龔辰，李秋生*，葉卓，張洪雙，李文輝，賀傳松，李英康

1 中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所，國(guó)土資源部“深部探測(cè)與地球動(dòng)力學(xué)”重點(diǎn)實(shí)驗(yàn)室，北京　100037　2　中國(guó)地震局地球物理研究所，北京　100081　3 國(guó)土資源實(shí)物地質(zhì)資料中心，河北燕郊　065201

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遠(yuǎn)震P波接收函數(shù)揭示的張家口(懷來)
—中蒙邊境(巴音溫多爾)剖面地殼厚度與泊松比

龔辰1，李秋生1*，葉卓1，張洪雙1，李文輝1，賀傳松2，李英康3

1 中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所，國(guó)土資源部“深部探測(cè)與地球動(dòng)力學(xué)”重點(diǎn)實(shí)驗(yàn)室，北京1000372中國(guó)地震局地球物理研究所，北京1000813 國(guó)土資源實(shí)物地質(zhì)資料中心，河北燕郊065201

摘要本文使用時(shí)間域迭代反褶積算法，從張家口(懷來)—巴音溫多爾一線布設(shè)的41個(gè)寬頻地震臺(tái)站、1年期間記錄的連續(xù)三分量數(shù)據(jù)中提取到高質(zhì)量的P波接收函數(shù)1844個(gè).用H-κ掃描方法獲得了測(cè)線下方Moho深度與波速比值(VP/VS)進(jìn)而計(jì)算出泊松比，用共轉(zhuǎn)換點(diǎn)(CCP)疊加方法獲得了沿測(cè)線Moho面起伏圖像.

結(jié)果顯示：(1)測(cè)線下方Moho深度平均40 km，僅各塊體邊界處出現(xiàn)Moho深度小尺度急劇變化.整體上，Moho面產(chǎn)狀相對(duì)于索倫縫合帶大致對(duì)稱，在縫合帶南側(cè)的溫都爾廟帶和白乃廟帶下方呈南傾趨勢(shì)，在縫合帶北側(cè)的寶力道帶、賀根山雜巖帶下方呈北傾趨勢(shì).(2)華北克拉通北緣泊松比總體較高，興蒙造山帶整體較低；各次級(jí)塊體內(nèi)部泊松比分布相對(duì)穩(wěn)定，塊體分界帶附近往往存在泊松比值的升降擾動(dòng).(3)整條測(cè)線地殼厚度和泊松比之間存在弱的負(fù)相關(guān)關(guān)系，表明存在構(gòu)造作用的影響.(4)整條測(cè)線泊松比呈現(xiàn)以索倫縫合帶南緣為對(duì)稱軸的非線性分布.

本文所獲得的地殼上地幔結(jié)構(gòu)以及泊松比分布特征，支持古亞洲(索倫)洋(南北)雙向俯沖，最終沿林西斷裂閉合的動(dòng)力學(xué)模式.

關(guān)鍵詞興蒙造山帶； 古亞洲洋； 接收函數(shù)； 地殼上地幔結(jié)構(gòu)； 泊松比

Structures and some major discontinuities beneath stations can be imaged by the receiver function method.In recent years,this method has become a mature and effective tool for studying the velocity structure of the crust and upper mantle.By the H-κ stacking method,the P-to-S converted waves at the Moho discontinuity and their multiple reflections are used to calculate crust thickness and VP/VSratio,which provide a way to study the rock composition of the crust and stress condition under stations.

Funded by National Natural Science Foundation of China,Institute of Geology,Chinese Academy of Geological Sciences deployed 41 portable broadband seismic stations from Huailai county in northern North China to Bayinondor town near the China-Mongolia border.The profile extends along the reflection and refraction profile funded by Sinoprobe project,crossing the Yinshan-Yanshan orogenic belt,the Inner Mongolia paleo-uplift,the Bainaimiao arc,the Ondor sum subduction accretion complex,the Solonker suture zone (the Solonder suture zone,located between the North China plate and Siberia plate,representing the closure of the Paleo-Asian Ocean),the Baolidao arc accretion complex,the Hegenshan ophiolite arc accretion complex and the Uliastai active continental margin from south to north.Recorders were set to the continuous recording mode and the sampling frequency was set to 50 Hz.The average distance between two neighbouring stations is 15 km and the length of the profile is about 650 km.Up to December 2014,we got 421 GB raw data in total.

First,we select 746 teleseismic events of mb>5 from 30 to 95 degrees and obtain their P waveforms from -20 s to 100 s with respect to the first P arrival time.Then,3-component waveform data with high signal-noise ratio are pre-processed (including mean removal and linear removal) and pre-filtering at 0.02～2 Hz and then rotated Z-N-E to the Z-R-T coordinate system.Finally we obtain 1844 P-wave receiver functions by the time domain iterative deconvolution method.After that we obtain the crustal thicknesses,VP/VSratios and Poisson ratios beneath the profile by the H-κ stacking method and image extension of the Moho interface by the Common Coversion Point (CCP) stacking method.The results are as follows:

(1) The Moho depth on average is 40 km along the profile.The crust thickness changes little across different tectonic blocks but shows evident variation at the boundaries between the blocks.The Moho dips southward beneath the Ondor sum belt and the Bainaimiao belt,while dips northward beneath the Baolidao belt and the Hegenshan belt.The Linxi fault seems to be as the boundary dividing the northern margin of North China Craton (NCC) and the southern CAOB.(2) The Poisson ratio shows a correlation with the tectonic unit along the profile.The averaged Poisson ratio in the northern margin of NCC is larger than the southern part of CAOB.The Poisson ratio changes little within the blocks except nearby the boundaries of different blocks.(3) There is a negative correlation between the crust thickness and the Poisson ratio that indicates the lateral heterogeneities of the crust structure resulted from the tectonic deformation.(4) Poisson ratio of the crust along the profile presents an approximately nonlinear symmetric shape relating to the south margin of Solonker suture zone.The characteristics of the crust and mantle structure and distribution of Poisson ratio along the profile support the geodynamic model that the Paleo-Asian Ocean subducted and closed at the southern margin of the Solon suture zone (along the Linxi fault).

1引言

中亞造山帶(CAOB)是西伯利亞古陸與塔里木—中朝板塊之間的古亞洲洋消減而形成的規(guī)模宏大的造山帶.該區(qū)域中-新生代處于亞歐大陸的核心地域，系統(tǒng)保存了亞歐大陸形成和演化的完整信息.復(fù)雜的地殼變形、強(qiáng)烈的殼幔相互作用和全球最典型的陸內(nèi)盆山體系，引起全球地質(zhì)學(xué)界的普遍關(guān)注(eng?r et al.,1993；eng?r and Natal′in,1996；Gao et al.,1994,1997,2004； Jahn et al.,2004；Zorin et al.,1989,2002,2003；Xiao et al.,2003,2004,2008,2009；Zhao et al.,2006；Kr?ner et al.2007；Mordvinova et al.,2007；Windley et al.,2007；Jian et al.,2008；Nielsen and Thybo,2009；Fullea et al.,2012；Li et al.,2013；Zhang et al.,2014；任紀(jì)舜等，1997；翟明國(guó)，2008；張風(fēng)雪等，2014；何靜等，2014)，一直是地球科學(xué)研究的前沿?zé)狳c(diǎn)之一.

興蒙造山帶是中亞造山帶的東段，包括加里東和海西等不同時(shí)期形成的造山帶.興蒙造山帶與華北克拉通北緣以白云鄂博—化德—赤峰斷裂為界.

圍繞興蒙造山帶的形成演化前人開展了大量的地質(zhì)調(diào)查和研究工作(Hsu et al.,1991； Xiao et al.,2003,2009；Jahn et al.,2004；Jong et al.,2006； Jian et al.,2008；李錦軼等，2009；徐備等，2014)并取得了重要進(jìn)展.索倫山蛇綠巖帶被認(rèn)為是古亞洲洋最終閉合的位置.然而，由于露頭稀少，地質(zhì)過程復(fù)雜，索倫山以東蛇綠巖呈多個(gè)條帶分布，導(dǎo)致對(duì)古亞洲洋最終閉合時(shí)間、古亞洲洋板片俯沖方向、主縫合帶確切位置以及碰撞后演化過程等問題尚存較大爭(zhēng)議.雖然地質(zhì)學(xué)家們依據(jù)所掌握資料的分析結(jié)果各自建立了構(gòu)造演化模式，但是這些模式并不兼容，造成這種狀況的重要原因之一是缺乏與地質(zhì)調(diào)查比例尺相當(dāng)?shù)纳畈康厍蛭锢碣Y料的約束，最基本的造山帶地殼結(jié)構(gòu)的幾何樣式尚未建立，從而制約著對(duì)該區(qū)構(gòu)造演化、地球動(dòng)力學(xué)和資源環(huán)境問題的深入探討.

多年來，我國(guó)對(duì)華北克拉通及北部鄰區(qū)進(jìn)行了大量探測(cè)研究(Li et al.,2006；Zheng et al.,2006,2007； Zhao et al.,2008； Chen et al.,2008; Chen,2010；Tian et al.,2011; 王峻等，2009；王未來等，2009； 陳凌等，2010a，2010b； 危自根等，2011； 葛粲等，2011；Lei,2012； Li et al.,2013； Zhang et al.,2014)，這些工作包括人工源地震探測(cè)和天然源地震觀測(cè).

寬頻帶地震流動(dòng)觀測(cè)剖面接收函數(shù)揭示燕山帶下方具有尖銳的莫霍面，地殼內(nèi)部結(jié)構(gòu)復(fù)雜，明顯區(qū)別于華北克拉通內(nèi)部和太行山(Zheng et al.,2007).人工地震寬角反射與折射地震剖面揭示燕山造山帶下方對(duì)應(yīng)3～5 km的莫霍面下凹(深度最大處達(dá)38 km)和相對(duì)薄的下地殼，暗示燕山帶仍保存著中生代區(qū)域收縮變形形成的地殼基本結(jié)構(gòu)，但已明顯受到區(qū)域伸展作用的影響(李秋生等，2008).幾條橫過燕山造山帶的寬角反射與折射地震剖面(孫武城等，1992；張先康等，1998)都表明其下地殼速度值(6.5 km·s-1)比正常大陸下地殼速度值(6.5～6.8 km·s-1)(邱瑞照等，2004；Wedepohl，1995)偏低，且存在自西向東地殼變薄的趨勢(shì).

大致與本文測(cè)線重合的Sinoprobe深反射地震剖面和寬角反射與折射剖面揭示了測(cè)線下方地殼的幾何學(xué)樣式和速度分布特征：突出的特征是Moho面在整條測(cè)線下方平緩延展，地殼厚度～40 km，僅陰山—燕山帶下方Moho面局部加深到46 km，在白乃廟帶下方可見Moho面微隆(39.5 km)；在整條測(cè)線下方，下地殼速度值偏低，速度分布變化劇烈；西拉木倫斷裂等斷裂帶下方存在莫霍面深度或結(jié)構(gòu)特征突變(Li et al.,2013；李文輝等，2014；李英康等，2014).在深反射地震剖面上可見下地殼存在一系列北傾的反射，在空間上其發(fā)育不受華北克拉通北緣斷裂(赤峰斷裂)的限制，一直延續(xù)到索倫縫合帶(張廣成等,2013).

但是上述觀測(cè)多數(shù)集中在華北克拉通主體范圍內(nèi)，僅有少數(shù)人工源地震剖面(如柏各莊—正藍(lán)旗剖面，昌黎—承德—達(dá)來諾爾剖面和遼寧東溝—內(nèi)蒙古烏珠沁旗剖面)和天然源地震剖面(如NCISP Ⅲ,Ⅵ剖面)延伸到華北克拉通北緣并進(jìn)入興蒙造山帶.華北克拉通北緣以北，即中亞造山帶南部的地殼結(jié)構(gòu)及其變化缺乏控制；Sinoprobe人工源深反射地震剖面和寬角反射與折射地震剖面揭示了測(cè)線下方地殼的變形樣式和速度分布特征，但仍缺乏地殼物質(zhì)組成和巖石圈結(jié)構(gòu)方面的信息，制約著華北克拉通北緣與中亞造山帶南部地殼演化、殼幔相互作用等地球動(dòng)力學(xué)問題的研究.

地殼厚度和泊松比是地殼結(jié)構(gòu)和屬性的重要參數(shù)，地殼厚度與泊松比的關(guān)系，對(duì)地質(zhì)構(gòu)造演化過程重建有較強(qiáng)的約束作用(李永華等，2006；嵇少丞等，2009；Tian and Zhang,2013).接收函數(shù)H-κ方法利用徑向接收函數(shù)的Ps、PpPs和PpSs/PsPs震相與直達(dá)P波的到時(shí)關(guān)系，共同約束Moho面深度與地殼平均速度比(VP/VS)，被認(rèn)為是求取地殼厚度與泊松比的最有效途徑之一(Zhu and Kanamori,2000；Tian and Zhang,2013；高延光和李永華，2014).近年來，眾多學(xué)者利用中國(guó)地震臺(tái)網(wǎng)(固定臺(tái))和流動(dòng)觀測(cè)資料，采用接收函數(shù)方法獲得了大量的地殼厚度與泊松比數(shù)據(jù)(王峻等，2009；張洪雙等，2009；李傳金等，2010；葛粲等，2011；武巖等，2011；危自根等，2011; 危自根和陳凌，2012；張廣成等，2013；吳慶舉和曾融生，1998；Pan and Niu,2011；許衛(wèi)衛(wèi)和鄭天愉，2005；劉瓊林等，2011)，這些地殼結(jié)構(gòu)和屬性參數(shù)(如地殼厚度、波速、泊松比分布等)為地震波場(chǎng)模擬和提高地震層析成像的橫向分辨力提供了重要基礎(chǔ)資料和約束.

張家口(懷來)—巴音溫多爾剖面近南北向大交角穿越華北克拉通北緣和興蒙造山帶的各個(gè)構(gòu)造帶(塊體)，測(cè)線兩側(cè)的蛇綠巖露頭和巖石地球化學(xué)和構(gòu)造研究程度較高，被認(rèn)為是研究古亞洲洋最終關(guān)閉的理想地區(qū)之一.寬頻帶地震觀測(cè)剖面大致重合人工源深反射地震和寬角反射與折射地震剖面布設(shè)，基于三種地震學(xué)方法的各自優(yōu)勢(shì)，有助于對(duì)測(cè)線下方地殼上地幔速度結(jié)構(gòu)的多側(cè)面成像和相互約束.

本文通過遠(yuǎn)震P波接收函數(shù)H-κ掃描和共轉(zhuǎn)換點(diǎn)(CCP)疊加獲得速度比(VP/VS)信息和Moho速度間斷面的產(chǎn)狀和起伏特征，結(jié)合前人的人工源地震剖面結(jié)果，討論華北克拉通北緣和興蒙造山帶地殼厚度和泊松比之間關(guān)系，及其隱含的大陸地殼形成和區(qū)域構(gòu)造演化過程的意義.

最新報(bào)道2011—2013年中國(guó)地震局地球物理研究所與蒙古科學(xué)院天文與地球物理研究中心合作在蒙古中南部(本研究區(qū)北段二連浩特西北)布設(shè)了69個(gè)臺(tái)站的陣列觀測(cè)(張風(fēng)雪等，2014；何靜等，2014)，上述工作為本研究提供了基礎(chǔ)和殼幔結(jié)構(gòu)的重要參考信息.

2數(shù)據(jù)與方法

2.1數(shù)據(jù)采集及預(yù)處理

由國(guó)家自然科學(xué)基金資助，中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所分別于2012年10月—2014年12月在華北北部懷來—張家口—蘇尼特右旗—二連浩特—中蒙邊境(巴音溫多爾)一線布設(shè)了41個(gè)寬頻地震流動(dòng)觀測(cè)臺(tái)(Reftek-130數(shù)字采集器配置Guralp CMG-3T(25臺(tái))或CMG-3ESP(16臺(tái))地震計(jì)).測(cè)線基本重合Sinoprobe反射/折射地震測(cè)線(Li et al.,2013；Zhang et al.,2014)(圖1)，采用連續(xù)記錄方式，采樣頻率50 Hz.自南向北橫跨陰山—燕山造山帶、內(nèi)蒙古地軸、白乃廟島弧、溫都爾廟增生雜巖帶、索倫縫合帶(被認(rèn)為是標(biāo)志古亞洲洋消亡，華北克拉通與蒙古微板塊(屬西伯利亞板塊)匯聚的板塊縫合帶)、賀根山雜巖帶，烏里雅斯太主動(dòng)大陸邊緣直至中蒙邊界，測(cè)線走向?yàn)镹W—SE向，臺(tái)站間距平均15 km，測(cè)線總長(zhǎng)約為650 km.至2014年12月底，采集到原始連續(xù)記錄數(shù)據(jù)421 GB.

研究選用來自USGS(美國(guó)地質(zhì)調(diào)查局)地震目錄的震中距在30°～95°，震級(jí)大于5級(jí)的地震事件746個(gè)(地震分布如圖2示)，從原始數(shù)據(jù)中截取P 波到時(shí)前20 s 至后100 s 數(shù)據(jù)，用以計(jì)算接收函數(shù).首先對(duì)截取的波形數(shù)據(jù)進(jìn)行挑選，選擇P波初至比較明顯的波形數(shù)據(jù)，然后對(duì)ZNE(垂直、南北、東西)三分量原始波形數(shù)據(jù)經(jīng)過去均值、去線性，預(yù)濾波采用0.02～2 Hz拐角頻率范圍的二級(jí)Butterworth帶通濾波，然后旋轉(zhuǎn)到ZRT(垂向、徑向和切向)坐標(biāo)系.

2.2P波接收函數(shù)分析

遠(yuǎn)震P波傳播到接收臺(tái)站下方時(shí)可認(rèn)為是近似垂直入射的，遠(yuǎn)震P波波形數(shù)據(jù)攜帶震源、傳播路徑以及接收區(qū)下方結(jié)構(gòu)等信息.Langston(1979)在Phinney(1964)的基礎(chǔ)上提出用等效震源的方法可以消除震源及傳播路徑等對(duì)波形的影響.本文利用時(shí)間域迭代反褶積算法實(shí)現(xiàn)接收函數(shù)的計(jì)算(Ligorría and Ammon,1999).在計(jì)算過程中，高斯系數(shù)α取2.5.

由近地表松散沉積地段臺(tái)站記錄計(jì)算出來的接收函數(shù)，其波形不同程度遭到畸變，導(dǎo)致H-κ方法 失效或結(jié)果的不確定.固定臺(tái)站觀測(cè)可通過另外選址或井下觀測(cè)從工程上較好地解決這個(gè)問題，流動(dòng)臺(tái)站觀測(cè)受投入成本的限制，目前可采取的措施只保留那些Ps和PpPs震相清晰且一致性較好的接收函數(shù)參與下一步分析計(jì)算，最終有1844個(gè)接收函數(shù)(如圖3)被用于求取地殼厚度和波速比.其中JIQN(7#)、BAYR(41#)臺(tái)站的接收函數(shù)如圖4所示.

2.3接收函數(shù)H-κ疊加掃描

圖1　華北北部寬頻地震流動(dòng)觀測(cè)臺(tái)站分布示意圖(構(gòu)造線據(jù)Xiao et al.,2003)藍(lán)色三角示意寬頻帶流動(dòng)臺(tái)站位置.陰影區(qū)示意索倫縫合帶；角圖示意研究區(qū)位置.F1：尚義—赤城—古北口—平泉斷裂；F2：赤峰—白云鄂博斷裂； F3：西拉木倫斷裂； F4：林西斷裂； F5：錫林浩特?cái)嗔眩?F6：二連浩特?cái)嗔眩?F7：查干敖包斷裂.

圖2　遠(yuǎn)震事件分布圖(MS>5,震中距 30°～95°)矩形框?yàn)檠芯繀^(qū).

遠(yuǎn)震P波穿過Moho面時(shí)一部分轉(zhuǎn)換為S波，還有一部分P波和發(fā)生轉(zhuǎn)換的S波會(huì)經(jīng)過地表及Moho的反射形成多次波，其中Pms、PpPms、PsPms、PpSms震相在接收函數(shù)中能量最強(qiáng).Zhu和Kanamori(2000)提出利用給定的平均P波速度以及多次波與直達(dá)P波的時(shí)間差可以求出地殼厚度(即H)與縱波橫波波速比(VP/VS，即κ).在一定范圍內(nèi)對(duì)H及κ進(jìn)行掃描，用掃描到的H、κ值求出Pms、PpPms、PsPms、PpSms震相與直達(dá)P波的到時(shí)差，繼而構(gòu)造目標(biāo)函數(shù)：

S(H，κ)=ω1r(TPms)+ω2r(TPpPms)

-ω3r(TPsPms+TPpSms),

(1)

ωi為各震相的權(quán)重系數(shù)，且ω1+ω2+ω3=1，由于Ps震相能量較強(qiáng)，選取ω1>ω2+ω3；TPs、TPpPs、TPsPs、TPpSs為各個(gè)震相相對(duì)于直達(dá)P波的到時(shí)差.當(dāng)三個(gè)震相交叉得到真實(shí)H、κ值時(shí)S(H，κ)最大.

H-κ掃描中，參考研究區(qū)人工源寬角反射地震 剖面探測(cè)結(jié)果(Li et al.,2013；李英康等，2014)，將研究區(qū)地殼平均P波速度取為6.3 km·s-1，地殼厚度變化的范圍取為10～70 km，P、S波波速比VP/VS搜索范圍定為1.5～2.0，另外，根據(jù)所得接收函數(shù)的特征(圖3)，經(jīng)多次測(cè)試，最終將Moho面的轉(zhuǎn)換波Pms震相和它的兩個(gè)多次波震相PpPms，PpSms+PsPms的疊加權(quán)系數(shù)分別定為0.6，0.3和0.1，實(shí)際上不同加權(quán)值組合對(duì)結(jié)果影響非常小(危自根和陳凌，2012).泊松比σ則由關(guān)系式：σ =0.5×[1-1/(κ2-1)]計(jì)算得到，其中JIQN、CHGD、SARL、BAYR四個(gè)臺(tái)站的接收函數(shù)H-κ掃描結(jié)果如圖5所示.

圖3　華北北部懷來—巴音溫多爾剖面接收函數(shù)圖上邊框字符為臺(tái)站代碼(參見表1)，縱軸為相對(duì)直達(dá)P波的到時(shí)，圖中為按每個(gè)臺(tái)站10個(gè)接收函數(shù)抽取的共計(jì)410個(gè)接收函數(shù).

圖4　JIQN(7#)、BAYR(41#)臺(tái)站的接收函數(shù)排列接收函數(shù)按反方位角(右側(cè)圖中紅色矩形表示)排列.黑色三角表示地震震中距分布.左圖左側(cè)數(shù)字為接收函數(shù)序號(hào)，橫軸為相對(duì)于直達(dá)P波的延遲時(shí)間.Moho轉(zhuǎn)換波Pms震相和它的兩個(gè)多次波震相(PpPms,PsPms+PpSms)用紅色虛線標(biāo)出.

圖5　H-κ方法獲得的地殼厚度和VP/VS波速比估計(jì)

對(duì)41個(gè)臺(tái)站接收函數(shù)進(jìn)行了H-κ掃描計(jì)算，結(jié)果如表1所示.

不同震相的有效識(shí)別對(duì)H-κ疊加結(jié)果的可靠性至關(guān)重要.由于測(cè)線跨度650 km，臺(tái)站場(chǎng)地條件、深部結(jié)構(gòu)沿測(cè)線變化較大，各臺(tái)站接收函數(shù)的數(shù)據(jù)質(zhì)量存在一定差別.Moho面轉(zhuǎn)換波Pms和多次反射波的波形特征和可識(shí)別程度，是臺(tái)站接收函數(shù)質(zhì)量評(píng)價(jià)的主要依據(jù).41個(gè)臺(tái)站中Pms和PpPms震相清晰且一致性較好的臺(tái)站有36個(gè)，用H-κ疊加方法獲得了可靠的地殼厚度和波速比，有5個(gè)臺(tái)站(表1中粗體所示)的遠(yuǎn)震接收函數(shù)波形受淺層多次波干擾嚴(yán)重，計(jì)算出來的H，κ值可信度較低，這些臺(tái)站主要集中在二連盆地(占4個(gè))和懷來盆地.

沿線地殼厚度值分布于37～43 km之間，整條測(cè)線地殼厚度平均40.3 km，接近全球陸殼平均厚度(39.2 km)(Christensen and Mooney,1995).所經(jīng)各構(gòu)造帶地殼厚度差異不大，起伏幅度±3 km，未見明顯山根.懷來盆地Moho深度37～39 km.華北克拉通北緣(陰山—燕山帶+內(nèi)蒙古地軸)Moho深度為37～43 km.索倫縫合帶Moho深度為 ～40 km.賀根山雜巖帶向北至中蒙邊境(烏里雅斯太)Moho深度為38～41 km.

沿線地殼波速比VP/VS分布在1.66～1.86之間，平均泊松比分布在0.215～0.297之間.

對(duì)各構(gòu)造帶內(nèi)的泊松比值做簡(jiǎn)單平均，不難發(fā)現(xiàn)，位于華北克拉通北緣的各構(gòu)造帶的泊松比普遍大于華北克拉通東部的泊松比平均值0.26 (王峻等，2009)，其中懷來盆地、陰山—燕山帶泊松比0.270，內(nèi)蒙古地軸平均0.281.而西拉木倫斷裂以北的各構(gòu)造帶普遍小于0.26，其中白乃廟帶平均0.252；溫都爾廟帶平均0.233；索倫縫合帶平均0.244；寶力道帶平均0.237；賀根山雜巖帶平均 0.261；烏里雅斯太帶平均0.259.

表1　沿線臺(tái)站下方地殼厚度(H)與泊松比(σ)及其與沿線構(gòu)造帶的對(duì)應(yīng)關(guān)系

在空間上，沿測(cè)線泊松比分布表現(xiàn)出與構(gòu)造單元的相關(guān)性：各構(gòu)造帶內(nèi)部泊松比(σ)分布相對(duì)穩(wěn)定，每一處σ值的突升或驟降，基本都能找到對(duì)應(yīng)的構(gòu)造邊界斷裂帶.

2.4共轉(zhuǎn)換點(diǎn)疊加

共轉(zhuǎn)換點(diǎn)(CCP)疊加技術(shù)被廣泛用于對(duì)較密集臺(tái)站下方速度界面的接收函數(shù)成像(Zhu and Kanamori,2000； Zhu,2000,2002； Zhu et al.,2006； Yuan et al.,1997,2002).

參考同測(cè)線人工地震寬角反射與折射剖面得到的速度模型(Li et al.,2013；李文輝等2014；李英康等，2014)，對(duì)IASP91全球速度模型的地殼部分進(jìn)行修正作為共轉(zhuǎn)換點(diǎn)疊加的背景速度模型.將地下空間劃分成沿測(cè)線方向1 km，深度1 km，寬度150 km(垂直于測(cè)線方向)的疊加單元，將臺(tái)站的每個(gè)接收函數(shù)按方位角及射線路徑排列并投影到深度域上，再對(duì)同一疊加單元的振幅進(jìn)行疊加，每一振幅都代表地下速度的變化情況.0～200 km深度范圍的CCP疊加結(jié)果如圖6所示.

圖6　臺(tái)站測(cè)線下方0～200 km CCP疊加剖面圖(成像位置如圖1中AA′、A′A′′所示)紅色代表P波從界面速度高一側(cè)向速度低一側(cè)的轉(zhuǎn)換；圖6a中小黑點(diǎn)代表H-κ疊加結(jié)果所得地殼厚度減去高程所得地殼埋深，小豎線代表誤差.F1：尚義—赤城—古北口—平泉斷裂； F2：赤峰—白云鄂博斷裂； F3：西拉木倫斷裂； F4：林西斷裂； F5：錫林浩特?cái)嗔眩?F6：二連浩特?cái)嗔眩?F7：查干敖包斷裂.

3討論與結(jié)論

3.1地殼厚度

整條測(cè)線地殼厚度平均40.3 km，接近全球陸殼平均厚度(39.2 km)，所經(jīng)各構(gòu)造帶地殼厚度差異不大，起伏幅度±3 km，僅不同構(gòu)造帶邊界保留了Moho深度的小尺度急劇變化.

最大的地殼厚度僅43 km，測(cè)線上有兩處連續(xù)3個(gè)臺(tái)站以上出現(xiàn)地殼厚度40～43 km，它們分別位于陰山—燕山造山帶由東西走向轉(zhuǎn)為北東走向的轉(zhuǎn)折部位和溫都爾廟拼貼帶的中部.

“燕山運(yùn)動(dòng)”期間，華北克拉通北緣經(jīng)歷了強(qiáng)烈的構(gòu)造變形，陰山—燕山造山帶廣泛發(fā)育指向華北克拉通內(nèi)部的逆沖斷層(趙越，1990；Meng,2003；鄭亞東等,2000)，地質(zhì)學(xué)家通過還原深源捕擄體和埃達(dá)克巖源區(qū)的溫度、壓力條件估算，陰山—燕山造山帶地殼厚度曾達(dá)到或超過50 km，現(xiàn)今的厚度可能是經(jīng)歷了新生代區(qū)域伸展后的殘余，區(qū)域伸展從東向西逐步減弱(李文輝等，2014).

溫都爾廟拼貼帶南部，靠近西拉木倫斷裂，可能存在P波速度局部高(李英康等，2014)、泊松比大的鎂鐵質(zhì)巖塊，北部Moho面有局部下凹，暗示地殼整體經(jīng)歷過強(qiáng)烈的擠壓縮短.

測(cè)線上小于地殼厚度平均值連續(xù)超過3個(gè)臺(tái)站間距的地段僅出現(xiàn)于懷來盆地，構(gòu)造上位于汾渭地塹—山西地塹與銀川—河套地塹的交疊部位，可能與鄂爾多斯周緣的新生代陸內(nèi)裂谷作用有關(guān).

3.2Moho形態(tài)與產(chǎn)狀

西拉木倫斷裂帶(圖6b，F(xiàn)3)以南，華北克拉通北緣，包括白乃廟弧、溫都爾廟雜巖帶的Moho面對(duì)應(yīng)的Pms震相明顯，深度一致性較好.在構(gòu)造帶內(nèi)部地殼厚度一致，較少小尺度起伏，更接近一級(jí)速度間斷面的尖銳和相對(duì)剛性的物理特征，與白乃廟弧、溫都爾廟雜巖帶較早拼貼到華北克拉通北緣，地殼經(jīng)歷了克拉通化有關(guān).

錫林浩特?cái)嗔?圖6b，F(xiàn)5)以北，寶力道帶、賀根山雜巖帶和烏里雅斯太構(gòu)造帶的Moho界面寬緩，但界面產(chǎn)狀穩(wěn)定.

索倫帶(圖6b，F(xiàn)4、F5之間)Moho面橫向急劇變化，與兩側(cè)各構(gòu)造帶的Moho界面特征形成鮮明對(duì)比.分辨率更高的深地震反射剖面顯示，帶內(nèi)Moho反射只能分段追蹤，其上覆地殼既有南傾也有北傾的反射和透明反射區(qū)，被認(rèn)為是鎂鐵質(zhì)下地殼殘塊、大陸邊緣沉積(增生楔)與后期伸展侵入的花崗巖混雜堆積(Zhang et al.,2014).

整體上，Moho面產(chǎn)狀相對(duì)于索倫縫合帶對(duì)稱，南側(cè)的溫都爾廟帶和白乃廟帶向南傾，北側(cè)的寶力道帶、賀根山雜巖帶向北傾.

Moho深度整體與地表高程呈鏡像對(duì)稱關(guān)系(圖7).

圖7　測(cè)線地殼厚度與地表高程示意圖F1：尚義—赤城—古北口—平泉斷裂； F2：赤峰—白云鄂博斷裂； F3：西拉木倫斷裂； F4：林西斷裂； F5：錫林浩特?cái)嗔眩?F6：二連浩特?cái)嗔?；F7：查干敖包斷裂.

3.3泊松比

泊松比已成為人類了解地球內(nèi)部物質(zhì)成分、構(gòu)造和物理狀態(tài)重要途徑之一(Holbrook et al.,1988；Ji and Salisbury,1993; Ji et al.,2002；Zandt and Ammon,1995；Christensen,1996).

泊松比的大小取決于巖石中P 波和S波速度的比值(VP/VS)，反映在擠壓與剪切條件下造巖礦物晶格原子間的性質(zhì)以及巖石內(nèi)部顯微構(gòu)造特征.例如，微裂隙的幾何形狀與空間分布、孔隙度以及各礦物相在三維空間中的連接性和連續(xù)性等(嵇少丞等，2009).泊松比的變化范圍介于-1～0.5之間，但對(duì)絕大多數(shù)巖石來說，泊松比介于0～0.5之間.

對(duì)造巖礦物的實(shí)驗(yàn)室測(cè)定的定性結(jié)論是泊松比主要與巖石的物質(zhì)組成相關(guān)，受溫度和壓力的影響較小(Christensen,1996).長(zhǎng)英質(zhì)巖中，巖石的泊松比隨石英含量的增加而減小，隨長(zhǎng)石含量的增加而增加；基性巖如輝長(zhǎng)巖、輝綠巖以及基性片麻巖不含石英，基性斜長(zhǎng)石所占比例較高，具有較大的泊松比(嵇少丞等，2009).新鮮的橄欖巖較基性巖具有較小的泊松比.蛇紋石的泊松比特別高，故橄欖巖的泊松比隨其蛇紋石化程度的增加而增大.榴輝巖的泊松比主要受其主要造巖礦物石榴子石和綠輝石的制約，所含石榴子石化學(xué)成分(Wang and Ji,1999)以及退變質(zhì)程度起決定性的作用(Ji et al.,2003).

從地震波接收函數(shù)分析得到的波速比(VP/VS)只是該臺(tái)站下面整個(gè)地殼的平均值.沿測(cè)線獲得的波速比(VP/VS)與地殼厚度呈負(fù)相關(guān)關(guān)系，如圖8所示.由關(guān)系式：σ =0.5×[1-1/(κ2-1)]可知，泊松比與地殼厚度也呈負(fù)相關(guān)關(guān)系.

圖8　測(cè)線地殼厚度與泊松比關(guān)系

泊松比與地殼厚度呈負(fù)相關(guān)關(guān)系反映構(gòu)造變形作用對(duì)地殼泊松比的影響.地殼由厚度不一、橫向起伏的不同巖性層疊合而成.在構(gòu)造擠壓縮短(增厚)或拉張伸展(減薄)過程中，應(yīng)變總是優(yōu)先集中于弱巖層面.相同溫度、壓力條件下，受到構(gòu)造擠壓的長(zhǎng)英質(zhì)巖石比起基性巖石更容易形成褶皺和推覆構(gòu)造，使長(zhǎng)英質(zhì)巖石在整個(gè)地殼柱體內(nèi)的所占比例增大，其結(jié)果導(dǎo)致地殼泊松比隨地殼厚度增加而減少.

圖9　波速比沿測(cè)線變化示意圖F1：尚義—赤城—古北口—平泉斷裂； F2：赤峰—白云鄂博斷裂； F3：西拉木倫斷裂； F4：林西斷裂； F5：錫林浩特?cái)嗔眩?F6：二連浩特?cái)嗔眩?F7：查干敖包斷裂.

流體或熔體能有效地影響巖石的泊松比.地下存在流體時(shí)，S波衰減很快，導(dǎo)致波速比高于周圍地區(qū)(Christensen and Fountain，1975)，二連浩特?cái)嗔?0.287)下方可能是斷裂帶破碎或裂隙流體的作用.

整條測(cè)線波速比(VP/VS)和泊松比呈現(xiàn)以索倫縫合帶南緣為對(duì)稱軸的非線性分布(圖9)：懷來盆地、陰山—燕山帶泊松比0.270，內(nèi)蒙古地軸平均0.281.而西拉木倫斷裂以北的各構(gòu)造帶普遍小于0.26，其中白乃廟帶平均0.252；溫都爾廟帶平均0.233；索倫縫合帶平均0.244；寶力道帶平均0.237；賀根山雜巖帶平均0.261；烏里雅斯太帶平均0.259.

這種分布，支持古亞洲(索倫)洋，分別向南、北雙向俯沖，最終沿林西斷裂閉合的動(dòng)力學(xué)模式.張北臺(tái)站附近泊松比奇高，與新生代漢諾壩裂隙溢流玄武巖對(duì)地殼的添加有關(guān).

綜合張家口(懷來)—中蒙邊境(巴音溫多爾)寬頻地震數(shù)據(jù)的遠(yuǎn)震P波接收函數(shù)結(jié)果和前人成果，可得以下結(jié)論：

(1) 測(cè)線下方Moho深度平均40 km，所經(jīng)各構(gòu)造帶地殼厚度差異不大，但各塊體邊界處存在Moho深度小尺度急劇變化.整體上，Moho面產(chǎn)狀相對(duì)于索倫縫合帶對(duì)稱，南側(cè)的溫都爾廟帶和白乃廟帶下方向南傾，北側(cè)的寶力道帶、賀根山雜巖帶下方向北傾.

(2) 沿測(cè)線泊松比分布表現(xiàn)出與斷層分割的構(gòu)造帶較好的相關(guān)性：華北克拉通北緣總體較高，興蒙造山帶整體較低；各構(gòu)造帶內(nèi)部泊松比分布相對(duì)穩(wěn)定，泊松比值的局部擾動(dòng)往往出現(xiàn)在各塊體邊界附近.

(3) 整條測(cè)線地殼厚度和泊松比之間存在弱的負(fù)相關(guān)關(guān)系.反映地殼結(jié)構(gòu)的橫向不均勻性很大部分是構(gòu)造變形的結(jié)果.

(4) 整條測(cè)線泊松比分布和地殼上地幔結(jié)構(gòu)特征支持古亞洲(索倫)洋，最終沿林西斷裂閉合的動(dòng)力學(xué)模式.

本文結(jié)果存在的不足之處是：以二連盆地為主的厚沉積層覆蓋區(qū)的部分臺(tái)站，接收函數(shù)的波形畸變，個(gè)別甚至無法獲得穩(wěn)定的H-κ結(jié)果.我們正在探索利用主動(dòng)源地震淺層層析成像的結(jié)果，改善較厚沉積層臺(tái)站的接收函數(shù)波形，以期獲得更加可靠的地殼厚度和泊松比圖像.

致謝感謝河北省張家口市國(guó)土資源局、內(nèi)蒙古自治區(qū)錫林郭勒盟國(guó)土資源局、烏蘭察布市國(guó)土資源局對(duì)野外工作的支持，感謝方霖、周大為、鈕增欣等野外臺(tái)站布設(shè)人員的辛勤勞動(dòng)，感謝劉啟民博士、鄭丹碩士在數(shù)據(jù)處理過程中給予的幫助，感謝審稿專家提出的建設(shè)性修改意見.

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(本文編輯何燕)

基金項(xiàng)目國(guó)家自然科學(xué)基金(41174081)和“深部探測(cè)技術(shù)與實(shí)驗(yàn)研究”專項(xiàng)(Sinoprobe-02-03,公益科研專項(xiàng)項(xiàng)目編號(hào)201311156)聯(lián)合資助.

作者簡(jiǎn)介龔辰，1989年生，中國(guó)地質(zhì)科學(xué)院與中國(guó)地質(zhì)大學(xué)(北京)聯(lián)合培養(yǎng)碩士研究生在讀，主要從事地殼上地幔結(jié)構(gòu)的寬頻帶地震探測(cè)研究.E-mail:gc_gongchen@163.com *通訊作者李秋生，1958年生，研究員，主要從事大陸深部結(jié)構(gòu)地震學(xué)探測(cè)與地球動(dòng)力學(xué)研究.E-mail:liqiusheng@cags.ac.cn

doi:10.6038/cjg20160312 中圖分類號(hào)P315

收稿日期2015-02-05，2015-10-28收修定稿

Crustal thickness and Poisson ratio beneath the Huailai-Bayinonder profile derived from teleseismic receiver functions

GONG Chen1,LI Qiu-Sheng1*,YE Zhuo1,ZHANG Hong-Shuang1,LI Wen-Hui1,HE Chuan-Song2,LI Ying-Kang3

1InstituteofGeology,ChineseAcademyofGeologicalSciences,KeyLaboratoryofEarthprobeandGeodynamics,MinistryofLandandResource,Beijing100037,China2InstituteofGeophysics,ChinaEarthquakeAdministration,Beijing100081,China3CoreandSamplesCenterofLandandResources,HebeiYanjiao065201,China

AbstractThe Central Asian Orogenic Belt (CAOB) between the Siberia paleo-continent and the North China paleo-continent has a close relationship with the evolution of the Paleo-Asian Ocean.The Xing′an-Mongolia Orogenic Belt (XMOB) as eastern part of the CAOB is located in North China.It is generally considered that the closure of the Paleo-Asian Ocean resulted in the formation of the XMOB,but there remain some disputes about the tectonic attribute of the northern margin of the North China Craton and where the Paleo-Asian Ocean finally closed and how it behaves due to complex evolution process,extensive coverage of Cenozoic sediments and the lack of high resolution deep exploration data.

KeywordsXing′an Mongolia Orogenic Belt； Paleo-Asian Ocean； Receiver function； Crust and upper mantle structure； Poisson ratio

龔辰，李秋生，葉卓等.2016.遠(yuǎn)震P波接收函數(shù)揭示的張家口(懷來)—中蒙邊境(巴音溫多爾)剖面地殼厚度與泊松比.地球物理學(xué)報(bào)，59(3):897-911,doi:10.6038/cjg20160312.

Gong C,Li Q S,Ye Z,et al.2016.Crustal thickness and Poisson ratio beneath the Huailai-Bayinonder profile derived from teleseismic receiver functions.Chinese J.Geophys.(in Chinese),59(3):897-911,doi:10.6038/cjg20160312.