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991.
M. E. Dickson 《The Australian geographer》2004,35(2):223-238
Lord Howe Island is a small eroded remnant of a Late Miocene shield volcano. A fringing coral reef dissipates wave energy along a portion of the shoreline, but the remainder of the coast is rugged with spectacular high basaltic sea cliffs. This paper investigates the evolution of talus slopes that occur beneath the loftiest cliffs, and places this analysis within the context of a longer history of island planation that has resulted in a wide truncated shelf around the island. During the Last Glacial, when the sea level was lower than at present, talus slopes accumulated around the extent of the island's cliffed coast because material eroded from cliffs by subaerial processes could not be removed by marine processes. The survival of these slopes during the Holocene has depended on a balance achieved between rates of subaerial and marine erosion. This balance is fundamentally influenced by cliff height, as cliffs higher than 200 m are plunging or veneered by talus slopes, whereas lower cliffs have erosional shore platforms. On comparison with published erosion rates from inland basalt scarps it appears that marine processes may account for over 90 per cent of the total cliff retreat that has occurred at Lord Howe Island, yet contemporary coastal morphology attests to the significance of subaerial processes in recent times. It is likely that marine cliffing was very rapid soon after volcanism ceased, but rates of erosion decreased through time as wave energy became increasingly attenuated across a widening planation surface, and as increasing cliff heights yielded greater quantities of talus that provided protection from rapid marine erosion. 相似文献
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四川盆地潜在震源区的细致划分 总被引:2,自引:0,他引:2
四川盆地包括川中台拱和川东陷褶束的范围,具有相对稳定的大地构造环境.鉴于该地区地震活动水平相对较低、发震构造不甚明确的特点,在编制1990年版中国地震烈度区划图时,划分的潜在震源区面积也相对较大.本文在总结四川盆地中强地震(Ms=4.7-5.7)发震构造条件的基础上,用地震构造类比原则对四川盆地的潜在震源区进行了重新划分.确定潜在震源区边界时,充分考虑了背斜轴走向、背斜构造的影响带宽度、小震活动密集条带和余震的分布范围.得到了四川盆地更加细致的潜在震源区划分方案.这对于低地震活动水平地区的潜在震源区的细化研究具有一定参考意义。 相似文献
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Abstract Calcretes can be observed on the surface of old moraines around Batura Glacier in the upper Hunza Valley, Karakoram Mountains, Pakistan. They develop as a calcareous crust cementing small gravels under boulders. In order to understand the genesis of the calcrete crust, a variety of methods were employed: (i) study of mineralogy and geochemistry of a calcrete crust precipitated on the lateral moraine using X-ray diffractometer and electron probe microanalysis; (ii) analysis of solute chemistry of surface water and ice bodies around the Batura Glacier; and (iii) accelerator mass spectrometry 14 C dating of the crust itself. The results indicate that the calcrete crust has definite laminated layers composed of a fine-grain and compact calcite layer, and a mineral fragment layer. The chemical composition of the calcite layer is approximately 60% CaO and 1% MgO. The mineral fragment layer consists of rounded grain materials up to 0.2 mm in diameter. It shows a graded bedding structure with fine grains of quartz, albite and muscovite. Meanwhile, as the Paleozoic Pasu limestone is distributed around the terminal of Batura Glacier, Ca cations dissolve in the melt water of the glacier. Accordingly, the calcrete crust is precipitated by decreases in CO2 partial pressure from glacier ice and evaporation of the melt water, including high concentration of Ca2+ at ephemeral streams and small ponds stagnating between the moraine and glacial ice. On the basis of the AMS 14 C age, the calcrete is considered to have formed approximately 8200 calibrated years bp under the Batura glacial stage. 相似文献
1000.
A detailed receiver function image of the sedimentary structure in the Bohai Bay Basin 总被引:5,自引:0,他引:5
Tianyu Zheng 《Physics of the Earth and Planetary Interiors》2005,152(3):129-143
If the site at which receiver functions are constructed is filled by sediments, then the waveforms from these receiver functions are dominantly controlled by the sedimentary structures within the first few seconds after the direct P arrival. Based on this observation, waveform data collected at 44 temporary seismic stations have been used to image the sedimentary structure of the Bohai Bay Basin, a major continental petroliferous basin in Eastern China. An adapted hybrid global waveform inversion method was applied to the receiver functions to extract structural information beneath each of the stations. The derived S-velocity structure provides for the first time, a basin-scale seismic image of detailed sedimentary stratification. The sedimentary cover of the basin is about 2-12 km thick, consisting of Cenozoic, Mesozoic, and Paleozoic strata from top to bottom. The structural features presented in the S-velocity image coincide quite well with the depression-uplift type of tectonic system in the Bohai Bay Basin. The reconstructed morphology of the sedimentary layers provides seismological evidence for the two-stage evolution of the intracontinental basin that were caused by an intensive tectonic regime transition in late Mesozoic immediately following the lithospheric reforming of the Eastern China continent. 相似文献