Electrical Structure Of The Altyn Tagh Fault At The Changma Section And Its Tectonic Significance

The geometry of faults is of great significance for evaluating propagation of ruptures;the internal structure of a fault is as important as its surface geometry for understanding the seismic behavior of the fault.Intracontinental strike-slip faults play a very important role in global plate tectonics,but there is not enough study of their deep structure,and no model can explain all intracontinental active strike-slip faults.Therefore,the study of the geometric structure of the large-scale active strike-slip fault zone in three-dimensional space can provide direct evidence for evaluating the fault propagate and seismic activity,and can also provide a new basis for the theoretical study of the strike-slip fault zone.The Altyn Tagh fault zone is the northern boundary of the Tibetan Plateau and one of the most important strike-slip faults in the Asian continent.Previous studies have shown that the left-lateral strike-slip rate in the eastern segment of the Altyn Tagh fault zone has obvious loss along the strike direction of the fault.There are two sharp decreasing positions in horizontal slip rates at Subei and Changma Dam.Because the Altyn Tagh fault zone intersects with the Qilian Mountains in geomorphology,and the internal faults of the Qilian Mountains also show convergence to the Altyn Tagh fault zone on the surface traces.Therefore,it was proposed in some geological studies that the strike-slip rate of the Altyn Tagh fault zone was decomposed at the triple junction where the Altyn Tagh fault and the Qilian mountain internal fault intersect,and a part of the slip rate was accomodated by the activity of the Qilian mountain internal fault.Some previous studies even pointed out that the activity of the Altyn Tagh fault zone controls the growth and deformation of the northern Tibetan Plateau.However,the fault activity within Qilian Mountain may be an inherited activity based on the early suture zone.Its relationship with the Altyn Tagh fault,the influence range of the Altyn Tagh fault zone,and the mechanism of the strike-slip rate decomposition need more evidences in deep structure.In this paper,taking the Changma section of the Altyn Tagh fault zone as a window,four magnetotelluric sounding profiles(numbering from west to east: L16-L19)across the Altyn Tagh fault zone were obtained.The two-dimensional electrical structure of the sections are analyzed to track the extension of the Altyn Tagh fault zone and the adjacent faults.The continuity and scale of the faults,and the relationship between the fault zones are also analyzed.At the same time,combined with the previous magnetotelluric sounding data,the three-dimensional electrical structure study was also carried out.The three-dimensional electrical structure provides evidence for the regional structure of the study area.Considering the research results of the slip rates of the faults,the continuity and spatial distribution of the Altyn Tagh fault zone in the Changma section are discussed.Considering the regional dynamics background,the variation in the strike-slip rate of the Altyn Tagh fault zone and its influence are also discussed,and clarifying whether the strike-slip activity of the Altyn Tagh fault controls the growth of the Qilian Mountains,or the north-south compression stress from the remote effect of the Indian-Eurasian collision caused the variation in slip rates of the Altyn Tagh fault?In the magnetotelluric data processing procedure,the instrument's own software and Robust processing method are used to obtain the apparent resistivity and phase data of the measuring sites.The phase tensor method and the phase sensitive skewness are used to analyze the dimensionality of the observation data and the electrical major direction of the unknown structure.The Groom-Bailey impedance tensor decomposition technique was used to decompose the impedance value along the direction of the measurement profiles,and the one-dimensional fitting method Rhoplus was used to perform a reasonable test on the apparent resistivity and phase data obtained after GB decomposition.The deviating points in the observed data according to the Rhoplus fitting curves and some ‘flying point' in low frequencies were deleted.In the two-dimensional inversion,the Nonlinear Conjugate Gradient Method(NLCG)was selected,and TE+TM data were used to obtain the electrical structure along the four profiles with a depth of 50 km in the survey area.The two-dimensional electrical profiles show that the middle and upper crust on the north side of the Altyn Tagh fault zone is dominated by continuous high-resistance bodies,while the deep electrical structure inside the Qilian Mountains has relatively complicated changes in the lateral direction.Such structure scenes are consistent with the regional tectonic background.That is,the eastern margin of the Tarim Basin north to the Altyn Tagh fault still shows good integrity,and the Qilian Mountains,which are the foremost growth fronts on the northern margin of the Tibetan Plateau,is experiencing strong deformation.In terms of the structure of the faults,the cutting depth of the Altyn Tagh fault zone along its strike direction has been significantly reduced in the L17 section on the west side of the Changma Basin.The electrical boundary related to the Altyn Tagh fault zone has shifted about 15 kilometers southward here,corresponding to a fault QingshixiaZhujiandashan fault(QZF),and connecting with the Changma Basin.The faults in the northern Qilian Mountains,including the Changma fault and the HanxiaDahuanggou fault,generally show a low-angle southward dip,cutting across the top of the high-resistivity anomalies in the electrical resistivity profiles.In order to further obtain the deep structural setting of the periphery of the study area,a dataset including all the sites in L2 and L4 and part sites in L3 were added to the present study.There are 186 magnetotelluric sites in total were used to cover the eastern section of the Altyn Tagh fault zone and its neighboring areas.The three-dimensional magnetotelluric inversion were performed on this dataset with the program Mod EM.Three-dimensional resistivity slices also show that the Altyn Tagh fault is the boundary,the electrical structure on the north side is relatively complete,and the electrical structure on the south side changes more drastically at the level of <30 km.The highresistivity body corresponding to the Tarim block has good integrity and extends below the Altyn Tagh fault zone.The Tarim and Beishan blocks have relatively high resistivity in the middle and lower crust.In the three-dimensional electrical slice,Qingshixia-Zhujiandashan fault also corresponds to the south boundary of the high-resistivity body,and the Changma Basin also corresponds to the low-resistivity anomaly.This feature can be extended to the depth of 30 km.On the whole,the trend of the cutting depth of the Altyn Tagh fault zone becoming smaller eastwards continues in this study.The Altyn Tagh fault zone cuts through the Aksai section at a depth of ?45 km,and passes through the Shibaocheng with a cutting depth of ?30 km.In the new sections of this paper,the Altyn Tagh fault is extending to a depth of 30-40 km in the profile L16;but it does not continue eastward.The Altyn Tagh fault corresponds to no obvious electrical boundary in profile L17.From profile L16 to L17,the electrical structure shows that the Altyn Tagh fault and the QingshixiaZhujiandashan fault correspond to the southern boundary of the same high-resistivity body,and they are interconnected in the deep electrical structure.Therefore,Qingshixia-Zhujiandashan fault can absorb the slip rates on the Altyn Tagh fault zone.The Changma fault and Qingshixia-Zhujiandashan fault are not clearly connected in geomorphology,but they are located on the west and south sides of the Changma basin,respectively.The Changma basin can be regarded as a step area in the direction of fault extension and becomes a link for stress propagation.However,the geological research results of the fault slip rates indicate the strike-slip rate on the east segment of the Changma fault is significantly greater than the loss in slip rates along the strike of the Altyn Tagh fault.Therefore,it can be considered that the Altyn Tagh fault has a limited contribution to the activity of the Changma fault.Combined with the regional deformation characteristics of the Qinghai-Tibet Plateau,especially the dynamics research results in the north,we propose that the activity of the main fault zone in the northern Tibetan Plateau is still affected by the remote compression effect caused by the Indian-Eurasian collision.The Altyn Tagh fault and the faults inside the Qilian Mountains are all in an oblique compression stress environment.Under this basic tectonic model,the Altyn Tagh Fault,QingshixiaZhujiandashan fault,the Changma Basin,and the Changma Fault form the decomposition-transformation-absorption system of slip rate.This system is only a local tectonic event comparing with the deformation mechanism of the entire northern margin of the Qinghai-Tibet Plateau.Whereas,due to the absorption effect of Qingshixia-Zhujiandashan fault,the loss of the strike-slip rate in the Altyn Tagh fault zone also indirectly led to the variation of the stress environment around the Changma basin east of the Qingshixia-Zhujiandashan fault.