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Zhang J.,Chinese Academy of Geological Sciences | Li J.-Y.,Chinese Academy of Geological Sciences | Liu J.-F.,Chinese Academy of Geological Sciences | Li Y.-F.,China Center for Earthquake Disaster Emergency | And 2 more authors.
Geological Bulletin of China | Year: 2013

Recent found pillow lava basalts in the southwestern Langshan and its preliminary dating has been reported here. Previous survey had assigned its age at the Early Paleozoic. 32 zircons from pillow lava basalts have been dated using LA-ICP-MS. Most zircons from basalts are captured from the basement, the distribution of ages and internal structures of which are complicated. Four groups of zircons have been distinguished by their internal structures. Among them the tabular zircons have been believed as the autogenetic zircons of pillow lava basalts, the ages of these zircons range from the end of Permian to the Early Triassic. The rest zircons in the samples are all from the Alashan basement, among which Paleoproterozoic and Newarchaeozoic zircons and Paleozoic zircons are most. The latest Late Permian Langshan basalts may developed in the extensional tectonics resulting from the delamination of the lithosphere after the closure of the Paleoasian Ocean, the extension of this period occurred along the whole north margin of the North China Plate including the Alashan block, which means that the Alashan block jointed with the North China Plate at least before the Late Paleozoic.

Zhang J.,Chinese Academy of Geological Sciences | Li J.,Chinese Academy of Geological Sciences | Liu J.,Chinese Academy of Geological Sciences | Li Y.,China Center for Earthquake Disaster Emergency | And 2 more authors.
Acta Petrologica Sinica | Year: 2012

Located between the North China Plate to the east, and the North Qilian Orogenic Belt to the southeast, the eastern Alxa Block is the key region to answer many controversies surrounding the tectonic setting of this region in the Early Paleozoic: among them the debates on the basement composition of the Alxa Block, whether the Alxa Block connected to the North China Plate in the Early Paleozoic, when exactly this event did happen, where the Lower Paleozoic sedimentary rocks along the southwestern Ordos had their provenances, and whether the so-called Helan Aulacogen formed in the Ordovician are the key questions to be solved. Laser-ablation inductively coupled plasma mass spectroscopy U-Pb dating of detrital zircons in two samples from Middle Ordovician sandstones to the west of Helan Shan located in the eastern Alxa Block yields a majority of 206Pb/238U ages between 900Ma and 950Ma with peak ages at 916Ma in Sample Alxa-1 and 953Ma in Sample Alxa-2, respectively, a smaller group yields 206Pb/U ages between 494Ma and 623Ma with peak ages at 505Ma and 588Ma in Sample Alxa-1 and 494Ma, 517Ma and 623Ma in Sample Alxa-2, respectively. Two samples all have a minor peak age at ca. 2500Ma with 2517Ma in Alxa-1 and 2552Ma and 2670Ma in Alxa-2. Moreover, there are a few zircons older than 3000Ma. The youngest 206Pb/ 238U ages of the two samples are similar with 451 ±8Ma in Alxa-1 and 483 ±4Ma in Alxa-2. These zircon U-Pb ages combined with several lines of sedimentary evidence imply that the so-called Helan Aulacogen did not exist in the Ordovician: a peripheral foreland basin related to the North Qilian Orogenic Belt developed instead, the Early Paleozoic detritals mainly came from the North Qilian Arc, the Neoproterozoic detritals mainly came from the Alxa Block, the detrital zircon ages along the whole western Ordos showing that the North Qilian Orogenic Belt (Arc) came closer to the Alxa Block, and the ocean between them closed gradually, and the basement of the Alxa Block is different from that of North China Plate, the Alxa Block underwent important tectono-magmatic events during the Paleozoic and Neoproterozoic, which have not found in the North China Plate. The Alxa Block may connect with the North China Plate during the Middle Ordovician or after it.

Lu M.-Y.,China Center for Earthquake Disaster Emergency | Zheng W.-H.,Huazhong University of Science and Technology | Chen L.-W.,Chinese Institute of Crustal Dynamics | Zhan Z.-M.,Chinese Institute of Crustal Dynamics | And 3 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2010

We use the three-dimensional non-linear dynamic finite element method to perform a modeling study on dynamical stress effects with a rhombic model and analyze the Tangshan M7. 8 earthquake in 1976. The result shows that stress wave is generated by shock force which is produced by strengthened moderately strong earthquakes before the Tangshan earthquake, when this wave encounters a fault, the reflected stress wave, becomes tensile due to half-wavelength loss, which causes cracks, local rupture and crack linkage in the rock by reducing rock friction. On the one hand, other earthquakes are triggered. On the other hand, energy for the Tangshan earthquake is accumulated. This is a process of positive feedback. The shock force from the Tangshan earthquake during processes of loading and unloading has a time difference, this forms the different phase of shock waves. The dynamical stress produced by stress wave, superposed on the confining pressure, makes confining pressure reduce and change at low values. These effects can reduce shear strength and make the initial fracture of the Tangshan earthquake propagate toward northeast and southwest respectively. The unloading of shock force generated by the Tangshan earthquake is equal to producing the effect of tensile stress wave, the reflected tensile stress waves, having lost half-wavelength, become compressive ones which cause confining pressure increase and rupture stop. Meanwhile during the process of dynamical stress propagation, its amplitude decays. So the aftershock occurrence of the Tangshan earthquake is a negative feedback process.

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