The continental margin of Uruguay: Crustal architecture and segmentation

The Uruguayan continental margin comprises three sedimentary basins: the Punta del Este, Pelotas and Oriental del Plata basins, the genesis of which is related to the break-up of Gondwana and the opening of the Atlantic Ocean. Herein the continental margin of Uruguay is studied on the basis of 2D multichannel reflection seismic data, as well as gravity and magnetic surveys. As is typical of South Atlantic margins, the Uruguayan continental margin is of the volcanic rifted type. Large wedges of seaward-dipping reflectors (SDRs) are clearly recognizable in seismic sections. SDRs, flat-lying basalt flows, and a high-velocity lower crust (HVLC) form part of the transitional crust. The SDR sequence (subdivided into two wedges) has a maximum width of 85 km and is not continuous parallel to the margin, but is interrupted at the central portion of the Uruguayan margin. The oceanic crust is highly dissected by faults, which affect post-rift sediments. A depocenter over oceanic crust is reported (deepwater Pelotas Basin), and volcanic cones are observed in a few sections. The structure of continental crust-SDRs-flat flows-oceanic crust is reflected in the magnetic anomaly map. The positive free-air gravity anomaly is related to the shelf-break, while the most prominent positive magnetic anomaly is undoubtedly correlated to the landward edge of the SDR sequence. Given the attenuation, interruption and/or sinistral displacement of several features (most notably SDR sequence, magnetic anomalies and depocenters), we recognize a system of NW-SE trending transfer faults, here named Río de la Plata Transfer System (RPTS). Two tectono-structural segments separated by the RPTS can therefore be recognized in the Uruguayan continental margin: Segment I to the south and Segment II to the north.     

SALTATING PARTICLES,PLAYA CRUSTS AND DUST AEROSOLS AT OWENS (DRY) LAKE,CALIFORNIA

As part of the multinational Lake Owens Dust Experiment (LODE), we have studied the generation of dust storms on the south sand sheet of Owens (dry) Lake, California, an anthropogenically desiccated playa reported to be the single greatest source of particulate matter in North America. During March 1993, we performed an intensive field study including eight significant dust storms, building on our prior work (1978–1984) and preliminary studies (1991–1992). We studied sources and magnitude of coarse saltating particles, the meteorological conditions that allow them to become mobile across the flat playa of Owens (dry) Lake, and how the motion of saltating particles across different types of playa surfaces results in the generation of PM₁₀ dusts (aerosol particles smaller than 10 μm aerodynamic diameter). Saltating grains of lacustrine sand and broken crust abrade and disaggregate the playa surface into fine aerosols, and the resulting PM₁₀ concentrations recorded during major dust storms are among the highest ever recorded in North America. On 23 March 1993, we measured a 2 h concentration on the playa of 40 620 μg m⁻³, as far as we can determine the highest ambient PM₁₀ value ever recorded in the U.S.A. Abrasion of salt-silt-clay crusts by saltation is shown to be responsible for all but a small part of one dust storm. The quantity ‘sand run’, saltating particle transport multiplied by wind run, is shown to be very closely correlated with dust aerosol concentration. Finally, we have established that on-lake bed studies are essential for quantitative prediction of dust events on the Owens (dry) Lake bed, despite the difficult conditions encountered.     

Geological and geophysical investigation of the Mid-Cayman Spreading Centre: seismic velocity measurements and implications for the constitution of layer 3

Summary . In this paper we present laboratory measurements of compressional and shear wave velocities of a diverse suite of gabbroic rocks collected from the walls of the Mid-Cayman Spreading Centre with DSRV Alvin. The degree of deformation and alteration affecting these gabbros is quite variable, and we believe that they are typical of plutonic rocks emplaced at shallow levels (upper portion of seismic layer 3 and shallower) of the oceanic crust. The compositional and textural variations are reflected in the wide range of laboratory velocities which span most of the range of seismic velocities reported for oceanic and ophiolite rock samples including basalts, gabbros, ultramafics, and their altered derivatives. Based upon the laboratory velocities and the geological setting of the Mid-Cayman gabbros, it is argued that no unique lithology, except anhydrous peridotite, can be unequivocally identified in the oceanic lithosphere from seismic velocity data alone. Furthermore, these data allow for the possibility of considerable lithologic heterogeneity within portions of the oceanic crust at the scale of a few centimetres to a few hundred metres. Such heterogeneities would go unrecognized because seismic refraction studies mask these variations resulting in a picture of apparent uniformity.     

青藏高原航磁异常及居里面研究

在对青藏高原航磁异常及地质构造特征进行定性分析的基础上，利用频率域三维磁性层反演方法对青藏高原居里面埋深进行了定量计算。反演结果表明，青藏高原居里面埋深相对较浅，一般为20～23km，羊八井、那曲地热田附近居里面埋深14～18km。依据反演结果对该地段地震测深的强反射亮点群及大地电磁测深的低阻高导构造的性质进行了讨论，并对青藏高原的地震及地壳隆升机制进行了探讨。     

中国大陆现今垂直形变特征的初步探讨

利用GPS观测资料进行大范围的垂直形变特征研究是本文研究的重点。通过网络工程基准站和基本站的GPS观测资料，绘制了中国大陆的垂直速率等值线图像，并对中国大陆垂直运动特征进行了定性的分析，结果表明：中国大陆垂直运动在70％以上的区域表现为沉降运动，在青藏高原和除北京外的整个华北块体及燕山块体的垂直运动表现为隆升的趋势。中国大陆西北和东南、西南和东北的整体垂直运动呈一种斜对称状态。对基于GPS观测所得的等值线图与基于水观测所得的等值线图进行了比较，比较结果显示，除了华南块体外，二者在整体趋势上比较一致。     

Oceanic basalt continuous thermal demagnetization curves

The palaeomagnetic standard technique of stepwise thermal demagnetization (STD), long regarded as unreliable for oceanic basalts that have undergone low temperature alteration, has recently been applied in a number of studies to characterize the natural remanent magnetization (NRM) of such rocks. In order to better understand STD data of oceanic basalts, and to possibly identify the magnetominerals that are carrying the NRM, we have carried out a number of continuous and STD experiments on seven oceanic basalt samples. During continuous thermal demagnetization (CTD), a sample is heated to a certain temperature and its NRM is measured during heating and subsequent cooling. Even when CTD reveals only titanomaghemite unblocking at 400°C as the remanence carrier, STD behaviour can be very complex and unblocking is observed at temperatures of up to 500°C and higher. CTD also allowed to identify a partial or full self-reversal of NRM due to interaction between two types of magnetominerals in one sample. The higher degree of maghemitization of smaller titanomaghemite grains with respect to larger ones, which are less efficient in carrying the remanence, was seen for three samples by a shift of 80°C between the strong field thermomagnetic curve and the NRM measured at elevated temperature. In several cases, the identification of the NRM-carrying magnetomineral was not possible from CTD data due to the ambiguity of Curie temperatures in the titanomagnetite/titanomaghemite system.     

广西钦州那必锰矿地质特征及成因认识

研究认为,那必锰矿为次生淋滤及堆积型矿床,其产出特点为：矿体主要产于紫色含锰硅质岩段（D3l^3）中,并受破碎带及层间裂隙控制,似层状产出,沿倾斜方向,延伸一般不会超过35m.其富集规律为：铁、锰矿沉积关系密切,菱锰矿与褐铁矿共生,但在表生淋滤环境下两者往往分离,锰质淋移至深部,而铁质则富集于上部.因此,含锰褐铁矿矿体常产于锰矿体的上部,可成为锰矿找矿的一种地表直接标志.     

海州—大悟地区沉积变质磷矿成矿预测与远景评价

通过对海州-大悟地区含磷地层岩性特征、地层层序、形成时代、岩相古地理环境等方面的分析研究,认为本区磷矿层位和相关成矿条件可与鄂西陡山沱期磷矿对比。大别山北部也具备一定的成磷条件。预测沭阳滥洪-华冲、霍山驼岭-金寨、广济牛头山-黄梅塔儿畈、应山-广水为4个找矿远景乐观的地区。