Himalayan structure and metamorphism south of the Main Mantle Thrust, Lower Swat, Pakistan

Abstract During the Eocene-Oligocene, the Indian plate collided with the Kohistan arc along the Main Mantle Thrust (MMT) zone. The structure of the Lower Swat rock sequence, on the Indian plate directly south of the MMT, is a dome with a basement of granitic gneiss and quartz-rich schist unconformably overlain by amphibolitic and calcareous schist. The earliest superposed small-scale folds (F1 & F2) represent a progressive F1/F2 deformation that is associated with a single set of WSW-vergent large-scale folds (termed F2). These folds are inferred to have developed during oblique, WSW-directed overthrusting of the MMT suture complex onto the Lower Swat rock sequence. Metamorphism began during F1/F2 as indicated by an S1 foliation that developed during biotite-grade metamorphism. S1 is preserved as a relict texture in porphyroblasts that grew during a subsequent interkinematic phase during garnet- and higher grade metamorphism. The dominant, regional foliation (S2) developed following the interkinematic phase. S2 is associated with transposition of S1 and rotation or dismemberment of porphyroblasts. Annealing recrystallization followed S2 and continued during F3 thereby destroying or masking possible pre-existing stretching fabrics. Superposed F3 folds are upright and open with N-S axial trends. They may correlate with early doming of the Lower Swat rock sequence and with strike-slip displacement in the northern part of the MMT zone, north of the Lower Swat area. F3 was followed by retrograde metamorphism and development of E-W-trending, S-vergent F4 folds. F4 may be associated with a final phase of southward directed thrusting and inactivity in the MMT zone. Correlation of published ⁴⁰Ar/³⁹Ar ages with the metamorphic fabrics suggests that F1/F2 and F3 occurred in the Eocene, and that F4 developed in the Oligocene. F4 is the earliest indication of southward verging structures on this part of the Indian plate.     

The relationship betweenin situ strain relaxation and outcrop fractures in the Potsdam Sandstone,Alexandria Bay,New York

Summary In situ strain was measured by overcoring foil-resistance strain gauge rosettes bonded to five outcrops of Potsdam Sandstone near Alexandria Bay, New York. Strain relaxation magnitude and orientation correlated with the area of the intact outcrop outlined by intersecting vertical fractures. The maximum expansion occurred at the outcrop with the largest area between intersecting fractures. Outcrops with more than one set of longer, open fractures or more complicated fracture patterns have lower recoverable strains. Strain relaxation was lowest next to a postglacial pop-up. The orientation of the pop-up indicated relief of an ENE directed compression, the direction also observed as the maximum expansion at the outcrop yielding the largest strain relaxation.Lamont-Doherty Geological Observatory Contribution No. 2456.     

A mechanism for strain relaxation of barre granite: Opening of microfractures

Summary Strain relaxation in the Barre Granite and surrounding metasediments in Vermont, was measured by overcoring strain gauge rosettes bonded to outcrop surfaces. The average maximum expansion upon relieving 15.2 cm diameter cores trends N55°W, while the average maximum expansion of 7.6 cm diameter cores coaxial with 15.2 cm cores trends N70°W. The maximum strain relief of the internal overcores is normal to the microfracture fabric. Therefore, the mechanism of strain relaxation is attributed to the opening of microfractures either parallel to the rift direction of the Barre Granite or parallel to the foliation of the metasediment. The lack of parallelism between the normal to the rift plane and the maximum expansion of the initial overcore suggests an extenrally applied strain superimeposed on the strain caused by opening of microfractures.Lamont-Doherty Geological Observatory Contribution No. 2455.     

构造地震及其可能震级的初步研究——以“人”字型构造为例

一个地区未来地震的可能震级的估计是地震危险性评价的一个极其重要的方面。本文根据已知的地质条件,利用线弹平面有限元方法计算区域构造应力场,并确定所积累的总应变能和产生的破裂区面积,然后利用经验公式估计出与断层活动有关的构造地震的可能震级     

Spatial and temporal variations in the compositions of Upper Miocene to Recent basic lavas in the northern main Ethiopian rift: Implications for the causes of Cenozoic magmatism in Ethiopia

The rate of lithospheric extension has previously been suggested as the most important factor governing the compositions of magmas generated in the Cenozoic Ethiopian volcanic province (CEVP). However, the distribution and chemistry of volcanic rocks extending from the western plateau margin at Addis Ababa to the rift floor in Nazret, northern sector of the main Ethiopian rift (MER), suggest that transitional magmatism in the region may have been triggered by an increase in the amount of lithospheric extension in the Early Pliocene. The rocks occur across an area of variable crustal thickness and show a general age progression from Upper Miocene (≤9 Ma) to Recent toward the rift. Alkalic basalts are extensive in the western part of the rift and along its margin but are found only locally within the rift, whereas transitional basalts are found within the rift only. Both types of basalts appear to have been derived from a common mantle source. In contrast, alkalic and transitional basalts on the Ethiopian plateau are mutually exclusive in terms of their spatial distribution, but exhibit a compositional contiguity which suggests that transitional magmas on the plateau formed at the expense of alkalic magmas, i.e. by equilibration of alkalic magmas at relatively shallow depth. The alkalic basalts bear clear record of a decrease in the degree of partial melting with time, suggesting that magmatism on the plateau was possibly triggered by a transient thermal anomaly.     

岩石变形实验中的微破裂与地震的联想

根据室内岩石变形实验中岩石的微破裂和应变能释放等情况，联想到地震发生的几种相似现象：（１）岩体在较低的状态下亦有可能发生地震；（２）小震的发生预示岩体内应变和应变能的释放，但应力却未必释放；（３）主震时就变能大量释放，但仍有相当多的应变能残留在岩体中，这应是老震区发生地震的原因之一。     

日本列岛及中国东南沿海地震对中国大陆地震活动的影响估计

１９９４－１９９５年初日本列岛及中国东南沿海发生一组大地震，其中包括引起重大破坏的阪神大地震。从地震横向迁移角度对这些大地震集中发生对大陆地震的影响进行了分析。大震引起的应变波动传播速度馒、周期长、能量大、影响距离和深度大，其波峰可能触发地震。据Ｂｏｔｔ和Ｄｅａｎ的非弹性介质应变波运动方程解释了从海沟到内陆的大震迁移，指出１９１８年以来迁移序列目前对山西地震带及贺兰山地震带有影响；１９９４－１９９５年的地震序列在２０２０年左右将对渤海──郯庐断裂产生影响，２０３０年左右对华北平原产生影响；目前可能激发广东、福建沿海发生中等地震。     

大同－阳高2次地震前长治电感地应力的异常特征

大同－阳高２次中强地震前，长治电感地应力曾出现明显的趋势异常和短临异常。本文就长治地应力的观测条件以及地震前后的异常特征进行了分析研究，认为对于山西地震带北部地区的地震监测预报有参考价值。     

1992年吉尔吉斯苏撒孟尔7.3级地震的地震构造与地震破裂

１９９２年８月１９日吉尔吉斯斯坦共和国苏撒孟尔盆地发生７．３级地震，主余震为６．７级。地震发生于北天山褶皱带内。震中烈度Ⅸ度。极震区内地震滑坡、崩塌呈ＥＷ向带状分布，并有地裂、泥砂喷发现象发生。主震和主余震的宏观震中区均产生地震断层陡坎。主震断层陡坎最高３．３５ｍ，陡坎总长度为１ｋｍ，陡坎走向为Ｎ６５°Ｅ～Ｎ８０°Ｅ，主震断层以逆推为主，并兼有右旋扭动。地震断层自南向北逆冲的水平距离是２ｍ左右。主余震断层陡坎长３ｋｍ，走向Ｎ６５°Ｗ，高７５ｃｍ左右。探槽开挖证实，主余震断层产状为Ｎ７２°Ｗ／１９２°∠１０°，地震时由南向北逆冲的水平距离是１ｍ。Ⅶ度以上等烈度线特征、地震地表破裂和震源机制解结果三者有关地震断错的结论一致     

广东省主要行业经济效益的比较研究

本文利用１９９３年主要行业经济效益统计指标，运用因子分析，聚类分析方法对广东省１８个主要行业的经济效益进行比较分析，抽取３个主因子，根据主因子得分，将广东省主要行业划分为４种类型，并分析各种类型的优势及存在的不足，进而提出调整，扶持，引导和促进主要行业发展的具体措施。