大别－苏鲁造山带与华北陆块东部岩石层三维速度结构

The 3-D velocity images of the crest and upper mantle beneath the region of 112° -124°E, 28°-39°N including the Dabie-Sulu orogenic belt are reconstructed by using 36405 P-wave arrivals of 3437 regional and 670 distant earthquakes during the period from 1981 to 1996, and gridding the area of 0.5° * 0.5°. The results of tomography demonstrate that: 1. The results of tomographic imaging show a broad heterogeneity in P wave velocity structure for the lithosphere beneath the Dabie-Sulu orogenic belt. 2. In the Dabie orogenic belt, the velocity patterns in the crust are different among various tectonic units. The Dabie and Qinling orogenic belts are remarkable in the tomographic images, and in mm the Hongan and Dabie blocks in the Dabie orogenic belt are also imaged very distinguishably. 3. A velocity (about 5.9～6.0 km/s) layer exists in the Dabie block at depth between 15～25 km, which is coincident with the low-resistance layer at the depth of 12-23 km, being inferred to be the tectonic detachment zone and suggesting that the extension detachment structure was formed in the middle crust. Beneath the southern and northerm Dabie tectonic units, the north-dipping high-velocity (at level of 6.5 ～ 6.6 km/s) block was developed in the crust, which might be correlated with the UHP rockswith low content of the meta-ultramafic rocks. This result is in agreement with the geological observation on the surface. 4. The velocity image at 40 km depth reveals the features at the top of mantle and the configuration of the Moho discontinuity. The depth of the Moho changes slightly along the trend of the orogenic belt. It in Hongan block is less than 40 km, but it is different in the western and eastern parts of the Dabie block, the former is more than 40 km, and the latter less than or equal to 40 km. The remnant of the mountain root exists between the Shangcheng-Macheng fault and the line of Huoshan-Yuexi-Yingshan in the Dabie orogenic belt, and beneath the southern and northern Dabie tectonic units. However, the thickness of the Moho is about 40 km and there is no obvious changes, which suggest that the Dabie orogenic belt has been experienced quite in the gravity equilibration. The Moho's depth in the Sulu is less than 40 km. 5. There is a dipping slab-like high-velocity body in the uppermost mantle. It is sandwiched by slow velocities and exists beneath the Dabie-Sulu orogenic belt in the range of depths between the Moho discontinuity and 110 km at least. This high-velocity body outlines a picture of the slab interpreted as the remnant of the Triassic subducted YZ. 6. The Sulu orogenic belt displays "crocodilian" velocity structure, the upper crust of the Yangtze thrusted over the Huabei crest, and the Huabei crust indented into the Yangtze crust, where the ancient subduction zone of the Yangtze lithosphere located. Based on the previous geological data, this structure is not related with the collision between the Yangtze and Sino-Korean Blocks, but caused by the sinistral offset of the Tan-Lu Fault. Studied on the velocity structure of the eastern Huabei lithosphere indicates: 1. The 'present-day' lithosphere of the eastern Huabei is between 40-100 km thick with greatly thinned lithosphere around the Bohai Sea. Generally, thickness of the lithosphere in this region decreased eastwards. 2. The attenuation of the lithosphere is attributed to the strongly uplift of the asthenosphere. In the area between the Taihang Mountains and the Tan-Lu Fault, there is a 'lever' with red low velocity belt, it is clearly defined, transverse continuity, depth between 100-150 km, local variations visible, and an upwards trend towards the Bohai Sea. Generally, the velocity structure in the mantle beneath the lithosphere displays irregular column-shape consisting of alternating high and low velocities, and when cold high velocity ancient lithosphere connects with the hot low velocity mantle materials forming precipitous compact structure. More heat pathways from the mantle occur towards the Tan-Lu Fault. 3. The strongly irregular characteristics of the contact between the asthenosphere and the lithosphere is induced by the long-term hot, chemical erosion and alteration on the contact. 4. There are still preserved high velocity lithosphedc root beneath Huabei with 'block-shape' distribution and surrounded by hot materials. Results of our studies indicate that the evolution models of the eastern China mantle are characterized by the direct contact between the uplifted lithosphere and the Huabei Craton accompanying the upwelling of the deep mantle materials. At the contact betwen the lithosphere and the asthenosphere, the upwelled mantle materials replaced and altered the lower lithosphere forming the metasome through the hot and chemical modifications impacted on the Craton lithosphere, and changed it into the lithosphere gradually, resulting in the lithospheric thinning. Thus, the lithospheric thinning is the result of the upwelling of the asthenosphere.