青藏高原中西部航磁异常的匹配滤波分析与构造意义

Matched-filter analysis of aeromagnetic anomaly in mid-western Tibetan Plateau and its tectonic implications

利用匹配滤波方法重新解释了青藏高原中西部航磁ΔT化极异常数据.结果显示,青藏高原中西部地区的区域场与局部场分离的最佳深度大约为19km,结合其他地球物理证据分析认为,该深度是上、中地壳间的重要分界面.分离后的区域场特征表明,在青藏高原中部存在一个北北东向的负异常带.该航磁负异常带显示,大量的热异常导致了青藏高原下的原始岩石圈地幔底部发生了热退磁作用.不论是区域场还是局部场都显示了班公湖-怒江缝合带南侧存在一个重要的隐伏断裂带(大致沿革吉北-改则南-错勤北-申扎北).

Aeromagnetic anomaly data of AT reduced to the pole in the mid-western Tibetan plateau are reprocessed, using matched-filter method, to extract local regional anomalous field from total field. The best depth of filter-windows is about 19 km. Based on other geophysical evidence, the depth of windows, 19 km, is considered an important boundary in the crust, which separates the upper crust from middle crust. The extracted regional anomaly field shows that there is an N-N-E trending strip of negative aeromagnetic anomaly in the central Tibet. The evidence from teleseismic P-wave tomography shows that the location of negative anomaly strip is the same as that of deformed lithospheric mantle of Indian plate, which became hollow, a scoop-like shape. A lot of thermal anomaly from deep athenosphere, trapped by the scoop-like Indian lithospheric mantle, are responsible for that thermal demagnetization in the original lithosphere under the central Tibet plateau, thus the negative anomaly strip was formed. Moreover, an anomaly of a fault exists in the local anomalous field, and also in the regional one. The fault locates between the northern margin of Lhasa terrane and south side of Banggonghu-Nujiang Suture, about along the north of Ge^＇gyai to south of Ge^rze^ to north of Coqe^n to north of Xainza, from west to east. Combined with a lot of data from surface geology, focal mechanisms, distribution of hypocenters, the latest regional geological survey and Bouguer gravity anomaly, the fault is the westem part of right-lateral strike-slip Jiali Fault.