南海海盆中南—礼乐断裂带研究进展*

文章首先论述了中南—礼乐断裂带的研究现状, 然后基于重力、磁力、地震剖面和地形等地球物理资料, 综合分析了中南—礼乐断裂带在南海海盆中的空间展布和内部构造形变特征。研究表明: 该断裂带在海盆中由北至南具有明显的分段性。北段(西北次海盆与东部次海盆北部之间)断裂带宽15km, 由(18°00'N, 115°30'E)向(17°30'N, 116°00'E)呈NNW向分布。南段(西南次海盆与东部次海盆之间)断裂带宽约60~80km, 由中沙海台东侧向礼乐地块西侧呈NNW向展布。中南—礼乐断裂带的主控断裂沿中南海岭呈NNW向分布。断裂带在南北两段的过渡区总体呈NNE向展布。断裂带两侧海盆的沉积厚度和洋壳厚度存在差异, 推断该断裂带对其东西两侧海盆的地质构造具有控制作用。根据地壳结构变化, 推测该断裂带至少是一条地壳级断裂。     

红河断裂带及其邻区的震源机制解特征及其反映的断裂活动分段性

对红河断裂带及其邻区219个地震的相关数据进行震源机制解分析,阐述了红河断裂带不同区段的地震分布特征及其地震类型的差异性,结合对研究区区域深部动力学条件的分析,从地震发生及其深部动力学特征分析红河断裂带活动的分段性特征,取得如下新认识:(1)红河断裂带北西段由于受印-藏碰撞影响而显现出挤压应力场特征,断裂活动具有逆断特征和局部拉张应力场下的正断特征;(2)中段作为华南亚板块与印支亚板块之间的主体剪切活动带,显现剪切应力场特征,断裂以剪切活动为主;(3)南东段在断裂带右旋走滑的基础上,受到深部物质抬升、岩石圈拉伸减薄的影响,而表现出张扭应力场性质,断裂活动显现张扭特征.     

南海北部晚中生代逆冲断裂带厘定与构造转换

古南海的构造演化对研究大陆边缘张裂和盆地形成模式有重大意义。关于南海的构造演化过程尚存有争议,基于对前人研究成果的收集及综合比对,同时结合地震剖面分析,对逆断层和褶皱等挤压构造进行识别以及展布的刻画;并对南海北部古逆冲断裂带的展布及活动进行了深入研究。在确定晚中生代南海北部区域构造应力背景的基础上,在研究区内识别出了一条中生代逆冲断裂带,并对其位置、展布、活动停止时间进行了分析,对南海晚中生代以来的构造演化提出了新的模式:晚中生代以前,南海北部地区构造应力以挤压为主,存在着碰撞造山过程,形成了岛弧型活动大陆边缘;晚白垩世至始新世,盆地应力背景由挤压向张裂过渡,并存在至西向东的转换延迟;渐新世以后逐步转为拉张应力背景。     

台湾海峡滨海断裂带地质构造特征

为深入研究滨海断裂带的地质构造特征,收集、整理了台湾海峡西部的地质、构造和地球物理等相关资料并进行综合分析和研究。结果显示:(1)牛山岛、兄弟屿、南澎列岛3个关键海岛上的地质和线性构造直接反映了滨海断裂带的产状、性质、分段分布及区域变化等特征;(2)台湾海峡西部存在3条主要断裂带,其中F1为滨海断裂带,属于浙闽隆起上隆中凹陷的西界控凹断裂,F2断裂带为浙闽隆起上隆中凹陷的东界控凹断裂,F3断裂带为台西盆地西界控盆断裂,3条断裂带及其内部发育的正断层组合整体组成了海峡西部断裂系,控制了台湾海峡西部区域隆、坳格局的形成;(3)滨海断裂带和海峡西部断裂系被4条NW向区域断裂带自北向南切割为5段,即平潭外海段、泉州外海段、厦门外海段、东山外海段和南澳外海段。     

哀牢山-红河断裂带中段应变分析

对哀牢山—红河断裂带中段和平—水塘剖面、墨江—元江剖面和其它地段的岩石应变及磁组构进行了分析，表明应变强度的校正磁各向异性度PJ从断裂带向西至三叠系明显降低，变形强度向西迅速减弱。磁化率椭球体主轴展布反映出剪切带内、外变形方式的改变。剪切带内，以水平走滑运动为主；向西则以水平缩短为主。在应变分析中，对断裂带内的S—C组构两组面理夹角、杏仁体和石榴石应变标志体进行了测量，结果显示，剪切带内的磁组构和岩石组构间关系较差。此外，还对这两条剖面中的三叠系进行了应变和磁组构分析，找出了它们与磁化率椭球体对应轴率间的相关性。     

红河断裂带中南段上新世以来构造隆升及成因

在数字高程模型(DEM)数据处理和前人磷灰石裂变径迹测年(AFT)数据的基础上,分析和识别了红河断裂带中南段(弥渡至河口段)夷平面和河流阶地的展布特征,对断裂带两盘的构造地貌进行了定量、半定量研究。研究表明,弥渡-元江段、红河-河口段南西盘夷平面高于北东盘150~840m,元江-红河段北东盘夷平面高于南西盘140~230m。红河断裂带中南段经历了7.36~11.9、3.6~4.9、1.6~2.5Ma 3个主要构造活动期次。估算了元江-红河段上新世晚期以来的隆升速率,南西盘为1.38~1.53mm/a,北东盘为1.46~1.59mm/a。红河断裂带元江-红河段北东盘处于小江断裂带和红河断裂带交汇区,其构造隆升是后二者共同作用的结果。     

营口-潍坊断裂带新生代运动学特征

营口-潍坊断裂带在新生代时期对渤海湾盆地东部构造、沉积及油气成藏规律等方面具有重要影响和控制作用.依据大量实际地震资料,从剖面上识别出了花状构造、丝带效应、反转构造等营口-潍坊断裂带新生代时期走滑活动的标志,在平面上识别出了4种组合构造样式,在此基础上,应用拉分盆地走滑量计算的理论模型,对营口-潍坊断裂带新生代时期的走滑位移量进行了初步估算,认为营口-潍坊断裂带在新生代时期具右旋走滑活动特征,其主要走滑活动表现为3期,分别为始新世早期、渐新世早-中期和上新世晚期-第四纪,新生代累计右旋走滑位移量约为10～20 km.     

印度-欧亚板块碰撞期间红河断裂带活动性的数值模拟研究

红河断裂带是一巨型走滑断裂带,在印度板块与欧亚板块碰撞过程中成为印支地块和华南地块之间相互运动的主位移带。通过有限差分数值模拟试验,结果表明,在碰撞期间,红河断裂带首先经历了左旋走滑运动,红河断裂带中央部分的左旋偏移量最大,达到约325km,印支地块向南东挤出,在5Ma BP后表现为右旋走滑,说明沿红河断裂带的剪切运动是红河断裂带西端与印度-欧亚碰撞边界之间距离的函数,随时间而减小并控制了印支地块与华南地块之间的相对运动。     

西菲律宾海中央断裂带地貌学研究

西菲律宾海中央断裂带位于西菲律宾海中部,为一ＮＷＷ—ＳＥＥ向伸展的长条形狭窄地带,东南面可延伸至帛琉海岭。因受强烈的构造断裂切割,海底地形崎岖,山顶至谷底之差达几百至１０００ｍ以上,长条形相间平行排列的海脊、海岭、高地和裂谷、洼地呈ＮＷＷ—ＳＥＥ向。其中的中央裂谷（也称海渊）位于断裂带的东南段,横断面呈“Ｖ”型,裂谷可被５５００ｍ的等深线圈闭,最大水深大于７０００ｍ,其相对深度达１０００～３０００ｍ。平行中央裂谷两侧的两条海脊是两条狭窄的高热流条带。中央断裂带的裂谷和海脊地貌是由于西北太平洋板块运动方向改变、受ＮＮＥ—ＳＳＷ向的张力作用所致,中央裂谷为一古扩张中心。     

梨树断陷断陷期小宽断裂带构造应力场数值模拟

在小宽断裂带构造解析基础之上,根据地质资料以及前人提供的岩石物理学参数建立了梨树断陷区的二维地质和数值模型。进行了构造应力场数值模拟,分析了梨树断陷小宽断裂带断陷期小宽断裂带断陷期在沙河子期(K_1sh)、营城期(K_1yc)、登楼库期(K_1d)的构造应力场特征。模拟结果显示:(1)沙河子期在左旋走滑应力状态下,小宽断裂带处于近NE—SW方向的拉张应力状态,剪应力在沿着断裂带SW—NE走向逐渐减小;(2)营城期在右旋走滑的应力状态下,小宽断裂带处于近NW—SE向的拉张力状态,剪应力最大值位于断裂带的最中间部位;(3)登楼库期在左旋走滑的应力状态下,断裂带处于SWW—NEE向的张拉应力状态,剪应力最大值位于断裂带西南段。     

Kane Fracture Zone

The Kane Fracture Zone probably is better covered by geophysical survey data, acquired both by design and incidentally, than any other fracture zone in the North Atlantic Ocean. We have used this data to map the basement morphology of the fracture zone and the adjacent crust for nearly 5700 km, from near Cape Hatteras to the middle of the Mesozoic magnetic anomalies west of Cap Blanc, northwest Africa. We use the trends of the Kane transform valley and its inactive fracture valley to determine the record of plate-motion changes, and we interpret the basement structural data to examine how the Kane transform evolved in response to changes in plate motion. Prior to about 133 Ma the Kane was a small-offset transform and its fracture valley is structurally expressed only as a shallow ( < 0.5 km) trough. In younger crust, the offset may have increased to as much as 190 km (present offset 150 km) and the fracture valley typically is up to 1.2 km deep. This part of the fracture valley records significant changes in direction of relative plate motion (5°–30°) near 102 Ma, 92 Ma, 59 Ma, 22 Ma, and 17 Ma. Each change corresponds to a major reorganization of plate boundaries in areas around the Atlantic, and the fracture-zone orientation appears to be a sensitive recorder of these events. The Kane transform has exhibited characteristic responses to changes in relative plate motion. Counterclockwise plate-motion changes put the left-lateral transform offset into extension, and the response was for ridge tips at the ridge-transform intersections to propagate across the transform valley and against the truncating lithosphere. Heating of this lithosphere appears to have produced uplift and formation of a well developed transverse ridge that bounds the inactive fracture valley on its older side. The propagating ridge tips also rotated toward the transform fault in response to the local stress field, forming prominent hooked ridges that now extend into or across the inactive fracture valley. Clockwise (compressional) changes in relative plate motion produced none of these features, and the resulting fracture valleys typically have a wide-V shape. The Kane transform experienced severe adaptions to the changes in relative plate motion at about 102 Ma (compressional shift) and 92 Ma (extensional shift), and new transform faults were formed in crust outside the contemporary transform valley. Subsequently, the transform offset has been smaller and the rates of change in plate motion have been more gradual, so transform-fault adjustment has been contained within the transform valley. The fracture-valley structure formed during extensional and compressional changes in relative plate motion can be decidedly asymmetrical in conjugate limbs of the fracture zone. This asymmetry appears to be related to the ‘absolute’ motion of the plate boundary with respect to the asthenosphere.     

Oceanic crust thickens approaching the Clipperton Fracture Zone

A multi-channel seismic reflection image shows the reflection Moho dipping toward the Clipperton Fracture Zone in crust 1.4 my old. This seismic line crosses the fracture zone at its eastern intersection with the East Pacific Rise. The seismic observations are made in travel time, not depth. To establish constraints on crustal structure despite the absence of direct velocity determinations in this region, the possible effects of temperature, tectonism, and anomalous lithospheric structure have been considered. Conductive, advective, and frictional heating of the old crust proximal to the ridge-transform intersection can explain <20% of the observed travel-time increase. Heating has a negligible effect on crustal seismic velocity beyond ~10 km from the ridge tip. The transform tectonized zone extends only 6 km from the ridge tip. Serpentinization is unlikely to have thickened the seafloor-to-reflection Moho section in this case. It is concluded that, contrary to conventional wisdom, the 1.4 my old Cocos Plate crust thickens approaching the eastern Clipperton Ridge-Transform Intersection. Increase in thickness must be at least 0.9 km between 22 and 3 km from the fracture zone.     

Fossil spreading center and faults within the Panama Fracture Zone

The north/south-trending Panama Fracture Zone forms the present eastern boundary of the Cocos Plate, with the interplate motion being right-lateral strike-slip. This fracture zone is composed of at least four linear troughs some hundreds of kilometers in length. Separate active or historic faults undoubtedly coincide with each trough. The greatest sediment fill is found in the easternmost trough. Surface and basement depths of the western trough are generally greater than those of the other three; the western trough contains the least sediment, and is most continually linear. Morphology and sediments suggest that the principal locus of strike-slip movement within the fracture zone probably migrated incrementally westward from one fault-trough to another. From north to south, the fracture zone apparently narrows from the continental intersection to approximately 5°30N, and again widens from about 5°N to at least 3°N. Residual E/W-trending magnetic anomalies are centered between two of the four troughs; sea floor spreading in a north-south direction is interpreted to have occurred between 5°30N and 7°N from 4.5 m.y. ago to 2 m.y. ago, with the symmetric center roughly coinciding with a rift valley at 6°10N, 82°30W.     

Paleocurrents in the Charlie-Gibbs Fracture Zone during the Late Quaternary

Planktonic foraminiferal and ice-rafted debris count data, as well as the mean size of mineral particles of the 10–63 μm fraction (sortable silt, \(\overline {SS} \)) were used as a proxy for surface and near-bottom paleocurrent intensity variations. The data obtained support our hypothesis about turbiditic origin of the lower (80–370 cm) section of the studied AMK-4515 core. Stratigraphic subdivision of the upper section (0–80 cm) makes it possible to allocate two marine isotope stages (MIS) covering the last 27 ka. The main intervals of the North Atlantic Polar Front (PF) migrations were recorded: south of the modern PF position during early MIS 2 (24–27 ka) with PF presence in the study area during MIS 2 (20–24 ka); south of the study area during the last glacial maximum (18–20 ka). Influence of the near-bottom currents within the investigated interval led to beginning of the channel-related drift formation on the northern slope of the southern channel of the Charlie-Gibbs Fracture Zone. There is a weak relationship between intensity of near-bottom contour currents and long-term climatic cyclicity. However, intervals corresponding to Heinrich events coincide with decrease in bottom currents activity.     

Magnetic study of serpentinized harzburgites from the Islas Orcadas Fracture Zone

Magnetic properties and bulk densities of 27 serpentinized harzburgite samples from the Islas Orcadas Fracture Zone, located in the vicinity of the Bouvet Triple Junction, have been measured and analyzed. Polished sections were examined using reflected light and scanning electron microscopy to characterize the size and geometric arrangement of opaque minerals. The relationship between the saturation magnetization (I_S) and remanent coercive force (H_R) is considered in terms of the amount of ferrimagnetic material and maghemitization. A suite of continental serpentinites from Canada is offered as contrast, to consider the role of weathering and maghemitization. Magnetite in the Islas Orcadas serpentinites is variably maghemitized, whereas continental serpentinites do not appear to contain maghemitized oxides. We verify this with optical microscopy, thermomagnetic analyses and cryogenic temperature cycling of saturation remanence. Maghemitization serves to reduce initial magnetic susceptibility, and introduce error in the use of I_S to evaluate the magnetic mode of magnetite. The presence of maghemite and the existence of a three dimensional vein network for magnetite geometry would suggest that magnetic hysteresis parameters can not reliably indicate grain size. Magnetic hysteresis ratios fall in a restricted range regardless of coercivity. The apparent grain size configured in a three dimensional vein network plus maghemitization might be responsible for this observation. Maghemitization does not affect thermal magnetic stability and enhances the geophysical importance of remanence in serpentinites. Paleomagnetic data suggest that important information about the geologic circumstances for oceanic rock serpentinization is embodied in the paleomagnetic records. This observation may be very important for generation of long wavelength aeromagnetic and possibly even satellite magnetic anomalies.     

The morphology and structure of the West O’Gorman Fracture Zone

The West O’Gorman Fracture Zone is an unusual feature that lies between the Mathematician Ridge and the East Pacific Rise on crust generated on the East Pacific Rise between 4 and 9 million years ago. We made a reconnaissance gravity, magnetic and Sea Beam study of the zone with particular emphasis on its eastern (youngest) portion. That region is characterized by an elongate main trough, a prominent median ridge and other, smaller ridges and troughs. The structure has the appearance of large-offset fracture zone, possibly in a slow spreading environment. However, magnetic anomalies indicate that the offset, if any, is quite small, and the spreading rate during formation was fast. In addition, the magnetic profiles do not support earlier models for a difference in spreading rate north and south of the fracture. The morphology of the fracture zone suggests that flexure may be responsible for some of the topography; but gravity studies indicate some of the most prominent features of the fracture zone are at least partially compensated. The main trough is underlain by a thin crust (or high density body), similar to large-offset fracture zones in the Atlantic, while the median ridge is underlain by a thickened crust. Sea Beam data does not unambiguously resolve between volcanism or serpentinization of the upper mantle as a mechanism for isostatic compensation. Why the West O’Gorman exists remains enigmatic, but we speculate that the topographic expression of a fracture zone does not require a transform offset during formation. Perhaps the spreading ridge was magma starved for some reason, resulting in a thin crust that allowed water to penetrate and serpentinize portions of the upper mantle.     

东太平洋克拉里昂-克利帕顿断裂区多金属结核保留区的气象状况和预报规律

为保障我国大洋矿产资源的勘探和开采,文章根据我国于2017年对东太平洋克拉里昂-克利帕顿断裂区多金属结核保留区开展首次调查的志愿船观测数据,结合实际天气和预报过程,在对观测数据进行处理和分析的基础上,总结调查期间该保留区的气象状况和预报规律。研究结果表明:受冷高压及其外围的影响,保留区风向和风速变化较大,在预报过程中须实时关注冷空气的中心位置、走向和持续时间,适时调整作业方案;保留区气压与天气系统吻合较好,气温和湿度受降水影响,且降水频繁;除10月可能受到飓风影响外,保留区11月风速通常比10月大。     

Depth to basement and geoid expression of the Kane Fracture Zone: A comparison

Geoid data from Geosat and subsatellite basement depth profiles of the Kane Fracture Zone in the central North Atlantic were used to examine the correlation between the short-wavelength geoid (=25–100 km) and the uncompensated basement topography. The processing technique we apply allows the stacking of geoid profiles, although each repeat cycle has an unknown long-wavelength bias. We first formed the derivative of individual profiles, stacked up to 22 repeat cycles, and then integrated the average-slope profile to reconstruct the geoid height. The stacked, filtered geoid profiles have a noise level of about 7 mm in geoid height. The subsatellite basement topography was obtained from a recent compilation of structure contours on basement along the entire length of the Kane Fracture Zone. The ratio of geoid height to topography over the Kane Fracture Zone valley decreases from about 20–25 cm km^-1 over young ocean crust to 5–0 cm km^-1 over ocean crust older than 140 Ma. Both geoid and basement depth of profiles were projected perpendicular to the Kane Fracture Zone, resampled at equal intervals and then cross correlated. The cross correlation shows that the short-wavelength geoid height is well correlated with the basement topography. For 33 of the 37 examined pro-files, the horizontal mismatches are 10 km or less with an average mismatch of about 5 km. This correlation is quite good considering that the average width of the Kane Fracture Zone valley at median depth is 10–15 km. The remaining four profiles either cross the transverse ridge just east of the active Kane transform zone or overlie old crust of the M-anomaly sequence. The mismatch over the transverse ridge probably is related to a crustal density anomaly. The relatively poor correlation of geoid and basement depth in profiles of ocean crust older than 130–140 Ma reflects poor basement-depth control along subsatellite tracks.     

Submersible observations of Equatorial Atlantic mantle: The St. Paul Fracture Zone region

The St. Paul F.Z. is a large structural domain made up of multiple transform faults interrupted by several Intra-Transform Ridge (ITR) spreading segments. Two regions were studied in details by submersible: (1) The ITR short (<20 km in length) segment near 0° 37N–25° 27W and 1° N–27° 42W and (2) The St. Peter and St. Paul's Rocks (SPPR) massif located at 29° 25W (¡3700 m depth). (1) The short ITR segments consist of a magma starved rift valley with recent volcanic activities at 4700 m depth. A geological profile made along the rift valley wall showed localized volcanics (basalts and dykes) which are believed to overlay and intrude the ultramafics. The geological setting and the high ultramafic/volcanic ratio suggest an extremely low magmatic supply and crustal-mantle uplift during lithospheric stretching and denudation. (2) The St. Peter and St. Paul's Rocks (SPPR) massif consists of a sigmoidal ridge within the active transform zone. The SPPR is divided into two different geological domains called the North and the South Ridges. The North Ridge consists of strongly tectonized fault scarps composed of banded and mylonitized peridotite, sporadic gabbros (3900–2500 m) and metabasalts (2700–1700 m). The South Ridge is less tectonized with undeformed, serpentinized spinel lherzolite (2000–1400 m) and basalts. Extensional motion and denudation accompanied by diapirism affected the South Ridge within a transform domain. Instead, the North Ridge was formed during an important strike-slip and faulting motion resulting in the uplifted portion of the St. Paul F.Z. transverse ridge. There is a regional compositional variation of the volcanics where E-MORBs and alkali basalts are produced on the SPPR massif and are comparable to the adjacent northern segments of the Mid-Atlantic Ridge. On the other hand, N and T- MORBs collected from the eastern part of the St. Paul F.Z. (25° 27W IRT) are similar to the volcanics from the southern segments of the MAR. The peridotites exposed in these provinces (SPPR and ITR) are similar in their REE and trace element distribution. Different degrees (3–15%) of partial melting of a mixed composite mantle consisting of spinel and amphibole bearing lherzolite veined with 5–40% clinopyroxenite gave rise to the observed MORBs and alkali basalts.     

Metalliferous Sediment and a Silica-Hematite Deposit within the Blanco Fracture Zone,Northeast Pacific

A Tiburon ROV dive within the East Blanco Depression (EBD) increased the mapped extent of a known hydrothermal field by an order of magnitude. In addition, a unique opal-CT (cristobalite-tridymite)-hematite mound was discovered, and mineralized sediments and rock were collected and analyzed. Silica-hematite mounds have not previously been found on the deep ocean floor. The light-weight rock of the porous mound consists predominantly of opal-CT and hematite filaments, rods, and strands, and averages 77.8% SiO₂ and 11.8% Fe₂O₃. The hematite and opal-CT precipitated from a low-temperature (≥ 115° C), strongly oxidized, silica- and iron-rich, sulfur-poor hydrothermal fluid; a bacterial mat provided the framework for precipitation.

Samples collected from a volcaniclastic rock outcrop consist primarily of quartz with lesser plagioclase, smectite, pyroxene, and sulfides; SiO₂ content averages 72.5%. Formation of these quartz-rich samples is best explained by cooling in an up-flow zone of silica-rich hydrothermal fluids within a low permeability system. Opal-A, opal-CT, and quartz mineralization found in different places within the EBD hydrothermal field likely reflects decreasing silica saturation and increasing temperature of the mineralizing fluid with increasing silica crystallinity.

Six push cores recovered gravel, coarse sand, and mud mineralized variously by Fe or Mn oxides, silica, and sulfides. Total rare-earth element concentrations are low for both the rock and push core samples. Ce and Eu anomalies reflect high and low temperature hydrothermal components and detrital phases.

A remarkable variety of types of mineralization occur within the EBD field, yet a consistent suite of elements is enriched (relative to basalt and unmineralized cores) in all samples analyzed: Ag, Au, S, Mo, Hg, As, Sb, Sr, and U; most samples are also enriched in Cu, Pb, Cd, and Zn. On the basis of these element enrichments, the EBD hydrothermal field might best be described as a base- and precious-metal-bearing, silica-Fe-oxide-barite deposit. Such deposits are commonly spatially and temporally associated with volcanogenic massive sulfide (VMS) ores. A plot of data for pathfinder elements shows a large hot spot at the northwestern margin of the field, which may mark a region where moderate to high temperature sulfide deposits are forming at depth; further exploration of the hydrothermal field to the northwest is warranted.