RAYLEIGH WAVE TOMOGRAPHY IN THE CRUST AND UPPER MANTLE OF THE DABIE-TANLU OROGENIC ZONE

Dabie Orogen has a series of special tectonic and geological features which make it important to the study of the tectonic evolution of mainland China and East Asia. The distribution of high pressure/ultra-high pressure metamorphic rocks discovered on the surface, the specific location of a series of deep and shallow sutures in the collisional convergence collage, and the seismogenic environment of shallow earthquakes attract many scientists continuously to study the interesting Dabie Orogen. In this paper, we used waveform records of 200 broadband seismic stations deployed by China Digital Seismograph Network and vertical component records of 21 mobile seismic stations located in the Dabie-Tanlu orogenic zone and its surrounding areas. Based on seismic ambient noise tomography, we have obtained the phase velocity distributions of Rayleigh surface wave with the periods between 8~40s, with the resolution higher than 50km. The high velocity anomalies are observed on the Hong'an-Tongbo region in the images of 8~16s phase velocity, which decreases with increasing periods. These high velocity anomalies are in consistence with the ultra-high pressure(UHP)metamorphic rocks of the region. It leads us an estimation of the extension of UHP metamorphic rocks at various depths. The distribution of these anomalies found in phase velocity maps of 8s to 16s indicates that the estimated depth is up to~20km. The horizontal distribution forms a heart shape, which is narrower on western side and wider on the eastern side. It is very much consistent with the surface observations. The whole shape is similar to a cone that laterally extends its wings on the southwest. It indicates that the high-pressure/ultra-high pressure metamorphic rocks had experienced quick exhumation after they broke and formed a drag at the tail, and the residual area formed by the fast exhumation was likely to be invaded by magma. We agree that it has experienced complex structural history, such as stretching, magmatic emplacement and tectonic extrusion, resulting in the high-pressure/UHP metamorphic rocks finally exhuming on the surface with the structural pattern of narrower on the western margin and wider on the eastern margin in the Hong'an-Tongbo area. The significant phase velocity difference from the period of 8s to 35s on both sides of the southern Tanlu fault zone enables us to infer that the Tanlu fault zone is a deep and huge fault, and the entire crust of the eastern zone of Dabie was cut by the Tanlu fault zone. It demonstrates that the Dabie block is separated from the northern Subei Basin and southern Yangtze blocks, which forms a seismogenic environment suitable for the generation of small-to-intermediate earthquakes in this region. Most of earthquakes in Anhui and adjacent provinces are distributed in those areas where the phase velocities changed dramatically, which are in consistence with the small faults of the upper crust in shallow layers of the Dabie-Tanlu orogenic belt. The shallow-source earthquakes mainly occur in velocity contrast regions, as demonstrated by the short period images. Earthquakes distribution and velocity maps show that the possible distribution of tiny faults of the upper crust can be roughly inferred from the geological structure. It helps to understand the seismogenic environment and seismic hazard in the Dabie areas. We conclude that the shallower faults with different velocity on either side of this region are still seismically active. These results have important significance for understanding the tectonic activity of the research areas.More detail work and further discussion are needed on the velocity structure of the Dabie orogen.