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苏君,季建清等——中国阿尔泰南缘构造变形层次与变形序列

已有 1277 次阅读 2021-7-2 09:31 |个人分类:论文推广|系统分类:论文交流

中国阿尔泰南缘构造变形层次与变形序列

苏 君 季建清 彭玉恒 邵媛燕

(北京大学地球科学与空间科学学院 北京 100871

Tectonic levels and deformation sequences of the southern margin of

Chinese Altai

Su Jun Ji Jianqing  Peng Yuheng Shao Yuanyan

(School of Earth and Space Sciences, Peking University, Beijing 100871)

摘 要 中国阿尔泰造山带发育了一套被认为是早古生代的变质岩系,保留了原始成分层理S0。岩石在下—中地壳层次经历了韧性变形及最深达到角闪岩相变质作用的改造。通过对岩石构造层次及变形序列分析,在造山带南缘识别出由深至浅、由早到晚发育的4个构造层次的变形样式,分别为流变构造、韧性剪切构造、褶皱劈理构造和断裂节理构造。流变构造以轴面NW走向、枢纽近直立的流动褶曲和NW向近直立片麻岩面理为特征,局部发育长英质和镁铁质矿物分异条带,相当于下构造层次变形(约15~18 km)。韧性剪切构造以NW-NNW向和NNE-NEE向近直立的糜棱岩面理和片理面为特征,相当于下-中构造层次变形(约8~12 km)。褶皱劈理构造对应产出轴向NW、枢纽近水平的等厚褶皱、NWWNNW向近直立劈理的产出,为中-上构造层次变形(约5~8 km)。上构造层次的断裂、节理构造在造山带南缘广泛发育,变形深度约浅于3~ 4 km。根据各变形几何学、运动学关系及变形次序,确认它们是3期构造应力场作用的结果。在二叠—三叠纪,阿尔泰经历NE向(47°~55°)挤压应力场,在下-中构造层次发育317°~325°走向的流动褶皱、片麻理构造S1-1306°~308°走向的左行和328°~332°走向的右行、近直立韧性剪切面理S1-2290°~295°走向的左行和350°~355°走向的右行、近直立片理面S1-3。在古近纪NW向挤压应力场下,中-上构造层次发育20°~22°走向左行和78°向右行片理面S2-110°~20°70°~78°近直立脆—韧性剪切面S2-25°~8°压扭性断裂构造S2-3。新近纪近N-S向应力场下,在浅地壳层次产出320°~325°340°~345°走向右行走滑断裂, NWW向破劈理和NE向节理构造S3。下构造层次变形被中-上构造层次变形叠加改造,体现变形深度层次与变形序列的一致性,反映了同构造挤压期的地壳抬升现象。

关键词 中国阿尔泰 构造层次 变形序列 变形深度

Abstract There is a set of metamorphic rocks in Chinese Altai Orogen, which is considered to be early Paleozoic and still retains the original bedding S0. The rocks have experienced ductile deformation and amphibolite facies metamorphism in the mid-low crustal. During the subsequent uplifting and exhumation, they are modified by late ductile or brittle deformation. Based on the analysis of tectonic level and deformation sequence, we recognize four tectonic levels for deformation units, flow deformation, ductile shear structure, fold-cleavage structure and brittle fractures, developed from deep to shallow and early to late. Flow deformation is characterized by the near-vertical, NW trending flow fold and gneissosity, with felsic and mafic mineral ribbons locally, which are developed at middle and lower tectonic level (15~18 km). Ductile shear deformation is featured with near-vertical NW-NNW and NNE-NEE-direction mylonite foliation and schistosity, which developed at mid-upper tectonic level (8~12 km). The NW-trending isopach fold, and the NWW and NNW-trending cleavage developed at middle-shallow tectonic level (5~8 km). Brittle faults and fractures of upper tectonic level, which formed at the depth shallower than 3~4 km, are widely developed in the southern margin of Altai. According to the sequence, geometry and kinematics of deformation structures, three phases of tectonic stress field are confirmed. During Permian to Triassic, Altai experienced NE-direction (47°~55°) compressive stress field, developing the S1-1 of 317°~325° trending flow fold and gneissosity, S1-2 of 306°~308° trending sinistral and 328~332° trending dextral ductile shear deformation, S1-3 of 290°~295° trending sinistral and 350°~355° trending dextral near-vertical schistosity, at the low-middle tectonic level. Under NW-direction compressive stress field during Paleogene, there developed the S2-1 of 20°~22° trending sinistral and 78° dextral schistosity, S2-2 of 10°~20° and 70°~78° trending brittle-ductile shear surface and S2-3 of 5°~8° trending trans-pressive fault. In Neogene, the S3 of 320°~325° and 340°~345° trending dextral faults, NWW-trending cleavage and NE-direction fractures were developed under the near N-S-direction stress field. The deformation of lower tectonic level is superimposed by the structure of upper tectonic level, which reflects the consistency of the deformation depth and deformation sequence, and revels the crustal uplift during tectonic compression.

Keywords Chinese Altai, Tectonic levels, Deformation sequence, Deformation depth

中国阿尔泰造山带构造变形复杂多样,且经历了多期地壳抬升作用,出露了大量花岗片麻岩穹隆、韧性剪切构造、断裂体系等多构造层次变形。早期变形样式解析认为,阿尔泰地区是在额尔齐斯俯冲带上发展起来的巨型复式推覆—滑脱构造,由4 个滑脱面和4个推覆体组成,自北东向南西呈背驮式叠瓦状依次斜冲(曲国胜等,1992;张朝文等,1992)。张进江等(1993)指出造山带SE段变形以逆冲推覆为主,而NW段变形以左旋走滑为主,形成腹地逆冲、前陆走滑、中间为过渡状态的运动特点,认为是斜向俯冲的影响。近年来学者通过岩石变质演化和变形分析认为,造山带在约400~390 Ma经历地壳伸展,发育NW走向近水平展布的混合岩(Jiang et al., 2019);晚泥盆纪经历了NW-SE向挤压变形,发育地壳尺度、NE轴向的近直立褶皱变形及花岗岩或混合岩穹隆, 290 ~270 Ma,遭受了NE-SW向挤压,形成NW向近直立高温变质带和褶皱构造(Zhang et al, 2015; Li et al., 2016Broussoll et al., 2018Jiang et al., 2019);该系列研究认为阿尔泰山是寒武至奥陶纪增生杂岩在遭受不同地壳层次变形变质作用后,水平并置的结果( Broussoll et al., 20182019Jiang et al., 2019)。以上研究中使用全地壳尺度的褶皱、逆冲推覆构造等变形样式来解释造山带的演化。

然而,由于岩石物理性质在浅地壳表现为脆性,在中-下地壳表现为塑形的特点,造山带地壳变形会在深度层次上发生分异,即特定深度发育特征变形样式。最初造山带地壳被分为上、中、下3个构造层次,分别为浅部脆性断裂作用域、中部等厚褶皱域和深部压扁作用和剪切作用域(Mattauer, 1980)。刘志成等(2018)结合造山带构造解析实例和前人研究结果,将地壳变形由浅至深分为5个单元:脆性断裂构造域,等厚褶皱构造域,相似褶皱构造域,韧性剪切构造域和流变构造域。构造层次模型是岩石物理力学属性及流变性质的体现,理想状态下,针对一期造山作用,地壳可以由浅至深形成较完整的构造层次。前人用全地壳尺度的等厚褶皱、劈理变形样式来解释阿尔泰造山带地壳演化,存在不合理性;中下地壳岩石处于韧性变形域,很难形成等厚褶皱、推覆断裂等扩容变形。同时,阿尔泰造山带多种构造层次变形出露地表是造山带高度演化的特点,对于露头尺度或小区域范围内,深层次变形往往比浅层次变形更早形成。由于构造抬升和地表剥蚀的共同作用,促使深部变形岩石逐步抬升至近地表,并在该过程中被浅部构造变形叠加改造。这可能是同造山期地壳快速抬升的结果(Su et al., 2020),也可能为不同应力场分期演化的结果,需要结合构造变形的几何学、运动学信息加以判别。

本文基于构造层次思想,在造山带南缘识别出4个不同深度层次的变形样式,确定它们是3期区域性构造应力场作用的结果,并且伴随同构造期地壳抬升,出现构造层次叠加现象。

图1-阿尔泰地质图-10.22.jpg

1 阿尔泰造山带简要地质图(据徐芹芹等,2015修改)

F1. 额尔齐斯断裂;F2. 科沙哈拉尔断裂;F3. 库尔特断裂;F4. 康布铁堡(阿巴宫断裂);F5. 巴寨断裂;

F6. 富蕴断裂;F7. 红山咀断裂;F8. 伊莱克断裂

Fig.1 Geological map of Altai Orogen (modified from Xu et al., 2015)



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