Cenozoic evolution of the central Myanmar drainage system: insights from sediment provenance in the Minbu Sub‐Basin

Located at the southern edge of the eastern Himalayan syntaxis, the Central Myanmar Basin (CMB) is divided into several Tertiary sub‐basins that have been almost continuously filled since the Indo‐Asia collision. They are currently drained by the Irrawaddy River, which flows down the eastern Tibetan Plateau and the Sino‐Burman Ranges. Tracing sediment provenance from the CMB is thus critical for reconstructing the past denudation of the Himalayan‐Tibetan orogen; it is especially relevant since a popular drainage scenario involves the capture of the Tsangpo drainage system in Tibet by a precursor to the Irrawaddy River. Here, we document the provenance of sediment samples from the Minbu Sub‐Basin at the southern edge of the CMB, which is traversed by the modern stream of the Irrawaddy River. Samples ranging in age from middle Eocene to Pleistocene were investigated using Nd isotopes, trace element geochemistry and sandstone modal compositions. Our data provide no evidence of a dramatic provenance shift; however, sandstone petrography, trace element ratios and isotopic values display long‐term trends indicating a gradual decrease of the volcanic input and its replacement by a dominant supply from the Burmese basement. These trends are interpreted to reflect the progressive denudation of the Andean‐type volcanic arc that extended onto the Burmese margin, along the flank of the modern Sino‐Burman Ranges, where most of the post‐collisional deformation of central Myanmar is located. Though our results do not exclude an ephemeral or diluted contribution from a past Tsangpo‐Irrawaddy connection, sedimentation rates suggest that this hypothesis is unlikely before the development of a stable Tsangpo‐Brahmaputra River in the Miocene. These results thus suggest that the central Myanmar drainage basin has remained restricted to the Sino‐Burman Ranges since the beginning of the India‐Asia collision.     

Inter-decadal Variability in Phytoplankton Community in the Middle Adriatic (Kaštela Bay) in Relation to the North Atlantic Oscillation

Evaluation of a 45-year data set of primary production (PP), a 30-year data set of phytoplankton biomass, and a 51-year data set of species composition shows an increase of phytoplankton biomass and abundance in the period from the mid-1980s to the mid-1990s. Phytoplankton biomass showed bimodal seasonal cycles, with winter and spring maxima, which did not change over the past 30 years. Diatoms were the most abundant functional group and they prevailed during the colder part of the year while the dinoflagellate contribution to the phytoplankton community increased in the warmer period from May to August. Diatoms showed a significant negative correlation with sea surface temperature (SST), while dinoflagellates were positively correlated with SST. An increase of phytoplankton abundance, particularly dinoflagellate, in the period from the mid-1980s to the mid-1990s coincided with years characterized by a high North Atlantic Oscillation (NAO) index. Primary production and chlorophyll a concentration in the spring period were negatively correlated with the NAO winter (DJFM) index, probably caused by increased precipitation associated with a low or negative NAO index. PP in winter during the mixing period was positively related to the NAO winter index associated with higher temperatures and dry conditions which brought more clear days and increased input of solar radiation.     

Influence of fluids and magma on earthquakes: seismological evidence

In this paper, we present seismological evidence for the influence of fluids and magma on the generation of large earthquakes in the crust and the subducting oceanic slabs under the Japan Islands. The relationship between seismic tomography and large crustal earthquakes (M=5.7-8.0) in Japan during a period of 116 years from 1885 to 2000 is investigated and it is found that most of the large crustal earthquakes occurred in or around the areas of low seismic velocity. The low-velocity zones represent weak sections of the seismogenic crust. The crustal weakening is closely related to the subduction process in this region. Along the volcanic front and in back-arc areas, the crustal weakening is caused by active volcanoes and arc magma resulting from the convective circulation process in the mantle wedge and dehydration reactions in the subducting slab. In the forearc region of southwest Japan, fluids are suggested in the 1995 Kobe earthquake source zone, which have contributed to the rupture nucleation. The fluids originate from the dehydration of the subducting Philippine Sea slab. The recent 2001 Geiyo earthquake (M=6.8) occurred at 50 km depth within the subducting Philippine Sea slab, and it was also related to the slab dehydration process. A detailed 3D velocity structure is determined for the northeast Japan forearc region using data from 598 earthquakes that occurred under the Pacific Ocean with hypocenters well located with SP depth phases. The results show that strong lateral heterogeneities exist along the slab boundary, which represent asperities and results of slab dehydration and affect the degree and extent of the interplate seismic coupling. These results indicate that large earthquakes do not strike anywhere, but only anomalous areas which can be detected with geophysical methods. The generation of a large earthquake is not a pure mechanical process, but is closely related to physical and chemical properties of materials in the crust and upper mantle, such as magma, fluids, etc.     

顾及水汽衰减因子的中国区域大气可降水量模型的建立与分析

提出一种顾及水汽衰减因子的PWV估算模型,通过输入地面大气水汽压和水汽衰减因子获得PWV,并选取2018年中国地区85个探空测站和7个IGS测站1 a的观测数据用于验证新模型的精度。结果表明,在已知当日水汽衰减因子的情况下,模型估算的PWV精度约为2 mm;也可通过GPT2w格网内插得到任意位置的水汽衰减因子,其结果精度与传统的一次多项式模型相当,但新模型的作用范围更广、适用性更强。