| About the formation mechanism, complex uplift and subsidence of Weihe faultdepression, the Qinling orogenic continental dynamics, the peripheral active fault system inOrdos and mosaic and contact relationships of North China platform and Qinling micro-blockeven and the Yangtze plate in the crustdeep, delamination&rheological of lithospheric, and theresearch of Moho fine structural form under Weihe basin, etc, there are still many key issuesremain unresolved. In response to these issues, this paper carried out deep seismic reflection,deep seismic wide-angle reflection/refraction and high resolution seismic refraction jointexploration and studies. Through the study of the combined profile of deep seismicexploration, attempts to identify active faults in the deep crust extension, the deep crust speedstructure, crustal fine structure, crust medium characteristics and deep, the shallow tectonicrelations, etc. of the Weihe Basin and adjacent areas, to provide a basis for the further study ofthe structural characteristics and continental dynamics and deep seismogenic tectonic setting,identify the seismogenic structure of the strong earthquake in the Weihe Basin.Deep seismic reflection survey lines laid between Chang’an District and Liquan Countyin the Weihe basin, the direction of the measuring line for the North West-South East. Southeastern end (0m Stake) of survey line is located in near Taiyigong Town, Chang’an District;north western end located in near Junma Town, Liquan County, the survey line length of69km, the observation point spacing of40m and shot spacing120m.Wide-angle seismic reflection/refraction survey line was laid between Xixia County,Henan and Changwu County, Shaanxi, the North West-South East direction of the measuringline. The southeast section of Wide-angle seismic reflection/refraction seismic line is theQinling-Huashan Mountain; the middle section of its is the Weihe basin; the northwest sectionof its is the southern margin of the Ordos Basin and the total length is about360km. Inaddition, between Lantian to Chunhua, the seismic line of the wide-angle seismicreflection/refraction laid for the high-resolution seismic refraction survey lines, its length is120km.High-resolution seismic refraction survey lines laid observation point156, average distance0.77km of the observation points,9shots (which seismic wide anglereflection/refraction profile sharing2shots), average distance12km of shots, to constitute amore complete multi-chase encounter Observing System. Wide-angle seismicreflection/refraction survey lines outside the high-resolution seismic refraction profile, laid220sets of instrument, with an average observation distance1.4km,5shots, average shotdistance about50km.Achieved academic achievements are as follows:1. Results of Lantian-Xi’an-Chunhua high-resolution seismic refraction survey showthat the structure of the local basement and cover possess the typical characteristics of thepartition. Profile on survey line stake number241km and341km can be divided into threedifferent blocks. Among them, the Qinling fold belt is in the south of the Stake number241km; Weihe Basin is between Stake number241km and341km; Ordos is in the north ofStake number341km. The boundaries of the three partitions are large fault zone or strongvelocity gradient zone. In Qinling fold zone and the Ordos platform, with the thin areacaprock, relatively high P-wave velocity, shallow basement, the structure is relatively simpleand complete. While in Weihe fault depression basin, with the thick sedimentary cover, mostup to6km or so, very low P wave velocity and deep buried basement, the rift basin is acomplex structure.2. P wave velocity structure of Xixia-Xi’an-Changwu Wide-angle seismicreflection/refraction sounding profiles, tectonic image reflected partition properties of thecrust and upper mantle in the region where the partition was consistent with the results of highresolution seismic refraction. The crustal thickness of Qinling fold belt is about37~38kmand the structure of the crust is relatively simple, crystalline basement buried shallow, evenexposed on the surface. The crustal thickness under Ordos platform is about42-43km and thestructure of the crust is relatively simple, crystalline basement buried shallow. Weihe Basincrustal thickness is about32~33km and the Moho under Weihe Basin on both sides of theOrdos platform and Qinling fold belt possess uplift phenomenon.3. It is speculated that Weihe fault, Lintong-Chang’an fault and Huashan piedmont faultmay extend to the bottom of the crust, the depth of about22km or so according to the deep,shallow velocity structure and the tectonic image of Weihe Basin. Stake310~330km of survey lines, there may be Moho dislocation. Along the Moho dislocation weak-surface,high-density upper mantle hot material penetrated into the lower crust.4. Combination of the Weihe fault basin oil drilling data, Changan-liquan deep seismicreflection sounding profiles reflect the bottom of Quaternary (TQ), the bottom of UpperTertiary (TN2), the bottom of Middle Tertiary (TN1), the bottom of Lower Tertiary (TE), thetop of crystalline basement (Tg), C interface, RB interface, Moho reflected wave group andthe tilt reflection events RA. According to the interpretation of deep seismic reflectionsounding profiles, the upper crust of Weihe fault depression basin is cut by a series of normalfaults of different penetration depth, forming the tectonic framework of the earth cuttingmosaic small graben or ladder-off bands; rift basin corresponds to the center, there is a cut towear a deep fault of the Moho.5. There are structural conditions of the strong earthquakes occurrence in the deep ofWeihe Basin where Moho of the relative Ordos block mutations uplift and upper mantlehigh-speed material emplaced in the lower crust are deep tectonic background of strongearthquakes occurred in the area. What Weihe fault, Lintong-Changan fault and F6faultrevealed by the deep seismic reflection profiles have cutting deep, large-scale features arelikely important seismogenic tectonic of strong earthquakes in the future, but also controlcenter fault structure of Weihe fault depression basin. Qianxian-Fuping fault, cuttingrelatively shallow, generally do not have the conditions of strong earthquakes occurred. |
PDF Full Text Request