A large volume of Early Cretaceous volcanic rocks are exposed in the central Lhasa subterrane, which are essential for models developed for understanding the evolution of Lhasa Terrane. However, the petrogenesis and geodynamic background of these rocks are still in debate. In this paper, we carried out a detailed study on the volcanic rocks of the Zenong Group, including andesite, dacite, and rhyolite from the Coqen area in the central Lhasa subterrane. Zircons from one dacitic tuff yields an age of 115.3 ± 1.1 Ma. All the rocks are enriched in light rare earth elements (LREEs), Th, U, and Pb and depleted in high field strength elements (HFSEs, e.g. Nb, Ta, P, and Ti). The dacite and rhyolite samples are high‐K calc‐alkaline to shoshonitic and are metaluminous to peraluminous. These felsic samples are characterized by negative Ɛ Nd (t) (−8.9 to −2.1) and Ɛ Hf (t) (−8.1 to −5.6) and have high radiogenic Pb isotopic composition ( 206 Pb/ 204 Pb = 18.6287–19.5578, 207 Pb/ 204 Pb = 15.7161–15.7720, and 208 Pb/ 204 Pb = 39.1440–40.6253). Particularly, a negative correlation between the Ɛ Nd (t) values and SiO 2 contents can be observed in these dacites and rhyolites. Meanwhile, the andesites exhibit similar Sr–Nd–Pb isotopic compositions to dacites and rhyolites (Ɛ Nd (t) = −4.3; 206 Pb/ 204 Pb = 18.7119–18.8575, 207 Pb/ 204 Pb = 15.7159–15.7203, and 208 Pb/ 204 Pb = 39.2944–39.6036). Our new data indicate that different geochemical reservoirs are involved in generating the ca. 115 Ma magmatism in the central Lhasa subterrane. It is likely that dacites and rhyolites are derived from the magma mixing between the ancient basement‐derived melts and mantle‐derived melts with varying degrees of partial melting. The andesites might originate from partial melting of an ancient lithospheric mantle that have been metasomatized by fluids and/or melts derived from the Tethyan oceanic slab. Our results, together with recent studies, suggest that the Zenong Group volcanic rocks in the Coqen area may represent the magmatic responses to the break‐off of the southward subduction of the Bangong–Nujiang Tethyan oceanic slab.
Abstract. We obtain the coseismic surface deformation fields caused by the Chile Mw8.3 earthquake on 16 September 2015 through analyzing Sentinel-1A/IW InSAR data from ascending and descending tracks. The results show that the main deformation field looks like a half circle convex to east with maximum coseismic displacement of about 1.33 m in descending LOS direction, 1.32 m in ascending LOS direction. Based on an elastic dislocation model in a homogeneous elastic half space, we construct a small-dip single plane fault model and invert the coseismic fault slip using ascending and descending Sentinel-1A/IW data separately and jointly. The results show that the patterns of the main slip region are similar in all datasets, but the scale of slip from ascending inversion is relatively smaller. Joint inversion can display comprehensive fault slip. The seismic moment magnitude from the joint inversion is Mw8.25, the rupture length along strike is about 340 km with a maximum slip of 8.16 m near the trench located at –31.04 N, –72.49 E, and the coseismic slip mainly concentrates at shallow depth above the hypocenter with a symmetry shape. The depth where coseismic slip is near zero appears to a depth of 50 km, quantitatively indicating the down-dip limit of the seismogenic zone. From the calculated coseismic Coulomb stress change, we find aftershocks locations correlate well with the areas having increased Coulomb stress and most areas with increased Coulomb stress appeared beneath the main shock fault plane.
Vertical coseisimic deformation near seismogenic fault provides meaningful information for understanding of rupture characteristics of the seismogenic fault and focal mechanism.Taking Wenchuan thrust earthquake for an example, we interpolate GPS horizontal observed deformation using Biharmonic spline interpolation and derive them into East-Westward or North-Southward deformation field.We first use reliable GPS observed value to correct InSAR reference point and to unify both GPS and InSAR coordinate frame, and then obtain a continuous vertical deformation field by combined calculation of GPS and InSAR LOS deformation field.The results show that the vertical deformation of both hanging wall and foot wall of the fault decreases rapidly, with deformation greater than 30cm within 50km across the fault zone.The uneven distribution of the vertical deformation has some peak values at near fault, mainly distributed at the southern section (Yingxiu), the middle (Beichuan) and the northern end (Qingchuan) of the seismogenic fault.
In this work we employ the pixel offset tracking technique to capture large displacements in incoherent zone nearby the Yingxiu-Beichuan fault caused by 2008 Wenchuan Ms8.0 earthquake. The used data of 6 tracks is from ALOS/PASAR dataset of Japan. The result shows that the entire surface rupture belt is 238km long, extending almost linearly in a direction NE42°. It is offset left laterally by a NW-striking fault at Xiaoyudong and turns at Gaochuan, where the rupture belt shifts toward south by 5km largely keeping the original trend. In terms of features of rupture traces, the rupture belt can be divided into 5 sections and 3 types. North to rupture belt, surface displacements are 2.95m on average, mostly in 2.0∼3.5m with the maximum 7.0∼9.0m at individual places nearby Beichuan. South to the rupture belt, the average displacement is 1.75m, dominated by 1.0∼2.0m, with 3.0∼4.0m at a few sites. Along the Guanxian-Jiangyou fault, there is a uplift zone in the radar line of sight, which is 66km long, 1.5∼6.0km wide, with vertical displacements about 2m but no observable rupture traces.
Numerous Paleozoic deposits have been found in the Tianshan Mountains. Postmineralization burial plays an important role in the preservation of Paleozoic epizonal deposits. However, the preservation mechanisms for mesozonal deposits in the Tianshan Mountains need further investigation. The late Paleozoic Katebasu gold–copper deposit (with a reported mineralization age varying from ∼ 270 Ma to ∼ 330 Ma) in the Chinese Western Tianshan is a mesothermal magmatic hydrothermal deposit that formed at mesozonal depths. The exhumation process and preservation mechanisms of this deposit remain ambiguous. In this study, (U–Th)/He and fission-track dating were applied to samples from a vertical profile to constrain the exhumation history of the Katebasu deposit. Apatite (U–Th)/He and fission-track ages vary systematically with elevation, ranging from 32.4 ± 9.0 Ma to 176.4 ± 18.0 Ma and 106.4 ± 3.1 Ma to 181.7 ± 5.1 Ma, respectively. Zircon (U–Th)/He ages range from 220.4 ± 11.0 Ma to 260.1 ± 17.4 Ma. The age–elevation relationship and inverse thermal modeling reveal that the Katebasu deposit underwent two phases of exhumation. The first phase of exhumation, which caused at least 4 km of erosion, occurred during the late Paleozoic to Early Jurassic. The second exhumation started during the Early Oligocene, resulting in ∼ 0.9 km of erosion. A protracted period of tectonic stability during the middle–late Mesozoic to early Cenozoic and limited exhumation during the late Cenozoic uplift played important roles in the preservation of the Katebasu deposit.
This work estimated the coseismic deformation field of the Yushu earthquake in 2010 using the C band ASAR data, and inverted the fault slip distributions and simulated the interferograms. The results demonstrated that the slip distributions were mainly presented in shallow depth above 15km.There were two slip-concentration areas, maximum slip was nearby the Jiegu town, and fault motion sense was left-lateral strike-slip, which were all reconciled with field observations.
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