哀牢山—红河断裂带新生代构造转换及其动力学机制

哀牢山—红河断裂带新生代构造转换及其动力学机制
黄学猛;许志琴;张进江
【期刊名称】《地球学报》
【年(卷),期】2017(038)0z1
【总页数】4页(P7-10)
【关键词】哀牢山—红河断裂带;构造转换;滇西北断陷盆地;东构造结;应力轨迹【作者】黄学猛;许志琴;张进江
【作者单位】地壳动力学重点实验室,中国地震局地壳应力研究所,北京 100085;中国地质科学院地质研究所,北京 100037;造山带与地壳演化教育部重点实验室,北京大学地球与空间科学学院,北京 100871;中国地质科学院地质研究所,北京 100037;造山带与地壳演化教育部重点实验室,北京大学地球与空间科学学院,北京 100871【正文语种】中文
【中图分类】P542;P548
哀牢山—红河断裂带位于青藏高原东南缘, 由青藏高原延入南海, 是一条分割华南地块与印支地块的构造分界线, 在地貌上也是一条醒目的分界线。

纵向上由4个北西向的长条状变质带组成(雪龙山、点苍山、哀牢山和Day Nui Con Voi变质带), 长约 1 200 km。

横向上分高级变质带和低级变质带,二者之间为倾向北东的哀牢山逆冲断裂带, 其中高级变质带主要由元古界高级片麻岩、混合岩、淡色花岗岩脉以及 S-L型糜棱岩组成, 低级变质带主要由古生界云母片岩、板岩、千枚岩和S型初糜棱岩组成, 夹杂着条带状基性岩与超基性岩。

断裂带整体往南东方向逐渐变宽。

西北段面理近直立, 线理近水平, 发育近直立的紧闭褶皱, 南东段较宽, 面理变平缓, 发育宽缓褶皱, 靠近北东侧面理倾向北东, 倾角较陡。

中生代时期, 该断裂带是印支地块与杨子地块俯冲碰撞边界线, 也称为金沙江—哀牢山—松马缝合带。

新生代以来, 受印度板块与欧亚板块持续挤压汇聚的影响, 青藏高原东南缘发生多期逃逸, 该断裂是藏东南地块挤出逃逸的边界, 协调着印支地块与杨子地块的相对运动。

新生代早期印支地块向南东逃逸, 该断裂带作为逃逸块体的北边界表现为左旋走滑; 随
着印度地块的持续向北推进, 新生代晚期华南地块(川滇地块)往南东逃逸,该断裂带
作为其南边界断裂表现为右旋走滑, 兼具正断分量。

哀牢山—红河断裂带新生代两期次运动与印度板块东构造结的运动轨迹、青藏高原的隆升、逃逸时序、南海的扩张皆具有密切关系。

对该断裂带转换时间和机制进行研究, 不仅有利于认识该断裂的演化历史, 而且有利于认识青藏高原的隆升、南海新生代构造演化历史。

此外,
对该断裂新生代构造转换的研究有利于对构造逃逸模式、均匀变形模式、旋转模式以及下地壳流模式的检验。

哀牢山—红河断裂带左旋走滑表现为韧性变形。

沿线糜棱岩、混合岩、片岩、千
枚岩、淡色脉体较为发育, xz面上各种指示左旋剪切运动方向的标志普遍发育。

千枚岩内绢云母40Ar/39Ar构造热年代学表明左旋剪切活动持续到13 Ma。

哀牢山—红河断裂带右旋走滑为脆性变形, 变形温度较低, 断裂破碎带较宽, 并且受早期韧性变形残留物质的影响, 其年龄限定以及位移标志体的选择具有较大的挑战。

该断裂右行走滑起始活动时间、位移量、以及左/右行运动转换的机制没有很好的认识。

滇西北地堑系和南东侧莺歌海盆地、北部湾盆地位于哀牢山—红河断裂带尾端, 以及中部元江—元阳一带位于该断裂的挤压阶区, 具有较厚且连续的新生代沉积, 较
好的记录了该断裂的活动历史, 是研究该断裂构造变形特征的理想场所。

本文拟通过断错微地貌观察、断层剖面分析、年代限定方法, 对断裂沿线、滇西北地堑系和南东侧莺歌海盆地、北部湾盆地的地层、地貌以及构造变形方面进行研究, 并结合
构造带的隆升历史, 以及区域隆升侵蚀年代、新生界地层的古地磁年代数据, 对该
断裂带新生代构造转换的时间进行限定。

此外, 结合区域应力场分析, 对其转换的
机制进行探讨。

这些研究对认识青藏高原东南缘新生代构造变形演化具有重要意义。

滇西北断陷系位于哀牢山—红河断裂带的西北尾端。

由于哀牢山—红河断裂带新
生代构造转换的发生, 新生代早期属于该断裂的尾端挤压区, 新生代晚期属于该断
裂带的尾端拉张区。

现今发育一系列张性断裂和拉分断陷盆地, 如大理盆地、剑川盆地、丽江盆地、鹤庆盆地、洱源盆地、程海盆地、宾川盆地、宁蒗盆地, 这些盆地内及其周缘发育较连续的新生代地层。

通过对滇西北断陷盆地系内新生界地层的构造变形研究, 发现中新世晚期发生过明显的构造转换, 由早期的挤压变形转换为
后期的伸展变形。

其中古新统至中新统下部地层主要表现为逆冲断层和褶皱变形, 并且受后期伸展变形改造,中新世晚期以后的地层仅表现为伸展变形, 发育高角度的正断层。

通过古地磁数据显示与伸展变形相关的地层底界年龄为7.6 Ma, 可以作
为哀牢山—红河断裂带右旋走滑导致西北尾端伸展构造开始的最小年龄。

莺歌海
盆地和北部湾盆地位于哀牢山—红河断裂带的南东尾端, 新生代早期表现为尾端拉张区, 新生代晚期表现为尾端挤压区。

该盆地具有较连续的新生代沉积, 新生界地
层厚约17～20 km, 记录着哀牢山—红河断裂带运动学历史, 是研究该断裂构造转换的理想场地。

莺歌海盆地盆地内NW向构造和北部湾盆地白龙尾岛上NE向构
造中新世中晚期发生构造反转。

根据上新统—更新统详细的生物地层年代限定、
沉积速率和构造反转后沉积厚度,估算构造反转的时间为 10～8 Ma, 表明哀牢山—红河断裂带右旋走滑启动的时间上限为10 Ma。

通过滇西北断陷盆地、莺歌海盆地和断裂沿线新生代地层沉积、变形调查, 找到了两期走滑及其配套变形的地质证据。

根据哀牢山—红河断裂带滇西北尾端、莺歌
海东南尾端构造转换及年代研究,并结合该断裂带左旋韧性变形结束的时代为13 Ma,综合判断哀牢山—红河断裂带新生代左/右行构造转换的时间为13～10 Ma。

该年代与鲜水河—小江断裂左旋走滑的启动时间以及青藏高原东南缘构造隆升与侵蚀下切时代具有较好的一致性。

构造反演以及震源机制解表明, 青藏高原东南缘的最大水平应力方向绕东构造结发生顺时针旋转。

此外, 青藏高原东南缘构造迹线的运动学反演表明, 随着东构造结大规模的北向推进, 东构造结周缘的块体及断裂发生大规模的顺时针旋转。

结合区域内构造应力场的格局以及断裂带旋转的时空演化关系, 以及哀牢山—红河断裂带与东构造结的相对位置, 对该断裂两期变形的动力背景进行探讨。

认为新生代早期, 哀牢山—红河断裂带走向近东西,位于东构造结的东北缘, 主要受近北东向或东西向挤压, 断裂带表现为左旋走滑兼挤压变形, 两侧的盆地同时着发育近南北向的褶皱变形, 滇西北受尾端挤压变形, 莺歌海盆地受尾端拉张变形。

新生代晚期哀牢山—红河断裂带走向转换为北西向, 位于东构造结东南缘, 主要受近南北向挤压, 该断裂表现为右旋走滑兼伸展变形, 滇西北尾端反转为拉张区, 南海尾端反转为挤压区。

从全球板块运动来看, 陆-陆碰撞和洋-陆俯冲沿特提斯造山带呈交替结构。

东南亚是全球新生代最活跃的构造地带之一, 东南亚构造域位于欧亚板块、印度板块和太平洋板块的交汇地带。

东南亚涉及到陆-陆碰撞、洋-陆俯冲、弧后扩张多个构造系统, 对其新生代构造转换的研究需要从全球的角度着手。

哀牢山—红河断裂带位于喜马拉雅与南海构造体系的枢纽地带, 其构造运动特征与喜马拉雅的陆-陆碰撞造山和东南亚的洋-陆俯冲具有密切联系。

根据印度板块与欧亚板块的俯冲、碰撞历史, 以及东印度板块向东南亚的俯冲、西太平洋板块向东南亚的俯冲构造特征, 综合认为东南亚地区经历过三期逃逸∶ 第一期为 36—23 Ma, 印度板块位于印支地块的西侧, 整个印支地块往南东逃逸, 其中南印支地块的南东向挤出最快, 北侧王朝断裂和哀牢山-红河断裂表现为左旋走滑; 第二期为23—13 Ma,随着印度板块持续向北推进, 越过王朝断裂, 北印支地块逃逸最快, 其南侧边界王朝断裂反转为右旋走滑, 北侧边界继续为左旋走滑; 第三期为 13—0 Ma, 随着印度板块和东构造结持续向
北推进, 越过哀牢山—红河断裂, 华南地块往南东逃逸最快,哀牢山—红河断裂反转
为右旋走滑。

根据以上时空关系的分析, 认为新生代以来, 随着印度板块与欧亚板
块的持续汇聚, 青藏高原东南缘南印支地块、北印支地块和华南地块发生三期递进式构造挤出。

构造挤出的动力来自印度-欧亚板块的持续汇聚挤压, 同时印度洋、西太平洋向东南亚的洋-陆俯冲、后撤、弧后扩张也为挤出逃逸的提供了自由的空间。

The Ailao Shan–Red River fault zone (ASRRFZ) in southeastern Tibet, which extends from the Tibet plateau to the South China Sea, is a geological and geomorphic boundary between South China block and Indochina block. This fault zone, about 1 200 km long, is longitudinally divided into the Xuelong Shan, Diancang Shan, Ailao Shan, and Day Nui Con Voi metamorphic massifs. This fault zone is transversely divided into the higher metamorphic belt to the northeast, the lower metamorphic belt to the southwest and reverse Ailao Shan reverse fault in between. The higher metamorphic belt is composed by Proterozoic gneiss, migmatite, leucogranitic veins and S-L type mylonite, while the lower metamorphic belt is composed by the Paleozoic schist, slate, phyllite, S type protomylonite and ribbons of mafic and ultramafic rocks. The ASRRFZ widens towards the southeast. Sub-vertical foliations and tight folds are developed in the northwestern segment of the ASRRFZ. Gently dipping foliations and wide open folds are developed in the southeastern segment of the ASRRFZ. Sub-horizontal lineations and NE-dipping high-angle foliations developed all along the fault. In the Paleozoic, the ASRRFZ, which is also called as the Jinshajiang–Ailao Shan–Songma suture zone, is the subduction-collisonal boundary between the Indochina and South
China block. In the Cenozoic, due to continuous convergence between the Indian and Eurasia plate, polyphase extrusion occurred in southeastern Tibet and the ASRRFZ acted as a transform fault accommodating the relative motion between the Indochina block and the South China block. The Indochina block was extruded to the southeast during early Cenozoic and the north boundary fault of the ASRRFZ activated as left-lateral motion. With the northward indentation of the Indian plate, the South China block (locally called as Chuandian block) extruded to the southeast during late Cenozoic time, and the southern boundary fault of the ASRRFZ slipped right-laterally with some normal component. The two phase motion of the ASRRFZ is in intimate relation with the movement trajectory of the eastern Himalayan Syntaxis, uplift of the Tibetan Plateau, extrusion sequence and the opening of the South China Sea. Studying the tectonic transition and dynamic mechanism of the ASRRFZ is not only beneficial to the recognition of the structural evolution of the ASRRFZ, but also beneficial to the recognition of the tectonic uplift of the Tibetan plateau and the Cenozoic tectonic evolution of the South China Sea. Additionally, studying the tectonic transition of the ASRRFZ is of great significance to the verification of the different tectonic models, such as the tectonic extrusion, diffuse deformation, tectonic rotation and lower crustal flow models.
The left-lateral motion of the ASRRFZ is behaved as ductile deformation. A series of ductile deformation related rocks, such as the mylonites, migmatites, schists, phyllites, and leucogranitic veins, are developed along
the fault, and left-lateral shearing indicators are pervasively developed on the XZ-surface of the strain ellipse. 40Ar/39Ar thermochronological dating of the sericite in the phyllite indicated that the termination of the left-lateral shearing of the ASRRFZ continued to 13 Ma. The right-lateral motion of the ASRRFZ is behaved as brittle deformation. Because of the low temperature of brittle deformation, wide deformation belt, residual anisotropy of ductile deformation, the age constrains and reliable piercing line of total offset is challenging for the dextral motion. The time of initiation and displacement of the right-lateral motion, and tectonic transition of leftto right-lateral motion of this fault have not been well recognized so far. The northwestern Yunnan extensional basins system (NYEBS) and the Yinggehai basin at the tips of the ASRRFZ, and the Yuanjiang-Yuanyang restraining bend area in the middle part of the ASRRFZ, all of which are characterized by continuous Cenozoic sediments and recorded the kinematic history of the ASRRFZ, are idea sites to study the structural deformation of the ASRRFZ. Based on the methods of micro geomorphology, fault profile analysis, age constraint, the sedimentation, geomorphology and structural deformation of the NYEBS, Yinggehai basin, Beibuwan basin and central segment of the ASRRFZ are studied, together with the tectonic uplift, regional incision, together with the exhumation of the ASRRFZ, regional uplift and incision, paleomagnetic data to constrain the time of the Cenozoic kinematic transition of the ASRRFZ. What’s more, the dynamic mechanism of this transition was discussed based on the regional tectonic stress. This research is of great significance to the
Cenozoic tectonic evolution of southeastern Tibet Plateau.
The northwestern Yunnan extensional basins system (NYEBS) is located at the northwestern tip of the ASRRFZ. Due to the kinematic transition of the ASRRFZ, the NYEBS belongs to the compressional deformation domain in the early Cenozoic and belongs to extensional deformation domain in the late Cenozoic. A series of extensional basins, such as the Dali basin, Jianchuan basin, Lijiang basin, Heqing basin, Eryuan basin, Chenghai basin, Bingchuan basin and Ninglang basin, are formed in the NYEBS with continuous Cenozoic sediments. Investigations of the structural deformation of the Cenozoic sediments in the NYEBS revealed that the tectonic transition from compression to extension in the NYEBS occurred at late Middle Miocene. The sediments from Paleocene to late Miocene was deformed by reverse faults and folds, and modified by later extension, while the sediments later than middle Miocene only recorded extension with high angle normal faults. Paleomagnetic data revealed that the basal age of the extension related sediments is 7.6 Ma. The above age can be regarded as the upper limit of the extension in the NYEBS associated with the initiation of the right-lateral motion of the ASRRFZ. Yinggehai and Beibuwan basins, which have experienced extension in the early Cenozoic and compression in the late Cenozoic, are located at the southeastern tip of the ASRRFZ. These basins, which are characterized by continuous Cenozoic sediments more than 17～20 km thick, recorded the kinematic history of the ASRRFZ and are the best sites to study the kinematic transition of the ASRRFZ. NW-trending structures in the Yinggehai basin
and NE-trending structures in the Bach Long Vi area of Beibuwan basin have experience tectonic reversion in the middle Miocene. Based on the chronostratigraphic data of the Neogene-Pleistocene sequence, sedimentary rate and thickness above the surface of the tectonic reversion, the time of the tectonic reversion was estimated to be 10～8 Ma. This age indicate that the upper limit of the tectonic transition of the ASRRFZ is 10 Ma.
Based on the sedimentation and tectonic analysis of the Cenozoic sediments of the northwestern extensional basin system in NW Yunnan and Gulf of Tokin, two sets of tectonics associated with the left- and right-lateral motion of the ASRRFZ are distinguished. Based on the tectonic transition and age constrains of the NYEBS and Yinggehai basin, together with the termination of left-lateral shearing of the ASRRFZ at 13Ma, the tectonic transition from left- to right-lateral motion of the ASRRFZ occurred at 13～10 Ma. This age is in accordance with the initiation of the Xianshuihe–Xiaojiang fault system and the tectonic uplift and incision of the southeastern Tibet Plateau.
Tectonic reconstruction and earthquake focal mechanism revealed that the maximum horizontal stress rotated around the eastern Himalayan Syntaxis (EHS). Additionally, kinematic reconstruction of the structure lines in southeastern Tibet revealed that, with the large-scale northward indentation of the EHS, the blocks and faults around the EHS have experienced large scale clockwise rotation. Based on the temporal and spatial evolution of the tectonic stress fields and faults, and the relative
locations between the ASRRFZ and the EHS, the dynamic background of the two stage kinematic activity of the ASRRFZ was discussed.
This paper will focus on the sedimentation, deformation and dating along the fault and the two ends to study the left- to right-lateral transition of the ASRRFZ. In the early Cenozoic, the ASRRFZ trends nearly EW located northeast of the EHS under nearly NE to EW compression. The ASRRFZ deformed by left-lateral transpression and the basins on both sides of the fault deformed by nearly N-S trending folding. The NYEBS was located at the extensional domain and the Yinggehai basin was located at the compressional domain associated with the left-lateral motion of the ASRRFZ.
From global tectonic perspective, continent-continent collision and continent-ocean subduction appeared alternatively along the Tethys orogenic belt. The Southeast Asia, which is located at the convergence area of the Eurasian, Indian and the Pacific plates, is tectonically one of the most active areas in the world. Structural evolution of the southeast Asia has respect to the continent-continent collision, ocean-continent subduction and back arc spreading tectonic systems, and studying the tectonic transition of the this area should be conducted from global perspective. The ASRRFZ is located at the transformation area of the Himalayan and the South China Sea tectonic system, and the kinematic activity of the ASRRFZ is correlated with the Himalayan continent-continent collisional orogenic belt and the subduction system in southeast Asia. Based on the subduction-collision history of the Indian and Eurasian
plates, subduction of the eastern Indian plate, western Pacific plate toward the southeast Asia, three stages of extrusion are proposed∶ in the first stage, the Indian plate is located at the west side of the Indochina block, the Indochina block extruded toward southeast with the south Indochina block the quickest, and the Wangchao fault and the ASRRFZ were slipped left-laterally during 36–23 Ma; in the second stage, with the continuous northward indentation of the Indian plate across the Wangchao fault, the north Indochina block extruded faster than the south Indochina block, and the southern boundary fault of Wangchao fault slipped right-laterally and the northern boundary of the ASRRFZ slipped left-laterally during 23–13 Ma; in the third stage, with the further northward movement of the Indian plate across the ASRRFZ, the South China block moved faster than its southern ones, and the ASRRFZ slipped right-laterally during 13–0 Ma. Based on the temporal and spatial analysis in southeast Asia, with the continuous convergence of Indian and Eurasia plates during Cenozoic, the south Indochina block, north Indochina block and South China block extruded in three stages. The dynamic mechanism of the tectonic extrusion in southeast Asia not only come from the continuous convergence between Indian and Eurasia plates, but also comes from the subduction of the Indian ocean, west Pacific toward the southeast Asia, and the rollback, back-arc spreading provided free spaces.
Acknowledgements: This study was supported by Central Public-interest Scientific Institution Basal Research Fund (No. ZDJ2016-18).。