青藏高原及周缘地壳浅层构造应力场量值特征分析

Analysis on magnitude characteristics of the shallow crustal tectonic stress field in Qinghai-Tibet plateau and its adjacent region based on in-situ stress data

以"中国大陆地壳应力环境基础数据库"中的实测地应力数据为基础,合理筛选出地理空间范围为21°N-40°N,73°N—110°N的近2000条数据,深度范围0~2 km.通过将研究区内实测地应力扣除重力影响,并考虑数据样本数量沿深度分布不均匀的问题,分析构造应力场的作用.重力影响的扣除采用海姆假说与金尼克假说两种模式估算其下限与上限,给出了青藏高原及周缘及青藏地块、南北地震带北、中、南段上构造应力和构造差应力随深度分布的特征和量值范围,结果显示:(1)青藏高原及周缘地区最大水平应力σ_H、最小水平应力σ_h随深度D呈线性增加:σ_H=22.115D+5.761、σ_h=14.893D+3.269;最大水平构造应力σ_T、最小水平构造应力σ_t的量值估算范围分别为4.609<σ_T<15.522D+4.609、3.121<σ_t<6.366D+3.121(D>0);构造差应力σ_T-σ_t=7.222D+2.492,地表值(D=0 km)为2.5 MPa左右,随深度增加以7.2 MPa.km^(-1)的梯度增大;(2)在测量深度范围内,青藏地块、南北地震带北、中、南段研究区σ_T、σ_t、σ_T-σ_t,随埋深均呈线性增大;D=1 km时,各地块σ_T的统计回归值中最大为30.1 MPa,最小为17.6 MPa,量值由大到小排序依次为:青藏地块、南北带北段、南北带中段、南北带南段;D=1 km时,各地块σ_T-σ_t,的统计回归值中最大为15.8 MPa,最小为8.9 MPa,量值由大到小排序依次为:青藏地块、南北带北段、南北带中段和南北带南段.总体表现为青藏地块强、南北带较弱的基本特征.(3)与南北带相比较,青藏地块地壳在从南向北的挤压作用下呈现出明显的"浅弱深强"特点.

Up to 2000 entries of in situ stress data measured by hydraulic fracturing and stress relief method within 21°N—40°N, 73°N—110°N, 0-2 km were selected from the ＂Database of Crustal Stress in China and Adjacent Area＂. After solving the problem of non-uniform distribution of stress data along the depth and removing the gravity contribution by limiting their upper and lower thresholds using the Heim＇s rule and Gennik＇s assumption, we studied the tectonic stress characteristics of the shallow crust in Qinghai-Tibet Plateau and its adjacent region. Our results show that： （1） In the entire study area, the maximum and minimum horizontal stress （σH, σh） in the shallow crust both increase linearly with depth （D） respectively： σH=22.115D＋5.761, σh=14.893D＋3.269. The estimated magnitude range of maximum and minimum horizontal tectonic stress （σT, σt） are 4.609〈 σT〈 15.522D＋4.609 （D 〉0） and 3.121〈 σt〈 6.366D＋3.121 （D 〉0） respectively. The difference of maximum and minimum horizontal tectonic stress （σT-σt） increases linearly with depth： σT-σt=7.222D＋2.492, i. e. the value of σT-σt on surface is about 2.5 MPa and the gradient of σT-σt is 7.2 MPa·km-1. （2） In the Qinghai-Tibet block and N-S seismic belt, σT, σt, σT-σt all increase linearly with depth. At 1 km depth, the maximum value of σT in those blocks is 30.1 MPa, while the minimum value is 17.6 MPa. The estimated values of σT in different areas decrease in the following order： Qinghai-Tibet, north segment of north-south seismic belt, middle segment of north-south seismic belt and south segment of North-South seismic belt. At 1 km depth, the maximum value of σT-σt in those blocks is 15.8 MPa, while the minimum value is 8.9 MPa. The descending order of the values in different areas is same as above. In general, the tectonic stress magnitude is much higher in Qinghai-Tibet block than in N-S seismic belt. （3） Compared with the N-S seismic belt, the northward crustal compression in the Qinghai-Tibet block is relatively lower in magnitude in the shallow subsurface and higher in deep, apparently displays a ＂weaker in the shallow but stronger in the deep＂ feature.