Peridotites from the southern Mariana forearc: Heterogeneous fluid supply in mantle wedge

Detailed petrological work was carried out on serpentinized peridotite dredged and sampled by submersible from the southern part of the Mariana Trench to reveal the nature of the mantle wedge in the southern Mariana forearc. The southern part of the Mariana Trench is important in that we should expect to find a transect of a typical island arc structure; that is, from east to west, the Mariana forearc, the Mariana arc proper, the Mariana Trough (active back-arc spreading center), and the West Mariana Ridge (remnant arc). The most striking feature of peridotites from the southern part of the trench is that primary hornblende is a major constituent mineral in many specimens. Thus, the peridotite samples are divided into anhydrous (A-type), hydrous (H-type) and intermediate (I-type) groups. Petrological data suggest that each type of peridotite is a residue of extensive partial melting in the upper mantle. It is argued here that the I- and H-type peridotites were modified from `proto-A-type peridotite' by fluid infiltration. The fluid was enriched in Al, Ti, Fe, and alkalis, and may have caused changes in mineral and bulk chemical compositions of the peridotites. A-type peridotite derives from the `proto-A-type peridotite' directly, without any fluid contamination. After the formation of the `proto-A-, I-, and H-type peridotites', lower-temperature fluids, probably of seawater origin, produced retrograde metamorphism and alteration including serpentinization. The mantle wedge in the southern Mariana forearc was heterogeneous in fluid supply.     

苏南孔山地区人为坡面草本植被的自然恢复

发现苏南孔山地区小型人为裸露坡面草本植被自然恢复过程受地形、母质、气候和周围植被覆盖等因素的影响,据此提出我国南方山地人为裸露坡面水土流失的防治要坚持边开发边治理原则,最大程度地降低人为造成的水土流失．     

四川盆地南北缘志留系的锶和碳、氧同位素演化及其地质意义

通过对四川盆地北缘广元地区及南缘綦江地区上奥陶统及下志留统地层的８７Ｓｒ／８６Ｓｒ、δ１３Ｃ及δ１８Ｏ的系统测定,取得了沿地层剖面变化的同位素组成规律。研究结果表明,在盆地南缘８７Ｓｒ／８６Ｓｒ值高于地质历史中海水的平均值,其原因是本区为受板块内古陆控制的缓坡环境,大量陆源锶的混入增加了８７Ｓｒ／８６Ｓｒ的值。８７Ｓｒ／８６Ｓｒ值在上奥陶统至下志留统地层交界处、Ｒｈｕｄｄａｎｉａｎ／Ａｅｒｏｎｉａｎ（鲁丹期／艾隆期）交界处具正峰波动,反映了此时海平面的短时下降。但从Ｒｈｕｄｄａｎｉａｎ至Ｔｅｌｙｃｈｉａｎ（特里奇期）,８７Ｓｒ／８６Ｓｒ逐渐降低。从Ａｅｒｏｎｉａｎ至Ｓｈｅｉｎｗｏｏｄｉａｎ（舍因伍德期）早期,盆地南北缘的δ１３Ｃ值处于逐渐上升的过程之中,而δ１８Ｏ则相反。同位素的演化特征说明本区当时为海进时期,海平面不断上升,与全球性海平面变化相吻合。由于受区域构造运动的作用,Ｓｈｅｉｎｗｏｏｄｉａｎ之后的全球持续海进对本区无影响。     

辽东-吉南地区早元古代变质地体的组成及主要特征

在辽东－吉南地区存在两个性质不同的变质地体：辽北地体和辽南地体,二者之间以断裂带或韧性剪切带相接。辽北地体包括北辽河群和老岭群,辽南地体包括南辽河群和集安群。不同变质地体内的早元古代变质岩系在地层系统及岩石组合、原岩建造及沉积环境、变质作用类型及ｐＴｔ轨迹、区域构造和岩浆活动等方面都存在明显的差别。它们在早元古代时期形成于不同的大陆边缘,具有完全不同的形成条件和演化历史。两个变质地体最晚在早元古代晚期经构造作用拼贴在一起。     

Palaeointensity studies of a late Permian lava succession from Guizhou Province, South China: implications for post-Kiaman dipole field behaviour

Palaeomagnetic results are presented from a volcanic sequence in Zhijin County, Guizhou Province, Southern China. The lavas and associated volcanic breccias comprising the sequence represent a southern extension of the Emeishan volcanic province. Biostratigraphic dating of interbedded limestone units and stratigraphic constraints indicate that the section formed during the late Permian (∼263–255  Ma), and is thus somewhat older than the Emei stratotype section in Sichuan Province, and close in age to reported estimates of the termination of the Permo-Carboniferous (Kiaman) reverse superchron. Rock magnetic analyses and reflected light microscopy indicate that the magnetic mineralogy of the lava units is dominated by fresh, primary magnetites containing a significant fraction of single-domain grains. Thermal demagnetization behaviour of the breccia units is poor, but most lava samples have one or two components of remanence above 250 °C. The normal polarity characteristic remanence held by the lavas implies a post-Kiaman age for this succession and suggests that the termination of the Kiaman occurred prior to 263  Ma, supporting recently published estimates. According to standard criteria, Thellier palaeointensity results from the lavas are of good quality and reveal that the dipole field strength was comparatively low shortly after the termination of the superchron. 80 per cent of samples record relative VDM values in the range 42–52 per cent of the present-day value, supporting recent studies of mid-Kiaman field intensity. This suggests that a low-energy dipole existed at least between 300 and 255  Ma and does not appear to have been confined to the stable reverse polarity interval.     

Utilization of Specific Attenuation for Rainfall Estimation in Southern China

This study uses rain gauge observations to assess the performance of different radar estimators R(ZH), R(KDP)and R(A) in estimating precipitation based on the observations of an S-band polarimetric radar over southern China during a typical convective storm and an extremely severe typhoon, i. e., Typhoon Manghkut. These radar estimators were derived from observations of a local autonomous particle size and velocity(Parsivel) unit(APU) disdrometer. A key parameter, alpha(α), which is the ratio of specific attenuation A to specific differential phase K_(DP) with three fixed values(α=0.015 dB deg~(-1), α=0.0185 dB deg~(-1) and α=0.03 dB deg~(-1)) was examined to test the sensitivity of the R(A) rain retrievals. The results show that:(1) All radar estimators can capture the spatio-temporal patterns of two precipitation events, R(A) with α =0.0185 dB deg~(-1) is well correlated with gauge measurement via higher Pearson's correlation coefficient(CC) of 0.87, lower relative bias(RB) of 16%, and lower root mean square error(RMSE) of 17.09 mm in the convective storm while it underestimates the typhoon event with RB of 35%;(2) R(A) with α=0.03 dB deg~(-1) shows the best statistical scores with the highest CC(0.92), lowest RB(7%) and RMSE(25.74 mm) corresponding to Typhoon Manghkut;(3) R(A) estimates are more efficient in mitigating the impact of partial beam blockage. The results indicate that α is remarkably influenced by the variation of drop size distribution. Thus, more work is needed to establish an automated and optimized α for the R(A) relation during different rainfall events over different regions.     

Seismicity Variations in the Southern Aegean,Greece, Before and After the Large (M7.7) 1956 Amorgos Earthquake Due to Evolving Stress

The largest earthquake (M₀=4.9·10²⁷ dyn·cm) of the 20^th century in the territory of Greece occurred south of Amorgos Island, causing extensive destruction in the southern Aegean area. It occurred on an ENE–trending normal fault that is seated parallel to the Islands southern coastline. Changes in the rates of moderate–size earthquakes (M 5.0) that occurred before and after the Amorgos earthquake, within circular regions centered on its epicenter with radii of 100, 150 and 200 km, are investigated. The rate for moderate–size events just before the main shock appears to be considerably increased when compared to those of either preceding or subsequent periods. Further inspection reveals that more evident seismicity fluctuations are attributed to distances exceeding 100 km. These changes may be indicative of a broad region that is approaching a high stress state prior to an eventual large earthquake. Close to the main event, that is, within the 100–km radius, a remarkable quiescence period lasting about two decades before its occurrence was observed. Changes in seismicity are discussed in combination with static stress changes calculated by the application of the stress evolutionary model that takes into account the coseismic slip associated with the larger events (M 6.5) since the beginning of the 20^th century and the tectonic loading on the major faults in the study area. These larger events, as with the intermediate magnitude seismicity taking place at distances exceeding 100 km and which encircled the quiescent area observed during the last 22 years before the Amorgos earthquake, are well correlated with stress-enhanced areas in each stage of the evolutionary model.     

The nature and origin of fissures in the East Anglian Coralline and Red Crags

Vertical or inclined planar fissures that may for the most part be classified as joints are commonly seen in exposures of both the Coralline and Red Crag in East Anglia. Measurements of fissure orientation reveal orthogonal patterns of alignment in both formations. The field relations of the fissures to the host Crag sediment suggest a tectonic rather than periglacial origin. It is proposed that the fissure system is the product of early Pleistocene tectonic flexuring in the area, on the western margin of the subsiding southern North Sea basin.     

BOOK REVIEWS

Book Reviewed in this article: Portable Utopia: Glasgow and the United States 1820–1920. Bernard Aspinwall. Studies in Russian Historical Geography. James H. Bater and R. A. French, eds. Water and Sanitation: Economic and Sociological Perspectives. Peter G. Bourne, ed. Green Politics. Fritjof Capra and Charlene Spretnak. The Southern Cone: Realities of the Authoritarian State. César N. Caviedes. Women in Third World Development. Sue Ellen Charlton. Land Management: New Directions. David Chiddick and Alan Millington, eds. Geography and Ethnic Pluralism. Colin Clarke, David Ley, Ceri Peach, eds. Geomorphological Hazards in Los Angeles. R. U. Cooke. Regional Impacts of United States-Mexico Economic Relations. Alfonso Corona and Lay James Gibson. eds. Patterns of Jobs and Geographic Mobility. Torvald Gerger. Cities and Sickness: Health Care in Urban America. Ann Lennarson Greer and Scott Greer, eds. Neighborhoods: Their Place in Urban Life. Howard W. Hallman. Development and the Rural-Urban Divide. John Harriss and Mick Moore, eds. Patterns of Undocumented Migration: Mexico and the United States. Richard C. Jones, ed. Central Place Theory. Leslie J. King. The Geography of Western Europe: A Socio-economic Survey. Paul L. Knox. Regions and Resources: Strategies for Development. David T. Krisge, Daniel A. Seiver, Oliver S. Goldsmith and Michael J. Scott. Geography of Crowding and Human Response. A Study of Ahmedabad City. K. M. Kulkarni. Haciendo Pueblo. The Development of a Guadalajaran Suburb. Kathleen Logan. The Imperial Lion: Human Dimensions of Wildlife Management in Central Africa. Stewart A. Marks. Water in the Hispanic Southwest: A Social and Legal History, 1550–1850. Michael C. Meyer. Imlil: A Moroccan Mountain Community in Change. James A. Miller. Tourism in Canada: Selected Issues and Options. Peter E. Murphy, ed. David Harvey's Geography. John L. Paterson. Raster Scanning, Processing and Plotting of Cartographic Documents. Donna J. Peuquet and A. Raymond Boyle. The USSR and the Muslim World: Issues in Domestic and Foreign Policy. Yaacov Ro'i, ed. Environmental Perception and Behavior: An Inventory and Prospect. Thomas F. Saarinen, David Seamon, and James L. Sell. Incised channels, Morphology, Dynamics, and Control. Stanley A. Schumm, Michael D. Harvey, and Chester C. Watson. The Gap Between Rich and Poor: Contending Perspectives on the Political Economy of Development. Mitchell A. Seligson, ed. Twentieth-Century Richmond. Planning, Politics and Race. Christopher Silver. Land Conservation and Development: Examples of Land-Use Planning Projects and Programs. F. R. Steiner and H. N. van Lier, eds. Biogeography: Recent Advances and Future Directions. J. A. Taylor, ed. Late Quaternary Environments of the Soviet Union. A. A. Velichko, ed. H. E. Wright, Jr. and C. W. Barnosky, eds.