土体新近纪构造结构面特征探讨

为阐明土体中存在的构造形迹对工程建（构）筑物地基的影响，将土体中的断层、构造节理和构造裂缝统称为土体新近纪构造结构面，用系统分析方法对土体新近纪构造结构面特性进行了分析。结果表明：土体构造结构面分布广泛且具隐伏性，是发震断裂在土体中留下的变形形迹，是近期构造活动最直接的反映，对工程地基土体的稳定性有重要影响。     

Miocene tectonics of the Maramures area (Northern Romania): implications for the Mid-Hungarian fault zone

The interplay between the emplacement of crustal blocks (e.g. “ALCAPA”, “Tisza”, “Dacia”) and subduction retreat is a key issue for understanding the Miocene tectonic history of the Carpathians. Coeval thrusting and basin formation is linked by transfer zones, such as the Mid-Hungarian fault zone, which seperates ALCAPA from Tisza-Dacia. The presented study provides new kinematic data from this transfer zone. Early Burdigalian (20.5 to ∼18.5 Ma) SE-directed thrusting of the easternmost tip of ALCAPA (Pienides), over Tisza-Dacia is linked to movements along the Mid-Hungarian fault zone and the Periadriatic line, accommodating the lateral extrusion of ALCAPA. Minor Late Burdigalian (∼18.5 to 16 Ma) NE-SW extension is interpreted as related to back-arc extension. Post Burdigalian (post-16 Ma) NE–SW shortening and NW–SE extension correlate with “soft collision” of Tisza-Dacia with the European foreland coupled with southward migration of active subduction. During this stage the Bogdan-Voda and Dragos-Voda faults were kinematically linked to the Mid-Hungarian fault zone. Sinistral transpression (16 to 12 Ma) at the Bogdan-Voda fault was followed by sinistral transtension (12–10 Ma) along the coupled Bogdan-Dragos-Voda fault system. During the transtensional stage left-lateral offset was reduced eastwards by SW trending normal faults, the fault system finally terminating in an extensional horse-tail splay.     

Structural,AMS and geochronological study of a laccolith emplaced during Late Variscan orogenic extension: the Rocles pluton (SE French Massif Central)

In the southern French Massif Central, the Rocles leucogranite of Variscan age consists of three petrographic facies; textural analysis shows that they experienced the same subsolidus deformation. New chemical U-Th-Pb dating on monazite yielded 324 ± 4 Ma and 325 ± 5 Ma ages for muscovite-rich and biotite-rich facies respectively. AMS-study results agree with petrostructural observations. The magnetic planar and linear fabrics, which correspond to the preferred orientation of biotite and muscovite, are consistent with the foliation and lineation defined by the preferred mineral orientation. This fabric developed during pluton emplacement. The accordance of this granite foliation with that observed in the host rock, suggests that the Rocles pluton is a laccolith, but its present geometry resulted from post-emplacement southward tilting due to the uplift of the Late Carboniferous Velay dome. Restoration of the primary geometry of the pluton and its country-rocks to a flat-lying attitude places the granite lineation close to the trend measured in other plutons of the area. This restoration further supports the interpretation of the Rocles laccolith as a pluton emplaced along a tectonic contact reactivated during the late-orogenic collapse of the Variscan Belt.     

西藏桑日县沃卡地堑的第四纪正断层活动及其机制探讨

遥感解译和地表调查结果发现,西藏桑日县的沃卡盆地构成了藏南近南北向裂谷带中最东端的错那-沃卡裂谷的北段,它是在该区近东西向的逆冲构造带停止活动之后,上地壳沿N108±1°E方向发生区域性的伸展变形所形成的第四纪活动明显的地堑式断陷盆地。晚第四纪期间,控制该盆地发育的主边界断裂带为整体呈北北东走向、倾向西侧、长50～60km的盆地东缘正断层。该断裂带也是1915年桑日M7.0级地震的控震断裂。断裂活动速率的估算结果表明,该断裂带MIS6以来的垂直活动速率介于0.4～0.9mm/a之间,末次冰期盛冰期以来断裂平均活动速率最合理的估计值为1.2±0.3mm/a。Q3晚期以来活动速率的明显增加可能标志着该断裂带全新世进入了地震丛集期。     

Evolution of Quaternary alluvial fans and terraces in the intramontane Pinjaur Dun, Sub-Himalaya, NW India: interaction between tectonics and climate change

Quaternary alluvial fans in the tectonically active Pinjaur Dun, an intramontane valley in the Sub‐Himalaya, were deposited in front of the Nalagarh Thrust and were influenced both by tectonics and glacial climate fluctuations. The surface morphology indicates that an earlier set of first‐order fans (Qf1) became entrenched and onlapped by a series of second‐order fans (Qf2). The younger fan segments were then cut by a pair of terraces (T1 and T2). Quartz optically stimulated luminescence dating establishes that the Qf1 aggradational phase was initiated before 96·5 ± 25·3 ka and terminated after 83·7 ± 16·3 ka. This was followed by a period of incision, before Qf2 fan deposition started at 72·4 ± 13·4 ka and continued until 24·5 ± 4·9 ka. Sediment was deposited on the T1 (upper) and T2 (lower) terraces at 16·3 ± 2·1 and 4·5 ka, respectively, recording a return to overall degradation punctuated by minor deposition on terraces. The period of incision separating the younger and older fan deposits coincided with enhanced SW monsoon precipitation. The subsequent development of the Qf2 fans and their progradation until 20 ka suggest erosional unloading of the thrust hangingwall during a tectonically quiescent phase. Toe cutting, deposition of axial river and lacustrine facies, and retreat of Qf2 around 45 ka, indicate fanward shift of the axial river due to tilting of the valley towards the NE in response to reactivation of the Nalagarh Thrust. The cessation of Qf2 deposition around 20 ka and the onset of through‐fan entrenchment suggest reduced sediment supply but relatively high stream power during the last glacial maxima (LGM). The prolonged stream incision since the cessation of Qf2 deposition, with only minor depositional phases at 16·3 ± 2·1 and 4·5 ka, resulted from high water discharge and low sediment input during intensification of the SW monsoon and vegetation changes in the hinterland.     

Active tectonic influence on the evolution of drainage and landscape: Geomorphic signatures from frontal and hinterland areas along the Northwestern Himalaya, India

The Kangra Re-entrant in the NW Himalaya is one of the most seismically active regions, falling into Seismic Zone V along the Himalaya. In 1905 the area experienced one of the great Himalayan earthquakes with magnitude 7.8. The frontal fault system – the Himalayan Frontal Thrust (HFT) associated with the foreland fold – Janauri Anticline, along with other major as well as secondary hinterland thrust faults, provides an ideal site to study the ongoing tectonic activity which has influenced the evolution of drainage and landscape in the region. The present study suggests that the flat-uplifted surface in the central portion of the Janauri Anticline represents the paleo-exit of the Sutlej River. It is suggested that initially when the tectonic activity propagated southward along the HFT the Janauri Anticline grew along two separate fault segments (north and south faults), the gap between these two fault and the related folds allowed the Sutlej River to flow across this area. Later, the radial propagation of the faults towards each other resulted in an interaction of the fault tips, which caused the rapid uplift of the area. Rapid uplift resulted in the disruption and longitudinal deflection of the Sutlej river channel. Fluvial deposits on the flat surface suggest that an earlier fluvial system flowed across this area in the recent past. Geomorphic signatures, like the sharp mountain fronts along the HFT in some places, as well as along various hinterland subordinate faults like the Nalagarh Thrust (NaT), the Barsar Thrust (BaT) and the Jawalamukhi Thrust (JMT); the change in the channel pattern, marked by a tight incised meander of the Beas channel upstream of the JMT indicate active tectonic movements in the area. The prominent V-shaped valleys of the Beas and Sutlej rivers, flowing across the thrust fronts, with Vf values ranging from <1.0–1.5 are also suggestive of ongoing tectonic activity along major and hinterland faults. This suggests that not only is the HFT system active, but also the other major and secondary hinterland faults, viz. the MBT, MCT, SnT, NaT, BaT, and the JMT can be shown to have undergone recent tectonic displacement.     

Topical problems of Sedimentology at the 17th International Sedimentological Congress

Information on the 17th International Sedimentological Congress held from August 27 to September 1, 2006, in Fukuoka, Japan is offered. The principal topics, sessions, and plans for future investigations, especially drilling on continents and in oceans with the new Japanese drilling ship Chikyu, are elucidated.     

Cenozoic graben facies in the Middle Amur sedimentary basin as inferred from seismic and electrical exploration

Most of the grabens of the Middle Amur sedimentary basin (MASB) are usually characterized by similar structures typical of continental rifts. Based on models elaborated for the sedimentary infill in well-studied rift basins, seismic facies developed in grabens of the MASB are first analyzed. The analysis revealed seismic facies characteristic of different parts of asymmetrical basins: facies of fans developed on steep slopes and facies of coastal shoals, alluvial plains, and deltas occurring on gentle slopes. Seismic facies peculiar of deep lacustrine settings that are considered promising with respect to oil-and gas-bearing source rocks is recognized. Correlation of seismic and drilling data confirms the correctness of the interpretations and shows that boreholes penetrated a thin sequence of deep-water lacustrine sediments. Using the vertical electrical sounding method, a low-resistivity sequence that is most promising for discovery of lacustrine sediments is defined in the southwestern and eastern parts of the MASB. The lowest values of the specific resistivity in this sequence (7–8 Ohm · m) are recorded in its southeastern part, where a borehole partly recovered a relatively thick silty-clayey sequence.     

Role of basin‐wide landslides in the formation of extensive alluvial gemstone deposits in Sri Lanka

The tectonically stable central highlands of Sri Lanka and its alluvial valleys are the source areas and sinks, respectively, for one of the most prolific Quaternary gemstone provinces in the world. However, the known ¹⁰Be/²⁶Al cosmogenic‐nuclide‐determined low natural (preanthropogenic) denudation rates of 2–11 mm kyr^?1, and resulting sediment fluxes, are grossly inadequate to deliver the vast throughputs of overburden required to concentrate the known gemstone deposits. Basin‐wide, unstable, slow‐moving channelized landslides and debris flows, aided by biotic factors, are the dominant mechanisms of mass‐wasting on hill‐slopes and bulk delivery of sediment to the alluvial valleys and fluvial networks. Channelization ensures modulated sediment transfer and run‐out during an erosional–depositional continuum. In a selected inventory of landslides, mobilized sediment volumes ranged from less than 1000 cubic metres to a maximum of ～800 000 cubic metres per event. Monsoonal rainfall (both cumulative seasonal and total daily thresholds) is the primary external factor, which interacts with colluvium thickness and steep slopes in triggering landslides. There are three to five ‘threshold’ rainfall events per year in the highlands that can be expected to generate landslides. They can occur under conditions of decreasing daily rainfall as the seasonal total rainfall increases. GIS databases show a very significant spatial overlap and direct causal linkage between several hundred landslide occurrences and the innumerable gem pits and mines in the catchments of the best known mining region of Sri Lanka. Landslide‐associated mass movements, besides providing significant numbers of gemstones to the alluvial valleys over time, are also a fundamental factor in the geomorphic evolution of the rugged central highland landscape. Rainfall‐driven landslide activity may be a natural geological response affecting erosional equilibrium in high‐relief tectonically stable terrains. Climatically forced base level changes will, over time, control sediment storage, removal or reworking in the valleys. Copyright © 2007 John Wiley & Sons, Ltd.