利用剪切波分裂研究内蒙古阿巴嘎地区上地幔各向异性

Upper mantle anisotropy beneath Abag area in Inner Mongolia from shear wave splitting

内蒙古阿巴嘎地区壳幔经历强烈变形,岩石圈变形机制尚不明确.利用布设在研究区的32个流动地震台站所记录到的远震剪切波数据,测量得到120对各向异性参数和113个无效分裂结果.结果表明,研究区快慢波延迟时间变化范围为0.4~1.4s,平均0.77±0.21s;各向异性快波方向变化范围为N101°E-N45°W.其中一组快波偏振方向为N82.0°E±12.3°,与区域内断裂走向平行,反映地幔矿物晶格定向排列;另一组快波方向集中位于华北克拉通内部,平均为N146.8°E±9.5°,平行于早白垩纪岩石圈伸展变形方向,推测由残留在岩石圈中的化石各向异性所引起.在研究区北部部分台站,只观测到无效分裂而没有观测到有效分裂结果,可能存在局部热地幔物质上涌.

Abag area suffered a strong crustal and mantle deformation,however,the mechanism of this deformation is poorly understood. A total of 120 pairs of shear wave splitting measurements and 113 null measurements are obtained at 32 portable seismic stations. Delay times vary from 0.4 s to 1.4 s with an average value of 0.77±0.21 s;while fast directions trend from N101°W to N45°E. One group of fast directions in line with the strike of regional faults,trending N82.0°E±12.3°,is caused by lattice-preferred orientation of mantle crystals;the other group of fast directions observed in the North China Craton with an average value of N146.8°E±9.5° are parallel to the direction of lithospheric deformation in Early Cretaceous,which may origin from fossil anisotropy remaining in the lithosphere. In addition,solely null measurements in the northern part of study area may suggest small-scale hot mantle upwellings which partially eroded the fossil anisotropy in lithosphere.