Distribution characteristics and geological implications of pockmarks and mud volcanoes in the northern and western continental margins of the South China Sea
Distribution characteristics and geological implications of pockmarks and mud volcanoes in the northern and western continental margins of the South China Sea
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发表时间：
2015
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通讯作者：
Chengzhu Jiang
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作者：
Chengzhu Jiang
利用新的高分辨率多波束测深数据和前人的研究资料，研究了麻坑和泥火山的分布特征，如位置、大小和形态，结合华南北方和西部陆缘的构造演化和地形特征，将华南大陆边缘划分为以台湾西南盆地为主体的北方陆缘和以中建南盆地为主体的西部陆缘中国南海（SCS），对这些特征进行了统计、分类，并与南海地质演化和油气资源分布相联系，南海被不同类型的大陆边缘所包围，北部为伸展大陆边缘，由台湾西南盆地（Ⅰ）、珠江口盆地（Ⅱ）和琼东南盆地（Ⅲ）组成;西部为走滑大陆边缘，包括北部湾盆地（Ⅳ）、莺歌海盆地（Ⅴ）和中建南盆地（Ⅵ）;东部为俯冲大陆边缘，南部为大陆碰撞边缘，上述盆地在南海演化过程中经历了几乎相同的阶段，其中包括古新世-中中新世裂谷作用、裂谷后热沉降作用和近5 Ma的新构造运动，在研究区划分出16个麻坑和泥火山分布区（a-p），其中a，B，c，d，1998- 1999年广州海洋地质调查局采集的数据。数据采集采用SeaBeam 2112采集系统，处理顺序为：导航滤波器，参数标定，换能器吃水修正，声速修正，数据滤波等，栅格网格尺寸约为100 m，垂直分辨率约为水深的3 ‰，利用多波束测深技术进行地貌研究，根据中国地质调查资料，运用统计和拟合的方法，研究了麻点和泥火山的分布特征和区域差异，麻坑的宽度和深度以及泥火山的直径和高度，不同地区麻坑和泥火山的特征有明显的区域差异，分别在结果剖面中表现出来，与西缘相比，北方边缘的麻坑数量少，宽度小，一般不超过100 m，少数巨型麻坑宽度在400~在琼东南盆地发现了1200 mare的麻坑，它们都倾向于发育在陆坡或陆棚坡折带附近，西缘麻坑在平面上呈圆形、椭圆形、新月形、细长形和不规则形等多种形态，且尺寸较大，它们的宽度通常超过1000分钟，深度通常超过100分钟。它们有的是复合的，有的成链排列或成群聚集。有的生长在峡谷或水道附近，在台湾盆地西南部，宽达100 m以上的泥火山广泛分布，拟合结果表明，各研究区麻点拟合曲线的斜率和决定系数（R2）不同，线性关系较差，但各研究区麻点均表现出较好的线性关系，反之，此外，根据统计结果，提出了一个综合的麻点分类方案：（a）平面形状，包括圆形、椭圆形、新月形、彗星形、细长形和不规则形麻点;（B）规模，包括小型、常规、巨型和巨型麻点;麻点的复合模式，包括复合麻点、麻点链和麻点群，不同地区麻点的不同拟合特征可以揭示不同的海底沉积、浅部构造活动、同一地区的泥火山具有良好的线性关系，说明泥火山受环境影响较小，形态一致性较强，结果表明，麻坑和泥火山的出现与台湾南部和西缘近5 Ma以来走滑断裂的活动有很强的相关性，在台湾西南盆地、莺歌海盆地和中建南盆地等走滑断裂带中分布广泛，规模较大，它们的分布特征，一方面可能表明有大量的流体逸出，另一方面可能表明持续的区域构造活动有利于浅层流体聚集，并使深层流体逸出而成藏，这可能成为油气资源勘探的重要标志。
New high resolution multi-beam bathymetry data and previous research data are used tostudy the distribution characteristics of pockmarks and mud volcanoes,such as location,size and morphology,in the northern continental margin consisting of Southwest Taiwan Basin and to the west and the western continental margin consisting of Zhongjiannan Basin and to the north.Combined with the tectonic evolution and topography of the northern and western margin of the South China Sea(SCS),this work made statistics,and classification to these characters,and linked them with the geological evolution of the South China Sea and the distribution of oil and gas resources.The SCS is surrounded by different types of continental margins.To the north is the extensional continental margin which consists of Southwest Taiwan Basin(Ⅰ),Pearl River Mouth Basin(Ⅱ)and Qiongdongnan Basin(Ⅲ);to the west is the strike-slip continental margin which consists of Beibuwan Basin(Ⅳ),Yinggehai Basin(Ⅴ)and Zhongjiannan Basin(Ⅵ);to the east is the subducted continental margin and to the south is the margin of continental collision.All the basins above have experienced almost the same stages during evolution of the SCS,which include Paleocene to mid-Miocene rifting followed by post-rift thermal subsidence and the neotectonic movement in recent ~5 Ma.In the study area,16 distribution areas(a—p)of pockmarks and mud volcanoes are recognized.The data of areas a,b,c,d,and h were collected from previous researches and the other data were collected by the Guangzhou Marine Geological Survey in 1998—1999.The acquisition system SeaBeam2112 was used to acquire data and the processing sequence includes:navigation filter,parameter calibration,correction for transducer draft,correction of sound velocity,data filtering and so on.The cell size of raster grids is ca.100 m and the vertical resolution is ca.3‰ of the water depth.The multi-beam bathymetry is utilized to study the morphology,size and shape of the pockmarks and mud volcanoes.Statistical and fitting methods are used to study their distribution features and regional differences according to the pockmarks′width and depth and the mud volcanoes′diameter and height.The characteristics of the pockmarks and mud volcanoes in different areas vary from area to area apparently,which are shown respectively in the result sections.Compared with the western margin,pockmarks in the northern margin are fewer and smaller with width of no more than 100 m.A few giant pockmarks with widths of 400~1200mare found in Qiongdongnan Basin.All of them tend to develop on the continental slopes or near the continental shelf break.Pockmarks in the western margin show various shapes in planar view,like circular,elliptical,crescent,elongated and irregular,and have larger size,which usually exceed 1000 min width and 100 min depth.Some of them are composited,arranged in a chain or gathered in a group.Some of them grow near canyons or channels,in the soft sediment deformation areas or beside the mud volcanoes.Mud volcanoes with widths more than 100 m widely spread singly or in groups in Southwest Taiwan Basin,Yinggehai Basin and Zhongjiannan Basin.Fitting results reveal that the slopes and coefficients of determination(R2)of pockmarks′fitting curves are different in each area,showing relatively bad linear relationship,however,the pockmarks of all the study areas show a good linear relationship.On the contrary,mud volcanoes in individual areas show good linear relationships,but all of them show a relatively bad linear relationship.Besides,according to the statistics result,a comprehensive pockmark classification scheme is proposed based on(a)their shape in planar view,which includes circular,elliptical,crescent,comet-shape,elongated and irregular pockmarks;(b)their magnitude,which includes small,conventional,giant and megapockmarks;and(c)their composite pattern,which includes composite pockmarks,pockmark chains and pockmark groups.Different fitting characteristics of pockmarks in different areas may reveal various seafloor sediments,shallow structural activities,continent slope dip and bottom current and so on.A good linear relationship of mud volcanoes in the same area may indicate that mud volcanoes suffer from minor environmental impact and have a strong coherence of morphologies,fluids components and fluids origin,which are probably different in other regions.Results indicate that the appearance of pockmarks and mud volcanoes are strongly coherent with the active activities of strike-slip faults in southern Taiwan and the western margin in recent ca.5 Ma.They are widespread with a relatively larger scale in strike-slip faults zones,like Southwest Taiwan Basin,Yinggehai Basin and Zhongjiannan Basin,but fewer and smaller in other extensional basins.Their distribution characteristics which may become important signs of hydrocarbon resources exploration,on one hand may indicate large quantities of fluid escape and on the other hand would indicate that continuous regional tectonic activities are in favor of shallow fluids accumulation and forming reservoirs resulted from deeply buried fluids escaping.