3.5 mW·m-2;陕北斜坡热流平均值:61.2±8.2 mW·m-2;天环拗陷平均值:55.4±5.5 mW·m-2;西缘逆冲带平均值:54.7±12 mW·m-2.纵向上也即南北向,盆地南部的渭北隆起平均值:64.9±3.6 mW·m-2;盆地北部的伊盟隆起平均值:70±10.2 mW·m-2.为了从 地热学角度探讨关于华北克拉通破坏问题,此次我 们更关注鄂尔多斯盆地东西向热流或者相关热状态的变化,而且惊奇地发现此次统计的鄂尔多斯盆地东部地表热流较高65 mW·m-2左右,西部热流相对较低55 mW·m-2,仅根据这些大地热流资料,可能会让我们思考华北克拉通西部的鄂尔多斯是否也像华北东部一样遭受了破坏?对于此困惑,作者于此暂不急于作答,需要后续对鄂尔多斯盆地进行进一步的岩石圈热结构等研究后,我们再给出来自地热学的相关认识.总之,这些大地热流基础数据是我们后续进行岩石圈热结构研究的基础.

3 数值模拟方法及热物性参数

如果二维区域内达到热平衡,温度分布服从以下控制方程:

式中:T为温度(℃);A为生热率(μW·m-3); k为热导率(W/(m·K));且T、Ak均为坐标(x,z)的函数,其中x代表横向距离(km),z代表纵向深度(km).该模型边界条件:

式中:T0是地表平均温度(℃); Qb是基底热流. 且0=z0zzb=180 km,0=x0xxl=653.17 km,xl为剖面的横向宽度.即以AB地质地球物理解释剖面作为数值模拟模型上部的沉积层(见图 3),下 部再加地壳和上地幔,从而构建一个横向宽为xl km,纵向深为180 km的岩石圈尺度地质-地球物理数值模拟模型,分别从图 3图 4可见其详细的沉积层和岩石圈分层结构信息.其中模拟模型的地壳底界为该剖面的Moho面底界深度,AB剖面的Moho面深度参考自中国及邻区地壳厚度分布(蔡学林等,2007),其结果与前人对中国地壳厚度的研究(曾融生等,1995; 嘉世旭和张先康,2005;朱介寿等,2006)较吻合但更为细致.其次,模拟计算采用的边界条件为:上边界取温度边界T0=10 ℃,下边界为热流边界Qb;左右边界为绝热条件.有限元数值模拟过程中,网格剖分中选择三角形剖分网格,模型由浅层到深层,网格剖分由细到粗.重点说明该模型底部热流边界需进行不断地试算拟合,当计算的地表热流与实测热流值拟合好后,试算可结束.此时即可得到地幔热流Qm.对构建的2-D岩石圈尺度地质-地球物理模型,在同时满足给定初始条件,边界条件 及剖面各层热物性参数条件下(见表 1表 2),根据 2-D稳态热传导方程的基本原理,基于Comsol Multiphysics软件进行二维有限元数值模拟计算,并以实测地表热流进行模拟约束,最终得到二维深部温度场及相关热状态分布特征.

图 3 东西向AB剖面地质地球物理解释剖面(李国玉和吕鸣岗, 2002)Fig. 3 Geological and geophysical interpretation section of east-westward profile (Li and Lü,2002)

图 4 二维岩石圈结构图(该模型从上至下依次为:沉积层(Sedimentary layer)、上地壳(Upper crust )、中地壳(Middle crust)、下地壳(Lower crust) 、地幔(Mantle);其中点划线为沉积层底界:沉积 层内分层则依据图3划分)Fig. 4 Two-dimensional profile of lithospheric structure

表 1 沉积层中各地层热物性参数 Table 1 Thermal parameters of each layer in deposition layer

表 2 剖面各构造层热物性参数 Table 2 Thermal parameters of each structural layer
under the deposition layer

稳定大陆地区的地表热流主要由来自上地幔的地幔热流和地壳内放射性生热所产生的地壳热流这两部分热流构成.地幔热流是一个能从本质上表征某一地区构造活动性的重要物理量(左银辉等,2013),它能反映该地区深部的区域热背景.由以上 模拟计算知,当计算的地表热流与实测热流值拟合好后,试算可结束时,此时可得到深部地幔热流Qm,然后由Qc=Q0-Qm即可计算地壳热流Qc(Hu and Wang,2000; 胡圣标等,1994),其中Q0为地表热流.

4 模拟计算结果及讨论 4.1 AB剖面热流及温度场分布特征

该东西向AB剖面自西向东依次穿越鄂尔多斯盆地的西缘逆冲带、天环拗陷、陕北斜坡和晋西挠褶 带等构造单元,且这些构造单元的平均地表热流分别为: 54.7 mW·m-2、55.4 mW·m-2、61.2 mW·m-2、68.2 mW·m-2,热流具有东高西低分布特征.在鄂尔多斯盆地大地热流密度分布的基础上,本文基于二维稳态热传导方程,根据研究区热导率、生热率等热物性参数,对横穿鄂尔多斯盆地西缘逆冲带、天环拗陷、陕北斜坡和晋西挠褶带等几个构造单元的AB剖面进行了二维温度场数值模拟研究,获得了关于其深部热结构认识.

结合鄂尔多斯盆地实测大地热流点分布(见图 2),AB剖面附近有13个实测热流值,以该实测热流值作为温度场数值模拟计算的地表约束条件(图 5中实测热流值以“+”表示),当计算的地表热流与实测的地表热流(Q0)拟合好后,此时试算结束.模拟结果显示(见图 5a):地幔热流(Qm)整体变化趋势比较平稳,自西向东热流值先有升高后稍微有降低,变化范围为:20.5~24.5 mW·m-2(占盆地平均地表热流小于40%),低于全球地幔热流的平均值28 mW·m-2(Turcotte and Schubert,1982),与Rudnick和Nyblade(1999)获得的太古代地质体的地幔热流17~25 mW·m-2相近,比中晚元古代稳 定陆块的地幔热流25~35 mW·m-2较低(Artemiva and Mooney,2001). 且剖面的壳幔热流比(Qc/Qm)介于1.51~1.84之间,表明来自盆地浅部地壳的热贡献作用较大,而来自深部的地幔热流贡献较少,盆地深部处于相对比较稳定的构造环境.地幔热流最高为陕北斜坡东部,而地幔热流最低为西缘逆冲带(见图 5a),即处于构造稳定的鄂尔多斯盆地深部在横向上也存在横向热作用不均匀性,东部地区相对西部构造热活动强烈.

图 5 (a) AB剖面实测热流、地表热流和莫霍面热流分布特征; (b) AB剖面莫霍面温度分布特征Fig. 5 (a) Heat flow distribution features of AB profile(Actual heat flow,Calculated surface heat flow and Moho heat flow);
(b) Calculated 2-D Moho temperature distribution along AB profile

由AB剖面深部温度场分布特征可知,深部温度自西向东先升高后稍微有降低(见图 6).由图 5b中Moho面温度分布知,该剖面Moho温度变化范围为610~700 ℃.图 6深部的温度场也是东部较高,西部较低.由图 6得知,假如将T=1300 ℃等温线作为约束来求取热岩石圈底界厚度,根据模拟计算所得的深部温度场分布特征可得出1300 ℃等温线所对应的热岩石圈厚度变化范围128.4~162 km,且热岩石圈厚度西部较厚,东部相对较薄;横跨鄂尔多斯 盆地的AB剖面其西部热岩石圈平均厚度大约160 km左右,而该剖面东部热岩石圈平均厚度约140 km. 造成热岩石圈厚度东西部差异,可能是受到西太平洋板块俯冲远程效应的影响.

图 6 AB剖面二维深部温度场分布特征Fig. 6 Calculated 2-D deep temperature distribution along AB profile
4.2 深部热结构特征

根据上述二维温度场模拟计算结果,归纳总结了盆地西缘逆冲带、天环拗陷、陕北斜坡和晋西挠褶带的深部热结构特征(见表 3).

表 3 AB剖面各构造单元热结构特征 Table 3 Thermal structure of each tectonic unit along AB profile

一个地区的深部热状态不仅与其深部构造运动 密切相关,而且还是决定该区地表热流的重要参数(邱楠生等,2004).上述热结构结果显示(见表 3),鄂尔多斯盆地各构造单元地幔热流(Qm)变化范围为21.2~24.5 mW·m-2,表现为鄂尔多斯盆地东部深部地幔热流较高,西部相对较低,但整体处于华北克拉通西部较为平缓稳定的深部热背景.鄂尔多斯盆地主体比较稳定,尽管长期受到加里东期、海西期洋盆发育到俯冲闭合形成造山带的影响和印支期后东部滨太平洋构造域和其西南的特提斯-喜马拉雅构造域的影响,但它仍然是一个稳定沉降、坳陷迁移的多旋回演化的克拉通内盆地.由于处于稳定的克拉通构造动力学背景下,故其深部地幔热流相对不高.鄂尔多斯盆地自西向东各构造单元的地表热 流变化范围为54.1~68.3 mW·m-2.对于鄂尔多斯盆 地东部热流较高的原因,并非来自地幔深部的热活动,因为该区地幔热流变化范围为20.5~24.5 mW·m-2(占盆地平均地表热流小于40%),低于全球地幔热流的平均值28 mW·m-2(Turcotte and Schubert,1982).从壳幔热流比(Qc/Qm):1.51~1.84,表明地壳热流对地表热流的贡献作用相对较大.且鄂尔多斯盆地东部的地壳热流相对较鄂尔多斯盆地西部 高.汪洋等(2000)利用热流和氦同位素比值(N(3He)/N(4He)比值)之间的关系得出的鄂尔多斯盆地壳幔热流比(1.9)也支持本文的“热壳冷幔”结果.赵孟为(1996)对鄂尔多斯盆地磷灰石裂变径迹资料进行了深入分析表明,最迟23 Ma以来盆地发生了一期由于快速抬升剥蚀事件盆地,且东部以95 m/Ma的速率抬升,造成约2000 m的剥蚀量;而盆地西部则以56 m/Ma的速率抬升,导致了约1000 m的剥蚀量.东西部的差异抬升剥蚀作用是造成东部热流较高的 另一原因.若暂以T=1300 ℃(Artemieva and Mooney,2001; Jaupart and Mareschal,1999)等温线得到了鄂尔多斯盆地各构造单元热岩石圈厚度变化范围:128.5~161 km,岩石圈厚度正常偏厚.后文将重点讨论热岩石圈厚度相关内容,此处不再赘述.

从上述鄂尔多斯盆地深部热结构特征,我们发现鄂尔多斯盆地各构造单元深部地幔热流变化范围:21.2~24.5 mW·m-2,比邱楠生(1998)早期对鄂尔多斯盆地研究的地幔热流Qm=33 mW·m-2明显偏低.我们的研究结果相对邱楠生的结果(1998)增加了热流数据和热物性参数,并采用二维温度场数值模拟研究,考虑了横向的不均匀性的影响.通过我们的研究表明鄂尔多斯盆地整体地幔热流相对较低.此时,我们不仅关注鄂尔多斯盆地较高的地表热流,更重要的是我们将把注意力转移到鄂尔多斯盆地深部的地幔热流,它相对影响因素较多的地表热流而言更能反映该区深部区域的热背景和深部的动力学环境.我们将鄂尔多斯盆地剖面热结构与中国大陆地区东西部其他盆地热结构特征进行了对比,得到中国大陆地区东西向剖面热结构特征,并对邱楠生(1998)早期研究进行了一定的修正(见图 7).中国大陆地区岩石圈热结构从东到西表现出有规律的变化:东部辽河盆地往西直到塔里木盆地,地幔热流所占的部分明显减小(见图 7),鄂尔多斯盆地东西部地幔热流比塔里木盆地较高,但比其西部的柴达木盆地稍微较低.总的来说,构造活动区来自深部的热流分量(即地幔热流)很大,如东部中新生代裂谷盆地;而构造稳定区来自深部的热流分量较少,如西部稳定区.剖面自东向西地幔热流几乎是逐渐降低的,辽河盆地为41 mW·m-2到塔里木盆地则降为20 mW·m-2.综上所述,我国东部裂谷盆地来自深部的地幔热流很大,约占盆地地表热流的63%(如辽河盆地);向西经华北盆地、鄂尔多斯盆地、柴达木盆地和塔里木盆地不断减小,且鄂尔多斯盆地尤其明显.对于稳定的塔里木盆地,地幔热流仅为20 mW·m-2(占盆地地表热流的45%),低于全球地幔热流的平均值28 mW·m-2(Turcotte and Schubert,1982).说明了构造活动区来自深部地幔的热量很大,而构造稳定区则来自深部地幔的热量较少.地幔热流是一个能从本质上表征某一地区构造活动性的重要物理量(左银辉等,2013),它能反映该地区深部的区域热背景.对于鄂尔多斯盆地,地幔热流20.5~24.5 mW·m-2(占盆地平均地表热流小于40%),也低于全球地幔热流的平均值28 mW·m-2(Turcotte and Schubert,1982),且与Rudnick和Nyblade(1999)获得的太古代地质体的地幔热流17~25 mW·m-2相近,比中晚元古代稳定陆块的地幔 热流25~35 mW·m-2较低(Artemiva and Mooney,2001),说明鄂尔多斯盆地深部处于相对稳定的热状态.汪洋(2000)利用该公式以及热流值估算了中国主要盆地的壳幔热流值; 其中,鄂尔多斯盆地qc/qm=1.9,济阳凹陷的qc/qm=0.66.根据现有的大陆地区地下流体3He/4He值(Polyak and Tolstikhin,1985; Oxburgh et al.,1986; Matthews et al.,1987; Oxburgh and O′Nions,1987; Möller et al.,1997),前人发现构造相对稳定地区的壳幔热流比值介于1.0~2.4,即地壳热流占地表热流值的50%~70%,地幔热流30%~50%; 而新生代以来活动的伸展盆地和火山带等地的壳幔热流比值一般介于0.4~1.0之间.汪洋(2000)的结果再次支持了本文中的鄂尔多斯地区qc/qm表现为“热壳冷幔”,表明该区处于构造相对稳定的地区.综上,来自鄂尔多斯盆地地幔热流和“热壳冷幔”的结果可以作为华北克拉通西部未破坏的地热学证据.

图 7 中国大陆地区沉积盆地东西向热剖面图

(修改自邱楠生,1998;其中q:地表热流,qm:地幔热流)
Fig. 7 East-westward thermal status profile of sedimentary basins in China

(Modified from Qiu,1998; q: Surface heat flow, qm: Mantle heat flow)
4.3 热岩石圈厚度及其与地震岩石圈厚度对比

1914年岩石圈的概念被提出,至今它一直是相对软流圈提出和讨论的.最初Barrell(1914)从力学强度(流变性)角度给出了两者的定义:岩石圈是具有高强度(高黏滞度,低流变性)的地球外壳,而其下的软流圈则强度较低且能够流动,可提供重力均衡补偿.随着20世纪60年代末板块构造理论的提出,岩石圈被赋予了新的含义.在板块构造理论框架下,岩石圈代表若干漂浮于软流圈之上的,在较长的地质时间尺度上保持刚性的块体(即板块).岩石圈厚度是岩石圈动力学中的一个基本问题,不同学者对岩石圈厚度的定义不同(Anderson,1995).目前有热岩石圈厚度、地震波速岩石圈厚度、幔源捕虏体岩石圈厚度、弹性岩石圈厚度、流变岩石圈厚度和电导率岩石圈厚度等(嵇少丞等,2008).其中,热岩石圈是指具有热传导温度梯度的地球外壳(White,1988),是地球最外面的热传导层,除浅部孔隙流体的对流作用外不存在热对流,其下部由于长时间尺度和高温的影响而表现出对流等流动性质.

热岩石圈的概念是指位于对流软流圈之上的热传导层(Morgan,1984),热岩石圈底界有三种确定方法:(1)当把某一绝热等温线当作岩石圈的底边界 温度,如1200 ℃(Petitjean et al.,2006)、1250 ℃(Lewis et al.,2003)、1280 ℃(McKenzie and Bickle,1988)、 1300 ℃(Artemieva and Mooney,2001; Jaupart and Mareschal,1999);(2)将热传导地温线与玄武岩固相线相交的深度定义为岩石圈底界面(Pollack and Chapman,1977Lachenbruch,1978Chapman and Furlong,1992),该方法多用于海洋(施小斌等,2000)和裂谷盆地,在克拉通不适用;(3)将热传导地温线与地幔绝热曲线相交的深度定义为岩石圈底界面(刘绍文等,2005; 臧绍先等,2002). 若本文对横穿鄂尔多斯盆地的AB剖面按第一种方法即取1300 ℃绝热等温线所对应的深度为热岩石圈底界,其变化范围为128.4~162 km,且厚度上表现为东薄西厚(见图 6),即热岩石圈厚度最薄处在陕北斜坡东部,靠近晋西挠褶带,约为128.5 km,对该盆地东西部热岩石圈厚度取均值,则鄂尔多斯盆地东部平均热岩石圈厚度为140 km左右,西部平均热岩石圈底界160 km左右.当采用第三种方法提出的T1=1200+0.5 z以及T2=1300+0.4 z两条绝热线分别作为“热”岩石圈底面温度的上限和下限(臧绍先等,2002),热岩石圈底界结果(见图 8ab)所示,鄂尔多斯盆地东部的热岩石圈厚度上下限(分别用表示)变化范围(见图 8a)为129~146 km,而其西缘热岩石圈厚度上下限变化范围(见图 8b)为154~171 km.当热岩石圈厚度取 ,即对于盆地东部晋西挠褶带的热岩石圈厚度,其热岩石圈厚度约为138 km,盆地西缘热岩石圈厚度底界约为163 km左右.此结果与T=1300 ℃绝热线温度确定的东西部热岩石圈厚度均值相差不大.它们均揭示鄂尔多斯盆地热岩石圈厚度东薄西厚且整体属于正常稍微偏厚的热岩石圈厚度.

图 8 鄂尔多斯盆地(a)西缘热岩石圈厚度(b)东缘热岩石圈厚度Fig. 8 Thickness of thermal lithosphere in Western part and (b) eastern part of Ordos Basin

尽管岩石圈定义很多,但热岩石圈与地震岩石圈常常用来互相对比.地震岩石圈指的是位于低速软流圈之上的高速盖层(Anderson,1995).就全球来看,两者在某些地区吻合得较好,而在另一些地区差异较大.事实上,纯传导的固体岩石圈与纯对流的流体软流圈之间存在一过渡层,即流变边界层(Sleep,2003;2006),其间传导与对流共同作用来传递热量.何丽娟等(2014)二维热传导/对流数值模拟研究指出正是热流变边界层的存在是导致热岩石圈和地震岩石圈差异的原因,并进一步指出流变边界层对岩石圈本身的结构特征并不敏感,而 主要受软流圈黏性系数控制.随着η从1×1021Pa·s 降低至1×1019Pa·s,流变边界层从130 km减薄至50 km,流变边界层的厚度与lg(η)成正比.流变边界层的存在是造成热岩石圈与地震岩石圈厚度差异的重要因素.

华北克拉通的地震结果表明,岩石圈厚度自西部鄂尔多斯的220 km向东逐渐减薄,至渤海湾盆地约80 km厚的岩石圈被视为华北克拉通遭受破坏的重要证据(Chen,2010; 朱日祥等,2011),热岩石圈同样也显示出西厚东薄的特征(图 9).通过前述研究知,关于热岩石圈厚度的定义方法尽管不少.但上文中以1300 ℃绝热等温线得到的鄂尔多斯盆地东西部平均热岩石圈厚度,与以地温线与地幔绝热曲线交点得到热岩石圈厚度上下限平均值结果相近.即鄂尔多斯盆地东部热岩石圈厚度底界平均为140 km,西部的底界厚度平均为160 km.来自地 震的结果显示鄂尔多斯地块西部下方存在约300 km 厚的前寒武纪大陆根( Li and Van DerHilst,2010),上地幔以高速异常为特征.也有学者指出鄂尔多斯盆地下方自东向西的高速根超过了200~250 km(Lebedev and Nolet,2003).根据上述计算知鄂尔多斯盆地西部的热岩石圈厚度底界深度平均约为160 km,再根据Li(2010)的地震结果,鄂尔多斯盆地西部地震岩石圈厚度与热岩石圈厚度差异最高将达到约为140 km(见图 9),若以Lebedev and Nolet(2003)的地震厚度结果,二者西部的差异也达约60~90 km.在鄂尔多斯盆地东部,本文以上述两种方法约束的热岩石圈厚度平均约为140 km,与前人研究结果较为接近,如以地温线与地幔绝热曲线交点定义的岩石圈底界面为140~157 km(刘绍文等,2005),以1350 ℃等温面计算出来的鄂尔多斯热岩石圈厚度为130~140 km(汪洋和程素华,2011),此时与地震岩石圈厚度相差70~90 km,与国际上其他克拉通的情况类似,相对鄂尔多斯盆地西部二者差异逐渐减小.并且两者之间的差异在汾渭地堑继续有所减小,热岩石圈厚度约为76 km(汪洋等,2001),地震岩石圈厚度约为140 km(Chen,2010).然而,在华北克拉通东部的渤海湾盆地,热岩 石圈厚度约为70~80 km,与地震岩石圈厚度相当接近(何丽娟等,2001汪洋等,2001刘绍文等,2005). 在大兴安岭、太行山、雪峰山以东区域的东亚裂谷带,如渤海湾盆地、松辽盆地,热岩石圈厚度在60~80 km(何丽娟等,2001汪洋等,2001),而地震学岩石圈厚度在80~100 km(朱介寿等,2006; Chen et al.,2009).下扬子地区苏北盆地的热岩石圈厚度 < 80 km(李成等,1996),地震学岩石圈厚度为75~100 km(陈沪生和张永鸿,1999).华北克拉通自西向东,地震岩石圈厚度与热岩石圈厚度差异不断减小(图 9).根据对鄂尔多斯盆地深部热结构特别是热岩石圈厚度与地震岩石圈厚度差异对比,结合何丽娟等(2014)进行的二维热传导/对流数值模拟研究,指出热流变边界层的存在是导致热岩石圈和地震岩石圈差异的原因,通过模拟计算并进一步指出流变边界层对岩石圈本身的结构特征(包括岩石圈热状态和固体岩石圈厚度)并不敏感,而主要受软流圈黏性系数控制.软流圈黏性系数越大,流变边界层越厚.当软流圈黏性系数从1×1021Pa·s降低至1×1019Pa·s时,流变边界层厚度也随之减薄,流变边界层的厚度与lg(η)成正比.且软流圈黏性系数特别容易受到水的影响,即水的加入会大大降低软流圈黏性系数.太平洋板块在中生代向欧亚板块下方快速俯冲,并停滞于660 km的地幔转换带,形 成大约宽1000 km的大地幔楔(Lei and Zhao,2005; Zhao et al.,2007).该俯冲下去的太平洋板块通过脱水作用在不同深度不断地释放富水流体,从而导 致大地幔楔内地幔对流变得更加剧烈(Komabayashi et al.,2004; Lei and Zhao,2005; Ohtani et al.,2004; Zhao et al.,2007).这种太平洋板块快速俯冲的深部脱水作用在Yonga俯冲带已被发现(Conder and Wiens,2006; Zhao et al.,1997).从搜集的各种资料来看,西太平洋深部地幔富水是大家的共识(Komiya and Maruyama,2007).实验室数据同样也证明,即使少量水也会显著降低橄榄岩和其他上地幔矿物的有效黏性系数(Hirth and Kohlstedt,1996).本文中热岩石圈与地震岩石圈底界自西向东的逐渐接近,暗示着流变边界层厚度自西向东的减薄,意味着华北克拉通岩石圈下部的软流圈地幔黏性系数自西向东逐渐降低,这可能与中生代太平洋俯冲脱水形成的低黏大地幔楔有关,从另一侧面印证了太平洋俯冲脱水作用对华北克拉通的影响.

图 9 华北克拉通热岩石圈与地震岩石圈底界对比(修改自何丽娟(2014),渤海湾至鄂尔多斯东地震结果来自Chen(2010),鄂尔多斯西地震结果来自 Li and Van DerHilst(2010))Fig. 9 Comparision between the thermal and seismic lithospheric bases for North China Craton(Modified from He(2014);The seismic results of Bohai Bay to east of Ordos basin from Chen(2010),while west of Ordos Basin from Li and Van DerHilst(2010))
5 结论

本文在最新且全面系统的大地热流密度研究基础上,通过二维温度场数值研究,获得了华北克拉通西部的鄂尔多斯盆地深部东西大剖面深部热结构特征.其地幔热流为21.2~24.5 mW·m-2,远低于华 北东部的高地幔热流,与Rudnick和Nyblade(1999)获得的太古代稳定地质体的地幔热流17~25 mW·m-2 相近;壳幔热流比(Qc/Qm)介于1.51~1.84之间,为“热壳冷幔”;鄂尔多斯盆地西部平均热岩石圈厚度约160 km,东部厚度约140 km,以上特征均表明位于华北克拉通西部的鄂尔多斯盆地处于相对稳定的深部动力学环境.华北克拉通地表大地热流高值自东向西不断减少,间接表明了太平洋板块俯冲作用对构造活动的影响;综合前人研究以及本文计算再次发现地震岩石圈和热岩石圈厚度确实存在差异,表明了流变边界层的存在,且华北克拉通自西向东,地震岩石圈厚度与热岩石圈厚度差异不断减小,意味着华北克拉通岩石圈下部的软流圈地幔黏性系数自西向东逐渐降低,这可能与中生代太平洋俯冲脱水形成的低黏大地幔楔有关,从另一侧面可能说明了太平洋俯冲脱水作用对华北克拉通的影响.

致谢 感谢两位匿名审稿专家提出的建设性意见!

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