Analysis of aeromagnetic anomalies over the central part of the Narmada-Son lineament

The Narmada-Son lineament (NSL) forms a major tectonic feature on the Indian subcontinent. The importance of this lineament lies in its evolution as well as its tectonic history. The lineament seems to have been active since Precambrian times. In order to understand the history of its evolution, it is necessary to know what igenous activity has been taking place along this lineament, and how the Deccan trap volcanics, which cover large areas along this lineament, have erupted.For the study of this problem an analysis of the aeromagnetic anomaly map lying between 76°15 to 77°30E and 21°45 to 22°50N has been carried out. Four different profiles (B ₁ B ₁,B ₂ B ₂,B ₃ B ₃ andB ₄ B ₄) have been drawn in N-S direction over this area and interpreted in terms of the intrusive bodies present within or below the surface of Deccan trap exposures. Inversion and forward modelling techniques have been adopted for interpretation purposes. An analysis of frequency spectra along the profiles has also been carried out to estimate the average depth of the different magnetic bodies. These results have been correlated with the available geological information. It has been found that most of the small wavelength anomalies are caused by dyke-like bodies within or below the Deccan trap at a depth of less than 0.5 km.     

不同类型低阻异常航电时间常数的特征分析

时间域航空电磁系统工作原理是机载发射线圈向地下发射脉冲电磁波,接收线圈观测断电后来自地下低阻体的二次感应电磁场,通过对观测数据进行处理解释达到探测地下低阻体的目的。结合近年来从事时间域航空电磁法探矿数据和资料解释的实践,分别对电力干扰、表层强低阻体、表层弱低阻体和地下低阻体航电异常响应的时间常数τ的特征进行了综合分析,总结出以下规律:电力干扰在τ图上一般显示为不连续的线性异常,电磁响应呈跳跃状的无规律衰减;低阻异常在τ图上一般显示为高值区,电磁响应呈指数衰减,导电性越好规模越大的低阻体衰减越慢。     

西藏林周县轮郎铅锌矿区地球物理异常特征评价及找矿意义

林周县轮郎铅锌矿区位于拉萨-林周-墨竹工卡铅锌成矿区中部,属雅鲁藏布江巨型铜多金属成矿带北亚带。根据该区地球物理测量成果,共圈出重力异常区10个、磁力异常区18个。通过对重力异常特征及磁力异常特征综合评价,分析出其中5个磁力异常区和重力异常有明显的对应关系,属矿致异常,具良好的找矿前景;并且得出该区高磁区、局部磁异常与局部重力高的结合部和重、磁双高地带均为找矿潜力区。     

Seismotectonic Reactions to the Arc-continent Convergence in Central Philippines

Abstract. High seismic activity in the Philippines originates from tectonic convergence related to surface and subsurface seismotectonic features. Based on earthquake data, the archipelago can be divided into the seismically-active Philippine Mobile Belt and the aseismic North Palawan Block. The latter represents a rifted continental fragment of the Eurasian margin that juxtaposed with the rest of island arc units in central Philippines. Earthquake hypocenter plots on planar and in vertical profiles show that the seismic events are associated with known seismotectonic features. In addition, data suggest that the collision zone between the North Palawan Block and the Philippine Mobile Belt is characterized by a decreased amount of hypocenters at > 100 km depths. Although field evidence favors the presence of a subducted slab or slabs beneath western central Philippines, these are difficult to image using the present seismicity distribution dataset.     

Seismicity of the Koyna region and regional tectonomagmatism of the Western Margin (India)

Recent findings on the Meso-Cenozoic tectonomagmatism and deep-seated anomalous geophysical structures suggest a close linkage between the seismicity of the Koyna region, the Westernghat uplift (WG-U) and associated thermomechanical and fluid activities. The WG-U seems to be the result of late Cretaceous thermal mobilization, erosion of the Deccan trap cover and superposition of compressional stress. The association of seismicity with uplift seems to result from movement of deep-seated heat and fluids/volatiles along the edges (or boundary faults) of the uplift; because the force required for crustal deformation depends on the relief. Observed gradients in relief may be attributed to the differential erosion-rates and heat inputs, due to the time gap of 50 Ma in the break-ups and plume activities on the eastern and western sides and consequence magmatism. Further, the geology and tectonics strongly indicate that the western margin (WM) is a relic of a mobile arm (MA), that included Madagascar, and which formed a part of the Proterozoic mobile belt of greater India (fort>85 Ma). The mobile nature of the WM facilitates mantle upwellings and transient elevation of isotherms at depth, raising the possibility of intermittent metamorphism and greater deformation.Superposition of the ongoing compression and uplift-induced forces make local permeability and pore-fluid pressure vital in triggering the seismic slip over the Peninsular shield. Certain representative model calculations have been carried out to estimate change in the e.m. induction characteristics caused by an intermittent hydraulic connectivity. The results show a drop in the resistivity which could be a useful monitoring index. The close connection of uplift and fluid activity as discussed here seems applicable for other active parts of the South Indian Shield (SIS) also.     

帕米尔强震活动对新疆地震活动的影响

分析了帕米尔强震活动对新疆地震活动的影响,认为帕米尔强震活动对新疆境内地震活动具有前兆意义;在各活动过程中,新疆境内强震活动在时空域中表现出较好的韵律性;标志震发生后的第1次强震活动位于天山地区将对其后的地震活动产生不同影响。     

Seismic observations at Lake Emosson,Switzerland

Since 1973 several seismic stations have been brought into operation at Lake Emosson, the last large conventional hydrodam in the Swiss Alps. The reservoir is situated in a seismically active zone which stretches from the Rhone valley in southwestern Switzerland into Haute Savoie, France. The monitoring program was started before the first impounding and continued during four full load cycles. A certain correlation is indicated between water level changes and the occurrence near the reservoir of weak seismic events (M_L<0) with high frequency content. A transient series of 30 tremors occurred in August 1974, during a period of rising water level. Local events of a different type frequently occur during the periods of decreasing water level. Their daily distribution indicates a mechanism which is caused by temperature variations.Contribution No. 131 of the Geophysical Institute, Karlsruhe University. Sonderforschungsbereich Felsmechanik of Karlsruhe University. Contribution No. 299 of the Institute for Geophysics, ETH Zürich.