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根据在阿德朗达克-西魁北克地震带及其邻近地区的59台短周期数字地震仪记录的直达P波走时观测值,用地震层析成像方法反演了该地区地壳三维速度结构.结果表明,地壳上层（0-5km）在阿德朗达克穹隆山的中南部地区出现正速度异常;第二层（5-10km）及第三层（10-15km）的速度横向变化较小,介质相对比较均匀;第四层（15-25km）出现显著的速度异常,一个是位于地震带中部的正速度异常（+4％）,它与布格重力正异常一致;另一个是位于阿德朗达克穹隆山下的负速度异常（-4％）.结合已有的地质及地球物理资料进行对比,可以认为阿德朗达克是一个正在发展的穹隆上升山,但其热源前锋尚未到达地面的大陆热点. 相似文献
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PHILIP L. GIBBARD RICHARD G. WEST 《Boreas: An International Journal of Quaternary Research》2000,29(4):329-336
There is a long-standing tradition of dividing Quaternary stratigraphical sequences on the basis of climate. This climatostratigraphical approach, begun on terrestrial sequences in the last century, led to the widespread adoption of glacial, interglacial, stadial and interstadial subdivisions as standard. However, the limitations of the approach resulted in its replacement by local sequences of chronostratigraphical stages from the 1950s. The subsequent establishment of the detail and complexity of ocean-core sequences has encouraged increasing use of the terminology developed for these records to the terrestrial and shallow marine succession. The view is expressed that it is advisable to separate and retain regional chronostratigraphies for each sequence-type, and that these should be correlated using event-based stratigraphy where possible. This should ensure both the highest precision and lack of ambiguity and a securely-based synthesis of the relations between the terrestrial and ocean isotope sequences, a synthesis which is fundamental to understanding the nature and detail of regional and global climate change. 相似文献
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R. G. WEST 《第四纪科学杂志》1996,11(2):167-167
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尽管Pn波品质因子（PnQ）对上地幔流变性质研究及核试验监测有着深刻含义,迄今它仍然是一个难以准确测定的地震参量.这是因为除了众所周知的影响所有短周期Q的干扰因素外,在所有主要的区域震相中,地震工作者对Pn波的传播模式及几何扩散函数G=d-k中的扩散率指数k了解最少.不同的研究者任意地假定不同的常数k,因此常算出非常不同的PnQ值（例如文献[1]、[2]）.实际上,理论研究已表明,k随频率而变,变化的形式取决于上地幔的速度结构.本文介绍用一推广的逆向双台法同时测定加拿大东部的PnQ和k值.我们的结果表明:1.加拿大东部的Pn波是由一系列的干涉潜波叠加而形成;2.视PnQ（散射和固有非弹性的综合效应）随频率而增加,在15Hz处接近2000. 相似文献
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It is now believed that the negative transients observed in coincident-loop transient electromagnetic (TEM) measurements are caused by polarizable bodies (bodies whose conductivity increases as a function of frequency). Ordinarily the TEM response of polarizable bodies is obtained by calculating the frequency-domain response at many frequencies and transforming it to the time domain via Fourier, Laplace or Hankel transforms. This is normally a computationally laborious task. However, for some simple non-polarizable bodies the time-domain response is analytical and can be computed easily. When these simple bodies are weakly polarizable an approximate response can be obtained by convolving the easily-calculated, non-polarizable response with the impulse response of the polarization. The approximate response is found to be very similar to the exact response for the polarizabilities normally seen in geological materials. 相似文献
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J. H. MARSH S. E. JOHNSON M. G. YATES D. P. WEST JR 《Journal of Metamorphic Geology》2009,27(8):531-553
A well preserved strain and reaction gradient records the progressive transformation of a megacrystic Kfs+Cpx+Opx+Bt1 ±Qtz syenitic pluton to a strongly sheared Kfs+Act+Bt2 +Ab+Qtz tectonite within the exhumed Norumbega Fault System, Maine, USA. Detailed microstructural analysis indicates that fracturing and localized fluid infiltration initiated the deconstruction of the existing K-feldspar and two-pyroxene load-bearing framework, and that feedback among metamorphic reactions, fabric development and enhanced permeability during progressive shearing led to the development of an interconnected, biotite- and actinolite-rich foliation. The activation of dislocation creep in biotite and quartz, and dissolution–precipitation creep in actinolite and feldspar, with increasing strain ultimately resulted in a transition from dominantly frictional to dominantly viscous deformation processes. Petrological data show that various scales of geochemical disequilibrium exist across the strain and reaction gradient, and that reaction progress was limited by slow chemical diffusion during the early stages of deformation. Petrological modelling results indicate that the existing plutonic assemblage was metastable at mid-crustal conditions, and that fluid infiltration and deformation allowed the product assemblage to advance towards chemical equilibration. Comparison of the observed microstructures and deformation mechanisms with experimental and numerical modelling results suggest that the development of an interconnected biotite-dominated fabric probably caused a major (up to three fold) reduction in bulk rock strength and localization of strain into the foliated margin. 相似文献
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IAN M. WEST 《Sedimentology》1973,20(2):229-249
Carbonate cementation of some carbonate and quartz sands in three raised beaches of temperate origins was investigated. The carbonate of the cements was found to have been derived from the dissolution of skeletal debris. The sandstones, so produced, now possess only low-magnesium calcite, but the original sediments, like adjacent modern beach and blown sands, probably contained low-magnesium calcite, aragonite and some high-magnesium calcite, all of skeletal origin. In meteoric water the dissolution has occurred of all carbonate within minute, tubulelike, volumes of sand. Concurrent deposition in adjacent volumes of sand of low-magnesium calcite formed cements that are irregularly nodular or uneven on a small scale. Aragonite within the minute nodules has been replaced paramorphically by low-magnesium calcite. Additional local carbonate cements were formed at later dates, around and within solution pipes. 相似文献
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With the increasing number of channels in AEM systems, computer data handling is becoming a necessity. The experience gained in processing of seismic and aeromagnetic data cannot be applied directly to low-frequency (100-5000 Hz) AEM methods. A novel scheme has been designed for AEM data processing and tested on 2900 km (1800 miles) of Input surveys. In the first step, the digital flight tapes are merged with digitized flight path recovery to form the primary data set. The validity of the raw data is controlled by the computer, but the interpreter has an option of checking them in perspective plots of channel amplitudes. The primary data set is reduced by processors which determine the location and type of anomalies and discard noise. Unlike the widely used deconvolution, the sequential processor determines first the anomaly location and then estimates parameters, such as peak amplitude, base width, and excess area, which are used as acceptance criteria. Interpretation parameters, such as σt, conductor depth, and dip are estimated by comparison with quantitative models. The recorded channel amplitudes are plotted together with the selected interpretation parameters in a profile form. The secondary data set which includes only the interpretation parameters for selected anomalies is graphically displayed as a schematic map of apparent σt. Elongated features are traced by a fan strike recognition routine and a trend map is automatically compiled and plotted. Disk storage is essential for second pass processing during which parts of the primary data set are searched for undiscovered anomalies matching the analyzed trend. The suggested procedure for AEM data processing is demonstrated on an Input MK V survey, Southern Indian Lake, Manitoba. 相似文献