摘要
渭河断陷的成因机理、复杂隆升与沉降、秦岭造山的大陆动力学问题、鄂尔多斯周缘的活动断裂系以及华北地台如何与秦岭微地块乃至与扬子板块的拼合及接触关系,其在地壳深部的岩石圈分层,岩石流变和莫霍面的最新构造形态等研究方面还存在许多关键问题没有解决?针对上述问题,本文在渭河盆地开展了深地震反射探测,在盆地及邻区包括秦岭褶皱系和鄂尔多斯地台区域开展了深地震宽角反射/折射和高分辨地震折射联合探测研究。通过深地震探测联合剖面研究,首次查明了渭河断陷盆地及邻近地区主要活动断层在地壳深部的延伸情况;地壳深部的速度结构,地壳精细结构,地壳介质特性和深、浅部的构造关系等,为进一步研究渭河盆地构造特征与大陆动力学以及深部孕震构造背景,判明渭河断陷盆地未来中强地震的发震构造提供依据。深地震反射测线布设在渭河盆地的长安区与礼泉县之间,测线方向为北西—南东向。南东端(0m桩号)位于长安区太乙宫镇沙场村附近,北西端位于礼泉县骏马镇付官寨附近,测线全长69km,观测点间距为40m,炮点间距为120m。地震宽角反射/折射测线布设在河南西峡县与陕西长武县之间,测线方向也为北西—南东向。地震宽角反射/折射测线的东南段为秦岭—华山山地,中段为渭河盆地,西北段为鄂尔多斯盆地南缘,全长约360km。此外,在地震宽角反射/折射测线的蓝田至淳化区段之间,还布设了为高分辨地震折射测线,高分辨地震折射的测线的长度为120km。高分辨地震折射测线布设观测点156个,观测点距平均0.77km,炮点9个(其中和地震宽角反射/折射剖面共用炮点2个),平均炮距12km,构成了较为完整的多重追逐相遇观测系统。地震宽角反射/折射测线在高分辨地震折射剖面之外,布设仪器220台套,平均观测点距1.4km,炮点5个,平均炮距约50km。取得的主要学术成果如下:1.蓝田—西安—淳化高分辨地震折射探测的结果表明,区内基底与盖层的结构具有典型的分区特性。大致以测线的桩号241km和341km为界,可分为三个不同的区块。其中,桩号241km以南是秦岭褶皱带区,桩号241km—341km之间是渭河断陷盆地,桩号341km以北则是鄂尔多斯地台区,三个分区的边界均为大断裂带或强速度梯度带。秦岭褶皱区和鄂尔多斯地台区的盖层薄,P波速度相对高,基底埋深浅,结构相对简单与完整,秦岭褶皱区的基底出露于地表;而渭河断陷盆地的沉积盖层厚,盖层最深可达6km左右,P波速度非常低,基底埋藏深,断陷盆地的结构甚为复杂。2.西峡—西安—长武地震宽角反射/折射探测剖面的P波速度结构、构造图像所反映的区内地壳、上地幔也具有明显的分区特性,分区的情况与高分辨地震折射的结果相一致。秦岭褶皱带的地壳厚度约37~38km,地壳的结构相对简单,结晶基底埋藏浅,以至出露于地表。鄂尔多斯地台的地壳厚度较大,约为42—43km,地壳的结构也相对简单,结晶基底埋藏浅。渭河断陷盆地的地壳厚度约为32~33km,渭河断陷盆地的莫霍界面相对两侧的鄂尔多斯地台和秦岭褶皱带明显产生了上隆现象。3.根据渭河断陷盆地的深、浅部速度结构、构造图象推测,渭河断裂、临潼—长安断裂和华山山前断裂可能延伸到了中地壳的底部,深度约为22km左右。在测线桩号310~330km之间,存在莫霍界面被错断的情况。沿着莫霍界面被错断的薄弱面,上地幔的高密度热物质侵入到下地壳中。4.结合渭河断陷盆地的石油钻井资料,长安—礼泉深地震反射探测剖面反映了第四系底面TQ、上第三系底面TN2,中第三系底面TN1,下第三系底面TE,结晶基底顶面Tg,C界面,RB界面,Moho界面的反射波组以及倾斜反射事件RA。据深地震反射探测剖面的解译,渭河断陷盆地的上部地壳被一系列穿透深度不等的正断层所切割,形成大地堑镶嵌小地堑或梯状断阶的构造格局;与断陷盆地的中心相对应,还存在一条切穿莫霍界面的深断裂。5.渭河断陷盆地存在发生中强地震的深部构造条件,莫霍界面相对鄂尔多斯地块突变隆起和上地幔高速物质侵位于下地壳,是该区中强性地震发生的深部构造背景;渭河断裂、临潼—长安断裂以及深地震反射剖面揭示的F6断裂很可能是未来中强性地震的重要孕震构造,也是控制渭河断陷盆地中心的断裂构造,这几条断裂具有切割深、规模大的突出特点。乾县—富平断裂切割比较浅,应不具备发生强烈地震的条件。
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.