雅浦海沟北部深渊和超深渊沉积物的物质组成、来源和形成过程研究

为探究雅浦海沟北部深渊、超深渊沉积物的组成、来源和形成特征,以1 cm分层对采自该海域不同深度的五根柱状沉积物样品的0~8 cm沉积层进行了扫描电子显微镜(SEM)观察和X射线能谱(EDS)分析,并且分析了这些样品的含水率,锰结核含量, Al、Ca、Fe、Mn、Ti、Mg等6种常量金属和Ba、Co、Cr、Ni、Pb、Sr、V、Cu、Zn等9种微量金属元素含量,以及总有机碳(TOC)含量等参数。结果表明,研究区域沉积物主要为深海软泥沉积,是不同年代沉积物的复杂混合,包含以盘星石为代表的颗石藻、海绵骨针、放射虫和硅藻等多种微体古生物化石以及辉石、重晶石、钛铁矿、长石等多种矿物。沉积组分主要来自生物源、火山源、陆源和海底热液,其中陆源沉积出现于海沟东侧深渊区。海沟西侧崖壁的沉积物比东侧崖壁的沉积物更容易发育锰结核。研究区域沉积物含水率较高并且颗粒较大,其含水率随深度增加呈减少趋势。研究区域超深渊站位沉积物的TOC和微体古生物化石含量均高于深渊站位,存在明显的漏斗效应。整体上海沟西侧崖壁沉积物含水率和TOC含量低于东侧崖壁的沉积物。自更新世以来,雅浦海沟北部的碳酸钙补偿深度(CCD)线从4 568 m以深变为4 435 m至4 568 m之间。研究区域的沉积环境为氧化环境,其0~8 cm沉积层的古老沉积物与现代沉积物发生了混合和再沉积作用,形成年代跨度极大。研究区域沉积物的形成受到海沟坡度、水动力环境、重力滑塌、浊流沉积、火山活动、漏斗效应等多种因素的显著影响。     

马尼拉海沟1.4 ka B.P.以来浊流事件沉积及其成因机制*

浊流是远距离沉积物运输的一种重要方式,海底浊流广泛存在于海底峡谷或海沟。马尼拉海沟位于南海东北部,是一条正在活动的板块汇聚边界。独特的地理位置(亚热带—热带)和气候条件(台风频发),使得马尼拉海沟浊流频发,然而,现今对马尼拉海沟的浊流研究甚少。本研究通过对马尼拉海沟北部水深3747 m处重力柱岩心(GEO6)进行高精度的粒度及沉积学特征分析,探讨马尼拉海沟浊流沉积规律。GEO6岩心底部细颗粒沉积物中浮游有孔虫的¹⁴C的AMS年龄为1405 a B.P.。高精度的粒度分析(0.25 cm)和沉积学特征显示: GEO6岩心记录有至少11次浊流沉积(T1-T11),且这些浊流都有明显的底部粒度最粗(砂质粉砂或砂)、向上粒度逐渐变细的正粒序特征,只有T8沉积体为反粒序特征,可能为异重流沉积。结合区域地质资料,本研究认为1.4 ka B.P.以来,研究区频繁的台风带来了大量陆源松散沉积物堆积在马尼拉海沟上游(高屏峡谷),不稳定的构造环境及地震频发导致这些松散沉积物垮塌并向下游马尼拉海沟输送,在海沟内形成频繁发育的浊流沉积体。     

Tomographic evidence for a slab tear induced by fossil ridge subduction at Manila Trench,South China Sea

A tomographic travel-time inversion has been applied to trace the subducted slab of the South China Sea (SCS) beneath the Manila Trench. The dataset, taken from the International Seismological Centre (1960–2008), is composed of 13,087 P-wave arrival times from 1401 regional earthquakes and 8834 from 1350 teleseismic events. The results image the different morphology of the subducted SCS slab as a high-velocity zone. The subducting angle of the slab varies along the trench: at 16° N and 16.5° N, the slab dips at a low angle (24° ~ 32°) for 20–250 km depth and at a moderate angle (50°) for ~250–400 km depth. At 17° N, the slab dips at a low angle (32°) to near 400 km depth, and at 17.5° N and 18° N the slabs are near vertical from 70 ~ 700 km depth, while at 20° N the high-velocity anomalies exhibit features from horizontal abruptly to near vertical, extending to 500 km depth. The dramatic steepening of the slab between 17° N and 17.5° N may indicate a slab tear, which is coincident with the axis of a fossil ridge within the SCS slab at around 17° N. In addition, low-velocity zones in the three profiles above 300 km depth may represent the formation of the slab window, induced by ridge subduction and slab tear, initiating upward mantle flow and resulting in the partial melting of the edge of the slab. The slab tear could explain the volcanic gap and geochemical difference between the extinct Miocene and Quaternary volcanoes in the Luzon Arc, the much higher heat flow around the fossil ridge, and the distribution of most of the adakites and the related porphyry Cu-Au deposits in the Luzon area. Based on the geometry and morphology of the subducted slab and certain assumptions, we calculate the initial time of ridge subduction, which implies that ridge subduction and slab tear possibly started at ~8 Ma.     

Petrogenesis and tectonic implication of lavas from the Yap Trench,western Pacific

We present major and trace element data of lava recovered from the northern Yap Trench in the western Pacific and discuss their petrogenesis and tectonic implications within the framework of interactions between the Caroline Ridge and Yap Trench. Rocks were collected from both landward and seaward trench slopes and exhibited geochemical characteristics similar to backarc basin basalt (BABB) and mid-ocean ridge basalt (MORB), including high Fe content, tholeiitic affinity, high TiO2 value at a given FeOT/MgO ratio, Ti/V ratio between 20 and 50, low Ba/Nb ratio and Th/Nb ratio, and trace element patterns commonly displayed by BABB and MORB, which are distinct from arc lava. These rocks seem to have been generated during mantle upwelling and decompression melting at a spreading center. However, compared with typical forearc lava produced by seafloor spreading in the Mariana forearc region, such as the early Eocene forearc basalts and late Neogene forearc lava in the southernmost Mariana Trench, the Yap Trench lava is derived from a more fertile mantle and feature a more minor subduction component; thus, they cannot be the products of forearc mantle decompression melting. We suggest that the landward slope lava represents backarc basin crust that was overthrust onto the forearc lithosphere during the collision of the Caroline Ridge with the Yap Trench (20–25 Ma), which played a key role in the evolution of the Yap subduction system. Moreover, the seaward slope lava represents the subduction plate crust that accreted onto the deep trench during the collision. This collision event resulted in the cessation of Yap Arc magmatism; thus, the Yap Trench volcanic rocks (<25 Ma) previously suggested to be arc magma products may actually represent the nascent island arc lava with a lower subduction component than in the mature Mariana Arc lava.     

马里亚纳“沟−盆”深水沉积环境稀土元素特征与物源约束

通过对西太平洋帕里西维拉海盆东南部的C-P19柱状样和马里亚纳海沟南坡的L3柱状样进行粒度和稀土元素地球化学分析,探讨了二者物源的异同。结果表明:研究区沉积物的稀土元素含量基本不受沉积物粒度控制,主要受控于物源变化。物源判别结果指示二者的物源具有同源性,其中西马里亚纳海脊剥蚀下来的火山物质对研究区的物质来源贡献最大。火山物质中的重矿物可以影响到稀土元素的含量,例如锆石的含量和稀土元素总量(ΣREY)的相关系数可达0.86。来自中国内陆黄土的陆源风尘物质对两个研究区的物源供给也有一定贡献,但其贡献程度较小。由于马里亚纳海沟南坡更加远离大陆且纬度更低,其接受的亚洲风尘也比帕里西维拉海盆和挑战者深渊更少。此外,南极底层水流经研究区,对海盆和海沟的沉积物都造成一定影响,而且在马里亚纳海沟南坡更加活跃,因而帕里西维拉海盆东南部的沉积物比马里亚纳海沟南坡更易于保存。     

Morphology and magnetic survey of the Rivera-Cocos plate boundary of Colima,Mexico

The propagation of the Pacific-Cocos Segment of the East Pacific Rise (EPR-PCS) has significantly altered the plate configuration at the north end of the Middle America Trench. This ridge propagation, the collision of the EPR-PCS with the Middle America Trench, the separation of the Rivera and Cocos plates and the formation of the Rivera Transform have produced a complex arrangement of morphotectonic elements in the area of Rivera-Cocos plate boundary, atypical of an oceanic transform boundary. Existing marine magnetic and bathymetric data has proved inadequate to unravel this complexity, thus, a dense grid of total field magnetic data were collected during campaigns MARTIC-04 and MARTIC-05 of the B/O EL PUMA in 2004 and 2006. These data have greatly clarified the magnetic lineation pattern adjacent to the Middle America trench, and have revealed an interesting en echelon, NE-SW oriented magnetic high offshore of the Manzanillo Graben. We interpret these new data to indicate that the EPR-PCS ridge segment reached the latitude (~18.3°N) of the present day Rivera Transform at about Chron 2A₃ (~3.5Ma) and propagated further northward, intersecting the Middle America Trench at about 1.7 Ma (Chron 2). At 1.5 Ma spreading ceased along the EPR north of 18.3°N and the EPR-PCS has since retreated southward in association with a southward propagation of the Moctezuma Spreading Segment. North of 18.3°N the seafloor near the trench has been broken into small, uplifted blocks, perhaps due to the subduction of the young lithosphere generated by the EPR-PCS.     

Indenter-tectonics in the Philippines: Example from the Palawan Microcontinental Block - Philippine Mobile Belt Collision

Abstract. The aseismic Palawan microcontinental block is an oceanic bathymetric high that has collided with the seismically-ac-tive Philippine Mobile Belt since the Early Miocene. Consequently, tectonic microblocks immediately north (Luzon) and south (Western Visayas Block) of the collision front rotated in opposite senses. The rotation led the microblocks to onramp adjacent strike-slip faults, and converted these to subduction zones, namely, the current Manila and Negros Trenches. In addition, the collision also initiated the southward propagation of a major left-lateral strike slip fault, the Philippine Fault Zone, and the Philippine Trench, which bounds the Philippine archipelago along its eastern boundary. Based on onshore and offshore data, the Philippine Fault Zone and the East Luzon Trough - Philippine Trench appears to also propagate northward. Furthermore, the opposite direction of propagation is also noted for the Manila and Negros Trenches from the locus of the collision in the Central Philippines to their northern and southern extensions, respectively. The ages of initiation of the Manila Trench (Early Miocene), Philippine Fault Zone (Middle Miocene) and Philippine Trench (Pliocene) as encountered along a west to east transect in the Central Philippines are consistent with the collision and subsequent indentation of Palawan with the rest of the Philippine Mobile Belt.     

Structural characteristics controlling the seismicity crustal structure of southern Japan Trench fore-arc region, revealed by ocean bottom seismographic data

The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the Japanese islands. Many earthquakes occur associated with plate convergence, and the hypocenter distribution is variable along the Japan Trench. In order to investigate the detailed structure in the southern Japan Trench and to understand the variation of seismicity around the Japan Trench, a wide-angle seismic survey was conducted in the southern Japan Trench fore-arc region in 1998. Ocean bottom seismometers (15) were deployed on two seismic lines: one parallel to the trench axis and one perpendicular. Velocity structures along two seismic lines were determined by velocity modeling of travel time ray-tracing method. Results from the experiment show that the island arc Moho is 18–20 km in depth and consists of four layers: Tertiary and Cretaceous sedimentary rocks, island arc upper and lower crust. The uppermost mantle of the island arc (mantle wedge) extends to 110 km landward of the trench axis. The P-wave velocity of the mantle wedge is laterally heterogeneous: 7.4 km/s at the tip of the mantle wedge and 7.9 km/s below the coastline. An interplate layer is constrained in the subducting oceanic crust. The thickness of the interplate layer is about 1 km for a velocity of 4 km/s. Interplate layer at the plate boundary may cause weak interplate coupling and low seismicity near the trench axis. Low P-wave velocity mantle wedge is also consistent with weak interplate coupling. Thick interplate layer and heterogeneous P-wave velocity of mantle wedge may be associated with the variation of seismic activity.