秦岭-大别及邻区背景噪声的瑞利波层析成像

Phase velocity tomography of Rayleigh in Qinling-Dabie and its adjacent areas using ambient seismic noise

秦岭—大别造山带西起青藏高原东北缘,东至郯庐断裂带,是华北板块和扬子板块之间的碰撞造山带.本文收集陕、豫、皖、赣、湘、鄂、渝等区域地震台网的160个宽频带地震台站连续两年地震背景噪声数据,用双台站互相关算法获得瑞利面波经验格林函数,提取相速度频散曲线,并根据面波层析成像反演得到秦岭—大别及邻区周期8~35 s范围内相速度分布图像.结果显示,大别地块在14 s相速度分布图中呈现低速异常,与8 s相速度分布图中的高速异常形成鲜明对比,反映大别HP/UHP(high pressure/ultrahigh pressure metamorphic rocks,高压/超高压变质岩)的影响仅存在于上地壳.25 s相速度图中,大致以太行一武陵重力梯度带为界,东部以高速异常为主,西部以低速异常为主,反映了地壳东薄西厚的结构特征.14~35 s相速度分布图显示郯庐断裂带南段东西两侧的显著差异,佐证了郯庐断裂带发生大规模左行平移运动时,其南段可能切入壳幔边界.同时,郯庐断裂带南段可能存在一个热物质上涌的通道,熔融的热物质通过该通道上升,混入大别地区的中下地壳,造成了红安一大别造山带的差异隆升.南秦岭与四川盆地东北部表现为低速异常,是否与青藏高原物质东流或者南秦岭的拆沉有关,还有待于进一步深入研究.

The Qinling-Dabie orogenic belt, extending from the northeastern margin of the Tibetan Plateau to the west of the Tan-Lu fault, was associated with the collision of Yangtze and North China blocks. We have calculated the phase velocity of Rayleigh waves at periods of 8~35 s in the Qinling-Dabie orogenic belt using data of ambient seismic noise for two years recorded at 160 broad-band stations of the China digital seismic network. Applying the two-station cross-correlated analysis to these data yields the empirical Rayleigh wave Green＇s functions. Phase velocity dispersion curves are measured for each interstation path by frequency-time analysis. The Rayleigh wave phase speed maps agree well with each other and show clear correlations with major tectonic structures. The Dabie block is characterized by high velocity anomalies at 8 s but slow velocity at 14 s, indicating the influence of high pressure （HP）/ultrahigh pressure （UHP） regions in the upper crust. At 25 s, the velocity varies from slow in the west to high in the east across the gravity gradient zone in the Taihang-Wuling Mountains. This pattern mainly reflects the effect of crustal thickness which is larger in the west and smaller in the east. The southern segment of the Tan-Lu fault shows different features on either side at 14~35 s, suggesting that the fault zone may extend down to the crust-mantle boundary. The slow velocity close to the fault is probably caused by the hot material upwelling. Obvious slow velocities at periods of 14~25 s in the South Qinling and northeast Sichuan basin are observed. We could not determine whether this low velocity zone is due to the Tibetan lower crustal flow and/or the delamination of the South Qinling. Inversion for the 3-D shear wave velocity structure in the crust and upper mantle is necessary to constrain the geodynamics of this region in the future.