Rates of active deformation in the Aegean Sea and surrounding regions

Abstract Average strain rates are calculated from earthquakes in the period 1908-81 that occurred in the Aegean Sea extensional region, and in the convergent zone associated with the Hellenic Trench. In spite of large uncertainties resulting from the use of an M_S : M_o relationship, seismic N-S extensional rates in the Aegean are in the region 20–60 mm yr^-1 whereas seismic shortening rates in the Hellenic Trench are less than about 15 mm yr^-1. This is surprising because Africa and Eurasia are known to be converging, not separating. This apparent anomaly is caused by most of the convergence in the Hellenic Trench occurring aseismically. By contrast, the seismic extensional rates in the Aegean agree quite well with those expected from other arguments. The present day extensional rates are sufficiently high for McKenzie's instantaneous stretching model to be applicable. There is some evidence that these high extensional rates have operated throughout the last 5 Myr.     

Surface wave tomography of the Barents Sea and surrounding regions

The goal of this study is to refine knowledge of the structure and tectonic history of the European Arctic using the combination of all available seismological surface wave data, including historical data that were not used before for this purpose. We demonstrate how the improved data coverage leads to better depth and spatial resolution of the seismological model and discovery of intriguing features of upper-mantle structure. To improve the surface wave data set in the European Arctic, we extensively searched for broad-band data from stations in the area from the beginning of the 1970s until 2005. We were able to retrieve surface wave observations from regional data archives in Norway, Finland, Denmark and Russia in addition to data from the data centres of IRIS and GEOFON. Rayleigh and Love wave group velocity measurements between 10 and 150 s period were combined with existing data provided by the University of Colorado at Boulder. This new data set was inverted for maps showing the 2-D group-velocity distribution of Love and Rayleigh waves for specific periods. Using Monte Carlo inversion, we constructed a new 3-D shear velocity model of the crust and upper mantle beneath the European Arctic which provides higher resolution and accuracy than previous models. A new crustal model of the Barents Sea and surrounding areas, published recently by a collaboration between the University of Oslo, NORSAR and the USGS, constrains the 3-D inversion of the surface wave data in the shallow lithosphere. The new 3-D model, BARMOD, reveals substantial variations in shear wave speeds in the upper mantle across the region with a nominal resolution of 1°× 1°. Of particular note are clarified images of the mantle expression of the continent-ocean transition in the Norwegian Sea and a deep, high wave speed lithospheric root beneath the Eastern Barents Sea, which presumably is the remnant of several Palaeozoic collisions.     

环中国南海周边国家及地区地缘经贸关系动态演化探析

基于1996~2016年联合国商品贸易数据库进出口贸易额,采用社会网络分析法和位序变动指数法,对环中国南海周边国家及地区的经贸网络关系展开时空演化的实证研究。结果表明：1）环中国南海周边国家及地区的整体经贸网络格局演化呈日益紧密的趋势,形成了以中国大陆和中国香港为核心,泰国、马来西亚、新加坡和印度尼西亚为次核心,澳门、越南、柬埔寨、文莱和菲律宾为边缘地带的相对稳定的网络结构特征;2）在整个网络的动态发展过程中,网络影响力高的中国大陆和中国香港、网络影响力低的文莱、柬埔寨和中国澳门在进出口网络中都具有很大程度的网络结构对等性,其他国家和地区的结构数值动态波动,在进、出口网络中扮演的角色差异明显,且不断产生新的结构对等性配对;3）自1996年以来环中国南海周边国家及地区的经贸互动规模和经贸总量持续增长,但中国大陆的进出口贸易重心已由中国香港和新加坡逐渐转向以越南、泰国和马来西亚等国为代表的新兴市场国家。地缘经贸关系整体空间呈现出以中国大陆和中国香港为中心,核心、次核心及边缘地带内部力量动态演化的网络结构,且中国大陆在进出口方面都显示出对其他国家和地区极其重要的影响力。     

Strike‐slip tectonics and basin inversion in the Western Mediterranean: the Post‐Messinian evolution of the Alboran Sea

A comprehensive interpretation of single and multichannel seismic reflection profiles integrated with biostratigraphical data and log information from nearby DSDP and ODP wells has been used to constrain the late Messinian to Quaternary basin evolution of the central part of the Alboran Sea Basin. We found that deformation is heterogeneously distributed in space and time and that three major shortening phases have affected the basin as a result of convergence between the Eurasian and African plates. During the Messinian salinity crisis, significant erosion and local subsidence resulted in the formation of small, isolated, basins with shallow marine and lacustrine sedimentation. The first shortening event occurred during the Early Pliocene (ca. 5.33–4.57 Ma) along the Alboran Ridge. This was followed by a major transgression that widened the basin and was accompanied by increased sediment accumulation rates. The second, and main, phase of shortening on the Alboran Ridge took place during the Late Pliocene (ca. 3.28–2.59 Ma) as a result of thrusting and folding which was accompanied by a change in the Eurasian/African plate convergence vector from NW‐SE to WNW‐ESE. This phase also caused uplift of the southern basins and right‐lateral transtension along the WNW‐ENE Yusuf fault zone. Deformation along the Yusuf and Alboran ridges continued during the early Pleistocene (ca. 1.81–1.19 Ma) and appears to continue at the present day together with the active NNE‐SSW trending Al‐Idrisi strike‐slip fault. The Alboran Sea Basin is a region of complex interplay between sediment supply from the surrounding Betic and Rif mountains and tectonics in a zone of transpression between the converging African and European plates. The partitioning of the deformation since the Pliocene, and the resulting subsidence and uplift in the basin was partially controlled by the inherited pre‐Messinian basin geometry.     

Travel times of Pn -waves in the Aegean and surrounding area

Summary. Based on accurately located 23 very shallow earthquakes ( h = 1–14 km) in northern and central Greece by portable networks of seismic stations and by the joint epicentre method, the travel times of the P_n -waves from the foci of these earthquakes to the sites of 54 permanent stations in the Balkan region have been determined. The travel times of P_n -waves in the central and eastern part of the area (eastern Greece, south-eastern Yugoslavia, the Aegean Sea, Bulgaria, southern Romania, western Turkey) fit a straight line very well with the P_n velocity equal to 7.9 ± 0.1 km s^-1. On the contrary, the travel times of P_n -waves to stations in the western part of the area (Albania, western Greece) do not fit this curve because the P_n -waves travelling to these stations are delayed by more than 1 s due to the thicker crust under the Dinarides–Hellenides mountain range. Time delays for P_n -waves have been calculated for each permanent station in the Balkan area with respect to the mean travel-time curve of these waves in the central and eastern part of the area. Corrections of the travel times for these delays contribute very much to the improvement of the accuracy in the location of the shallow earthquakes in the Aegean and surrounding area.     

域外国家与南海周边地缘经济联系强度演化与态势评估

随着全球经济重心逐渐向亚太地区转移以及区域地缘经济联系日趋紧密,南海战略地位日益凸显,南海周边已成为全球重要战略区域和亚洲最重要的国际地缘问题区域。运用社会网络分析法、修正引力模型等方法,从贸易、投资、旅游等视角切入探究域外国家与南海周边区域的地缘经济联系演变及态势。结果表明：① 2005—2019年域外国家与南海周边区域的贸易联系日益紧密但贸易合作的非均衡化现象较为突出,域内出口贸易网络呈现以中国为核心,老挝、文莱和柬埔寨为边缘地带的相对稳定网络结构特征。② 域外国家对南海周边区域的投资规模显著提升但投资规模的两极化差异态势较为明显,投资区位呈现由高度集中向渐进式扩散转变态势。③ 域外国家入境旅游流持续增长,旅游网络由中国为单一核心演变为中国和泰国为双核心的网络结构,旅游目的地日趋多元化。④ 域外国家与南海周边区域地缘经济联系强度有所降低,但与东盟9国地缘经济联系强度整体上升;虽然以美日为代表的域外国家逐渐减弱与中国的地缘经济联系,但地缘经济联系网络整体仍然呈现以中国为核心,以泰国和新加坡为次核心的网络结构。     

Response of active tectonics on the alluvial Baghmati River, Himalayan foreland basin, eastern India

Active tectonics in a basin plays an important role in controlling a fluvial system through the change in channel slope. The Baghmati, an anabranching, foothills-fed river system, draining the plains of north Bihar in eastern India has responded to ongoing tectonic deformation in the basin. The relatively flat alluvial plains are traversed by several active subsurface faults, which divide the area in four tectonic blocks. Each tectonic block is characterized by association of fluvial anomalies viz. compressed meanders, knick point in longitudinal profiles, channel incision, anomalous sinuosity variations, sudden change in river flow direction, river flow against the local gradient and distribution of overbank flooding, lakes, and waterlogged area. Such fluvial anomalies have been identified on the repetitive satellite images and maps and interpreted through DEM and field observations to understand the nature of vertical movements in the area. The sub-surface faults in the Baghmati plains cut across the river channel and also run parallel which have allowed us to observe the effects of longitudinal and lateral tilting manifested in avulsions and morphological changes.     

The structure of the Berzeliustinden area: Evidence for thrust wedge tectonics in the Tertiary fold-and-thrust belt of Spitsbergen

The Berzeliustinden area forms part of the Tertiary fold-and-thrust belt of western Spitsbergen. The relations of a basement-involved thrust fault, decollement structures, and a fault repeating part of the stratigraphy are investigated. The deforming mechanism is thought to be'wedging'of a basement-involved thrust block into bituminous shaly beds of the overlying strata. The thrust fault thus does not continuously cut through to the surface, but lifts the overlying strata, forming a backward directed bedding-parallel thrust fault on top of the wedge. The presence of two bituminous shale formations, both potential splitting mediums for the wedge, complicates the structures. Many structural observations from adjacent areas of the fold-and-thrust belt also fit with this model. It is suggested that thrust wedges are common tectonic elements in the belt and might also be present further east beneath the relatively undeformed Tertiary strata of central Spitsbergen.     

Active tectonics of an apparently aseismic region: distributed active strike-slip faulting in the Hangay Mountains of central Mongolia

We identify and describe a series of east–west left-lateral strike-slip faults (named the Songino-Margats, the Hag Nuur, the Uliastay and the South Hangay fault systems) in the Hangay mountains of central Mongolia: an area that has little in the way of recorded seismicity and which is often considered as a rigid block within the India–Eurasia collision zone. The strike-slip faults of central Mongolia constitute a previously unrecognized hazard in this part of Mongolia. Each of the strike-slip faults show indications of late Quaternary activity in the form of aligned sequences of sag-ponds and pressure-ridges developed in alluvial deposits. Total bed-rock displacements of ∼3 km are measured on both the Songino-Margats and South Hangay fault systems. Bed-rock displacements of 11 km are observed across the Hag Nuur fault. Cumulative offset across the Uliastay fault systems are unknown but are unlikely to be large. We have no quantitative constraint on the age of faulting in the Hangay. The ≤20 km of cumulative slip on the Hangay faults might, at least in part, be inherited from earlier tectonic movements. Our observations show that, despite the almost complete absence of instrumentally recorded seismicity in the Hangay, this part of Mongolia is cut through by numerous distributed strike-slip faults that accommodate regional left-lateral shear between Siberia and China. Central Mongolia is thus an important component of the India–Eurasia collision that would be overlooked in models of the active tectonics based on the distribution of seismicity. We suggest that active faults such as those identified in the Hangay of Mongolia might exist in other, apparently aseismic, regions within continental collision zones.     

Impact of normal faulting and pre‐rift salt tectonics on the structural style of salt‐influenced rifts: the Late Jurassic Norwegian Central Graben,North Sea

Studies of salt‐influenced rift basins have focused on individual or basin‐scale fault system and/or salt‐related structure. In contrast, the large‐scale rift structure, namely rift segments and rift accommodation zones and the role of pre‐rift tectonics in controlling structural style and syn‐rift basin evolution have received less attention. The Norwegian Central Graben, comprises a complex network of sub‐salt normal faults and pre‐rift salt‐related structures that together influenced the structural style and evolution of the Late Jurassic rift. Beneath the halite‐rich, Permian Zechstein Supergroup, the rift can be divided into two major rift segments, each comprising rift margin and rift axis domains, separated by a rift‐wide accommodation zone – the Steinbit Accommodation Zone. Sub‐salt normal faults in the rift segments are generally larger, in terms of fault throw, length and spacing, than those in the accommodation zone. The pre‐rift structure varies laterally from sheet‐like units, with limited salt tectonics, through domains characterised by isolated salt diapirs, to a network of elongate salt walls with intervening minibasins. Analysis of the interactions between the sub‐salt normal fault network and the pre‐rift salt‐related structures reveals six types of syn‐rift depocentres. Increasing the throw and spacing of sub‐salt normal faults from rift segment to rift accommodation zone generally leads to simpler half‐graben geometries and an increase in the size and thickness of syn‐rift depocentres. In contrast, more complex pre‐rift salt tectonics increases the mechanical heterogeneity of the pre‐rift, leading to increased complexity of structural style. Along the rift margin, syn‐rift depocentres occur as interpods above salt walls and are generally unrelated to the relatively minor sub‐salt normal faults in this structural domain. Along the rift axis, deformation associated with large sub‐salt normal faults created coupled and decoupled supra‐salt faults. Tilting of the hanging wall associated with growth of the large normal faults along the rift axis also promoted a thin‐skinned, gravity‐driven deformation leading to a range of extensional and compressional structures affecting the syn‐rift interval. The Steinbit Accommodation Zone contains rift‐related structural styles that encompass elements seen along both the rift margin and axis. The wide variability in structural style and evolution of syn‐rift depocentres recognised in this study has implications for the geomorphological evolution of rifts, sediment routing systems and stratigraphic evolution in rifts that contain pre‐rift salt units.