兴蒙造山带及邻区地壳速度结构特征

兴蒙造山带位于中亚造山带东段, 作为古亚洲构造域的重要组成部分, 由西伯利亚板块与华北板块碰撞拼合而成, 其经历了大陆裂解、洋盆扩张、洋壳俯冲消减和碰撞拼合造山等复杂的构造演化过程.为了利用壳幔结构约束造山带演化的深部过程, 跨越华北地块北缘、松辽—锡林浩特地块、兴安地块以及索伦—西拉木伦缝合带和二连—贺根山缝合带, 实施了一条520 km长的深地震测深剖面, 获得了高质量的人工源大当量的宽角反射和折射地震资料, 并采用地震动力学射线方法获得地壳速度结构.结果显示: (1)研究区地壳平均速度为6.15~6.3 km·s^-1, Pn波速度为7.8~8.2 km·s^-1; (2)地壳厚度约为36.1~42.2 km, 最厚位置(~42.2 km)对应地表大兴安岭主峰, 说明大兴安岭在此位置存在山根; (3)地壳速度在1.5~6.8 km·s^-1范围内, 认为在该区地壳内不存在洋壳物质; (4)主要断裂带或缝合带位于速度等值线变化剧烈的梯度带上; (5)速度结构显示研究区具有明显的横向分区和纵向分层的特点.地壳内速度剧烈变化特征表明兴蒙造山带的地壳物质组成不均匀, 尤其中下地壳, 速度等值线起伏剧烈.这种复杂的地壳速度结构应该与中生代以来多板块汇聚引发的多期区域性伸展和挤压作用有关.

Crustal velocity structure of the Xingmeng orogenic belt and its adjacent areas

The Xingmeng orogenic belt (XOB) is located in the eastern part of the Central Asian orogenic belt. As one of the most important parts of the Paleo-Asian tectonic domain, the XOB resulted from a collision between the Siberian plates and the North China plates, which has experienced complex tectonic evolution from continental breakup, through oceanic spreading and subduction to the continent orogeny. In this study, to better constrain the deep process of the XOB by understanding the crust-mantle structures, we deployed a 520km-long deep seismic sounding profiling across the northern margin of the North China block, through the Songliao-Xilinhot block and the Xing'an block to the Solonker-Xar Moron suture zone and the Erlian-Hegenshan suture zone. We obtained high-quality and large equivalent wide-angle reflection and refraction data that were excited by artificial sources. With the additional application of the seismic dynamic ray method, the resultant crustal velocity show: (1) the average crustal velocity in the study area is 6.15~6.3 km·s-1, and the velocity of the Pn wave is 7.8~8.2 km·s-1; (2) crustal thickness is 36.1~42.2 km, and the thickest crust (42.2 km) was found beneath the peak of the Great Xing'an Range, indicating an existence of the mountain root of the Great Xing'an Range; (3) crustal velocity ranges from 1.5 km·s-1 to 6.8 km·s-1, a feature indicating the absence of oceanic crust; (4) The main fault zones or suture zones are coincident with the gradients that possess sharp changes of the velocity isoline; (5) The velocity structure of the crust-upper mantle appears with lateral variation in segmentation and vertical stratification. The big difference in crustal velocity indicates heterogeneity in the crustal composition of the XOB. Especially for the mid-to-lower crust, there is a sharp variation in crustal velocity. The overall complex velocity structures may have resulted from the multi-stage extensional and compressional tectonic events associated with the blocks since the Mesozoic.