李仕虎,黃寶春,朱日祥*
1中國(guó)科學(xué)院地質(zhì)與地球物理研究所巖石圈演化國(guó)家重點(diǎn)實(shí)驗(yàn)室,北京 100029
2中國(guó)科學(xué)院研究生院,北京 100049
青藏高原東南緣構(gòu)造旋轉(zhuǎn)的古地磁學(xué)證據(jù)
李仕虎1,2,黃寶春1,朱日祥1*
1中國(guó)科學(xué)院地質(zhì)與地球物理研究所巖石圈演化國(guó)家重點(diǎn)實(shí)驗(yàn)室,北京 100029
2中國(guó)科學(xué)院研究生院,北京 100049
本文在總結(jié)青藏高原東南緣近年來(lái)地質(zhì)研究進(jìn)展的基礎(chǔ)上,從古地磁學(xué)的角度討論其新生代以來(lái)的構(gòu)造運(yùn)動(dòng)特征.結(jié)果表明:相對(duì)穩(wěn)定的歐亞大陸,新生代以來(lái)山泰地塊發(fā)生了約20°~80°順時(shí)針旋轉(zhuǎn),局部地區(qū)旋轉(zhuǎn)量甚至高達(dá)135°,且中部地區(qū)的旋轉(zhuǎn)量明顯高于南北地區(qū);印支地塊經(jīng)歷了~30°的順時(shí)針旋轉(zhuǎn);川滇地塊的順時(shí)針旋轉(zhuǎn)量沿102°E經(jīng)度線由南向北由30°逐漸減?。涣硪环矫?,古地磁數(shù)據(jù)還揭示出山泰地塊新生代以來(lái)發(fā)生了~8°的南向滑移運(yùn)動(dòng).旋轉(zhuǎn)量隨時(shí)間的變化表明主要構(gòu)造旋轉(zhuǎn)發(fā)生在始新世與中中新世之間,與哀牢山—紅河斷裂的左行走滑時(shí)間相一致.這表明青藏高原東南緣的新生代構(gòu)造運(yùn)動(dòng)具有差異性和復(fù)雜性,現(xiàn)今國(guó)際流行的擠出逃逸、地殼縮短增厚及下地殼流模式均有其局限性.值得注意的是,青藏高原東南緣可靠的新生代古地磁數(shù)據(jù)在時(shí)空分布上的嚴(yán)重不足,制約了我們對(duì)印度與歐亞大陸碰撞在青藏高原東南緣的運(yùn)動(dòng)學(xué)響應(yīng)過(guò)程的深入探討和正確理解.因此,進(jìn)一步對(duì)該地區(qū)新生代地層開(kāi)展深入細(xì)致的古地磁學(xué)等綜合研究,無(wú)疑具有重要的科學(xué)意義.
青藏高原東南緣,古地磁,構(gòu)造旋轉(zhuǎn),走滑逃逸
青藏高原的隆升是始于古近紀(jì)初印度與歐亞大陸碰撞的結(jié)果[1-5].這一碰撞不僅造成了亞洲大陸內(nèi)部強(qiáng)烈的構(gòu)造變形[6-8],而且對(duì)區(qū)域甚至全球氣候變化都有著深遠(yuǎn)影響[9-12].早期研究表明,印度與歐亞大陸碰撞以來(lái)的構(gòu)造縮短量達(dá)到2600km左右[13-14].而關(guān)于青藏高原構(gòu)造縮短變形的端元模式主要有兩種:地殼縮短增厚和走滑逃逸.地殼縮短增厚模式認(rèn)為歐亞大陸地殼類似于一粘滯性薄板,碰撞所造成的構(gòu)造縮短主要由地殼增厚和一系列逆沖斷層所吸收,加厚的高原地殼向東擴(kuò)展.早期沒(méi)有物質(zhì)向東逃逸,晚期盡管有物質(zhì)向東運(yùn)動(dòng),但其變形是一種連續(xù)的內(nèi)部應(yīng)變[15-16].同時(shí),青藏高原東部、東南緣地區(qū)因右行剪切而繞東喜馬拉雅構(gòu)造節(jié)發(fā)生大規(guī)模的順時(shí)針旋轉(zhuǎn),旋轉(zhuǎn)速度達(dá)到每百萬(wàn)年1°~2°[17].而走滑逃逸模式則認(rèn)為印度向歐亞大陸的俯沖可簡(jiǎn)化為塊體的剛性運(yùn)動(dòng);塊體間的匯聚由斷層所圍限的剛性地塊的側(cè)向走滑逃逸來(lái)調(diào)節(jié)[18-21].首先,印支地塊向東南方向走滑逃逸并伴隨大規(guī)模順時(shí)針旋轉(zhuǎn),其走滑的東、西邊界分別為左行走滑的哀牢山—紅河斷裂和右行走滑的實(shí)皆斷裂(或高黎貢走滑斷裂)[22-24].該期走滑可能造成了中國(guó)南海的擴(kuò)張[18-19,25].其次,隨著印度板塊不斷向北擠壓,青藏高原東北部和華南、華北地塊開(kāi)始第二期走滑.其南邊界為由左行走滑轉(zhuǎn)為右行正斷的紅河斷裂,北邊界為左行的阿爾金斷裂帶.華北地塊內(nèi)部山西地塹的裂開(kāi)以及西伯利亞板塊南緣貝加爾湖的形成等均是其遠(yuǎn)程效應(yīng)[18].
盡管上述兩種變形模式均能解釋青藏高原的很多地質(zhì)現(xiàn)象,但不足之處顯而易見(jiàn).譬如走滑逃逸模式難以解釋高原內(nèi)部南北向裂谷的產(chǎn)生、GPS觀測(cè)到的斷層走滑速率與地質(zhì)推測(cè)速率之間的巨大差異[26-32];而縮短增厚模式則很難解釋大規(guī)模走滑的發(fā)生[19,25].為此,Royden等根據(jù)野外觀測(cè)、GPS 測(cè)量和數(shù)值模擬提出了介于上述兩種模式之間的下地殼流動(dòng)模式[33-34].該模式認(rèn)為早第三紀(jì)時(shí)(約50~20Ma),青藏高原中、北部以構(gòu)造擠壓縮短為主,而東、東南部在印度與歐亞板塊碰撞擠壓和西太平洋板塊俯沖弧后擴(kuò)張的雙重作用下向東、東南方向發(fā)生走滑逃逸.晚第三紀(jì)時(shí)(約20~15Ma以后)青藏高原整體快速隆升,中部發(fā)生東西向伸展.由于西太平洋板塊俯沖速度的減慢甚至停止,東向逃逸主要集中在高原東北緣地區(qū),且多被區(qū)域性的逆沖和褶皺所吸收.此時(shí)由于高原的抬升,青藏高原的下地殼變熱變軟,變軟的下地殼向東、東南緣發(fā)生塑性流動(dòng),而上地殼向東南的移動(dòng)則被順時(shí)針旋轉(zhuǎn)和鮮水河—小江斷裂的走滑所調(diào)節(jié).
由此可見(jiàn),青藏高原東南緣是全球晚新生代以來(lái)構(gòu)造最活躍的地區(qū)之一,也是解決青藏高原演化的關(guān)鍵地區(qū)之一.因此從20世紀(jì)80年代開(kāi)始,青藏高原東南緣就成為地球科學(xué)家關(guān)注的焦點(diǎn).地質(zhì)研究表明,青藏高原東南緣晚新生代地殼沒(méi)有明顯的擠壓縮短,主要是以沿大型斷層的走滑和塊體的構(gòu)造旋轉(zhuǎn)為主[35-51].而上述模型爭(zhēng)論的焦點(diǎn)就是地質(zhì)時(shí)期是否發(fā)生過(guò)大規(guī)模的走滑逃逸和構(gòu)造旋轉(zhuǎn).古地磁研究作為定量恢復(fù)古構(gòu)造運(yùn)動(dòng)最有效的手段而成為解決這一爭(zhēng)論的關(guān)鍵.為此,近年來(lái)在該地區(qū)積累了大量的古地磁數(shù)據(jù)[52-92].然而,由于采樣位置和采樣地層時(shí)代的差異、數(shù)據(jù)本身的可靠性、以及局部新構(gòu)造運(yùn)動(dòng)的影響,不同作者所得出的結(jié)論亦存在很大差異,甚至相互矛盾.為此,本文擬在對(duì)青藏高原東南緣近年來(lái)的地質(zhì)研究進(jìn)展進(jìn)行扼要分析的基礎(chǔ)上,對(duì)青藏高原東南緣晚中生代以來(lái)已有的古地磁數(shù)據(jù)進(jìn)行篩選分析,依據(jù)可靠的古地磁數(shù)據(jù),探討青藏高原東南緣晚新生代的構(gòu)造運(yùn)動(dòng)和其對(duì)青藏高原構(gòu)造運(yùn)動(dòng)的啟示,以及今后古地磁學(xué)研究亟待解決的關(guān)鍵問(wèn)題.
青藏高原東南緣由一系列斷層所夾的次級(jí)塊體拼合而成[28],主要包括川滇、山泰和印支地塊三個(gè)次一級(jí)構(gòu)造單元(圖1).川滇地塊是揚(yáng)子板塊的西南邊緣部分,被北西—北北向的鮮水河—小江斷裂與揚(yáng)子板塊主體隔開(kāi),其西南以哀牢山—紅河斷裂帶為邊界.山泰地塊的西部邊界為高黎貢—實(shí)皆斷裂帶;該地塊可進(jìn)一步劃分為分屬于岡瓦納大陸的保山地體和屬于特提斯的蘭坪—思茅地體,二者之間由近南—北向花崗巖體組成的昌寧—孟連縫合帶分開(kāi)[93-95].山 泰 地 塊 和 印 支 地 塊 由 平 行 于 Nan-Uttaradit縫合帶的北東—南西向奠邊府左行走滑斷裂隔開(kāi)[25].青藏高原東南緣大面積出露中、新生代紅層,并直接覆蓋于古生代地層之上[95].受新生代構(gòu)造活動(dòng)影響,這些紅層在山泰和川滇地塊內(nèi)被擠壓成一系列北西—北北西向的褶皺和逆沖斷層[25,95].各主要邊界斷裂帶的活動(dòng)歷史簡(jiǎn)介如下:
作為調(diào)節(jié)青藏高原物質(zhì)向東運(yùn)動(dòng)和繞東喜馬拉雅構(gòu)造節(jié)順時(shí)針旋轉(zhuǎn)的邊界[38,96],鮮水河—小江斷裂帶從西北向東南依次為北西—南東向的甘孜斷裂、鮮水河斷裂、近南—北向的安寧—?jiǎng)t木河斷裂和小江斷裂[35,38,97].小江斷裂向南由西向東 分為綠 汁江、易門、普渡河、西小江和東小江斷裂,其中綠汁江斷裂又稱元謀斷裂.許志琴等[98]在康定發(fā)現(xiàn)一個(gè)平行于該斷裂的大型花崗巖體,認(rèn)為該花崗巖體為同剪切巖漿侵入產(chǎn)物,其侵位年齡為鮮水河斷裂的走滑開(kāi)始時(shí)間.U-Pb和Rb-Sr同位素以及40Ar/39Ar年齡研究表明,其侵入和冷卻年齡分別為10~12Ma和5Ma[99-101],因此很多學(xué)者認(rèn)為10~12Ma可能為鮮水河—小江斷裂左旋走滑的開(kāi)始時(shí)間.最近Wang等[102]通過(guò)對(duì)甘孜—玉樹(shù)地區(qū)鮮水河斷裂帶的花崗巖進(jìn)行40Ar/39Ar和磷灰石熱年代學(xué)研究表明鮮水河—小江斷裂帶走滑運(yùn)動(dòng)分為兩期,早期開(kāi)始于13Ma,切穿了宕江、甘孜、貢嘎山,到達(dá)清河—鹽源地區(qū);晚期從5Ma到現(xiàn)在,斷裂穿過(guò)玉樹(shù)、甘孜、貢嘎山到達(dá)昆明地區(qū).鮮水河—小江斷裂甘孜段左行走滑位移量為78~100km,其中有60km的走滑轉(zhuǎn)移至鮮水河—小江斷裂上,局部地區(qū)走滑被伸展和擠壓構(gòu)造所吸收,但是在整個(gè)斷裂帶上總走滑位移量是一定的[36,38].
哀牢山—紅河斷裂由西北向東南依次為雪龍山、點(diǎn)蒼山、哀牢山、Day Nui Con Voi(DNCV)四個(gè)變質(zhì)帶,其早期變形形式為左行走滑,后期為右行正斷[19,25].目前研究較深入的是點(diǎn)蒼山和哀牢山變質(zhì)帶.點(diǎn)蒼山變質(zhì)帶主要由一套深變質(zhì)巖組成,包括副片麻巖、眼球狀片麻巖、云母片巖、角閃石片巖等[103];哀牢山變質(zhì)帶由西南部的低級(jí)變質(zhì)帶和東北部的高級(jí)變質(zhì)帶組成,東北的高級(jí)變質(zhì)帶由角閃巖-綠片巖相的副片麻巖、角閃巖、大理巖和花崗巖組成,西南的低級(jí)變質(zhì)帶由低綠片巖相片巖、千枚巖和板巖組成.帶內(nèi)巖石均經(jīng)歷韌性左行剪切,形成具透入性面理和線理的糜棱狀片麻巖[19,25].
哀牢山—紅河斷裂帶作為走滑逃逸模式早期擠出的東邊界而成為研究焦點(diǎn),但目前關(guān)于該斷裂的性質(zhì)、走滑時(shí)間和方式仍存在很大爭(zhēng)議.Tapponnier等[19]認(rèn)為該斷裂是一巖石圈規(guī)模的大型走滑斷裂,但Jolivet等[104]則認(rèn)為其僅為一上地殼斷裂.Harrision等、陳文寄等、李齊等[39,109-111]指出哀牢山變質(zhì)帶走滑具有轉(zhuǎn)換拉張性質(zhì),在走滑的同時(shí)經(jīng)歷了穿時(shí)性的勻速擴(kuò)張?zhí)瑥臇|南以4.5cm/a的速率向西北傳遞,與由南海磁異常條帶所預(yù)測(cè)的哀牢山—紅河斷裂走滑速率3~5cm/a相一致[112],支持?jǐn)D出逃逸導(dǎo)致中國(guó)南海的擴(kuò)張.而 Wang and Burchfiel、Schoenbohm 等[35,115]則 認(rèn) 為 哀 牢 山—紅河斷裂帶為一轉(zhuǎn)換壓縮斷裂.
Tapponnier等、Leloup 等、Gilley 等[19,25,103,105]認(rèn)為左行走滑開(kāi)始于35Ma,主走滑時(shí)間為22~17Ma,與斷裂兩側(cè)火山巖的年齡相一致[106-108].但是,Chung 等、Wang 等[42,113]認(rèn)為走 滑時(shí)間應(yīng)晚于30Ma.Wang 等[43,44]進(jìn) 一 步 通 過(guò) 對(duì) 最 東 南 端DNCV變質(zhì)巖的40Ar/39Ar定年得出走滑開(kāi)始于27.5Ma,晚于南海擴(kuò)張的時(shí)間(34Ma),因此南海擴(kuò)張與走滑逃逸無(wú)關(guān).Searle[114]則認(rèn)為走滑開(kāi)始時(shí)間甚至晚于21Ma.
另一方面,通過(guò)斷裂帶兩側(cè)相關(guān)標(biāo)志地質(zhì)體的對(duì)比,Leloup等[25]認(rèn)為左行走滑位移為700±200km,最大位移約1150km.Chung等[113]認(rèn)為左行走滑量約為600km.然而,Searle[114]則認(rèn)為上述標(biāo)志體都不可靠,哀牢山—紅河斷裂帶左行走滑位移量仍不清楚.哀牢山—紅河斷裂在約5Ma時(shí)由左行走滑轉(zhuǎn)為右行走滑且兼具正斷分量[19,25],走滑的位移量在幾公里至幾十公里[45,96,116],但也有學(xué)者認(rèn)為轉(zhuǎn)換時(shí)間可能為12或16Ma[101].
除哀牢山—紅河斷裂帶和鮮水河—小江斷裂帶外,青藏高原東南緣還發(fā)育實(shí)皆斷裂、高黎貢山剪切帶、沖山剪切帶、Wang Chao斷裂、Three Pagodas斷裂等大型走滑剪切斷裂以及南亭、孟興、南馬等小型斷裂(圖1).實(shí)皆斷裂和高黎貢山剪切帶作為印支地塊逃逸的西邊界,主要為右行剪切.實(shí)皆斷裂的走滑與安達(dá)曼海的擴(kuò)張有關(guān),開(kāi)始于15Ma;而高黎貢斷裂的走滑時(shí)間和哀牢山—紅河斷裂帶相同[23,117].但是,高黎貢剪切帶現(xiàn)今為一不活動(dòng)剪切帶,而GPS觀測(cè)證實(shí)實(shí)皆斷裂現(xiàn)今仍以18mm/a的速度運(yùn)動(dòng)[118].最新研究表明,沖山剪切帶為一既有左行走滑又有右行走滑的剪切帶,其活動(dòng)時(shí)間至少開(kāi)始于34Ma,甚至早于41Ma,結(jié)束于17Ma,與哀牢山—紅河剪切帶早期左行走滑的時(shí)間相同,這表明夾于哀牢山—紅河斷裂和高黎貢山剪切帶之間的地塊在逃逸時(shí)并不是一個(gè)剛性塊體,至少被沖山剪切帶分為兩部分[22].Wang Chao和 Three Pagodas斷裂左行走滑停止于30.5Ma[41].南亭、孟興、南馬等小型斷裂都為NE—SW走向,現(xiàn)今表現(xiàn)為左行走滑,但早期可能為右行走滑,在距今5~20Ma時(shí)走滑形式發(fā)生反轉(zhuǎn)[119].Wang等、Schoenbohm等[38,96]認(rèn)為這些小型斷裂可能是鮮水河—小江斷裂跨過(guò)哀牢山—紅河斷裂后的延伸,這一結(jié)論得到GPS觀測(cè)結(jié)果的進(jìn)一步證實(shí)[120].
圖1 青藏高原東南緣構(gòu)造圖.(a)青藏高原東南緣構(gòu)造簡(jiǎn)圖(修改自[25]).ASRR:哀牢山—紅河剪切帶;DS:點(diǎn)蒼山變質(zhì)帶;XLS:雪龍山變質(zhì)帶;DNCV:Day Nui Con Voi變質(zhì)帶;XSHF:鮮水河—小江斷裂;SF:實(shí)皆斷裂;KLF:昆侖斷裂;JLF:嘉黎斷裂;WCF:Wang Chao Fault;TPF:Three Pagodas Fault.實(shí)線箭頭代表實(shí)測(cè)古地磁偏角,虛線代表以思茅(23.5°N、100.5°E)為參考點(diǎn),以歐亞大陸視極移曲線為參考極所計(jì)算出的期望古地磁偏角;紫色、紅色、藍(lán)色、綠色分別代表侏羅紀(jì)、白堊紀(jì)、古近紀(jì)、新近紀(jì);黑色(紅色)實(shí)點(diǎn)代表古地磁所記錄的向北(向南)的緯向運(yùn)動(dòng).(b)走滑逃逸模式示意圖[18-19].Fig.1 Sketch map of the tectonic setting of the southeast margin of the Tibetan plateau.(a)Sketch map of the tectonic setting of the southeast margin of the Tibetan plateau(modified from [25]).ASSR:Ailao Shan-Red River Shear Zone;DS:Diancang Shan Shear Zone;XLS:Xuelong Shan Shear Zone;DNCV:Day Nui Con Voi Shear Zone;XSHF:Xian Shui He-Xiao Jiang Fault;SF:Sagaing Fault;KLF:Kunlun Fault;JLF:Jiali Fault;WCF:Wang Chao Fault;TPF:Three Pagodas Fault.Solid arrows represent measured paleomagnetic declination,dotted lines represent the expected paleomagnetic declination,they were calculated from the apparent polar wander path(APWP)of Eurasia as a reference pole and Simao(23.5°N,100.5°E)as a reference site.The purple,red,blue,and green represent Jurassic,Cretaceous,Paleogene,and Neogene data,respectively.The black (red)dots represent the northward (southward)latitudinal displacement recorded by paleomagnetic data.(b)Sketch map of the extrusion model on refs[18-19].
值得注意的是,所有由野外地質(zhì)觀測(cè)推斷出的大型走滑斷裂平均走滑速率均遠(yuǎn)高于GPS觀測(cè)到的走滑速率[26-32],而斷層的走滑速率直接被用來(lái)約束重建青藏高原的構(gòu)造演化模型[29].這一差異或表明這些斷層在地質(zhì)時(shí)期走滑速率比現(xiàn)今要大,或表明野外地質(zhì)對(duì)斷層走滑時(shí)間和位移量的厘定存在很大誤差.由此可見(jiàn),盡管前人對(duì)青藏高原東南緣走滑斷裂開(kāi)展了大量研究,但迄今對(duì)其走滑時(shí)間、走滑位移量及其在青藏高原構(gòu)造演化中的作用還很不清楚,仍有待于多學(xué)科的進(jìn)一步綜合研究.
走滑逃逸模式的關(guān)鍵在于青藏高原東南緣在新生代沿哀牢山—紅河斷裂發(fā)生了大規(guī)模的南向逃逸和構(gòu)造旋轉(zhuǎn).因此,自從Tapponnier等[18]提出走滑逃逸模式以來(lái),青藏高原東南緣就成了古地磁學(xué)研究的熱點(diǎn).Achache等[52]首次對(duì)青藏高原周緣白堊紀(jì)和新生代已有古地磁數(shù)據(jù)進(jìn)行了總結(jié),提出青藏高原東南緣的印支地塊相對(duì)于歐亞大陸存在緯向上的南移(-5.5±10.2°)和順時(shí)針旋轉(zhuǎn)(29±16.2°),支持走滑逃逸模式.然而,該數(shù)據(jù)少、誤差很大,尤其是緯向運(yùn)動(dòng)量在古地磁誤差范圍內(nèi).此后的二十多年,大量學(xué)者對(duì)青藏高原東南緣新生代以來(lái)的運(yùn)動(dòng)模式進(jìn)行了廣泛的古地磁學(xué)研究.但已有數(shù)據(jù)的質(zhì)量參差不齊,比如有的數(shù)據(jù)未進(jìn)行系統(tǒng)退磁[79],有的則可能受到了重磁化的影響[53].因此,為了更好地約束青藏高原東南緣的構(gòu)造運(yùn)動(dòng)模式,我們對(duì)迄今已發(fā)表侏羅紀(jì)以來(lái)的古地磁數(shù)據(jù)按照如下可靠性標(biāo)準(zhǔn)進(jìn)行篩選:
(1)采樣點(diǎn)大于3且經(jīng)過(guò)系統(tǒng)退磁;
(2)α95小于15°;
(3)通過(guò)褶皺檢驗(yàn)或倒轉(zhuǎn)檢驗(yàn)或者其它檢驗(yàn)方法.
同時(shí),本文選用歐亞大陸200Ma以來(lái)的視極移曲線[121]作為參考極重新計(jì)算各個(gè)地塊相對(duì)于穩(wěn)定歐亞大陸的構(gòu)造旋轉(zhuǎn)和緯向位移量;各個(gè)特定時(shí)期的參考極以10Ma為窗口進(jìn)行Fisher平均(表1).相對(duì)旋轉(zhuǎn)和緯向運(yùn)動(dòng)計(jì)算采用Butler方法[122],誤差采用Demarest方法[123],計(jì)算結(jié)果見(jiàn)表2和圖1,以箭頭表示古地磁偏角,其與由歐亞大陸視極移曲線得出的期望古地磁偏角之差代表旋轉(zhuǎn)量.
表1 歐亞大陸的視極移曲線[121]Table 1 The APWP of Eurasia[121]
山泰地塊是青藏高原東南緣古地磁數(shù)據(jù)積累最多的地區(qū),由符合標(biāo)準(zhǔn)、重新計(jì)算后的古地磁結(jié)果(表2)隨采樣點(diǎn)緯度的分布(圖2)可以發(fā)現(xiàn):除Mae Sot、景東和下關(guān)采點(diǎn)外,山泰地塊相對(duì)歐亞大陸經(jīng)歷了約20°~80°順時(shí)針旋轉(zhuǎn),局部地區(qū)旋轉(zhuǎn)量甚至高達(dá)135°,且中部地區(qū)旋轉(zhuǎn)量明顯高于南北地區(qū).為此,一些研究者結(jié)合區(qū)域地質(zhì)背景,提出山泰地塊早期整體先經(jīng)歷了~30°的順時(shí)針旋轉(zhuǎn),后期由于南亭、孟興、南馬等小型斷裂的活動(dòng)[119],中部地區(qū)經(jīng)歷 內(nèi)部變 形 又 發(fā) 生 了 進(jìn) 一 步 旋 轉(zhuǎn)[67-69,77-78].在 緯 向運(yùn)動(dòng)方面,大部分白堊紀(jì)古地磁結(jié)果支持山泰地塊白堊紀(jì)以來(lái)相對(duì)歐亞大陸經(jīng)歷了~8°(~900km)的南向滑移,但侏羅紀(jì)古地磁結(jié)果卻顯示山泰地塊自侏羅紀(jì)以來(lái)相對(duì)歐亞大陸發(fā)生大規(guī)模的北向運(yùn)動(dòng),可能與特提斯洋的演化有關(guān);同時(shí)新生代古地磁結(jié)果也顯示山泰地塊新生代以來(lái)相對(duì)歐亞大陸發(fā)生了北向運(yùn)動(dòng).
圖2 山泰地塊相對(duì)歐亞大陸的旋轉(zhuǎn)和緯向運(yùn)動(dòng).圖中菱形和實(shí)心圓圈分別代表中生代和新生代古地磁結(jié)果;條形棒代表95%置信區(qū)間內(nèi)的誤差;括號(hào)外字符為采樣點(diǎn)名稱縮寫(xiě),括號(hào)內(nèi)為樣品時(shí)代.Fig.2Relative rotation and latitudinal displacement of the Shantai terrane respect to the Eurasian plate.Diamonds and circles represent Mesozoic and Cenozoic paleomagnetic results,respectively.Error bars represent 95%confidence intervals.The labels represent the abbreviation of sampling localities and ages are given in parenthesis.
印支地塊上積累的可靠古地磁數(shù)據(jù)相對(duì)較少,同樣將計(jì)算后的結(jié)果(表2)投影在采樣點(diǎn)緯度的坐標(biāo)系上(圖3).結(jié)果表明:除Song Da采樣點(diǎn)外,侏羅紀(jì)和白堊紀(jì)的古地磁結(jié)果顯示印支地塊侏羅紀(jì)以來(lái)相對(duì)歐亞大陸經(jīng)歷了~30°的順時(shí)針旋轉(zhuǎn),但是越南、呵叻高原以及Mae Moh新生代古地磁結(jié)果顯示印支地塊第三紀(jì)以來(lái)相對(duì)歐亞大陸無(wú)明顯的構(gòu)造旋轉(zhuǎn).緯向運(yùn)動(dòng)方面,只有呵叻高原早白堊世和Borikhanxay晚侏羅-早白堊世的結(jié)果顯示印支地塊白堊紀(jì)以來(lái)相對(duì)歐亞大陸經(jīng)歷了約6°~8°的南向滑移;而Khorat plateau和Muang Phin采點(diǎn)早侏羅紀(jì)的結(jié)果則顯示印支地塊侏羅紀(jì)以來(lái)相對(duì)歐亞大陸經(jīng)歷了~20°的北向漂移.此外,Song Da采點(diǎn)白堊紀(jì)古地磁結(jié)果顯示該地區(qū)白堊紀(jì)以來(lái)相對(duì)歐亞大陸無(wú)明顯的緯向運(yùn)動(dòng);新生代古地磁結(jié)果也均顯示印支地塊新近紀(jì)以來(lái)相對(duì)歐亞大陸無(wú)顯著緯向運(yùn)動(dòng).
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圖3 印支地塊相對(duì)歐亞大陸的旋轉(zhuǎn)和緯向運(yùn)動(dòng)KP:呵叻高原;NT:泰國(guó)北部;B:Borikhanxay;SD:Song Da;MP:Muang Phin;MM:Mae Moh;V:越南.其余同圖2.Fig.3 Relative rotation and latitudinal displacement of the Indochina terrane relative to the Eurasian plate.The symbols and labels are the same as in Fig.2.
川滇地塊可靠古地磁數(shù)據(jù)亦較少.為了便于討論,暫將鮮水河-小江斷裂以北的古地磁數(shù)據(jù)也一并投影在采樣點(diǎn)緯度的坐標(biāo)系內(nèi).從表2和圖4可以看出,川滇地塊相對(duì)歐亞大陸的順時(shí)針旋轉(zhuǎn)量沿102°E經(jīng)度線由南向北由30°逐漸減小,鮮水河-小江斷裂以北轉(zhuǎn)為逆時(shí)針旋轉(zhuǎn),推測(cè)可能是川滇地塊受山泰、印支地塊擠出拖拽影響的結(jié)果[55,74].緯向運(yùn)動(dòng)方面,除楚雄采點(diǎn)外,所有結(jié)果均顯示川滇地塊相對(duì)歐亞大陸經(jīng)歷了顯著的北向運(yùn)動(dòng),新生代結(jié)果尤為明顯.
圖4 川滇地塊相對(duì)歐亞大陸的旋轉(zhuǎn)和緯向運(yùn)動(dòng).灰色矩形框代表鮮水河—小江斷裂帶的位置.Fig.4 Relative Rotation and Latitude displacement of Chuandian terrane respect to the Eurasian block.The grey rectangle represents the location of XSHF.
古地磁結(jié)果揭示了山泰、印支、川滇地塊在新生代經(jīng)歷了差異性順時(shí)針旋轉(zhuǎn),這種旋轉(zhuǎn)的差異性說(shuō)明印度與歐亞大陸的碰撞對(duì)青藏高原東南緣不同地區(qū)產(chǎn)生的影響存在區(qū)域差異性,這也與青藏高原東南緣局部地區(qū)構(gòu)造背景的差異性有關(guān),比如思茅地體中部發(fā)育大量左行走滑斷裂,而呵叻高原則相對(duì)為一剛性的整體.下關(guān)、景東、Mae Sot、Song Da等地區(qū)古地磁結(jié)果表明其相對(duì)歐亞大陸無(wú)顯著的構(gòu)造旋轉(zhuǎn).進(jìn)一步分析發(fā)現(xiàn)上述地區(qū),古地磁采樣剖面均非??拷呋瑪嗔褞Вㄒ?jiàn)圖1),其不旋轉(zhuǎn)或者旋轉(zhuǎn)量很小可能是受走滑斷層的影響[127-128],也有可能是這些走滑斷層作為差異性旋轉(zhuǎn)的解耦帶,本身并沒(méi)有旋轉(zhuǎn)[64].
目前,僅有山泰地塊白堊紀(jì)和印支地塊部分白堊紀(jì)古地磁數(shù)據(jù)反映白堊紀(jì)以來(lái)其相對(duì)歐亞大陸經(jīng)歷了約800km的南向移動(dòng),這一方面可能是印支和川滇地塊白堊紀(jì)數(shù)據(jù)太少,另一方面也可能是印支地塊并非作為一個(gè)整體發(fā)生走滑逃逸,只有部分地塊發(fā)生了擠出逃逸的緣故;比如最新研究發(fā)現(xiàn),印支地塊最南端的Kontum塊體白堊紀(jì)之后,同樣相對(duì)于歐亞大陸發(fā)生了9.2°±4.9°的南向移動(dòng)[129].山泰地塊的擠出邊界為哀牢山—紅河斷裂,而印支地塊 的擠出 邊 界 則 位 于 Song Ma斷 裂 以 西[70,130-131].這一結(jié)果表明走滑逃逸模式在適用范圍上有其局限性,而完全否認(rèn)走滑發(fā)生的地殼增厚模式同樣也有局限性.
此外,無(wú)論是山泰地塊還是印支地塊,其侏羅紀(jì)和新生代古地磁結(jié)果都顯示其相對(duì)歐亞大陸發(fā)生了大規(guī)模的北向運(yùn)動(dòng).Achache and Courtillot[132]對(duì)呵叻高原晚三疊的古地磁研究得出印支地塊晚三疊世相對(duì)歐亞大陸存在1650±850km的北向位移,這與本文的結(jié)果(~15°)基本一致.印支和山泰地塊侏羅紀(jì)的北向運(yùn)動(dòng)有兩種可能:
其一,由于印支、山泰地塊和華南板塊在侏羅紀(jì)以前已經(jīng)碰撞拼合完畢[133],而華南、華北板塊在晚侏羅世才拼合為一個(gè)整體[134-135],華南、華北與西伯利亞直至早白堊世才完全拼合成為一個(gè)整體[136],所以印支、山泰地塊的北向運(yùn)動(dòng)可能是其與華南板塊一起北向運(yùn)動(dòng)與華北板塊在晚侏羅世完成碰撞拼合,或者是印支地塊與華南、華北地塊一起北向運(yùn)動(dòng)與西伯利亞板塊發(fā)生碰撞拼合.
其二,山泰地塊和印支地塊在侏羅紀(jì)時(shí)尚未與華南地塊完全拼合,在侏羅—白堊紀(jì)時(shí)其處于拉薩和羌塘地塊之間,新生代時(shí)由于印度與歐亞大陸的碰撞走滑逃逸至現(xiàn)今位置[56].目前尚沒(méi)有更多的證據(jù)表明哪一種原因更有可能,因此需要更多的中生代古地磁數(shù)據(jù)以確定青藏高原東南緣各個(gè)微板塊與華南大陸的拼合歷史.
新生代印支、山泰和川滇地塊相對(duì)歐亞大陸大規(guī)模向北運(yùn)動(dòng)與現(xiàn)有地質(zhì)背景相矛盾,造成這種矛盾的原因可能有很多,但一種最可能的解釋是新生代沉積物受到了傾角淺化的影響而沒(méi)有實(shí)際構(gòu)造意義.事實(shí)上,新生代沉積物磁傾角淺化現(xiàn)象在新生代快速沉積的中亞地區(qū)非常普遍[137-144].可靠的新生代火山巖的古地磁資料也許是解決這一問(wèn)題的關(guān)鍵[139,143].
另一方面,為反映青藏高原東南緣構(gòu)造旋轉(zhuǎn)量隨時(shí)間的變化,我們把所有可靠古地磁結(jié)果投影在時(shí)間坐標(biāo)系內(nèi)(圖5),發(fā)現(xiàn)青藏高原東南緣整體經(jīng)歷了30°~40°的順時(shí)針旋轉(zhuǎn),局部地區(qū)的旋轉(zhuǎn)量高達(dá)100°以上.侏羅紀(jì)到古新世-始新世所記錄的構(gòu)造旋轉(zhuǎn)量幾乎沒(méi)有顯著區(qū)別,說(shuō)明青藏高原東南緣構(gòu)造旋轉(zhuǎn)發(fā)生在始新世以來(lái);同時(shí),地層時(shí)代分別為14.1~12.0Ma的 Mae Moh盆地和~5Ma的元謀盆地未觀察到明顯的構(gòu)造旋轉(zhuǎn)[73,146-147],這很可能說(shuō)明青藏高原東南緣構(gòu)造旋轉(zhuǎn)主要發(fā)生在始新世—中中新世之間.這一旋轉(zhuǎn)時(shí)間和哀牢山—紅河斷裂左行走滑時(shí)間相一致,而中中新世以后無(wú)明顯的構(gòu)造旋轉(zhuǎn)與GPS觀測(cè)現(xiàn)今較低的走滑速率相一致,說(shuō)明青藏高原東南緣主要的走滑逃逸和旋轉(zhuǎn)發(fā)生在中中新世以前.Dupont-Nivet等[148]對(duì)青藏高原東北緣西寧—蘭州盆地古地磁研究同樣得出西寧—蘭州盆地的構(gòu)造旋轉(zhuǎn)發(fā)生在始新世—中新世之間.如果上述結(jié)論正確的話,則意味著西寧-蘭州盆地所代表的青藏高原東北緣與青藏高原東南緣在新生代擁有同樣的旋轉(zhuǎn)歷史.然而,這一推論正確與否仍有待于進(jìn)一步研究.
圖5 青藏高原東南緣相對(duì)歐亞大陸旋轉(zhuǎn)量隨采樣時(shí)間的變化.字符代表采樣點(diǎn)名稱的縮寫(xiě)(同圖2,3,4).豎直和水平細(xì)線分別代表在95%置信區(qū)間內(nèi)旋轉(zhuǎn)量的誤差和采樣地層的時(shí)間跨度范圍(見(jiàn)表1).Fig.5 Observed relative rotations of the southeastern margin of the Tibetan plateau with respect to the Eurasian block as a function of age.The labels represent the abbreviation of sampling localities(same as Figs.2,3and 4).Vertical and horizontal thin lines represent 95%confidence intervals in rotation and uncertainty in age or age span(see Table 1).
首先,迄今為止仍沒(méi)有地質(zhì)證據(jù)表明青藏高原東北緣與東南緣在新生代具有共同的構(gòu)造演化史.其次,相比于東北緣,青藏高原東南緣新生代古地磁數(shù)據(jù)還非常少.而且Mae Moh盆地和元謀盆地的古地磁結(jié)果均來(lái)自磁性地層學(xué)的結(jié)果,這些結(jié)果所揭示的構(gòu)造旋轉(zhuǎn)樣式和旋轉(zhuǎn)量能否適用于采樣剖面之外的塊體,仍值得進(jìn)一步研究.特別是區(qū)域范圍內(nèi)普遍缺少始新世—中新世古地磁數(shù)據(jù),使得上述推論缺少直接證據(jù);此外,青藏高原東南緣新構(gòu)造運(yùn)動(dòng)非常復(fù)雜,不僅可劃分為印支、山泰和川滇等多個(gè)次級(jí)地塊,而且地塊內(nèi)部斷層交錯(cuò)、變形強(qiáng)烈,基本不具有剛性塊體的特點(diǎn),因而為數(shù)不多的幾個(gè)采點(diǎn)或采樣剖面很難客觀地描述整個(gè)塊體的運(yùn)動(dòng)學(xué)特征.因此,盡管到目前為止青藏高原東南緣已經(jīng)積累了不少古地磁數(shù)據(jù),但是未來(lái)對(duì)該地區(qū)的古地磁研究,尤其是新生代古地磁研究,仍然亟需加強(qiáng).
5.1 印支、山泰、川滇地塊新生代以來(lái)相對(duì)歐亞大陸發(fā)生了顯著的差異性順時(shí)針旋轉(zhuǎn);山泰地塊南向運(yùn)動(dòng)顯著.
5.2 靠近走滑斷層一般沒(méi)有明顯的旋轉(zhuǎn),這說(shuō)明走滑斷層可能是差異性旋轉(zhuǎn)的解耦帶.
5.3 侏羅紀(jì)古地磁結(jié)果暗示歐亞大陸在晚侏羅紀(jì)以后在動(dòng)力學(xué)意義上才成為一個(gè)整體;而新生代古地磁結(jié)果所揭示的青藏高原東南緣相對(duì)歐亞大陸的北向位移則可能是沉積物磁傾角淺化所致.
值得注意的是現(xiàn)有青藏高原東南緣古地磁數(shù)據(jù)還很匱乏,尤其是晚新生代古地磁數(shù)據(jù).因此,進(jìn)一步對(duì)新生代地層開(kāi)展深入細(xì)致的古地磁學(xué)研究,對(duì)更好地理解印度與歐亞大陸碰撞對(duì)青藏高原東南緣的影響具有重要的科學(xué)意義.
致 謝 與鄧成龍研究員、姚海濤、黃晟、李金華博士的討論使作者獲益匪淺,G Peterson博士對(duì)論文英文摘要進(jìn)行了語(yǔ)言修改,作者對(duì)此深表感謝.
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Paleomagnetic constraints on the tectonic rotation of the southeastern margin of the Tibetan Plateau
LI Shi-Hu1,2,HUANG Bao-Chun1,ZHU Ri-Xiang1*
1 State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics of the Chinese Academy of Sciences,Beijing100029,China
2 Graduate University of the Chinese Academy of Science,Beijing100049,China
In this paper we present a comprehensive summary of the geological evolution of the southeastern margin of the Tibetan Plateau and a detailed reanalysis of previously published paleomagnetic data.We focus on the Cenozoic,which represents a period during which the southeastern margin of the Tibetan Plateau was one of the most active tectonic regions due to the India-Eurasia collision.Our analysis indicates that,since the Cenozoic,with respect to the stable Eurasian block,the Shantai terrane experienced a clockwise rotation of~20°—80°,with some areas experiencing clockwise rotation by as much as 135°,and the rotation of the central part of the terrane is higher than that in the north and south of the terrane;the Indo China terrane rotated~30°clockwise and the rotation of the Chuandian terrane decreased from 30°along the longitude 102°E from south to north.Of the three terranes only the Shantai terrane recorded a~8°southward translation.The variation of rotation versus time indicates that the main rotation of the southeastern margin of the Tibetan Plateau occurred between Eocene and mid-Miocene,which is in accordance with the sinisterly slip of the Ailao Shan-Red River fault zone.This complex tectonic history,revealed by paleomagnetism,cannot be fully explained by the commonly accepted models for the formation of the Tibetan Plateau,such as crustal thickening,lateral extrusion or lower crustal flow.Reliable Cenozoic paleomagnetic data in the southeast margin of Tibetan Plateau are scarce.Therefore,to better evaluate the effects of the India-Eurasian collision on the southeastern margin of the Tibetan Plateau,additional and more detailed paleomagnetic studies of Cenozoic rocks from this region are essential.
Southeastern margin of the Tibetan Plateau,Paleomagnetism,Tectonic rotation,Extrusion
10.6038/j.issn.0001-5733.2012.01.008
P318,P541
2011-04-06,2011-06-28收修定稿
國(guó)家自然科學(xué)基金(40221402)資助.
李仕虎,男,1985年出生,博士研究生,從事構(gòu)造古地磁學(xué)研究.E-mail:lsh917@m(xù)ail.iggcas.ac.cn
*通訊作者 朱日祥,男,研究員、中國(guó)科學(xué)院院士,主要從事古地磁及地球動(dòng)力學(xué)研究.E-mail:rxzhu@m(xù)ail.iggcas.ac.cn
李仕虎,黃寶春,朱日祥.青藏高原東南緣構(gòu)造旋轉(zhuǎn)的古地磁學(xué)證據(jù).地球物理學(xué)報(bào),2012,55(1):76-94,
10.6038/j.issn.0001-5733.2012.01.008.
Li S H,Huang B C,Zhu R X.Paleomagnetic constraints on the tectonic rotation of the southeastern margin of the Tibetan Plateau.Chinese J.Geophys.(in Chinese),2012,55(1):76-94,doi:10.6038/j.issn.0001-5733.2012.01.008.
(本文編輯 劉少華)