Seismicity of Garfagnana-Lunigiana (Tuscany, Italy) as recorded by a network of semi-broad-band instruments

Eight seismic stations equipped with5-second sensors have recently beeninstalled in the Lunigiana-Garfagnana areato monitor activity in this sector of theNorthern Apennines shaken by the 5.0 Mdearthquake of October 1995. The stations(RSLG network) represent an eastern branchof the RSNI (Regional Seismic network ofNorthwestern Italy) managed by Dipteris,Universita' di Genova.The installation of this dense networkallowed the operators to improve themagnitude detection level, providinginformation about the seismicity of thearea down to magnitude 1.0. Severalanalyses have been carried out to evaluatethe performance of the network and thereliability of the computed locations. Itseems that the epicentral location forevents occurring inside the network isalready constrained using 8–10 P+S phases,but usage of more readings ensuresreliability of depth and the reduction oflocation errors.Though not conclusive, as the network hasonly been fully operating for two years, apreliminary study carried out on a selecteddataset of high-quality locations confirmed that the seismicity of theGarfagnana-Lunigiana is characterised by alow-to-medium magnitude level and it issubdivided into two bands, a superficialone about 30–35 km thick and the secondbelow 50 km. This distribution, confirmedby other studies carried out in the past,reflects the complex structure of this areawhere two tectonic plates (European andAdriatic) meet.     

Prediction of the maximum credible ground motion in Singapore due to a great Sumatran subduction earthquake: the worst‐case scenario

Although Singapore is located in a low‐seismicity region, huge but infrequent Sumatran subduction earthquakes might pose structural problems to medium‐ and high‐rise buildings in the city. Based on a series of ground motion simulations of potential earthquakes that may affect Singapore, the 1833 Sumatran subduction earthquake (M_w=9.0) has been identified to be the worst‐case scenario earthquake. Bedrock motions in Singapore due to the hypothesized earthquake are simulated using an extended reflectivity method, taking into account uncertainties in source rupture process. Random rupture models, considering the uncertainties in rupture directivity, slip distribution, presence of asperities, rupture velocity and dislocation rise time, are made based on a range of seismologically possible models. The simulated bedrock motions have a very long duration of about 250 s with a predominant period between 1.8 and 2.5 s, which coincides with the natural periods of medium‐ and high‐rise buildings widely found in Singapore. The 90‐percentile horizontal peak ground acceleration is estimated to be 33 gal and the 90‐percentile horizontal spectral acceleration with 5% damping ratio is 100 gal within the predominant period range. The 90‐percentile bedrock motion would generate base shear force higher than that required by the current design code, where seismic design has yet to be considered. This has not taken into account effects of local soil response that might further amplify the bedrock motion. Copyright © 2002 John Wiley & Sons, Ltd.     

印度与欧亚板块俯冲—碰撞过程岩石圈热结构模拟

研究中采用了非线性非稳态的二维热传导－平流方程，首次考虑了俯冲角度及地球物质材料的性质对摩擦生热率的影响，对拉萨地块中部100－40Ma期间印度与欧亚板块俯冲－碰撞过程岩石热结构演化过程数值模拟进行了尝试，在模拟板块俯冲－碰撞过程中高原中部热结构的同时探讨了岩石圈的拆层作用对高原（尤其是对拉萨地块）热结构的影响。模拟结果显示，在俯冲阶段岩石圈热结构能较好地解释冈底斯花岗岩带中花岗闪长岩及其中铁镁质微粒包体存在壳幔二元体系这一认识。     

Geophysical profiling across the Sulu ultra-high-pressure metamorphic belt, eastern China

The largest ultra-high pressure metamorphic (UHPM) belt in the world is located along the Dabie–Sulu region, which tectonically belongs to the east part of the central orogenic belt of China. Integrated geophysical investigations of using deep seismic reflection, MT, and geothermal observations have been carried out in the Sulu area since 1997. The results of integrated interpretation suggest the existence of three features: (1) a rift beneath the Lianshui basin by the Jiashan–Xionshui fault; (2) a special crustal pattern, called the magmatic multi-arch structure occurs beneath the northern Sulu UHPM zone; and (3) a northwest-dipping regional thrust crosses the Sulu crust, representing the intracontinental subduction of the Yangtze craton beneath the Sulu metamorphic belts after collision between the Yangtze and Sino-Korean cratons. A magmatic multi-arch structure consists of some arched reflectors that occur in both the lower and the upper crust where arched reflectors coincide with granitoid plutons. The multi-arch structures are common in eastern China where many Mesozoic granitoid plutons of different scales occur. The crustal structures in the Sulu metamorphic belts resulted from intensive dynamic processes following the Triassic collision between the Yangtze and Sino-Korean cratons. The formation and exhumation of UHPM rocks followed the collision, and then intracontinental subduction of the Yangtze craton beneath the Dabie–Sulu terranes took place in the early and middle Jurassic. In the late Jurassic, the Sulu lithosphere turned to an extensional regime, large-scale granitic intrusions occurred in eastern China; these likely resulted from lithospheric thinning and asthenospheric uplifting. The granitic intrusions came to a climax during the Cretaceous and were followed by rifting along existing faults in the early Eogene, resulting in many petroleum basins. The granitoid emplacement that generated the magmatic multi-arch structure and the rift were consequences of the lithospheric thinning process, and deep intracontinental subduction of the Yangtze craton beneath the Sulu metamorphic belt might partially contribute to the lithospheric thinning.     

Geochemistry of the subduction-related magmatic rocks in the Dahong Mountains, northern Hubei Province--Constraint on the existence and subduction of the eastern Mianlüe oceanic basin

The existence and subduction of the eastern Mianlue oceanic basin in the south Qinling belt are keys to understand the Qinling orogen. Based on geological mapping, several volcanic slices have been identified in Tumen, Zhoujiawan, Xiaofu and Yuantan areas, which distribute in the northern margin of the Dahong Mountains (DHM), and thrust into the Sanligang-Sanyang fault. These slices consist mainly of diabases, basaltic-andesitic lavas, pyroclastic rocks and a minor tuff. The geochemistry of the basalts, andesites, and diabases is characterized by depleting in Nb and Ta, enriching in Th and LILE (e.g.K, Rb, Ba), and undifferentiating in HFSE. These geochemical characteristics suggest that the original magma of these rocks was derived from a mantle wedge above a subduction zone, and formed in an island-arc setting in Carboniferous-early Triassic. Comparing with the ophiolites and island-arc volcanic rocks in Mianxian-Lueyang area to the west, it is reasonable to consider that there had been an oceanic basin connecting with the Mianlue ancient ocean to the westward, distributing along the south edge of the Tongbai-Dabie block. In view of the ophiolite in Huashan area and these island-arc volcanic rocks along the north of the Dahong Mountains, it is suggested that there had been a plate tectonic evolutionary history with oceanic basin rifting and subduction in this region.     

Cross-section through the frontal Japan Trench subduction zone: Geochemical evidence for fluid flow and fluid–rock interaction from DSDP and ODP pore waters and sediments

Abstract Fluids and sediments from Deep Sea Drilling Project/Ocean Drilling Program Legs (56, 57, 87 and 186) along a transect extending from the subducting plate, across the midslope and upper slope of the Japan Trench forearc were analyzed for B and B isotopes in order to assess their composition and fluid–sediment interaction. At the reference Site 436 on the subducting plate, changes in B contents and B isotopes are controlled by the lithology and diagenesis only. The midslope Sites 440 and 584 showed stronger variations in the B geochemistry, which can be related to diagenesis and tectonic dewatering along faults. The strongest changes in the B geochemistry were observed on the upper slope Sites 1150 and 1151, where profound down‐hole freshening (chlorinities as low as ～310 mmol) coincides with a B enrichment (up to 9.3 × seawater concentration). The B isotope pore fluid profile of Site 1150 displayed a bimodal variation with depth, first increasing to values more positive than seawater, then shifting to lower signatures typical for deep‐seated fluids, whereas Site 1151 showed a constant B decrease with depth. Sites 1150 and 1151 sediments showed B increases with depth to values as high as ～164 p.p.m. and isotopic compositions ranging from ～+4 to ?9‰. A linear decrease in B_solid/B_fluid ratio, suggests that B geochemistry of the upper slope sites is controlled by fluid–rock interaction and deep‐seated fluid flow, whereas constant B_solid/B_fluid ratios were observed at the reference site on the incoming plate. This fluid overprint is probably caused by normal faults in the sediment cover which might be interconnected to deep thrusts in the underlying Cretaceous accreted wedge. This suggests that the erosive Japan Trench margin is characterized by back‐flux of deep‐seated, B‐enriched fluids into the ocean, which is facilitated by extensional normal faulting as a result of tectonic erosion and subsidence.     

Accretion and postaccretion tectonics of the Calamian Islands, North Palawan block, Philippines

Abstract   Upper Paleozoic to Mesozoic sedimentary sequences of chert (Liminangcong Formation), clastics (Guinlo Formation) and a number of limestone units (Coron Formation, Minilog Formation and Malajon Limestone) constitute the accretionary complex of the North Palawan block, Philippines. Based on chert-to-clastic transitions from different stratigraphic sequences around the Calamian Islands, three accretionary belts are delineated: the Northern Busuanga Belt (NBB), the Middle Busuanga Belt (MBB) and the Southern Busuanga Belt (SBB). The accretion events of these belts along the East Asian accretionary complex, indicated by their sedimentary transitions, began with the Middle Jurassic NBB accretion, followed by the Late Jurassic MBB accretion and the Early Cretaceous SBB accretion. Several limestone blocks that formed over the seamounts became juxtaposed with chert–clastic sequences during accretion. During the Late Cretaceous, accretion-subduction along the East Asian margin subsided bringing tectonic stability to the region. The seafloor spreading during the mid-Oligocene disconnected the entire North Palawan block from the Asian mainland and then migrated southward. The collision between the North Palawan block and the Philippine Island Arc system in the middle Miocene generated a megafold structure in the Calamian Islands as a result of the clockwise turn of the accretionary belts in the eastern Calamian from originally northeast–southwest to northwest–southeast.     

Thermal effects of massive CO2 emissions associated with subduction volcanism

Large volumes of CO₂ are emitted during volcanic activity at convergent plate boundaries, not only from volcanic centres. Their C isotopic signature indicates that this CO₂ is mainly derived from the decarbonation of subducted limestones or carbonated metabasalts, not as often admitted from magma degassing. On the example of Milos (Aegean Sea) it is argued that these fluids originate from intermediate depth in the mantle and carry sufficient heat to account for the generation of subduction-related magmas, as well as for the geothermal manifestations at the surface. The heat that is required for the decarbonation reactions is drawn by conduction from a wide zone surrounding the subducting slab and then rapidly transported upward by convection of the mixed CO₂–H₂O fluids that originate from the sediments in the slab. The transport takes place in a focused way through ‘chimneys’ in the upper mantle, where magmas are generated by the introduced heat and water. In the crust, the hot fluids cause thermal-dome-type metamorphism. In volcanic areas, magmas are commonly held responsible for the major part of heat transfer from the mantle to the surface. Here it is argued that most of the heat transfer is by hot gases. To cite this article: R.D. Schuiling, C. R. Geoscience 336 (2004).     

塔北隆起-库车坳陷区中新生代基底-盖层构造变形机理

利用三维建模软件获取了塔北隆起-库车坳陷区现今及不同时期基底构造格局，为基底构造分析提供了新的技术方法。研究表明，三叠纪-古近纪，塔里木盆地北部块体的基底构造基本处于稳定状态，基底-盖层构造变形不明显；新近纪，克拉苏背斜带以北的塔里木盆地基底明显向天山倾斜，库车基底俯冲变形导致基底转折带逐渐向南迁移，库车前陆盆地形成；第四纪时期，库车-拜城盆地的基底整体向天山下倾斜，基底构造的强转折带迁移到塔北隆起带；新近纪以来库车坳陷的基底转折期与塔北隆起盖层的负反转构造形成期在时间上一致，在空间上，库车坳陷的基底在强烈沉降俯冲的同时，相邻塔北隆起的基底相对隆起，形成库车新生代前陆盆地的前隆；库车前陆冲断带的形成是在库车基底斜坡上伴随天山的推覆而形成的；塔北隆起为库车前陆盆地的前隆，基底俯冲变形及其转折带迁移导致塔北隆起盖层负反转构造的形成。     

大别-苏鲁造山带大理岩中榴辉岩包体的锆石U-Pb年龄及其地质意义

大别-苏鲁造山带大理岩中榴辉岩包体的锆石有两种类型，为变质增生锆石和变质重结晶的残留碎屑锆石。变质增生锆石和完全变质重结晶锆石都具有较低的U、Th含量和Th/U比值，且给出两组变质年龄，分别为240.9±4.8~25.1±3.1 Ma和223.7±4.2~226.0±3.9 Ma。年龄为240.9±4.8~245.1±3.1 Ma的变质锆石的生长对应于板块俯冲过程中高压榴辉岩相变质作用期间的流体活动，而223.7±4.2~226.0±3.9 Ma的变质锆石的生长可能对应了板块折返早期的流体活动。这些年龄同样可以很好地用来限定其寄主大理岩的变质演化历史。