秦岭造山带的东西向地壳速度结构特征

Characteristics of crustal velocity structure along Qingling orogenic belt

跨越东、西秦岭造山带的深地震叫深剖面沿近东一西向布设长约560km。沿测线采用6个1.5～2.0吨的爆炸震源激发地震波，使用260套数字地震仪接收，取得了较高质量的地震资料。通过资料分析扣处理，识别出沉积层及结晶基底的折射波（Pg）、上地壳底面的反射波（P2）、中地壳底面的反射波（P3）、下地壳内的反射波（P4）、莫霍面的反射波（Pro）乖首波（Pn）六个震相。采用地震动力学射线方法（seis88）得到的地壳速度结构表明：（1）在秦岭造山带内反射界面起伏剧烈，西部在略阳-勉县以西明显抬升，断差约3～4km。东部在旬阳-白河县以东呈斜坡状抬升，总体特征呈中间深，两侧浅的态势。东部的速度大于西部，速度差为0.02～0.05km／s。（2）上地壳在略阳-白河县段、中地壳在西乡东-白河县段的速度等值线明显起伏上隆，说明在深度25km之上的区域速度极不均匀，地表上略阳、西乡东对应于勉一略缝合带、犬巴山弧的位置。下地壳的速度等值线变化形态与反射界面形态基本一致。（3）莫霍面与地表高程呈镜像，深度为42～54km，地壳平均速度较大为6.44～6.48km／s。东、西秦岭的地壳厚度变化较大，分界位置大致在勉县一略阳和西乡东一石泉西。勉略带以西为西秦岭造山带，莫霍面深度为52～54km，最深处在略阳一勉县地区为54km。西乡东一石采西以东为东秦岭造山带，莫霍面深度为42～49km，西乡～石泉附近为49～49.5km，安康附近为44.7km，最浅处在十堰附近为42km.东、西秦岭造山带之间是扬子板块的北部边缘带，莫霍面深度为48～49km莫霍面整体形态呈现起伏的向西倾斜台阶式的增深特点，东西深度相差10～12km.（4）勉一略缝合带以西的地壳增厚，可能是由青藏高原隆升及向东北缘的扩张引起的。总之，沿秦岭造山带的东一西方向的地壳结构比较复杂，它不l吾l于板块碰撞作用形成的盆山结构。莫霍面首波（Pn）在210km之后出现，速度为7.85～8.01km／s。

A deep seismic sounding profile across the East and West Qinling Orogenic belt, the length is 560 km along the Near East-West direction layout. The recording of seismic waves from 6 explosions was conducted of 260 recorders along seismic line. The P-wave field provided good quality data for most of the profile. The authors processed, analyzed, and modeled the data which was collected during the wide-angle reflection and refraction effort. There are six phases, arrivals of refracted and reflected waves from sediments and basement （Pg）, the reflected wave from the bottom of upper crust （P2）, the bottom of middle crust （P3）, in lower crust （P4）, Moho （Pm） and head wave （Pn） of Moho. Crustal velocity structure which was obtained by seismic dynamics ray method （seis88） showed that： （1） In the Qinling orogenic belt reflecting interface dramatic ups and downs, the west areas of Lueyang- Mianxian uplift obviously, the depth difference is about 3 4 kin, the east area from Xunyang to Baihexian was sloping uplift of the westward tilt, the overalI characteristics shows that the middle deep and shallow on both sides. In the crust, the layer velocity of the east area is greater than the west part, the velocity difference is 0. 02～0. 05 km/s. （2） Lueyang-Baihe segment in upper crust and Xixiang East-Baihe segment in middle crust, the velocity contours presents the significant fluctuations and uplifting, illustrated that the velocity is very inhomogeneous in the depth area above 25 km.Lueyang, Xixiang East corresponds to Mian-Lue suture zone, Dabashan arc position on the surface. The velocity contours morphological changes consistent with the reflection interface morphology in lower crust. （3） Moho and surface elevation display mirroring, the depth range is 42～54 km, the average velocity of crust is greater from 6. 44 to 6. 48 km/s. East, West Qinling crustal thickness variation is larger, the approximate location of the boundary between Lueyang- Mianxian and xixiang east-shiquan west, respectively. The west part of Mian-Lue suture zone called the West Qinling orogenic belt, the Moho depth is 52 ～ 54 km, the deepest in Lueyang-Mianxian area is 54 km. The east area of Xixiang- Shiquan known as the East Qinling orogenic belt, Moho depth is 42～49 krn, Xixiang-Shiquan is 49～49.5 km, near Ankang is 44. 7 km, the most shallow in the near Shiyan is 42 km. Is the northern marginal zone of the Yangtze plate between East and West Qinling orogenic belt, the Moho depth is 48～49 km. The overall shape of Moho has shown that the features of the rolling and westward tilt of a stepped, the depth gradually increase, the deuth difference reaches 10～12 km between East and West Qinling orogenic belts. （4） The crustal thickening of the west area of Mian-Lue suture zone may be caused by the uplifting of the Tibetan Plateau and expansion to the northeast margin. In short, the crustal structure of East-West direction along Qinling orogenic belt is more complex, which is different from the Mountain-basin structure formatted by the plate collision. The head wave （Pn） of Moho appears after 210 km, the velocity is 7. 85～8. 0 km/s.