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. 相似文献
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. 相似文献
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. 相似文献
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. 相似文献
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. 相似文献