Structural Characteristics Of Baoji Sag In Weihe Basin

The western Weihe basin is located at the intersection of the northern and middle sections of the north and south seismic belts in China.There are abundant helium resources in Weihe basin.The western area of Weihe basin is an important exploration area for helium resources in the basin.Therefore,it is of great significance to study the tectonic characteristics of the western Weihe basin.Previous researches have done a lot of work in this area,but at present there are different understandings on the basic geological problems such as stratigraphic development characteristics,basement properties and tectonic unit division in this area,which makes the development of many related works in this area inconvenient.In order to further deepen the research on the basic geology and tectonic geology in this area which had been disputed by predecessors,it provides some geophysical basis for the study of the related geological problems in this area.This paper by using magnetic data,the Weihe basin by moving average,vertical second derivative,the total level of derivative,horizontal first-order method such as the directional derivative of magnetic data processing,combined with drilling,four new magnetotelluric sounding(MT)on the implementation of section materials and regional geological research results,the systematic analysis of the characteristics and genetic analysis of gravity magnetic anomaly in this region,concluded that the fracture structure in the study area.Based on the gravity-magnetoelectric comprehensive inversion,the characteristics of strata distribution are discussed,the basement properties are determined,and the tectonic units are divided.The results show that the gravity and magnetic anomalies in the study area are distinct,with low gravity anomalies in the south and north,and high gravity anomalies in the central and western regions.The distribution of gravity field is "two low and one high".There is an obvious magnetic low value area in the middle of the study area,the magnetic high anomaly in the north is caused by the Archean Eonothem metamorphic rocks in the southwest margin of the ordos basin,and the magnetic high anomaly in the southeast is caused by the early Paleozoic Erlangping group and Triassic monzonite granite.In the Longxian-Fengxiang area,it is reflected as low resistivity anomaly and high gravity anomaly,the Cenozoic is relatively thin,the basement burial depth is relatively shallow,and there is cretaceous in some areas.In the Qianyang area,it is reflected as high resistivity anomaly and high gravity anomaly,and the surface outcropping is complex,including Cambrian,Ordovician,Triassic,cretaceous,Jurassic,etc.,and its basement is Mesozoic-Paleozoic-Proterozoic.In the west of Fufeng in the south of Qishan,the resistivity value drops sharply compared with that in the north and south,so it is speculated that the basement is Proterozoic metamorphic rock.The northern part of Baoji-Meixian is characterized by high resistivity anomaly and high gravity anomaly,the cretaceous system is exposed in the northern part,and the basement is Paleozoic-Proterozoic.In the south of Baoji-Meixian,there are high resistivity anomalies and low gravity anomalies.The main conclusions are as follows:(1)The main fault strike developed in the study area is NW,and the main fault strike is consistent with the regional tectonic strike,which reflects the regional tectonic strike in the study area.In addition,secondary fractures in the NE and NW directions that cut off the main faults were also developed in the study area.Secondary faults are relatively small.(2)The west of the Weihe basin basement properties is complex.Mesozoic mainly distributed in Longxian-Fengxiang area,the Mesozoic-Paleozoic-Proterozoic are mainly located in the area of Qianyang,Proterozoic is mainly distributed in the south of Qishan and the west of Fufeng,Paleozoic-proterozoic is mainly located in Baoji-Meixian,granite is mainly distributed in the south of Baoji-Meixian.(3)The fluctuation of basement in the study area is different,the southern granite develops,the eastern Paleozoic buried depth fluctuates greatly,the central Cenozoic thickness fluctuates little,and the northern Mesozoic and Paleozoic develop.(4)The whole research area is the uplift area in the west of Weihe basin,which can be divided into three sub-structural units: Baoji sag,Qianyang uplift and Fengxiang sag.