蒙古—鄂霍茨克構造帶中段構造變形及動力學特征

黃始琪, 董樹文*, 張福勤, 苗來成, 朱明帥

1)中國地質科學院, 北京 100037; 2)中國科學院地質與地球物理研究所, 北京 100029

蒙古—鄂霍茨克構造帶中段構造變形及動力學特征

黃始琪1), 董樹文1)*, 張福勤2), 苗來成2), 朱明帥2)

1)中國地質科學院, 北京 100037; 2)中國科學院地質與地球物理研究所, 北京 100029

蒙古—鄂霍茨克構造帶作為中亞造山帶的重要組成部分, 其構造變形和動力學特征一直是地質界關注的問題。沿著該構造帶中段, 對5個韌性變形點及1個脆性變形點進行詳細解析, 揭示了該構造帶變形及動力學特征。B型褶皺、揉皺、A型褶皺、礦物拉伸線理、S-C組構都顯示了該構造帶明顯的NW—SE剪切作用。剪切方向穩(wěn)定而單一, 未發(fā)現(xiàn)多方向變形疊加現(xiàn)象, 可能指示了蒙古—鄂霍茨克構造帶的形成過程為一期主要的俯沖碰撞或多期同向的俯沖碰撞。對蒙古—鄂霍茨克構造帶形成時間和動力學背景進行了討論, 認為該構造帶主要形成于中晚侏羅世—早白堊世東亞多向匯聚動力學背景之下。對構造帶內地質點mg6脆性斷層面上滑動矢量進行了統(tǒng)計和古應力場反演, 得出兩期古構造應力場, 一期為NW—SE擠壓, 一期為近E—W擠壓。NW—SE擠壓應力場可能對應了中晚侏羅世—白堊紀古太平洋板塊向西俯沖對中亞地區(qū)的遠程影響; 而近 E—W 向擠壓可能反映了早新生代印度—歐亞板塊碰撞對中亞地區(qū)的遠程效應。

蒙古—鄂霍茨克構造帶; 韌性變形; 脆性變形; 古應力場; 東亞多向匯聚; 蒙古

Key words:Mongolia-Okhotsk collisional belt; ductile deformation; brittle deformation; stress field; East Asian multi-direction convergence; Mongolian

中亞造山帶位于西伯利亞板塊與塔里木和華北板塊之間, 主要由華北板塊和西伯利亞板塊的俯沖增生所形成, 是世界上最大的增生型造山帶(Seng?r et al., 1993; Wickham et al., 1996; Jahn et al., 2000a, b; Kovalenko et al., 2004; Xiao et al., 2008, 2009a, b)。蒙古—鄂霍茨克構造帶是中亞造山帶的重要組成部分(圖 1), 在東亞大陸形成演化的歷史上占有極為重要的地位(李錦軼等, 2009)。根據(jù)巖石建造, 趙越等(1994)認為該構造帶是華北板塊和西伯利亞板塊之間的最后縫合帶。蒙古—鄂霍茨克構造帶主要分布在東經96°—130°, 北緯46°—58°之間的俄羅斯和蒙古境內, 西起蒙古中部的杭蓋山脈, 東至鄂霍茨克海的烏達海灣, 總體呈北東—南西走向, 長約3000 km, 寬約300 km, 北部為西伯利亞板塊及其增生邊緣, 南部為中朝—蒙古板塊及其以北的造山帶與地塊鑲嵌構造區(qū), 東部為太平洋板塊。蒙古—鄂霍茨克洋俯沖時間一直存在爭議, Seng?r等(1993)認為在埃迪卡拉紀—晚二疊世; Parfenov等(2003)認為在泥盆紀—早三疊世; Gordienko等(2010)和Bussien等(2011)認為在泥盆紀—二疊紀; Metelkin等(2010)認為在晚石炭世—晚侏羅世; Zhao等(1990)、Enkin等(1992)、Kuzmin等(1996)以及 Zorin(1999)認為在早二疊世—中侏羅世; Zonenshain等(1990)認為在三疊紀—晚侏羅世。蒙古—鄂霍茨克洋的最終俯沖關閉時間也存在爭議, Zonenshain等(1990)認為西側關閉時間為三疊紀—晚侏羅世; Zorin(1999)和Parfenov等(1999)認為發(fā)生在中晚侏羅世。許多學者認為東段的封閉時間更晚, 應該在晚侏羅世—早白堊世(Sengor et al., 1996; Yakubchuk et al., 1999; Kravchinsky et al., 2002a; Cogné et al., 2005; 李錦軼, 1998, 2013)。盡管蒙古—鄂霍茨克構造的開啟和最終閉合時間存在爭議, 而蒙古—鄂霍茨克洋兩側板塊的碰撞是一個自西向東的順時間旋轉碰撞得到地質學者的普遍認同(Zorin, 1999)。這種碰撞過程有的學者認為主要是晚元古代—石炭紀的一次單階段俯沖碰撞(Seng?r et al., 1993, 1996); 有的學者則認為是多階段俯沖碰撞(Badarch et al., 2002; Filippova et al., 2001; Kr?ner et al., 2005, 2007; Windley et al., 2007; Zorin et al., 2007)。Donskaya等(2013)認為石炭紀蒙古—鄂霍茨克洋殼高角度俯沖, 導致褶皺變形, 雙重推覆構造,及最后地殼加厚。正常角度的俯沖發(fā)生在晚二疊世到晚三疊世, 導致大量侵入巖和火山巖的產出。碰撞過程的不同, 對應的巖石變形和反映的動力學特征也不同, 通過對露頭尺度巖石的變形解析有助于揭開真實的地質演化過程。而碰撞之后該構造帶是否受后期其它區(qū)域應力作用影響, 可以通過脆性斷層的古應力場反演來加以解析。本文基于野外巖石脆韌性變形數(shù)據(jù)的收集和分析, 對蒙古—鄂霍次克構造帶中段巖石變形及其對應的動力學特征做了基本分析, 以揭示蒙古—鄂霍茨克構造帶碰撞變形及動力學過程。

圖1 蒙古—鄂霍茨克構造帶及其鄰區(qū)構造簡圖(據(jù)Donskaya et al., 2013修改)Fig. 1 Simplified tectonic map of the Mongolia–Okhotsk collisional belt and its adjacent areas (modified after Donskaya et al., 2013)

蒙古—鄂霍茨克構造帶中段主要由杭蓋—肯特區(qū)、奧倫島弧區(qū)和南部的阿穆爾區(qū)組成(圖1)。杭蓋—肯特區(qū)主要為古生代濁積巖盆地, 而其具體時代及屬性存在一定爭議。Minjin等(2006)認為盆地內烏蘭巴托附近存在中—晚泥盆世混雜巖; 而有的學者認為盆地內泥盆紀增生雜巖與早古生代增生雜巖伴生, 石炭紀濁積巖不整合于下伏增生雜巖之上(Dorjsurend et al., 2006); Tomas等(2008)對杭蓋—肯特盆地古生代地層的碎屑鋯石研究, 證明該濁積巖盆地形成于 345 Ma后的早石炭世晚期, 碎屑鋯石還揭示了濁積盆地與下伏前早石炭世地質體間的廣泛不整合。奧倫島弧區(qū)超過 1000 m厚的泥盆系—石炭系鈉質玄武巖、碧玉、超鎂鐵質巖覆于前寒武紀的結晶基底上, 石炭系之上是二疊紀到三疊紀的陸源磨拉石建造(莫申國等, 2005)。本文研究區(qū)域主要在蒙古境內阿穆爾板塊。阿穆爾板塊是前蘇聯(lián)學者Zonenshain和Savostin于1981年首次提出的, 主要用于解釋從貝加爾裂谷以東沿斯坦諾夫山地的地震活動條帶(Zonenshain et al., 1981)。阿穆爾板塊的西部邊界為貝加爾裂谷, 向北經過斯坦諾夫山地,然后向東, 沿日本島以西一系列的正斷層南下, 在日本本州中部與南海地槽相連, 而后向西經朝鮮半島南端由渤海進入中國, 經山西地塹北部、鄂爾多斯北端向西北與貝加爾裂谷相接, 形成一個覆蓋中國東北及華北部分地區(qū)、朝鮮半島、日本西南部、俄羅斯東南部及蒙古西部的巨大構造單元(許厚澤等, 2004)。阿穆爾地區(qū)主要為元古代基底及石炭紀至二疊紀的火成巖帶(Denise et al., 2011)。據(jù)Ren等(2013)和任紀舜等(2013)亞洲地質編圖結果顯示,沿著蒙古—鄂霍茨克構造帶廣泛發(fā)育巖漿巖, 西段及中段主要為石炭紀—二疊紀花崗巖和花崗閃長巖, 往東巖漿巖年齡逐漸變新, 出現(xiàn)侏羅紀花崗巖和花崗閃長巖。

1 韌性變形及動力學特征

由南西往北東, 沿著蒙古—鄂霍茨克構造帶中段, 分析了 5個重要韌性變形點的形態(tài)學和動力學特征。顯示明顯的NW—SE剪切作用。mg7位于東經 110°12′12″, 北緯 47°23′43″, 蒙古木倫市西南,巖性為泥盆紀(Bussien et al., 2011)灰黑色變泥質巖,綠片巖相, 原巖可能為凝灰質砂巖。發(fā)生明顯的斷滑式褶皺, 反映NW—SE剪切作用(圖2)。mg8與mg7相鄰, 位于東經 110°13′47″, 北緯 47°23′39″。該處泥盆紀變泥巖內侵入角閃輝長巖脈, 巖脈產狀為345°/80°, 寬約3 m。巖脈內長石發(fā)生強的韌性變形, 形成揉皺和 A型褶皺, 指示 NW—SE剪切(圖3)。該點亦見蛇紋巖, 巖石片理化明顯, 且片理發(fā)生褶皺變形, 指示NW—SE剪切作用(圖4)。mg11位于東經111°45′41″, 北緯48°47′11″, 蒙古烏蘭河東。巖性為泥盆紀(Bussien et al., 2011)云母石英片巖,巖內石英脈發(fā)生揉皺, 石香腸構造, 及 A 型褶皺,指示NW—SE剪切作用(圖5, 6, 7)。mg14位于東經112°53′19″, 北緯 49°22′20″, 巴彥烏拉北, 蒙古—鄂霍茨克構造帶北部, 該處為二疊紀(Bussien et al., 2011)云母石英片巖, 片理化明顯, 暗色礦物形成拉伸線理, 片理總體產狀為: 325°/19°, 線理總體產狀為: 325°/20°。S-C組構反映NW—SE剪切作用(表1, 圖7)。mg16位于mg14北側, 東經112°52°11″, 北緯 49°25′33″, 蒙古與俄羅斯交界處, 緊鄰蒙古—鄂霍茨克縫合帶。巖性為二疊紀(Bussien et al., 2011)黑云斜長角閃片巖, 糜棱巖化。面理總體產狀為: 331°/24°, 線理產狀為: 321°/18°。S-C組構, 反映NW—SE剪切作用(表2, 圖8)。

圖2 點mg7 B型斷滑褶皺(位置見圖1)Fig. 2 B-fold of schist at Site mg7(see Fig. 1 for the location)

圖3 點mg8輝長巖脈內長石脈發(fā)生柔皺(位置見圖1)Fig. 3 Crumple structure of feldspar veins in the gabbro dike at Site mg8 (see Fig. 1 for the location)

2 脆性變形及動力學特征

巖石韌性變形后, 抬升至較淺地表, 受區(qū)域應力作用, 易發(fā)生脆性變形而改造原有韌性變形(孟憲剛等, 2001)。通過統(tǒng)計分析脆性斷層面上滑動矢量的運動特征, 反演古構造應力場。本文使用斯諾維尼亞?alohar等(2007)開發(fā)的應力場反演軟件, 基本原理為安德森模式和庫倫摩爾破裂準則。

圖4 點mg8蛇紋巖發(fā)生柔皺(位置見圖1)Fig. 4 Crumple structure in serpentinite at Site mg8 (see Fig. 1 for the location)

圖5 點mg11片巖內A型褶皺(位置見圖1)Fig. 5 A style fold in schist at Site mg11(see Fig. 1 for the location)

圖6 點mg11片巖內長英質脈發(fā)生柔皺(位置見圖1)Fig. 6 Crumple structure of felsic veins in schist at Site mg11 (see Fig. 1 for the location)

圖7 點mg11片巖內過渡型A型褶皺(位置見圖1)Fig. 7 Transitional A-fold of schist at Site mg11 (see Fig. 1 for the location)

表1 點mg14面理及線理統(tǒng)計結果Table 1 Statistics of foliations and lineations at Site mg14

圖8 點mg14韌性變形(位置見圖1)Fig. 8 Ductile deformation at Site mg14 (see Fig. 1 for the location)

圖9 點mg16韌性變形(位置見圖1)Fig. 9 Ductile deformation at Site mg16 (see Fig. 1 for the location)

mg6位于東經 110°07′26″, 北緯 47°23′25″, 蒙古木倫市西南, 蒙古—鄂霍茨克構造帶中段南部,該處泥盆紀地層內傾入后期輝長巖, 并發(fā)生糜棱巖化, 后期脆性斷層切割輝長糜棱巖。對斷層面上滑動矢量統(tǒng)計分析以及古應力場反演, 得出一期 NW—SE擠壓古應力場和一期近E—W擠壓古應力場。NW—SE應力場最大主應力軸產狀為309°/2°, 中間主應力軸和最小主應力軸產狀分別為 40°/24°和215°/66°(表3, 圖10)。NE—SW向擠壓應力場最大主應力軸產狀為 268°/17°, 中間主應力軸和最小主應力軸產狀分別為5°/24°和145°/60°(表4, 圖11)。

3 討論與構造意義

蒙古—鄂霍茨克構造帶的形成是經歷了一次主要俯沖碰撞還是經歷多次俯沖碰撞一直存在爭議(Seng?r et al., 1993, 1996; Badarch et al., 2002; Filippova et al., 2001; Kr?ner et al., 2005, 2007; Windley et al., 2007; Zorin et al., 2007; Donskaya et al., 2013)。如果是經歷了多次大規(guī)模方向不同的俯沖碰撞, 則巖石韌性變形可能會體現(xiàn)出多期構造作用的疊加現(xiàn)象, 如多期拉伸線理疊加, 多方向 A型褶皺疊加等, 而如果是同方向的多期俯沖碰撞, 原有的韌性變形只是被后期構造作用加強, 不易識別多階段的疊加現(xiàn)象。沿著蒙古—鄂霍茨克構造帶所分析的mg7、mg8、mg11、mg14、mg16五個韌性變形點, 皆顯示一致的NW—SE剪切作用。mg7、mg8所見的B型和A型褶皺未見多方向疊加現(xiàn)象, mg14、mg16面理上并未見多方向線理疊加現(xiàn)象, 因此, 推斷蒙古—鄂霍茨克構造帶是一次主要俯沖碰撞或多期同方向俯沖碰撞的產物。

表3 點mg6反映NW—SE擠壓應力場斷層滑動矢量統(tǒng)計Table 3 Results of fault–slip analysis and stress orientations at Site mg6 defining a compressional regime with NW–SE compression

表4 點mg6反映近E—W擠壓應力場斷層滑動矢量統(tǒng)計Table 4 Results of fault-slip analysis and stress orientations at Site mg6 defining a compressional regime with E–W compression

圖10 mg6 NW—SE擠壓應力場及應力摩爾圓圖析(位置見圖1)Fig. 10 Fault-slip data and computed stress axis of NW–SE compression and its stress Moore circle analysis at Site mg6 (see Fig. 1 for the location)

圖11 mg6近E—W擠壓應力場及應力摩爾圓圖析(位置見圖1)Fig. 11 Fault-slip data and computed stress axis of E–W compression and its stress Moore circle analysis at Site mg6 (see Fig. 1 for the location)

蒙古—鄂霍茨克洋東段最終閉合時間一直持續(xù)到晚侏羅世—白堊紀, 對應了中國的燕山運動,董樹文等(2000, 2007, 2008)和Dong等(2013)認為燕山運動不僅僅局限于中國東部地區(qū), 而是一個全球范圍的大構造事件, 其影響遠遠超出中國東部的范圍, 是東亞多板塊的多向匯聚事件。約起始于170 Ma, 多板塊幾乎同時向東亞地區(qū)發(fā)生俯沖或推覆碰撞, 在克拉通和剛性盆地周緣形成環(huán)形造山帶,如四川盆地和鄂爾多斯盆地周邊的造山帶。受此構造事件影響, 西伯利亞克拉通周緣也形成大規(guī)模環(huán)形山系。西伯利亞板塊向南運動逆沖到華北—蒙古板塊基底和被動大陸邊緣沉積地層之上, 以及導致蒙古—鄂霍茨克構造帶北西側結晶基底推覆于中侏羅世含煤沉積巖之上(Zorin, 1999), 意味著該構造帶形成的主要階段為中晚侏羅世。該逆沖推覆系統(tǒng)構成蒙古—鄂霍茨克構造帶主枝, 據(jù)上地殼地球物理圖像, 蒙古—鄂霍茨克構造帶杭蓋地區(qū)水平位移量為150 km, 肯特地區(qū)為100 km。奧倫島弧在碰撞前位于西伯利亞板塊和華北蒙古板塊之間, 碰撞時脫離其基底推覆于華北—蒙古板塊之上, 其水平位移量為200 km(Zorin, 1999)。西伯利亞板塊向南運動與華北—蒙古板塊碰撞的同時, 侏羅紀末期, 北美洲板塊以及科利馬—奧莫隆復合地塊向西運動與其碰撞形成近南北走向的維爾霍揚斯克沖斷褶皺帶(Vladimir et al., 2003)。維爾霍揚斯克褶皺帶內古生代到早中生代幾個深水沉積盆地在晚中生代板塊碰撞作用下發(fā)生強烈的褶皺變形, 也說明維爾霍揚斯克沖斷褶皺帶主要形成于晚中生代(Eugene et al., 1986)。另外, 蒙古—鄂霍茨克構造帶兩側大量發(fā)育130 Ma碰撞后的伸展盆地(Yannick et al., 2013), 也意味著晚侏羅世—早白堊世在該區(qū)發(fā)生了強烈的碰撞擠壓事件。

越來越多的事實證明發(fā)生在中晚侏羅世的東亞多向匯聚構造事件影響范圍十分廣泛, 造成東亞地區(qū)強烈構造變形, 其背后有著深刻的地球動力學背景與動力來源(董樹文等, 2008)。自侏羅紀以來,東亞地區(qū)大量巖石圈物質俯沖到地幔之中, 是地球上俯沖巖石圈物質最大量的地區(qū)(Maruyama et al., 2007)。而華北與揚子陸塊碰撞造山作用, 使得中國東部巖石圈厚度曾經達到150~200 km, 可能是東亞匯聚的先兆(董樹文等, 2008)。隨后發(fā)生的東亞和中國東部巨厚巖石圈的垮塌、拆沉和斷離, 導致了超高壓巖石的折返, 軟流圈物質側向補償, 牽引了太平洋板塊向西俯沖, 印度洋板塊向 NE俯沖, 西伯利亞陸塊與華北陸塊碰撞(董樹文等, 2008), 甚至北美洲板塊向西與西伯利亞板塊碰撞(Vladimir et al., 2003), 形成一個多板塊在中晚侏羅世同時向東亞地區(qū)匯聚的格局。

mg6通過脆性斷層反演出一期NW—SE擠壓應力場, 很可能對應了晚侏羅世到早白堊世古太平洋俯沖對中亞造山帶的遠程影響。印度—歐亞板塊的碰撞及之后的合并對亞洲新生代以來的地質、構造、地球動力學, 甚至氣候都產生了巨大影響(Johan et al., 2007; Tapponnier et al., 1982, 2001; Yin, 2006; Yin et al., 2000; 張岳橋等, 2012)。中亞造山帶雖遠離印度—歐亞板塊, 其晚期的再活動也明顯受到這次強烈構造活動的遠程影響(Molnar et al., 1975, 1977; Tapponnier et al., 1979; Peltzer et al., 1988)。研究區(qū)mg6通過脆性斷層反演的一期近 E—W 擠壓, 很可能反映了印度—歐亞板塊碰撞的遠程效應及原有蒙古—鄂霍茨克構造帶邊界限制作用的聯(lián)合影響。

4 結論

蒙古—鄂霍茨克構造帶作為中亞造山帶的重要組成部分, 其變形和動力學特征一直是地質界關注的問題, 沿著該構造帶中段, 對該構造帶進行韌性和脆性變形分析, 主要得出以下結論:

1)對蒙古—鄂霍茨克構造帶5個韌性變點進行了形態(tài)學動力學解析, 顯示NW—SE剪切作用。剪切方向單一, 未發(fā)現(xiàn)多方向的變形疊加現(xiàn)象, 可能指示了蒙古—鄂霍茨克構造帶的形成經歷了一期強烈的俯沖碰撞或多期同向的俯沖碰撞作用。

2)對蒙古—鄂霍茨克構造帶形成時間和動力學背景進行了討論。該構造帶主要形成于中晚侏羅世—早白堊世東亞多向匯聚動力學背景之下。

3)對mg6脆性斷層滑動矢量進行了統(tǒng)計和古應力場反演, 得出兩期古構造應力場, 一期為 NW—SE擠壓, 一期為近E—W擠壓。NW—SE擠壓應力場可能對應了晚侏羅世古太平洋板塊對中亞地區(qū)的遠程影響, 而近E—W向擠壓可能反映了早新生代印度—歐亞板塊碰撞對中亞地區(qū)的遠程效應。

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Tectonic Deformation and Dynamic Characteristics of the Middle Part of the Mongolia–Okhotsk Collisional Belt, Mongolia

HUANG Shi-qi1), DONG Shu-wen1)*, ZHANG Fu-qin2), MIAO Lai-cheng2), ZHU Ming-shuai2)
1) Chinese Academy of Geological Sciences, Beijing 100037; 2) Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029

As an important part of the Central Asian Orogenic Belt, the Mongolia–Okhotsk collisional belt has attracted much attention for its tectonic deformation and dynamic characteristics. Along the middle part of the Mongolia–Okhotsk collisional belt, five ductile deformation sites and a brittle deformation site were analyzed to reveal its tectonic deformation and dynamic features. B style fold, crumple structure, A style fold, mineral stretching lineation and S-C fabric indicate NW–SE shearing. This information reveals that might have existed a large collision or multi-periodic collisions in the same direction, which resulted in the formation of the Mongolia–Okhotsk collisional belt. The forming time and global tectonic settings as well as the dynamic origin of the Mongolia–Okhotsk collisional belt were discussed. This tectonic belt was mainly formed during the middle Jurassic-early Cretaceous period under the tectonic setting of the East Asian multi-direction convergence. The brittle deformation of Site mg6 was analyzed and two paleo-stress fields were restored, i.e., the NW–SE compression stress field and the E–W compression stress field. The NW–SE compression stress field might have resulted from the distant effect of the westward subduction of the Paleo-Pacific plate during the Jurassic and Cretaceous, whereas the E–W compression stress field probably resulted from the distant effect of the India-Asia collision during the early Cenozoic.

P542; P541

A

10.3975/cagsb.2014.04.03

本文由國家專項“深部探測與實驗研究”(編號: SinoProbe-08-01)資助。

2013-07-11; 改回日期: 2014-04-15。責任編輯: 張改俠。

黃始琪, 男, 1984年生。博士研究生。構造地質專業(yè)。通訊地址: 100037, 北京市西城區(qū)百萬莊大街 26號。電話: 010-68999606。E-mail: qi283463544@163.com。

*通訊作者: 董樹文, 男, 1954 年生。研究員, 博士生導師。長期從事構造地質與碰撞造山帶研究。通訊地址: 100037, 北京市西城區(qū)百萬莊大街26號。電話: 010-68999606。E-mail: swdong@cags.ac.cn。