雪峰山構(gòu)造帶古構(gòu)造應(yīng)力場

蘇金寶, 張岳橋, 董樹文, 李 勇, 李建華, 馬收先, 崔建軍

1)中國地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所, 國土資源部新構(gòu)造運(yùn)動與地質(zhì)災(zāi)害重點實驗室, 北京 100081; 2)中國地質(zhì)科學(xué)院, 北京 100037

雪峰山構(gòu)造帶古構(gòu)造應(yīng)力場

蘇金寶1), 張岳橋1), 董樹文2), 李 勇1), 李建華1), 馬收先1), 崔建軍1)

1)中國地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所, 國土資源部新構(gòu)造運(yùn)動與地質(zhì)災(zāi)害重點實驗室, 北京 100081; 2)中國地質(zhì)科學(xué)院, 北京 100037

華南地區(qū)構(gòu)造復(fù)雜, 中生代動力體制經(jīng)歷了從特提斯構(gòu)造域向濱太平洋構(gòu)造域的轉(zhuǎn)換。雪峰山位于華南內(nèi)部, 其隆升機(jī)制也存在廣泛爭議, 對雪峰山及其鄰區(qū)古應(yīng)力場研究, 為華南大陸構(gòu)造演化動力機(jī)制提供了依據(jù)。古應(yīng)力反演顯示, 中生代以來, 研究區(qū)受到NE向、近SN向、NW向、NNE向四期的構(gòu)造擠壓。NE向與近SN向是華南塊體印支期順時針旋轉(zhuǎn)以及古特提斯洋閉合的結(jié)果, NW向擠壓是古太平洋板塊俯沖作用導(dǎo)致, 而NNE向擠壓則與印、藏碰撞的遠(yuǎn)程效應(yīng)有關(guān)。

印支期擠壓; 燕山期擠壓; 華南地塊

中生代是華南構(gòu)造的重要變革時期, 華南的主要構(gòu)造格局被認(rèn)為定型于中生代的印支與燕山構(gòu)造事件的陸內(nèi)造山作用(Chen et al., 1991; Wang et al., 2005; Shu et al., 2006)。但印支與燕山構(gòu)造事件對華南的作用時限、作用強(qiáng)度和范圍還存在爭議, 使得對華南陸內(nèi)造山有不同的解釋。如Li等(2007)指出華南1300 km寬的陸內(nèi)造山是開始于二疊世的古太平洋板塊平俯沖作用; 早中生代華南大陸由于軟流圈上涌而伸展(Gilder et al., 1996; Sun et al., 2005), Zhou等(2006)認(rèn)為古太平洋俯沖應(yīng)在早中侏羅世; 還有學(xué)者認(rèn)為古太平洋板塊俯沖的時代是中晚侏羅世(董樹文等, 2007; 張岳橋等, 2009)。華南大陸的早中生代構(gòu)造巖漿活動歸功于華南與華北板塊的碰撞以及印支塊體與華南大陸的碰撞(Li et al., 1998; Wang et al., 2007; Shu et al., 2009; Zhou et al., 2006), Wang等(2005)認(rèn)為雪峰山是這一時期陸內(nèi)斜向擠壓變形的產(chǎn)物, 然而這難以解釋這些碰撞造山如何傳播到塊體內(nèi)部, 以及造山帶走向與碰撞帶走向近垂直(Li et al., 2007)。李三忠等(2011)認(rèn)為雪峰山是NNE向的構(gòu)造線在印支晚期由于揚(yáng)子地塊順時針旋轉(zhuǎn)變位為NEE向, 隨后又疊加了第二幕NEE向構(gòu)造線形成。張岳橋等(2009)通過地表形跡學(xué)分析提出華南印支期是近南北向擠壓, 而燕山期轉(zhuǎn)為NW向或近東西向擠壓。古應(yīng)力場反演能為解決華南相對華北塊體旋轉(zhuǎn)前后的應(yīng)力變化、華南動力方向來源提供依據(jù), 因而本文著重對雪峰山構(gòu)造帶中生代古應(yīng)力場進(jìn)行反演, 進(jìn)而分析華南大陸動力演化模式。

圖1 研究區(qū)平面地質(zhì)圖Fig. 1 Geological map of the study area

揚(yáng)子與華夏塊體的拼合時間一般認(rèn)為是在格林威爾期(ca. 1.1～0.9 Ga, Shui, 1987; Chen et al., 1998; Li et al., 2002; Ye et al., 2007)或新元古0.87～0.82 Ga (Zhao et al., 1999; Wang et al., 2004; Zheng et al., 2007)。揚(yáng)子變質(zhì)基底主要由云母片巖、片巖、絹云母泥砂巖復(fù)理石及硅質(zhì)巖和成層較好的雜砂巖-板巖組成(Yan et al., 2003; Shu et al., 2006, 2008)。揚(yáng)子與華夏塊體碰撞后發(fā)育為新元古—古生代裂谷, 中心為最大可達(dá)13 km深海碳酸巖碎屑巖和邊部為2～5 km厚的淺海碳酸巖沉積(Wang et al., 2003; Wang et al., 2007)。沉積蓋層主要是寒武系黑色頁巖、砂巖、灰?guī)r夾白云巖, 奧陶系厚層灰?guī)r夾白云巖粘土粉砂巖, 志留系頁巖及砂巖, 泥盆系砂巖與粉砂巖, 石炭系碎屑巖與灰?guī)r, 二疊系灰?guī)r等(Yan et al., 2009)。早三疊系為灰?guī)r夾泥灰?guī)r頁巖, 此后以陸相地層為主。研究區(qū)經(jīng)歷多期構(gòu)造事件, 如加里東構(gòu)造事件使晚泥盆系角度不整合在志留系之上(Charvet et al., 1996; Shu et al., 2006; Sun et al., 2005)。接下來為三疊系的脆韌性變形及華南廣泛巖漿事件被認(rèn)為是印支事件的產(chǎn)物(e. g. Yang et al., 1982; Li et al., 2006; Wang et al., 2001; Wang et al., 2005; Zhou et al., 2006)。燕山期形成后碰撞陸內(nèi)山間盆地與陸內(nèi)伸展盆地(Shu et al., 2009)。

研究區(qū)恰位于揚(yáng)子與華夏的拼合帶, 是以NE走向為主的構(gòu)造帶(圖1), 雪峰山以東為上古生界地層為主的湘中褶皺帶, 并發(fā)育疊加褶皺。雪峰山出露前寒武系基底, 上覆下古生界地層, 殘留的上古生界地層相對較少, 中生代盆地不整合在其上。

1 古應(yīng)力分析與分期

1.1 古應(yīng)力方法

一般而言, 地殼在一定演化階段, 構(gòu)造應(yīng)力場是相對統(tǒng)一的, 其主應(yīng)力方向基本保持不變, 因此可通過多種地質(zhì)方法反演古構(gòu)造應(yīng)力場方向。在脆性變形區(qū), 較為常用的方法是通過觀測斷面上的擦痕滑動失量來反演古構(gòu)造應(yīng)力場(Carey, 1979; Angelier, 1984; Mercier et al., 1987)。用各種滑動構(gòu)造標(biāo)志判別斷層的運(yùn)動方向, 通過計算機(jī)計算各觀測點三軸主應(yīng)力方向。再結(jié)合疊加擦痕、斷層相互的切割關(guān)系, 確定構(gòu)造應(yīng)力場演化序列。古應(yīng)力反演主要是通過T-TECTO軟件, 它是應(yīng)用Gauss方法(?alohar et al., 2007)對斷裂滑動數(shù)據(jù)進(jìn)行古應(yīng)力與運(yùn)動分析, 最佳擬合應(yīng)力方向, 并成功應(yīng)用于構(gòu)造地質(zhì)研究中(如?alohar et al., 2008, 2010)。

華南地區(qū)晚古生代至中三疊世地層(D2-3—T2),通常稱為印支構(gòu)造層, 其底部不整合超覆在下古生界加里東構(gòu)造層之上, 泥盆系底部為一套河流相砂礫巖沉積, 向上主體為淺海相碳酸鹽沉積, 其中有海陸交互相沉積夾層。晚三疊世至早中侏羅世地層(T3—J1-2)構(gòu)成了早燕山構(gòu)造層, 主體為陸相沉積。

圖2 板溪群s0被s1構(gòu)造置換, 顯示NW—SE向擠壓Fig. 2 Structural replacement of s0 by s1 in Banxi Group, showing NW–SE trending compression a-F29點; b-F30點; c-F34點; d- F35點(位置見圖3) a-F29; b-F30; c-F34; d-F35 (locations as for Fig. 3)

我們對湘桂褶皺帶與雪峰山地區(qū)相應(yīng)的印支與燕山構(gòu)造層以及后期的白堊系中構(gòu)造斷層與擦痕等進(jìn)行了測量并做古應(yīng)力的恢復(fù), 印支構(gòu)造層灰?guī)r較多, 因而易產(chǎn)生擦痕, 而燕山構(gòu)造層限于出露地層少而相對擦痕要少一些, 如圖版I所示。為了避免層面滑動對應(yīng)力判別上的失準(zhǔn), 因而在擦痕測量中,擦痕均來自垂直或斜交層面的斷層面上。我們假設(shè)地層在受到擠壓前是水平沉積, 初始擠壓會使斷層面上的擦痕與地層近于平行, 而當(dāng)?shù)貙玉薨欁冃?再次受到擠壓時擦痕近似與地層斜交。這樣在測量擦痕的同時, 我們可人為的將擦痕分為兩組, 即平行地層層面s0的與不平行s0的兩組擦痕。在測量擦痕的過程中, 對同一構(gòu)造面疊加的擦痕進(jìn)行先后分期, 最終可算出擠壓應(yīng)力的先后順序。

1.2 古應(yīng)力分期

通過測量反演, 研究區(qū)自中生代以來至少經(jīng)歷了NE向、近SN向、NW或近東西向、NNE向擠壓等4期構(gòu)造事件。

在測量XX122點中, 不同構(gòu)造面上反演出三期的構(gòu)造擠壓(如圖版I-c, d, e)。平行s0的擦痕反演擠壓應(yīng)力為NE—SW向擠壓, 不平行s0的擦痕反演出NE向與近SN向擠壓。我們認(rèn)為平行s0擦痕代表地層初始NE向擠壓時地層平緩產(chǎn)生, 隨擠壓加大,地層褶皺并使擦痕與s0不平行, 近南SN向擠應(yīng)該在北東向擠壓之后。

XX159點, 在同一構(gòu)造面上產(chǎn)生了兩期疊加的擦痕(圖版I-a), 早期向北傾的擦痕被晚期向南傾的擦痕所疊加, 對所測擦痕反演, 早期顯示NE向擠壓,晚期為NNE向。同時還反演出近南北向的擠壓應(yīng)力。此時還不能判斷NNE向擠壓與近SN向擠壓先后。

華南褶皺帶表現(xiàn)為NE走向, NW向擠壓是普遍存在的, 雪峰山主要呈北東向構(gòu)造。雪峰山出露的老地層板溪群, s0被s1置換, s1產(chǎn)狀總體指向NW—SE, 如圖2所示, 說明NW向構(gòu)造擠壓的強(qiáng)烈。在應(yīng)力反演中也表現(xiàn)出NW向的擠壓如xx164點(圖版I-b)。此處擦痕反演出三期擠壓, 而平行s0擦痕反演出一期NE向擠壓和一期NW向擠壓, 推測NE向為最早階段的擠壓, 這次擠壓并未使該處地層強(qiáng)烈傾斜。在此之后, 地層受到北西向擠壓, 因而產(chǎn)生的擦痕同樣與s0近于平行。此處還發(fā)育一些逆斷層, 斷層向西傾(圖版I-b), 在斷層面上測得擦痕反演顯示地層受到NNE方向擠壓。這期NNE向擠壓應(yīng)發(fā)生在NW向擠壓之后。

在XX171點中(圖版I-f), 平行s0擦痕反演出一近SN向的擠壓, 同時不平行s0擦痕反演出NNE向擠壓, 說明近SN向擠壓應(yīng)在NNE向擠壓之前。至此推斷XX159點(圖版I-a)近SN向擠壓應(yīng)發(fā)生在NNE向擠壓之前, 結(jié)合區(qū)域構(gòu)造事件, 近SN向擠壓應(yīng)發(fā)生在NW向擠壓之前。

結(jié)合上述, 我們分析出四期的構(gòu)造擠壓, 先后為NE向、近SN向、NW向、NNE向, 數(shù)據(jù)如表1。圖3顯示所有測試點位置以及古應(yīng)力狀況。自中生代以來, 研究區(qū)經(jīng)歷兩大構(gòu)造域, 最初的NE向與近SN向擠壓很可能是與古特提斯洋閉合有關(guān); 而NW向擠壓則是太平洋板塊向歐亞大陸俯沖的結(jié)果; 最新的NNE向擠壓可能是喜山期印藏碰撞遠(yuǎn)程效應(yīng)的影響。喜山期的NNE向擠壓也可在喜山構(gòu)造層(J2-K)中反演得以確認(rèn), 我們在J2與K地層中測得ym315點與ym340點(如圖3)也反演出NNE向擠壓,說明白堊之后是存在一期NNE向擠壓。

圖3 古構(gòu)造擠壓應(yīng)力場Fig. 3 The principal compressional stress filed

2 討論

華南大陸四期構(gòu)造擠壓的識別為探討華南中生代構(gòu)造體制轉(zhuǎn)換和大地構(gòu)造過程提供了關(guān)鍵的構(gòu)造地質(zhì)學(xué)依據(jù)。從大地構(gòu)造發(fā)展角度看, 華南地區(qū)四

期的構(gòu)造擠壓反映了印支期以來華南所受擠壓應(yīng)力變化。印支期華南塊體順時針旋轉(zhuǎn)與華北塊體拼貼,形成以東西向構(gòu)造為主的特提斯構(gòu)造域, 擠壓應(yīng)力為NE向后變?yōu)榻黃N向。燕山期, 特提斯構(gòu)造域向濱太平洋構(gòu)造域轉(zhuǎn)換形成以NW向擠壓為主的NNE向構(gòu)造。華南印支—早燕山構(gòu)造層中的疊加褶皺構(gòu)造(張岳橋等, 2009, 2012)是擠壓應(yīng)力變化的直接結(jié)果。喜山期, 中國東部存在一些構(gòu)造擠壓, 盆地中出現(xiàn)了構(gòu)造反轉(zhuǎn), 如濟(jì)陽坳陷等(Su et al., 2009; 蘇金寶等, 2011), 這些擠壓事件很有可能是與印藏碰撞的遠(yuǎn)程效應(yīng)有關(guān)。在測得擠壓應(yīng)力時NNE向擠壓出現(xiàn)較晚, 在白堊盆地中也反演出這期的擠壓應(yīng)力,其時代顯然要晚于印支期擠壓, 因此我們把NNE向擠壓定義為喜山期的構(gòu)造事件。

表1 湘中褶皺帶與雪峰山地區(qū)斷層滑動失量及其構(gòu)造應(yīng)力場反演結(jié)果Table 1 Fault slip vectors measured in central Hunan and Xuefeng Mountain regions and their inverted results of stress fields

續(xù)表1

華南地區(qū)印支期近東西褶皺構(gòu)造帶的形成與華南大陸南、北邊緣地塊碰撞作用有關(guān)。碰撞使大別山、蘇魯造山帶出露的超高壓變質(zhì)帶年齡峰值約為240 Ma(Li et al., 1993; Hacker et al., 1998), 這期擠壓使華南地塊發(fā)生擠壓增厚, 并導(dǎo)致華南印支期過鋁質(zhì)花崗巖的發(fā)育(Wang et al., 2007)。然而雪峰山在印支期構(gòu)造線為NNE向, 二者同期的構(gòu)造線近乎垂直(Wang et al., 2005; 李三忠等, 2011)。李三忠等(2011)認(rèn)為揚(yáng)子地塊北緣與秦嶺—大別山帶印支期變形方位一致的變形區(qū)域只限制在其南側(cè)50 km以內(nèi), 其余的揚(yáng)子地塊內(nèi)部的印支期變形方位和秦嶺—大別造山帶無關(guān)。Li等(2007)提出華南1300 km褶皺帶是與二疊紀(jì)開始的古太平洋平俯沖有關(guān), 但因在中國東部沒有發(fā)現(xiàn)印支期俯沖證據(jù)而受質(zhì)疑。古應(yīng)力結(jié)果揭示, 中生代以來華南大陸內(nèi)部變形印支期是與特提斯閉合產(chǎn)生的NE向與近SN向擠壓所致, 這也使雪峰山形成左行走滑逆沖構(gòu)造(Wang et al., 2005)。華南擠壓方向轉(zhuǎn)化為NW向為主是受太平洋板板俯沖影響, 俯沖的時間顯然不支持發(fā)生在印支期。

3 結(jié)論

雪峰山及鄰區(qū)印支—燕山構(gòu)造層(D-K)反演出四期構(gòu)造擠壓, 分別為NE向、近SN向、NW向、NNE向。早期NE向后轉(zhuǎn)為近SN向, 這與印支期華南塊體順時針旋轉(zhuǎn)與古特提斯閉合有關(guān)。燕山期受太平洋板塊俯沖影響, 古應(yīng)力轉(zhuǎn)為NW向擠壓。最后一期為NNE向擠壓, 我們認(rèn)為它是與喜山期印藏碰撞的遠(yuǎn)程效應(yīng)引起。

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圖版說明

圖版I Plate I

a-XX159點反演出三期構(gòu)造應(yīng)力, 擦痕顯示早晚兩期擦痕疊加;

b-XX164點反演出三期構(gòu)造擠壓, 逆斷層對應(yīng)于晚期NNE向擠壓;

c-XX122點平行于s0的擦痕反演出的擠壓;

d-XX122點不平行s0的擦痕反演出的NE向擠壓;

e-XX122點不平行s0的擦痕反演出的近SN向擠壓;

f-XX171點構(gòu)造變形及反演的三期擠壓應(yīng)力(位置見圖3)

a-3 stages of stress at XX159 and superimposition of two stages of vectors;

b-3 stages of compressive stress at XX164 and thrust showing NNE-trending compression;

c-stress inverted from vectors parallel to s0 at XX122;

d-NE-trending stress inverted from vectors unparallel to s0 at XX122;

e-nearly NS-trending stress inverted from vectors unparallel to s0 at XX122;

f-structural deformation and 3 stages of compressional stress at XX171

Paleo-structural Stress Field in Xuefengshan Tectonic Belt, South China

SU Jin-bao1), ZHANG Yue-qiao1), DONG Shu-wen2), LI Yong1), LI Jian-hua1), MA Shou-xian1), CUI Jian-jun1)
1) Key Laboratory of Neotectonic Movement and Geohazard, Ministry of Land and Resources, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081; 2) Chinese Academy of Geological Sciences, Beijing 100037

The structure of South China is complex, and its tectonic regimes underwent a tremendous change from the Tethys tectonic domain to the Pacific tectonic domain during the Mesozoic period. The Xuefengshan tectonic belt (XTB) is located in the interior of South China and its uplift mechanism is debated hotly. The study of stress fields on XTB and its adjacent areas provides a key basis for the dynamic mechanism of tectonic evolution of South China. The inverted stress shows that the study area has experienced four stages of compressive stress since Mesozoic, i.e., NE-trending, nearly NS-trending, NW-trending and NNE-trending compressive stresses. The NE-trending and nearly NS-trending stresses resulted from clockwise rotation of South China and closing of Tethys ocean. The NW-trendiang compression was induced by subduction of paleo-Pacific Ocean towards Eurasia continent, whereas the NNE-trending compression might have been related to the far-distance effect of collision between the India and the Tibet plates.

Indosinian compression; Yanshannian compression; South China block

圖版I Plate I

P547; P553

A

10.3975/cagsb.2013.06.04

本文由國家專項“深部探測與實驗研究”第八項目第一課題(編號: SinoProbe-08-01)和國家自然科學(xué)基金項目(編號: 41202154)聯(lián)合資助。

2012-08-27; 改回日期: 2013-02-09。責(zé)任編輯: 魏樂軍。

蘇金寶, 男, 1980年生。博士。主要從事盆地與造山帶構(gòu)造研究。E-mail: jin.su@163.com。