The Features Of Airborne Gravity And Magnetic Anomalies In Western Afghanistan And Its Tectonic Implications

Gravity and magnetic methods play an important role in tectonic studies,mineral exploration,and petroleum exploration and so on.In western Afghanistan,the tectonic framework and crustal structure remain much unknown due to few large-scale geophysical surveys and lack of the previous researches.In this research,the highresolution airborne gravity and magnetic data measured by the USGS during 2006-2009 were used and interpreted to reveal several crustal structures in western Afghanistan.These structures include depths of Moho and basement interfaces,the extension of major fautls,and crustal density and magnetic structures.This research will provide significant evidence and supports for understanding the tectonic framework in western Afghanistan,and also may help whit the mineral and petroleum exploration in this region.Firstly,the anomaly separations on the complete Bouguer gravity anomalies were performed to obtain the Moho gravity anomalies,and then the interface depth inversion on the Moho anomalies was performed to obtain the Moho depths in the study area.The results demonstrate that the Moho depths in western Afghanistan range from 32 km to 43.3 km,and they are high in the northeast area while low in the southwest area,indicating that the crustal thickness getting thinners from northeast to southwest.Secondly,the anomaly separation on the complete Bouguer gravity anomalies was performed to obtain the gravity basement anomalies,and then the interface depth inversion on the basement anomalies was performed to obtain the depths of gravity basement(crystal basement)in the study area.Such processing and inversion on the magnetic RTP anomalies were also performed to obtain the depths of magnetic basement.The results show that the depths of both gravity and magnetic basements range from 4 km to 10 km,but they vary from each other due to various related physical properties.The gravity basement presents a trend of deep-shallow-deep from north to south,while the magnetic basement presents a feature of deep-shallow from north to south of the study area.Thirdly,the anomaly separation on the complete Bouguer gravity anomalies was performed to obtain the regional and residual anomalies,and then the derivative analyses on these anomalies was performed to reveal the gravity linear structures in the study area.Such processing on the magnetic RTP anomalies were also performed to reveal the magnetic linear structures in the study area.The results illustrate that quite a few of gravity linear structures trending northeastward are presented in the whole area,implying the complex deformation happened for the whole area.However,magnetic linear structures trending northeastward are presented mainly in the southwest area,implying that magmatic activity is strong in the southwest area while it is weak in the northeast area.Based on these linear structures,the possible extensions of four major faults(Bandi Bayan fault,Helmand fault,Darshan fault and Mokur fault)were deduced in the study area,all of which trend in the NE-SW direction and extend from shallow to deep in the crust.Finally,the 3-D inversion respectively on the complete Bouguer gravity anomalies and the magnetic RTP anomalies was performed,and the 3-D crustal density and magnetic structures in western Afghanistan were obtained.The 3-D crustal density structure reveals the thickness variation of the sedimentary strata and the uplift and depression of the basement in the study area,and also demonstrate that the density distribution is heterogeneous in the northeast area from shallow to deep.The 3-D crustal magnetic structure reveals several strong magmatic belts trending in the NE-SW direction from shallow to deep in the southwest area while weak magmatic activity in the northeast area.