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Fine crust structures of Xi′an sag in the Weihe basin revealed by a deep seismic reflection profile

REN Jun1,2,FENG Xi-Jie1,WANG Fu-Yun3,PENG Jian-Bing2,LIU Chen1, DAI Wang-Qiang1,LI Gao-Yang1,ZHANG Yi1,MA Ji1 1 Earthquake Administration of Shaanxi Province,Xi′an,710068,China 2 School of Geological Engineering and Geomatics,Chang′an University,Xi′an 710064,China 3 Geophysical Prospecting Center,China Earthquake Administration,Zhengzhou 450002,China  
In 2005,a cross the Xi′an sag,a NW-direction and 69 km-long deep seismic reflection profile was completed.It is first time to get the fine structures of the crust,the distribution of main faults and image of tectonics relation between deep and shallow structure in this region.CMP time-stack section of seismic reflection shows the crust is divided into two parts by a reflection event C as the boundary.Upper crust consists of multiple nearly horizontal reflection belts,with good segmental continuity,local existence the fault offset reflection belts or the characteristics of morphological mutations that clearly depict Xi′an sag Cenozoic sedimentary layers,the bottom boundary of sag,and the geometry and relationships of the Weihe Fault,Lintong-Chang′an fault zone and piedmont of Qinling fault zone.C is the reflection event in the crystallization crust,with the reflection band width of about 0.5s,the deepest of the bottom boundary of about 6.5s at stake No.30 km,slow uplift to the northwest to the 5.5 s,rapid uplift to the south east to the 5.5 s.There are two distinct lower crustal reflection events RA and RB: RB is the local reflect group at stake No.40-47 km;RA is the band width of about 2 s,which inclines to sag.RA is a strong reflection under C and above Moho south of the stake 34 km.Arguably,the upper crust is above the C interface in the whole profile,but from south of stake 34 km,crust under C interface is divided into middle crust and lower crust by RA interface.From north of stake 34 km,RB is weak,unclear reflection group,so it cannot be a distinct interface to divide middle and lower crust.On both sides of stake No.38 km.Moho reflection is of different forms,but there is also a significant fault to offset in big stake number direction.Moho reflection in the two-way travel time(TWT) for the 11-14 s is a level piecewise continuous reflection transition zone,with width of about 3 s in small stake number direction.Moho reflection is a about 2 s wide,piecewise continuous transition zone and tilts to the big stake number direction.Its shape is almost same as reflection event RA.According to seismic velocity data,these depth of top boundaries of reflective belts are:10.5-13.5 km(reflective belt C),20.3-21.5 km(reflective belt RB),16.8-34.3 km(reflective belt RA) and 32-36.7 km(reflective belt Moho) or so.The authors think the same shape of reflection events of RA and Moho is likely the relic of ancient Qinling ocean to dive to the North China platform.The fracture tectonic in this district developed,according to the reflection wave group characteristics of CMP stack-time section of deep reflection profile and total 11 faults(F1-F11) explained by the obvious characteristics of fracture.In addition,the depth Weihe fault and Lintong-Chang′an fault to the reflective belt C in Xi′an Cenozoic sag and the presence of Moho fault indicates that the region has strong present crustal activity,being worth noted as the future strong earthquake area.
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