祁連山北緣玉門—北大河斷裂晚第四紀(jì)活動(dòng)特征

劉興旺, 袁道陽(yáng), 邵延秀, 張 波

(1.蘭州大學(xué)西部環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室,甘肅 蘭州 730000; 2.中國(guó)地震局蘭州地震研究所,甘肅 蘭州 730000;3.蘭州地球物理國(guó)家野外科學(xué)觀測(cè)研究站,甘肅 蘭州 730000)

祁連山北緣玉門—北大河斷裂晚第四紀(jì)活動(dòng)特征

劉興旺1,2,3, 袁道陽(yáng)2,3, 邵延秀2,3, 張 波2,3

(1.蘭州大學(xué)西部環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室,甘肅 蘭州 730000; 2.中國(guó)地震局蘭州地震研究所,甘肅 蘭州 730000;3.蘭州地球物理國(guó)家野外科學(xué)觀測(cè)研究站,甘肅 蘭州 730000)

通過(guò)衛(wèi)星影像解譯、野外實(shí)地調(diào)查并結(jié)合前人研究成果,對(duì)位于祁連山北緣的玉門—北大河斷裂晚第四紀(jì)構(gòu)造活動(dòng)特征進(jìn)行研究。結(jié)果表明,玉門—北大河斷裂為一條全新世活動(dòng)的逆沖斷裂,該斷裂西起玉門青草灣,向東經(jīng)老玉門市、大紅泉止于骨頭泉,全長(zhǎng)約80 km,整體走向NWW。根據(jù)斷裂的幾何結(jié)構(gòu)及活動(dòng)習(xí)性可將其分為三段:東段構(gòu)造形態(tài)簡(jiǎn)單連續(xù),為逆沖斷層陡坎為主的古地震地表破裂帶;中段結(jié)構(gòu)復(fù)雜,由多條次級(jí)斷層組成,以逆沖擴(kuò)展為主;西段未出露地表而成為盲斷裂-褶皺帶。通過(guò)對(duì)斷層陡坎差分GPS測(cè)量及相應(yīng)地貌面年代測(cè)試,得到斷裂晚更新世以來(lái)逆沖速率約為(0.73±0.09) mm/a。

玉門—北大河斷裂; 斷錯(cuò)地貌; 晚第四紀(jì); 滑動(dòng)速率

0 引言

新生代以來(lái),印度板塊與歐亞板塊擠壓碰撞,青藏高原開始形成并分階段隆升[1-5],同時(shí)受區(qū)域擠壓應(yīng)力作用的影響,青藏塊體持續(xù)向北東方向推擠擴(kuò)展,在其前緣形成了一系列擠壓逆沖斷裂帶,祁連山北緣斷裂即為其重要的逆沖斷裂帶之一[6-10]。位于河西走廊盆地南緣西段的玉門—北大河斷裂是祁連山北緣斷裂帶的重要組成部分。根據(jù)前人研究,酒西盆地南緣斷裂由多條逐漸向盆地內(nèi)部擠壓逆沖的斷裂系組成,構(gòu)成了一個(gè)大型的推覆構(gòu)造[11],其前鋒斷裂即為玉門—北大河斷裂,其構(gòu)造活動(dòng)特征反映了酒西盆地南緣斷裂的最新活動(dòng)特性。

對(duì)于玉門—北大河斷裂,前人曾開展過(guò)一些研究工作,但對(duì)其活動(dòng)時(shí)代和活動(dòng)性質(zhì)都有不同的認(rèn)識(shí)。國(guó)家地震局地質(zhì)研究所等[7]認(rèn)為其活動(dòng)時(shí)代為中更新世末以前,晚第四紀(jì)以來(lái)不活動(dòng);陳杰等[12]認(rèn)為斷裂自中更新世以來(lái)的活動(dòng)從未停止過(guò),以擠壓逆沖和褶皺變形為主;陳文彬等[8]認(rèn)為其最新活動(dòng)時(shí)代為晚更新世末,全新世期間不活動(dòng);閔偉等[9]、陳柏林等[13]認(rèn)為其為一條全新世活動(dòng)斷裂帶,并初步得到其逆沖速率和古地震事件。對(duì)其活動(dòng)性質(zhì)的研究也有兩種不同的認(rèn)識(shí):有研究者認(rèn)為該斷裂為逆沖斷層,無(wú)走滑分量[7,14-15];也有研究者認(rèn)為該斷裂有左旋走滑的分量[7-8,13]。這些認(rèn)識(shí)上的差異反映了對(duì)該斷裂研究缺少有力的最新活動(dòng)證據(jù)。筆者通過(guò)對(duì)前人資料的收集和實(shí)地野外調(diào)查,認(rèn)為玉門—北大河斷裂為一條全新世活動(dòng)的逆沖斷裂,斷裂沿線未發(fā)現(xiàn)左旋走滑的地質(zhì)地貌證據(jù),并通過(guò)斷層陡坎差分GPS測(cè)量及相應(yīng)地貌面測(cè)年得到了斷裂晚更新世以來(lái)的逆沖滑動(dòng)速率。

1 玉門—北大河斷裂分段活動(dòng)特征

玉門—北大河斷裂分布在祁連山西端的北部邊緣,是酒西盆地與祁連山的分界線,也是祁連山北緣斷裂帶的重要組成部分。該斷裂西起青草灣,向東經(jīng)老玉門市、青頭山、大紅泉、小紅泉、止于骨頭泉一帶,整體走向NWW,傾角20°～60°,總長(zhǎng)約80 km(圖1)。斷裂穿過(guò)了石油河、白楊河及北大河等河流,為一條逆沖推覆斷裂。根據(jù)斷層幾何展布特征及活動(dòng)特點(diǎn)可將該斷裂分為東、中、西3段。各段的幾何結(jié)構(gòu)與活動(dòng)特征敘述如下:

1.1 東段(F2-1)

斷裂東段西起西溝礦以西,向東經(jīng)北大河到骨頭泉一帶,長(zhǎng)約20 km(圖1)。該段斷裂幾何形態(tài)較為簡(jiǎn)單,展布于祁連山山前洪積扇上,連續(xù)發(fā)育,距山邊約有2 km,以斷層陡坎形態(tài)保存。根據(jù)各洪積扇面的新老程度,陡坎高度從小于1 m至7 m不等(圖2),大部分陡坎高度在2 m左右,在最新的沖洪積扇上陡坎高度不足1 m(圖2S1、S8)。部分?jǐn)鄬佣缚捕晤愃乒诺卣鸬乇砥屏褞?保留有較為新鮮的陡坎自由面。在骨頭泉附近沖溝內(nèi)發(fā)現(xiàn)天然斷層剖面,揭露的地層如圖3所示:U1 全新統(tǒng)松散黃土;U2 崩積楔,含礫石較多;U3 灰色土層;U4 土灰色松散砂礫石層,粒徑較大;U5 灰色沖積砂礫石層,粒徑小,近斷層側(cè)褶皺彎曲,斷層帶內(nèi)礫石定向;U6 青灰色砂礫石層,粒徑大,輕微膠結(jié)。斷裂活動(dòng)形成一個(gè)斷層帶,其間礫石定向排列,斷層逆沖于原先地表土層之上并形成崩積楔,被最新沉積的全新世松散黃土覆蓋,已接近地表,斷層上盤靠近斷層處礫石層產(chǎn)生褶皺變形,斷層傾角35°,斷距約1 m。根據(jù)斷錯(cuò)地貌特征及剖面判斷,該段斷裂為全新世活動(dòng)斷裂。

1.2 中段(F2-2)

斷裂中段西起青頭山,向東經(jīng)大紅泉、小紅泉,東到西溝礦以西,長(zhǎng)約20 km。該段斷層連續(xù)性不好,沿走向由6個(gè)不連續(xù)的雁列狀次級(jí)斷層組成(圖1)。地貌上表現(xiàn)為斷層三角面和不同高度的斷層陡坎,陡坎一般發(fā)育于各沖溝兩側(cè)階地之上,其保存高度明顯高于東段。在南山村南,發(fā)源于祁連山的一條未知名河流兩側(cè)共發(fā)育3級(jí)階地,T2、T3階地之上均保留斷層陡坎。T2階地主要分布于河流西岸,其上保留的斷層陡坎高度約8 m[圖4(a)],T3階地主要分布于河流東岸,其上陡坎高約20 m,這種斷層陡坎高度的變化表明斷裂在晚第四紀(jì)以來(lái)是持續(xù)活動(dòng)的。河道內(nèi)出露斷層剖面,第三系紅色砂巖逆沖于第四系砂礫層之上,斷層向上延伸已近地表,傾角約25°[圖4(b)]。

在大紅泉沖溝西壁出露斷層剖面,第三系紅色砂巖形成背斜褶皺并派生若干張性小斷層,未見主斷層(圖5),但主斷層的活動(dòng)使得其上覆礫石層產(chǎn)生撓曲、位錯(cuò)及定向排列,據(jù)此可以推斷主斷層古地震事件。根據(jù)前人的研究[8],從剖面上可分辨出兩次古地震事件:第一次事件發(fā)生在距今(26.8±2.1) ka,造成覆蓋于N面上礫石層的錯(cuò)動(dòng)及定向排列;第二次事件發(fā)生在距今(14.1±1.1) ka,造成礫石層與上覆砂礫石層的不整合面發(fā)生撓曲錯(cuò)動(dòng)。根據(jù)野外實(shí)地考察,發(fā)現(xiàn)灰色砂礫石層上部一套淡粉色粉砂層也發(fā)生了彎曲變形,根據(jù)其年代,推測(cè)斷裂最新活動(dòng)發(fā)生在距今(4.9±0.7) ka以來(lái)。

圖1 玉門—北大河斷裂地質(zhì)構(gòu)造圖Fig.1 The geological tectonic map of Yumen—Badahe fault

圖2 玉門—北大河斷裂東段影像及陡坎測(cè)量(紅色箭頭為斷層通過(guò)位置)Fig.2 Satellite image and fault scarp measurement at the eastern segment of Yumen—Beidahe fault

圖3 骨頭泉斷層剖面照片(鏡像NW)及素描Fig.3 Fault section photo (view to NW) and sketch at Gutouquan

圖4 南山村斷層地貌Fig.4 Fault landform at Nanshan village

圖5 大紅泉斷層剖面(據(jù)文獻(xiàn)[8]修改)Fig.5 Fault section at Dahongquan village (After reference[8])

1.3 西段(F2-3)

斷裂西段西起青草灣,向東經(jīng)老玉門市到白楊河以東一帶,長(zhǎng)約40 km。該段斷裂未出露地表而成為盲斷裂-褶皺帶,但物探及鉆孔資料不僅證實(shí)了斷層的存在,而且其表現(xiàn)出逆沖的特征。斷裂在該段橫切了石油河和白楊河等河流,這些河流都發(fā)育有河流階地,而河流階地是記錄構(gòu)造變形過(guò)程的良好地貌證據(jù),利用階地變形資料研究活動(dòng)斷裂也是活動(dòng)構(gòu)造研究的一種重要手段[16-18]。陳杰等[12]根據(jù)石油河階地變形資料,分析認(rèn)為該段斷裂自中更新世以來(lái)有持續(xù)活動(dòng)。Hetzel等[19]對(duì)石油河的階地開展了差分GPS測(cè)量及年代學(xué)研究,認(rèn)為石油河低階地可能存在著微小變形,且根據(jù)變形階地的年代可以推斷斷裂在全新世有過(guò)活動(dòng)。2002年12月14日玉門發(fā)生MS5.9地震,有研究者認(rèn)為其發(fā)震斷裂為玉門—北大河斷裂南側(cè)的旱峽大黃溝斷裂[20];榮代潞等[21]通過(guò)對(duì)主震及余震序列的精定位,分析認(rèn)為在北東向的壓應(yīng)力作用下,玉門—北大河斷裂和旱峽—大黃溝斷裂在深部匯合的斷層面上發(fā)生逆推錯(cuò)動(dòng),共同參與了此次地震。因此,玉門—北大河斷裂為此次地震的發(fā)震斷裂之一,國(guó)內(nèi)類似的活動(dòng)擠壓構(gòu)造帶也有發(fā)生這種“褶皺地震”的例子[22-23]。

綜上所述,玉門—北大河斷裂為一條全新世活動(dòng)斷裂,野外考察發(fā)現(xiàn)斷裂以逆沖推覆為主要活動(dòng)特征,斷裂沿線未發(fā)現(xiàn)左旋走滑的證據(jù)。

2 玉門—北大河斷裂滑動(dòng)速率的厘定

沿祁連山前的河西走廊發(fā)育有一系列規(guī)模不等的洪積扇,活動(dòng)斷裂常發(fā)育于洪積扇之上,斷層陡坎是識(shí)別斷裂位置的重要標(biāo)志。在斷裂中段的大紅泉村,發(fā)源于祁連山的一條大沖溝西側(cè)發(fā)育有多級(jí)階地[圖6(a)],在斷層通過(guò)處形成了高度不等的斷層陡坎,階地越老,陡坎越高[圖6(b)]。根據(jù)航片解譯及野外實(shí)地考察,在該沖溝西側(cè)至少發(fā)育了5級(jí)階地,均為基座階地,基座為第三系紅色砂巖。T1及T4階地發(fā)育于斷層南側(cè),未穿過(guò)斷層。T2階地可分為T2a和T2b兩級(jí)亞階地,拔河高度分別約為11.5 m和13 m,T3階地拔河高度約21 m,T5階地拔河高度約37.5 m。野外利用差分GPS對(duì)各級(jí)階地的斷層陡坎進(jìn)行了實(shí)地測(cè)量[圖6(a) P1～P4],得到其高度分別為(9.8±0.4) m、(12.4±0.3) m、(18.5±1) m和(30±2) m[圖6(c)]。

圖6 大紅泉地貌解譯及陡坎測(cè)量Fig.6 Geomorphic interpretation and fault scarp measurement at Dahongquan village

為確定各階地年代,野外采集了系列樣品[圖6(a)]。各級(jí)階地面之上幾乎無(wú)黃土覆蓋,其上有大量裸露的小礫石,適合利用宇宙成因核素測(cè)量階地的暴露年齡。我們?cè)诟麟A地面上采集了大量(約50塊)富含石英的礫石,直徑約3～5 cm,同時(shí)在河道內(nèi)也采集了一個(gè)樣品,用于計(jì)算10Be繼承濃度。樣品前期處理由中國(guó)地震局地質(zhì)研究所地震動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室完成,之后10Be與9Be比值由法國(guó)地球科學(xué)與環(huán)境研究與教育歐洲中心宇宙成因核素實(shí)驗(yàn)室測(cè)定。根據(jù)測(cè)試結(jié)果,計(jì)算各級(jí)階地暴露年齡T2a為(12.6±0.7) ka、T2b為(17.5±0.8) ka、T3為(24.2±0.8) ka、T5為(44.9±1.7) ka。結(jié)合斷層陡坎高度,得到斷裂的逆沖速率分別為(0.78±0.05)、(0.71±0.04)、(0.76±0.05)和(0.67±0.04) mm/a,這些滑動(dòng)速率在誤差范圍內(nèi)大致一致,晚更新世以來(lái)平均逆沖速率為(0.73±0.09) mm/a,表明該斷裂晚更新世期間逆沖速率較為均勻。需要指出的是,由于斷裂下盤各階地面被同一期洪積扇覆蓋,得到的斷層陡坎高度較真實(shí)值可能偏小,因此該滑動(dòng)速率可作為斷裂的最小滑動(dòng)速率。

3 結(jié)論與討論

根據(jù)前人資料、航衛(wèi)片解譯及野外實(shí)地考察,綜合判斷玉門—北大河斷裂為一條全新世活動(dòng)的斷裂,其活動(dòng)特征有明顯的分段性。東段幾何形態(tài)簡(jiǎn)單,發(fā)現(xiàn)有古地震地表破裂帶的遺跡,斷層陡坎上保留新鮮的自由面;中段形態(tài)復(fù)雜,由多條次級(jí)斷層組成,以逆沖擴(kuò)展為主,斷層三角面和高陡坎為其顯著的地貌特征;西段未出露地表,形成盲斷層-褶皺帶,為2002年玉門5.9級(jí)地震的發(fā)震斷裂之一。

玉門—北大河斷裂表現(xiàn)為逆沖推覆特征,斷裂沿線未發(fā)現(xiàn)左旋走滑的證據(jù),根據(jù)斷層陡坎測(cè)量及年代學(xué)測(cè)試,得到斷裂晚更新世以來(lái)逆沖速率約為(0.73±0.09) mm/a。祁連山北緣斷裂帶由一系列構(gòu)造背景類似的斷裂組成,玉門—北大河斷裂東側(cè)的佛洞廟—紅崖子斷裂的逆沖速率為(0.61±0.28) mm/a[24],榆木山北緣斷裂為(0.55±0.15) mm/a[25],張掖斷裂為0.6～0.9 mm/a[10],這些斷裂的滑動(dòng)速率大致相當(dāng),反映了祁連山北緣斷裂帶在區(qū)域構(gòu)造活動(dòng)上具有整體性和協(xié)調(diào)性。

致謝:文中樣品前期處理由中國(guó)地震局地震動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室完成,測(cè)試由法國(guó)地球科學(xué)與環(huán)境研究與教育歐洲中心宇宙成因核素實(shí)驗(yàn)室完成,在此表示感謝。

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Characteristics of Late Quaternary Activity of Yumen—Beidahe Fault in North Margin of Qilian Mountain

LIU Xing-wang1,2,3, YUAN Dao-yang2,3, SHAO Yan-xiu2,3, ZHANG Bo2,3

(1.KeyLaboratoryofWesternChina'sEnvironmentalSystemwiththeMinistryofEducation,LanzhouUniversity,Lanzhou73000,Gansu,China; 2.LanzhouInstituteofSeismology,ChinaEarthquakeAdministration,Lanzhou730000,Gansu,China; 3.LanzhouNationalObservatoryofGeophysics,Lanzhou730000,Gansu,China)

As the boundary of the Qilian Mountain and Jiuxi Basin, the Yumen—Beidahe fault is located along the western part of the southern margin of the Hexi Corridor, and is an important part of the north Qilian Mountain fault system. The Yumen—Beidahe fault extends from Qingcaowan in the west in an ESE trending for 80 km to near Gutouquan. Considering its geometric structure and active behavior, the fault can be divided into three segments. The eastern segment, whose length is about 20 km, is located on the two sides of Beidahe and about 2 km from the Qilian Mountain. This fault segment is preserved in the form of continuous fault scarp, whose height ranges from less than 1 m to 7 m. At the most recent alluvial fan, some fault scarps still retain a fresh free surface, which suggests that the segment is an earthquake rupture zone. The structure of the middle segment is complex and mainly exhibits thrust characteristics. Its length is about 20 km and it comprises six discrete faults. The topography is expressed as a fault triangle and a fault scarp of different heights. The fault section suggests that the latest earthquake event occurred about 5 000 years ago. The western segment, whose length is about 40 km, does not appear on the ground and becomes a blind thrust-fold belt. Some rivers, such as the Shiyou and Baiyang Rivers, cross over this blind thrust-fold belt. The terraces of these rivers have been described as fold deformation in previous work. This fault segment is also the seismogenic structure of the 2002 YumenMS5.9 earthquake. Based on previous data and field surveys, we believe the Yumen—Beidahe fault to be an active thrust fault in the Holocene, and there is no geological or geomorphic evidence of any left-lateral slip along the fault. According to a differential GPS survey of the fault scarps and geomorphic surface dating, we infer the average thrust rate of the Yumen-Beidahe fault since the late Pleistocene to be about (0.73±0.09) mm/a.

Yumen—Beidahe fault; faulted landform; late Quaternary; slip rate

2016-04-20 基金項(xiàng)目：中國(guó)地震局行業(yè)專項(xiàng)(201408023);國(guó)家自然科學(xué)基金青年基金(41402186) 作者簡(jiǎn)介：劉興旺(1980-),男,副研究員,主要研究方向?yàn)榛顒?dòng)構(gòu)造及地貌。E-mail:lxw_27@163.com。

P534.63;P542+.3

A

1000-0844(2016)06-0948-07

10.3969/j.issn.1000-0844.2016.06.0948