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地壳和上地幔结构及其性质对于理解地球浅部构造演化过程具有非常重要的指示作用.南海地区位于亚洲东南部,其构造演化长期以来受到了特提斯和太平洋构造域的控制.晚中生代以来,南海北部经历了主动大陆边缘向被动大陆边缘的演化,并且发生了多期次拉张构造运动,形成了一系列裂陷和盆地.近几年来,地震学探测方面的丰硕成果为理解此过程提供了有效的约束.(1)地壳结构方面而言:海底地震探测结果表明,南海北部陆缘地壳厚度范围为7～32 km,最厚和最薄处分别对应着东沙隆起和白云凹陷.深地震反射/折射剖面结果显示南海东北部大陆架和洋陆过渡带的下地壳底部发育有明显的高速层,厚度为0～10 km,且向洋盆减薄并尖灭,此特征与张裂运动后岩浆底侵过程一致.东沙隆起的上地壳岩性为长英质,表明该区12 km厚的下地壳高速层可能与中生代太平洋板块俯冲产生的火山弧有关.(2)在岩石圈尺度上:面波成像结果显示,南海北部的岩石圈比周边陆块的要薄一些,大致在60～75 km之间,为中生代俯冲形成的软弱带.(3)上地幔结构方面:体波层析成像结果表明在南海北部地区地幔转换带内存在高速异常,三重震相波形拟合则进一步约束了该高速异常体,即厚度为209～225 km,P波高速异常为1.5％～3.5％,其可能为中生代太平洋俯冲板片或伴随有大陆岩石圈拆沉体,也可能是古南海北部板片的俯冲残留体.滞留板片带来的低温异常导致410 km间断面抬升了 5 km,660 km间断面下沉了 5～15km.与此同时,在地幔转换带之上存在厚度为60～75 km,P波低速异常为2.0％～2.5％的低速层,其可能与地幔转换带内俯冲板片脱水有关. 相似文献
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南海北部东沙海底冲蚀河谷及其成因探讨 总被引：3，自引：0，他引：3
对覆盖南海北部东沙区的50多条多道地震剖面进行了解释.通过剖面解释,发现东沙隆起后,南海北部海底地形发生了较大变化.这其中包括东沙岛在陆架坡折处的隆起,以及沉积地层以较大的幅度向上翘起.向上翘起的地层在很大的范围内遭受海底剥蚀.在东沙隆起区的北侧,海底发育下切沟.本文解释为现代海底在更强烈的剥蚀条件下形成的海底冲蚀地形,称为海底冲蚀沟.海底冲蚀沟分布于东沙岛的北侧,大致与陆架坡折线,以及600m等深线相平行,其宽度一般在20km左右,NE-SW向长度可达200km,本文称为东沙海底冲蚀河谷.无论是冬季还是夏季,东沙岛以北的吕宋海峡都有黑潮分支从太平洋贯入南海.这一黑潮分支在冬季演化为太平洋-印度洋贯通流,在夏季演化为吕宋海峡次表层流.东沙海底冲蚀谷的分布与太平洋-印度洋贯通流,以及吕宋海峡次表层流的位置完全一致.本文认为,黑潮分支在遇到南海北部陆坡的阻挡后,黑潮水体由陆坡向陆架堆积,并因水深变浅,水体通过空间变小而使流速加大,从而有能力对海底产生剥蚀.东沙区海底地层的剥蚀,以及东沙海底冲蚀谷的发育都是黑潮分支在东沙陆架区形成的强底流和海底长期耦合作用的结果. 相似文献
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《中国科学:地球科学》2017,(4)
台湾岛为华夏陆块东缘自中生代晚期以来的年轻活动造山带,在东海-南海陆架地质背景上,台湾岛具有多重特殊性:除缺失始新世晚期层序外,始新世-更新世间有连续陆相至浅海相层序出露;记录"中国海"的发展及东海-南海陆架沧海桑田变迁;快速的隆升速率及重要的海洋沉积物贡献源;活跃的地震活动;既有被动又有活动大陆边缘层序;从台湾-吕宋间海域到岛上同时进行俯冲-碰撞-仰冲-张裂构造活动,因此有完整的威尔逊构造旋回地质纪录."台湾岛什么时候出露?"及"台湾岛怎么形成?"是大家关心台湾岛地质的基本问题.如果把东海陆架一半水深(60m)的海水拿掉,150km宽的台湾海峡就是台湾岛跟大陆相连的脐带;如果把台湾岛恢复到未造山前的原始形貌,台湾岛就是东海连结到南海间的脐带.但是为什么在东海及南海的陆架上只有台湾岛有比较大范围的出露?显然是有一个特定的地质作用在这个特殊的脐带位置发生,那就是早中新世19~18Ma开始的南海俯冲及随后6.5Ma开始的北吕宋火山岛弧与俯冲的欧亚大陆发生了斜向碰撞.台湾岛在晚中新世(~6.5Ma)时开始出露,最早出露部位就在中央山脉.中央山脉为中生代白垩纪的活动大陆边缘,随后拼合为欧亚大陆一部分,经新生代古近纪断陷构造(50~39Ma)、渐新世-中期中新世南海张裂(33~16Ma)、早中新世(19~18Ma)至今欧亚大陆-南海岩石圈沿马尼拉海沟向东俯冲于西进的花东海盆-菲律宾海板块之下,形成中新世增生楔,及6.5Ma至今由北向南迁移的斜向弧陆碰撞,导致中央山脉隆升出露海平面之上,形成原始台湾岛.由此原始台湾岛,台湾河流系统开始发生,并将侵蚀物经河流向西供输到原始台湾海峡前陆盆地,同时也向东供应到北吕宋海槽弧前盆地.随着弧陆碰撞的持续进行,南海东北部陆架上层序被推覆并出露在中央山脉西侧,台湾岛出露的面积也随之变大;1Ma的吕宋火山岛弧北段更向西仰冲,形成海岸山脉,拼贴在中央山脉东侧;最后又因1Ma冲绳海槽弧后张裂,中央山脉东北部又发生断陷,产生宜兰平原,形成今日台湾岛的形貌. 相似文献
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潮汕坳陷MZ-1井揭示的中生界为深入分析南海北部晚中生代的构造演化提供了关键性的资料.基于MZ-1井的标定，开展了系统的地震剖面构造-地层解释，在中生代地层内识别出Tm30区域性不整合面，同位素定年确定该界面发育于早白垩世末至晚白垩世初，落实了潮汕坳陷上白垩统的分布.此外，在研究区西南部识别出大型的兴宁—东沙逆冲推覆带，主要由多条NW—SE向延伸、西倾的叠瓦状逆冲断层及其伴生的不对称褶皱组成，其明显控制了上白垩统厚度分布.由此可见，上白垩统构造层不具有张裂盆地的典型特征，因此南海北部主动陆缘向被动陆缘的转换不会早于晚白垩世末.研究认为，在南海地区特提斯残留洋盆关闭的总背景下，在约80 Ma时期，南海地块与华南陆块强烈碰撞挤压，在靠近碰撞带处的礼乐滩、潮汕坳陷西南部形成褶皱冲断构造体系，进而控制了潮汕坳陷晚白垩世周缘前陆盆地的发育. 相似文献
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南海东部的板块汇聚带是了解南海和菲律宾海晚中新世以来构造演化的一个重要窗口.针对这一区域地壳运动的研究,获得了该区内微块体的现时地壳运动特征及其动力机制,在此基础上结合俯冲板片形态和震源机制资料提出了该区晚中新世以来的板块汇聚作用特征.研究发现:晚中新世晚期,菲律宾海板块西缘在南、北部的西向运动均受到限制的情况下,位于中间区域的吕宋岛北部则由于其西侧相对自由而继续往NW方向运动.该过程中,菲律宾大断裂等汇聚带内部的走滑断裂对于协调不同块体之间的地壳运动速度差异有着重要作用.受南、北阻挡的影响,中间部分的西向运动速率呈现出中间大(吕宋岛北部)两端小的特征.因而北吕宋西侧的马尼拉海沟也以相对于南部更快的速率不断向NW迁移.然而俯冲的南海岩石圈受其下方SE向地幔流的影响,未能发生相应的俯冲板片后撤,而是在两板块之间的直接接触面形成强烈推挤并发生反向弯曲.结合这一板块作用特征认为,马尼拉海沟现今的构造形迹是在上述背景下北段多次向NW方向变迁形成的,而双火山弧的形成则主要是由菲律宾海板块在吕宋岛弧南、北部的西向运动速率差异引起的. 相似文献
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中国台湾的大地构造格局和晚中生代以来的演化过程一直是地质研究的难点和热点,特别是对其构造单元的类型、性质和演化过程存在较多争论.本文系统综述了50多年来已有中国台湾大地构造格局和重建晚中生代以来构造演化史的主要认识,特别是近10年来新的研究成果,侧重探讨了板块运动和弧陆碰撞对中国台湾大地构造形成过程中起到的作用.研究认为中国台湾总体经历了4个关键期:(1)晚中生代太平洋活动大陆边缘时期(约80 Ma),在华南陆缘形成造山带和俯冲沟弧盆系统;(2)古近纪到早中新世被动大陆边缘时期(约80～16.5 Ma),形成新生代台西盆地和台西南盆地雏形;(3)中-晚新世板块汇聚初期(16.5～6 Ma),形成的马尼拉海沟北段俯冲增生楔为中央山脉、恒春半岛前身,向南连接恒春海脊,部分南海洋脊因碰撞挤压形成中国台湾东部蛇绿岩套;(4)晚中新世末以来活跃造山期(6 Ma至今),菲律宾海板块朝欧亚大陆向西碰撞和向北俯冲,分别形成西部麓山带、雪山山脉、高屏斜坡和大屯火山区—琉球火山弧、宜兰平原—冲绳海槽;北吕宋岛弧向欧亚大陆碰撞和逆冲,形成花东纵谷和利吉混杂岩体、海岸山脉.中国台湾地区在同裂谷(syn-rift)时期的晚白垩系到古近系、后裂谷(post-rift)时期的中新统、造山定型期的下中新统和第四系地层发育了三套主要含油气系统.从大地构造格局和演化来认识中国台湾独特的发生、发展规律,从而理解其独有油气和天然气水合物成藏规律,这对该区油气和天然气水合物的寻找和评价具有指导意义. 相似文献
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《中国科学:地球科学》2017,(5)
位于青藏高原东北缘的祁连山由一系列平行的山系及其山间盆地相间组成,其新生代山体隆升与盆地演化是关系到青藏高原隆升扩展乃至中国一二级阶梯地势形成的关键问题.本文以祁连山腹地及外围新生代沉积盆地为研究对象,综合考虑区域位置、盆地形成与演化等要素,基于不同盆地区域地层对比,进行动态地层区划,进而恢复祁连山地区构造-沉积演化过程.结果表明,古近纪早期(古-始新世)祁连山地区为一隆起带,南北两侧的柴达木盆地与酒泉盆地以及腹地兰州-西宁盆地独立演化;渐新世青藏高原隆升并向东北方向扩张逆冲,西秦岭北缘贵德-西宁-兰州-临夏压陷盆地形成,祁连山东部地区压陷盆地全面接受沉积,祁连山西部地区的南祁连古隆起与北祁连古隆起之间苏里盆地开始形成并接受沉积.中新世祁连山地区陆内挤压造山与盆地裂解,拉脊山、青海南山、积石山、六盘山等山体相继隆起,将古近纪统一大型压陷盆地分割为贵德、共和、青海湖、西宁、兰州、临夏及天水等盆地,分别归属新近纪贵德-西宁-兰州地层区中各地层分区,疏勒南山隆起将苏里盆地裂解为多个小盆地,同归新近纪苏里地层区.与此同时,南、北祁连开始向外逆冲扩展,柴达木盆地与酒泉盆地范围较古近纪缩小,且在盆地边缘形成次一级沉降中心,分属新近纪柴达木地层区和肃北-酒泉地层区. 相似文献
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台湾岛南部海域的前碰撞构造地球物理特征 总被引：1，自引：0，他引：1
在2001年进行了一次从南海跨越吕宋岛弧到西菲律宾海的海洋地球物理调查, 获得了一条高质量深反射地震剖面. 以此地震剖面的解释为基础, 结合重磁数据的正反演, 系统分析了台湾岛南部海域的区域构造地球物理学特征, 特别是相邻块体之间碰撞拼接前的构造关系以及不同块体的构造属性, 以帮助更好地认识形成台湾岛造山带的早期弧陆碰撞过程. 马尼拉增生楔的组成及自西向东的递进变形表明南海的俯冲经历过多个次级俯冲活跃阶段. 北吕宋海槽内存在活跃的前碰撞地壳缩短过程, 导致地层倾角产生6°~13°的倾斜, 但是由地层倾斜而造成的缩短量只占区域总缩短量的很小的一部分. 吕宋岛弧的东翼比岛弧的其他部分显得更为活跃, 在东翼的弧内盆地内存在活动断裂与褶皱作用. 磁反演结果表明吕宋岛弧可能经历了多期次的岩浆过程, 造成磁性横向不均一性. 布格重力异常及重力正反演结果显示花东海盆具有明显高于南海和吕宋岛弧的地壳和上地幔密度, 且与南海北部之间存在较大的海底深度差异, 表明花东海盆与南海北部不可能如前人认为的在整体或部分上曾经属于同一个洋壳. 加瓜海脊作为一片隆升的洋壳, 可能由早期菲律宾海板块在加瓜海脊东侧短暂的西北向俯冲有关. 所有证据表明台湾岛南部海域正在进行多块体的拼接, 为一大陆增长的典型地区. 相似文献
10.
东亚大陆的新生代构造演化受两大地球动力系统所控制:印度-欧亚板块的碰撞及陆内汇聚体系、西太平洋-印度尼西亚板块俯冲消减体系。从晚白垩纪到古新世期间,温暖宽阔的新特提斯洋分割着欧亚大陆和印度次大陆,并且向北俯冲消减于欧亚板块之下。与此同时,太平洋板块继续向西俯冲消减于欧亚板块之下,随着俯冲速率的大幅度降低,俯冲边界发生海沟后撤(trench rollback),使得欧亚大陆东边界开始形成一系列NNE走向的弧后拉张盆地。尽管印度与欧亚大陆碰撞的起始时间仍有争议,但至少强烈碰撞发生在距今45~55Ma期间。陆-陆碰撞及印度板块持续的楔入作用导致了新特提斯海的退出,青藏高原南部和中部的地壳增厚,并隆起形成"原青藏高原"。碰撞及其强烈的楔入作用还导致了青藏高原南部岩石圈块体向SE方向的大规模挤出。青藏高原南部块体的挤出时间与西太平洋-印度尼西亚海洋俯冲消减带的加速后撤是一致的,表现为沿消减带上盘弧后盆地的快速拉张和裂陷,构成具有成因联系的"源-汇关系"。距今20~30Ma期间,随着青藏高原大规模南东挤出的减弱,碰撞和楔入引起了向NE方向挤压的增强,导致了青藏高原本身向S和向NE方向的扩展。构造变形向南迁移到主边界逆冲推覆带,向北扩展到昆仑山断裂,造成柴达木盆地、河西走廊、陇西盆地开始接受最初的新生代沉积,形成青藏高原东北缘的大规模晚新生代沉积盆地群。西太平洋-印度尼西亚板块的海沟后撤大幅度减速或停止,直接导致了日本海扩张的停止,华北盆地裂陷期终止,进入整体热下沉阶段。大约距今10Ma以来,青藏高原内部的高海拔地区晚中新世以来开始出现近SN向的拉张,形成一系列SN向裂谷以及NW向右旋和NE向左旋的共轭走滑断裂系。与此同时,青藏高原向周边生长扩展,祁连山快速隆起形成高原北边界,龙门山也第2次加速隆升,与四川盆地形成近4 000m的地貌高差。在东部,沿西太平洋-印度尼西亚板块俯冲消减带的运动开始加速,不仅弧后拉张作用停止,一些早新生代的拉张盆地还发生反转而遭受到挤压缩短作用。 相似文献
11.
马尼拉俯冲带是南海的东部边界，记录了南海形成演化的关键信息，同时也是地震和海啸多发区域.本文利用过马尼拉俯冲带北段的高分辨率多道地震剖面，分析了研究区内海盆和海沟的沉积特征，精细刻画了区内增生楔前缘的构造变形、结构以及岩浆活动特征.研究区内增生楔下陆坡部分由盲冲断层、构造楔和叠瓦逆冲断层构成，逆冲断层归并于一条位于下中新统的滑脱面上，滑脱面向海方向的展布明显受到增生楔之下埋藏海山和基底隆起的影响；上陆坡的反射特征则因变形强烈和岩浆作用而难以识别；岩浆活动开始于晚中新世末期并持续至第四纪.马尼拉俯冲带北段增生楔的形成时间早于16.5 Ma，并通过前展式逆冲向南海方向扩展；马尼拉俯冲带的初始形成时间可能在晚渐新世，而此时南海海盆扩张仍在持续.南海东北缘19°N-21°N区域为南海北部陆坡向海盆的延伸，高度减薄的陆壳的俯冲造成马尼拉海沟北段几何形态明显地向东凹进. 相似文献
12.
13.
K–Ar ages have been determined for 14 late Miocene to Pliocene volcanic rocks in the north of the Kanto Mountains, Japan, for tracking the location of the volcanic front through the time. These samples were collected from volcanoes located behind the trench–trench–trench (TTT) triple junction of the Pacific, Philippine Sea, and North American plates. This junction is the site of subduction of slabs of the Pacific and the Philippine Sea plates, both of which are thought to have influenced magmatism in this region. The stratigraphy and K–Ar ages of volcanic rocks in the study area indicate that volcanism occurred between the late Miocene and the Pliocene, and ceased before the Pleistocene. Volcanism in adjacent areas of the southern NE Japan and northern Izu–Bonin arcs also occurred during the Pliocene and ceased at around 3 Ma with the westward migration of the volcanic front, as reported previously. Combining our new age data with the existing data shows that before 3 Ma the volcanic front around the TTT junction was located about 50 km east of the preset‐day volcanic front. We suggest that northward subduction of the Philippine Sea Plate slab ended at ～3 Ma as a result of collision between the northern margin of the plate with the surface of the Pacific Plate slab. This collision may have caused a change in the subduction vector of the Philippine Sea Plate from the original north‐directed subduction to the present‐day northwest‐directed subduction. This indicates that the post ～3 Ma westward migration of the volcanic front was a result of this change in plate motion. 相似文献
14.
珠江口盆地及其邻近地区的活动断裂与地震活动 总被引：1，自引：0，他引：1
珠江口盆地位于南海北部，属于大陆地壳的陆缘张裂型盆地，其形成与地幔向上隆起和地壳拉张减薄有密切关系，自中新世纪晚期以来，珠江口盆地的坳陷为沉积区，盆地的隆起及外围为剥蚀区。 相似文献
15.
Xu ZhaoKai Li AnChun Jiang FuQing Li TieGang Meng QingYong Jin Ning 《中国科学D辑(英文版)》2007,50(8):1179-1188
From systemic research of microstructure, geochemistry, uranium-series and 10Be isotope dating on a new-type deepwater ferromanganese crust from the East Philippine Sea, the paleoenvironment evolu-tion of the target area since the terminal Late Miocene was recovered. The vertical section changes of microstructure and chemical composition are consistent in the studied crust, which indicate three major accretion periods and corresponding paleoenvironment evolution of the crust. The bottom crust zone was formed in the terminal Late Miocene (5.6 Ma) with loose microstructure, high detritus content and high growth rate. Reductions of mineral element content, accretion rate and positive Ce-anomaly degree at 4.6 Ma indicate temporal warming, which went against the crust accretion and finally formed an accretion gap in the terminal Middle Pliocene (2.8―2.7 Ma). The more active Antarctic bottom sea-waters in the Late Pliocene (2.7 Ma) facilitated the fast transfer to the top pure crust zone. Hereafter, with the further apart of volcanic source and the keeping increase of eolian material (1.0 Ma), although surrounding conditions were still favorable, mineral element content still shows an obvious reducing trend. It thereby offers new carrier and data for the unclear paleoceanographic research of the target area since the terminal Late Miocene. 相似文献
16.
塔里木盆地的高分辨率沉积记录对于理解青藏高原隆升、亚洲内陆干旱化乃至全球气候变化至关重要.建立可靠的地层年代标尺对于研究塔里木盆地晚新生代沉积环境演化、构造运动及古气候变化具有重要意义.本文对塔里木盆地东北缘库尔勒地区的两个全取心钻孔ZK3（深500 m）、ZK5（深300 m）进行详细的磁性地层学研究，结果表明，ZK3孔中更新统底界为54.8 m，下更新统底界为167.0 m，上新统底界为432.0 m，钻孔底部年龄约为6.2 Ma，属上中新统上部；ZK5孔中更新统底界为64.7 m，下更新统底界为241.5 m，钻孔底部年龄约为3.2 Ma，属上上新统.基于上述磁性地层年代标尺，通过沉积速率分析发现ZK3孔在3.0—3.6 Ma之间沉积速率明显增大，反映了塔里木盆地北部天山在此期间的快速隆升.通过东西部多个盆地地质剖面沉积速率的对比分析发现，这期构造活动在区域上具有准同期活动特征，在时代上与晚中新世以来青藏高原快速隆升的时代一致，可能与青藏高原的隆升扩展效应有关. 相似文献
17.
《Earth and Planetary Science Letters》2005,229(3-4):259-271
High-resolution reconstruction of Benioff zone depth–dip angle trajectory for Burma–Java subduction margin between 2° and 17°N Lat. reveals two major episodes of plate geometry change expressed as abrupt deviation in subduction angle. Estimation of effective rate of subduction in different time slices (and then length of subducted slab) allowed drawing of isochrones in Ma interval through these trajectories for the time period 5–12 Ma. With these isochrones, the deformation events on the subducting Indian plate are constrained in time as of 4–5 and 11 Ma old. This well-constrained time connotation offered scope for the correlation of slab deformation events with the well-established two-phase opening history of the Andaman Sea. While the 11 Ma event recorded from southern part of the study area is correlated with early stretching and rifting phase, the 4–5 Ma event is interpreted as major forcing behind the spreading phase of the Andaman Sea. Systematic spatio-temporal evaluation of Indian plate obliquity on the Andaman Sea evolution shows its definite control on the early rifting phase, initiated towards south near northwest Sumatra. The much young spreading phase recorded towards north of 7° Lat. is possibly the result of late Miocene–Pliocene trench retreat and follow-up transcurrent movement (along Sagaing and Sumatran fault system) with NW–SE pull-apart extension.Nonconformity between plate shape and subduction margin geometry is interpreted as the causative force behind Mid-Miocene intraplate extension and tearing. Enhanced stretching in the overriding plate consequently caused active forearc subsidence, recorded all along this plate margin. Initial phase of the Andaman Sea opening presumably remains concealed in this early–middle Miocene forearc subsidence history. The late Miocene–Pliocene pull-apart opening and spreading was possibly initiated near the western part of the Mergui–Sumatra region and propagated northward in subsequent period. A temporary halt in rifting at this pull-apart stage and northeastward veering of the Andaman Sea Ridge (ASR) are related with uplifting of oceanic crust in post-middle Miocene time in form of Alcock and Sewell seamounts, lying symmetrically north and south of this spreading ridge. 相似文献
18.
菲律宾群岛受到欧亚板块、菲律宾海板块和印度-澳大利亚板块的碰撞作用，地质环境复杂，构造因素多样.尽管近几年来已经有了少数关于该区域层析成像的研究，但这些研究的区域主要集中在马尼拉海沟、吕宋岛及中菲律宾地区，而关于群岛周围其他海沟和南菲律宾地区的讨论相对较少.到目前为止，还没有同时获得过关于菲律宾群岛深部纵、横波速度结构的研究，本次研究通过反演155779条P波震相和59642条S波震相，同时获得了菲律宾群岛从地表至150 km深度的纵、横波速度结构.地震层析成像结果表明该地区的壳幔速度结构具有较强的不均一性，地壳内部存在着广泛的低速异常，而表征俯冲板块的高速异常则沿着群岛周边的海沟展布.南海块体在马尼拉海沟中段的俯冲角度和俯冲活动性比南段小；菲律宾海板块在东吕宋海槽南段微弱的俯冲作用很有可能同本哈姆海台的碰撞有关.菲律宾群岛大部分MW>6.0的强震沿着各个板块的边界发生，体现出菲律宾海板块同欧亚板块之间的强耦合作用，群岛西侧的南海块体在马尼拉海沟16°N-20°N之间呈现出的弱耦合状态可能跟北吕宋地区的拉张应力环境有关，南海块体在16°N以南的地区同上覆块体之间的耦合作用较强；群岛东侧的菲律宾海板块在14°N以北的地区没有强震发生，它与菲律宾群岛之间的耦合程度从北向南逐渐增强，在12°N以南的地区要强于12°N以北的地区；此外苏禄海盆和菲律宾构造带之间也存在着强耦合关系. 相似文献
19.
The Longxi region contains different kinds of Cenozoic sediments, including eolian deposits, reworked loess, fluvial and lacustrine deposits. The provenance evolution of these sediments is of great significance in exploring the uplift, tectonic deformation and associated with geomorphic evolution of the Northeastern Tibetan Plateau. In this paper, we used the single-grain zircon provenance analysis to constrain the provenances for the Paleogene alluvial conglomerates and for the Neogene fluvial-lacustrine sediments, and compared them with results from the loess deposits since the Miocene. The results show that: (1) the Paleogene alluvial conglomerates contain a large number of detrital zircons ranging from 560 to 1100 Ma that were derived from the Yangzi Block. However, the sediments of early Miocene have much fewer zircons of this age span, which are characterized by an abundance of zircon ages in the ranges of 200–360 Ma. This indicates that the Paleogene alluvial conglomerates mainly come from the middle and/or southern West Qinling, and the early Miocene sediments are primarily from the northern West Qinling; (2) Late Neogene fluvial sediments (11.5 Ma onward) in Tianshui-Qinan region are dominated by zircon ages of 380–450 Ma. This zircon population is similar to that of the exposed intrusive rocks of southern part of the Liupan Mountains, implying that the southern part of Liupan Mountains probably had already uplifted by 11.5 Ma; (3) Late Miocene lacustrine sediments in Tianshui region have a zircon age spectra that is remarkably different from coeval fluvial deposits, but is similar to the zircon age distributions of the Miocene loess in Qinan region, late Miocene-Pliocene Hipparion red clay and Quaternary loess. This indicates that fine particles within these Miocene lacustrine sediments in Tianshui region may be dominated by aeolian materials. This study reveals that provenance changes of Cenozoic sediments in Tianshui-Qinan region and its geomorphic evolution are closely related to the multi-stage uplift of the Northeastern Tibetan Plateau. In particular, the major uplift of the Northern Tibetan Plateau during late Oligocene-early Miocene may have not only provided the source areas and wind dynamic conditions for the deposits of the Miocene loess, but also provided the geomorphic conditions for its accumulation. 相似文献
20.
Abstract We carried out paleomagnetic measurements and K–Ar dating on Neogene andesitic lavas and sills of the Shigarami Formation in North Fossa Magna, central Japan. The Shigarami Formation is distributed in the axial part of the Komiji Syncline in the folding zone of the southwestern North Fossa Magna. Results of the present study indicate that the Komiji Syncline was formed shortly after 4.42 ± 0.12 Ma during the Pliocene. The sedimentary rocks of the Shigarami Formation consist of shallow marine and fluvial deposits. Intrusions of andesitic sills are found in the shallow marine deposits and two andesitic lava flows are present in the fluvial deposits. Oriented samples were taken from the sills at four sites and from the lavas at three sites. The samples produced stable remanent magnetization through stepwise alternating field and thermal demagnetizations. Results of a positive fold test indicate that the stable remanent magnetizations concentrate around a mean reversed polarity of declination = 169.0°, inclination = ?58.5° and 95% confidence limit = 9.0° after corrections have been made according to the direction of the bedding of the sedimentary rocks. Four fresh samples were selected for K–Ar dating from the samples used for paleomagnetic measurements. The groundmass of three samples taken from the sills yield ages of 4.42 ± 0.12, 4.49 ± 0.22 and 4.69 ± 0.13 Ma, whereas the one taken from the lower lava has an age of 5.91 ± 0.26 Ma. We believe that the Komiji Syncline was formed after the emplacement of lavas and sills in the area, because the descending Miocene strata were folded concordantly with the Shigarami Formation. The Pliocene and Pleistocene strata rest unconformably on the folded strata. The deformation might have progressed during the Pliocene, then slowed down in the Early Pleistocene. Our results suggest that the northwestward motion of the Philippine Sea Plate and the collision of the Tanzawa Block affected not only the South Fossa Magna, but also the North Fossa Magna. 相似文献