Petrology and Geochemistry of the Volcanic Rocks Dredged from the Geophysicist Seamount in the Kurile Basin: Evidence for the Existence of Thinned Continental Crust

Dredged samples from the Geophysicist seamount volcano in the northeastern part of the Kurile Basin include volcanic and volcanoclastic rocks ranging from basalt to andesite. The rocks have geochemical features typical of high-K island-arc calc-alkaline volcanism. They are enriched in LILE and depleted in Zr, Ti, Nb, Ta and Y. The chondrite-normalized REE patterns are characterized by enrichment of LREE similar to those of island-arc lava from the submarine volcanoes of rear-arc zone of the Kurile Island Arc. The volcanic rocks have a wide range of ⁸⁷Sr/⁸⁶Sr ratios (0.70287-0.70652), varying ¹⁴³Nd/¹⁴⁴Nd and Pb isotopic ratios. Their trace-element compositions and Sr-Nd-Pb isotope signatures may be explained by a small addition of crustal continental component to mantle-derived magmas that suggest the existence of thinned continental basement under the eastern part of the Kurile Basin.     

Flexure of the Indian plate and intraplate earthquakes

The flexural bulge in central India resulting from India's collision with Tibet has a wavelength of approximately 670 km. It is manifest topographically and in the free-air gravity anomaly and the geoid. Calculations of the stress distribution within a flexed Indian plate reveal spatial variations throughout the depth of the plate and also a function of distance from the Himalaya. The wavelength (and therefore local gradient) of stress variation is a function of the effective elastic thickness of the plate, estimates of which have been proposed to lie in the range 40–120 km. The imposition of this stress field on the northward moving Indian plate appears fundamental to explaining the current distribution of intraplate earthquakes and their mechanisms. The current study highlights an outer trough south of the flexural bulge in central India where surface stresses are double the contiguous compressional stresses to the north and south. The Bhuj, Latur and Koyna earthquakes and numerous other recent reverse faulting events occurred in this compressional setting. The N/S spatial gradient of stress exceeds 2 bars/km near the flexural bulge. The overall flexural stress distribution provides a physical basis for earthquake hazard mapping and suggests that areas of central India where no historic earthquakes are recorded may yet be the locus of future damaging events.     

Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.     

Microearthquakes and faulting in the southern Okinawa Trough

Southern Okinawa Trough represents an early stage of back-arc rifting and is characterized by normal faulting and microearthquakes. Earthquake distribution and deep structure of fault was investigated to clarify active rifting in the southern Okinawa Trough, where two parallel grabens are located. A network of ocean bottom seismometers (OBSs) that displayed the hypocenters of 105 earthquakes were observed for a period of 4 days in southern-graben (SG). Most of the microearthquakes occurred in a cluster about 7 km wide, which on a cross-section striking N45°E dips 48° to the southwest. Relocated hypocenters, which are recorded by a local seismic network, show scattered distribution around the southern-graben. There are no remarkable surface faults in the southern-graben. On the other hand, the recalculation of hypocenter locations of 1996 earthquakes swarm recorded by a local seismic network suggests that the swarm is associated with normal faulting on the southern side of northern-graben (NG). Thus, the undeveloped southern-graben is located to the south of the developed northern-graben. Southward migration of rifting, which may be caused by migration of volcanism, could thus be occurring in the southern Okinawa Trough. The extension rate computed for the southern Okinawa Trough from the fault model of the northern-graben is 4.6 cm/year, which is 59–102% of the extension rate (GPS measurements). This result indicates that the majority of extensional deformation is concentrated within the center of the northern-graben in the Okinawa Trough.     

中国大陆地震构造研究回顾

简要回顾了近4年来(1999～2002年)我国在地震构造方面研究的进展，重点介绍了在地壳上地幔结构、活动断裂和大地震构造背景、地壳形变和数值模拟方面的工作．在我国境内发生的绝大多数地震都属于大陆地震的范畴，因此,中国地震学家把研究的重点放在大陆地震上．中国在九五期间完成的中国数字地震观测系统和中国地壳运动观测网络重大科学工程为地震构造的研究提供了大量的基础资料．因此，４年来的地震构造研究取得了相当大的进展．      

渤海湾盆地区燕山期构造特征与原型盆地

结合近几年来渤海湾盆地区深层地震勘探与解释的成果，重点论述了渤海湾盆地区燕山期构造特征与盆地原型，提出燕山期构造变形样式总体在纵向上可分为三个构造层，分别称为上部、中部和下部；横向上总体可分为三个构造带，西部为向西逆冲的薄皮逆冲带，中部为冲断—走滑带，东部为厚皮褶皱—冲断带，主体由两期挤压方向皆为NW—SE向的褶皱—逆冲变形形成；并将其演化分为三个阶段：燕山早期、燕山中期和燕山晚期。但是，渤海湾盆地区燕山期的构造变形特征和原型盆地有所变化，其空间上的差异是基底构造格局及其空间差异叠合的结果。综合其它研究结果还表明，渤海湾盆地区燕山期构造是在西太平洋大陆边缘弧的挤压构造背景下，陆内壳下拆沉和壳内挤出逃逸构造的综合动力作用下形成的。     

华北地台北缘西段铜多金属矿床的热水沉积成因

早元古代后狼山成矿区和于陆缘拉张拗陷阶段，一直延续至晚元古代末期。狼山群食矿岩系中存在的限层变基性火山岩，热水沉积岩和条纹条带状构造，胶状构造均显示狼山成矿区在１８００－９５０Ｍａ期间处于高地热异常的被动大陆边缘。     

The Origin of the Early Proterozoic Kuandian Complex: Evidence from Geochemistry

The Kuandian Complex is scarcely preserved Early Proterozoic volcanic suite, formed2.3-2.4 Ga ago. It is located in an Early Proterozoic mobile belt bounded by the ArchaeanRangrim and Ryonggang Blocks of the northeastern Sino-Korean Craton. The Complex ismainly made up of amphibolites, gneisses, leucoleptite, leptite and layered granite. Petrologicaland geochemical studies show that the protoliths of the Complex are mainly assoctations ofbimodal volcanics and anorogenic granites. The Kuandian amphibolites are depleted in Nb, Ta,P and Ti, and enriched in LILE, e.g. K, Rb and Cs, with pronounced depletion of Sr relative toNd and Pb; La/Nb ratios are higher than 1(1.75 to 5.18). The trace element patterns of theamphibolites are similar to continental flood basalts formed by the Gondwana break-up, suchas those in South Karoo and Tasmania, which shows continental contamination. ε_(Nd) valuesranging from 0.70 to 1.94 of the Kuandian amphibolites and the relationships between Nb/Yband La/Yb suggest that contamination of basaltic magma happened in the mantle, rather thanalong the conduit. Isotope ratios of ~(208)Pb/~(204)Pb, ~(207)Pb/ ~(204)Pb, ~(206)Pb/~(204)Pb, ~(143)Nd/~(144)Ndand ~(87)Sr/~(86)Sr indicate that the magma was derived from a contaminated mantle source likeDMM or a mixture of DMM and EM2. The Kuandian Complex has Dupal anomaly, as is thecase with some continental basalts in the south hemisphere, e.g. in South Karoo and Tasmania.Petrochemical modelling proposes that the Kuandian gneiss, granite, and amphibolite camefrom the same parental magma, being products of strong fractional crystallization. Protoliths ofthe Kuandian Complex were formed in extensional tectonic setting during the transition fromcontinental crust to oceanic crust. The formation of the Kuandian Complex indicates that 2.3or 2.4 Ga ago tectonic evolution of the Sino-Korean craton was different from that of otherwell-studied Precambrian cratons, e.g. the North American shield, European platform andAustralian continent in that strong volcanic eruption resulted in its accretion. Besides, the con-taminated magma source with a Dupal anomaly for the Complex indicates that crust-mantleconvection whose scale was similar to that of the present plate tectonics had occurred at leastbefore the formation of the Kuandian Complex (2.3-2.4 Ga B.P).     

中国东部中新生代盆地成因及其地球动力学

中国东部大兴安岭－太行山－武陵山重力梯度带一线以东至太平洋西岩，大陆连缘中新生代盆地系的形成，中生代岩浆活动，盆地基本特征同Ｍｏｈｏ、岩石圈和软流圈及上地幔基本特征的内在联系等都表明，它们在时空和成因上有一定联系，可以纳入同一地球动力学过程之中，这种动力学机制可能与岩石圈界，地壳底界的拆离作用有关。     

一个被动大陆边缘的“开”“合”史及找矿标志——以赣西北震旦系至下古生界为例

本文根据赣西北地区震旦系至志留系基本层序和板内构造演化,可把古板块构造发展划分为4个阶段:开裂期(Z_1l-Z_2p)以中、小规模的重力流和凝缩段为特征;成熟发展期(∈_1-∈_3~1),以沉降沉积作用为主;萎缩期(∈_3~2-O_2h)以大规模重力流和混生动物群为特征;闭合期(O_3-S_3)以双幕式沉积为特征。同时阐述了在板块发展的各个阶段,不能忽视其成矿作用。