Chemical composition of upper crust in eastern China

In an area of 3.3×10⁶ km² within eastern China, 28 253 rock samples were collected systematically and combined into 2718 composite samples which were analyzed by 15 reliable methods using national preliminary certified reference materials (CRMs) for data quality monitoring. The average chemical compositions of the exposed crust, the sedimentary cover and the exposed basement as well as the upper crust for 76 chemical elements in eastern China are given. A key basic geology projcct supported by Ministry of Geology and Mineral Resources of China.     

Paleoclimatological and paleoenvironmental records since 4 000 a B. P. in sediments of alpine lakes in Taiwan

The 3 310-m-high Chia-min Lake records the climatic history since 4 ka B. P. in Taiwan. The warm/wet period before 2.2 ka B.P. seemed to correspond to the later part of the Holocene Megathermal, and the cold/dry period during 0–2.2 ka B. P. corresponded to the Katathermal. Before the termination of the Megathermal, an especially warm and humid segment (2.2–2.4 ka B. P.) emerged. The paleoclimatic records from Yuen-yang and Chi-tsai Lakes support the notion that the Megathermal in Taiwan terminated during 2—2.3 ka B. P. A warm segment (820–1 320 AD) in the Katathermal could be considered the Medieval Warm Period. The climate turned cold and dry after 1 320 AD and this indicated the onset of the Little Ice Age. These paleoclimatic variations are also in good agreement with those recorded in Great Ghost Lake.     

Crustal structure of Dabieshan orogenic belt

The crustal structures ofP velocity and density on the deep seismic sounding profile across the Ilabieshan orogenic belt are presented. There is a 5-km-thick crustal “root” between the Yuexi and Xiaotian where the elevation is highest on the profile. An apparent Moho offset of 4. 5 km beneath the Xiaotian-Mozitan fault marks the paleo-suture of the Triassic collision. A high-velocity anomaly zone at the depth below 3 km beneath the ultra-high pressure (UHP) zone may be correlated to the higher content of UHP metamorphic rocks. Project supported by the National Natural Science Foundation of China and the Joint Earthquake Science Foundation.     

Numerical modeling of 3-D terrain effect on MT field

Using the boundary element method, the numerical modeling problem of three-dimensional terrain effect on magnetotelluric (MT) field is solved. This modeling technique can be run on PC in the case of adopting special net division. The result of modeling test for 2-D terrain by this modeling technique is basically coincident with that by 2-D modeling technique, but there is a great difference between the results of 3-D and 2-D modeling for 3-D terrain. Project supported by the National Natural Science Foundation of China.     

Magnetostratigraphy and palaeoclimate of Red Clay sequences from Chinese Loess Plateau

Two Red Clay profiles near Xi’an and Xifeng were investigated in an attempt to determine magnetostratigraphic and palaeoclimatic records. The results show that aeolian dust accumulation and the related East Asia palaeomonsoon system had begun by 6.5 Ma, and it is deduced that the Tibetan Plateau had reached a significant elevation at that time. The late Tertiary palaeoclimatic history of the Red Clay as reflected by magnetic susceptibility is reconstructed during the period of 6.5–2.5 Ma. Stepwise increase in susceptibility of aeolian dust accumulation appears to have a close correlation to the uplift processes of the Tibetan Plateau. The remarkable increase of aeolian dust accumulation at 3.2 Ma appears to be due to the influence of global ice volume on the East Asia monsoon. Palaeomonsoon variation during the late Tertiary as recorded in the Red Clay sequences from the Chinese Loess Plateau can be regarded as the product of a number of interacting factors, such as uplift of the Tibetan Plateau, solar radiation, global ice volume, etc. Project supported by the National Natural Science Foundation of China and the Foundation of Xi’an Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences.     

Properties of wave velocity for two types of granitoids at high pressure and temperature and their geological meaning

The wave velocity for two types of granitoids was measured using the analytic method of full-wave vibration at high pressure and high temperature. The laws of velocity changes for them differ with the pressure boost and temperature rise, and the velocity change of S-type is more violent than that of I-type. The “softening point” of compressional wave velocity (V μ) is also revealed during the measurement for two types of granitoids imitating the pressure and temperature at a certain depth. But the depth of “softening”, V_p after “softening” and the percentage of V_p’s drop around the “sofrening point” for two types of granitoids are obviously different. The depth of “softening” is 15 km approximately and V_p after “softening” is 5.62 km/s for S-type granitoid. But for I-type granitoid the depth of “softening” is 26 km approximately and V_p after “softening” is 6. 08 km/s. Through careful analysis of rock slices after the experiment, it was found that the “softening” of elastic-wave velocity is caused by the partial melting of granite. Combined with the results of geophysical prospecting, these results suggest that the low-velocity layers developing in the interior of Earth crust are related to thc partial melting of different types of granitoids. The formation of the low-velocity layer in the upper-middle Earth crust is closely related to the development of S-type granitoid, but that in the lower Earth crust is closely related to the development of I-type granitoid.     

Main models and distribution patterns of gas reservoirs in large-medium gas fields of China

Based on characteristics and trap types of gas reservoirs in large and medium gas fields in China, 4 gas reservoir models have been established:(i) structural trap gas reservoir model I, formed earlier than or simutaneously with generating of gases; (ii) structural trap gas reservoir model II, formed later than generating of gases; (iii) fossil weathered residuum gas reservoir model; and (iV) mud diapir abnormal temperature and pressure gas reservoir model. Distribution patterns of large-medium gas fields are described with the concept of “sealed compartment”. It is concluded that the inner-compartment area, marginal area of the compartment and the areas between two overlapped sealed compartments are the most favourable areas for discovering large-medium gas fields. Project supported by the “85–102” Chinese National Key Science and Technology Project.