Through tectonostratigraphic analysis of the nonmarine, intracontinental Songliao basin in northeast China, four episodes of deformation are recognized: mantle upwelling, rift, postrift thermal subsidence and structural inversion.The episodes are related to regional geodynamics and plate motions. Each episode is associated with a speci¢c stratigraphic signature.The ¢rst period of deformation occurred during the Middle and Late Jurassic when asthenospheric upwelling heated, thinned and stretched the lithosphere.These events may have been caused by the narrowing of the Okhotsk Sea through subduction.This deformation is characterized by doming, extension, widespread volcanism and intrusion, and erosion.Volcanics inter¢nger with alluvial fan and alluvial plain facies systems tracts.The second rifting episode began in the latest Jurassic and continued into the Early Cretaceous. It resulted in the formation of a large number of isolated, NNE-trending fault blocks of 'basin-and-range' style. Rifting may have been caused by the formation and subduction of the Izanagi and Paci¢c Plates. Coal-bearing £uvial, £oodplain, lacustrine and fan-delta strata and widespread volcanic rocks ¢lled the fault-block basins.Volcanic strata hundreds to several thousand meters thick in the Huoshiling and Yingcheng Formations record multiple intrusive events during the rifting stage in the basin.These events were concurrent with episodes of intrusion and volcanic eruption in northeast China.The third phase of regional postrift deformation and subsidence, which began with the Lower Cretaceous Denglouku Formation, was caused by lithospheric cooling and extension, modulated by multiple compressional events. Subsidence in the Songliao basin permitted accumulation of thick postrift deposits, in contrast with other Cretaceous basins in Mongolia and northeast Asia.Three compressional episodes, which episodically interrupted the long-term cooling subsidence, originated from development of the Okhotsk suture and subduction of the Paci¢c plate. In the Early Cretaceous, pronounced compression originated from closure of Okhotsk Sea, forming the mountain ranges of Daxinganling, which provided sediment to the northern part of the basin. In the Late Cretaceous, the intensity of compression from the Paci¢c margin increased through time, causing westward migration of depocentres and uplift in the east until the end of Cretaceous. Postrift strata, typically 3000^4000 m thick with a maximum thickness of 6000 m, extend beyond the rift blocks and onlap the basin margins to form a large uniform basin. Early thermal subsidence strata include alluvial fan, £uvial, £oodplain, shallow lacustrine and delta facies tracts, overlain by large deltaic and lacustrine facies. Late postrift environments featured by large lakes in the basin centre rimmed by delta, £uvial and £oodplain environments. Re£ection seismic pro¢les show that strong structural inversion, including folding and uplift, began at the end of the Nenjiang Formation and culminated at the end of the Cretaceous...
Summary A mid Proterozoic volcanic series, the Xionger Group, in the eastern Qinling Mountains of central China, may be one of the earliest products of plate subduction in eastern Asia. The main rock types of the 1710 Ma Xionger Group are andesites and dacites of the calcalkaline series. The rocks of the Xionger Group have trace element contents and REE patterns which are comparable to those of lavas from modern active continental margins, implying that the Xionger Group volcanics had a similar tectonic setting. It appears that there was an important event in China at 1800–2000 Ma, after which plate tectonics with the Wilson cycle was the main contributor to crustal growth.
Abstract:There is a cross‐cutting relationship between the E‐W trending structures and the NE‐trending structures in the northern Longmen‐Micang Mountains region, which reflects possible regional tectonic transition and migration. Apatite fission track (AFT) analyses of 15 samples collected from this area yield apparent ages varying from 30.3±4.2 Ma to 111.7±9.0 Ma and confined‐track‐lengths ranging from 10.6±0.3 μm to 12.4±0.1 μm. Four specific groups were identified on the basis of the Track Age Spectrum Calculation (TASC) patterns, i.e., 143–112 Ma, 93.6–88 Ma, 42–40 Ma and ∼25.6 Ma. These age groups correspond to the spatial distributions of datasets and may represent four tectonic events. Together with the regional deformation patterns, the four age groups are interpreted to indicate tectonic superposition, transition and migration during the Meso‐Cenozoic with the following possible order: (1) the Micang Mountains belt was dominated by the E‐W trending structure during 143–112 Ma; (2) the contraction of the Longmen Mountains belt from the NW to the SE during 93.6–88 Ma led to the superposition of the NE‐trending structures over the E‐W trendinding structures; (3) dextral strike‐slip shear dominated the Longmen Mountains belt at 42–40 Ma; (4) westward migration of the active tectonic belt occurred from 93.6–25.6 Ma in a break‐back sequence in the northern Longmen Mountains belt. The Late Cenozoic tectonics in the northern Longmen Mountains belt are characterized by the dextral strike‐slip shear and the occurrence of westward break‐back sequence of deformations. As a result, north‐south differences in deformations along the Longmen Mountains belt were intensified since the Miocene time and strains were mainly accumulated in the hinterland of the Longmen Mountains instead of being propagated to the foreland basin.
The Liangshan and Qixia formations in the Sichuan Basin of central China were formed in the earlier middle Permian. Based on outcrop observation of the Changjianggou section at Shangsi, Guangyuan region and 3rd‐order sequence division in typical drillings, one‐dimensional spectrum analysis has been used to choose the better curve between the natural gamma ray spectrometry log(ln (Th/K)) in Well‐Long17 and the gamma ray log(GR) in Well‐Wujia1, respectively, for identifying Milankovitch cycles in Sequence PSQ1 which comprises the Liangshan and Qixia formations, and then to identify the variation in the Milankovitch cycle sequences. On this basis, the system tract and 4th‐order sequence interfaces in Sequence PSQ1 were found via two‐dimensional spectral analysis and digital filtering. Finally, a high‐frequency sequence division program was established. Among these cycles, long eccentricity (413.0 ka) and short eccentricity (123.0 ka) are the most unambiguous, and they are separately the major control factors in forming 4th‐order (parasequence sets) and 5th‐order (parasequences) sequences, with the average thicknesses corresponding to the main cycles being 11.47 m and 3.32 m in Well‐Long17, and 14.21 m and 3.79 m in Well‐Wujia1, respectively. In other words, the deposition rate in the beach subfacies is faster than that of the inner ramp facies. The ln(Th/K) curve is more sensitive than the GR as the index of relatively ancient water depth in carbonate deposition. One‐dimensional spectrum analysis of ln(Th/K) curve could distinguish the Milankovitch cycle sequences that arose from the Precession cycle (20.90 ka), with a much higher credibility. Sequence PSQ1 in Well‐Long17 contains 10 4th‐order sequences, and the growth span of Sequence PSQ1 consisting of the Liangshan and Qixia formations is about 4.13 Ma. The single deposition thickness of the long eccentricity cycle sequence has the characteristics of thinning and then thickening in the two‐dimensional spectrum, which could be used to identify the system tract interface of the 3rd‐order sequence. The precession sequence thickness remains stationary. As a result, the early deposition rate in the mid‐Permian of the Sichuan basin was very slow, remaining nearly stationary, and this reflects a sustained depositional environment. Whole‐rock carbon and oxygen isotope curves could also prove this point Milankovitch cycle sequence studies provide a basis for paleoenvironmental analysis and, as such, can be used to analyze ancient climate change, calculate deposition rate and deposition time, and carry out fine isochronous stratigraphic correlation.
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