Detachment of lithosphere beneath the middle and lower Yangtze region and its effect to mineralization

There exists a mineralization zone in the middle and lower Yangtze region. Previous studies have shown that this zone might be caused by the rich eruption of magma during the Mesozoic period. We have applied the teleseismic tomography method to determine a 3-D P-wave velocity structure of the mantle down to 500 km depth beneath this region by using over 16,000 P-wave arrival times collected from 678 teleseismic events recorded at 66 portable and 51 static seismic stations in this region. The relative residual times used for the tomography were calculated by using the multi-channel cross-correlation method which increases the efficiency and the data precisions up to 0.01 seconds. The grid spaces are chosen as 0.5°×0.5° horizontally and 50-100 km vertically. From the cross-sections views along profile-lines norm to the Tanlu fault (Figure 1), there exist higher velocity anomalies at the depths between 300 and 400 km and at depth less than 100 km and the low velocity anomalies locates between these two higher anomalies. Combining with other previous studies, these higher and lower anomalies are explained as the lithosphere and the asthenosphere respectively. The velocity structure seems to show that the lithosphere is split into two parts due to the incursion of the upwelling asthenosphere. It is also noted that the lithosphere beneath the Tanlu Fault seems to be thickened. Some previous studies show that the Tanlu Fault may be the channel of magma eruption, which resulted in the rich mineralization in this region. Our results show that the detachment of lithosphere might cause the upwelling of asthenosphere and the upwelling materials may consist of the erupted magma. In a word, the detachment of lithosphere has played an important role for rich mineralization in middle and lower Yangtze River. Figure 1 Cross-section views of velocity anomalies along three profile lines shown at the top. The red and blue colours represent the lower and higher anomalies respectively, and the colour bar is shown at the top. The inversed triangles approximately indicate the positions of cross-points between the profile-lines and the Tanlu Fault. TLF-Tanlu Fault; CHF-Chuhe Fault; JNF-Jiangnan Fault; YZR-Yangtze River.