Deep seismic sounding in the region of the Mirnyi kimberlite field indicates that the boundary velocity of the uppermost mantle is elevated (vb=8.6–8.8 km/sec) and extremely variable near the Mir kimberlite pipe. These velocity heterogeneities are probably associated with the kimberlite magmatism and may be useful in the identification of other kimberlite fields. 相似文献
The VRANCEA99 seismic refraction experiment is part of an international and multidisciplinary project to study the intermediate depth earthquakes of the Eastern Carpathians in Romania. As part of the seismic experiment, a 300-km-long refraction profile was recorded between the cities of Bacau and Bucharest, traversing the Vrancea epicentral region in NNE–SSW direction.
The results deduced using forward and inverse ray trace modelling indicate a multi-layered crust. The sedimentary succession comprises two to four seismic layers of variable thickness and with velocities ranging from 2.0 to 5.8 km/s. The seismic basement coincides with a velocity step up to 5.9 km/s. Velocities in the upper crystalline crust are 5.9–6.2 km/s. An intra-crustal discontinuity at 18–31 km divides the crust into an upper and a lower layer. Velocities within the lower crust are 6.7–7.0 km/s. Strong wide-angle PmP reflections indicate the existence of a first-order Moho at a depth of 30 km near the southern end of the line and 41 km near the centre. Constraints on upper mantle seismic velocities (7.9 km/s) are provided by Pn arrival times from two shot points only. Within the upper mantle a low velocity zone is interpreted. Travel times of a PLP reflection define the bottom of this low velocity layer at a depth of 55 km. The velocity beneath this interface must be at least 8.5 km/s.
Geologic interpretation of the seismic data suggests that the Neogene tectonic convergence of the Eastern Carpathians resulted in thin-skinned shortening of the sedimentary cover and in thick-skinned shortening in the crystalline crust. On the autochthonous cover of the Moesian platform several blocks can be recognised which are characterised by different lithological compositions. This could indicate a pre-structuring of the platform at Mesozoic and/or Palaeozoic times with a probable active involvement of the Intramoesian and the Capidava–Ovidiu faults. Especially the Intramoesian fault is clearly recognisable on the refraction line. No clear indications of the important Trotus fault in the north of the profile could be found. In the central part of the seismic line a thinned lower crust and the low velocity zone in the uppermost mantle point to the possibility of crustal delamination and partial melting in the upper mantle. 相似文献
本文对白令海北部陆坡B2-9站位沉积物岩芯开展了高分辨率的生物标志物分析,获得了研究区近一万年来陆源长链正构烷烃(简写为烷烃)的输入及其源区植被结构的变化等相关记录。结果表明,nC27是烷烃中最高的主碳峰,对烷烃总量的贡献也最大,这可能与源区木本植物的丰度及其分布有关;nC23的含量也较高,可能主要是来源于北半球沿海地区广泛分布的一类沉水植物。全新世期间,烷烃总量分别在7.8 ka B.P.,6.7 ka B.P.和5.4 ka B.P.经历了三次阶梯状的下降过程,呈现出四个相对稳定的阶段,可能主要受控于早全新世海平面上升以及源区气候环境和植被分布的变化。烷烃的分子组合特征各参数(如CPI、ACL以及nC31/nC27等)的变化则表明,烷烃主要来自陆生高等植物,且全新世期间植被结构较为稳定,木本植物占据优势。此外,在几个较短的时期内,烷烃总量及其分子组合特征等参数的变化具有同步性,表明在这些时期特殊的气候条件下,源区木本植物烷烃对烷烃总量的贡献率的增加可能低于草本植物烷烃和化石烷烃。 相似文献