巴颜喀拉地块东部及邻区重力均衡与岩石圈有效弹性厚度

Gravity isostasy and effective elastic thickness of the eastern Bayan Har block and adjacent areas

基于SIO(Scripps Institute of Oceanography)最新全球重力和高程模型,计算了巴颜喀拉地块东部及邻区的布格重力异常、均衡重力异常、岩石圈有效弹性厚度及荷载比.结合大地热流、地震速度结构、地震活动和断裂构造分布等,分析了地壳均衡状态和岩石圈有效弹性厚度、地质构造单元间的差异及与地震活动的相关性特征.研究结果表明,该区域布格重力变化范围约为-500~0 mGal(1 mGal=10-5 m·s^-2,下同),在巴颜喀拉块体东部区域形成弧形重力梯度带,近年来的中强地震活动频发于该梯度带不同部位,应与其应力依次释放有关;均衡重力异常结果表明,其变化范围约为-80^+100 mGal,且大部分区域处于±20 mGal以内的被认为处于重力均衡的状态,重力非均衡(正或负)多出现于块体边界带附近,地震多发生在靠近块体边界的均衡重力异常(正或负,主要为正)区域内;巴颜喀拉地块东部及邻区岩石圈有效弹性厚度(Te)为10~65 km,不同构造单元之间Te空间分布差异明显,较低的T e值出现在龙门山构造带附近,T e值为20 km左右,岩石圈荷载加载比为0.5~0.8,表明现今的岩石圈挠曲状态主要由莫霍面加载形成.进一步分析表明,巴颜喀拉地块东部挤压增生与横向流动同时发生,是造成该区域地震发生与重力均衡异常高值重合、岩石圈有效弹性厚度和大地热流值较低的主要原因.本文获得的地壳均衡特征及岩石圈有效弹性强度结果,加深了对巴颜喀拉东部及邻区岩石圈构造演化过程的认识.

This study is based on SIO(Scripps Institute of Oceanography)global gravity and digital elevation data,Bouguer gravity anomalies,isostatic gravity anomalies,lithospheric effective elastic thickness and loading ratio of the eastern Bayan har block and adjacent areas.Combined with the surface heat flow,seismic velocity structure,earthquake activity and fault structure distribution,the isostatic state of the crust and the effective elastic thickness of the lithosphere,the differences between geological tectonic units and the correlation with earthquake activity are analyzed.The results show that the regional Bouguer gravity variation range is about-500~0 mGal(1 mGal=10-5 m·s-2,the same below),and the moderate-major earthquakes in the eastern Bayan Har block are mainly distributed around the arc-curved gravity gradient zone.The release of accumulated tectonic stress in different parts of the gradient zone cause earthquakes;the isostatic gravity anomalies vary from-80 to+100 mGal,and most of the regions are within±20 mGal,which are considered to be in a state of gravity equilibrium.Gravity anomalies(positive or negative)are mainly distributed around several major fault zones in the region,which have a high correlation with seismic activity.The effective elastic thickness(T e)of the lithosphere in the eastern Bayan Har block and adjacent areas is 10~65 km,the difference in T e distribution space between different structural units is obvious.The lower T e value appears in the Longmenshan fault zone,with a T e value of about 20 km,and the lithosphere loading ratio is 0.5~0.8,which indicates that the current lithospheric deflection state is mainly formed by Moho surface loading.Further analysis shows that the simultaneous extrusion and lateral flow of the Bayan Har block occur at the same time,which is the main reason for the high coincidence of the earthquake occurrence and the isostatic gravity anomalies,the effective elastic thickness of the lithosphere and the low geothermal flow value.The crustal isostatic characteristics and lithospheric strength obtained in this work would help the understanding of the lithospheric tectonic evolution process in the eastern Bayan Har block and adjacent areas.