Gravity field of Ascension Island,South Atlantic

A gravity survey of Ascension Island was carried out in 1964, a total of 210 stations being occupied. The results suggest that magma has reached the surface via three distinct conduits, the easternmost of which has been associated with extrusion of trachytic lavas. A regional decrease in gravity from west to east across the island is probably due to the proximity to the east of the crest of the Mid Atlantic Ridge, underlain by low density mantle.     

Stratigraphic constraints on suture models for eastern Indonesia

Although collision in eastern Indonesia is now accreting the Australian continent to Southeast Asia, the small North and South Banda oceanic basins within the suture zone are interpreted as Late Cenozoic extensional features. Stratigraphic columns from the surrounding islands conform to one of three generalised patterns, two of which can be related to the margins of SE Asia (Sundaland) and the Australian continent, respectively. The third system, which is dominant in the outer Banda Arc and eastern Sulawesi, is associated with a microcontinent that was rifted from Australia in the Jurassic, drifted northwards ahead of Australia in the Cretaceous and collided with the Sundaland Margin in the Paleogene. Subsequent collapse of the resulting collision orogen led to rapid extension and the formation of the Banda Sea behind the Outer Banda Arc thrust belt. Eastern Indonesia thus duplicates a pattern familiar in the Mediterranean. The Tertiary compressional structures of the region cannot be explained solely in terms of the most recent collision, which began only in the Pliocene.     

Zoogeography of marine Bryozoa around South Africa

The zoogeography of marine Bryozoa around South Africa was investigated using published distribution records, museum catalogues, and an examination of previously unworked bryozoan material in (mostly) museum collections. Although a total of 276 valid species are recognised, it was not possible to unambiguously assess geographic patterns of diversity. At all depth zones examined (shore and inner-shelf, 0–30 m; mid- and outer-shelf, 31–350 m; bathyal, >500 m), there was a clear geographic structure to communities that mirrored established regional patterns of biogeography. Too few samples were collected from the shelf edge (351–500 m) and they were consequently excluded from zoogeographic analysis. Communities on the shore and inner-shelf and on the mid- and outer-shelf were more similar to each other than they were to bathyal communities, and the pronounced structure in bathyal communities suggests heterogeneity in the deep sea around South Africa.     

Estimation of southern resident killer whale exposure to exhaust emissions from whale-watching vessels and potential adverse health effects and toxicity thresholds

Southern resident killer whales in British Columbia and Washington are exposed to heavy vessel traffic. This study investigates their exposure to exhaust gases from whale-watching vessels by using a simple dispersion model incorporating data on whale and vessel behavior, atmospheric conditions, and output of airborne pollutants from the whale-watching fleet based on emissions data from regulatory agencies.Our findings suggest that current whale-watching guidelines are usually effective in limiting pollutant exposure to levels at or just below those at which measurable adverse health effects would be expected in killer whales. However, safe pollutant levels are exceeded under worst-case conditions and certain average-case conditions. To reduce killer whale exposure to exhaust we recommend: vessels position on the downwind side of whales, a maximum of 20 whale-watching vessels should be within 800 m at any given time, viewing periods should be limited, and current whale-watch guidelines and laws should be enforced.     

火山岩风化壳储层发育模式——以三塘湖盆地马朗凹陷石炭系火山岩为例

中国的火山岩油气勘探近年来进展非常快,并不断在许多盆地发现了优质火山岩储层,其中风化壳型储层作为非常重要的火山岩储集体类型而倍受重视。三塘湖盆地马朗凹陷石炭系风化壳型储层储集空间以溶蚀孔洞缝为主,储层物性非常好。通过火山岩油气勘探中的野外露头观察、钻井取心、镜下薄片鉴定,及主量元素、微量元素等分析化验资料,并结合火山岩岩石学特征和物性特点,初步建立了该区的风化壳储层的发育模式,将火山岩风化壳储层在垂向上自上而下划分为五个带: ①最终分解产物带; ②水解带; ③淋滤带; ④崩解带; ⑤未风化带(母岩)。淋滤带储集物性最好,该区风化淋滤是改善储层的关键。     

The Vrancea seismic zone and its analogue in the Banda Arc, eastern Indonesia

It is now widely, although not universally, accepted that the Carpathian orogen marks the site of an arc–continent collision that followed the subduction of a now vanished small ocean basin. Seismic tomography has defined a high-velocity anomaly in the upper mantle similar to those associated with subduction zones worldwide. There is, however, no recognisable Wadati–Benioff Zone (WBZ), and intermediate-depth seismicity is confined to a relatively small, roughly cylindrical and vertically elongated region beneath the extreme southeastern corner of the mountain chain. There is no consensus in the published studies as to the origin of this ‘Vrancea Zone’.

The Banda Sea region of eastern Indonesia has sometimes been cited as an analogue for the Pannonian/Transylvanian basin and the enclosing Carpathian orocline, but at first sight the patterns of seismicity appear very different. Intermediate depth seismic activity defines a subducted slab that dips north, south and west beneath the Banda Sea, a configuration explained as a consequence of the rapid expansion of the sea during roll-back subduction. If the similar scenario proposed for the Carpathians is correct, then it is the absence of a Carpathian WBZ that is actually anomalous. Closer examination of Banda Arc seismicity shows that it can be divided into two parts, these being a scoop-shaped WBZ and an adjacent ‘Damar Zone’ of much more intense intermediate-depth activity. At its eastern end the Damar Zone merges with the WBZ, but in the west there is evidence for separation from it. A plausible explanation of this pattern is that a lower layer of the downgoing slab is peeling away from the remainder.

The Banda/Australia collision is now almost complete and the activity in the WBZ proper can be expected to decrease. Damar Zone activity, on the other hand, may persist for a much longer period, migrating towards the foreland as the detaching layer separates from the remainder of the subducted lithosphere. In a few million years the seismicity of the Banda region could well resemble the present day seismicity of the Carpathian orogen.     

贺兰山北段牛头沟金矿区围岩的原岩恢复、时代及其地质意义

牛头沟金矿区位于华北陆块鄂尔多斯地块西缘贺兰山北段之基底杂岩带,赋矿地层为一套古元古界宗别立群第二亚群(Pt1Z2)中-深程度变质岩系,主要岩石类型是黑云斜长片麻岩和变粒岩。岩石地球化学特征分析及原岩恢复说明,蚀变岩型金矿体的围岩由正、副变质岩类共同组成,其中变粒岩的原岩为沉积碎屑岩,黑云斜长片麻岩的原岩为花岗闪长质侵入岩。离子探针锆石U-Pb定年表明,表壳岩的形成时代小于2120Ma,花岗闪长岩锆石U-Pb年龄为1950±8.9Ma,辉绿岩脉的侵入与花岗闪长岩属同期。综合研究表明,矿区变质岩的原岩形成时代不是前人认为的晚太古代,而是古元古代。同时应将属于正变质岩类黑云斜长片麻岩(原岩为花岗闪长岩)从宗别立群(Pt1Z)地层中分离出来,作为侵入岩单元考虑。