苏鲁造山带区域地壳山根结构特征

本利用苏鲁大别造山带及其邻区的三维P波速度资料，详细对比研究了苏鲁与大别超高压变质带莫霍面深度和深部P波速度结构分布特征。结果表明，尽管苏鲁、大别超高压变质带都具有上地壳明显高速且上凸；中地壳增厚；下地壳埋藏较深且下凹等共同的P波速度结构特征，与大别地区相比较，苏鲁超高压变质带还存在着独特的区域性特征。从地貌上看，苏鲁地区山脉已经基本消失。苏鲁超高压变质带的地壳厚度为32～33公里，深于其周围地区2～3公里，但是莫霍面下凹程度远不如大别地区，造山带地壳山根已逐步趋向消失。苏鲁地区上地壳P波速度高于大别，比其周围地区约快1～1．2km/s，有可能显示了该区有更多高速、高密度的超高压变质岩折返到上地壳与地表的岩石物性效果。大别造山带山脉依然存在，莫霍面下凹更明显，沿NWW向串状残留地壳山根最深为37～38公里，深于其周围地区3～4公里。对比研究结果表明，由于区域构造运动的作用，苏鲁大别造山带中的不同地段，在其造山、演化过程中也存在着差别。苏鲁的造山运动起始虽略晚于大别，但结束的更快，比大别更早进入了造山运动的后期。分析促使苏鲁造山运动进程加速的主要构造原因可能有两点，郯庐断裂带的左旋走滑运动以及通过中国华北区域的大范围NW-SE向扩张应力场的影响。大区域构造背景加速了苏鲁造山带地表高山侵蚀过程的同时，随着山根浮力的不断减弱，地壳深部山根逐渐趋向消失。地壳速度结构特征有可能反映了苏鲁造山带的地壳山根随着地表山脉的侵蚀而减弱，趋向消失的过程。

Regional structure characteristics of crustal root of mountain beneath the Sulu orogenic belt

The depth distribution of Moho discontinuity and P-wave velocity structure beneath the Orogenic Belt and the ultra-high-pressure metamorphic belts (UHPM) in the Sulu and Dabie regions were analyzed in the present analysis, respectively. The results suggest that the orogenic structure beneath the Sulu region evidently differ from those beneath the Dabie region, although there are some similar structure characteristics in the two regions. The mountains already are worn away in the Sulu region. The crustal thickness is about 32 - 33 km, greater than those in its surrounding areas. The Moho discontinuity beneath the Sulu orogenic region does not obviously cave in. The crustal root of mountain probably has exhumed. The upper crustal velocity in the Sulu region is about 5. 6 -6.2 km/s, being 1-1. 2km/s greater than those in its surroundings and also greater than that in the Dabie region. This velocity difference between the Sulu and Dabie regions shows that more UHPM rocks of high velocity and density in the mantle exhumed into the upper crust and the surface in the Sulu region. The Dabie mountains, however, towers aloft still. The Moho discontinuity shaping like a series of ellipsoids along the NWW direction beneath Dabie caves in obviously. The remnant crustal root of old mountain is as deep as 38 km. These results show that the orogenic motions and the evolutions in the Sulu region are different from those in the Dabie orogen region although they are in identical orogenic belt. The orogeny started late in Sulu, the orogeny, however, ends early. The orogeny in Sulu entered in the late period of the orogeny much earlier than that in Dabie. The early exhumation of the crustal root of old mountain in Sulu may probably relate to the left turning strike-slip motions of the Tanlu fold and the effects of the extensional tectonic stress field in NW-SE in East China. The crustal root of old mountain gradually exhumes in response to the decrease of buoyancy of the crustal root, as the continuing erosion progressively wore away surface relief in the Sulu region because of the tectonic motions in the wide region. The velocity structure characteristics in the present analysis probably reflect the exhuming procedure of the crustal root with the wearing away surface relief due to the erosions.