祁連山中段白堊紀以來階段性構造抬升過程的磷灰石裂變徑跡證據

戚幫申, 胡道功, 楊肖肖, 張耀玲, 譚成軒, 張　鵬, 豐成君

1)中國地質科學院地質力學研究所, 北京 100081;　2)國土資源部新構造與地質災害重點實驗室, 北京 100081; 3)中國科學院地質與地球物理研究所, 北京 100029

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祁連山中段白堊紀以來階段性構造抬升過程的磷灰石裂變徑跡證據

戚幫申1, 2), 胡道功1)*, 楊肖肖3), 張耀玲1), 譚成軒1, 2), 張鵬1, 2), 豐成君1, 2)

1)中國地質科學院地質力學研究所, 北京 100081;2)國土資源部新構造與地質災害重點實驗室, 北京 100081; 3)中國科學院地質與地球物理研究所, 北京 100029

摘要:祁連山作為青藏高原的東北邊界, 是研究青藏高原隆升和擴展的重要區域, 利用磷灰石裂變徑跡分析反映的祁連山地區白堊紀以來階段性隆升和擴展新認識對理解青藏高原的隆升過程有重要的意義。分別采自南祁連陸塊、疏勒南山—拉脊山縫合帶、中祁連陸塊和北祁連縫合帶22個樣品的磷灰石裂變徑跡年齡介于(124±11) Ma與(13±2) Ma之間, 平均徑跡長度介于(13.6±2.3) μm和(10.3±1.8) μm之間。時間-溫度反演模擬結果表明祁連山地區至少經歷了3個重要構造活動階段: 1)白堊紀早期(>(129±14)~(115±17) Ma)祁連山隆升, 南祁連陸塊和疏勒南山—拉脊山縫合帶的冷卻速率及剝蝕速率均較大, 并且祁連山南部可能率先抬升而初步構成高原的東北邊界; 2)白堊紀中晚期—中新世((115±17)~(25±7) Ma)祁連山構造平靜, 南祁連陸塊和疏勒南山—拉脊山縫合帶冷卻速率及剝蝕速率均較低; 3)中新世以來祁連山由南向北逐漸擴展, 構造活動強烈而最終形成盆-山構造地貌格局。祁連山白堊紀早期的快速冷卻過程可能是受拉薩地塊和羌塘地塊碰撞的影響; 中新世以來向北擴展則主要是受印度—歐亞板塊碰撞的影響。

關鍵詞:裂變徑跡分析; 磷灰石; 白堊紀; 新生代; 祁連山

本文由中國地質調查局天然氣水合物資源勘查與試采工程國家專項“祁連山凍土區天然氣水合物資源勘查(力學所)”(編號: GZHL20120301)資助。

青藏高原的隆升不僅改變了亞洲大陸的構造地貌格局, 并對南亞乃至全球氣候產生了重大影響(England and Houseman, 1988; Prell and Kutzbach, 1992; Raymo and Ruddiman, 1992)。因此, 青藏高原隆升的時間和幅度一直是地學界研究的焦點之一,有些學者認為由于中特提斯洋沿班公—怒江縫合帶閉合和拉薩地塊與羌塘地塊的碰撞影響, 青藏高原可能于印度—歐亞板塊碰撞之前便已隆升(Murphy et al., 1997; Schneider et al., 2003; Kapp et al., 2005; Guynn et al., 2006; Otofuji et al., 2007; Volkmer et al., 2007; Li et al., 2013a); 有些學者認為青藏高原開始隆升于印度板塊沿雅魯藏布江縫合帶與歐亞板塊碰撞時, 主要的隆升時代為古新世—始新世(Yin et al., 2002; Spicer et et al., 2003; Wang et al., 2008); 還有很多學者認為青藏高原的東部和北部直至中新世晚期才快速隆起(Turner et al., 1993; An et al., 1999;萬景林等, 2001; 王瑜等, 2002; Zheng et al., 2006;張培震等, 2006), 另有認為直到~4 Ma以來青藏高原才開始整體快速隆起(Li and Fang, 1999)。除了青藏高原隆升時間上的爭議, 其隆升空間分布上也存在很多不同的觀點, 如由南向北逐步隆升(Meyer et al., 1998; Tapponnier et al., 2001)、青藏高原中部先隆起后向南北兩側擴展(Wang et al., 2008)以及高原南部與北部整體隆升(Yin and Harrison, 2000; Yin et al., 2002)。祁連山處于阿拉善和柴達木地塊之間,構成青藏高原的東北緣(圖1A, B), 研究其構造地貌演化特征對認識青藏高原隆升和擴展的過程有著重要意義。

祁連山構造帶寬約500 km, 整體走向NW—SE,共分為南祁連陸塊、蔬勒南山—拉脊山縫合帶、中祁連陸塊和北祁連縫合帶四個構造單元(張雪亭等, 2007)。北祁連縫合帶(北祁連新元古代—早古生代縫合帶)主體位于托來山南緣并呈北西向分布于中祁連陸塊和阿拉善陸塊間, 南部大致以中祁連北緣斷裂為主斷裂構成中祁連陸塊與北祁連縫合帶的分界線, 北部以祁連山北緣斷裂與河西走廊分離(Liu and Gao, 1998; Pan et al., 2013)。中祁連陸塊是一個陸塊與巖漿弧疊置的構造單位, 夾持于北祁連縫合帶與蔬勒南山—拉脊山縫合帶之間, 呈北西西向分布于托勒南山—大通山一帶。南祁連陸塊呈北西西向介于中祁連南緣斷裂(疏勒南山—拉脊山縫合帶主斷裂)與宗務隆—青海南山斷裂之間, 沿居洪圖—陽康—化隆一帶分布(張雪亭等, 2007)。

三疊紀以來, 幾個陸塊已經拼合到歐亞板塊的南緣, 印支期中祁連和南祁連地塊的整體抬升, 導致南祁連和中祁連盆地由海相沉積轉變為陸相沉積,中三疊世海相地層與晚三疊世阿塔寺組陸相沉積建造多表現為平行不整合接觸; 侏羅紀煤系地層與晚三疊世尕勒得寺組陸相地層同樣表現為平行不整合接觸(張雪亭等, 2007), 表明上侏羅統享堂組沉積之后的燕山運動奠定了祁連山中生代的構造格局, 強烈的構造活動形成中祁連以及南祁連廣泛分布的寬緩褶皺和斷裂構造; 新生代以來受印度—歐亞板塊碰撞的影響, 并在阿爾金走滑斷裂和昆侖走滑斷裂的控制下, 形成大范圍的殼內滑脫構造, 從而導致青藏高原北部的強烈地殼增厚和構造隆升(Meyer et al., 1998; Tapponnier et al., 2001), 祁連山地區受一系列NNW和NWW向的逆沖斷裂影響而隆起, 并形成盆-山相間的構造地貌格局。

圖1　研究區位置及地質構造簡圖Fig. 1　Location and simplified geological map of the study areaA-祁連山構造綱要簡圖(據Vincent and Allen, 1999; Gehrels, 2003; Yue et al., 2005; Bovet et al., 2009; 張雪亭等, 2007修改); B-青藏高原北部構造綱要圖及磷灰石裂變徑跡研究點分布及結果(斷裂分布據Jolivet et al., 2001; Gaudemer et al., 1995; Meyer et al., 1996, 1998; Lasserre et al., 1999); C-研究區地質圖(據張雪亭等, 2007修改)及磷灰石采樣地點和結果

磷灰石裂變徑跡的年齡和徑跡長度分布特征為巖石溫度冷卻貫穿部分退火帶(PAZ)提供量化信息, 記錄了巖石從溫度~110℃到60℃(Gallagher et al., 1998)的熱史情況, 而這一過程一般被認為是受地區的隆升或剝蝕的結果, 因此磷灰石裂變徑跡分析可以有效地約束地區剝蝕或隆升的熱演化歷史(Green, 1988; Johnson, 1997; Ventura et al., 2001)。目前, 低溫年代學研究對祁連山隆升歷史的認識還存在較大爭議, 主要的隆升時代可包括: 晚白堊紀(Jolivet et al., 2001; Pan et al., 2013; Li et al., 2013b)、始新世晚期—漸新世(Yin et al., 2002)和中新世(Jolivet et al., 2001; 萬景林等, 2001; 王瑜等, 2002;陳正樂等, 2002; Zheng et al., 2006)。以往的青藏高原北部低溫年代學研究集中于主要斷裂的周圍(George et al., 2001; 萬景林等, 2001, 2010; Jolivet et al., 2001; 王瑜等, 2002; 陳正樂等, 2002; Wang et al., 2006; 拜永山等, 2008; Zheng et al., 2010; Li et al., 2013b; Pan et al., 2013; 孫岳等, 2014), 特別是針對祁連山地區的研究集中于該地區的東緣、北緣、西緣(圖1B), 而祁連山構造帶內部的研究較為匱乏。因此, 本文通過對祁連山中段取樣進行磷灰石裂變徑跡分析, 恢復祁連山地區白堊紀—新生代的熱史,為研究祁連山地區構造活動提供科學依據。

1　樣品采集和方法

22個裂變徑跡樣品分布貫穿南祁連陸塊、中祁連陸塊、疏勒南山—拉脊山縫合帶和北祁連縫合帶,樣品巖性有砂巖、凝灰巖、流紋巖、花崗巖、閃長巖、英安巖等。樣品首先經過粉碎、分選和自然晾干, 經傳統方法粗選, 再利用電磁選、重液選、介電選等手段, 對礦物顆粒進行單礦物提純, 分離出磷灰石單礦物顆粒。分別用環氧基樹脂和聚四氟乙丙烯透明塑料片將磷灰石固定, 制作成光薄片, 并研磨拋光揭示礦物顆粒內表面。磷灰石樣片在恒溫25℃的7%的HNO3溶液中蝕刻30 s以揭示自發徑跡(Yuan et al., 2003)。將低鈾白云母片(<4×10-9)作為外探測器蓋在光薄片上, 緊密接觸礦粒內表面, 與CN5(磷灰石)標準鈾玻璃(Bellemants et al., 1995)一并接受熱中子輻照(Yuan et al., 2006)。然后在25℃條件下的40%HF中蝕刻白云母外探測器20 min揭示誘發徑跡。最后需要在高精度光學顯微鏡100倍干物鏡下觀測統計裂變徑跡。應用IUGS推薦的Zeta常數標定法計算出裂變徑跡中心年齡。實驗中根據標準磷灰石礦物的測定, 加權平均得出Zeta常數值(Hurford and Green, 1983; Hurford, 1990)。由于磷灰石中裂變徑跡退火存在各向異性(Green et al., 1986),因此選擇平行c軸的柱面來測定水平封閉徑跡長度、自發徑跡密度和誘發徑跡密度。

表1　祁連山磷灰石裂變徑跡年齡Table 1　Apatite fission-track data from the Qilian Mountain

2　結果

22個樣品的磷灰石裂變徑跡年齡介于(124± 11) Ma到(13±2) Ma之間, 平均徑跡長度介于(10.3±1.8) μm到(13.6±2.3) μm之間(表1)。僅樣品B325-3和B412-1測試的磷灰石顆粒數目小于20,其余樣品均超過20粒, 并且大多數樣品的圍限徑跡測試條數超過50條, 數據質量較好。

χ2統計法可判斷樣品中各單顆粒年齡在多大程度上可作為具有單一平均年齡來看待(Galbraith and Laslett, 1993)。從單顆粒的自發和誘發裂變徑跡數可計算出P(χ2), 是單顆粒年齡與所有顆粒的平均年齡符合的幾率量度。P(χ2)>5%表示各單顆粒年齡的差別屬于統計誤差范圍, 應作為具有單一平均值看待, FT年齡采用池年齡(Pooled Age); P(χ2)<5%表示各單顆粒年齡確有分散, FT年齡采用中心年齡(Central Age)(Sobel et al., 2006a, b)。統計結果表明共有18個樣品通過了χ2檢驗(圖2), 4個樣品單顆粒年齡分散程度高于一般范圍, 雷達輻射圖顯示這4個樣品的變化范圍為±140 ~ ±25 Ma, 和所有通過χ2檢驗的樣品的年齡變化范圍相近。裂變徑跡年齡分布基本不受巖性的控制, 例如侏羅紀的砂巖和奧陶紀的花崗巖具有相似的裂變徑跡年齡, 所有的樣品均小于其形成年齡。并且平均徑跡長度為(13.6± 2.3) μm 和 (10.3±1.8) μm之間, 指示這些樣品均經歷形成之后的溫度貫穿PAZ的過程(Gleadow et al., 1986; Yuan et al., 2006; Yuan et al., 2007)。

3　磷灰石裂變徑跡年齡的意義

總的來看, 裂變徑跡年齡和海拔相關關系以及徑跡的平均長度與裂變徑跡年齡相關關系不強(圖3A, B), 但按照SW—NE向從采樣所屬不同的構造單元位置來看, 可以發現中祁連陸塊、疏勒南山—拉脊山縫合帶和南祁連陸塊的海拔-年齡呈正相關,相關關系較好(圖3C)。中祁連陸塊裂變徑跡年齡為(96±9) Ma和(13±2) Ma之間, 南北差異較大, 南部的裂變徑跡年齡明顯老于北部地區, 裂變徑跡年齡較小((60±5)~(13±2) Ma)的樣品主要在中祁連北緣斷裂附近(圖3C), 這主要是受新生代以來中祁連北緣斷裂構造活動的影響。因此, 祁連山地區同一個構造單元具有相似的剝蝕速率, 不受差異熱狀態的干擾, 樣品裂變徑跡年齡受樣品相對于主要斷裂的位置影響(Yuan et al., 2006)。

圖2　祁連山磷灰石裂變徑跡輻射圖Fig. 2　Radial plots of single grain ages for the 22 samples from the Qilian Mountain

雖然磷灰石裂變徑跡的退火特征受化學組成(Barbarand et al., 2003)和礦物特征的影響(Carlson et al., 1999)。但這方面的影響相對較弱(Pan et al., 2013), 地殼剝蝕和區域性的構造活動均可能造成年齡-海拔之間的相關性不強(Green et al., 1986)。因此, 不能簡單地將平均裂變徑跡年齡等同于重要的構造事件, 需要結合熱史模擬分析來進一步探討裂變徑跡年齡所代表的巖石熱演化過程。

利用AFTSolve軟件及Ketcham等(1999)模型進行熱史模擬(Ketcham et al., 2003), 模擬次數為10000次, 模擬的評價標準包括K-S檢測和GOF檢測, 當GOF≥0.05, 模擬的曲線被認為是可以接受的, 當GOF≥0.5, 模擬的曲線被認為是好的模擬曲線(Ketcham, 2005)。每次模擬, 都假設樣品實測裂變徑跡年齡的1.5倍時地溫達200~160℃以致樣品完全退火, 在實測裂變徑跡年齡的時間樣品處于PAZ(110~60℃), 而現今處于地表的~20~0℃地溫為另一個限制條件(Pan et al., 2013)。模擬結果見圖4,由于北祁連縫合帶內僅有樣品B054-1, 而單個樣品不足以代表區域的熱史, 因此本文不對其模擬。

模擬結果顯示, 所有樣品的K-S檢測和GOF檢

圖3　祁連山磷灰石裂變徑跡年齡、平均徑跡長度和海拔的關系Fig. 3　Relationship between AFT age, mean track length and elevation

A-裂變徑跡年齡和海拔的關系; B-裂變徑跡年齡和平均徑跡長度的關系; C-樣品分布位置、裂變徑跡年齡和海拔的關系

A-relationship between AFT age and elevation; B-relationship between AFT age and mean track length; C-plot showing relationship between elevation, main faults, AFT ages and samples’ location, from the South Qilian fold belt to North Qilian suture zone in SW-NE direction測均大于0.5, 模擬質量較高且較為可信。根據所有樣品的時間-溫度(t-T)最佳的模擬曲線(圖4), 每條最佳模擬曲線可以分離出3個限制點, 第一個限制點為巖石降溫至~110℃的年齡, 第二個限制點為巖石冷卻速率由快轉慢, 第三個限制點為巖石降溫速率由慢轉快。采自不同地點樣品的最佳模擬曲線限制點可以構成出3組(圖5), 第一組是溫度降溫至~110℃的時候, 南祁連陸塊為(124±7) Ma, 疏勒南山—拉脊山縫合帶為(129±14) Ma。第二組限制點為當巖體冷卻速度從快轉慢的時候, 南祁連陸塊為(117±8) Ma, 古地溫達(79±15)℃ ; 疏勒南山—拉脊山縫合帶內的樣品為(115±17) Ma, 古地溫為(74±9)℃。第三組限制點為巖體冷卻速率從慢再一次轉快的時候, 南祁連陸塊為(25±7) Ma, 古地溫達(54±9)℃ ; 疏勒南山—拉脊山縫合帶為(17±9) Ma,古地溫為(56±11)℃。中祁連陸塊于中新世晚期((10±7) Ma)所有樣品可見快速冷卻, 中新世之前的t-T模擬曲線比較分散, 可能中祁連地塊在中新世之前存在更為復雜的熱史, 直到中新世晚期((10±7) Ma)中祁連地塊整體出現快速冷卻剝蝕。模擬結果中小的冷卻事件被忽略(Pan et al., 2013), 因為這些事件可能受退火模型不穩定的影響(Ketcham et al., 2009)。

祁連山的平均地溫梯度大致為25℃/km(Hu et al., 2000), 結合時間-溫度(t-T)最佳的模擬曲線結果,可估算出每個樣品的冷卻速率(△溫度/△時間)和侵蝕速率(冷卻速率/地溫梯度)。結果表明, 南祁連陸塊白堊紀早期((124±7) Ma至(117±8) Ma), 冷卻速率為~3.4℃/Ma, 剝蝕速率達~0.13 mm/a; 白堊紀早期—中新世早期((117±8) Ma至(25±7) Ma), 冷卻速率為~0.2℃/Ma, 剝蝕速率為~0.01 mm/a; 中新世以來((25±7) Ma至現今), 冷卻速率為~2.0℃/Ma, 剝蝕速率為~0.08 mm/a(圖5A)。疏勒南山—拉脊山縫合帶白堊紀早期((129±14) Ma至(115±17) Ma), 冷卻速率為~2.7℃/Ma, 剝蝕速率達~0.11 mm/a; 白堊紀早期—中新世((115±17) Ma至(17±9) Ma), 冷卻速率為~0.1℃/Ma, 剝蝕速率為~0.01 mm/a; 中新世以來((17±7) Ma至現今), 冷卻速率為~4.6℃/Ma,剝蝕速率為~0.18 mm/a(圖5B)。中祁連陸塊中新世晚期以來((10±7) Ma至現今)存在快速冷卻剝蝕作用, 冷卻速率為~11.9℃/Ma, 剝蝕速率為~0.47 mm/a(圖5C)。

4　討論

磷灰石裂變徑跡記錄了祁連山的熱史情況, 為分析該地區冷卻剝蝕歷史提供定量信息。結果表明,祁連山白堊紀以來至少經歷了3個重要的構造活動階段: ①白堊紀早期隆升; ②白堊紀中晚期—中新世早期構造平靜; ③中新世以來向北逐漸擴展。

圖4　祁連山中段熱模擬結果(時間-溫度模擬曲線和徑跡分布)Fig. 4　Modeled inverse t–T paths and length distribution for samples from the middle segment of the Qilian Mountain

圖5　祁連山熱模擬最佳時間-溫度曲線Fig. 5　The best-fit line of the time-temperature modeling for samples from the Qilian MountainA-南祁連陸塊熱模擬最佳時間-溫度曲線; B-疏勒南山—拉脊山縫合帶熱模擬最佳時間-溫度曲線; C-中祁連陸塊熱模擬最佳時間-溫度曲線

4.1祁連山白堊紀早期隆升

裂變徑跡結果顯示, 祁連山在白堊紀早期((129±14)~(115±17) Ma)南祁連和疏勒南山—拉脊山縫合帶均存在快速冷卻的過程(圖5), 并且大柴旦逆沖斷裂帶兩側剝蝕速率亦存在明顯差異(Jolivet et al., 2001), 低溫年代學數據指示祁連山在白堊紀早期很可能已經隆起, 初步構成青藏高原的東北邊界。同時, 南祁連陸塊的磷灰石裂變徑跡年齡明顯老于中祁連陸塊以及北祁連縫合帶(Pan et al., 2013; Li et al., 2013b), 表明祁連山南部可能率先隆起, 從而導致南祁連侏羅系與白堊系僅出露零星露頭, 與白堊系主要分布在祁連山北部和東部的特征相一致(張雪亭等, 2007)。

阿爾金斷裂東段地區的火山巖主要分布在阿爾金斷裂與祁連山西端交匯的山前盆地(酒西盆地)與山間盆地(昌馬盆地)中, 均為一套偏堿性基性火山巖, 其Ar-Ar測年結果顯示本區巖漿活動分為100~120 Ma和~82 Ma兩期巖漿活動(李海兵等, 2004)和阿爾金斷裂白堊紀再次強烈走滑活動相一致(金山口北坡糜棱巖化的加里東花崗巖白云母形成時代為89.2 Ma和斷層附近侏羅系兩個韌性變形樣品中云母形成時代91.7 Ma和97.7 Ma; Liu et al., 2001), 并且在祁連山北部山前早白堊地層中出現軟沉積變形, 古斜坡指向也反映祁連山的抬升(李海兵等, 2004)。河西走廊地區在白堊紀早期出現類似磨拉石的粗碎屑沉積(Vincent and Allen, 1999)印證了本次構造活動。祁連山白堊紀早期的隆升主要受拉薩地塊和歐亞板塊碰撞的影響(Vincent and Allen, 1999; Jolivet et al., 2001; 李海兵等, 2004),祁連山很可能已經初步隆起, 構成青藏高原東北邊界雛形。

4.2祁連山白堊紀中晚期—中新世早期構造平靜

白堊紀中晚期—中新世南祁連陸塊和疏勒南山—拉脊山縫合帶的冷卻速率(~0.2℃/Ma; ~0.1℃/Ma)及剝蝕速率(~0.01 mm/a; ~0.006 mm/a)均比較低, 徑跡長度較短以及呈寬緩不對稱正態分布的特征均指示該地區長時間處于PAZ(圖5)。AFT熱年代學證據表明祁連山的東緣于±83 ~ ±24 Ma期間的冷卻速率及剝蝕速率(~0.6℃/Ma; ~0.017 mm/a)亦較低(Pan et al., 2013), 本時期祁連山南北兩側的大柴旦地區和河西走廊盆地構造活動由擠壓轉變為伸展, 沉積速率較低(Vincent and Allen, 1999)。因此,白堊紀中晚期—中新世祁連山地區普遍構造活動較弱。雖然晚始新世—漸新世(距今約35.3~32.6 Ma)河西走廊盆地和中祁連木里盆地內白楊河組(E3b)和火燒溝組(E2-3h)均呈輕微角度不整合接觸(戴霜等, 2005; 戚幫申等, 2013), 阿爾金北緣山脈和黨河南山在~40 Ma出現快速冷卻剝蝕(Jolivet et al., 2001; 孫岳等, 2014)均顯示青藏高原北部在始新世晚期—漸新世早期存在構造變形與隆升, 但祁連山構造帶內部的磷灰石裂變徑跡數據沒有顯示本次隆升過程, 結合碳氧同位素估算的古近紀的古海拔較低以及火燒溝組和白楊河組均為河湖相的沉積特征(戴霜等, 2005; 戚幫申等, 2013; 戚幫申等, 2015),可以得出祁連山本次隆升的幅度不大。總體上祁連山地區白堊紀晚期—中新世早期構造平靜。

4.3祁連山中新世以來向北逐漸擴展

印度板塊與歐亞板塊碰撞的時間目前還存在很大的爭議(Beck et al., 1995; Lee and Lawver., 1995; Patzelt et al., 1996; Searle et al., 1997; Rowley, 1998; Zhang and Scharer, 1999), 首次影響到高原北部導致地殼縮短與增厚的時間也是爭論的焦點之一, 如~40 Ma(Jolivet et al., 2001)、~30 Ma(Mock et al., 1999)、25~20 Ma(Sobel and Dumitru, 1997)以及~4 Ma(Li and Fang, 1999; 李吉均等, 2001)。始新世—漸新世高原北部普遍存在構造活動, 但河西走廊盆地和中祁連木里盆地內白楊河組(E3b)和火燒溝組(E2-3h)均呈輕微角度不整合接觸(戴霜等, 2005;戚幫申等, 2013), 構造活動的強度可能不大。上新世—第四紀(~3.6 Ma)的隆升證據主要是依靠沉積相變化得出, 然而這也可能是受氣候變化因素的影響(Zhang et al., 2001), 缺乏更多構造變形證據的支持(張培震等, 2006)。前人的研究發現祁連山及鄰區于中新世中晚期存在“準同期”(~8 Ma)的強烈構造變形, 并通過逆沖斷裂和褶皺變形等方式, 使山脈隆升與沉積盆地消亡(Turner et al., 1993; 張培震等, 2006), 同時期紅黏土在六盤山地區沉積, 這主要受高原腹地海拔達到一定臨界值的影響, 引起高原東北部的氣候和環境方面的變化(An et al., 1999; 宋友桂等, 2001), 受此影響祁連山地區新生代河湖相沉積的碳氧同位素(δ13C和δ18O)于中新世中期亦出現明顯的變化(Dettman et al., 2003; 戚幫申等, 2015)。自上新世早期以來, 位于祁連山南北兩側的柴達木盆地和河西走廊沉積速率明顯加快(Metivier et al., 1998), 由于柴達木盆地和河西走廊地區與其附近的盆地無物質交換, 沉積速率加快指示構造活動加強(Jolivet et al., 2001)。因此, 中新世以來為祁連山地區主要隆升階段, 并且斷裂活動的時間以及區域冷卻歷史顯示中新世以來祁連山具有向北擴展的規律。

祁連山構造帶內有若干條北西—北西西向的逆沖斷裂帶, 自南向北包括柴北緣逆沖斷裂帶、南山逆沖斷裂帶、中祁連南緣逆沖斷裂帶、拉脊山逆沖斷裂、中祁連北緣逆沖斷裂帶和祁連山北緣逆沖斷裂帶等(Yin et al., 2002; Li et al., 2015)。生物地層學、沉積學以及熱年代學研究表明柴達木北緣斷裂在~40 Ma之前就已經開始活動(Jolivet et al., 2001; Yin et al., 2002), 南山逆沖斷裂帶自漸新世(~33 Ma)便已經存在(Wang, 1997; Rumelhat, 1998; Yin et al., 2002), 拉脊山逆沖斷裂帶卻從~22 Ma開始活動(Lease et al., 2011), 北祁連逆沖斷裂帶則直至8.3~ 0 Ma開始活動(Tapponnier et al., 1990; Yang et al., 2007), 故祁連山構造帶內的逆沖斷裂活動時間表現由南向北擴展的規律。

區域性冷卻歷史同樣顯示祁連山地區中新世以來具有向北擴展的規律。南祁連陸塊于(25±7) Ma冷卻速率明顯加快(圖5A), 表明祁連山的南部地區在始新世晚期—漸新世早期存在構造變形與隆升。疏勒南山—拉脊山縫合帶于(17±9) Ma開始快速冷卻剝蝕(圖5B), 而中祁連陸塊直至(10±7) Ma冷卻速率和剝蝕速率才明顯加快, 北祁連亦于±20~ 10 Ma以來轉入快速冷卻(George et al., 2001; Pan et al., 2013), 而高原北緣直到9~7 Ma發生快速蝕頂過程(George et al., 2001; 萬景林等, 2001; 王瑜等, 2002; 陳正樂等, 2002; Zheng et al., 2006, 2010)。由此可見, 中新世以來祁連山的構造變形具有向北逐漸擴展的規律, 這主要受印度—歐亞板塊碰撞的影響, 通過一系列的逆沖斷裂和下地殼廣泛的滑脫作用向北擴展(Bovet et al., 2009)。

5　結論

祁連山構造帶內部不同地點的磷灰石裂變徑跡分析結果表明, 祁連山白堊紀以來至少經歷三個重要的構造活動階段: 1)白堊紀早期(>(129±14) ~(115±17) Ma)隆升, 祁連山地區受拉薩地塊和羌塘地塊的碰撞影響而出現隆升剝蝕, 南北磷灰石裂變徑跡年齡的差異顯示祁連山南部較北部率先隆起,導致祁連山南部出現白堊系沉積間斷, 此時的祁連山可能已經構成青藏高原的東北邊界; 2)白堊紀中晚期—中新世早期((115±17)~(25±7) Ma)祁連山處于構造平靜期, 此時不論是南祁連陸塊還是疏勒南山—拉脊山縫合帶的樣品冷卻速率和剝蝕速率都很低; 3)中新世以來(<(25±7) Ma—今)祁連山由南向北逐漸擴展, 祁連山強烈隆起并導致柴達木盆地和河西走廊地區沉積速率加快以及祁連山地區新生代湖相沉積的碳氧同位素變化, 并形成和現代地貌相近的盆-山構造地貌格局。

致謝: 中國地質科學院地質力學研究所吳中海研究員及各位評審專家給予本文的建設性意見和重要指導, 以及中國地質科學院趙珍博士, 中國地質大學(北京)李波碩士、趙釗碩士、高雪咪碩士、于航碩士、田珺碩士, 長江大學徐久晟碩士和李丹江碩士等參與野外取樣工作, 磷灰石裂變徑跡測試由中國地質大學(北京)袁萬明教授協助完成, 謹表謝意。

Acknowledgements:

This study was supported by China Geological Survey (No. GZHL20120301).

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Apatite Fission Track Study of the Cretaceous–Cenozoic Stepwise Uplift of the Middle Segment of the Qilian Mountain

QI Bang-shen1, 2), HU Dao-gong1)*, YANG Xiao-xiao3), ZHANG Yao-ling1), TAN Cheng-xuan1, 2), ZHANG Peng1, 2), FENG Cheng-jun1, 2)
1) Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081; 2) Key Laboratory of Neotectonic Movement & Geohazard, Ministry of Land and Resources, Beijing 100081; 3) Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029

Abstract:The Qilian Mountain constitutes the northeastern margin of the Tibetan Plateau, and hence

characteristics of its tectonic activity recorded by apatite fission track (AFT) analysis play an important role in understanding the uplift and growth of the Tibetan Plateau. 22 samples collected for AFT analysis from the Qilian Mountain belt were located along NS-trending transect across southern Qilian fold belt, Shule Nanshan-Laji Shan suture zone, Central Qilian massif and North Qilian suture zone. AFT ages range from (13±2) Ma to (124±11) Ma, and mean track lengths range from (10.3±1.8) μm to (13.6±2.3) μm. Samples from the same tectonic unit have positive correlation between AFT ages and elevations, whereas samples with younger ages ((60±5) Ma to (13±2) Ma) are clustered around North Central Qilian fault. On the basis of the measured apatite fission track age data, the inversion simulation was used to analyze the thermal history of the Qilian Mountain. The best-fit line ofthe time-temperature modeling results suggest that at least three cooling periods have occurred since early Cretaceous: 1) Rapid cooling in the Qilian Mountain during early Cretaceous (>(129±14) Ma to (115±17) Ma). The cooling rates and exhumation rates of South Qilian fold belt and Shule Nanshan -Laji Shan suture zone were great, suggesting that the Qilian Mountain formed the northeastern margin of the Tibetan Plateau during early Cretaceous; 2) From middle Cretaceous to Miocene ((115±17) Ma to (25±7) Ma), the cooling rates and exhumation rates of South Qilian fold belt and Shule Nanshan-Laji Shan suture zone were quite low, implying that the tectonic activity of the Qilian Mountain was weak during middle Cretaceous to Miocene; 3) Since Miocene time, timing of both thrust activities and regional rapid cooling event shows that the Qilian Mountain experienced north-eastward rise and growth, which is in line with the hypothesis that the Qilian Mountain was formed by thrusting within the Qaidam crust along a large decollement in the lower crust that progressively propagated north-eastward, the Qilian Mountain was uplifted considerably since Miocene, forming basins-mountains tectonic landforms. Early Cretaceous rapid cooling event in the Qilian Mountain probably resulted from the docking of the Lhasa block to the south, and the rapid cooling since Miocene may be the result of the docking of the India-Asia collision, representing the main uplift of the Qilian Mountain.

Key words:fission-track analysis; apatite; Qilian Mountain; Cretaceous; Cenozoic

*通訊作者:胡道功, 男, 1963年生。研究員。主要從事新構造與活動構造研究。E-mail: hudg@263.net。

作者簡介:第一 戚幫申, 男, 1988年生。博士研究生。主要從事區域地殼穩定性評價、工程地質和地質災害研究。

通訊地址:100081, 北京市海淀區民族大學南路11號。E-mail: qibangshen@126.com。

收稿日期:2015-05-24; 改回日期: 2015-09-30。責任編輯: 魏樂軍。

中圖分類號:P597.3; P542.1

文獻標志碼:A

doi:10.3975/cagsb.2016.01.05