Crustal and upper mantle S-wave velocity structure beneath the Bransfield Strait (West Antarctica) from regional surface wave tomography

Regional surface wave tomography in the sub-Antarctic Scotia Sea is helpful in revealing the nature of the crust and the S-wave seismic velocity profile beneath the Bransfield Strait. The joint use of our regional network, global seismographic network stations and local temporary arrays provide better lateral resolution than that obtained in our previous studies concerning the Scotia Sea region.Tomographic analysis of data obtained using 10 broad band seismic stations and more than 300 regional events, shows that the Bransfield Basin is characterised by a strong group velocity reduction of 8% with respect to the surrounding areas, in the period range from 15 s to 50 s.The crustal and upper mantle models of the eastern, central and western Bransfield Basin are obtained by joint inversion of Rayleigh and Love local dispersion curves from 15 s to 50 s. In addition our data set is expanded to a broader period interval (1–80 s), in central Bransfield Strait in order to better constrain the upper mantle and shallow crust.The main results can be summarized as follows: (a) the crust thins distinctly from W toward E; the variation is consistent with the type of volcanism, earthquake distribution and bathymetric observations, (b) low upper mantle velocities (soft lid) extend down to depths exceeding 70 km as a consequence of elevated temperatures, (c) the crust beneath the central Bransfield Basin displays continental characteristics with a gradually increasing S-wave velocity distribution versus depth analogous to the East African Rift structure of Kenya, (d) negative velocity gradients are present in the lower crust beneath the eastern Bransfield Basin; these could be interpreted as magmatic bodies originating from decompression melting of the mantle.     

Numerical simulation of rifting in the northern Viking Graben: the mutual effect of modelling parameters

Numerous basin modelling studies have been performed on the Viking Graben in the northern North Sea during the past decades in order to understand the driving mechanisms for basin evolution and palaeo temperature estimations. In such modelling, it is important to include lithospheric flexure. The values derived for the lithospheric strength by these studies vary considerably (i.e. up to a factor of 30). In this study, which is based on new interpretation of a regional transect, we show that both the estimated value of the effective elastic thickness and the derived β-profile are dependent on the assumed value of the depth of necking. The use of models that implicitly set the level of necking at a depth of 0 km generally leads to an underestimation of the lithospheric strength, and an overestimation of the thinning factors. In the northern Viking Graben, a necking depth at intermediate crustal levels gives results comparable to the observations. Extension by faulting is modelled to be a significant factor. In conclusion, rifting in the northern Viking Graben can be explained with various models of effective elastic thicknesses (EET) varying from 1 km for a zero necking depth to the depth of the 450 °C isotherm for an intermediate level of necking.It is also shown that the development of the basin during the post-rift phase cannot be explained by pure shear/simple shear extension. Two mechanisms are proposed here to explain the post-rift subsidence pattern, namely intra-plate stress and phase boundary migration. The two extreme models for EET mentioned above (1 km for a zero necking depth to the depth of the 450 °C isotherm for an intermediate level of necking) give very different responses to compressional stress, the latter gives basically no response for realistic intra-plate stress.     

渤海第三纪裂谷扩张的地球物理分析

根据重磁计算的结果，对渤海第三纪裂谷扩张问题做了分析。在研究中还注意参考了渤海地热流和地壳上地幔电性高导层抬升的研究成果。认为由于深部岩浆的上涌，在渤海及其相邻的陆地共有渤中、渤海湾、庙西、辽东湾和垦利５个较大的地幔柱拱起，它们在华北新生代拗陷区形成了一个走向明显、形态独特、规模较大而又相对集中的地幔柱群     

山东沭河裂谷带早白垩世晚期恐龙足迹群与古地理背景*

山东沭河裂谷带由北至南发育8个早白垩世晚期(大盛期）的恐龙足迹群化石点,其中5处为本研究首次报道。沭河裂谷带足迹化石点多,分布广,层位多,足迹数量和造迹恐龙种类丰富。郯城新发现的单个蜥脚类巨型足迹直径近1.0m,可能仅次于前人描述过的兰州西南盐锅峡下白垩统河口群内中国最大的蜥脚类足迹化石。沭河裂谷带的恐龙足迹产出于大盛群田家楼组,地质年代为110～100Ma,属于早白垩世晚期Aptian晚期—Albian期。恐龙足迹一般为凹形印痕,造迹者以蜥脚类恐龙为主,其次为鸟脚类和兽脚类。足迹多保存在发育波痕、泥裂、雨痕等沉积构造的细砂—粉砂岩沉积物层面。足迹和行迹特征显示造迹恐龙为群居和在栖息地漫步行走状态。足迹原位保存特征及赋存地层沉积学研究表明,早白垩世中晚期气候处于由温暖潮湿变得炎热干旱的过渡期,湖泊、三角洲广布,恐龙栖息于湖滨环境。沭河裂谷带早白垩世晚期恐龙足迹群的发现和综合研究进一步证实,中国早白垩世恐龙动物群有着更广阔的地理分布和相似的古地理环境。     

A mantle- and a lower crust-derived bimodal suite in the Yusufeli (Artvin) area, NE Turkey: trace element and REE evidence for subduction-related rift origin of Early Jurassic Demirkent intrusive complex

The Yusufeli area, in the Eastern Black Sea Region of Turkey, contains a crystalline complex that intruded into the Carboniferous metamorphic basement and is composed of two intrusive bodies: a gabbro-diorite and a tonalite-trondhjemite. The mafic body (45–57 wt% SiO₂) displays a broad lithological spectrum ranging from plagioclase-cumulate to quartz diorite. Primitive varieties of the body have Mg-number, MgO and Cr contents that are close to those expected for partial melts from mantle peridotite. Data are consistent with the magma generation in an underlying mantle wedge that was depleted in Zr, Nb and Ti, and enriched in large ion lithophile elements (K, Rb, Ba, Th). However, high Al₂O₃, CaO and generally low Ni (<65 ppm) contents are not in agreement with the unfractionated mantle-derived primitive magmas and require some Al₂O₃- and CaO-poor mafic phases, in particular, olivine and orthopyroxene. Absence of orthopyroxene in crystallization sequence, uralitization, and a common appearance of clinopyroxene surrounded by hornblende imply an anhydrous phase fractionated from highly hydrous (5–6%) parent. Geochemical modelling suggests derivation by 15–20% melting of a depleted-lherzolitic mantle. The tonalite-trondhjemite body (58–76 wt% SiO₂) ranges in composition from quartz diorite to granodiorite with a low-K calc-alkaline trend. Although LILE- and LREE- enriched characteristics of the primitive samples imply a metasomatic sub-arc mantle for their source region, low MgO, Ni and Cr concentrations rule out direct derivation from the mantle wedge. Also, lack of negative Eu anomalies suggests an unfractionated magma and precludes a differentiation from the diorites of mafic body, which show negative Eu anomalies. Their Na enrichments relative to Ca and K are similar to those of Archean tonalites, trondhjemites and granodiorites and Cenozoic adakites. However, they exhibit important geochemical differences from them, including low-Al (<15 wt%) contents, unfractionated HREE patterns and evolution towards the higher Y concentrations and lower Sr/Y ratios within the body. All these features are obtained in experimentally produced melts from mafic rocks at low pressures (≤5 kbar) and also widespread in the rocks of arc where old (Upper Cretaceous or older) oceanic crust is being subducted. Major and REE modelling supports formation of the quartz dioritic parent to the felsic intrusive rocks by 70% partial melting of a primitive gabbroic sample (G694). Therefore, once taking into account the extensional conditions prevailing in the Pontian arc crust in Early Jurassic time, former basic products (gabbros) seem to be the most appropriate source for the tonalite-trondhjemite body. Magmatic emplacement of stratigraphically similar lithologies in the Pulur Massif, just southwest of the Yusufeli, was dated to be 184 Ma by the ⁴⁰Ar/³⁹Ar method on amphibole, and is compatible with the initiation of Early Jurassic rifting in the region.     

Evolution of the Bükk Mountains (NE Hungary) during the Middle–Late Triassic asymmetric rifting of the Vardar-Meliata branch of the Neotethys Ocean

Based on sedimentological and biostratigraphic investigations of the Middle–Late Triassic successions of the Bükk Mountains, the evolution of an upper plate margin of a rifting area was reconstructed. The Middle Anisian shallow water carbonates are succeeded by terrestrial sediments. Simultaneously with the uplift, volcanic activity starts, indicating the beginning of the rifting. The emersion was followed by rapid subsidence in the Late Anisian–Early Ladinian which corresponds to the synrift stage. Based on facies distribution of Ladinian–Carnian sediments, the half-graben structure of the basement can be outlined. Coeval existence of platforms and basins is characteristic of this period. From the end of the Fassanian, the subsidence slows down: postrift stage. At this time the thermal cooling controls the subsidence of the area. During the Late Triassic, the edges of the platforms were gradually drowned and basins conquered bigger and bigger areas. Sediments deposited on the southern shelf of the opening Vardar-Meliata branch of the Neotethys Ocean show features characteristic to the upper plate part of a rifting area, whereas sediments of the northern shelf show features characteristic to the lower plate. The opening of the Vardar-Meliata branch of the Neotethys Ocean follows the asymmetric rifting model of Wernicke (Can J Earth Sci 22:108–125, 1985) and Dixon et al. (Tectonics 8(6):1193–1216, 1989).     

Geology,mineralogy and geochemistry of the Kekem dyke swarm (Western Cameroon): Insights into Paleozoic–Mesozoic magmatism and geodynamic implications

The broadly N70°–90°E-trending dykes swarm at Kekem cut across the Paleoproterozoic-to-Achean terranes of West Cameroon remobilized during the Pan-African orogeny. They are picrite basalts and basalts with tholeiitic/transitional affinity, as shown by mineralogical and geochemical data, with variable major and trace element contents, MgO ranges from 7.3 to 12.4 wt.%, Cr from 190 to 411 ppm, Ni from 15 to 234 ppm. All the dykes are light REE enriched with La_N/Yb_N values of 5.3–8.1, suggesting a co-magmatic origin. They originated from a 2.8% partial melting of a spinel-mantle source with no or little crustal input. The geochemical features of Kekem dykes are similar to those of Paleozoic and Mesozoic dykes recorded in North and Central Africa, suggesting multiple reactivations of pre-existing fractures that resulted in the fragmentation of western Gondwana and the opening of Central and South Atlantic Oceans.     

The post-Variscan development of the British Isles within a regional transfer zone influenced by orogenesis

The break-up of Pangaea after the Variscan Orogeny included rifting extending southwards from the Barents Sea via the Norwegian–Greenland Rift and into the North Sea, and northwards from the Central Atlantic. These two major rift systems interacted to form an approximately 1200-km-wide transfer zone across the British Isles, where a complex network of basins developed during the Mesozoic. Fault patterns were commonly controlled by reactivation of Precambrian, Caledonian and Variscan structures. The two main rift systems were unable to breach this regional transfer zone, where the crust had been thickened by the Caledonian and Variscan orogenies, until the Eocene. Breaching did not occur down the North Sea and through the English Channel because of Alpine contraction in NW Europe. Instead, breaching occurred around the west of Ireland and NW Scotland, so the British Isles remained connected to Europe rather than to the North American Plate.