Zhourong Cai

Geochronology of the Dong Tso Ophiolite and the Tectonic Environment

The wedge shaped Dong Tso ophiolitic block is distributed near the transition point from the western to the middle sub–belt of the Bangong–Nujiang suture zone. The ophiolite is characterized by well–developed cumulate rocks that are mainly composed of cumulate and massive gabbros. In the cumulate gabbros, the adcumulate amphiboles are distributed extensively around the plagioclase and residual pyroxene grains; hence, the rocks are named adcumulate amphibole–gabbro. In this study, the formation age of the ophiolite has been estimated to be 166 ± 4 million years (Ma) by the sensitive high–resolution ion microprobe (SHRIMP) II U–Pb isotopic analysis of the zircons from the adcumulate amphibole–gabbro; the 40Ar/39Ar plateau age was estimated to be 148.19 ± 1.53 Ma, which should represent the emplacement time of the ophiolite, by isotopic dating of the pure amphibole mineral from the amphibole–schist. Two different suits of volcanic lavas have been recognized in this work. The purple colored pillow basalts have high TiO2 and P2O5 contents, and are rich in light rare earth elements (LREEs), large–ion lithospheric elements (LILEs) and high–field–strength elements (HFSEs), the characteristics that are the typical of the oceanic island basalt (OIB). On the other hand, other massive basaltic andesites of celadon color are poor in MgO; rich in Fe2O3, LREEs, LILEs, and HFSEs; and especially characterized by negative Nb and Ta anomalies, the properties that establish the andesites as continental arc volcanic rocks. It is concluded that hotspots had developed in the old Dong Tso basin, the oceanic basin that had been developing from middle Jurassic (166 Ma) or even before and emplaced northward in late Jurassic (about 148 Ma).

Geochronology, geochemistry, and Sr–Nd–Pb isotopes of Cretaceous granitoids from western Tibet: petrogenesis and tectonic implications for the evolution of the Bangong Meso-Tethys

Abstract New zircon U–Pb age data, whole-rock major and trace elemental data, and Sr–Nd–Pb isotopic data of granitoids from the Woruo and Bailong areas in the southern margin of the Qiangtang Block (western Tibet, China) provide important evidence for northward subduction of the Bangong Meso-Tethyan oceanic lithosphere. Five granitoid samples are dated at 120.7–112.7 Ma. These granitoids mostly belong to calc-alkaline and high-K calc-alkaline series, and show strong enrichments in large-ion lithophile elements (e.g. Rb, Ba, and Th), depletions in Nb and Ta, and negative correlations between P2O5, TiO2, MgO, and SiO2 contents, consistent with the evolution trend of I-type magmas. The samples are characterized by a wide range of Sr–Nd–Pb isotopic compositions (initial εNd values from −0.06 to 5.84, initial 87Sr/86Sr from 0.704458 to 0.705596, (206Pb/204Pb)0 from 18.5436 to 19.2465, (207Pb/204Pb)0 from 15.6115 to 15.6712, and (208Pb/204Pb)0 from 38.6427 to 39.6942). The trace elemental and Sr–Nd–Pb isotopic data suggest that these samples are derived from a mixture of lower crust and mantle materials. Together with the published data, these Early Cretaceous magmatic rocks represent a continental arc setting formed by the northward subduction of the Bangong Meso-Tethys during the Early Cretaceous (120–112 Ma).

Development features of volcanic rocks of the Yingcheng Formation and their relationship with fault structure in the Xujiaweizi Fault Depression, Songliao Basin, China

The Xujiaweizi Fault Depression is located in the northern Songliao Basin, Northeast China. The exploration results show that the most favorable natural gas reservoirs are in the volcanic rocks of the Yingcheng Formation (K1yc). Based on seismic interpretation, drill cores and the results of previous research, we analyzed the distribution of faults and the thickness of volcanic rocks in different periods of K1yc, and studied the relationship of volcanic activities and main faults. Volcanic rocks were formed in the Yingcheng period when the magma erupted along pre-existing fault zones. The volcanic activities strongly eroded the faults during the eruption process, which resulted in the structural traces in the seismic section being diffuse and unclear. The tectonic activities weakened in the study area in the depression stage. The analysis of seismic interpretation, thin section microscopy and drill cores revealed that a large number of fractures generated in the volcanic rocks were affected by later continued weak tectonic activities, which greatly improved the physical properties of volcanic reservoirs, and made the volcanic rocks of K1yc be favorable natural gas reservoirs. The above conclusions provide the basis to better understand the relationship of the volcanic rock distribution and faults, the mechanism of volcanic eruption and the formation of natural gas reservoirs in volcanic rocks.

Initial Rifting Process and Dynamics Mechanism of Huaguang Sag: Evidence from a Numerical Modeling Method

Huaguang Sag is located in the deep seawater area of Qiongdongnan Basin, and its tectonic position belongs to the intersection of NE-trending, SN-trending and NW-trending tectonic systems in the continental margin of the Northwest South China Sea. To investigate the initial rifting process and further more the dynamics mechanism of Huaguang Sag, this paper sets up the structure model of basement which mainly makes up with several depression-controlling faults, and simulates the initial rifting process of Huaguang Sag by the FLAC software. The simulation results show that only affected by the S-N trending extensional stress, the rifting center appears in northern boundary basement faults (two NEE-trending and NWW-trending faults) of Huaguang Sag while does not take place at the NNE-trending and NE-trending basement fault zone in the middle sag, and doesn’t match the current pattern that the basement fault plays a main role in controlling the sediment. In the other case, affected by the S-N trending and E-W trending extensional stress at the same time, the areas of the northern boundary faults zone and internal NNE-trending basement faults zone come to be rifting center quickly, the sedimentary is controlled by the main basement faults to different degrees, and is consistent with the tectonic-sedimentary framework of Huaguang Sag which obtained by the data of geophysical interpretation. In combination with the analysis of regional tectonic background, the paper proposes that two remote tectonic effects occurred by the collision of India-Eurasian Plate: One remote effect was the rotational extrusion of IndoChina Block, which led to form a series of NE-trending and NNE-trending basement faults, as well as the E-W trending tensile stress field in Huaguang Sag. The other remote effect was that the deep mantle material of South China Block flowed southward, which resulted in the S-N trending extensional rifting of the lithosphere in northern South China Sea, and finally formed a series of EW-trending and NEE-trending basement faults and the S-N trending tensile stress field in Huaguang Sag. Affected by the above tensile stress fields and the basement faults, the initial rifting occurred in E-W and nearly S-N directions along the pre-existed basement faults (the weak structural zones) in Huaguang Sag.

Yinggehai basin gas exploration: Comparison with Jiyang depression

Yinggehai (莺歌海) basin and Jiyang (济阳) depression experienced similar tectonic evolution, which is mainly controlled by the strike-slip faults. The strike pull-apart basins are characteristic by multiple deposition cycles, migration of deposition and subsidence center, and diversity depositional systems. Furthermore, both basins show abnormal formation pressure. Compared with the oil and gas-rich Jiyang depression, Yinggehai basin developed the similar geological background that is favorable to the formation of funnel-shaped meshwork-carpet subtle reservoirs. Overpressure diapir body is the core of hydrocarbon accumulation in central diaper zone of Yinggehai basin. Driven by high pressure, oil and gas migrate along the funnel-shaped passage system into the overlying low-potential zone formed. The overlying sand bodies of overpressure diapirs are the favorable gas exploration zone.

Textural and map contrasts of the subduction-collision boundary between the Philippine Arc and the Sunda margin

The trench-arc-basin system of Southeast Asia is one of the most developed areas in the world, and there is a subduction-collision boundary which is still active nowadays between the Philippine Arc and the Sunda margin. Based on the previous research and the latest geological and geophysical data, the spatial distribution characteristics as well as the deformation characteristics of tectonic units on both sides of the tectonic boundary are systematically analyzed in this paper; we propose that the boundary begins from the east side of Taiwan coast plain in the northeastern South China Sea, then extends southward along the Manila Trench, through the thrust fault zone of Mindoro island, joints to the Negros Trench at the east edge of Sulu Sea, then the boundary goes through the thrust fault zone of Zamboanga and connects with the Cotabato Trench. According to the crust properties and interaction methods of the tectonic units on both sides of the boundary, the tectonic boundary can be divided into three types: arc-continent collision type, trench-subduction type and transitional type, and the different types of tectonic boundary have different significant characteristics in the geological and geophysical data. By the comparative analysis on the structural characteristics of the different sections of the boundary as well as the tectonic units on both sides of the boundary, we suggest that the crust properties on both sides of the boundary and the ancient tectonic position of the micro-continents of the west boundary are the main dynamic mechanism which leads to the significantly subsection characteristics of the tectonic boundary.

An Analysis of Cenozoic Petroleum Structural Mineralization in Kunbei Oil-Gas Pool, Southwestern Qaidam Basin, China

The Qaidam basin is a plateau basin inside the Tibetan plateau with very rich oil-gas resources. It has had a history of some forty years of exploration and geological research (Fu Suotang, et al., 2013; Chen Shijia, et al., 2012), despite the production of oil and gas inside the basin and extensive exploration since the late 1980s, but the proven rate is very low because of the complex accumulation conditions. For quite a long period the main exploration objectives were in the Cenozoic sedimentary sub-sag, and less attention has been paid to the thrust structure belt of basin margin, Recently, the discovery of a large oil-gas pool termed the Kunbei oil-gas pools made a great breakthrough in Qinghai oil field, this large oil-gas pool located in the Kunbei thrust structure belt of southwest margin of Qaidam basin, and characteristiced by shallow buried, big storage and high single well production, buried hill, anticline, fault block, structure-lithology and other types oil and gas reservoirs have been found. Preliminary studies results made it clear that the oil-gas pools formed with the collocation of long distance stepwise migration, continue filling and dynamic accumulation, it presents absolutely as a new special type of petroleum structural mineralization in China. However, there have not been reports or publications discussing the formation of oil-gas accumulation and the process of petroleum system crossing and superimposing. The purpose of this paper is to present the characteristics and formation mechanism of this oil-gas pool using an integrated geological, geophysical and geochemical approach based on data from relatively recently published papers and current research. 2 Structural Mineralization model