帕米尔—兴都库什地区构造应力场反演及拆离板片应力形因子特征研究

从Global CMT目录搜集了1976年1月至2016年6月之间的震源深度大于70km的255个震源机制解,用阻尼应力反演方法,分70～160km和170～310km两个深度,计算了帕米尔—兴都库什地区的构造应力场;同时以10km为间隔计算了兴都库什地区深度介于70～310km之间的应力形因子.得到以下初步结论:兴都库什板片向下俯冲和帕米尔地区断裂带的横向拉张,可能是导致应力场不同的原因.兴都库什俯冲带与帕米尔俯冲带碰撞,导致交汇地区(37°N—37.5°N)的应力场参数突变.兴都库什俯冲板片受到深部温度、压力等因素,出现薄弱面进而形成拆离板片.其脱离了主俯冲板片的束缚后,重力的上下拉张作用导致空区附近张轴倾伏角接近90°,拆离板片俯冲至上地幔不连续面,导致板片部分熔融进而应力形因子随着深度变小.而拆离板片受到地幔挤压其内部发生破碎,其压应力轴由西部的NS到东部NW-SE方向偏转及纵向张应力轴倾伏角变小.

Inversion for the tectonic stress field and the characteristic of the stress shape factor of the detachment slab in the Pamir-Hindu Kush area

This work collected focal mechanism solutions of 255 events with depths larger than 70 km in the Pamir-Hindu Kush from Global CMT catalogue January 1976 to June 2016. By using the method of regional damped stress field inversion, the tectonic stress fields in the depths of 70~160 km and 170~310 km are determined, respectively. The stress shape factor is calculated with an interval 10 km in depth range 70~310 km beneath the Hindu Kush area. Results show that subduction of the detachment slab in Hindu Kush and the horizontal extension of fault zones under Pamir may cause different stress regimes in Pamir and Hindu Kush. The stress field parameters are dramatically changed in the collision area of the Pamir and Hindu Kush nearby 37°N-37.5°N. The detachment slab in the subduction slab may be formed due to the weakness resulted from the high temperature and high pressure. After the detachment slab separates from the main body, the extensional axis shows nearly vertical near the seismic gap under the gravity pulling of the detaching slab that dives to the discontinuity in upper mantle. Heated by high temperature, the detached slab is in partial melting state, resulting in decrease of the stress shape factor with depth. The interior of the detached slab would be broken under compression in upper-mantle, which makes the compressive axis rotate from NS in west to NW-SE in east and the dip of the extensional axis become smaller.