中国大陆造山带地壳密度结构特征

Crustal Density Structures beneath Orogenic Belts of Chinese Continent

本文探讨中国大陆造山带地壳介质密度的变化特征和不同类型,并用大陆动力学机制解释这些特征产生的原因。根据重力场小波变换的尺度—源深度转换律,进行地面重力异常场多尺度分解,取得了反映中国地壳不同埋藏深度的小波细节,揭示了造山带地壳的密度结构。和克拉通地体不同,中国大陆的造山带下地壳密度都偏低,只有古生代块体弱碰撞形成的摺皱山脉是例外。这些造山带大都与古大洋封闭和俯冲碰撞作用有关。古大洋封闭后在地壳中留下的裂隙和海水,会造成地壳密度降低。中国大陆造山带地壳密度结构大致可以分四类。上中下地壳密度都偏低的属于第一类造山带。第二类造山带上中地壳密度偏低,而下地壳密度偏高。第三类造山带上中地壳密度偏高,而下地壳密度偏低。第四类造山带上中下地壳密度都偏高。通过板块构造原理和岩石物理规律对青藏高原造山带地壳密度结构分类的解释可见,青藏高原南部,中部和北部地壳密度结构分属三类不同的造山带,体现印度次大陆和欧亚板块碰撞不同阶段作用造成的结果。根据地壳密度变低和厚度是否加大,还可以识别板沿和板内造山带。

Objectives ： This work shows the results of computing and interpretation of the these results based on geodynamics belts. the 3D density disturbance images in Chinese continent and petro-physics, especially for crust beneath orogenic Methods： We apply the scale--depth relationship of gravity anomalies and the multi-scale wavelet for extracting crustal density information from surface Bouguer gravity data of Chinese continent, and presents source disturbance images of the upper, middle and lower crust, which represent the three dimensional density structures in the studied area. Results： The density structures beneath orogenic belts can be divided into four types： （1） relatively low in whole crust; （2） relatively low in the upper and middle crust, but high in the lower crust; （3） relatively high in the upper and middle crust, but low in the lower crust; and （4） relatively high in whole crust. In southern, middle and northern of Qingzang Plateau, the orogenic belts belong to three different types of the density structures respectively. Based on geodynamics and petro-physics, we infer this arrangement was the result of the collision between India and Eurasia plates. Based on the low-density crustal structures and thickness, one can recognize most of the plate-margin or intra-plate orogenic belts. Conclusions： As the results shows, different from craton terranes, the lower crust of the orogenic belts is usually of low density, except few fold maintains created by weak collision between two small blocks during the Paleozoic.