Journal: |
Acta Geologica Sinica 2018 No.1
clicks:161 |
Title:
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Author:
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LI Dahu1, 2, LIAO Hua1, *, DING Zhifeng2 , ZHAN Yan3 , WU Pingping4 , XU Xiaoming2 and ZHENG Chen2 |
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Abstract:
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The special seismic tectonic environment and frequent seismicity in the southeastern margin of
the Qinghai–Tibet Plateau show that this area is an ideal location to study the present tectonic movement
and background of strong earthquakes in mainland China and to predict future strong earthquake risk
zones. Studies of the structural environment and physical characteristics of the deep structure in this
area are helpful to explore deep dynamic effects and deformation field characteristics, to strengthen our
understanding of the roles of anisotropy and tectonic deformation and to study the deep tectonic
background of the seismic origin of the block’s interior. In this paper, the three-dimensional (3D) P-wave
velocity structure of the crust and upper mantle under the southeastern margin of the Qinghai–Tibet
Plateau is obtained via observational data from 224 permanent seismic stations in the regional digital
seismic network of Yunnan and Sichuan Provinces and from 356 mobile China seismic arrays in the
southern section of the north–south seismic belt using a joint inversion method of the regional
earthquake and teleseismic data. The results indicate that the spatial distribution of the P-wave velocity
anomalies in the shallow upper crust is closely related to the surface geological structure, terrain and
lithology. Baoxing and Kangding, with their basic volcanic rocks and volcanic clastic rocks, present
obvious high-velocity anomalies. The Chengdu Basin shows low-velocity anomalies associated with the
Quaternary sediments. The Xichang Mesozoic Basin and the Butuo Basin are characterised by lowvelocity anomalies related to very thick sedimentary layers. The upper and middle crust beneath the
Chuan–Dian and Songpan–Ganzi Blocks has apparent lateral heterogeneities, including low-velocity
zones of different sizes. There is a large range of low-velocity layers in the Songpan–Ganzi Block and the
sub–block northwest of Sichuan Province, showing that the middle and lower crust is relatively weak.
The Sichuan Basin, which is located in the western margin of the Yangtze platform, shows high-velocity
characteristics. The results also reveal that there are continuous low-velocity layer distributions in the
middle and lower crust of the Daliangshan Block and that the distribution direction of the low-velocity
anomaly is nearly SN, which is consistent with the trend of the Daliangshan fault. The existence of the
low-velocity layer in the crust also provides a deep source for the deep dynamic deformation and seismic
activity of the Daliangshan Block and its boundary faults. The results of the 3D P-wave velocity structure
show that an anomalous distribution of high-density, strong-magnetic and high-wave velocity exists
inside the crust in the Panxi region. This is likely related to late Paleozoic mantle plume activity that led
to a large number of mafic and ultra-mafic intrusions into the crust. In the crustal doming process, the
massive intrusion of mantle-derived material enhanced the mechanical strength of the crustal medium.
The P-wave velocity structure also revealed that the upper mantle contains a low-velocity layer at a
depth of 80–120 km in the Panxi region. The existence of deep faults in the Panxi region, which provide
conditions for transporting mantle thermal material into the crust, is the deep tectonic background for the area’s strong earthquake activity. |
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