The Himalayas

After splitting from Gondwanaland, India drifted northwards to collide with the Asian landmass about 40 million years ago. The intervening Tethys ocean was closed by northwards subduction beneath southern Tibet, and the collision created the Himalayan orogenic belt. Continuing northward movement of India at a rate of about 5 cm per year over the last 40 million years has caused it to indent Asia, and the resultant massive shortening is expressed by thrusting of the northern margin of India, by faulting and earthquakes in the Himalayas and China, by rifting and faulting in Tibet, and by the uplift of the Himalayas which is still continuing at rates of up to several millimetres per year.     

URBAN DEVELOPMENT AND REDEVELOPMENT IN SAN FRANCISCO*

ABSTRACT. San Francisco has arisen as an “instant city” not once but three times: after the California Gold Rush (1849–1875), following the 1906 earthquake and fire (1906–1930), and with the modern high-rise city (1960–1985). These pivotal periods of intense city building, each about twenty-five years in duration, have been separated by a generation of relative stability. Current redevelopment activity raises an intriguing historical-geographical question: Is a fourth instant city in the making?     

Pellets, ooids, sepiolite and silica in three calcretes of the southwestern United States

Pellets and ooids are widespread and locally abundant in mature calcrete profiles in the Argus Range, California; near Wickieup, Arizona; and in Kyle Canyon, Nevada. Most concentrations of pellets and ooids either overlie laminar calcrete at various levels in the calcrete profile or fill subhorizontal fractures in the petrocalcic horizon. In all three profiles the petrocalcic horizon has been thickened by the pelletal, chemically deposited fracture fillings. Pellets range from 0.02 to 8.0 mm in diameter and consist principally of micritic calcite and sepiolite. Ooid coatings are chiefly calcite and opal or calcite and sepiolite. The pellets represent small concretions, some of which grew by accretion, either in void space or by displacing adjacent sediment, and the others of which were formed by cementation of pellet-shaped bodies of porous micrite. Ooid coatings with opal or sepiolite may have been deposited as a gel with sufficient strength for surface tension to thin the coatings over angular corners of nuclei so as to increase the roundness and sphericity of the particles. Major problems in calcrete genesis are (1) the cause of subhorizontal fractures and the mechanism for widening a fracture as sediment accumulates in it and (2) what determines the deposition of calcite, sepiolite, and opal as pellets and ooid coatings or as laminar layers.     

Late-glacial marine erosion in Scotland

A pronouaced raised reck platform and cliff that occur on part of the west coast of Scotland ale generally considered to have been formed before the last glaciation. It is argued in this paper that they were formed in late-glacial times, mainly during the Loch Lomond (Upper Dryas) Readvance. The features arc correlated with an extensive buried and/or submerged marine erosion feature in South-East Scotland. It is also suggested that the Scottish features correlate with the 'Main line' (P₁₂ shoreline) of northern Norway.     

A box model for non-entraining, suspension-driven gravity surges on horizontal surfaces

The propagation of and the deposition from a turbulent gravity current generated by the release of a finite volume of a dense particle suspension is described by a box model. The approximate model consists of a set of simple equations, a predetermined, depth-dependent leading boundary condition and one experimentally determined parameter describing the trailing boundary condition. It yields predictions that agree well with existing laboratory observations and more complex theoretical models of non-eroding, non-entraining, suspension-driven flows on horizontal surfaces. The essential features of gravity-surge behaviour have been observed and are captured accurately by the box model. These include the increased rate of downstream loss of flow momentum with increased particle setting velocity, the existence of maxima in the thickness of proximal deposits, and the downstream thinning of distal deposits. Our approximation for the final run-out distance, x_r, of a surge in deep water is given by x_r3(g'_oq³_o/w²_s)^1/5, where g'_o is the initial reduced gravity of the surge, q_o the initial two-dimensional volume, and w_s the average settling velocity of the particles in the suspension. A characteristic thickness of the resulting deposit is given by φ_oq_o/x_r'where ø_o is the initial volumetric fraction of sediment suspended in the surge. Our analysis provides additional insight into other features of gravity-surge dynamics and deposits, including the potential for the thickening of currents with time, the maintenance of inertial conditions and the potential for strong feedback in the sorting of particle sizes in the downstream direction at travel distances approaching x_r. Box-model approximations for the evolution of gravity surges thus provide a useful starting point for analyses of some naturally occurring turbidity surges and their deposits.     

Late Quaternary mass movement on the lower continental rise and abyssal plain off Western Sahara

The late Quaternary development of part of the lower continental rise off Western Sahara has been determined from an investigation of short (< 2 m) gravity cores collected from a deep-sea channel, the interchannel areas and an abyssal hill, between 30 and 33°N. Stratigraphic analysis is based on systematic variations in abundances of particular coccolith species and pelagic sediment types, referenced to the oxygen isotope time-scale. During the last 73 000 years deposition in the channel has included volcaniclastic sand/silt turbidites and minor marl turbidites as well as pelagic sediments. The interchannel area has fewer turbidites, and the sands present were probably deposited from turbidity currents which spilt over the channel sides. The last‘event’ to give rise to sands in the channel and interchannel area occurred about 45 000 years ago. Although the channel has been inactive as an area of turbidity current deposition for the last 20 000 years, sands were deposited elsewhere on the lower rise, indicating that turbidity current transport routes have varied in time. Turbidity current deposition on the abyssal plain and low-lying continental rise appears to be related to distinct sliding events involving transport of material from various sources. Thin marl turbidites are interbedded with pelagic sediments in the area of sediment drape. There is a strong correlation between these and the thick marl turbidites on the abyssal plain, suggesting that the same turbidity current‘events’, occurring about once every 25 000 years, gave rise to both sets of deposits. The thinner units probably represent deposition from the outer parts or tails of the large turbidity flows. The turbidites occur at glacial/interglacial transitions, suggesting that the slides that created them were triggered by mechanisms related to climatic change. Several volcaniclastic sand/silt units within the channel and in interchannel areas occupy mid-stage stratigraphic positions, perhaps indicating a different triggering mechanism for slides around volcanic islands. A debris flow deposit (debrite), between 30°N, 21°W and 31°N, 24°W, is related to the Saharan Sediment Slide, a major mass movement feature on the continental slope over 1000 km to the southeast. Stratigraphic correlations indicate that this slide produced a large turbidity current as well as a debris flow.     

Fossil charcoal: a plant-fossil record preserved by fire

Small pieces of black organic material are common in many post-Devonian sequences, both sedimentary and volcanic. While many of these are coalified plant fragments, others are fossil charcoal, also known as fusain. Charcoalification preserves exquisite detail of the plant ultra-structure and is best viewed by scanning electron microscopy.     

一个关于临界地城的临界区域判别的方法

由临界地震的概念和外推Ｂｏｗｍａｎ等人关于孕育未来的地震半径的思想,发展出一个临界圆重叠方法来地寻找未来强震的临界区域位置,模拟实验的对比检验表明这种方法是有效的,使用主震前一定时间内地震目录,这个方法能找出临界区域,而未来震中位置将位于临界区域内或其边缘。使用新西兰和中国新疆南部的中强震目录检验,该方法可以在震前较准确地定位出强城的临界区域位置,使用中国１９９５年至１９９９年８月的小震目录,给出